X-ray diffraction provides a direct measurement of residual strain on a material by directly measuring the expansion or compression of lattice plane spacings in the grains of the material. X-ray diffraction is a non-destructive and widely accepted method for the measurement of strain. Residual stress is computed from the measured residual strain.
Conventional X-ray diffraction measures residual strain on bulk materials. The diffraction lines of interest are produced from the entire area illuminated by the X-ray beam, which is usually a line source approximately 2 mm by 12 mm. Penetration depth of the X-ray beam provides the third dimension for the volume being tested. When a point source with micro-optics is incorporated, a beam size as small as 50 μm in diameter can be precisely positioned on the sample, and the instrument can be aligned to within 10 μm accuracy. This microdiffraction technique is necessary to measure stress on small parts or on specific small areas on larger parts. Applications include stress on specific locations on medical implants or on composite parts used in the microelectronics industry.
A video microscope is used to precisely position the microscopic X-ray beam on the sample. This is a critical step, since it not only reduces error associated with sample positioning in the X-ray beam, but also ensures that the exact area of interest is illuminated. The following illustration shows the microbeam used as an X-ray probe to measure stress on a 304 stainless steel wire filament of 0.005" in diameter. A beam of appropriate size (100 μm pinhole) is selected for the experiment. The area illuminated by the X-ray beam as well as the penetration depth are labeled.
This microbeam must then be precisely positioned on the sample area of interest. This is accomplished via "X-ray microdiffraction optics". In principle, the X-ray beam and an optical axis with high magnification zoom capability can be aligned to intersect at one point in space within a 10 mm certainty. This point in space must intersect the center of the circles on the goniometer used for the diffraction experiment. Cross hairs are positioned on the video image and displayed on the terminal screen, which, in principle, exactly correspond to the intersection of the incident X-ray beam and the video optical axis.
The limitation on the sample size and positioning is the accuracy of the instrumentation, which is measured by such variables as the size (diameter) and position of the incident beam and the sphere of confusion of the goniometer (accuracy of the circle center positions). Conventional X-ray diffraction instruments have a number of moving parts, which can include the X-ray beam, the detector, and the goniometer base which positions the sample in the X-ray beam. All of these positioning parts introduce sources of geometrical error. Microdiffraction X-ray instruments use pinhole optics, and the instrument can be aligned with accuracy to within 10 mm. Incident beam collimators and a high magnification video camera are used to accurately and precisely position the sample area of interest in the X-ray beam. A two-dimensional detector (RAPID II imaging plate) is used to collect the entire diffraction cone while reducing the number of moving axes and associated uncertainty.