Electronic versions of our publications have been created for distribution directly from our Web Site. The documents listed below, which have been converted to PDF format files for viewing, downloading and printing, may be selected by clicking on their associated numbers. PDF documents can be viewed within Web browser windows that are compatible with Netscape Navigator or Internet Explorer. You may need to download and install the free Adobe Acrobat Reader.

Technical Papers

(265) N. Jayaraman, P. Prevéy, "Case Studies of Mitigation of FOD, Fretting Fatigue, Corrosion Fatigue and SCC Damage by Low Plasticity Burnishing in Aircraft Structural Alloys," Proceedings of USAF ASIP, Memphis, TN, Nov. 29 to Dec. 1, 2005

Surface enhancement technologies such as shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB) can provide mitigation of foreign object damage (FOD), fretting fatigue, corrosion fatigue, and stress corrosion cracking (SCC) damage. However, to be effective, the compressive residual stresses must be retained in service for successful integration into aircraft structural design, and the process must be affordable and compatible with the manufacturing environment. LPB provides high magnitude deep thermally and mechanically stable compression, and is performed on CNC machine tools. LPB provides a means to extend the lives of both new and legacy aircraft structural components. Improving fatigue performance by introducing deep stable layers of compressive residual stress avoids the generally prohibitive cost of modifying either material or design.

 

(264) N. Jayaraman, P. Prevéy, "An Overview of the use of Engineered Compressive Residual Stresses to Mitigate SCC and Corrosion Fatigue," Proceedings of 2005 Tri-Service Corrosion Conference, Orlando, FL, Nov. 14-18, 2005.

Mechanical suppression of corrosion fatigue and stress corrosion cracking (SCC) through low plasticity burnishing (LPB) has been demonstrated in several systems. LPB can be used to impart controlled compressive residual stresses of desired depth, magnitude and location in structural components. For example, in 300M aircraft landing gear steel, LPB produced over 1.2 mm of compression and in turn completely mitigated corrosion fatigue and SCC, even in the presence of simulated foreign object damage (FOD). In precipitation hardenable stainless steels used for turbine engine compressor blade applications, full mitigation of corrosion fatigue damage and FOD has been achieved. In friction stir processed (FSP) aluminum alloys, LPB mitigated the corrosion fatigue damage by completely suppressing the tensile residual stresses associated with the FSP. A comprehensive design procedure has been developed to determine the optimum location and magnitude of compression. Effects of compensatory tension and part distortion are analyzed with the use of several design tools like Fatigue Design Diagram and finite element analysis (FEA). Actual measurements of residual stresses and corrosion fatigue tests are used to validate the initial design. LPB has been applied to successfully mitigate FOD and/or SCC in 300M landing gear steels, corrosion fatigue in turbine engine compressor blade 17-4PH and Custom450 stainless steels, and aircraft structural aluminum alloys AA7075-T6 and AA2219-T8751. In this paper, selected examples demonstrating the mitigation of active corrosion fatigue, restoration of fatigue performance after severe pitting due to prior corrosion exposure, and complete prevention of SCC by LPB treatment are presented.

 

(263) D. Hornbach, P. Prevéy, and E. Loftus, "Application of Low Plasticity Burnishing (LPB) to Improve the Fatigue Performance of Ti-6Al-4V Femoral Hip Stems," Symp. On Fatigue & Fracture of Medical Metallic Materials & Devices, Dallas, TX, Nov. 10, 2005

Low plasticity burnishing (LPB) is a surface enhancement method that produces a deep layer of compressive residual stress with minimal cold working and an improved surface finish. Extensive fatigue testing, performed on numerous metal alloys in simulated environmental conditions, demonstrates that LPB significantly improves fatigue strength of highly stressed components. LPB is a flexible process, capable of being implemented on a wide variety of CNC machine tools. A product-specific LPB process was developed and applied to the modular neck taper junction of a Ti-6Al-4V total hip prosthesis (THP). LPB produced a compressive residual stress field with an improved surface finish, which enhanced component fatigue strength and resistance to fretting damage. X-ray diffraction (XRD) residual stress measurements, made before and after LPB application, are shown. High cycle fatigue (HCF) results obtained on LPB-processed hip stems are shown along with baseline data for unprocessed hip stems. HCF tests demonstrate complete elimination of fretting fatigue failures in the LPB processed area of the taper junction and a substantial increase in overall THP fatigue strength.

 

(262) N. Jayaraman, P. Prevéy and R. Ravindranath, "Improved Damage Tolerance of Ti-6Al-4V Aero Engine Blades and Vanes using Residual Compression by Design," Proceedings of NATO RSV, Granada, Spain, Oct. 3-7, 2005.

The deep stable layer of compressive residual stress produced by low plasticity burnishing (LPB) has been demonstrated in laboratory testing to improve damage tolerance in engine alloys IN718, Ti-6Al-4V, Ti-6-2-4-6, and 17-4PH. This paper describes the fatigue and FOD tolerance benefits afforded by LPB treatment of a Ti-6Al-4V first stage fan blade and vane. FOD sensitive blades and vanes removed from fielded engines were LPB processed to protect the leading edge of the blade and the trailing edge of the vane. Both components were fatigue tested in cantilever bending mode at R>0 using specially designed test fixtures. FOD was simulated with machined notches for the blade and electrical discharge machined (EDM) notches for the vane. Residual stress and cold work distributions were measured using x-ray diffraction mapping techniques. LPB produced a zone of nominally -100 ksi (-690 MPa) through-thickness compression in the leading edge of the blade and trailing edge of the vane. The HCF strength for LPB processed blades was 125 ksi (860 MPa) without FOD, and equal or greater than the as-received blades for FOD up to 0.050 in. (1.3 mm) deep - an order of magnitude improvement in damage tolerance. For both vanes and vane simulation specimens with 0.020 in. (0.5 mm) deep FOD, the HCF strength after LPB was over 4 times higher than the unprocessed counterparts. The HCF performance was largely unaffected by FOD up to 0.030 in. (0.7 mm) deep. If the traditional design criterion of Kt=3 is used, both the LPB processed blade and vane could be considered tolerant of even 0.10 in. (2.5 mm) deep FOD.

(261) J. Cammett, P. Prevéy and N. Jayaraman, "The Effect of Shot Peening Coverage on Residual Stress, Cold Work, and Fatigue in a Nickel-base Superalloy," Proceedings of ICSP 9, Paris, Marne la Vallee, France, Sept. 6-9, 2005.

The goal of this work was to determine effects of shot peening (SP) coverage on the compressive residual stress magnitude, depth, and relaxation of residual stresses due to thermal exposure, as well as fatigue strength of IN718, a nickel-base superalloy. The residual stress-depth profiles (both depth of compression and magnitude) for coupons shot peened to different coverage levels of 82% (0.2T where T = time to achieve full area coverage) to 400% (4T) show a slight trend of increased depth of compression with increase in coverage. Though having similar residual stress distributions, the coupons exhibited markedly different cold work distributions. While 82% (0.2T) coverage resulted in less than five percent cold work, increasing coverage to 400% (4T) resulted in cold work as high as thirty-five percent. The heavily cold worked surfaces of the higher coverage coupons exhibited significant relaxation of surface residual stresses, accompanied by corresponding reduction of cold work upon thermal exposure at 525°C for 10 hours. In contrast, the low cold work associated with lower SP coverage resulted in little relaxation of residual stresses under the same conditions. These observations were consistent with findings in other alloy systems. High cycle fatigue (HCF) performance at 525°C showed little dependence on peening coverage. Even with deeper compression achieved through low plasticity burnishing (LPB), the 525C fatigue performance of IN718 was only marginally improved. There may be other controlling elevated temperature fatigue mechanisms, such as oxidation, operating here that do not depend on residual stresses. HCF behavior at room temperature for the LPB treatment was significantly better than for SP treatment.

 

(260) P. Prevéy, N. Jayaraman and J. Cammett "Overview of Low Plasticity Burnishing for Mitigation of Fatigue Damage Mechanisms," Proceedings of ICSP 9, Paris, Marne la Vallee, France, Sept. 6-9, 2005.

Surface enhancement technologies such as shot peening (SP), laser shock peening (LSP), and low plasticity burnishing (LPB) can provide substantial fatigue life improvement. However, to be effective, the compressive residual stresses that increase fatigue strength must be retained in service. LPB provides thermally stable compression and can be performed in conventional machine shop environments on CNC machine tools. LPB enables the extension of component service lives fatigue limited by various damage mechanisms including foreign object damage (FOD), corrosion fatigue, pitting, and fretting. The thermal and mechanical stability of the compressive layer are briefly reviewed. The LPB process, tooling, and control system are briefly described. Four representative applications are presented: thermal stability in IN718, improved damage tolerance in Ti-6-4 fan blades, mitigation of fretting fatigue damage in Ti-6-4, and improved corrosion fatigue in 17-4PH stainless steel.

 

(259) N. Jayaraman, P. Prevey, N. Ontko, M. Shepard, R. Ware and J. Coate, "Comparison of Mechanical Suppression by Shot Peening And Low Plasticity Burnishing to Mitigate SCC and Corrosion Fatigue Failures in 300M Landing Gear Steel," Proceedings of ICSP 9, Paris, Marne la Vallee, France, Sept. 6-9, 2005.

300M steel is widely used in aircraft landing gear because of its unique combination of strength and fracture toughness, but is vulnerable to foreign object damage (FOD), corrosion fatigue, and stress corrosion cracking (SCC) failures with potentially catastrophic consequences. The fatigue, corrosion fatigue in salt water, and SCC performance of LPB processed 300M steel was compared with shot peened (SP) and low stress ground (LSG) conditions. LPB, with and without simulated FOD, produced deep residual compression that dramatically improved both the HCF and corrosion fatigue strength. The fatigue strength of LSG and SP treated surfaces was drastically reduced by salt and FOD exposure with no discernible endurance limit for corrosion fatigue conditions. SCC testing of LPB treated landing gear sections at 1030 to 2270 MPa (150 to 180 ksi) static loads was terminated after 1500 hrs without failure, compared to failure in as little as 13 hours without treatment. Mechanistically, the deep compressive surface residual stresses from LPB treatment mitigated both the individual and synergistic effects of corrosion fatigue and FOD. LPB also reduced the surface stress well below the SCC threshold for 300M, even under high tensile applied stresses, effectively suppressing the SCC failure mechanism.

 

(258) C.Giummarra, H.Zonker, "Improving the Fatigue Response of Aerospace Structural Joints," Proc. ICAF 2005, Hamburg, Germany, 2005.

The effect of various surface treatments on the fretting fatigue and joint fatigue performance of a 7xxx series aluminum alloy was investigated with the objective to reduce the nucleation and growth of fretting cracks and enhance the fatigue life of aerospace joints. The results indicate that anodizing does not influence the fretting fatigue performance and the type of anodizing does not affect the joint fatigue life. UltraCem coating inhibited fretting crack nucleation in the fretting specimen, increasing the fatigue life. Shot peening increased the fretting fatigue life significantly due to the compressive residual stresses it imparts; however, the stresses were not deep enough to influence the fretting cracks which nucleated in the hole bore of the joint specimens. Laser peening and low plasticity burnishing induce deeper compressive residual stresses than shot peening, which appear to inhibit the growth of fretting cracks in both the fretting and joint specimens, resulting in a significant fatigue life improvement.

(257) P. Prevey, N. Jayaraman, R. Ravindranath, "Use of Residual Compression in Design to Improve Damage Tolerance in Ti-6Al-4V Aero Engine Blade Dovetails," Proc. 10th Nat. HCF Conf., New Orleans, LA, Mar. 8-11, 2005.

The deep stable layer of compressive residual stress produced by low plasticity burnishing (LPB) has been demonstrated to improve the damage tolerance in engine alloys IN718, Ti-6Al-4V, and 17-4PH. This paper describes the application of LPB to the dovetail bedding surface of a Ti-6Al-4V fan blade to mitigate the adverse effects of fretting-induced microcracks. Blades removed from fielded engines were LPB processed to protect the dovetail region of the blade and specially designed feature specimens were used to simulate the dovetail region of the blades. Both feature specimens and actual dovetail sections were fatigue tested in cantilever bending mode at a stress ratio, R of 0.5 using specially designed test fixtures. Coalescing microcracks were simulated with electrical discharge machined (EDM) notches. Residual stress and cold work distributions were measured using x-ray diffraction mapping techniques.

LPB produced compression in the dovetail region up to a depth of 0.065 in. The HCF performance with EDM notches up to 0.040 in. deep was tested. LPB processed specimens with 0.020 in. deep EDM notch showed an endurance limit of 100 ksi, greater than that of the baseline undamaged surface. LPB treated blades and feature specimens with 0.030 in. and 0.040 in. deep notches showed endurance limits of 60 and 45 ksi, respectively. LPB was shown to fully mitigate the fretting debit, whether applied before or after the fretting damage occurred.

Linear elastic fracture mechanics analysis including the residual stress fields confirms the HCF performance in the presence of high residual compression. A novel approach for determining the residual stress field design to provide a desired fatigue life and microcrack tolerance is introduced.

(256) P. Prevey, N. Jayaraman, R. Ravindranath, "Design Credit for Compressive Residual Stresses in Turbine Engine Components," Proc. 10th Nat. HCF Conf., New Orleans, LA, Mar. 8-11, 2005.

The high cycle fatigue (HCF) performance of turbine engine components has long been improved by the introduction of a surface layer of compressive residual stress, usually by shot peening. However, credit is not generally taken for the improved fatigue performance in component design. Laser shock processing (LSP) and low plasticity burnishing (LPB) provide impressive fatigue and damage tolerance improvement by introducing deep or through-thickness compression in fatigue critical areas, but have been applied primarily to improve existing, rather than new, designs. This paper describes a design methodology to allow credit to be taken for beneficial residual stresses in component design to achieve a required or optimal fatigue performance.

The fatigue design methodology is based on an extension of the traditional Haigh Diagram to include compressive mean stresses. The Smith Watson Topper equation (or other similar equations by Walker or Jasper) is used in combination with Neuber’s rule to account for both the stress ratio, R, and stress concentration factors from notches and cracks. The extension of the Haigh Diagram into the compressive mean stress region and the effect of stress concentration factors lead to the identification of a safe range of mean and alternating stresses in which there can be no Mode I crack growth. This in turn is used to determine the minimum and optimum compressive residual stresses needed to mitigate different damage conditions in terms of k f.

Case studies are presented illustrating the design approach forTi-6Al-4V turbine engine compressor blade and vane edges to mitigate FOD and fan blade dovetail surfaces to mitigate fretting damage.

 

(255) D. Hornbach, P. Prevey, M. Blodgett, "Practical Application of Nondestructive Residual Stress Measurements by X-ray Diffraction," ASNT Fall Conference, Las Vegas, NV, Nov. 15-16, 2004.

A modified Integral Method was investigated as a means to nondestructively measure the subsurface residual stress distribution. The technique has been demonstrated to be feasible in aluminum alloys by comparison to established destructive measurement methods.

In the current effort a thorough study of higher energy radiations was conducted to obtain deeper penetrating radiations on titanium and nickel base alloys. Higher energy radiation used in conjunction with the modified Integral Method would provide nondestructive subsurface residual stress measurement in components composed of these alloys. Results of the study show a nondestructive x-ray residual stress method providing measurements to depths of 0.0028 to 0.003 in. (51 to 76 m m) is not technically feasible.

(254) P. Prevey, N. Jayaraman, N. Ontko, M. Shepard, R. Ware, J. Coate, "Mechanical Suppression of SCC and Corrosion Fatigue Failures in 300M Steel Landing Gear," Proceedings of ASIP 2004, Memphis, TN, Nov. 29 to Dec. 2, 2004.

300M steel is widely used in landing gear because of its ultra high strength with high fracture toughness, but is vulnerable to both corrosion fatigue and stress corrosion cracking, with potentially catastrophic consequences. Plating and shot peening surface treatments currently used to extend life are only partly effective. A surface treatment is needed that will mitigate foreign object damage (FOD), corrosion fatigue and stress corrosion cracking. This paper describes the use of low plasticity burnishing (LPB) to improve damage tolerance and to mechanically suppress stress sensitive corrosion failure mechanisms.

The fatigue and corrosion fatigue performance of LPB processed 300M steel was compared with shot peened (SP) and low stress ground (LSG) conditions. LPB produced residual compression to a depth of 1.27 mm (0.050 in.), and shot peening only 0.127 mm (0.005 in.), an order of magnitude less. LPB treatment dramatically improved both the high cycle fatigue (HCF) performance and corrosion fatigue strength, with and without simulated FOD. LPB treated specimens with 0.020 in. deep FOD exhibited a definite endurance limit of 1035 MPa (150 ksi) even under corrosion fatigue conditions. Stress corrosion cracking (SCC) testing of LPB treated landing gear sections at 1030 to 2270 MPa (150 to 180 ksi) static loads was terminated after 1500 hrs without failure, compared to failure in as little as 13 hours without treatment. Corrosion and FOD caused early crack initiation and growth, dramatically decreasing fatigue performance. Deep surface compressive from LPB mitigated both the individual and synergistic effects of corrosion fatigue and FOD. LPB reduced the surface stress well below the SCC threshold for 300M, even under high tensile applied loads, effectively mechanically suppressing the SCC failure mechanism.

(253) P. Prevey, N. Jayaraman, and R. Ravindranath, "Incorporation of Residual Stresses in the Fatigue Performance Design of Ti-6Al-4V," Proceedings of the Intl. Conf. on Fatigue Damage of Structural Materials V, Hyannis, MA, Sept. 19-24, 2004.

The high cycle fatigue (HCF) performance of turbine engine components has long been improved by the introduction of a surface layer of compressive residual stress, usually by shot peening. However, credit has not been taken for the improved fatigue performance in component design; rather shot peening is used primarily as an additional safe guard against fatigue failure. Recently, laser shock processing (LSP) and low plasticity burnishing (LPB) have been shown to provide spectacular fatigue and damage tolerance improvement by introducing deep or through-thickness compression in fatigue critical areas. These new processes have been introduced primarily to improve an existing inadequate design, and credit for the fatigue benefits is not taken in the initial design. This paper describes a design methodology and testing protocol to take credit for beneficial residual stresses in component design to achieve a required or optimal fatigue performance.

A protocol has been developed for designing a residual stress distribution using surface treatments to achieve a targeted HCF performance. The protocol is applied to a 1 st stage Ti-6Al-4V compressor blade to provide the optimal leading edge damage tolerance. The use of finite element modeling (FEM), linear elastic fracture mechanics, and x-ray diffraction (XRD) mapping of the residual stress field to develop an LPB generated residual stress distribution is described. A novel adaptation of the traditional Haigh diagram to estimate the compressive residual stress magnitude for optimal fatigue performance is introduced. Fatigue results on both blade-edge feature samples and fretting damaged samples with various k f are compared with analytical predictions provided by the design methodology.

(252)P. Prevey, N. Jayaraman, and R. Ravindranath, "HCF Performance and FOD Tolerance Improvement in Ti-6Al-4V Vanes with LPB Treatment," Proceedings of the 42nd AIAA Aerospace Sciences Meeting & Exhibit, Reno, NV, Jan. 5-8, 2004.

Mechanical surface treatments that introduce a layer of residual surface compression improve high cycle fatigue (HCF) performance. If the depth of compression extends through the thickness of blade or vane edges, foreign object damage (FOD) tolerance can be dramatically improved. The effect of low plasticity burnishing (LPB) on the HCF performance and FOD tolerance of a first stage Ti-6Al-4V turbine engine vane have been investigated in both tension-tension (R=0.1) and fully revered bending (R=-1). Actual vanes from fielded engines and blade-edge feature samples were fatigue tested with FOD simulated by EDM notches.

The fatigue strength for LPB processed blades increased over 4-fold for both vanes and vane-edge feature specimens with FOD 0.020 in. deep, and was undiminished by 0.030 in. deep FOD. Assuming a Kt = 3 HCF performance criteria, LPB provided tolerance of FOD up to 0.10 in. deep. The beneficial through-thickness compression was retained even for compressive loading in fully reversed bending. The fatigue and FOD tolerance improvement are shown by linear elastic fracture mechanics modeling to be due to the deep stable compressive layer produced by LPB.

(251) D.Hornbach, P. Prevéy, and M. Blodgett, "Development of Nondestructive Residual Stress Profile Measurement Methods - The Integral Method,"
Proceedings of the Review of Progress in Quantitative Nondestructive Evaluation, Vol. 24, Eds. D.O. Thompson and D.E. Chimenti, July 25-30, 2004, Golden, CO.

In the current effort a thorough study of higher energy radiations was conducted to obtain deeper penetrating radiations on titanium and nickel base alloys. Higher energy radiation used in conjunction with the modified Integral Method would provide nondestructive subsurface residual stress measurement in components composed of these alloys.

(250) P. Prevéy, N. Jayaraman, N. Ontko, M. Shepard, R. Ware, and J. Coate, "Mitigation of SCC and Corrosion Fatigue Failures in300M Landing Gear Steel using Mechanical Suppression,"
Proceedings of the 6th Aircraft Corrosion Workshop, August 24-27, 2004, Solomons, MD.

The fatigue and corrosion fatigue performance of LPB processed 300M steel was compared with shot peened (SP) and low stress ground (LSG) conditions. LPB produced residual compression to a depth of 1.27 mm (0.050 in.), and shot peening only 0.127 mm (0.005 in.), an order of magnitude less. LPB treatment dramatically improved both the HCF performance and corrosion fatigue strength, with and without simulated FOD. The corrosion fatigue strengths of LSG and SP surfaces decreased dramatically, to only 20% and 50%, respectively, of the baseline strength, with no discernible endurance limit behavior under corrosion fatigue conditions. The fatigue behavior was even worse with FOD, simulated with a 0.5 mm (0.020 in.) deep EDM notch, both in air and exposed to salt. In contrast, LPB treated specimens with FOD exhibited a definite endurance limit of 1035 MPa (150 ksi) even under corrosion fatigue conditions. SCC testing of LPB treated landing gear sections at 1030 to 2270 MPa (150 to 180 ksi) static loads was terminated after 1500 hrs without failure, compared to failure in as little as 13 hours without treatment.

Mechanistically, the effect of corrosion and FOD resulted in early crack initiation and growth, thus resulting in dramatic decrease in fatigue performance. Despite the existence of similar corrosion conditions, the deep compressive surface residual stresses from LPB treatment mitigated both the individual and synergistic effects of corrosion fatigue and FOD. The deep compressive layer produced by LPB reduced the surface stress well below the SCC threshold for 300M, even under high tensile applied loads, effectively mechanically suppressing the SCC failure mechanism.

(249) P. Prevéy, J. Cammett, "The Influence of Surface Enhancement by Low Plasticity Burnishing on the Corrosion Fatigue Performance of AA7075-T6,"
International Journal of Fatigue, Elsevier Science Ltd., Ed. Prof. K.L. Reifsnider, Vol 26/9, pp 975-982, Sept. 2004.

The restoration of fatigue performance by LPB processing of AA7075-T6 after severe pitting in salt fog was previously described. This paper describes benefits of introducing a deep compressive residual stress by LPB on fatigue strength after salt fog pitting and corrosion fatigue (under active corrosion) performance. Since LPB processing was performed in a conventional CNC machining center, it offers a cost effective and practical alternative to alloy substitution or component re-design as a means of improving the structural integrity of aging aircraft.

(247) P. Prevéy, N. Jayaraman, R. Ravindranath, "Low Plasticity Burnishing (LPB) Treatment to Mitigate FOD and Corrosion Fatigue Damage in 17-4 PH Stainless Steel,"
Proceedings Tri-Service Corrosion Conference, Las Vegas, NV, Nov. 17-21, 2003.

Corrosion fatigue strength (in the presence of active corrosion medium of 3.5% NaCl solution) of the LSG material showed a drop of nearly 33% from the baseline material without corrosion; LPB material showed corrosion fatigue strength nearly the same as the baseline material without corrosion. While the introduction of a simulated FOD on the LSG dramatically decreased the fatigue strength to less than 15 ksi (~100 MPa), LPB retained nearly 90% of the fatigue strength of the baseline material without corrosion.

Mechanistically, the effect of corrosion and FOD resulted in early crack initiation and growth, thus resulting in a dramatic decrease in fatigue performance. Despite the existence of similar corrosion conditions, the deep compressive surface residual stresses from LPB treatment helped to mitigate the individual and synergistic effects of corrosion fatigue and FOD.

(246) N. Jayaraman, P. Prevéy, "Application of Low Plasticity Burnishing (LPB) to Improve the Corrosion Fatigue Performance and FOD Tolerance of Alloy 450 Stainless Steel,"
Proceedings Tri-Service Corrosion Conference, Las Vegas, NV, Nov. 17-21, 2003.

Mechanistically, the effect of corrosion and FOD resulted in early crack initiation and growth, and dramatically decreasing the fatigue performance. Despite the existence of similar corrosion conditions, the deep compressive surface residual stresses from LPB treatment helped to mitigate the individual and synergistic effects of corrosion fatigue and FOD.

(245) P. Prevéy, N. Jayaraman, R. Ravindranath, "Mitigation of FOD and Corrosion Fatigue Damage in 17-4 PH Stainless Steel Compressor Blades with Surface Treatment,"
Proceedings 9th National HCF Conference, Pinehurst, NC, Mar. 16-19, 2004.

(244) P. Prevéy, N. Jayaraman, M. Shepard, "Improved HCF Performance and FOD Tolerance of Surface Treated Ti-6-2-4-6 Compressor Blades,"
Proceedings 9th National HCF Conference, Pinehurst, NC, Mar. 16-19, 2004.

(243) P. Prevéy, N. Jayaraman, R. Ravindranath, "Introduction of Residual Stress to Enhance Fatigue Performance in the Initial Design,"
Proceedings Turbo Expo 2004, Vienna, Austria, Jun. 14-17, 2004.

(242) P. Prevéy, N. Jayaraman, J. Cammett, "Mitigation of Active Corrosion Fatigue and Corrosion Pit Initiated Fatigue in AA7075-T6 with Low Plasticity Burnishing," Proceedings ASIP Conference, Savannah, GA, Dec. 2-4, 2003

(241) M. Shepard, P. Prevéy, N. Jayaraman, "Effect of Surface Treatments on Fretting Fatigue Performance of Ti-6Al-4V,"
Proceedings 8th National Turbine Engine HCF Conference, Monterey, CA, April 14-16, 2003

(240) P. Prevéy, N. Jayaraman, R. Ravindranath, "Effect of Surface Treatments on HCF Performance and FOD Tolerance of a Ti-6Al-4V Vane,"
Proceedings 8th National Turbine Engine HCF Conference, Monterey, CA, April 14-16, 2003

Mechanical surface treatments including shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB) have been shown to introduce compressive residual stresses that improve high cycle fatigue (HCF) performance and foreign object damage (FOD) tolerance. Shot peening has been widely used to introduce a shallow (<250 mm) beneficial compressive layer, at the expense of roughening and heavily cold working the surface. Minimizing cold work during surface enhancement has been shown to improve both thermal and mechanical stability of the compressive layer. Compression deeper than achievable by shot peening has been shown to dramatically improve FOD tolerance in Ti-6Al-4V.
(239) P. Prevéy and M. Mahoney, "Improved Fatigue Performance of Friction Stir Welds with Low Plasticity Burnishing: Residual Stress Design & Fatigue Performance Assessment," Proceedings Thermec 2003, Madrid, Spain, July 7-11, 2003.

Although friction stir welding (FSW) produces minimal distortion, residual stresses are created that impact fatigue and stress corrosion performance. X-ray diffraction residual stress and cold work mapping methods used at Lambda Research for FSW studies are described. Post weld surface enhancement processing can be used to place the FSW region in compression to improve fatigue performance. Deep compressive residual stress distributions produced by low plasticity burnishing (LPB) designed to improve fatigue and corrosion fatigue performance in aluminum alloy FSW are described. The LPB tooling and FSW processing are presented. Corrosion fatigue testing used for FSW samples is described and compared to fracture mechanics based fatigue life predictions calculated for the measured residual stress distributions. Residual stress and corrosion fatigue results for aluminum alloy 2219-T8751 are presented.

 

(238) P. Prevéy, R. Ravindranath, M. Shepard, T. Gabb, "Case Studies of Fatigue Life Improvement Using Low Plasticity Burnishing in Gas Turbine Engine Applications," Proceedings of ASME Turbo Expo 2003, Atlanta, GA, June 16-19, 2003.

Surface enhancement technologies such as shot peening, laser shock peening (LSP), and low plasticity burnishing (LPB) can provide substantial fatigue life improvement. However, to be effective, the compressive residual stresses that increase fatigue strength must be retained in service. For successful integration into turbine design, the process must be affordable and compatible with the manufacturing environment. LPB provides thermally stable compression of comparable magnitude and even greater depth than other methods, and can be performed in conventional machine shop environments on CNC machine tools. LPB provides a means to extend the fatigue lives of both new and legacy aircraft engines and ground-based turbines. Improving fatigue performance by introducing deep stable layers of compressive residual stress avoids the generally cost prohibitive alternative of modifying either material or design.

 

(237) P. Prevéy, D. Hornbach, R. Ravindranath, J. Cammett, "Application of Low Plasticity Burnishing to Improve Damage Tolerance of a Ti-6Al-4V First Stage Fan Blade,"
Proceedings 44th AIAA/ASME/ASCE/AHS Structures, Structural Dynamics, & Materials Conf., Norfolk, VA, April 7-10, 2003.

This paper describes the application of Low Plasticity Burnishing (LPB) to increase the damage tolerance and fatigue strength of a Ti-6Al-4V fan blade that is fatigue life limited by the occurrence of leading edge foreign object damage (FOD) as small as 0.1mm (0.005 in.). The size and location distributions of service generated FOD were documented; no FOD exceeded a depth of 0.5mm (0.020 in.). LPB processing of the fan blade leading edge was therefore designed to provide tolerance of 0.5 mm deep FOD. A zone of -100 ksi through-thickness compression was achieved extending back 6.3 mm chord-wise from the leading edge along the lower half of the blade from the platform to the mid-span damper. Residual stress distributions were measured as functions of depth and position along the leading edge using x-ray diffraction mapping. HCF testing was with the leading edge cantilever loaded under a sustained mean stress (R=0.1). FOD was simulated with 60-degree "V" notches machined into the leading edge at the point of maximum applied stress. The 620 MPa (90 ksi) endurance limit of as-received blades was reduced to less than half by 0.5mm. FOD. LPB produced an HCF strength of 861 MPa (125 ksi) without FOD, and strengths equal or greater than the as-received blades for FOD up to 1.3 mm (0.050 in.) deep - an order of magnitude improvement in damage tolerance. Fatigue life modeling confirmed the HCF strength achieved, and suggests FOD tolerance can be further increased by optimizing the size and shape of the compressive zone.

 

(236) N. Jayaraman, P. Prevey, M. Mahoney,"Fatigue Life Improvement of an Aluminum Alloy FSW with Low Plasticity Burnishing,"
Proceedings 132nd TMS Annual Meeting, Mar. 2-6, 2003, San Diego, CA.

Friction stir welding provides a new technology for solid state joining of a wide variety of aluminum alloys that cannot be joined with conventional fusion welds. However, recent work has shown that significant tensile residual stresses are developed in the stirred region with local tension maxima at the transition between the stir and heat-affected zones. Residual tension at the edges of the stir zone has been associated with stress corrosion cracking and corrosion fatigue crack growth initiation. This fatigue debit has been overcome using low plasticity burnishing (LPB) to introduce a deep surface layer of compressive residual stress. LPB processing after friction stir welding has increased the high cycle fatigue endurance of aluminum alloy FSW by 80%. However, the LPB processing parameters have not yet been optimized to produce the maximum achievable fatigue life.

A linear elastic fracture mechanics approach is applied to calculate the fatigue crack growth rates and fatigue lives of friction stir welded 2219-T8751 aluminum with initiation from salt fog pitting. The analysis is performed with and without the including of the deep compressive residual stresses produced by low plasticity burnishing (LPB). Calculated fatigue lives are compared to fatigue data developed in four-point bending at R = 0.1 following 100 hr. salt fog pitting corrosion. The results indicate that the improved fatigue life achieved with LPB after friction stir welding can be explained by delayed crack initiation and retardation of growth in the deep compressive layer produced by LPB. The fatigue crack growth analysis provides a theoretical basis for understanding the improved fatigue life realized with LPB and for estimating the residual stress distribution that will provide the highest achievable fatigue strength.

 

(235) P.S. Prevey and J.C. Cammett, "The Effect of Shot Peening Coverage on Residual Stress, Cold Work and Fatigue in a Ni-Cr-Mo Low Alloy Steel,"
Proceedings International Conference on Shot Peening, Garmisch-Partenkirchen, Germany, Sept. 16-20, 2002.

The underlying motivation for this work was to test the conventional wisdom that 100% coverage by shot peening is required to achieve full benefit in terms of compressive residual stress magnitude and depth as well as fatigue strength. Fatigue performance of many shot peened alloys is widely reported to increase with coverage up to 100%, by many investigators and even in shot peening manuals. The fatigue strength of some alloys is reported to be reduced by excessive coverage Aerospace, automotive, and military shot peening specifications require at least 100% coverage. Internal shot peening procedures of aerospace manufacturers may require 125% to 200% coverage. Most of the published fatigue data supporting the 100% minimum coverage recommendation was developed in fully reversed axial loading or bending with a stress ratio, R= Smin / Smax, of -1.

 

(234) Paul S. Prevéy, Doug Hornbach, Terry Jacobs, and Ravi Ravindranath, "Improved Damage Tolerance in Titanium Alloy Fan Blades with Low Plasticity Burnishing,"
Proceedings of the ASM IFHTSE Conference, Columbus, OH, Oct. 7-10, 2002

Low Plasticity Burnishing (LPB) has been applied to produce a layer of deep high magnitude compressive residual stress in the leading edge of Ti-6Al-4V first stage fan blades. The goal was to improve damage tolerance from 0.13 to 0.5 mm (0.005 to 0.02 in.). LPB processing of the airfoil surface was performed on a conventional four-axis CNC mill. The LPB control system, tooling, and process are described. A zone of through-thickness compression on the order of -690 MPa (-100 ksi) was achieved extending 2.5 mm (0.10 in.) cord-wise from the leading edge and along the lower half of the blade from the platform to mid-span damper. Cantilever fatigue testing was performed at R=0.1 using FOD simulated by a 60 degree "V" notch. The processing provided co, an order of magnitude improvement in damage tolerance. The benefits of the deep layer of surface compression were confirmed through fatigue performance modeling.

 

(233) Paul S. Prevéy, Doug Hornbach, Perry Mason, and Murray Mahoney, "Improving Corrosion Fatigue Performance of AA2219 Friction Stir Welds with Low Plasticity Burnishing,"
Proceedings of the ASM IFHTSE Conference, Columbus, OH, Oct. 7-10, 2002.

Low Plasticity Burnishing (LPB) has been investigated as a post-weld surface treatment to improve the corrosion fatigue performance of friction stir welded (FSW) AA2219 plate. Welds were fabricated from 9.5 mm thick AA2219-T8751 plate. Residual stress distributions mapped by x-ray diffraction through the thickness of the weld indicate zones of tension parallel to the weld line at the interface of the weld and parent metal on both the advancing and retreating sides of the weld stir zone. Maximum tension ranged from 100 MPa at the surface to 200 MPa at mid-thickness. LPB placed the weld region in high compression on the order of -400 MPa. Fatigue testing was performed in four-point bending at R=0.1 under constant stress. The corrosion fatigue performance after 100 hours in 3.5% salt fog improved from 175 MPa for a milled surface to 225 MPa after LPB processing. LPB provided nearly complete mitigation of the pitting corrosion damage, with comparable fatigue performance regardless of salt fog exposure. Theoretical predictions of the fatigue life improvement from LPB are also shown and compared to the empirical fatigue data. LPB appears to provide a practical weld post-treatment for improved corrosion fatigue performance of aluminum alloy FSW's.

 

(232) Paul S. Prevéy and John T. Cammett, "Restoring Fatigue Performance of Corrosion Damaged AA7075-T6 and Fretting in 4340 Steel with Low Plasticity Burnishing,"
Proceedings 6th Joint FAA/DoD/NASA Aging Aircraft Conference, San Francisco, CA, Sept 16-19, 2002.

Corrosion related fatigue in aluminum structural alloys and fretting damage in high strength steels are primary failure mechanisms that reduce the structural integrity of aging aircraft. A chemically active environment, susceptible material and static and/or alternating tensile stresses are all required for failure. Conventional approaches to mitigate corrosion and fretting related failure mechanisms address either elimination of the corrosive environment with coatings, substitution or modification of alloys, or changes in design, all expensive solutions. This paper describes an alternate approach, employing surface enhancement by low plasticity burnishing (LPB) to introduce a deep, stable layer of compressive residual stress to eliminate the tensile stresses necessary for failure without altering either material or design.
The restoration of fatigue performance by LPB processing of severely salt fog pitted AA7075-T6 was previously described. That work has been extended to investigate the effects of introducing a deep layer of compressive residual stress with LPB on the fatigue performance of AA7075-T6 prior to salt fog pitting, and prior to active corrosion fatigue in the absence of pitting, and prior to fretting of 4340 steel. The endurance limit of the baseline machined 7075-T6 surface was reduced from 205 MPa (30 ksi) to 103 MPa (15 ksi) by either 100 hr salt fog pitting or active corrosion in 3.5% NaCl during fatigue. LPB prior to corrosive exposure increased the endurance limit for 100 hr pitted samples to 310 MPa (45 ksi), and for active corrosion fatigue to 275 MPa (40ksi). LPB processing was performed in a conventional CNC machining center. LPB prior to fretting in 4340 steel increased the HCF strength 25%, eliminating the fretting fatigue debit. Surface enhancement of fatigue critical aircraft structural components with LPB offers a cost effective and practical alternative to alloy substitution or component re-design as a means of improving the structural integrity of aging aircraft.

 

(231) Paul S. Prevéy, Doug Hornbach, John Cammett, and Ravi Ravindranath, "Damage Tolerance Improvement of Ti-6-4 Fan Blades with Low Plasticity Burnishing,"
Proceedings 6th Joint FAA/DoD/NASA Aging Aircraft Conference, San Francisco, CA, Sept 16-19, 2002.

Low plasticity burnishing (LPB) has been demonstrated to increase the damage tolerance of Ti-6Al-4V fan blades by an order of magnitude. First stage Ti-6Al-4V fan blades were LPB processed using a conventional 4-axis CNC machine tool. LPB dramatically improved surface finish with negligible blade distortion and produced compressive residual stresses of -690 MPa (-100 ksi) through the entire thickness of the blade leading edge. Fatigue testing demonstrated that the deep compression of LPB provided a 3X improvement in HCF endurance limit, complete tolerance of FOD up to 1.3 mm (0.050 in.) deep, and an order of magnitude improvement in fatigue life. Damage tolerance for the TI-6AL-4V fan blade was improved by an order of magnitude.
The benefit of the LPB generated compressive layer in improving damage tolerance was confirmed using the fatigue crack growth code AFGROW. Crack growth modeling indicates tolerance of deeper FOD is achievable with optimization of the depth and magnitude of the residual stress field. LPB has been demonstrated to be an effective and affordable means of improving the damage tolerance of Titanium alloy fan and compressor blades. Application of LPB during manufacturing and overhaul operations could significantly reduce the cost of engine inspection and maintenance while improving fleet readiness.

 

(230) Paul S. Prevéy and John T. Cammett, "The Influence of Surface Enhancement by Low Plasticity Burnishing on the Corrosion Fatigue Performance of AA7075-T6,"
Proceedings 5th International Aircraft Corrosion Workshop, Solomons, Maryland, Aug. 20-23, 2002.

The restoration of fatigue performance by LPB processing of AA7075-T6 after severe pitting in salt fog has been previously described that work has now been extended to investigate the effect on fatigue strength of introducing a deep layer of compressive residual stress by LPB prior to salt fog pitting. The effect of prior LPB treatment of non-corroded surfaces on subsequent corrosion fatigue performance has now also been studied. The endurance limit of the baseline machined surface was reduced from 205 MPa (30 ksi) to 103 MPa (15 ksi) by either pitting during 100 hr. salt fog exposure or from active corrosion in 3.5% NaCl during fatigue cycling. LPB prior to exposure increased the endurance limit for 100 hr. salt fog pitted samples to 310 MPa (45 ksi) and for active corrosion fatigue samples to 275 MPa (40 ksi). LPB processing was performed in a conventional CNC machining center. Surface enhancement of fatigue critical aircraft structural components with LPB offers a cost effective and practical alternative to alloy substitution or component re-design as a means of improving the structural integrity of aging aircraft.

 

(229) John Cammett, "Quality Assurance of Shot Peening by Automated Surface and Subsurface Residual Stress Measurement,"
The Shot Peener, Vol. 15(3) September 2001, pp 7-8.

Shot peening is frequently used to produce compressive residual stress in the surface layer of components for fatigue life enhancement and suppression of stress corrosion cracking (SCC). Shot peening is controlled by monitoring Almen intensity. Almen intensity is determined from the arc heights produced in series of at least four Almen strips peened for progressively longer times on one side of the strips. There is, however, no simple relationship between the Almen intensity and the residual stress distribution produced in the 1070 steel Almen strip. Arc height in Almen strips is a function of the induced total strain energy, or the area under the residual stress-depth distribution. Furthermore, quite different residual stress distributions can produce the same Almen strip arc height. Shot peening to the same Almen intensity using different shot sizes will also generally produce different subsurface residual stress distributions. The depth and magnitude of compression developed in a component being shot peened, generally having mechanical properties very different from the Almen strip, cannot be determined simply from the response of a steel Almen strip identically peened. Therefore, the only reliable method of controlling shot peening of a component is by measuring the subsurface residual stress distribution.

 

(228) J.C. Cammett and P.S. Prevey, "Fatigue Strength Restoration in Corrosion Pitted 4340 Alloy Steel via Low Plasticity Burnishing".

Low plasticity burnishing (LPB) is a surface enhancement process with significant economic and physical attributes that make it attractive for component repair/refurbishment applications in aging aircraft. The current work addresses the efficacy of fatigue strength restoration by applying LPB directly to a corroded surface without first removing damaged layers. Compressive residual stresses of the order of material yield strength in quenched and tempered, 38 HRC 4340 steel were achieved via LPB on as-corroded surfaces and sub-surface layers. The total depth of compression was about 1.25 mm (0.05 in.).

Corrosion damage from 100 and 500-hour salt fog exposures reduced the 107-cycle fatigue strength respectively by about 25 and 50 percent relative to the as machined uncorroded fatigue strength. LPB applied to the corroded surfaces after superficial cleaning to remove loose corrosion product restored the fatigue strength of the 100-hour exposed material to 110 percent of the as-machined, uncorroded level. Fatigue strength restoration was 85 percent in 500-hour exposed material. Similar degrees of fatigue strength restoration were achieved in the finite life regime as well. Fractography revealed that fatigue failures of salt fog-exposed specimens initiated at corrosion pits. Fatigue failures in LPB treated corroded specimens also initiated at corrosion pits. Nonetheless, fatigue strengths were greatly improved by such treatment.

 

(227) P.W. Mason and P.S. Prevey "Iterative Taguchi Analysis: Optimizing the Austenite Content and Hardness in 52100 Steel"
Journal of Materials Eng. & Perf., Vol. 10(1), February 2001, ASM, Materials Park, OH, pg. 14-21.

Three iterations of Taguchi designed experiments and analyses were used to determine optimal thermal treatments for minimizing retained austenite content while maximizing Rockwell hardness (HRC) in AISI 52100 bearing steel. Experimental variables chosen for this study included austenitizing and tempering temperatures, tempering time and cold treatment. After one iteration, tempering temperature and cold treatment were seen to have the greatest effect on austenite content while austenitizing and tempering temperatures had the greatest influence on hardness. After the second and third experimental iterations, two thermal treatments were noted each producing hardness of 58-59 HRC in combination with zero retained austenite as measured by x-ray diffraction.

 

(226) Paul S. Prevey, "The Effect of Low Plasticity Burnishing (LPB) on the HCF Performance and FOD Resistance of Ti-6Al-4V"
Proceedings: 6th National Turbine Engine HCF Conference, March 5-8, 2001, Jacksonville, FL.

Low Plasticity Burnishing (LPB) has been developed as a rapid, inexpensive surface enhancement method adaptable to existing CNC machine tools. LPB produces a deep layer of compression with minimal cold work of the surface, comparable to laser shock peening (LSP), but can be incorporated into manufacturing operations at lower cost. Minimizing cold work during surface enhancement has been shown to improve both thermal stability at engine temperatures and resistance to overload relaxation accompanying foreign object damage (FOD).

Recent research leading to the development of a practical LPB demonstration facility and tooling is described. The mechanism for compressive residual stress development during LPB has been studied with elastic-plastic finite element modeling. DOE methods have been utilized to optimize compressive magnitude and depth with minimum cold work. Using optimum burnishing parameters, compression on the order of the material yield strength can be achieved to depths exceeding 0.040 in. (1mm) with low cold work.
Residual stress and cold work distributions developed by LPB in Ti-6Al-4V are compared to traditional shot peening and LSP. The compressive layer produced by LPB is shown to be resistant to both thermal and overload relaxation. After exposure to engine temperatures, the high cycle fatigue (HCF) strength at 2x106 cycles after LPB is 40% higher than 8A shot peening. FOD 0.010 in. deep reduces the HCF strength of shot peened Ti-6Al-4V by 50% but has no significant effect on fatigue life after LPB. HCF life improvement and FOD tolerance are attributed to the deep compressive layer produced by LPB.

 

(225)Douglas J. Hornbach and Paul S. Prevey, "The Effect of Prior Cold Work on Tensile Residual Stress Development in Nuclear Weldments"
Proceedings: PVP 2001, July 22-26, 2001, Atlanta, GA.

Austenitic alloy weldments in nuclear reactor systems are susceptible to stress corrosion cracking (SCC) failures. SCC has been observed for decades and continues to be a primary maintenance concern for both pressurized water and boiling water reactors. SCC can occur if the sum of residual stress and applied stress exceeds a critical threshold tensile stress. Residual stresses developed by prior machining and welding can accelerate or retard SCC depending on their sign and magnitude.

The residual stress, cold work and yield strength distributions on the inside diameter of an Alloy 600 tube J-welded into a pressure vessel were determined by a combination of x-ray diffraction (XRD) and mechanical techniques. A novel method was used to relate the XRD line broadening to the percent cold work or true plastic strain in the Alloy 600 tube. The accumulated cold work in the layers deformed by prior machining, in combination with the true stress-strain relationship for Alloy 600, was used to determine the increase in yield strength as a result of deformation due to machining and weld shrinkage. The yield strength of the deformed layer was found to be well in excess of the bulk yield for the alloy, and is therefore capable of supporting residual stresses correspondingly higher.

Tension as high as +700 MPa, exceeding the SCC threshold stress, was observed in both the hoop and axial directions on the inside diameter of the Alloy 600 tubing adjacent to the weld heat affected zone (HAZ). The cold worked high tensile zones correlated with the locations of field SCC failures. The tensile residual stresses are shown to result from a combination of the high cold working from initial machining followed by weld shrinkage. The development of surface tension during weld shrinkage has been modeled using finite element methods, and the benefits of minimizing or removing the cold worked layer prior to welding are demonstrated. Further laboratory studies showing the influence of prior cold working on the formation of residual stresses following bulk plastic deformation are presented.

 

(224) Paul S. Prevey, "X-ray Diffraction Characterization of Crystallinity and Phase Composition in Plasma-Sprayed Hydroxylapatite Coatings"
Journal of Thermal Spray Technology, ed. C.C. Berndt, Metals Park, OH, ASM, Vol. 9(3), Sept, 2000, pp. 369-376.

Orthopedic and dental implants consisting of a metallic substrate plasma-spray coated with hydroxylapatite (HA) are currently used in reconstructive surgery. The crystalline phases present in the calcium phosphate ceramic and the degree of crystallinity must be controlled for medical applications. X-ray diffraction (XRD) is routinely employed to characterize the phase composition and percent crystallinity in both biological and sintered HA. However, application of the same XRD methods to plasma-sprayed coatings is complicated by the potential presence of several crystalline contaminant phases and an amorphous component.

(223) Paul S. Prevey, "The Effect of Cold Work on the Thermal Stability of Residual Compression in Surface Enhanced IN718"
Proceedings of the 20th ASM Materials Solutions Conference & Exposition, St. Louis, MO, Oct. 10-12, 2000.

Surface enhancement, the creation of a layer of residual compression at the surface of a component, is widely used to improve the fatigue life in the automotive and aerospace industries. The compressive layer delays fatigue crack initiation and retards small crack propagation. The benefits of surface enhancement are lost if the compressive layer relaxes at the operating temperature of the component. Surface enhancement methods producing minimal cold work are shown to produce the most thermally stable compression. The residual stress and cold work distributions developed in IN718 by shot peening, gravity peening, laser shock peening (LSP) and low plasticity burnishing (LPB) are compared. Estimation of cold work (equivalent true plastic strain) from x-ray diffraction line broadening is described. Thermal relaxation at temperatures ranging from 525C to 670C is correlated to the degree of cold working of the surface, independent of the method of surface enhancement. Highly cold worked (> 15%) shot peened surfaces are found to relax to half the initial level of compression in minutes at all temperatures investigated. The rapid initial relaxation is shown to be virtually independent of either time or temperature from 525C to 670C. The LPB process is described with application to IN718. High cycle fatigue performance after elevated temperature exposure is compared for surfaces treated by LPB and conventional (8A intensity) shot peening.

 

(222) Paul S. Prevey, "Low Cost Corrosion Damage Mitigation and Improved Fatigue Performance of Low Plasticity Burnished 7075-T6"
Proceedings of the 4th International Aircraft Corrosion Workshop, Solomons, MD, Aug. 22-25, 2000.

Low plasticity burnishing (LPB) has been investigated as a surface enhancement process and corrosion mitigation method for aging aircraft structural applications. Compressive residual stresses reaching the alloy yield strength and extending to a depth of 1.25 mm (0.050 in.), deeper than typical corrosion damage, is achievable. Excellent surface finish can be achieved with no detectable metallurgical damage to surface and subsurface material. Salt fog exposures of 100 and 500 hrs. reduced the fatigue strength at 2x106 cycles by fifty-percent. LPB of the corroded surface, without removal of the corrosion product or pitted material, restored the 2x106 fatigue strength to greater than that of the original machined surface. The fatigue strength of the corroded material in the finite life regime (104 to 106 cycles) after LPB was 140 MPa (20 ksi) higher than the original uncorroded alloy, and increased the life by an order of magnitude. Ease of adaptation to CNC machine tools allows LPB processing at costs and speeds comparable to machining operations. LPB offers a promising new technology for mitigation of corrosion damage and improved fatigue life of aircraft structural components with significant cost and time savings over current practices.

 

(221) Paul S. Prevey, et.al., "FOD Resistance and Fatigue Crack Arrest in Low Plasticity Burnished IN718" Proceedings of the 5th National High Cycle Fatigue Conference, (2000).

Surface enhancement methods induce a layer of residual compressive stress to improve fatigue life. Shot peening is inexpensive and widely used, but the associated cold work accelerates relaxation of the compressive layer at turbine temperatures and increases sensitivity to overload relaxation. "Deep rolling" burnishing methods produce deep compression, but with cold work comparable to shot peening. Laser shock peening (LSP) produces deep compression with minimal cold work and impressive FOD resistance, but is costly and presents logistical problems in manufacturing.

Low Plasticity Burnishing (LPB) has been investigated as a rapid, inexpensive surface enhancement method. Preliminary results indicate depth and magnitude of compression comparable to LSP. Compression reaching the alloy yield strength and extending to a depth of 1.2 mm (0.047 in.) is achievable with cold work of less than 4%. Excellent surface finish can be achieved with no detectable metallurgical damage. Ease of adaptation to CNC machine tools allows LPB processing at costs and speeds comparable to machining operations.

The LPB process is described with application to IN718. Thermal stability at engine temperatures is compared to conventional shot peening. Resistance to 0.13 and 0.25 mm (0.005 and .010 in.) deep sharp notch FOD was achieved, even after exposure to engine temperatures. Elevated temperature crack growth data are presented showing the arrest of existing 0.46 mm x 0.91 mm (0.018 x 0. 036 in.) fatigue cracks by the deep compressive layer.

(220) Douglas J. Hornbach, Paul S. Prevéy, "Development of Machining Procedures to Minimize Distortion During Manufacture," Heat Treating, Proceedings of the 17th Heat Treating Society Conference and Exposition, ASM, Metals Park, OH, (1998), pp.13-18.

Distortion during machining can result in high scrap rates and increased manufacturing costs. Distortion results from either the introduction or elimination of residual stresses during manufacture. Residual stresses which are induced in the surface by machining and grinding , or throughout the body by welding or heat treatment, can generally be measured and controlled. Distortion caused by re-equilibration after removal of stressed material during machining is more difficult to avoid, and is the primary cause of scrap in precision components.

Heat treatment required to develop desired mechanical properties will generally produce residual stress distribution. During machining, the distortion of a part depends upon the geometry, order of removal, and stress state in the material removed. If the change of shape which occurs is not accommodated, the part may be scrapped during machining. Measurement of the initial residual stress distribution and the use of finite element modeling allow the development of machining procedures which minimize distortion.

Examples of the residual stress distributions typically seen in heat treated components and the development of finite element models to minimize distortion are presented. Control of distortion is demonstrated with a detailed example of machining a nickel-base superalloy turbine disk from a quenched forging.

(219) Paul Prevéy, Douglas Hornbach, Perry Mason, "Thermal Residual Stress Relaxation and Distortion in Surface Enhanced Gas Turbine Engine Components,"
Proceedings of ASM Materials Week, Indianapolis, IN, September 15-18, 1997.

Compressive residual stresses are often deliberately induced in the surfaces of turbine engine components, using a variety of surface enhancement methods, to improve fatigue life. Thermal stress relaxation can occur in both the Ti and Ni alloys used in compressor and turbine stages. Nonuniform relaxation of the compressive layer can cause distortion of the critical aerodynamic shapes of thin blades, potentially effecting engine performance.

A detailed study of the thermal relaxation of the layer of compression induced by shot peening, gravity peening and laser shocking in Ti-6Al-4V and Inconel 718 at engine temperatures is summarized. Both the magnitude and rate of relaxation were found to depend primarily on the degree of cold working developed during processing. Compression in highly cold worked surfaces relaxed extremely rapidly in both alloys. Half the initial compression may be lost in less than 10 minutes even at moderate engine temperatures. Finite element estimates of the distortion resulting from nonuniform thermal relaxation in a hypothetical airfoil geometry is presented.

(218) P.A. Sherburne, D.J. Hornbach, R.A. Ackerman, A.R. McIlree, "Residual Stresses in OTSG Tube Expansion Transitions,"
Proceedings of the Eighth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, NACE, TMS, Amelia Island, FL, August 10-14, 1997.

A section of once through steam generator (OTSG) tubing, approximately 33 cm. long and encompassing the roll expansion region in the upper tubesheet (hot leg), was removed from the Davis Besse Nuclear Power Station in April 1996 for laboratory failure analysis. Rotating pancake coil (RPC) inspection of this region had determined that an axial defect was present in the expansion transition approximately 2.5 cm below the primary face of the hot leg tubesheet. Examination of the pulled tube section identified the defect as an ID-initiated primary water stress corrosion crack (PWSCC) approximately 78% throughwall and 2.3 mm long. Additional axial cracks with less throughwall extent were also observed within the roll transition.

During fabrication, OTSGs were subjected to a full-vessel stress relief at 593&deg; - 621&deg;C for ~12 to 15 hours. This practice was shown in early testing to reduce the residual stresses caused by roller expansion. Operating experience for OTSG tubing has demonstrated that it is more resistant to stress corrosion cracking than low temperature mill annealed Alloy 600 tubing used in early recirculating-type steam generators. Thus, the appearance of PWSCC in this tube was not expected.

A review of manufacturing records for the Davis Besse OTSGs revealed that this specific tube end had been repaired following stress relief and hydrotest operations. It was not clear from the records, however, if the tube had been rerolled as part of the repair procedure. Based on this uncertainty, a project was initiated to measure the residual stress distribution and cold work in the Davis Besse roll transition and in assorted rolled tube mockups, using X-ray diffraction techniques and finite element analysis. The objective was to determine if the Davis Besse tube had been rerolled following stress relief and thus more likely to experience PWSCC. Results confirm a definite effect of rerolling the tube following stress relief on both cold work and residual stresses. It was further concluded that the Davis-Besse tube data has a high degree of correlation with the reroll data.

 

(217) Douglas J. Hornbach, Paul S. Prevéy, "Tensile Residual Stress Fields Produced in Austenitic Alloy Weldments,"
Proceedings of Energy Week Conference & Exhibition, Houston, TX, January 28-30, 1997, American Society of Mechanical Engineers, American Petroleum Institute, pp. 183- 188.

Residual stresses developed by prior machining and welding may either accelerate or retard stress corrosion cracking (SCC), in austenitic alloys, depending upon their magnitude and sign. A combined x-ray diffraction (XRD) and mechanical technique was used to determine the axial and hoop residual stress and yield strength distributions into the inside diameter surface of a simulated Alloy 600 penetration J-welded into a reactor pressure vessel. The degree of cold working and the resulting yield strength increase caused by prior machining and weld shrinkage were calculated from the line broadening distributions. Tension as high as +700 MPa was observed in both the axial and hoop directions at the inside diameter adjacent to the weld heat affected zone (HAZ). Stresses exceeding the bulk yield strength develop due to the combined effects of cold working of the surface layers during initial machining, and subsequent weld shrinkage. Cold working produced by prior machining was found to influence the final residual stress state developed by welding.

 

(216) Rod McGregor, Doug Hornbach, Usama Abdelsalam, Paul Doherty, "Experimental Residual Stress Evaluation of Hydraulic Expansion Transitions in Alloy 690 Steam Generator Tubing,"
Proceedings of the Seventh International Symposium on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, NACE, Breckenridge, CO, August 7-10, 1995.

Residual stresses developed by prior machining and welding may either accelerate or retard stress corrosion cracking (SCC), in austenitic alloys, depending upon their magnitude and sign. A combined x-ray diffraction (XRD) and mechanical technique was used to determine the axial and hoop residual stress and yield strength distributions into the inside diameter surface of a simulated Alloy 600 penetration J-welded into a reactor pressure vessel. The degree of cold working and the resulting yield strength increase caused by prior machining and weld shrinkage were calculated from the line broadening distributions. Tension as high as +700 MPa was observed in both the axial and hoop directions at the inside diameter adjacent to the weld heat affected zone (HAZ). Stresses exceeding the bulk yield strength develop due to the combined effects of cold working of the surface layers during initial machining, and subsequent weld shrinkage. Cold working produced by prior machining was found to influence the final residual stress state developed by welding.

 

(215) Paul S. Prevéy, Perry W. Mason, Douglas J. Hornbach, and James P. Molkenthin, "Effect of Prior Machining Deformation Upon the Development of Tensile Residual Stresses in Weld Fabricated Nuclear Components,"
Journal of Materials Engineering and Performance, Vol. 5, 1, February 1996, pp. 51-56.

Austenitic alloy weldments in nuclear systems may be subject to stress corrosion cracking (SCC) failure if the sum of residual and applied stresses exceeds a critical threshold. Residual stresses developed by prior machining and welding may either accelerate or retard SCC, depending upon their magnitude and sign. A combined x-ray diffraction and mechanical procedure was used to determine the axial and hoop residual stress and yield strength distributions into the inside diameter surface of a simulated Alloy 600 penetration J-welded into a reactor pressure vessel. The degree of cold working and the resulting yield strength increase caused by prior machining and weld shrinkage was calculated from the line broadening distributions.
Tensile residual stresses on the order of +700 MPa were observed in both the axial and hoop directions at the inside diameter surface in a narrow region adjacent to the weld heat affected zone (HAZ). Stresses exceeding the bulk yield strength were found to develop due to the combined effects of cold working of the surface layers during initial machining, and subsequent weld shrinkage. The residual stress and cold work distributions produced by prior machining were found to strongly influence the final residual stress state developed after welding.

 

(214) Paul S. Prevéy, "Current Applications of X-ray Diffraction Residual Stress Measurement,"
Developments in Materials Characterization Technologies, eds. G.F. Vander Voort and J.J. Friel, Materials Park, OH: American Society of Metals, pp. 103-110.

A brief theoretical development of x-ray diffraction residual stress measurement is presented emphasizing practical engineering applications of the plane-stress model, which requires no external standard. Determination of the full stress tensor is briefly described, and alternate mechanical, magnetic, and ultrasonic methods of residual stress measurement are compared.
Sources of error arising in practical application are described. Subsurface measurement is shown to be necessary to accurately determine the stress distributions produced by surface finishing such as machining, grinding, and shot peening, including corrections for penetration of the x-ray beam and layer removal.
Current applications of line broadening for the prediction of material property gradients such as yield strength in machined and shot peened surfaces, and hardness in steels are presented. The development of models for the prediction of thermal, cyclic, and overload residual stress relaxation are described.

 

(213) Douglas J. Hornbach, Perry W. Mason, and Paul S. Prevéy, "X-Ray Diffraction Characterization of Residual Stress and Hardness Distribution in Induction Hardened Gears,"
Proceedings of the First International Conference on Induction Hardened Gears and Critical Components, Indianapolis, IN, May 15-17, 1995, Gear Research Institute, pp. 69-76.

Accurate knowledge of the subsurface residual stress and hardness distributions is required for failure analysis, fatigue life prediction and process control of induction hardened components. X-ray diffraction (XRD) provides a powerful tool for the simultaneous determination of both the macroscopic residual stress and hardness distributions through the case and into the core of induction hardened parts. A procedure for developing the empirical relationship between diffraction peak width and mechanical hardness is described.

Subsurface XRD residual stress measurement requires layer removal and correction for the resulting stress relaxation. The corrections may dominate the results obtained at depths near the case/core interface. Traditional closed-form corrections may be inadequate when applied to gear teeth. A novel finite element analysis (FEA) correction technique applicable to arbitrary geometries and stress distributions is presented and described. Examples of the determination of the residual stress and hardness distributions through the case of induction hardened gears are presented.

 

(212) J.F. Hall, J.P. Molkenthin, P.S. Prevéy, and R.S. Pathania, "Measurement of Residual Stresses in Alloy 600 Pressurizer Penetrations,"
Conference on the Contribution of Materials Investigation to the Resolution of Problems Encountered in Pressurized Water Reactors, Paris: Societe Francaise d'Energie Nucleare, Sept. 12-16, 1994.

Alloy 600 penetrations in several pressurized water reactors have experienced primary water stress corrosion cracking near the partial penetration J-welds between the Alloy 600 and the cladding on the inside diameter of the components. The microstructure and tensile properties indicated that the Alloy 600 was susceptible to primary water stress corrosion cracking (PWSCC) providing that a high tensile stress (applied + residual) was present.
The residual stress distributions at the inside diameter surface and at different depths below the surface were measured in Alloy 600 nozzle and heater sleeve mockups. Surface residual stresses on the nozzle mockup ranged from -350 to +830 MPa. For the heater sleeve mockup, the surface residual stresses ranged from -330 to +525 MPa. In the areas of high tensile residual stress, for the most part, the residual stresses decreased with increasing depth below the surface. For the nozzle and heater sleeve mockups, the percent cold work and yield strength as a function of depth were determined.

 

(211) J.F. Hall, J.P. Molkenthin, and P.S. Prevéy, "XRD Residual Stress Measurements on Alloy 600 Pressurizer Heater Sleeve Mockups,"
Proceedings of the Sixth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors, San Diego, CA, 1993, TMS, ANS, NACE, pp. 855-861.

Alloy 600 penetrations in several pressurized water reactors have experienced primary water stress corrosion cracking near the partial penetration J-welds between the Alloy 600 and the cladding on the inside diameter of the components. The microstructure and tensile properties indicated that the Alloy 600 was susceptible to primary water stress corrosion cracking (PWSCC) providing that a high tensile stress (applied + residual) was present.

The residual stress distributions at the inside diameter surface and at different depths below the surface were measured in two Alloy 600 heater sleeve mockups. Surface residual stresses ranged from 340 to 690 MPa. For the most part, the residual stresses decreased with increasing depth below the surface. For the heater sleeve mockups, the percent cold work (i.e. true plastic strain) and yield strength as a function of depth were determined. As a result of pre-reaming and welding the heater sleeves, the amount of plastic strain and yield strength increased to a nominal depth of 0.025 cm. The true plastic strain and yield strength decreased with increasing depth below the surface.

 

(209) Paul S. Prevéy, "Problems with Non-Destructive Surface X-Ray Diffraction Residual Stress Measurement,"
Practical Applications of Residual Stress Technology, ed. C. Ruud, Materials Park, OH: American Society for Metals, 1991, pp. 47-54.

Because surface measurements are non-destructive, x-ray diffraction is often considered as a method of residual stress measurement for quality control testing. Unfortunately, errors caused by the presence of a subsurface stress gradient as well as difficulties in interpreting surface results often limit the usefulness of surface data. The magnitude of the potential errors, both in measurement and in interpretation, depends upon the nature of the subsurface residual stress distribution which can only be determined destructively. Although residual stress distributions subject to these problems are commonly encountered in practice, the question of the validity of non-destructive surface results is seldom adequately considered.

Examples are presented showing common residual stress distributions produced by grinding, nitriding and shot peening which are subject to errors in measurement and/or interpretation when measured only at the surface. The methods for determining the subsurface residual stress distributions and correction for penetration of the x-ray beam are discussed along with examples of their application. The need to determine the subsurface stress distribution in order to verify the accuracy of surface measurements is emphasized.

 

(208) Paul S. Prevéy and Perry W. Mason, "The Use of X-Ray Diffraction to Determine the Triaxial Stress State in Cylindrical Specimens,"
Practical Applications of Residual Stress Technology, ed. C. Ruud, Materials Park, OH: American Society for Metals, 1991, pp. 77-81.

A method of determining the axial, circumferential and radial residual stress distributions in cylindrical specimens is described. The axial and circumferential residual stresses are measured directly by x-ray diffraction at the free cylindrical surface exposed by machining and electropolishing. The radial stress component is then calculated from an integral of the circumferential stress at the free surface as a function of depth by the method of Moore and Evans.
The method is applicable only to cylindrical samples with rotationally symmetrical stress distributions from which complete cylindrical shells are removed for subsurface measurement. The method does not require prior knowledge of the stress-free lattice spacing, and thus provides a means of verifying neutron and x-ray diffraction methods of full tensor stress determination. The stress -free lattice constant, do, is also calculated as a function of depth from the sum of the principal stresses.

Application of the method, to determine the triaxial residual stress distribution in an induction hardened 1045 steel multi-axial fatigue speciment, is described. The variation in the stress-free lattice spacing of the (211) planes with depth is estimated through the hardened case and into the core material.

 

(207) Paul S. Prevéy, "Residual-Stress Distributions Produced by Strain-Gage Surface Preparation,"
Proceedings of the 1986 SEM Spring Conference on Experimental Mechanics, Society for Experimental Mechanics, Inc., Bethel, CT, (1986) pp. 216-223.

Abrasion of a metallic surface to improve bonding during strain gage installation is generally thought to produce negligible effect on the measurement of applied or residual stresses by blind hole drilling. However, residual stresses induced by surface abrasion may affect residual stress measurements in shallow subsurface layers of residual stress fields produced by processes such as grinding and shot peening.

The residual stress and cold work distributions produced by four methods of abrasive surface preparation and etching were studied by x-ray diffraction in fully annealed AISI 1018 steel. Abrasion of the surface was found to alter the residual stresses near the sample surface. The surface residual stresses produced by abrasion ranged from tension to compression with magnitudes as high as 80% of the yield strength. Cold work was induced to depths of 20 to 60 mm. Etching produced low magnitude surface stresses and negligible cold work.

 

(206) Paul S. Prevéy, "A Method of Determining the Elastic Properties of Alloys in Selected Crystallographic Directions for X-Ray Diffraction Residual Stress Measurement,"
Advances in X- Ray Analysis, Vol. 20, ed. H.F. McMurdie, New York, NY: Plenum Press, 1977, pp. 345-354.

A technique and apparatus are described for obtaining the elastic constant E/(1+v) in selected crystallographic directions for the purpose of calibrating x-ray diffraction residual stress measurement methods. The preparation of a simple rectangular beam specimen with two active electrical resistance strain gages applied to the test surface is described. Samples are clamped in a diffractometer fixture designed to minimize displacement errors, and loaded in four-point bending to several stress levels below the proportional limit. A method is described for calculating E/(1+v) and an estimate of the experimental error.

Values of E/(v+1) obtained for several alloy-(hkl) combinations are presented. The results indicate that several alloys of current commercial interest exhibit significant elastic anisotropy.

 

(205) Paul S. Prevéy, "The Use of Pearson VII Distribution Functions in X-Ray Diffraction Residual Stress Measurement,"
Advances in X-Ray Analysis, Vol. 29, ed. C.S. Barrett, New York, NY: Plenum Press, 1986, pp. 103-111.

The fitting of a parabola by least squares regression to the upper portion of diffraction peaks is commonly used for determining lattice spacing in residual stress measurement. When Ka techniques are employed, the presence of the Ka doublet is shown to lead to significant potential error and non-linearities in lattice spacing as a function of Sin2y caused by variation in the degree of blending of the doublet. An algorithm is described for fitting Pearson VII distribution functions to determine the position of the Ka1 component, eliminating errors caused by defocusing of diffraction peaks of intermediate breadth. The method is applied to determine the subsurface residual stress distribution in ground Ti-6Al-4V, comparing directly the use of parabolic and Pearson VII peak profiles, and is shown to provide precision better than 1% in elastic constant determination.

 

(203) Paul S. Prevéy, "Surface Residual Stress Distributions in As-Bent Inconel 600 U-Bend and Incoloy 800 90-Degree Bend Tubing Samples,"
Workshop Proceedings: U-Bend Tube Cracking in Steam Generators, Electric Power Research Institute, Palo Alto, CA, 1981, pp. 12-3 to 12-19.

Selected data showing typical macroscopic residual stress distributions in u-bent Inconel 600, and 90 deg. bends in Incoloy 800 are presented. The results indicate regions of both high magnitude tension and compression in the longitudinal direction around the circumference of the bends at the apex.
The microscopic residual stress, or percent plastic strain and macroscopic residual distributions in the surface of cross-roll straightened and ground Inconel 600 tubing are described. The results indicate a compressive surface layer accompanied by a yield strength gradient from 90 ksi at the surface to 30 ksi at a depth of 0.003 in.

 

(202) Paul S. Prevéy, "X-Ray Diffraction Characterization of Residual Stresses Produced by Shot Peening,"
Shot Peening Theory and Application, series ed. A. Niku-Lari, IITT-International, Gournay-Sur-Marne, France, 1990, pp. 81-93.

A brief overview of the theory and practice of x-ray diffraction residual stress measurement as applied to shot peened materials is presented.
The unique ability of x-ray diffraction methods to determine both the macroscopic residual stress and the depth and magnitude of the cold worked layer produced by shot peening is described. The need to obtain a complete description of the subsurface residual stress distribution, in order to accurately characterize the residual stress distributions produced by shot peening, is emphasized.
Non-destructive surface residual stress measurements are shown to generally be inadequate to reliably characterize the residual stresses produced by shot peening. Practical applications of x-ray diffraction methods for quality control testing are considered.
Examples are presented for steel and nickel base alloys.

 

(201) Paul S. Prevéy, "The Uniformity of Shot Peening Induced Residual Stress,"
Residual Stress for Designers and Metallurgists, ed. L.J. Vande Walle, Metals Park, OH: American Society for Metals, 1981, pp. 151-168.

Two rectangular samples of ASTM SA 508 Class 2 steel, stress relieved and shot peened to 14-16A intensity, were examined in detail to determine the principal macroscopic residual stress distribution. The uniformity of the shot peening induced macroscopic residual stresses with orientation in the plane of the surface and as a function of depth were examined and compared. The microscopic residual stress (plastic deformation) distribution was determined as a function of depth, and compared for the two samples.
The calibration technique to determine the single crystal elastic constants in the (211) direction and verification of the values obtained by comparison with mechanically measured applied stress are discussed.
The results indicate variation in the magnitude of the subsurface compressive macroscopic residual stress with direction in the plane of measurement for either sample of less than 12 ksi. The mean value of the macroscopic stress distributions for the two samples examined differed by less than the same amount at any depth examined. The microstress distribution was found to vary essentially linearly as a function of depth, reaching a negligible amount immediately beneath the microscopically compressive surface layer. The microstress distributions in the two samples examined were identical within the limits of experimental error.

 

(200) Paul S. Prevéy, "X-Ray Diffraction Residual Stress Techniques,"
Metals Handbook: Ninth Edition, Vol. 10, ed. K. Mills, Metals Park, OH: American Society for Metals, 1986, pp. 380-392.

In x-ray diffraction residual stress measurement, the strain in the crystal lattice is measured, and the residual stress producing the strain is calculated, assuming a linear elastic distortion of the crystal lattice. Although the term stress measurement has come into common usage, stress is an extrinsic property that is not directly measurable. All methods of stress determination require measure-ment of some intrinsic property, such as strain or force and area, and the calculation of the associated stress.
Mechanical methods (dissection techniques) and nonlinear elastic methods (ultrasonic and magnetic techniques) are limited in their applicability to residual stress determination. Mechanical methods are limited by assumptions concerning the nature of the residual stress field and sample geometry. Mechanical methods, being necessarily destructive, cannot be directly checked by repeat measurement. Spatial and depth resolution are orders of magnitude less than those of x-ray diffraction.

All nonlinear elastic methods are subject to major error from preferred orientation, cold work, temperature, and grain size. All require stress-free reference samples, which are otherwise identical to the sample under investigation. Nonlinear elastic methods are generally not suitable for routine residual stress determination at their current state of development. In addition, their spatial and depth resolutions are orders of magnitude less than those of x-ray diffraction.

To determine the stress, the strain in the crystal lattice must be measured for at least two precisely known orientations relative to the sample surface. Therefore, x-ray diffraction residual stress measurement is applicable to materials that are crystalline, relatively fine grained, and produce diffraction for any orientation of the sample surface. Samples may be metallic or ceramic, provided a diffraction peak of suitable intensity and free of interference from neighboring peaks can be produced in the high back-reflection region with the radiations available. X-ray diffraction residual stress measurement is unique in that macro-scopic and microscopic residual stresses can be determined nondestructively.

Macroscopic stresses, or macrostresses, which extend over distances that are large relative to the grain size of the material, are of general interest in design and failure analysis. Macrostresses are tensor quantities, with magnitudes varying with direction at a single point in a body. The macrostress for a given location and direction is determined by measuring the strain in that direction at a single point. When macrostresses are determined in at least three known directions, and a condition of plane stress is assumed, the three stresses can be combined using Mohr's circle for stress to determine the maximum and minimum residual stresses, the maximum shear stress, and their orientation relative to a reference direction. Macrostresses strain many crystals uniformly in the surface. This uniform distortion of the crystal lattice shifts the angular position of the diffraction peak selected for residual stress.

(99) Residual Stress Measurement for Quality Control of Shot Peening