Effect of Loading Condition on Stress Intensity Factor for Threaded Fasteners Under Helix Angle Condition

Abstract

The effect of loading condition on the stress intensity factor (SIF) solution for a metric threaded bolt with helix angle consideration is investigated. Available SIF solutions for a nut loaded bolt do not consider the effect of helix angle of thread. Various loading conditions such as: (i) far field loading, (ii) thread face loading without helix angle, (iii) thread face loading with helix angle and (iv) nut loading with helix angle are considered in the present study. 3-D contact analysis is carried out to observe the stress distribution between the bolt and nut interface. A crack was introduced at the first thread of the bolt so that the crack faces experience opening mode of fracture under nut loading condition. The SIF estimated by each loading condition was compared with the SIF values computed taking into consideration the nut loading with helix angle. SIF solutions obtained under far field loading condition are lower than those obtained under other loading conditions at short crack depths (a/d = 0.1); at deep crack depths (a/d = 0.5), SIF obtained under nut loading condition are lower than those obtained under other loading conditions. At deep crack depths (a/d = 0.5) the effect of loading condition on SIF is more pronounced for an elliptic crack (a/c = 0.2) than semi circular crack (a/c = 1). Due to the combined effect of mode II and mode III fracture which is caused by helix angle, non symmetric distribution of SIF was observed along the crack front. It is noted that, crack growth rate derived from SIF under nut loading condition is lower than published data at the middle region (P/P0 = 0).