Simulation of micro cracks introduced by environmental stress in amorphous plastics

Abstract

Environmental stress crack resistance (ESCR) is a major problem especially for amorphous polymers. The interaction between aggressive substances and both mechanical external and residual stresses can lead to a fracture of the plastic part. In a previous study, a method was developed to measure the ESCR. The specimens are bent over a circular jig to introduce external stresses and a softener is applied as aggressive substance. After a certain time, the samples fracture giving a measure for the ESCR. In this study, this method is under further investigation using the example of acrylonitrile butadiene styrene. Before the sample fractures, there are several micro cracks visible on the surface. The growth of these micro cracks was measured by dye penetrant testing and the shape of the cracks was determined with computer tomographic measurements. From these measurements, artificial samples were constructed for a simulation. Through the simulation, the stresses in the samples were determined to get an insight into the failure mechanism of the samples. It has been shown that the number of the micro cracks in a sample is constant at all times even though there is a great variation in the number of cracks in the different samples. The cracks grow almost linearly with time. This means that all cracks initiate in one specimen at the same time and in the following phase these cracks grow while no more new cracks initiate. This can be explained by the simulation showing that the stress accumulates at the tips of the cracks and the tensile stress at the surface of the specimens decreases. These results were compared with the results from a stress relaxation test which also showed a decrease in the tensile stress at the surface with time, hence verifying the simulative results.