Variation of the fatigue lifetime with the initial notch geometry of circular notched bar specimens

Circular notched bar (CNB) specimens are often used for studying the brittle failure of pipe-grade polymeric materials such as polyethylene (PE). However, there are some technical difficulties, such as the alignment of the centreline of a CNB specimen with that of a circular notch, the installation of a razor blade to fabricate a circular notch, etc., in making the axisymmetric initial notch for CNB specimens. Though a slightly asymmetric initial notch is fabricated for a CNB specimen, the final failure of the CNB specimen under fatigue loads can be severely eccentric. In this case, the fatigue lifetime of the CNB specimen with asymmetric crack growth is varied significantly comparing with that of the CNB specimen with symmetric crack growth. In this study, the effect of two cases of non-circular initial notch geometry, i.e., (a) an asymmetric circular notch (Case I) and (b) an elliptical notch (Case II), of the CNB specimen on the fatigue lifetime is investigated using finite element analysis. The variation of stress intensity factors during crack growth is investigated to understand the fatigue crack growth of the CNB specimen with two cases of slightly asymmetric initial notch geometries.