Stress intensity factor calculation for autofrettaged tube test-specimens using finite element method

The residual stresses, which are generated due to an autofrettage process, in a cracked autofrettaged tube may cause partial crack tip closure. In such a case, the superposition of the stress intensity factor due to both mechanical loading and residual stresses will not produce accurate results. It is, therefore, important to consider the crack closure effect in the calculation of the stress intensity factor. This has been done, in this paper, by using contact elements on the crack faces in order to prevent overlapping. In a first step, the residual hoop stresses for different degrees of autofrettage are calculated from a two-dimensional axisymmetric non-linear elastic-plastic finite element model. Elastic perfectly plastic material behaviour is assumed. The finite element results are in good agreement with the analytical solution found in the literature. For the stress intensity factor calculation, three different autofrettaged test-specimens are considered, namely ring, C-shaped and split ring. Crack length to specimen width ratio between 0.1 and 0.8 and several degrees of autofrettage are considered. Contact elements on the crack faces are used in order to prevent the overlapping and account for any possible partial crack surface closure. A spring and gap in series define the contact element. The stress intensity factor is calculated from the opening displacements behind the crack tip using the two-point displacement formula and the one-point formula.