Failure Mechanisms in Compression-Loaded Composite Laminates Containing Open and Filled Holes

Abstract

Filled hole-based compression allowable strengths for carbon/epoxy laminates are not widely used in airframe design, due to a lack of understanding of geometryrelated effects and failure mechanisms. The effects of fastener hole-filling, hole clearance, and clamp-up torque upon the filled hole compression strength of composite laminates were investigated experimentally and analytically. Tests were conducted using open and filled hole specimens with defined hole clearances spanning the range permitted in Class 1 structural holes (nominal bolt diameter +0.003/-0.000 inch). All filled hole specimen configurations demonstrated compression strengths in excess of those for open hole specimens, due to the development of a load path through the fastener. Hole clearance was found to be the primary factor governing filled hole strength; fastener torque had relatively little effect. A semiempirical strength prediction methodology was developed using 2-D finite element analysis and ply-level quadratic failure theory. Good agreement between experimental data and predictions was demonstrated for T800/3900-2 tape laminates.