Assessment of equivalent initial flaw size estimation methods in fatigue life prediction using compact tension specimen tests

To assess the efficiency of the Equivalent Initial Flaw Size (EIFS) concept in the life estimation of mechanical components, a novel approach is outlined in this paper. For this purpose, experimental tests are conducted on the compact tension specimens made of 4340 steel and the number of cycles required for the crack to grow from the end of the notch up to the fracture of the specimen is counted. In fact, the fatigue cycling for the pre-crack initiation is a part of cycle count procedure and it is assumed that an initial micro-crack exists at the end of the notch, which grows due to fatigue loading, the length of which is estimated using the EIFS method. Three methods of back extrapolation, Kitagawa–Takahashi diagram, and time to crack initiation are used in order to estimate EIFS and their results are compared. For estimating EIFS by Kitagawa–Takahashi diagram, the threshold value of the stress intensity factor (ΔKth) is required. In this paper, ΔKth is estimated by both K-increasing and K-decreasing methods. The experimental results show that the value of ΔKth estimated by the K-increasing method is lower than that estimated by K-decreasing method. It is observed that the estimated EIFS by back extrapolation method and TTCI method is dependent on the loading amplitude, while the estimated EIFS by Kitagawa–Takahashi diagram is identical for all loading amplitudes and it can be considered as the material property. The predicted life based on the Kitagawa–Takahashi method is in relatively good agreement with the experimental results. However, the TTCI method does not have sufficient accuracy especially in low amplitude fatigue.