Fracture
Plastic Pipes Conference Association # 1998 Gothenburg
Venizelos, Greenshields, Ivankovic
Over 30 years ago, fast brittle fracture was identified as an inherently catastrophic failure mode in pressurised plastic gas pipelines. Although fast cracks seemed difficult to initiate, subsequent rapid crack propagation (RCP) was thought to be possible at pressures lower than the rated pressure, imposed by design against slow crack growth. RCP is characterised by long cracks extending axially at high speed (typically exceeding 100 m/s). There exists a critical pressure p, above which a crack, once initiated, can propagate indefinitely. Rapid escape of compressed gas during such a failure could cause severe structural damage to roads and pavements above the pipeline; the possibility of a fuel gas explosion increases the overall destructive potential. A crack might be initiated from a defective butt fusion weld joint or an external impact from a ‘third party’ and no amount of care can completely eliminate the risk. The study of RCP in gas pipes therefore aims to ensure that a fast-running crack will always arrest, even under the most extreme operating conditions.
In the early 70s, British Gas responded quickly to minimise any risk of RCP failure in polyethylene (PE) vas pipelines. They introduced a special design code [1] and later imposed pressure restrictions on larger pipe sizes and used a fracture toughness parameter to determine operating pressures. Once _ these precautionary measures had been put in place, research began in earnest with the construction of a full-scale test facility at Spadeadam, UK. The results of early experiments gave an invaluable insight into the problem [2] but the preparation time and cost of each test inevitably made progress slow.
In 1987, Imperial College began to develop a laboratory test to reproduce the full-scale RCP phenomenon in short, manageable pipe specimens (Fig. 1). The Small-Scale Steady-State (S4) test was quickly developed to the point where it seemed to reflect reliably the full-scale performance of different pipe materials [3]. Several research institutions followed in producing their own versions of the S4 test and a growing interest in RCP research led to the adoption of the S4 test as an ISO standard method.
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