Analytical model for failure bending moment of straight pipes including residual stress and ovality

When a pipe breaks the fracture starts in the vicinity of an initial or nucleating defect, such a defect being usually located near to the weldment. Generally, this means that breakage is an elastic-plastic process taking place in areas with heterogeneous properties. This complexity is increased by the residual stress in the weldment, which is always difficult to evaluate. Moreover, the breakage is usually associated with a gross loss of circularity, independent of the pipe dimensions. In this paper a four-parameter model is given which describes four fundamental aspects of the pipe section at the moment of breakage. These parameters are: (a) the effective stress in the uncracked area as defined by M.F. Kanninen, (b) the effective stress in the cracked area as defined by K. Hasegawa, and two new parameters to describe (c) residual stresses and (d) ovality. Additionally, a simplified approach to describe crack growth is included in the model. This approach requires two new parameters, namely the minimum defect length and the effective defect area at the failure instant. (author)