Fracture initiation in polybutylene tubing in potable water applications

The fracture initiation in polybutylene (PB) tubing is usually observed in the form of a microcrack network within a layer of degraded PB at the inner surface of the tubing exposed to flowing water. Chemical degradation of PB is manifested in a reduction of molecular weight, increase of crystallinity and density of the material, a subtle increase in yield strength, and a dramatic reduction in toughness. An increase in material density, i.e., shrinkage of the degraded inner layer of the tubing constrained by the outer layer of unchanged material results in a buildup of tensile stress due to an increasing incompatibility between the inner and outer layers of the PB tubing. These stresses combined with preexisting manufacturing and service stresses, as well as reduction of toughness, result in fracture initiation. A mathematical model of the foregoing process is presented in this paper. An equation for determination of the critical level of degradation at the onset of fracture is derived. The critical level of degradation leading to fracture depends on the rates of toughness deterioration and build-up of the degradation related stresses as well as on manufacturing and service stresses. A method for the evaluation of the time interval prior to fracture initiation is also outlined.