Probing the Elastic-Plastic, Time-Dependent Stress Response of Test Fasteners Using Finite Element Analysis

Abstract

The evolution of global and local stress/strain conditions in test fasteners under test conditions is investigated using elastic-plastic, time-dependent finite element analyses (FEA). For elastic-plastic response, tensile data from multiple specimens, material heats and test temperatures are integrated into a single, normalized flow curve from which temperature dependency is extracted. A primary creep model is calibrated with specimen- and fastener-based thermal relaxation data generated under a range of times, temperatures, stress levels, and environments. These material inputs are used in analytical simulations of experimental test conditions for several types of fasteners. These fastener models are constructed with automated routines and contact conditions prescribed at all potentially mating surfaces. Thermal or mechanical room temperature preloading, as appropriate for a given fastener, is followed by a temperature ramp and a dwell time at constant temperature. While the amount of thermal stress relaxation is limited for the conditions modeled, local stress states are highly dependent upon geometry (thread root radius, for example), preloading history and thermal expansion differences between the test fastener and test fixture. Benefits of this FE approach over an elastic methodology for stress calculation will be illustrated with correlations of stress corrosion cracking (SCC) initiation time and crack orientations in stress concentrations.

Initiation of Fatigue Cracks in Bolted Joints

“…4). The same effect takes place in the matter of subcritical growth associating with stress corrosion cracking [6].…”

“…Fatigue cracking usually occurs on the notch root of the loaded flank corresponding to the first thread inside the bolt-nut joint [1,2,5,6]. Fatigue crack propagates with an angle of between 15-30º in relation to the transversal section of the bolt, towards the part of the bolt loaded by the nut [7].…”