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32. Characterization of the Residual Stress, Corrosion Fatigue Strength and Stress Corrosion Cracking Behavior in Shot Peened and Low Plasticity Burnished 300M Landing Gear Steel

Stress corrosion cracking (SCC), corrosion fatigue, and foreign object damage (FOD) are generally recognized as significant degradation processes that affect naval aircraft landing gear components. Ultrahigh strength steels such as 4340, AF1410, and 300M are widely used in applications where a combination of high strength and fracture toughness is needed. Most of these ultrahigh strength steels have been known to be susceptible to SCC and corrosion fatigue. The phenomenon of SCC is generally understood to be the result of a combination of susceptible material, corrosive environment, and tensile stress above a threshold. Introduction of residual compressive stresses in metallic components has long been recognized to lead to enhanced fatigue strength. The goal of this research was to measure the residual stresses imparted by the LPB process and conventional SP surface treatment and characterize the influence of the two treatments upon the mechanisms of corrosion fatigue, FOD and SCC in 300M steel.

31. Measurement of Residual Stresses Using Ring-Core Technique

Various methods exist to determine residual stresses in metallic engineered components. Some methods are more practical than others, depending upon the material, geometry and the desired locations and depths of measurement. X-ray diffraction (XRD) provides an accurate and well established method of determining the residual stress distributions produced by various types of processes. The macroscopic residual stress and information related to the degree of cold working can be obtained simultaneously by XRD. XRD is applicable to most polycrystalline materials, metallic or ceramic, and is nondestructive at the sample surface. However, XRD is limited in characterizing the residual stresses in coarse grain materials such as castings and weldments. Mechanical techniques, which involve removing material and monitoring strain relaxation, can often provide the only means of determining deep stresses in coarse grain components. The ring-core method is a mechanical technique used to quantify the principal residual stresses within a specified depth of material. The technique is based upon linear elastic theory and consists of dissecting a circular plug containing a strain gage. Examples of residual stress measurements obtained by ring core are presented.

30.  Fatigue Life Improvement and Mitigation of Pitting Corrosion Damage of Friction Stir Welded 2219-T8751 Aluminum Alloy by Low Plasticity Burnishing

Salt spray corrosion pits are a common site of fatigue crack initiation in aluminum alloy aircraft components. Salt corrosion pitting occurs during exposure to the marine atmosphere and results in intergranular corrosion to a depth dependent on the time of exposure, temperature, and the service environment of the aircraft. The pronounced fatigue strength reduction caused by salt pit corrosion is well established for both steels [1] and aluminum alloys, and typically reduces the endurance limit to nominally half of the uncorroded value. Friction stir welding (FSW) allows the welding of even dissimilar aircraft alloys with up to 30% weight reduction and reduced manufacturing and maintenance costs. However, the FSW process has been shown to produce zones of tensile residual stress at the edges of the stir zone. Pitting and stress corrosion cracking have been observed to follow these regions of tension. X-ray diffraction residual stress and high cycle fatigue test results are shown for FSW and FSW + LPB 2219-T8751 aluminum alloy.

29. Characterization of Tensile Residual Stresses in 7050-T7651 Aluminum Friction Stir Welds

Salt spray corrosion pits are a common site of fatigue crack initiation in aluminum alloy aircraft components. Salt corrosion pitting occurs during exposure to the marine atmosphere and results in intergranular corrosion to a depth dependent on the time of exposure, temperature, and the service environment of the aircraft. The pronounced fatigue strength reduction caused by salt pit corrosion is well established for both steels and aluminum alloys, and typically reduces the endurance limit to nominally half of the uncorroded value. A new surface enhancement technology called low plasticity burnishing (LPB) has been developed which can provide a layer of surface compression of sufficient depth to effectively eliminate the influence of the salt pit corrosion. X-ray diffraction residual stress and high cycle fatigue test results are shown for friction stir welded (FSW) and FSW + LPB 7050-T7651 aluminum.

28. The Effect of Prior Cold Working on the Development of Tensile Residual Stress Following Bulk Deformation

Cold work produced during machining or shot peening can have a dramatic influence on the formation of tensile residual stress following bulk plastic deformation. Components subjected to applied stress levels that cause localized or wide spread yielding may develop high tensile residual stresses in the previously cold worked surface layers. Tensile residual stress development due to mechanical overload was recently investigated at Lambda Research. A controlled laboratory test was conducted in which Alloy 718 samples were processed to induce varying degrees of cold working. The samples were then further deformed to produce different degrees of bulk plastic deformation. The residual stress, cold work and yield strength were measured using an x-ray diffraction (XRD) technique and correlated to the applied plastic strain.

27. Automated Surface and Subsurface Residual Stress Measurement for Quality Assurance of Shot Peening

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. The depth and magnitude of compression developed in the component being shot peened, generally having mechanical properties very different from the Almen strip, cannot be determined simply from the response of the steel Almen strip. Lambda Research has the capability to quantify residual stress distributions using a unique automated apparatus. The StressPro™ allows the residual stress in one sample to be measured while layers of material are being electrochemically removed from a second sample.

26. Effect of Low Plasticity Burnishing (LPB) on the HCF Life of IN718

High cycle fatigue (HCF) ultimately limits the performance of aircraft and automotive components, increases maintenance costs and reduces the service life. The HCF life of automotive and aerospace components can be improved by inducing a surface layer of compressive residual stress using a surface enhancement finishing treatment. The compressive layer resists both crack initiation and small crack propagation. A novel surface enhancement method termed low plasticity burnishing (LPB), developed at Lambda Research, is presented.

25. Efficiently Optimizing Manufacturing Processes Using Iterative Taguchi Analysis

Taguchi experimental methods are now widely used in many industries to efficiently optimize the manufacturing process. An iterative approach allows multiple complex properties to be rapidly optimized at minimal cost. Lambda Research offers studies designed to optimize properties such as residual stress, retained austenite, phase composition, texture and cold working measurable by x-ray diffraction based upon Taguchi techniques. A Taguchi study of the influence of several heat treatment parameters on the percent retained austenite in 52100 steel is presented.

24. Minimizing Distortion in Machining

Distortion of the work piece during machining can result in costly increases in scrap rates. Residual stresses may be present in the original forging or casting. Machining away portions of the material relieves these residual stresses and the re-equilibration of the remaining residual stresses distorts the shape of the work piece. A method combining residual stress measurement and finite element analysis, to minimize distortion, is presented.

23. Lambda's Year in Review, 1998

A review of the events and hi-lights of 1998 are presented.

22. Qualitative Phase Analysis of Nitrided Steels

Many automotive, aerospace, and machine components are subject to wear and abrasion. Common methods of preventing wear and abrasion consist primarily of case hardening methods, particularly carburizing, induction hardening, and flame hardening. Nitriding is one method of case hardening which results in very low distortion because quenching is not required. Furthermore, nitriding does not result in phase transformations because it relies upon significantly lower temperatures, typically ranging from 500 to 550 C. X-ray diffraction can be used to identify which phases are present at the surface and the distribution of those phases with depth below the surface of the nitrided component. Lambda Research has developed techniques for making measurements as a function of depth in increments as small as 2.5 μm (0.0001 in.).

21. Lambda's Year in Review

A review of the events and hi-lights of 1997 are presented.

20. Automated Measurement of Subsurface Residual Stress Distributions

The measurement of residual stress distributions with depth is critical to understanding the impact of machining, shot peening, induction hardening, and similar processes on fatigue performance. Lambda Research has developed a revolutionary new apparatus which enables fully automated residual stress profiles to be obtained. The StressPro device, for which a patent is pending, allows one specimen to be measured while layers of material are removed from a second.

19. Residual Stress Contour Mapping

Traditional residual stress measurement by x-ray diffraction provides a single value of the residual stress in one direction at each point of measurement, similar to the results obtained from a single electrical resistance strain gage grid. Often, however, the nature of the residual stress distribution and the locations of maximum stress are not known. For example, stresses developed by quenching, welding, or forming of complex shapes may produce stress fields which defy prediction. Mapping of entire stress fields would be the ideal solution to this class of problems but until now has been prohibitively expensive for most applications using traditional means. Lambda has developed a novel translation table to allow the automated mapping of the residual stresses. The apparatus has three translational degrees of freedom to allow stress distributions to be mapped on either flat or moderately curved surfaces in planes parallel to the original surface.

18. Customized Literature Search Services for X-ray Diffraction Applications & MaterialsTechnology

As technology progresses, there is an increasing need to keep up with the latest changes and innovations. However, most people don't have the time or the resources to effectively collect and review all of the literature available, or to maintain a complete technical library. In an effort to address the needs of our clients Lambda offers a new customized literature search service for those who require library services pertaining to x-ray diffraction and materials technology.

17. Finite Element Correction for Stress Relaxation in Complex Geometries

Subsurface x-ray diffraction residual stress measurements are necessary to fully assess the magnitude and depth of stress distributions. Layers of material must be removed for subsurface measurement electrolytically to avoid plastic deformation. Re-equilibration after layer removal alters the residual stress in the exposed layers, leading to potential errors in measurement. The resulting errors increase with depth and stress magnitude, and must be corrected in proportion to the relaxation caused by layer removal. A novel correction technique, developed at Lambda Research for layer removal stress relaxation using finite element analysis (FEA), is presented.

16. The Identification of Crystalline Phases in Boiler Scale Deposits by X-Ray Diffraction

The analysis of boiler scale water deposits is an important tool both for maintenance of power generation systems and for failure analysis. Loss of heat transfer efficiency, increased fuel consumption, reduction in pipe carrying capacity, and the increased likelihood of pipe and boiler failures result from excessive boiler scale deposits. X-ray diffraction (XRD) qualitative phase analysis can be used to identify the crystalline compounds present in the boiler scale. A discussion of the x-ray diffraction method and examples are shown.

15. Effect of Prior Machining on Residual Stress in Welds

Tensile residual stresses partial J-weld penetrations in nuclear pressure vessels can cause stress corrosion cracking in the heater sleeves. X-ray diffraction residual stress and cold work measurements, made on an Alloy 600 heater sleeve welded into a massive steel pressure vessel, are shown. Measurements were made on the inside diameter of the heater sleeve as a function of axial position and depth into the wall.

14. Analysis of Radially Symmetric Textures Using Partial Pole Figures

Many manufacturing processes, including wire drawing, rolling, and chemical vapor deposition (CVD), tend to orient crystals in the material with a specific crystallographic plane parallel to a certain direction. The presence and degree of this preferred crystal orientation or texture can have a profound effect on the material’s mechanical, physical and/or electrical properties. X-ray diffraction methods of measuring the texture by means of pole figures are discussed.

13.Quantitative Analysis of Trace Amounts of Quartz and Cristobalite

Silica, SiO 2 is one of the most abundant materials on earth. Amorphous silica is used in a wide variety of applications ranging from metal casting cores to the manufacture of orthopedic implants. Small amounts of crystalline silica may form during the manufacture of the amorphous material. Crystalline forms of silica, particularly alpha quartz, have long been associated with the development of silicosis in exposed workers and silica has been identified as a material, which is potentially carcinogenic to humans. An x-ray diffraction technique used to identify and quantify the crystalline phases present in the fused silica is presented.

12. Quantitative Phase Analysis of Ceramic Coatings

Medical and dental implants employing hydroxylapatite (HA) plasma-sprayed coatings on a structural substrate are widely used in orthopedic and dental applications. Both the crystallinity of the coating and the presence of contaminant phases are believed to affect the biological response to the ceramic coating. A rigorous quantitative x-ray diffraction phase analysis method, capable of quantifying any or all of the contaminate phases, and determining the percent crystallinity from the total fraction of crystalline phases is presented.

11. Relocation of Lambda Research Corporate Headquarters and Expansion of Facilities

Lambda Research announces the move of our corporate headquarters to a newly expanded laboratory facility.

10. Problems with Non-Destructive Surface X-ray Diffraction Residual Stress Measurement, Part 3

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 and difficulties in interpreting surface results often limit the usefulness of surface data. Many material removal and surface treatment processes produce subsurface stress distributions which vary significantly within the depth of penetration of the x-ray beam and can cause significant experimental error in the measurement of the surface stress. Residual stress results demonstrating this point are shown.

9. Problems with Non-Destructive Surface X-ray Diffraction Residual Stress Measurement, Part 2

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 and difficulties in interpreting surface results often limit the usefulness of surface data. Many machining and grinding practices produce variations in the surface residual stresses that are so large that surface results are of little value. Residual stress data demonstrating this point are shown.

8. Problems with Non-Destructive Surface X-ray Diffraction Residual Stress Measurement, Part 1

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 and difficulties in interpreting surface results often limit the usefulness of surface data. The surface residual stresses present on many samples of practical interest are not representative of the processes that produced them. Residual stress results demonstrating this point are shown.

6. Pole Figure Determination

Because of anisotropy of individual crystals, the physical properties of most polycrystalline material are dependent upon any preferred orientation or “texture” which may be developed during manufacture. The preferred orientation is best quantified by a pole figure. Pole figures are either stereographic or equal area plots of a specific (hkl) pole (or crystallographic direction) density for the individual crystals comprising the sample. Pole figures obtained at Lambda Research are presented.

5. Residual Stress Distributions in Inconel 600 Tubing

Inconel 600 tubing used in steam generators can be susceptible to stress corrosion cracking providing that a high tensile residual stress is present. Residual stress and cold working was mapped around u-bend tubing using x-ray diffraction. A summary of the results is presented

4. Retained Austenite Measurement

Retained austenite strongly influences the properties of steel. The fatigue life, ductility, toughness, hardness, yield strength and machinability, can all depend upon the austenite content. X-ray diffraction is generally considered the most reliable technique to measure the percent retained austenite. X-ray diffraction retained austenite results obtained in accordance with E975 are presented.

3. Principal Residual Stress Determination

As a tensor property, the macroscopic residual stress will generally vary with direction at any point on a body. Processes that are inherently directional, such as forming, grinding or turning, may induce residual stress, which differ markedly in magnitude and even sign with direction in the plane of the surface. A technique of determining the complete stress field, as well as the principal maximum and minimum residual stresses, in the plane of the surface is presented.

2. X-Ray Diffraction Determination of Subsurface Cold Work Distributions

Plastic deformation, or cold working, of metals produces extensive dislocation networks within individual grains. The dislocation networks define “crystallites” which are near perfect crystalline subgrains. As cold work increases, the average size of the crystallites is reduces and the average microstrain within the crystallites increases. Both reduced crystallite size and increased microstrain result in broadening of the x-ray diffraction peak. A method of using Person VII fitting and x-ray diffraction line broadening to measure the percent cold work or plastic strain of Inconel 718 is shown.

1. Simultaneous Determination of Residual Stress and Hardness in Steels

X-ray diffraction can be used to determine both the macrostress and microstress. The macrostress is a tensor property and is equal to the sum of the residual and applied stresses present in the irradiated area. Diffraction peak broadening, produced by microstresses, can be treated as a scalar property, dependent on the average dislocation density and microstrain of the diffracting crystals. In martensitic steels, the martensite phase transformation produces microstresses and line broadening, which can be related directly to hardness. A method of using recently developed Pearson VII fitting function and x-ray diffraction line broadening to measure the hardness of steels is presented.