Crack propagation calculation for aluminium aircraft structures considering the influence of load sequences

Abstract

The load sequence has an important influence on the crack propagation in aircraft structures. The estimation of this influence is a particular challenge in the life time prediction of aircraft components. Current calculation methods consider this inadequately. Therefore a new calculation method was developed to determine the crack propagation step-by-step for each load point considering the influence of former loads on the current crack propagation rate. This method bases on the assumption that the current load distribution in front of the crack tip determines the effective stress intensity factor directly for the current load step. Therefore, it is mandatory to possess a very detailed knowledge of the stress-strain-behaviour of the used aluminium alloy under cycled loadings. The mathematical description of the load distribution in the structure is carried out for the ligament in front of the crack tip by a function of the distance to the crack tip by special assumptions. These functions result from FE analyses or a specific simplified simulation method. Supported by particular weight functions the effective stress intensity factor for the current load step is calculated directly from the current load distribution function. Based on the effective stress intensity factor the crack propagation rate is calculated by the Paris-equation. The applicability of the new crack propagation calculation method was demonstrated by a number of plausibility checks and an extensive test program. The analysis of the test and calculation results shows very good correlations.