Assessment of the Behavior of Exhumed Cast Iron Pipes Using Distributed Strain Sensing

Abstract

To improve failure predictions and replacement decisions for cast iron pipes, a better understanding of their capacity and failure mechanisms is required. A series of ring bearing and four-point bending tests were conducted on exhumed small diameter cast iron pipe specimens instrumented with distributed fiber optic sensors (DFOS). Dynamic DFOS enabled the measurement of longitudinal and circumferential strains around and along each specimen including at the critical location which, due to the material variability of cast iron, could not be determined prior to testing. The ring bearing tests were used to estimate the modulus of rupture, initial elastic modulus, and secant modulus for each pipe. Additionally, variation within the same pipe and between pipes was characterized. The four-point bending tests illustrated the value of DFOS, which captured the non-linear development of strain at different locations as a function of load as well as the ratio of longitudinal and circumferential strains (the Poisson’s ratio). The location of failure along the pipe did not correlate with the location of the peak strain, which further highlights the value of DFOS and the need to account for material variability in assessments of cast iron pipe strength.