Fatigue Life of Lugs : A finite element-based model for fatigue life calculations of lugs with arbitrary geometry and load sequence

Sammanfattning

Lugs are commonly used in aircraft structures to transmit loads between parts through a bolt or pin connection. As a result, they are frequently subjected to cyclic loading and therefore susceptible to fatigue failure. The current lug fatigue life calculation method is based on fatigue test data of lugs, and is only applicable for straight lugs, subjected to a longitudinal load with constant amplitude. This makes the method limited and unnecessarily conservative, due to the assumption that the load is applied in a single direction. The aim has therefore been to expand the method with model, valid for any lug geometry and load sequence. The proposed model builds upon an existing method where the fatigue life of a lug is predicted based on the fatigue life of a tested reference lug, through scaling of geometrical and stress quantities. It further introduces a comparison between the stress concentration factors, enabling fatigue life predictions of lugs with arbitrary geometry and load angles by finite element calculations of the tangential stress in the lug. Additionally, damage theory has been applied to predict the fatigue of lugs subjected to arbitrary load sequences by calculating the cumulative damage in each point along the hole edge of the lug. The validity of the proposed model has been assessed by comparing the predicted fatigue life of lugs to test data, available for a few different lug geometries and different constant load angles. The calculation model showed a promising agreement between the predictions and test results. To validate the implementation of damage theory, tests with varying load angle and amplitude must be performed.