Elastic-Plastic Behaviour of Structural Members and Systems with Crack Damage (1st Report)

Abstract

Crack damages are sometimes detected in offshore structures for petroleum activities. In this paper, influences of cracks on the performance of structural members and systems are theoretically investigated. A computer program SYSTAN 2 based on the Idealized Structural Unit Method is developed for an elastic-plastic analysis of two-dimensional structural systems containing cracked members. The influence of cracks on the full plastic strength interaction relationships is considered.Example calculations are carried out with SYSTAN 2. First, the ultimate strength of a both ends simply supported tubular member with a circumferential through-thickness crack is evaluated under axial load. It has been found that compressive ultimate strength depends on the slenderness ratio of a member and the magnitude of initial deflection as well as the crack size. On the other hand, tensile ultimate strength depends only on the size of a crack.A quasi-dynamical analysis is also performed for the transverse section of a semi-submersible drilling unit with assumed crack damages in the lower horizontal brace. The following observations have been made : (1) The intact structure behaves elastically under the extreme cyclic wave loading.(2) With a moderate crack damage in the brace, the cumulated plastic deformation at every wave cycle levels off after some wave cycles.(3) With a more significant crack damage, the progressive plastic collapse of the transverse section takes place.Further work is needed to account for the elastic-plastic crack tip behaviour and the crack growth effect.