Sven-Erik Sällberg, Kristian Thörnblom, Hans Sandberg
Papers # 2010 Vancouver
As trenchless installation techniques and coarse-grained backfill materials are gaining in interest for plastics pipelines, the influence from surface damage on the technical lifetime is becoming more and more important. For non-pressure applications, such as gravity sewers, this aspect is relevant, e.g., when considering severe deflection of pipes with scratched surfaces, or point-loads from stone indentations. The present study investigated the influence from scratches and indentations on the technical lifetime of polypropylene pipes under stress relaxation load cases with constant deformations. Two polypropylene materials were selected. One block co-polymer high-modulus material used for commercial drainage and sewerage pipes, and one mineral filled PP containing 30 % chalk designed for profile extrusion. Testing of notched samples for one year showed no crack propagation in any of the materials or load cases. Compared to un-notched samples, the deflection force relaxation was slightly accelerated. The mineral filled PP exhibited plastic hinging and clear crazing at the deflection tests, and severe debonding between filler and matrix under the indenter, but no through-wall cracks were seen. The fracture mechanisms and excellent durability under relaxation previously seen for PP was thus confirmed, even with significant stress concentrations added to the load case.