Reliable Measurement of Residual Stress for Understanding Fatigue Performance in Different Metallic Materials

Micro-slotting, a relaxation residual stress measurement technique, has recently been shown to be an effective method for measuring local residual stresses in a variety of materials. This technique relies on a Scanning Electron Microscope-Focused Ion Beam (SEM-FIB) dual-beam system for milling and imaging, Digital Image Correlation (DIC) software to track displacements due to relaxation of residual stresses after milling, and finite element analysis (FEA) for displacement-stress correlation and calculation of the original stress state in the imaged region. In our previously presented study, sub-surface micro-slotting measurements were shown to agree well with X-ray diffraction data on surface machined and shot peened Ti-6Al-4V planar coupons, and the method was successfully used to capture residual stress profiles around drilled and cold worked holes. Electron Backscatter Diffraction (EBSD) was used to estimate the depth of plastic deformation on each sample by evaluating pattern quality parameters as a function of distance from the sample surface or hole edge. Correlation of the elastic and plastic strain data allowed for interpretation of fatigue life differences as a result of the different processing conditions. For this presentation, the combination of the micro-slotting residual stress measurement method and the semi-quantitative EBSD approach is extended to different metallic materials in order to assess differences in fatigue life and to show the versatility of the two techniques. EBSD will also be used to obtain grain orientation information in the measurement regions in an attempt to account for local differences in elastic properties.