Known Residual Stress Specimens Using Opposed Indentation

In order to test new theories for residual stress measurement or to test the effects of residual stress on fatigue, fracture, and stress corrosion cracking, a known stress test specimen was designed and then fabricated, modeled, and experimentally validated. To provide a unique biaxial stress state, a 60 mm diameter 10 mm thick disk of 316L stainless steel was plastically compressed through the thickness with an opposing 15 mm diameter hard steel indenters in the center of the disk. For validation, the stresses in the specimen were first mapped using time-of-flight neutron diffraction and Rietveld full pattern analysis. Next, the hoop stresses were mapped on a cross section of two disks using the contour method. The contour results were very repeatable and agreed well with the neutron results. The indentation process was modeled using the finite element method. Because of a significant Bauschinger effect, accurate modeling required testing the cyclic behavior of the steel and then modeling it using a Chaboche-type combined hardening law. The model results agreed very well with the measurements. The duplicate contour measurements demonstrated stress repeatability better than 0.01% of the elastic modulus and allowed discussion of implications of measurements of parts with complicated geometries.