A new engineering approach to predict the hydrostatic strength of uPVC\ud pipes

Extruded unplasticised Poly(Vinyl Chloride) (uPVC) pipes are certified using pressurised pipe tests.\ud During these tests the pipes are subjected to a certain temperature and internal pressure, while the\ud time-to-failure, the time at which the internal pressure drops due to rupture or fracture, is measured.\ud These tests are time consuming and are therefore costly. To circumvent these costs a model-based\ud approach is proposed where the time-to-failure is predicted. The input parameters for this approach\ud can be determined using short term measurements. The approach uses the observation that the timeto-\ud failure kinetics of uPVC pipes subjected to an internal pressure is independent of the type of failure\ud mode (ductile, semi-ductile or brittle). This supports our statement that the underlying mechanism\ud that initiates failure is similar for these types of failure. Local deformation of the material up to a\ud critical value of the anelastic strain is believed to determine the start of failure of the material. This\ud critical strain appears to be constant for the testing conditions used during this study. A pressure\ud modified Eyring expression is employed to calculate the strain rate resulting from the applied stress\ud at a certain temperature. The time-to-failure follows from the calculated strain rate and the critical\ud strain of the material. This approach has been verified against literature data and shown to hold\ud quantitatively. Furthermore, the model seems to hold for different processing conditions

A new engineering approach to predict the long-term hydrostatic strength of unplasticized poly(vinyl chloride) pipes

Extruded polymer pipes are qualified using pressurized pipe tests. With these tests the long-term hydrostatic strength is determined by subjecting the pipes to an internal pressure, while measuring the time-to-failure. Although these tests can be accelerated (at higher temperatures), they remain time consuming and require a spacious experimental setup. To circumvent this costly method a model based approach is proposed by which the long-term hydrostatic strength is predicted. Using short term measurements, the input parameters for this approach can be determined. In this engineering approach the effects of physical aging are included. The approach is capable to quantitatively predict the (long-term) failure time of pipe sections under internal pressure

The effect of physical aging on the embrittlement of steam-sterilized polycarbonate

Polycarbonate is known to suffer from dramatic reductions in ductility upon exposure to hot, humid environments, such as during steam sterilization. Two phenomena have been proposed to be the main causes of this embrittlement: hydrolysis and microcavity formation. The present study focuses on a third phenomenon, whose contribution to the embrittlement has until now been considered insignificant: (physical) aging. By studying the influence of steam sterilization on the tensile deformation behavior of polycarbonate, it is shown that aging actually is one of the dominant factors in the embrittlement

Premature failure of poly-L/D-lactide bioresorbable spinal cages; Pittfalls in designing in time-dependent materials

The premature in-vivo mechanical failure of PLDLA bioresorbable spinal cages is investigated. The instantaneous strength of the cages is more than adequate to sustain the forces encountered in the specific loading condition. It is now shown that the high rate dependence of thematerial is the cause of the failure, pointing out that the time-dependentnature of polymeric materials can lead to unexpected failure