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A failure analysis of two broken bolts from an Army tank recoil mechanism was performed by the U.S. Army Research Laboratory (ARL), Weapons and Materials Research Directorate. The bolts failed at the head-to-shank radius during installation at Aniston Army Depot. A total of 69 additional bolts from inventory and the field were also characterized and tested for comparison. Optical and electron microscopy of the broken bolts showed topographies and black oxide on the fracture surface consistent with the characteristics of quench cracks. The crack origin was located within a region covered with a heavy black oxide where fracture occurred as a result of intergranular decohesion. Heat treatment tests of the material confirmed the black oxide to be formed during the tempering operation of ∼677°C. The remaining fracture surface failed in a ductile fashion. The cause of failure was attributed to preexisting quench cracks which should have been detected by the required 100% magnetic particle inspection conducted during manufacturing. These cracks propagated during installation, causing the bolt heads to sever. Recommendations were provided to screen inventory to prevent cracked parts from entering fielded tanks and improve control of manufacturing and inspection procedures. These included machining the head-to-shank radii to specification, tighter control of the magnetic particle inspection process, the use of dull cadmium plate to mitigate the potential for delayed failures due to hydrogen embrittlement or stress corrosion cracking, an alternative to electrolytic cadmium plate (such as vacuum deposition), and finally, replacing all existing bolts in the field and in inventory with new or reinspected bolts.
failure analysis, fractography, quench cracks, AISI 8740 steel
Chief, Materials Analysis Group, U.S. Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, MD