Fatigue Fracture of an Aluminum Main Support Bridge for a Helicopter

This paper summarizes the results of a failure analysis investigation of a fractured main support bridge made of 7075 aluminum alloy from an army helicopter. The part, manufactured by “Contractor IT,” failed component fatigue testing while those of the original equipment manufacturer (OEM) passed. Metallurgical data collected during this investigation indicated that the difference in fatigue life between the components fabricated by IT and by OEM may be attributable to a difference in dimensions at the web where fatigue crack initiation occurred. The webs of the two OEM parts examined had cross-sectional thicknesses significantly larger than the web cross-sectional thicknesses of the IT components. Recommendations included changing the web reference dimension of 0.38 in. to include a tolerance range based upon a fracture mechanics model. Also, the shot peening process should be controlled especially at the critical areas of the web, to assure complete coverage and proper compressive residual stresses.