Bearing strength and failure behavior of bolted composite joints (part I: Experimental investigation)

The experimental investigations described in this paper were conducted in order to study the strength and failure of mechanically fastened composite joints. Two different types of polymer-matrix-based Carbon Fiber Reinforced Plastic laminates were selected to evaluate the effect of resin properties on bearing response. A special measurement system using a non-contact electro-optical extensometer to measure hole elongation is proposed. During static tensile testing, the response of bearing damage was monitored by acoustic emission measurement. Several specimens were observed by means of soft X-ray radiography and SEM at different loading levels to assess internal damage. The observations indicate that the bearing failure can be outlined as a process of compressive damage accumulation, and can be divided into the following four stages: damage onset; damage growth; local fracture; structural fracture. Major features of bearing failure include fiber micro-buckling, matrix cracking, delamination and out-of-plane shear cracking. Bearing strength and failure mode should also depend on the lateral constraints and the “toughness” of different polymer-matrix based laminates.