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In 1988, inspectors discovered widespread cracking in the main landing gear actuator support beam in a U.S. Navy fighter aircraft. Fractography confirmed metal fatigue as the cause. The beam was made of D6AC steel, a high-strength material.
A flange with a small radius behind the attachment lug was the location of crack origination. A second radius had introduced a multiplication of stress concentrations. Some fixes were evaluated and ruled inviable due to structural constraints or economic considerations.
The solution was to “beef up” (reinforce) the backup structure with a 17-7 stainless-steel plate. Interference fit fasteners were used to stiffen the surrounding structure. A finite element analysis was performed to analyze the rework design of the support structure. This helped in fracture mechanics calculations, which are included. Critical crack size and crack growth life determinations were made. This paper describes the investigations, analyses, and development of the solution to the cracking. Discussion of the causes of the cracking was given an emphasis. Questions for the students have been added.
fatigue, crack initiation or nucleation, crack propagation, fracture mechanics, repairs, stress concentration, aircraft
President, Fatigue Concepts, El Dorado Hills, CA
Senior structures engineer, U.S. Navy, Norfolk, VA