Residual stress measurement

Residual stress measurement quantifies the locked-in stress that welding and forming leave in a structure after the external load is removed, since welding shrinkage can leave tensile stress near the yield strength at the weld toe that adds directly to service stress and drives fatigue cracking. The two field methods are X-ray diffraction, which reads the lattice strain in the surface grains non-destructively, and the hole-drilling strain-gauge method (ASTM E837), which relieves stress around a small drilled hole and infers it from the measured relaxation. Knowing the residual field guides post-weld heat treatment and fatigue assessment. High weld-toe tensile residual stress is why fatigue S-N curves are built on as-welded details.