The development of the crack shape in a thick wall pressure vessel is important in fitness-for-service evaluations such as leak-before-break. In this work the crack tip stress field for surface cracks in a thick plate is investigated using 3-D finite element analyses. The mean stress around the crack from the deepest point to the free surface is investigated for a semi-circular surface breaking crack. A new empirical procedure is developed that allows ductile crack growth to be modelled on the basis of standard deep and shallow cracked fracture toughness test data. In conjunction with this procedure the results show that the crack growth will be suppressed at the deepest segments and the crack will preferentially grow in direction of 45°–70° measured from the deepest point in full plasticity.

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