Assessing Life Prediction Methodologies for Fasteners Under Bending Loads

New polyimide matrix composite materials are leading candidates for aerospace structural applications due to their high strength to weight ratio and excellent mechanical properties at elevated temperatures. The high fatigue resistance of these composites often results in the bolts being the weak link of a structure. Aircraft-quality bolts made of 4340 steel with a minimum UTS = 1241 MPa (180 ksi) were tested in three-point bend fatigue. Two life prediction methodologies were accessed for bending stress: S-N curves and fracture mechanics. The tensile S-N curve from the Mil-Handbook-5 conservatively predicts the bending fatigue life and run-out stress. Crack growth data, in the form of da / dN versus ΔK, from the Damage Tolerant Design Handbook was converted to a versus N data using five geometric correction factors. None of the five correction factors accurately predict crack growth, but all five correction factors did conservatively predict crack growth.