Thread lap behavior determination using finite-element analysis and fracture mechanics techniques

A new aerospace bolt thread lap inspection criterion is proposed based on fatigue crack propagation trajectories predicted by finite-element analysis (FEA) and fracture mechanics techniques. Fastener producers and end users incur high costs due to thread lap problems. Current thread lap inspection criteria are 30 to 40-years-old, and are based on an intuitive understanding of potential thread lap behavior. These inspection criteria are ambiguous and make inspection difficult. Some costs can be avoided and quality can be improved by redefining acceptable and unacceptable thread laps. Specifically, these inspection criteria can be improved by using fracture mechanics and FEA techniques developed in the last 30 years. Two-dimensional, axisymmetric, full nut-bolt-joint geometry, FEA models with elastic-plastic material properties, and contact elements at the thread interfaces were analyzed using ANSYS FEA software. Singular crack tip elements were used. Laps of expected geometries were built into the models, and then assumed to propagate as fatigue cracks. Cracks were iteratively grown using the maximum tangential principal stress criterion to determine the direction of crack propagation. Certain types of thread laps were shown to propagate in such a way that bolt failure would not occur. Conversely, other types of laps were predicted to propagate in such a way that bolt failure would be expected. A simpler, less subjective criterion for inspecting laps is presented. This criterion takes advantage of the benign laps. The proposed method allows any lap contained entirely within a particular zone to be acceptable.