Cracking in cargo aircraft main landing gear truck beams due to abusive grinding following chromium plating

While a wheel was being replaced on the subject aircraft, a crack was found on the rear axle bore of the left-hand main landing gear truck beam. This part had been overhauled 11 months earlier. One year later, while the subject aircraft was being parked, two loud bangs were heard coming from the right-hand main landing gear. Upon inspection, the right-hand truck beam was found cracked longitudinally at two locations on the rear axle bore. Microscopic examination revealed two crack origins, one on each side of the bore. Both cracks propagated from corrosion pits under the chromium plating in a stable intergranular mechanism. The final overload fracture produced quasi-cleavage features. Nital etch, following removal of the chromium plating, revealed areas of overtempered and untempered martensite indicative of heat damage incurred during abusive grinding. The hardness of the material in the heat-affected areas and in the areas adjacent to the origins was lower than that of the surrounding tempered martensite structure. These heat-affected areas were located in the chromium plating runout plateau adjacent to the counterbore transition radius and exhibited numerous thermally induced secondary cracks. It was concluded that the cause of failure was improper overhaul process, which left grinding burns and cracks beneath the chromium coating. Subsequently, electrolyte that penetrated through these cracks promoted the formation of pits beneath the coating, which served as preferred sites for failure initiation.