Influence of Circumferential Flaw Length on Internal Burst Pressure of a Wall-Thinned Pipe

Abstract

This paper examines the effect of the circumferential angle of a flaw θ on the internal burst pressure pf of pipes with artificial wall-thinned flaws. The effect of θ has conventionally been regarded as unimportant in the evaluation of the pf of wall-thinned straight pipes. Therefore, a burst pressure equation for an axial crack inside a cylinder (Fig. 1, left), such as Kiefner’s equation (Kiefner et al., 1973), has been widely applied (ANSI/ASME B31.G., 1991; Hasegawa et al., 2011). However, the following implicit assumptions notably exist when applying the equation to planar flaws in situations with non-planar flaws. 1) The fracture mode of the non-planar flaw under consideration is identical to that of the crack. 2) The effect of θ on pf, which is not considered for an axial crack, is small or negligible. However, the experimental results from the systematic burst tests for carbon steel pipes with artificial wall-thinned flaws examined in this paper showed that these implicit assumptions may be incorrect. In this paper the experimental results are evaluated in further detail. The purpose of the evaluation was to clarify the effect of θ on pf. Specifically, the significance of the flaw configuration (axial length δz and wall-thinning ratio t1/t) was studied for its effects on θ　and pf. In addition, a simulation of this effect was conducted using a large strain elastic-plastic Finite Element Analysis (FEA) model. As observed from the experimental results, θ　tended to affect pf in cases with large δz, and t1/t was also correlated with a decrease in pf with an increase in θ. These tendencies were successfully simulated by the large strain elastic-plastic FEA model. The observed effects demonstrate that the burst pressure predicted for a crack with identical ligament thickness decreases with an increase in θ, so that the effect of θ on pf should be taken into consideration when evaluating pf