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Residual stresses affect such important materials design
properties as fatigue life, fracture strength, onset of yield, and microcracking. Of all the residual stress measuring techniques, diffraction methods are widely accepted as the most general and reliable nondestructive method of quantifying the residual stress tensor. Diffraction methods use the atomic planes of the crystalline grains within the material as very sensitive strain sensors. Analysis techniques permit separation of long range macrostresses and short range or grain-to-grain microstresses. Crystallographic texture, or the non-random orientation of grains, is also measured in the RSUC for correlation with materials performance. The RSUC facilities include laboratory x-ray, synchrotron x-ray, and neutron diffraction instruments. Neutron diffraction is complementary to the traditional x-ray diffraction because of the great differences in a material's absorption of the two radiation types. Neutrons can penetrate many millimeters into most engineering materials, while x-rays are absorbed within a surface layer several micrometers thick. Thus, neutron diffraction can be used to probe strains throughout the volume of a material while X-rays are used to measure the strain in a shallow layer very near the surface. |
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