An attempt to use scratch tests to predict the residual lifetime of unplasticised poly(vinyl chloride) pipes

This paper reports on a procedure that can potentially predict the residual lifetime of low-pressure uPVC pipes in a non-destructive way. Ageing of these materials is characterised by a change in yield stress and fracture behaviour. The search for a method being able to non-destructively evaluate the yield stress and the fracture behaviour led to the study of scratching of the polymer surface. According to an extensive study by Atkins and Liu, the scratching behaviour is a function of yield stress, fracture toughness and the attack angle of the cutting tool.Experiments reported in this paper give an evaluation of the scratched area as a function of the attack angle for uPVC samples having different ageing times. Existing analyses for metals are adapted for polymer specific properties, such as the pronounced strain rate dependence of yield stress and visco-elastic recovery. The suggested adaptations are successful in the sense that the resulting analysis is capable of describing the forces during scratching and the resulting cross-sectional area of the groove. Although the scratching technique was not able to discriminate between differences in the scratch force or cross-sectional area of the groove of the different ageing times, it was possible to get an estimate of the fracture toughness from the analysis. Therefore, scratching is a promising technique to determine the fracture toughness for ductile materials for which it is difficult to obtain accurate results with conventional techniques.