The drilling-induced failure mechanisms in T800/924C toughened carbon-epoxy composite materials

The process of drilling of toughened carbon fiber composite materials presents numerous problems during structural assembly. The extremely abrasive nature of carbon fibers along with the softer resin quickly dull sharp tools, split, tear, pullout, and push-in the fibers on the hole boundaries. The presence and growth of such flaws seriously impairs the structural stability, durability, and reliability particularly under fatigue loading. This research aims to investigate the failure modes, pattern, and sequence of damage mechanisms in toughened carbon-epoxy composite in relation to the drilling dynamics. The surface morphology of the damaged fibers in the sectioned holes was examined by scanning electron microscopy (SEM). The drill forces were determined by drill dynamometer to investigate the drilling thrust and torque. The photoelastic stress analysis was used to determine the strains around hole of the lowest ply. The most critical failure mode was found to be shear crimping of −45° fibers due to microbuckling. This resulted in formation of damage pits that were spaced apart periodically at an angle of 45° on hole boundaries. The through-the-thickness drilling forces caused delamination in the resin rich region of −45°/90° interlayer. The SEM, dynamometer, and photoelastic strain results were correlated to predict the onset of failure modes. The results have been explained in the light of analytical models based on fracture mechanics. Measures have been suggested for minimizing the damage on carbon-epoxy composite hole boundaries.