Effect of impact face damage on strength of sandwich composites

A low energy impact face defect is likely to decrease the face buckling load of a sandwich composite to. P = Po (1--Wom/Wm) t. where P0 is the buckling load without initial damage, and wOm and wm are theinitial and failure values of displaceme nt amplitude at the centre of the defect. This expression represents the critical lateral face load incompression, and is essentially unchanged regardless whether the structure is asandwich panel or a strut on elastic foundation. Experimental studies suggest that the diameter of the impact delaminationdefect du ~IN where ~is the impact energy and N is the number of equal impacts. This corresponds to the critical energy criterion for fatigue. If w0m ~l, where 21 is the wavelength of the initial irregularities, and l du, then wOm ~. This would agree with both the equation (1) and the experimentally fitted equation. P = Po · (1 ~'JN/K). where K is a constant for similar composites. If the wavelength is proportional to the diameter of the impact damage, then wOm dU2. However, a relation wOm du3 is suggested by another experimentally fitted equation. P = Po · [1 (1.5 · dU/b)3]. where b is the effective width under loading. The observed correlation 'between defect size and strength loss is somewhat worse with the last equation. However, in practice using the measured defect size du from NDT inspections is more attractive than the usually unknown impact energy in the evaluation of strength loss due to impacts.