Effect of Helix Angle on the Stress Intensity Factor of a Cracked Threaded Bolt

Abstract

The fracture behavior of a crack in a threaded bolt depends on the stress intensity factor (SIF). Available SIF solutions have approximated the threaded bolt as a circular groove, thus, the SIF predominantly corresponds to the opening mode, mode-I. As a thread in a bolt has a helix angle, the crack propagates under mixed mode conditions (opening, sliding and tearing), esp. when the crack sizes are small. This paper presents the results of SIF solutions for a part-through crack emanating from a Metric threaded bolt. A 3D finite element model with preexisting flaws was generated to calculate the SIF values along the crack front. Crack aspect ratios in the range of (0.2 < (a/c) < 1) and crack depth ratios in the range of (0.1 < (a/d) < 0.5) (where “a” is crack length, “c” is semi major axis of ellipse and “d” is minor diameter of the bolt) were considered along the crack plane for the SIF estimation. The SIF values at the midregion decreases with an increase in aspect ratio (a/c), and SIF increases when the crack depth ratio (a/d) increases in the midregion. Close to the free edges, higher SIF values was observed for crack depth and aspect ratios ranging between 0.2 and 0.6 compared to midregion. In the crack surface region, up to a crack depth ratio of 0.25, significant influence of mode-II and mode-III fracture was noted for shallow cracks (a/c < 0.2). Significant influence of mode-II and mode-III fracture was observed for semicircular cracks (a/c = 1) beyond the crack depth ratio of 0.3.