An Analysis of a Wire-Wrapped Mechanical Crack Arrester for Pressurized Pipelines

Because of the difficulties which have been encountered in efforts to establish fracture toughness standards for arrest of running shear fractures in pressurized pipelines, the possibility of introducing mechanical crack arresters on such pipelines is currently being explored. The purpose here is to discuss the mechanics of the crack arrest process for a mechanical arrester which is fabricated by loosely thread wrapping ductile steel wire rod around a segment of the pipe. The net effect of the wire rod arrester as the crack approaches is to substantially increase the axial in-plane strain in the pipe wall adjacent to the fracture path from the value it would have in the absence of the arrester. The result is that, in the presence of the arrester, the growth of a circumferential tensile fracture can become more favorable than continued growth of the axial tensile fracture, and a criterion for arrest of the axial fracture based on this result is proposed. Through an analysis of rapid crack propagation in a pressurized pipeline, the arrest criterion leads to an expression for the minimum ductility of the wire rod necessary for crack arrest. The required minimum ductility for the wire rod is estimated for system parameters corresponding to available full-scale test data, and the results compare favorably with the independently measured actual ductility of the arrester material.