Reliable Measurement of Sub-Surface Residual Stress for Understanding Fatigue Performance

Reliable measurement of residual stress is critical to both fatigue life prediction and understanding crack growth in failed components.  Micro-slotting, a relaxation residual stress measurement technique, has been shown to be an effective method for measuring near surface and sub-surface residual stresses in a variety of materials. The micro-slotting method 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 the presented study, FEA is used to assess the sensitivity of the calculated stress to possible sources of variability within the method, and an identified best practice micro-slotting procedure is employed to estimate the sub-surface residual stress distributions in surface machined and shot peened Ti-6Al-4V plates and around drilled and cold worked holes. In order to observe the relationship between elastic and plastic strain, semi-quantitative data obtained from Electron Backscatter Diffraction (EBSD) maps is utilized to estimate the depth of plastic deformation for the different processing conditions. Pattern quality parameters and grain misorientation data are evaluated as a function of distance from the sample surface or hole edge in order to identify regions of higher strain. The collective understanding of residual stress and extent of plastic deformation around the holes allows for interpretation of differences in fatigue life and crack growth behavior.