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

The effect of the circumferential angle of a flaw θ on the internal burst pressure pf of pipes with artificial wall-thinned flaws is examined.When evaluating pf of wall-thinned straight pipes, the effect of θ has been conventionally not regarded as important. Therefore, a burst pressure equation for an axial crack inside a cylinder (Fig. 1, left), such as Kiefner’s equation [1] is widely used [2], [3]. However, it should be noted that there exist the following implicit assumptions when applying the equation for planar flaws to non-planar flaws.1) The fracture mode of a non-planar flaw under consideration is identical with that of the crack.2) The effect of θ, which is not considered for an axial crack on pf, is small or negligible.However, from the systematic burst test results of carbon pipes with artificial wall-thinned flaws, Meshii [4] showed that these implicit assumptions may not be correct. On the other hand, the significance of the effect of the fracture mode on pf and the condition for θ to affect pf are not clear.Therefore, in this paper, Meshii’s experimental results are evaluated in farther detail. The purpose of the evaluation was set to clarify the effect of θ on pf. Specifically, the significance of flaw configuration (axial length δz and wall-thinning ratio t1/t) was studied in relation to θ and pf. In addition, a simulation of the effect by a Finite Element Analysis (FEA) was attempted.From the experimental results, θ tended to affect pf in cases with large δz, and t1/t also was correlated to a decrease in pf with the increase of θ. These tendencies were successfully simulated by the elastic-plastic FEA.This effect means the burst pressure predicted for a crack with identical ligament thickness decreases with the increase of θ, so that the effect by θ on pf should not be ignored.Copyright © 2012 by ASME