Cumulative damage
Damage accumulation describes the gradual growth of microscopic damage from repeated mechanical or thermal loading. Each load or heat cycle creates tiny cracks or structural changes that add up. Once the damage reaches a critical size, the component fails without warning.
Physical-Chemical Effect Mechanism
Each load cycle creates tiny plastic deformations at notches, inclusions, or grain defects. These deformations tear open the crystal lattice and form microcracks. At the crack tip, a high stress concentration develops, making the crack grow a bit with each additional cycle. Thermal cycling additionally stresses the microstructure through expansion and contraction. This way, damage accumulates continuously.
Metallurgical Relevance & Welding Risks
Welds almost always contain small imperfections like porosity, slag inclusions, or undercuts. These act as stress concentrators and kick off damage accumulation. Additionally, welding residual stresses add a preload that combines with service loads and accelerates damage. In the heat-affected zone, coarse grain or brittle martensite can further promote crack propagation. This lowers the number of cycles the part can withstand, causing earlier failure.
Shop Practice & Action Items
Avoid sharp notches and undercuts at the weld transition. Grind the weld transitions smooth to reduce stress concentration. Pick a filler metal that's tough enough for the base material. For high cycle counts, post-weld heat treatment to relieve residual stresses is a good idea. Document the number and magnitude of load cycles to estimate remaining life. Check critical welds regularly for cracking.