Repeated impact behaviour of fully thermoplastic laminates

The use of laminated composites are spreading in engineering applications, respect to heavier metallic materials, thanks to their excellent advantages of weight/strength and weight/stiffness ratio. Even if up to now the attention was focused on fibers reinforced with thermosetting matrix, in the last years composites made with thermoplastic matrix have gained consideration. This is mainly due to their advantages in terms of low density and, more important, recyclability. The use of thermoplastics composites are of interest not only for the replace of the non structural parts, but also for the structural components located in areas potentially subjected to impacts. Depending on the design geometry and the field of application, some composite components may be subjected to repeated impacts at localised sites either during fabrication, routine maintenance activities or during service conditions. Even though the impact damage associated to the single impact event maybe slight, the accumulation of the damage over time may seriously impair the mechanical performance of the structure. In this paper, experimental data of repeated impact tests performed on fully thermoplastic thick laminates are presented. Repeated impacts at different energy levels are considered. The experimental data are analyzed evaluting the damage index (DI). This parameter has been previously defined and used for thermosetting composites in order to overcome shortcomings of the damage degree (DD) in case of thick composite laminates. The rate of initial steady damage accumulation as well as the onset of severe damage modes are analyzed and discussed in the paper. When applied to repeated impact tests, the DI allows to distinguish between an initial steady phase of damage progression and the onset of severe damage mechanisms. For the initial damage, the damage index increases linearly with the impact number whereas the severe damage mechanisms lead to laminate performation in few impacts. In this work, the values of the DI at different stages are discussed: at first impact, in the steady phase and at the onset of the unsteady phase. Moreover, the total energy absorbed by the laminate in the steady phase is also computed for all the performed tests and compared with the results of other thermosetting composites previously tested by other researchers.