Effect of Loading Frequency on the Fatigue Crack Growth Rate of Type 304L(N) Material

Abstract

The aim of this study was to study the effect of loading frequency on the fatigue crack growth rate (FCGR) behaviour of Type 304L(N) austenitic stainless steel material. Compact tension (C(T)) specimens machined from the solution annealed pipe were used for carrying out the FCGR tests as per the standard procedure of ASTM E647. Constant amplitude loading method with varying frequency (0.01–10 Hz) was followed for cyclic loading during the tests. The tests were carried out at load ratio 0.1 and ambient temperature. Experimental results show that FCGR increases with decrease in frequency for a given stress intensity factor range. Increase in FCGR is very significant from high to low frequency. Fracture surface examinations of tested specimens were carried out using scanning electron microscope to understand the increased crack growth at lower frequency. The fracture surfaces revealed the presence of facets which have feature of brittle fracture. These facets could be the locations for the formation of deformation-induced-martensite (DIM). The facets are relatively harder (320 HV) as compared to matrix of the material (155 HV) which may have led to higher crack growth rate. Area covered by these facets is more for fracture surface tested at lower frequency. FCGR of present test study is also compared with FCGR curve for austenitic stainless steel given in ASME Section XI. ASME FCGR curve is comparable with that of the FCGR curve of loading frequency 10 Hz. However, ASME FCGR would lead to over-prediction of fatigue crack growth life for lower loading frequency scenario.