X-ray diffraction is commonly used to measure a materials crystal structure, crystal lattice orientation, and the spacing of the lattice planes. Our X-ray systems are designed to measure the spacing of one or two specifically selected lattice plane reflections for a given material. The systems use various X-ray tubes selected to optimize these measurements for ferritic steel, stainless steel, aluminum, or magnesium alloys. The system tracks the lattice spacing from the initial to current spacing while the specimen is under applied load in the plane of the sheet. By tilting the measurement system from the normal direction of the sheet toward the direction of loading the distribution of strains can be measured and used to derive the full stress tensor just inside the sheet surface.
We use this method to measure the average stress for plane-stress experiments in the plastic range while the specimen are held under load. The two systems are associated with our Marciniak and cruciform mechanical testing machines which can apply plane-stress in fixed and variable stress ratios, respectively, between the principal axes of the sheet material. The measurements are made at a point typically between 0.5 mm to 3 mm in diameter, and can be repositioned to interrogate multiple points on the specimen surface. This permits measurements of the multiaxial stress field around features (e.g. hole or localized neck) in the specimen.