Experimental optimization of the impact energy absorption of epoxy–carbon laminates through controlled delamination

In this paper, the correlation between interlaminar fracture toughness and impact energy absorption for the fracture of epoxy–carbon laminates was studied. Carbon fibres–epoxy cross-ply prepreg layers were interleaved with thin (26 μm) poly(ethylene-terephthalate) (PET) films. Before the composite preparation, circular holes 1 mm in diameter were drilled in the PET films at several densities (from 0 up to 44 holes/cm2) in order to selectively increase the interlaminar contact area between the epoxy–carbon laminae. In this way, the interlaminar contact area was gradually varied from 0%, corresponding to the case in which non-perforated PET films were used, up to 100% in the case of non-interleaved laminates. The Mode I interlaminar fracture toughness of the resulting laminates was determined according to the ASTM D-5528-01 standard test method on double cantilever beam (DCB) specimens. The critical values of the strain energy release rate determined at the point at which the load versus opening displacement curve becomes non-linear (GIC,NL) resulted to vary from 40 up to 260 J/m2, depending on the interlaminar contact area. All the laminates were then characterized by three point bending tests performed both under quasi-static (5 mm/min) and impact (2 m/s) loading conditions. The elastic modulus of the laminates resulted to be practically independent of the level of interlaminar adhesion, while the bending strength decreased as the interlaminar fracture toughness decreased. The total energy to fracture evaluated under impact conditions showed a non-monotonic correlation with the interlaminar fracture toughness, reaching a maximum level in correspondence of a GIC,NL value of about 60 J/m2. At the same time, the ductility index, i.e. the ratio between the propagation and the initiation energies, evaluated by instrumented Charpy impact tests, markedly increased as the interlaminar fracture toughness decreased.