Room Temperature Creep At Crack Tip In SUS304 Stainless Steel And Its Influence On Fatigue Crack Growth

With the micromation of mechanical components and the improvement of measurement accuracy, room temperature creep has been observed in many kinds of metals. The influence of room temperature creep on the application of materials in engineering has received more and more attention as a new issue. On the other hand, fatigue and overload are very familiar phenomena. However, researches about room temperature creep are now focused on round tension test specimens and applied in particular situation. For these reasons, SUS304 stainless steel is selected as experimental material and made into compact tensile samples in order to investigate room temperature creep at crack tip and its influence on subsequent fatigue crack growth based on fracture mechanics. The main equipment is MTS810 (materials test system).Time-dependent deformation at crack tip under constant stress intensity factor following fatigue crack growth under constant stress intensity factor range was detected at room temperature. That is to say, room temperature creep occurred at crack tip. The creep deformation increases with the increasing of overload ratio and constant stress intensity factor. But with the increasing of load rate, creep deformation decreases before a critical rate, but increases after that. Unloading and then reloading to prior stress intensity factor after the first creep test, room temperature creep would occur again. But creep strain was much less than that during the first creep test within the same time. After room temperature creep, the crack tip became blunt and a demarcation line is observed on the fracture surface.Crack propagation mode will change after room temperature creep. Compared to single wave overload, room temperature creep under the same overload ratio will insult in lower minimal growth speed and longer overload-affected distance. Following the same basic cyclic pattern, room temperature creep under higher overload ratio will result in more serious retardation. In addition, load ratio would affect the retardation after room temperature creep. It turns out that crack closure is the main reason for retardation, and crack tip blunting, crack deflection and strain-induced martensite would reinforce it.