Modelling damage and residual indentation from composites impact

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Damage tolerance is a major concern with composite laminates. For low velocity impact external damage may be invisible, or barely visible, whereas significant internal damage may have occurred. In order to assess this, the Compression strength After Impact (CAI) test has been developed and is the industry standard to experimentally quantify loss in strength of a composite laminate after damage from low velocity impact. Essentially, this test impacts plates with various levels of low energy. The plate with a 0.5mm indent is selected, and loaded under edgewise compression to failure. This failure load gives the residual strength after impact which must exceed the maximum structural design load; thus a structure is proven safe to fly with barely visible damage.Numerous researchers have investigated the CAI test to numerically predict intra- and inter-ply damage during impact. A few researchers, including IFB, have also had limited success to couple impact damage modelling with post-impact compression strength analysis. Despite these efforts one important aspect has been overlooked to date; namely, to predict local indentation accurately with respect to impact energies. This is not a trivial problem; it will require 3D modelling and constitutive laws for damage, plasticity and relaxation after load removal; also, a strong coupling with accurate prediction of internal damage will be essential. This is the aim of this research proposal. An extensive materials test program will be undertaken from which new constitutive models will be developed to characterise the local impact crushing and time dependent relaxation unloading of locally impacted composites. The work would advance general scientific knowledge of interaction between surface and internal damage due to low velocity impact and would be an essential step toward a full virtual simulation methodology for the CAI test procedure. Although research will focus on the CAI test configuration the topic is relevant to many other applications involving relatively low impact; for example hailstone impact with wind turbine blades and general impact of foreign bodies against composite laminates.