Effect of Mean Stress on Small Fatigue Crack Growth Rate on Low Carbon Steel with Several Simulated HAZ Heat Treatment

摘要

In this study, to clarify the effect of microstructure of Heat Affected Zone (HAZ) and residual stress of weld on small fatigue crack growth behavior, uniaxial fatigue testing with surface small fatigue crack growth observation were carried out for low carbon steel with three simulated HAZ heat treatment under several mean stress conditions. As a results, ε~na type small crack growth law was also effective to HAZ microstructure under zero mean stress. Under tensile mean stress condition, fatigue fracture life and small fatigue crack growth life were decreased. In addition, it is clarified that small fatigue crack growth rate acceleration under tensile mean stress was able to be evaluated by using Smith-Watson-Topper equivalent strain ε_(eq) which is obtained from mean stress dependency of fatigue fracture life for smooth specimen and ε_(eq)~na type modified small crack growth law was proposed. Furthermore, it is clarified that ε_(eq) was proportional to ε_(eff) which was evaluated by crack opening point measured with Digital Image Correlation technique. It is considered that equivalent strain ε_(eq) implicitly includes small crack growth life and effect of crack closure.