Durability of polypropylene pipes for non-pressure applications – influence from surface damage and point loads

Session 5B
12:00 AM

Sven-Erik Sallberg, SP Technical Research Institute of Sweden

As trenchless installation techniques and coarse-grained backfill materials are gaining in in-terest 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 sur-faces, 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 % talc designed for profile extrusion. 110 mm pipes made from these materials were sub-jected to a constant deflection of 25 %. Along the lines of maximum tensile strain (the waist-line of the pipes), V shaped notches were machined. The deflection force relaxation was measured and compared to previous results from un-notched samples. A second set of pipes were subjected to indentation tests, where spherical indenters were pressed into the pipe wall to depths corresponding to 10 %, 15 % and 25 % of the pipe diameter. Both the deflection and indentation tests were undertaken on virgin pipes and on pipes aged 12 months in water at +95 ºC.

Testing 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.

Sven-Erik Sällberg Research Engineer – buried pipelines, SP
Kristian Thörnblom Research Engineer – polyethylene materials, SP
Hans Sandberg Application development & technical service engineer, Borealis AB