Prediction of static failure in titanium based fibre metal laminates

Abstract

Fibre Metal Laminates (FMLs) made of titanium and carbon-fibre reinforced epoxy have the potential to reduce the mass of structures that would otherwise be made of conventional materials. This material also exhibits good fatigue performance and resistance to localised and impact damage. These properties make the material a good candidate for airframes. The properties of the material also make it suitable for military vehicles that require light weight armour. The design of components that include FMLs requires a model to predict the failure under static loads. The static failure modelling of titanium / carbon based FMLs has seen relatively little research. A failure model for FMLs will include different failure criteria for each of the composite and metallic constituents. The evaluation of the best constituent criteria for inclusion into a titanium / carbon FML failure model has also seen little research. A model is therefore presented which predicts the failure progression of a titanium / carbon FML. The predictions cover the range from the initial failure in the material up to the point of final collapse. The individual failure criteria for the material constituents are selected on the basis of their applicability to the modelling such an FML. The accuracy of the proposed progressive failure model is investigated by two methods. The model is first verified by laboratory level tests and thereafter validation is carried out by means of field trials that are representative of actual service conditions. The laboratory level verification includes the design of various types of test samples that will exhibit different failure modes. These samples are tested under load where both the initial and final failure modes are recorded. The initial failure modes are detected through the simultaneous examination of the data from the strain output, acoustic monitoring, micrographic examination and micro-focus X-Ray examination. The results from the tests are compared with the progressive failure modes predicted by the model, where there is good correlation. The validation of the failure model under realistic field conditions is achieved through the development and testing of armour panels that meet the need for light weight protection against improvised explosive devices. The static progressive failure model is used as an input to the design the armour panels. Additional predictive ballistic models are required for the balance of the design calculations. These ballistic models do not exist for FMLs and therefore they are developed for this work. The armour panels are then tested against a representative threat. Additional ballistic theory is also developed for the analysis of the test data in order to determine the performance of the armour. The correlation between the predictions and test results is good.