Lifetime Assessment of Load‐Bearing Polymer Glasses: The Influence of Physical Ageing

Abstract

The timescale at which ductile failure occurs in loaded glassy polymers can be successfully predicted using the engineering approach presented in a previous publication. In this paper the influence of progressive physical ageing on the plastic deformation behaviour of unplasticised poly(vinyl chloride) (uPVC) is characterised and incorporated in the existing approach. With the modification it is possible to quantitatively predict long‐term failures which show a so‐called endurance limit. The predictions are compared with failure data of uPVC specimens which were subjected to constant or dynamic loads. In dynamic loading conditions a second type of failure mode was observed: fatigue crack growth. A brief study on the influence of the frequency and stress ratio of the applied stress signal shows that crack growth failure is not expected to occur within experimentally reasonable timescales for constant loading conditions.magnified image

Influence of physical aging on impact embrittlement of uPVC pipes

“…The aging kinetics of this water pipe grade were found to differ significantly from a pipe grade used for gas distribution pipes, which was characterised elsewhere. 7 For the operators of the gas distribution network, the influence of this difference on the prediction of the ductile to brittle transition is of interest. As a first approximation, the same critical yield stress was used to calculate the evolution of the transition temperature.…”

“…The proposed hypothesis can only be employed when the influences of strain rate, temperature and physical aging on the yield behaviour of uPVC are characterised, as performed in previous works. 7,25 In these papers, the yield behaviour of uPVC was assumed to be thermorheologically simple: only one relaxation mechanism, the a process related to the glass transition temperature, was considered to contribute to the yield behaviour. This assumption holds for low strain rates and/or moderate temperatures.…”

“…7,25 Remarkably, the aging kinetics is significantly different. The activation energy for the gas pipe grade is about half the value found for the material used in this paper.…”

Direct comparison of the compliance method with optical tracking of fatigue crack propagation in polymers

“…The rate of physical ageing depends on temperature, but also on the stress. [41,[44][45][46][47] Figure 10 illustrates the occurrence of physical ageing during fatigue loading by measuring the yield stress after a certain time under load. Physical ageing is observed already at room temperature after relatively short time-scales.…”

Fatigue life prediction via the time‐dependent evolution of linear and nonlinear mechanical parameters determined via Fourier transform of the stress

“…Due to the long‐time duration of fatigue tests in combination with their large experimental uncertainty, testing, and evaluating materials is time and resources consuming . However, the predictive power of the results for a single sample or application is limited, since each test must be interpreted against a statistical background, and a more precise prediction is generally not possible.…”