Initial quality of advanced joining concepts

This initial fatigue quality of three types of aircraft construction were studied. The three types included conventional mechanically-fastened joints, adhesively-bonded joints, and monolithic aluminum castings. The objectives are to obtain data for setting initial flaw assumptions for U.S. Air Force damage tolerance specifications, and to develop a method for comparing the relative merit of competing structural concepts. Two hundred test elements representing these joining concepts were prepared and tested under realistic spectrum load histories. Nondestructive inspections were performed on all specimens, but no correlation to crack growth performance was found. Crack growth data were obtained by fractographic examination and analyzed using the equivalent initial flaw size (EIFS) concept. Statistical distributions, representing the variation in EIFS and in crack growth rate, were obtained. Adhesively-bonded structure was found to give the best overall combination of benefits. The scatter in crack growth was highest in castings, which limits reliability at high stresses. An improved methodology was developed for comparing structural performance and efficiency. The methods include consideration of initial material and manufacturing quality, and can be used to quantify reliability at any confidence level and service time.