Stress Intensity Factor Approximations for Cracks Located at the Thread Root Region of Fasteners

Abstract

Nondimensional stress intensity factors have been numerically determined for a straight-fronted edge crack located at the thread root of a fastener. An influence-function or weight-function approach was used in determining KI for several fastener sizes: diameters ranging from 1 to 6 in. (2.5 to 15.2 cm) and thread pitches of 4, 8, and 12 threads per inch. Several analytical approximations are used in simplifying the stress analysis problem and the development of the weight function. The results for the nondimensional stress intensity factor, KI/σ(πa)1/2, are provided as a function of crack depth, bolt size, and thread pitch. The effects of bolt size and thread form play an important role only when cracks are relatively small. These results indicate that local differences caused by thread form and root radius are limited to crack depths within 2% of the net section. All major contributions to elevation in KI, caused by the presence of the threads appear to diminish for cracks extending beyond 25% of the minor diameter.

Investigation on Flexural Fracture Behaviour of Bolted Spherical Joints with Crack Propagation in Screw Threads

“…Alternatively, the observation of the finite element modelling in Figure 15 seems to correlate well with that reported in Ref. [ 36 ], in which root radius effects are nearly attenuated when a / D is greater than 0.015. Therefore, it can be concluded that the effect of ρ is negligible on the SIFs of the bolted spherical joints, with not very shallow crack propagation at the root of the first internal thread of the high-strength bolt outside the junction with the screw thread of the bolted sphere.…”

Effect of microstructure and alloying elements on the resistance of fastener grade steels to hydrogen assisted cracking

“…A threaded fastener can essentially be compared to a round bar, typically stressed in uniaxial tension, with several notches that act as stress risers [5]. A fastener usually fails at one of three locations: a thread root radius, the thread to shank radius, or the head to shank fillet [6]. highlighting the stress concentrations.…”

“…The amount of threads that are engaged during the loading of the fastener will affect the stress concentration at the notch roots. If more threads are engaged, then the stress concentration is lower, due to an even distribution of the stresses [6]. Also, the first engaged thread on the fastener typically experiences the highest stress concentrations [7].…”

“…Also, the first engaged thread on the fastener typically experiences the highest stress concentrations [7]. The analysis of a single grooved round bar is often adequate when theoretically calculating stress concentrations for threaded fasteners, because the one notch will experience a higher stress than each of a series of threads [6]. The stress concentration factor, K T , is defined as the ratio of the maximum stress at a notch root to the nominal stress.…”

“…Calculation of the theoretical stress concentration depends solely on the geometry of the specimen and can only be applied when there is no plastic deformation. The general equation for calculating K T is shown as Equation 2-1, while a more specific equation for calculating K T for the notch geometry is shown in Figure 2-2 can be seen in Equation 2-2 [6].…”