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.

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