Fatigue crack growth in fiber metal laminates : experiment and modeling

Abstract: Fiber Metal Laminates (FML) is a class of materials with highly resistant fatigue behavior, developed primary for the aerospace industry. The laminates are comprised of alternating layers of high strength aluminwn sheet and fiber reinforced polymers. The high fatigue resistance is mainly provided by the fatigue resisting fibers bridging the metal fatigue crack.Several works to describe and to prediet the fiber bridging phenomena and their effect on crack growth rate of the laminate have been published in the literature. Some of these studies have used a modeling approach, where with the help of geometry and constitutive relations the effect or interaction of the fiber bridging and the metal crack propagation rate has been evaluated. Others have used an experimental approach, where the metal crack propagation rate is measured for different load ranges, or with modified reinforcement, to be compared with the behavior of non-reinforced aluminum sheets, so called monolithic sheets.In this work, an experimental method has been developed and used with the purpose of increasing the understanding of the stress field around a metal crack in fiber metal laminates. The experimental results and the assumptions in the method have been compared with FE analysis. Knowledge of the stress field around a metal crack for fiber-metal laminates is useful for further development in modeling the fiber bridging phenomena in fiber metal laminates.In order to prove the validity of an effective stress intensity range model found in the literature, a parametric study of some of the key variables of the mode! has been performed. Crack growth tests have been performed to give crack growth data for the fiber-metal-laminates and for the metal sheet material, used in the fiber metal Jaminates. The idea was to compare the fiber-metal-laminates with the meta! sheet material in respect of the crack growth rate - stress intensity range relation.A comparison between different residual stress measurement methods on FML has been made. The methods used were X-ray diffraction, neutron diffiaction and strain measurement throughout destructively produced stress release. Good agreement was found between the different methods.Neutron diffraction residual stress measurements have also been performed on fatigue-damaged material, specifically in the wake of a fatigue-induced metal crack in the laminate.A 3D FEA of a fatigue crack has been made with different fatigue crack geometries. The results has been compared with experimental data for the two crack fronts (delamination growth and metal crack growth).In order to magnify the role of the delamination shape on metal crack growth in FML, further crack growth tests have been performed. For different levels of the alternating loading different delamination shapes were developed. By changing the level of the alternating loading for some tests, a comparison ofthe metal crack growth at small delamination damage and larger delamination damage could be made. The experimental crack growth results are compared with 3D FEA and other models found in the literature.