Damage Initiation and Evolution in Composite Laminates

Abstract

This study investigates the identification of damage initiation strain levels in marine composite laminates. Loss of stiffness during tensile tests, acoustic emission monitoring, and load/reload tests combined with a damage model, were used to identify damage initiation strain levels on stitched biaxial and woven roving Eglass/epoxy laminates. These laminates provided similar fibre orientation and different fabric architectures. Cyclic tests and residual strength tests were conducted to confirm the identified damage initiation strain levels. Tensile tests were conducted to failure to obtain the basic mechanical properties of the different laminates considered. The loss of stiffness as well as the acoustic emission data monitored during the tensile tests provided a range of strain levels where the damage initiation occurred for the woven and stitched laminates. The load/reload tests were performed at different load levels to characterise the loss of transverse stiffness of the laminates. The results were combined with a damage model to identify damage initiation strain levels and the evolution of the damage for each laminates. The cyclic tests were performed at the identified damage initiation strain levels and at higher strain, to verify the strain levels obtained with the techniques used previously. The cycled specimens were then subjected to residual strength tests to assess the loss of stiffness and strength for the different cyclic strain levels. The results analysis of the tensile test, the acoustic emission monitoring and the damage model provided similar estimation of damage initiation strain levels for each type of laminate. The techniques also identified damage initiation strain levels to be lower for the woven laminates than for the stitched laminates. The residual strength tests confirmed the loss of mechanical properties for both laminates when laminates were cycled for higher strain levels than the estimated damage initiation strain levels. The loss of mechanical properties was greater for the woven laminates than for the stitched laminates at identical cyclic strain levels. The techniques identified in this study are proposed to be used in a methodology to identify damage initiation strain levels for a wide range of laminates to allow designers to improve the design of marine composite laminates.