Meaning
Time-dependent reduction of internal residual stress field values within a bulk engineering component occurs without changes in external dimensions. The process of macro-stress relaxation converts elastic strain into plastic deformation over time when a component is exposed to elevated thermal or mechanical conditions. It operates across long-range macroscopic distances, affecting entire forged cross-sections, welded assemblies, and cold-worked fasteners.
The scope excludes micro-stress relaxation occurring at sub-grain scales between individual dislocations or lattice defects.
Thermal Activation
Elevated thermal exposure increases atomic mobility, allowing dislocation movement and creep strain mechanisms to proceed at lower applied stress levels. Thermally induced macro-stress relaxation occurs rapidly during stress-relief annealing operations in industrial furnaces. The magnitude of stress reduction depends directly on exposure temperature, initial residual stress magnitude, and material yield properties at temperature.
Prolonged thermal exposure eventually reduces macro-stresses to an asymptotic minimum baseline.
Stress Assessment
X-ray diffraction techniques and neutron diffraction mapping measure macroscopic residual stress gradients non-destructively. Evaluating macro-stress relaxation involves measuring lattice strain shifts before and after thermal exposure cycles. Hole-drilling strain gauge methods provide destructive verification by recording elastic strain release as material is removed.
Diffractometer calibration ensures precise peak shift measurements across complex part geometries.
Component Retention
Loss of residual compressive surface stresses reduces the structural benefit provided by shot peening or surface rolling operations. Uncontrolled macro-stress relaxation reduces clamping force in bolted flange connections over extended service periods.
