Critical Fracture Processes in Army Cannons: A Review

Abstract

Fast fracture in cannons can be well described using elastic-plastic fracture toughness, in combination with comparisons of cannon section size relative to the size required to maintain plane strain fracture. Fatigue fracture of cannon tubes is modeled from results of full-size fatigue tests that simulate cannon firing. These tests are also the basis of fatigue-intensity-factor modeling of fatigue life, which incorporates material strength, initial crack size and Bauschinger-modified autofrettage residual stress into life predictions. Environment-assisted fracture in the thermally damaged near-bore region of fired cannons is shown to be controlled by hydrogen. High strength cannon steels are susceptible to hydrogen; cannon propellant gases provide the hydrogen; and the source of sustained tensile stress is the near-bore thermal damage and compressive yielding. A thermo-mechanical model predicts tensile residual stress of similar depth to that of observed hydrogen cracks. Coating fracture in the thermal-damage region of fired cannons is characterized and modeled. The Evans/Hutchinson slip zone concept is extended to calculate in-situ coating fracture strength from observed crack spacing and hardness in the damaged region.