Abstract
The fatigue life of an externally cracked modern tank gun barrel is controlled by the prevailing combined stress intensity factor (SIF) <em>K</em><sub><em>IN</em></sub>, which consists of two components:<em>K</em><sub><em>IP</em></sub>-the SIF caused by internal pressure; <em>K</em><sub><em>IA</em></sub>-the positive SIF due to the tensile residual stresses induced by autofrettage. <em>K<sub>IA</sub></em> values for a single external radial semi-elliptical crack originating at the outer surface of an autofrettaged gun barrel were calculated for a large number of crack configurations by Perl and Saley. In order to assess the combined effect of overstraining and the pressurizing of the barrel during firing, values of <em>K</em><sub><em>IP</em></sub>, the SIF caused by internal pressure, and those of <em>K</em><sub><em>IN</em></sub>, the combined SIF, are evaluated. The 3D analysis is performed using the finite element method (FEM) employing singular elements along the crack front. The novel realistic overstrain residual stress fields, incorporating the Bauschinger effect, for the three types of autofrettage, Swage, Hydraulic and Hill's, previously developed, are applied to the barrel. The RSFs are simulated in the finit element (FE) analysis using equivalent temperature fields. Values of <em>K</em><sub><em>IP</em></sub> and <em>K</em><sub><em>IN</em></sub> are evaluated for a typical barrel of radii ratio <em>R</em><sub><em>0</em></sub>/<em>R</em><sub><em>i</em></sub>=2, crack depths (<em>a/t</em>=0.005-0.1), crack ellipticities (<em>a/c</em>=0.2-1.0), and five levels of the three types of autofrettage, (<em>ε</em>=40%, 60%, 70%, 80%, and 100%). A detailed analysis of the effect of the above parameters on the prevailing SIF is conducted. All three types of autofrettage are found to have a detrimental effect on the barrel's fatigue life. However, the magnitude of life reduction is autofrettage-type dependent. In the case of external cracking, Hydraulic autofrettage is found to be somewhat superior to Swage autofrettage, and Hill's autofrettage is found to be non-realistic. Finally, the results accentuate the importance of the three dimensional analysis and the incorporation of the Bauschinger effect.
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