The effects of through-thickness compression on the interlaminar shear response of laminated fiber composites

The effects of through-thickness compression on the interlaminar shear response of laminated fiber Composites were studied. The combined stresses were generated using a hollow cylindrical specimen that was subjected to axial compression and torsion. For both glass- and carbon-fiber composites, through-thickness compression resulted in a significant enhancement in the interlaminar shear stress and strain at failure. Under moderate compression levels, the failure mode changed from elastic to plastic. An attempt was made to predict the observed increase in shear strength for carbon fiber-epoxy laminates using three-dimensional lamina failure criteria. Although all the failure theories correctly predicted the trend of increasing shear strength with compression, none were able to predict the full extent of the observed strength increase. These results indicate that improved models are needed for determining failure under a combined state of interlaminar stress. The experimental results demonstrate that there are significant gains to be made in improving interlaminar strengths of composite structures by applying through-thickness compression. This effect could be exploited for improved strength and possibly improved fatigue life of composite joints and other regions in structures where interlaminar stress states are critical.