The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.

Fracture toughness of AZ31B magnesium alloy subjected to quasi-static loading was investigated by infrared thermography. The results showed that temperature evolution around the crack propagation path during fracture underwent three stages: initial steady stage, monotonic increase stage and final steady stage. The temperature increase at the beginning of stage II is nearly corresponding to the initiation of unstable crack propagation. And based on this phenomenon, a method applying infrared thermography to estimate fracture toughness of AZ31B magnesium alloy was proposed. Fracture toughness was calculated through infrared thermography, which was in good agreement with the result determined by traditional standard method. Finally, the fracture mechanism was investigated.