Investigation Into the Effect of Thread Root Condition on the High Cycle Fatigue Performance of a Metric Threaded Fastener

Abstract

This paper presents an experimental investigation into the effect of thread root condition on the high cycle fatigue behavior of a tightened M12 metric fastener under a fully reversed, cyclic load. Two conditions were mechanically created at the fastener thread root. The first had an elastic condition with no localized plastic straining, and the second had localized plastic straining at the thread root. The elastic or plastic condition at the thread root was created by tightening the metric fastener into a fabricated steel joint at several different input or tightening torque levels prior to the application of a fully, reversed cyclic load. The fully reversed cyclic load was applied and a staircase methodology used in accordance to International Standard (ISO) 3800:1993(E). Fatigue results were also evaluated in conformance with (ISO) 3800:1993(E) to determine fatigue strengths at five million cycles. All fatigue tests were run at a rate of 50 Hz under a simple, sinusoidal wave form until either failure (rupture) or five-million cycles was attained and the test suspended. The fatigue strength values were then plotted on a graph of applied stress amplitude versus nominal mean stress level commonly referred to as a Modified Goodman diagram for bolted joints. The plotted fatigue strength data was not linear but displayed a knee or a bend. This knee or bend represents a transition point in the fatigue behavior of the threaded fastener. To describe the fatigue behavior, a bi-linear model has been applied. This bi-linear model is based on the local condition at the thread root. The knee represents a transition in condition from elastic to plastic behavior at the thread root. The result of this study shows that fastener fatigue behavior was influenced by the condition at the thread root.

Investigating Fatigue Mechanisms and Crack Growth in 20mntib Steel High-Strength Bolts: An Experimental and Simulation Study

“…When the number of loading cycles reaches 2 × 10 6 , the fatigue strength of the 20MnTiB steel bolt is 1.68 times greater than that of the 35K steel bolt. This is because fatigue properties are associated with the yield strength to ultimate strength ratio of the material: a higher ratio facilitates stress redistribution of the material at the notch or thread root [39]. The yield strength to ultimate strength ratio of the 20MnTiB and 35K steel bolts is 0.91 and 0.75 [38], respectively, with the former being 1.21 times higher than the latter, resulting in superior fatigue properties for the 20MnTiB steel bolt.…”

Effects of root radius and pitch difference on fatigue strength and anti-loosening performance for high strength bolt–nut connections

“…Therefore, it should be noted that the stress data plotted beyond the line ((σ m /σ sl )+(σ a /σ w ) > 1) does not represent the real fracture at the bolt thread. Several previous studies investigated the endurance limit after a certain amount of mean stress level (Burguete et al, 1995; Gunn, 1955; Hashimura et al, 2019; Munn et al, 2010). In this paper, the Soderberg line of plain specimen is used to discuss the relative hazard at the bolt roots.…”