Damage Mechanisms and Energy Absorption in Composite Laminates Under Low Velocity Impact LoadsDynamic Failure of Composite and Sandwich Structures

An extensive study of the behaviour of composite laminates subjected to dynamic loads was carried out by the authors many years in order to understand the complex mechanisms of damage initiation and propagation under low velocity impact loads. A review of the main results is hereafter presented. The problem is that many parameters are involved in an impact and the induced damage is very complex and not always visible. The present research efforts were undertaken to supply semi empirical and analytical models for the prediction of the impact response in terms of load curve, damage, involved energies and forces, independently of the particular laminate, its thickness and stacking sequence, matrix type and content, fibre type and architecture, fibre orientations and impact conditions such as tup and support diameter, load speed. Experimental tests were carried out on different material systems varying the initial kinetic energy until the complete penetration. This allows the study of the start and propagation of the failure modes. From the load-deflection curves recorded, all the impact parameters involved like first failure and maximum load and energy, absorbed and penetration energy, were obtained. The influence of the thickness and stacking sequence so that the composite system, constrain condition and tup diameter on the impact parameters was evaluated. Destructive and non-destructive techniques were adopted to investigate the failure modes and the observed damage was correlated to the relative energies. The analysis highlighted the importance of the penetration energy, Up. An elastic solution available for circular isotropic plates loaded at the centre was modified to model the indentation and applied to the prediction of the load-displacement curve necessary to know the energy that cause the first failure. Interestingly, the force required for damage initiation under form of delamination was found to increase at the increasing of the thickness, t, following a power law whose exponent is very close to 1.5 of the contact law.