Predicting fatigue crack initiation and propagation in Glare reinforced frames

Abstract

The most highly loaded frames of the newly developed Airbus A400M transport aircraft have glass reinforced aluminium laminate (Glare) straps attached to them to increase their resistance to fatigue damage. These straps were designed using static strength requirements for the frame. Basic justification was done in two steps. Firstly, the frame load was divided between frame and flange on the basis of in-plane stiffness of these parts. Secondly, a fatigue analysis was performed on the strap using an empirical model for fatigue of Glare laminates under tension. This report contains a thorough explanation of the development of a code which can be used to analyse the fatigue crack initiation and propagation characteristics of the combination of frame flange and Glare strap at once under variable amplitude loading. The choice was made for a modular implementation of a combination of analytical models. This allows the programme to analyse a large variety of cases and allows it to be adapted easily in the case it is decided to continue its development. The fatigue crack initiation model works in three stages. First, it computes the highest stress cycle occurring in the metal sheets of the laminate, second, it corrects this cycle to match the available SN-data, and third, it finds the corresponding cycles to initiation using the nearest matching SN-curve. Fatigue crack growth calculations are based on an analytical model which applies displacement compatibility at the delamination boundary, to solve for the crack tip stress intensity and subsequently calculate crack- and delamination growth characteristics. These displacements originate from four phenomena. One, the crack opens due to far-field stress in the metal layers. Two, there is also a crack opening effect because of the release of residual stress due to crack- and delamination growth. Three, crack closing occurs due to the bridging effect of the fibre layers. Four, the fibre layers elongate and deform due to the bridging load they sustain. The influence of variable amplitude loading on the laminate is taken into account by implementing the Wheeler yield zone model. Correct working of the code was verified by assessing whether the modules produced results equal to the codes they were based upon. The parts of the code that were made from scratch were verified by checking whether these parts reacted to input parameter changes in a way that matched expectations. Validation with a variety of test results proved that all the modules and combinations thereof produce results which corresponded to the experiments. The code that was developed for the research described in this report was used to evaluate Airbus' method of justification of their design of the Glare reinforced frame. It was shown that the method of justification is unconservative, which is likely due to the assumption that the load through the flange is redistributed to the strap. A study on ways to improve the design of the Glare reinforced frame flange showed, that the performance of the Glare-reinforced frame flange could be improved by increasing the amount of fibre layer in the cross section relative to the amount of metal.