The effect of external stiffening elements on the fatigue crack growth in Fibre Metal Laminate

Fibre Metal Laminates (FML’s) have been developed in the past to increase the fatigue characteristics of laminated aluminium structures by adding fibres in the bond line. The fibres are insensitive to the occurring fatigue stresses in FML’s and bridge the fatigue cracks in the metal layers by restraining the crack opening. This results in a complex mechanism of crack growth in the metal layers and delamination growth at the interface between metal and fibre layers that, if in optimal balance, result sin the excellent fatigue characteristics for which FML’s are known. The complex mechanism has been investigated by many researchers in the past and is understood with the development of a recent analytical and physical sound prediction model. The main conclusion from previous research is that FML’s can not be dealt with as a monolithic material, but have to be treated as a composite with metal and fibre reinforced polymer behaviour. To make the step towards full FML structures possible, the effect of external stiffening elements on the fatigue crack growth mechanisms inside the FML must be fully understood. The approach generate this understanding is two-fold. First, fatigue crack growth experiments have been performed on three types of Centre Crack Tension specimen, with different symmetric titanium strap geometries. The first type consists of bonded straps at the edges of the specimen, to investigate the effect of a crack approaching stiffeners. The second type consist of an intact strap bonded over the fatigue crack centre to investigate the bridging effect of the strap, while the third type consist of a central strap which is cracked together with the fatigue crack in the FML. This type is investigated to understand the effect of the additional load from the cracked strap on the fatigue crack in the FML substrate. To make a proper correlation, tests have also been performed on these three specimen types with a monolithic aluminium substrate, to distinct the FML fatigue mechanisms from the monolithic aluminium behaviour. Second, the available analytical model has been modified to describe the effect of the three strap geometries. Correlation between the model and the experimental measurements provided additional information about the bridging- and delamination mechanisms inside the FML substrate. This paper presents the experimental work and the analysis and modelling work. Fatigue crack growth results will be presented in form of crack growth rates, crack opening contours and delamination shapes. The analysis will be presented with the correlation between prediction and measurements and with the additional calculated bridging stresses