Issue 33

A. Winkler et alii, Frattura ed Integrità Strutturale, 33 (2015) 262-288; DOI: 10.3221/IGF-ESIS.33.32 281 temperature. Our intermediate conclusion is that accelerated testing of plastics will be very hard to conduct and to justify. The strong load amplitude dependence is also clear and present. Figure 23 : Temperature change as a function of frequency, all other properties constant. Figure 24 : Temperature change as a function of load, all other properties constant. The dependencies illustrated were based on strongly simplified behavioural laws, we assumed linear elasticity, one single elasticity layer, and linear viscoelasticity. Changing any of these assumptions is going to change the behaviour, either making the temperature change larger or smaller. What is also of critical importance is how the temperature is diffused into the remainder of the part, and convected to the environment. Depending on this behaviour we will see changing temperature behaviour at a material point. Summarizing our findings, we can distinguish three load application speeds: 1. The load is applied very slowly. Energy dissipation occurs, but there is sufficient time for the heat to diffuse through the whole part. This means that we may consider the problem as isothermal. We note that this requires very low frequency loading. This case would produce the longest fatigue life. 2. The load is applied at a moderate speed. We can observe a higher rate of energy dissipation, but also insufficient time for all the heat to diffuse away. Thus, the local temperature rises. The temperature rise in turn lowers the viscosity, as well as the effective stiffness. This shortens the life of the part significantly, since the viscoplastic effects become more dominant and as such cause more internal damage. 3. The load is applied at a high speed. The result is a high rate of energy dissipation, and because there is no time for the heat to diffuse away, the process becomes adiabatic. We observe a rapid rise in temperature, and will typically see low cycle fatigue failure. From the summary we can draw a set of conclusions: