Thesis defence M. Gupta: laminates
12 May 2017 15:00 - Location: Aula, TU Delft - By: Webredactie TU Delft
Directionality of Damage growth in Fibre Metal Laminates and Hybrid Structures. Promotor: Prof.dr.ir. R. Benedictus (LR).
Fibre-metal laminates (FMLs) have been studied intensively for the past three decades because of their enhanced fatigue properties compared to monolithic metals. Most of these studies have focused on the fatigue damage under in-axis loading. These studies led to the application of FMLs in the aircraft structure in the early 21st century. However, the main application remains limited to the aircraft fuselage where the loading direction remains mostly constant. The few studies in the damage directionality of FMLs show that crack paths in FMLs under off-axis loading can undergo small deflections in biaxial GLAss REinforced aluminium (Glare) grades but show a significant amount of deflection in uniaxial Glare grades. In order to extend FML application to other parts of the aircraft structure where the loading direction is not constant or where uniaxial Glare is required – like aircraft wings - more understanding is required about the directionality of damage in FMLs under off-axis loading. To this effect the present research in damage directionality of FMLs under off-axis loading was undertaken. The thesis begins with an introduction to the problem of damage directionality in FMLs under off-axis loading. The problem raises the scientific question, ‘What mechanisms govern the crack path deflection in FMLs under off-axis loading’. Thereafter, a brief introduction to FMLs and these damage mechanisms are presented. Subsequently, various crack path theories used in monolithic metals are presented because from previous studies, it was concluded that the crack growth in metal governs the path taken by the damage. It was concluded that although the specimen is under uniaxial loading, the off-axis fibres induce shear loading at the crack tip due to laminate orthotropy and a transverse fibrebridging component. Hence, it was concluded that the mixed-mode theory using the Maximum Tangential Stress (MTS) is the most suitable theory for predicting crack paths. Because the previous research in FMLs was done using Stress Intensity Factors (SIFs), the mixed-mode approach using SIFs was deemed suitable to develop the analytical model. To verify the presence of mixed-mode loading– both tensile and shear loading – at the crack tip in FMLs under the application of uniaxial loading, Digital Image Correlation (DIC) was utilized. Additional to the DIC tests, more tests were performed under fatigue loading to observe the crack path behaviour in various Glare grades under a wider range of off-axis angles. Thereafter, the analytical model was developed. To develop the analytical model, the previously developed fatigue model for in-axis loading was modified. The modification was necessary to include the effect of transverse fibre-bridging and laminate orthotropy, and the resulting mixed-mode ratio was used to predict the fracture angle. Finally, it is briefly shown that the model developed for fatigue will not work under quasi-static loading and using a nonlinear plasticity based model would be a better approach. In conclusion, the thesis showed that the crack paths in FMLs under fatigue loading deflect because of the presence of mixed-mode loading at the crack tip. The amount of deflection depends on the mixed-mode ratio induced which, in turn depends on the Glare grade and the off-axis angle.
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