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 hole 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 test fixture was designed to load the test specimen at the geometric center in the thickness direction minimizing the single-shear bending problem. Protruding-head and countersunk-head fastener tests were conducted 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 edge distance-to-diameter ratios, 1.0, 1.5, 2.0, and 3.0, were tested for each factor. Three replicate tests were conducted for each set of parameter combinations. An analysis-of-variance of the data was conducted for each factor to determine whether or not that factor significantly influenced ultimate joint strength. Fisher's Protected Least Significant Difference procedure was then applied to the geometric and laminate lay-up parameter combinations for each factor to identify the parameter combinations where significant joint strength differences exist between the standard and nonstandard hole joints.