Orthopedic and dental implants consisting of a metallic substrate plasma spray coated with hydrox... more Orthopedic and dental implants consisting of a metallic substrate plasma spray coated with hydroxyapatite (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. Repeability and accuracy are demonstrated with powder mixtures of known composition.
The present study builds upon the work of Maitra and English and postulates that anodic polarizat... more The present study builds upon the work of Maitra and English and postulates that anodic polarization of a galvanic couple falling between two breakdown potentials could cause significant intergranular corrosion but minimal general pitting of the matrix of 7000 series aluminum alloy. The crack-like intergranular fissures, visually like stress corrosion cracking (SCC) or intergranular cracking (IGC), would be difficult to detect and independent of applied or residual stress. Component failure initiating from the fissures could be incorrectly identified as SCC. SCC mitigation by the introduction of surface compressive residual stresses would be ineffective against a failure mechanism indifferent to applied or residual stress fields. In this study, the Maitra-English grain boundary attack mechanism was observed in potentiodynamic testing of a proprietary 7000 series aluminum alloy, producing both crack-like fissures along grain boundaries and more general pitting. Galvanic couple testin...
Selected data showing typical macroscopic residual stress distributions in u-bent Inconel 600, an... more 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.
The two-angle and Sin2ψ methods of x-ray diffraction, residual stress measurement have been shown... more The two-angle and Sin2ψ methods of x-ray diffraction, residual stress measurement have been shown to yield anomalous results When applied to samples which have been plastically deformed uniaxially in tension or by rolling. These anomalous stresses, which are not true mechanical macrostresses, have been shown to arise from a nonlinear dependence,of d, the interplanar spacing employed for stress measurement, upon Sin2 ψ. A new method of x-ray diffraction stress measurement has been developed by Marion and Cohen which aLlows the separation of the Linear and nonlinear components of the dependence of d upon Sin2 ψ.The two-angle and the Marion-Cohen x-ray diffraction methods are applied to the measurement of residual stresses in the machined and shot peened surfaces of steel and aluminum samples. The results indicate that machining deformation results in a nearly linear dependence of d upon Sin2ψ , and onLy a slight deformation texture. The two x-ray methods yield stress values in ground,...
Two rectangular samples of ASTM SA 508 Class 2 steel, stress relieved and shot peened to 14-16A i... more 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 di...
... SSC susceptibility in high strength API P110 grade tubular products prevents their use in 100... more ... SSC susceptibility in high strength API P110 grade tubular products prevents their use in 100% H2S sour environments at temperatures less than 79°C ... Samples were processed on a CNC mill or lathe to allow positioning of the LPB tool in a series of passes along the region to ...
Because surface measurements are non-destructive, xray diffraction is often considered as a metho... more Because surface measurements are non-destructive, xray 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 resi...
The reason for cold working the surface of a metal component is to develop a compressive layer th... more The reason for cold working the surface of a metal component is to develop a compressive layer that will delay fatigue crack initiation and retard small crack propagation. However, the benefits of surface enhancement are lost if the compressive layer relaxes at the operating temperature of a component such as a turbine blade. To find ways to prevent such relaxation, a study was conducted at NASA Glenn to establish an improved method for producing a heat-resistant compressive layer with minimal plastic deformation. The study encompassed shot peening, gravity peening, laser shock peening, and low-plasticity burnishing (Fig. 1). This article compares the results of thermal relaxation studies, high-cycle fatigue testing, and fatigue crack growth measurements after application of the various treatments. They show that the deep compressive layer produced by low-plasticity burnishing provides the best resistance to the growth of surface cracks.
Orthopedic and dental implants consisting of a metallic substrate plasma spray coated with hydrox... more Orthopedic and dental implants consisting of a metallic substrate plasma spray coated with hydroxyapatite (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. Repeability and accuracy are demonstrated with powder mixtures of known composition.
The present study builds upon the work of Maitra and English and postulates that anodic polarizat... more The present study builds upon the work of Maitra and English and postulates that anodic polarization of a galvanic couple falling between two breakdown potentials could cause significant intergranular corrosion but minimal general pitting of the matrix of 7000 series aluminum alloy. The crack-like intergranular fissures, visually like stress corrosion cracking (SCC) or intergranular cracking (IGC), would be difficult to detect and independent of applied or residual stress. Component failure initiating from the fissures could be incorrectly identified as SCC. SCC mitigation by the introduction of surface compressive residual stresses would be ineffective against a failure mechanism indifferent to applied or residual stress fields. In this study, the Maitra-English grain boundary attack mechanism was observed in potentiodynamic testing of a proprietary 7000 series aluminum alloy, producing both crack-like fissures along grain boundaries and more general pitting. Galvanic couple testin...
Selected data showing typical macroscopic residual stress distributions in u-bent Inconel 600, an... more 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.
The two-angle and Sin2ψ methods of x-ray diffraction, residual stress measurement have been shown... more The two-angle and Sin2ψ methods of x-ray diffraction, residual stress measurement have been shown to yield anomalous results When applied to samples which have been plastically deformed uniaxially in tension or by rolling. These anomalous stresses, which are not true mechanical macrostresses, have been shown to arise from a nonlinear dependence,of d, the interplanar spacing employed for stress measurement, upon Sin2 ψ. A new method of x-ray diffraction stress measurement has been developed by Marion and Cohen which aLlows the separation of the Linear and nonlinear components of the dependence of d upon Sin2 ψ.The two-angle and the Marion-Cohen x-ray diffraction methods are applied to the measurement of residual stresses in the machined and shot peened surfaces of steel and aluminum samples. The results indicate that machining deformation results in a nearly linear dependence of d upon Sin2ψ , and onLy a slight deformation texture. The two x-ray methods yield stress values in ground,...
Two rectangular samples of ASTM SA 508 Class 2 steel, stress relieved and shot peened to 14-16A i... more 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 di...
... SSC susceptibility in high strength API P110 grade tubular products prevents their use in 100... more ... SSC susceptibility in high strength API P110 grade tubular products prevents their use in 100% H2S sour environments at temperatures less than 79°C ... Samples were processed on a CNC mill or lathe to allow positioning of the LPB tool in a series of passes along the region to ...
Because surface measurements are non-destructive, xray diffraction is often considered as a metho... more Because surface measurements are non-destructive, xray 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 resi...
The reason for cold working the surface of a metal component is to develop a compressive layer th... more The reason for cold working the surface of a metal component is to develop a compressive layer that will delay fatigue crack initiation and retard small crack propagation. However, the benefits of surface enhancement are lost if the compressive layer relaxes at the operating temperature of a component such as a turbine blade. To find ways to prevent such relaxation, a study was conducted at NASA Glenn to establish an improved method for producing a heat-resistant compressive layer with minimal plastic deformation. The study encompassed shot peening, gravity peening, laser shock peening, and low-plasticity burnishing (Fig. 1). This article compares the results of thermal relaxation studies, high-cycle fatigue testing, and fatigue crack growth measurements after application of the various treatments. They show that the deep compressive layer produced by low-plasticity burnishing provides the best resistance to the growth of surface cracks.
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