Notch FCI life under constant amplitude uniaxial loads

Purpose - The purpose of this paper is to estimate the number of cycles for fatigue crack initiation in notched plates. Design/methodology/approach - Previously published experimental results for tests with initially uncracked U-shaped notches of different sizes and geometries are utilized. The present work defines the fatigue initiation period, N-i, as number of loading cycles required for the formation of a through-thickness Mode I crack at the notch root macroscopically propagating along the plate width. Thus, backward extrapolation of the observed early decreasing fatigue growth rates of the initiated crack at the notch root on the specimen surface to zero surface crack size enables the estimation of N-i. Findings - A parameter based on the normal strain range and the maximum normal stress acting at the notch root best fitted Ni. Research limitations/implications - This paper provides an estimation of Ni in 3mm thick mild steel plates subjected to constant amplitude uniaxial cyclic stresses. Initiated-but-non-propagating cracks possibly found at the notch surface and at the notch root along the plate width are distinguished. Originality/value - Based on the present analysis, a threshold stress is devised to represent the minimum stress level required for the appearance of a through-thickness crack at the notch root running along the specimen width. Notched plates simulate many engineering applications particularly in aerospace industry.