What is a typical indicator of fatigue in boiler tubes?

A thin-lipped rupture in boiler tubes is a typical indicator of fatigue because it suggests that the material has undergone repeated stress cycles. This kind of failure often occurs in areas that experience high thermal and pressure fluctuations, leading to the development of small cracks that can grow over time due to continuous loading. The "thin-lipped" aspect points to the failure's propensity to occur at lower stress levels, typical in metal fatigue scenarios.

In contrast, uniform thinning typically indicates wear or corrosion rather than fatigue, as it signifies an even loss of material throughout the tube. Circumferential cracks may suggest other failure modes such as stress corrosion cracking or thermal cycling but are not solely indicative of fatigue. Surface delamination generally refers to the separation of layers in composite or layered materials and is not a direct indicator of fatigue in typical metallic boiler tubes.