A STATISTICAL-ANALYSIS OF BUSHING, SLEEVE, AND OVERSIZE HOLE EFFECTS ON THE BOLTED JOINT STRENGTH OF GRAPHITE-EPOXY LAMINATES

A test program was conducted to determine the influence of metallic ho le inserts (bushings and sleeves) and over-sized fastener holes on the ultimate strength of bolted joints in IM7/8551 graphite/epoxy. Single -shear specimens were used to simulate in-service constraints; the tes t fixture was designed to load the test specimen at the geometric cent er in the thickness direction minimizing the single-shear bending prob lem. Protruding-head and countersunk-head fastener tests were conducte d on laminate lay-ups with ply percentages of 42/50/8 and 12/80/8. The width-to-diameter ratio was held constant at 4.0, but four distinct e dge distance-to-diameter ratios, 1.0, 1.5, 2.0, and 3.0, were tested f or each factor. Three replicate tests were conducted for each set of p arameter combinations. An analysis-of-variance of the data was conduct ed for each factor to determine whether or not that factor significant ly influenced ultimate joint strength. Fisher's Protected Least Signif icant Difference procedure was then applied to the geometric and lamin ate lay-up parameter combinations for each factor to identify the para meter combinations where significant joint strength differences exist between the standard and nonstandard hole joints.