Evaluation of the Influence of Residual Stresses on Ductile Fracture

In this work, the significance of residual stresses on ductile fracture is investigated by a set of experiments that are analyzed by finite element simulations. The treatment of residual stresses as expressed in fracture assessment procedures such as R6 is believed to be very conservative for ductile materials, when fracture occurs at high primary loads. Earlier numerical studies have reinforced this belief. This is supported in the current study. Tests on notched 3PB specimens with and without residual stresses were conducted on two ferritic steels. The residual stresses were introduced by applying a compressive preload on notched specimens. The tests were designed to achieve crack initiation at load levels around the plastic limit load. The crack growth in the tests was measured by a compliance method and by color marking of the crack surface. The crack tip driving force J was evaluated numerically for specimens with and without residual stresses. The experimental results show that the residual stresses clearly contribute to J at low primary loads. However, this contribution diminishes as the primary loads increase. The experimental results were also compared with results evaluated using the R6 procedure. These comparisons revealed overly high conservatism in R6 for cases with residual stresses compared to the ones for cases without residual stresses where less conservatism was evident.