Fatigue Crack Growth in Open and Nut-Loaded Bolts with and without Pretension Loading

Abstract

Results of an experimental study to understand crack growth in open threaded bolts and nut-loaded bolts with and without pretension are presented in this paper. Experimental crack growth rates are compared with predicted solutions obtained from known stress intensity factor solutions. For the experiment, a specially designed fixture was used for loading the bolt and the level of pretension was varied by controlling the displacement on the arm of the C section of the fixture. Programmed Hi—Lo fatigue load sequence was applied on a bolt to ensure that decodable marker bands are left behind on the fracture surface, to study the crack growth characteristics. Fatigue crack growth rate was estimated using a scanning electron microscope by identifying crack increment during high stress ratio cycles. This has been compared with predicted crack growth rates using available stress intensity factor solutions. Crack growth at the center of the bolt correlates well with Toribio's (Int. J. Fract., Vol. 53, 1992, pp. 367–385) finite element based predictions for open threaded bolts. Crack growth rate is much higher at the surface than at the center. In case of nut-loaded bolts, available K solutions are found to be inadequate to provide an accurate estimate of crack growth rates at the center of the bolt. Further study is required to characterize the effect of friction, pitch angle, and thread geometry to understand crack growth in bolts.