Fatigue fracture behavior under thermal cyclic loads: Experimental methods, physical regularities, and damage mechanisms

Thermal fatigue and creep-thermal fatigue are two typical fatigue fracture behaviors under thermal cyclic loads. This study focuses on three aspects. i. Experimental methods. A thermal cyclic experimental method suitable for thin-walled structures with holes was developed, and a tubular specimen with test holes was designed. By comparing different test hole configurations, it was found that the conical hole specimen shows the best performance in both crack growth and observation. ii. Physical regularities. Experiments were conducted with temperature range, mean temperature, and high-temperature hold time as the control variables. The influence patterns of these variables were empirically explained. The data demonstrated that the temperature range and high-temperature hold time are the main factors accelerating crack propagation. The increase in mean temperature reduced the fatigue life, but the effect was relatively weak. iii. Damage mechanisms. Microstructural observation and elemental analysis were conducted, and the damage mechanisms for both behaviors were summarized. The initiation and propagation of thermal fatigue cracks are primarily governed by fatigue behavior, with transgranular fracture as the dominant damage mode. Crack initiation in creep-thermal fatigue is still dominated by fatigue behavior. However, crack propagation is driven by the combined effects of creep, fatigue, and oxidation, resulting in intergranular fracture as the primary damage mode.