J-integral Based Fracture Assessment of Pressure Vessels Made of AA2219 Material Using Surface Crack Tension Test Data

Launch vehicles of ISRO make use of pressure vessels made of aluminium alloy AA2219 to store propellants. These are all welded constructions with weld defects encountered during inspection. These defects can cause a catastrophe if they propagate across the vessel during pressure testing or while in service under the influence of tensile stresses. Due to this reason, structural integrity evaluation of the tanks in the presence of defects is of utmost importance from both economic and safety considerations, especially for human rated launch vehicles. Though AA2219 is fairly ductile and exhibits significant stable crack growth prior to failure, crack growth is unacceptable for critical aerospace hardware. Prediction of stable crack growth initiation and failure can be made using either of single parameter or two parameters fracture theories. In the former, stress intensity factor, energy release rate, J-integral etc. are the parameters considered. In this paper, a novel approach is proposed to predict the criticality of part-through cracks in welds. In this work pressure vessel made of AA2219 material is considered. This method uses the fracture parameter ‘J-integral’ at the crack front, evaluated through finite element analyses. Based on simulation of tension tests carried out on Surface Crack Tension (SCT) specimens by NASA, a critical value of J-integral value is finalized. For actual hardware, the J-integral value at the crack front is evaluated at welds. This is compared with the critical value to conclude on the integrity of the vessel weld in the presence of the surface crack. The advantage of this methodology is that it can be used for fracture evaluation of hardware made of any ductile material with the availability of SCT test data.