Strain field and scattered intensity profiling with energy dispersive x-ray scattering

Abstract

Two powerful synchrotron x-ray scattering techniques for residual strain depth-profiling and tomography-like scatter-intensity profiling of materials are presented. The techniques utilize energy dispersive x-ray scattering, from a fixed microvolume, with microscanning of the specimen being used to profile its interior. The tomography-like profiles exploit scattering-cross-section variations, and can be contrast enhanced by separately monitoring scattering from different crystal structures. The strain profiling technique is shown to finely chronicle the internal strain variation over several mm of steel. Detailed strain profiling for a cantilever spring demonstrates the interplay of residual and external stresses in elastic/plastic deformation. Since surface compression, by shot peening, is a classic method to fortify against fatigue failure, the strain profile for a shot-peened, surface-toughened material is determined and discussed in terms of a simple elastic–plastic stress/strain model. Finally the lattice strains in a WC/Co coated steel composite material are discussed.

A two-scale methodology for determining the residual stresses in polycrystalline solids using high energy X-ray diffraction data

“…The collection time for a typical X-ray experiment is significantly less than that needed for an experiment conducted using neutrons and monochromatic X-ray area detector technology also makes it possible to collect data in many directions simultaneously. Energy dispersive X-ray methods employing polychromatic beams also hold significant potential (Croft et al, 2002;Korsunsky et al, 2002) but have their own limitations related to collecting complete strain data in many directions.…”

Nondestructive residual strain measurement using high energy x-ray diffraction

“…Those high intensities allow for 20 mm penetration in aluminum using 80 kev beam (26) and 3D strain maps can be constructed for depths up to 100 µm in most practical materials using 5 keV to 14 keV synchrotron source (27). White synchrotron beams with energies ranging from 10 keV to 200 keV were used to perform energy dispersive fixed angle strain diffraction measurement through several millimeters of steel (6). More measurements on steel showed penetration depths up to 300 µm using energies up to 70 kev and up to 30 µm using 25 keV (28-31).…”

Synchrotron Based X-Ray Strain Mapping in Fatigued Materials Subjected to Overload

“…The strain variation from position to position in the sample is then determined from the shifts in Bragg peak energy via the relation eh=(Ad/ do)hkl=(AE/ E 0 )hk 1 the a sample is translated through the beam. Here Ad = d-d 0 is the change in the lattice plane spacing, do is the lattice spacing of the stress-free materials, AE = E 0 -E is the corresponding Bragg peak shift and E 0 is the center of gravity of peak of the stress-free material [1][2]. A point far from the crack tip was chosen to determine the E 0 value.…”

“…The application of synchrotron based energy dispersive X-ray diffraction (EDXRD) to a measurement of local strain fields in the vicinity of fatigue cracks was discussed in a previous article by the authors [1][2]. The technique exploits the high intensity/energy white beam (30-200 KeV) x-rays of the X17 wiggler beamline at the Brookhaven National Synchrotron Light Source (NSLS).…”

“…Understanding the important "overload effect" is also a first step in the direction of eventual understanding of the fatigue crack growth under arbitrary variable amplitude loading conditions. Our group has been involved in the ongoing development of high-energy synchrotron radiation xray diffraction techniques to map local strain fields in general [1], and in the vicinity of fatigue cracks in particular [2]. In this report we apply these techniques to a series of compact tension (CT) geometry 4140 steel test specimens which have been fatigued at constant amplitude, subjected to a single overload cycle and further fatigued to various distances (or times) beyond the overload point (see for example Figure P1.…”