## Laboratory Services Technical PapersHCF results for surface treated IN718 Effect of simulated FOD on HCF results for surface treated IN718. The SP specimens showed a drop in fatigue strength at 107 cycles of 50% from 690 MPa (100 ksi) to about 345 MPa (50 ksi). In contrast, the LPB treated specimens showed less than 15% drop in fatigue strength from about 900 Mpa (130 ksi) to about 800MPa (115 ksi).## (No. 261) “The Effect of Shot Peening Coverage on Residual Stress, Cold Work, and Fatigue in a Nickel-base Superalloy"
J.T. Cammett, U.S. Naval Aviation Depot, Cherry Point, NC, USA
(Excerpt) 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 Error in measured RS vs. Bragg angleError in measured residual stress vs. Bragg angle for 0.01 deg. error in fitted peak position showing large errors at low angles. ## (No. 255) "Practical Application of Nondestructive Residual Stress Measurements by X-ray Diffraction"
Douglas J. Hornbach, Vice President, Director of Laboratory Operations, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Error in measured RS vs. Bragg angleError in measured residual stress vs. Bragg angle for 0.01 and 0.02 deg. error in fitted peak position for Ti alloys. ## (No. 251) "Development of Nondestructive Residual Stress Profile Measurement Methods - The Integral Method"Douglas J. Hornbach, Vice President, Director of Laboratory Operations, Lambda Research, Inc.
(Excerpt) 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. ## (No. 235) "The Effect of Shot Peening Coverage on Residual Stress, Cold Work and Fatigue in a Ni-Cr-Mo Low Alloy Steel"
Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) RS distributions in 1018 carbon steelResidual stress distributions in shot peened 1018 carbon steel with a hardness of Rb 80. ## (No. 229) "Quality Assurance of Shot Peening by Automated Surface and Subsurface Residual Stress Measurement"
John T. Cammett, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Error in measured RS vs. Bragg angleError in measured residual stress vs. Bragg angle for 0.01 and 0.02 deg. error in fitted peak position for Ti alloys. ## (No. 227) "Iterative Taguchi Analysis: Optimizing the Austenite Content and Hardness in 52100 Steel"
Perry W. Mason, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) RS distributions in 1018 carbon steelResidual stress distributions in shot peened 1018 carbon steel with a hardness of Rb 80. ## (No. 224) "X-ray Diffraction Characterization of Crystallinity and Phase Composition in Plasma-Sprayed Hydroxylapatite Coatings"
Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. To overcome the complexities of characterizing plasma-sprayed HA coatings, an external standard method of XRD quantitative analysis has been developed that can be applied nondestructively. Data collection and reduction strategies allowing separation of intensity diffracted from commonly occurring phases and the amorphous fraction are presented. The method is applied to coating samples, and detection limits and sources of error are discussed. Repeatability and accuracy are demonstrated with powder mixtures of known composition. (See the full paper) FEM of forging geometryFinite element model of forging geometry with gray elements indicating material to be removed, and white elements signifying the final disk geometry. ## (No. 220) "Development of Machining Procedures to Minimize Distortion During Manufacture"
Douglas J. Hornbach, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Airfoil cross section distortionAirfoil cross section distortion of shot peened Inconel 718 FEA model. ## (No. 219) "Thermal Residual Stress Relaxation and Distortion in Surface Enhanced Gas Turbine Engine Components"Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Primary defect in OTSG tubingPrimary defect in Davis-Besse Nuclear Power Station - Once through steam generator (OTSG) Tube No. 58/119, after bending (10X). ## (No. 218) "Residual Stresses in OTSG Tube Expansion Transitions"P.A. Sherburne, Framatone Technologies, Inc.
(Excerpt) 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° - 621°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. (See the full paper) Complex cold work distributionsVariation in degree of cold work (equivalent accumulated true plastic strain) with axial displacement and depth in the HAZ and machined regions of the low angle side of 50 deg. Alloy 600 J-weld mockup.## (No. 217) "Tensile Residual Stress Fields Produced in Austenitic Alloy Weldments"Douglas J. Hornbach, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Generator tubesheet secondary-sideIn a steam generator, the transition region between the expanded and unexpanded tube has been particularly problematic because of its higher tensile residual stresses and its location at the tubesheet secondary-side where it is exposed to corrosive conditions. ## (No. 216) "Experimental Residual Stress Evaluation of Hydraulic Expansion Transitions in Alloy 690 Steam Generator Tubing"
Rod McGregor, Babcock & Wilcox International
(Excerpt) 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. (See the full paper) Alloy 600 50-deg. heater sleeveAlloy 600 50-deg. heater sleeve penetration J-weld mockup specimen geometry. Several J-weld heater sleeve mockups were prepared to simulate as accurately as possible the actual fabrication procedures used in the pressure vessel penetrations which had experienced stress corrosion cracking in service. ## (No. 215) "Effect of Prior Machining Deformation Upon the Development of Tensile Residual Stresses in Weld Fabricated Nuclear Components"Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Plane stress at a free surfacePlane stress at a free surface showing the change in lattice spacing with tilt ψ for a uniaxial stress σφ parallel to one edge.## (No. 214) "Current Applications of X-ray Diffraction Residual Stress Measurement"
Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) ## (No. 213) "X-Ray Diffraction Characterization of Residual Stress and Hardness Distribution in Induction Hardened Gears"Douglas J. Hornbach, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) ## (No. 212) "Measurement of Residual Stresses in Alloy 600 Pressurizer Penetrations"
J.F. Hall, ABB-CE
(Excerpt) 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. (See the full paper) True stress-strain curves for Alloy 600Engineering and true stress-strain curves for Alloy 600 heater sleeve material. ## (No. 211) "XRD Residual Stress Measurements on Alloy 600 Pressurizer Heater Sleeve Mockups"
J.F. Hall, ABB-CE
(Excerpt) 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. (See the full paper) RS distribution in ground steelResidual stress distribution with depth below a ground surface on hardened steel. ## (No. 209) "Problems with Non-Destructive Surface X-Ray Diffraction Residual Stress Measurement"
Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) ## (No. 208) "The Use of X-Ray Diffraction to Determine the Triaxial Stress State in Cylindrical Specimens"
Paul S. Prevéy, Lambda Research, Inc. Practical Applications of Residual Stress Technology, ed. C. Ruud, Materials Park, OH: American Society for Metals, 1991, pp. 77-81. 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. (See the full paper) RS on 1018 steel SEM sampleLongitudinal residual stress and (211) peak breadth distributions produced by strain-gage installation on a 1018 steel SEM round-robin sample. ## (No. 207) "Residual-Stress Distributions Produced by Strain-Gage Surface Preparation"
Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Four-Point Bending ApparatusWhen positioned in the four-point bending apparatus, the samples were stressed to approximately 5%, 40%, and 75% of the yield strength of the alloy.## (No. 206) "A Method of Determining the Elastic Properties of Alloys in Selected Crystallographic Directions for X-Ray Diffraction Residual Stress Measurement"
Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Ground Ti-6Al-4VThe residual stress distributions, showing the mean value and standard deviations, are presented for both parabolic and Cauchy profiles as functions of depth. ## (No. 205) "The Use of Pearson VII Distribution Functions in X-Ray Diffraction Residual Stress Measurement"
Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) System for stress measurementThe coordinate system used for defining the stress measurement sites on bent tubing samples. ## (No. 203) "Surface Residual Stress Distributions in As-Bent Inconel 600 U-Bend and Incoloy 800 90-Degree Bend Tubing Samples"
Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Shot peened 5056 AluminumLinear Dependence of Lattice Spacing with Sine-Squared-Psi in Shot Peened 5056 Aluminum. ## (No. 202) "X-Ray Diffraction Characterization of Residual Stresses Produced by Shot Peening"
Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) "Stiff-back" specimenA "stiff-back" specimen which could be loaded directly in tension was instrumented with strain gages on both sides of the region irradiated during x-ray diffraction applied stress measurement. ## (No. 201) "The Uniformity of Shot Peening Induced Residual Stress"Paul S. Prevéy, Lambda Research, Inc. 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. (See the full paper) X-ray diffraction RS measurementBasic geometry of the single-angle technique for x-ray diffraction residual stress measurement Np, normal to the lattice planes; Ns, normal to the surface.## (No. 200) "X-Ray Diffraction Residual Stress Techniques"Paul S. Prevéy, Lambda Research, Inc.
(Excerpt) 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. (See the full paper) Comparable surface stressesComparable surface stresses produced by radically different processing showing the need for subsurface measurement. ## (No. 99) "Residual Stress Measurement for Quality Control of Shot Peening"Lambda Research, Inc. The best developed and most accurate means of measuring shot peening residual stress distributions with depth is by x-ray diffraction (XRD). XRD procedures have been established by the SAE, and have been widely used since the 1970's for the determination of subsurface stress distributions in automotive and aerospace applications. (See the full paper) |