Use of Optical Fibers to Investigate Strength Limit States for Pressure Pipe Liners Spanning across Circular Perforations

Local strain concentrations that develop where cured-in-place pipe liners span across perforations in the wall of the cast iron host pipe have been identified as a key limit state. However, difficulties measuring strain have resulted in minimal experimental support for existing theoretical findings. This paper reports on the use of distributed strain sensing with polyimide optical fibers to quantify strain profiles along the inside surface of a liner and the outside where the liner is exposed at a perforation through the pipe wall. Measured strains are compared to theoretical calculations to assess performance of the design model currently used for selecting liner thickness. Results obtained confirm that the peak axial strain location moves from the center of the perforation to the edge at perforation diameters above the limiting flat plate criterion. The measurements also support the use of current ASTM design rules for pressure pipe liners spanning across small sized perforations (up to 50-mm diameter in a 154-mm diameter pipe). However, ASTM design produces unconservative estimates for larger perforations.