AbstractThe ingestion of small hard body particles at high velocities leads to Foreign Object Damage (FOD), against which composite structures are required to be resilient as the development of failure modes and their interactions can result in considerable loss in mechanical performance. Extensive knowledge of the damage process in composites and advanced numerical simulation tools can help to improve FOD resistance at an early design stage. In this work, the behaviour of carbon/epoxy laminates subjected to high velocity oblique impacts is studied. Impact tests at the edge and centre of quasi-isotropic IM7/8552 laminates using a 3 mm steel cube projectile have been conducted over a range of velocities. The extent of impact damage, characterised in terms of fibre failure and delamination area, was observed to increase with increasing impact energy. Beyond the velocity at which laminate perforation was observed, the delamination area decreased with increasing impact energy. To predict the dynamic response of the carbon/epoxy laminates, a Hi-Fidelity Finite Element Method was employed, which combines failure criteria based on Weibull theory and maximum stress, where multiple split paths and delamination were modelled using a cohesive interaction, together with an automated unit cell meshing technique. The results from the baseline impact model were in good agreement with the experimental results for some impact energies. The baseline impact model was further investigated by changing the orientation of the projectile before impact and by varying the number and position of cohesive layers. While the extent of fibre failure was significantly affected by changes in projectile orientation, the extent of delamination was observed to be more dependent on the number of cohesive layers utilised.
The combination of high velocity impacts using small and light projectiles and the subsequent reduction in tensile strength has not received much attention, this research aims at investigating the residual tensile strength of impacted laminates and the damage development due to tension. The tensile strength of the impacted laminates was observed to be strongly dependent on the extent of fibre failure. The contribution of impact-induced delamination to the residual strength has been discussed. Impacted laminates under tension have also been analysed numerically, where the failure stress is strongly reliant on the length of 0° splits. To find a simpler and quicker way to assess the effect of impact damage, this work takes a new approach in comparing machined notches as a potential equivalent for impact damage. As the impact velocity increases, edge machined notches are a very good equivalent for edge-impacted laminates. Generally, all composite structures are shielded for protection against impacts and/or erosion, and to this extent, an experimental high velocity impact study has been conducted on carbon/epoxy laminates with an edge shield. At high impact velocities, the edge shield was observed to significantly reduce the extent of impact damage and the resulting tensile strength.
|Date of Award||23 Jan 2020|
|Supervisor||Michael R Wisnom (Supervisor) & Stephen R Hallett (Supervisor)|