Characterisation of fibre metal laminates under thermomechanical loadings

Fibre metal laminates, such as Arall or Glare, can offer improved properties compared to monolithic materials. Glare for example shows improved fatigue, residual strength, burn-through, impact and corrosion properties with respect to aluminium 2024, together with a considerable weight reduction and competitive costs. A large research program has proven the technological readiness of Glare and the fibre metal laminate has seen its application today in the primary structure of the Airbus A380 super jumbo. However, the effect of temperature on the performance of the fibre metal laminates has not been fully characterised. Differences in thermal expansion coefficients cause residual stresses after curing of the laminate. In service the temperature of the aircraft skin can vary between -55 up to 70 C due to solar radiation and convection, which will affect the thermal and mechanical properties of Glare. A detailed understanding of the behaviour of these laminates is necessary for further improvement of their performance and durability. With the increase in complexity of structures and material systems, the need for powerful design tools becomes evident. In this thesis, the thermo-mechanical behaviour of fibre metal laminates has been characterised via experimental testing and numerical modelling. Experimental tests have been performed to determine the temperature-dependent thermal and mechanical behaviour of unidirectional (UD) glass-fibre epoxy. Calculations based on these test results at room temperature and 80 C for the tension and shear stiffness of three different composite laminate lay-ups showed a good agreement with experimental test results. The UD glass-fibre epoxy data is used as input for the finite element model, together with aluminium 2024-T3 data from the literature. Glare laminates with a special lay-up have been experimentally tested to determine the effect of temperature and mechanical loadings on the laminate characteristics. The test results show that the off-axis and temperature effect can give a reduction of 24% in ultimate strength at room temperature due to off-axis loading and a further reduction of 17% at 80 C temperature. For standard Glare from the literature, where tests at elevated temperature have only been performed in fibre direction, the strength and stiffness reductions are at most 12% compared to room temperature. Numerical simulation is a very powerful tool to investigate the behaviour of materials and structures. Therefore, a thermo-mechanical finite element model, based on a solid-like shell element and including thermal expansion and heat transfer, has been developed to capture the behaviour of Glare in a fully three-dimensional state. The through-the-thickness temperature and stress distributions can thereby be determined, which allows for a straight-forward implementation of damage and plasticity models. Moreover, the solid-like shell element is ideal for thin-walled (aerospace) structures since it can have high aspect ratios without showing Poisson thickness locking, which occurs in standard continuum elements, and can have multiple layers in one element. To account for physical nonlinearities, a strain hardening model for the aluminium 2024-T3 and an orthotropic damage model for the UD glass-fibre epoxy layers in Glare are used. The strain hardening behaviour of aluminium has been modelled with a yield function based on an isotropic Von Mises plasticity formulation. An exponentially saturating hardening law has been assumed, which gives a good agreement with the experimental aluminium 2024-T3 stress-strain curve. A return-mapping algorithm is used to project the stress back onto the yield surface when the stress state violates the loading condition. The concept of continuum damage mechanics is used, with a separate damage parameter for fibre and matrix, to describe the appearance of microcracks that lead to ultimate failure. The equivalent strain measure is obtained by rewriting the yield function of the orthotropic Hoffman plasticity model into a strain-based format. The damage parameters are directly implemented into the stiffness matrix to avoid undesirable coupling terms in the damage matrix. The simulations of the shear and tensile test in transverse direction show a good fit with the experimental curves for the UD glass-fibre epoxy. The transient behaviour is captured by taking the heat capacity, inertia forces and damping into account. Park's method is used to solve the dynamic system of equations. The good performance of the thermomechanical solid-like shell element and the transient solver have been demonstrated for a single element under thermo-mechanical loadings and the snap-through of a cylindrical panel subjected to a concentrated load. Via a number of benchmark tests for practical applications the obtained numerical model is compared with the experimental test results. Bluntnotch test simulations have been performed on Glare3-3/2-0.4 and on a special Glare laminate (tested at 0, 45, 67.5, and 90 off-axis angle), which show a good agreement with experimental results. Simulations of off-axis tensile tests on a 0/90 composite, tensile tests on standard Glare laminates, and off-axis tensile tests on special Glare laminates with additional fibre layers in -45 and 45 direction, also showed a good agreement with experimental results. The thermo-mechanical solid-like shell element and the experimentally obtained material data, presented in this thesis, together create a powerful simulation tool for the effective and accurate characterisation of fibre metal laminates under thermo-mechanical loadings.Aerospace Engineerin