Crack Path Bifurcation at a Tear Strap in a Pressurized Stiffened Cylindrical Shell

A finite element model of a fracture test specimen is developed using the STAGS computer code (STructural Analysis of General Shells). The test specimen was an internally pressurized, aluminum cylindrical shell reinforced with two externally bonded aluminum tear straps around its circumference. The shell contained an initial, axial through-crack centered between the straps. The crack propagated slowly in the axial direction as the pressure increased above a certain value until a maximum pressure was attained, and then the crack propagated dynamically. The tear straps sufficiently toughened the shell such that the dynamic crack path bifurcated near the edges of the straps. The bifurcated crack branches ran circumferentially, parallel to the straps causing the shell wall to flap open.

The STAGS analysis for the static equilibrium configurations of the fractured shell include geometric nonlinearity and elastic-plastic material behavior. The crack tip opening angle (CTOA) is used in the criterion for ductile crack growth, and the critical value of the CTOA is determined by correlating the STAGS predictions of the stable portion of the crack growth curve (internal pressure versus half crack length) to the test. With the employment of a new STAGS algorithm, the complete axial crack growth curve, including both the stable and unstable portions, through the tear strap is obtained. The complete axial crack growth curve indicates that crack growth through the strap is unlikely. STAGS models with long cracks which bifurcate at various half crack lengths are developed to assess the location of crack bifurcation. Three different stress based crack turning criteria are investigated from the axial crack growth results as a second method for assessing a location of bifurcation. The bifurcation analyses and stress based turning criteria corroborate the experimentally measured bifurcation point. A parametric study is then conducted to determine the influence of tear strap thickness and width on the location of crack bifurcation.