Plastic pipeline systems have now become dominant for fuel-gas and water distribution
networks. Although they have an impressive service record failures do
occur, with Rapid Crack Propagation being characterised as the least probable but
most potentially catastrophic one. This study investigates the effect of structural
morphology and bulk residual strains on the RCP performance of polyethylene
pipes, and proposes a new methodology for predicting a safe service envelope.
During crack propagation in PE pipes, the fracture surface has two distinct regions;
plane strain and plane stress. In addition to the Instrumented Charpy, Reversed
Charpy, High Speed Double Torsion, Dynamic Mechanical Analysis and uniaxial
tensile testing, S4 tests of extruded pipe specimens were employed in order to
evaluate the structural and fracture parameters of pipe grade resins in these two
fracture modes on pipe. A new experimental technique, which modified the pipe
bore crystallinity without altering the residual strain field (as evaluated from slit
ring tests) showed that the bore surface layer properties had much less influence
on RCP than previously thought. Parallel with the experimental work, modeling
of the fracture mechanisms was also undertaken. Using previous models in the
field, such as the adiabatic decohesion model, the plane strain fracture toughness
was evaluated while the plane stress fracture toughness was evaluated either from
the Reversed Charpy or from the stability of adiabatic drawing in a tensile test.
A mixed mode, temperature sensitive toughness was finally evaluated, leading to
an overall fracture properties assessment for polyethylene pipes which could be
compared directly to the crack driving force during RCP in pipe. By employing a
new mathematical approach, which incorporated both the effects of residual strains
and pipe stiffness behind the pressure decay length, a previous basic analytical
RCP model was further developed and compared to more elaborate finite element
and finite volume solutions. The new results were also compared to S4 experiments
using high-speed photography and showed that the new methodology could be
employed by the end user even when testing facilities are not directly available