Fatigue and Hydrogen Cracking in Cannons With Mechanical and Thermal Residual Stresses

Abstract

Abstract : Bauschinger-modified autofrettage residual stresses are used to improve the fatigue intensity factor model for fatigue life of cannon pressure vessels. Effects of yield strength and initial crack size are included with applied and residual stress. In an S-N description of cannon tube life that matches full-scale cannon fatigue test results with an R2 correlation of O.92. Thermally induced residual stress near the bore of a fired cannon is modeled by finite-difference calculations of temperature and mechanics calculations of transient thermal stress and resulting residual stress. Temperature-dependent thermal and physical properties are used, and the temperature distributions are validated by direct comparison with the known temperatures and the observed depths of microstructural damage and transformation in fired cannons. Calculations of fatigue life and yield pressure for a range of applied pressure, diameter ratio, yield strength, and percent autofrettage agree well with measurements frem full-scale cannons. Increased life is predicted for increases in yield strength and percent autofrettage, although the Bauschinger effect signiflcantiy reduces the amount of life Increase for autofrettage above 50%. The combined effects of mechanically Induced residual stress and thermally induced residual stress on cannon fatigue life are calculated, using an Increased initial crack size to account for thermal residual stress. Calculations of fatigue life are presented for a range of applied pressure and for selected gas temperatures.