INNER SURFACE CRACKS IN AN INTERNALLY PRESSURIZED CYLINDER ANALYZED BY A THREE DIMENSIONAL DISPLACEMENT-HYBRID FINITE ELEMENT METHOD.

Stress intensity factors of unpressurized and pressurized surface cracks at the internal surface of an internally pressurized cylinder are determined directly by a three-dimensional displacement-hybrid finite element method. The finite element method is based on a modified variational principle of the potential energy, with arbitrary element interior displacements, interelement boundary displacements and element boundary tractions as variables. The special crack front element used in this analysis contains the inverse square root singularity in stresses where the three stress intensity factors, K//I, K//I//I, and K//I//I//I are quadratically variable along with the unknown displacements. Stress intensity factors for semi-elliptical cracks with crack aspect ratios of b/a equals 0. 5 and 1. 0, and at crack depths of 50 percent and 80 percent of the cylindrical wall thickness, are shown for internally pressurized cylinders with outer to inner diameter ratio, R//o/R//i equals 2.