Issue 33

A. Winkler et alii, Frattura ed Integrità Strutturale, 33 (2015) 262-288; DOI: 10.3221/IGF-ESIS.33.32 286 It is clear that da/dN analysis seems to provide the largest potential for future physically founded fatigue analyses of plastics, but from all the points mentioned, it is also obvious that further research is necessary in this field. C ONCLUSIONS AND RECOMMENDATIONS e have studied the morphological behaviour of plastics and seen that the material behaviour depends on the manufacturing process of the component. We also have seen that the temperature plays a crucial role in any material behaviour of plastics, and thus also in its fatigue behaviour. We have given an overview of how classical fatigue algorithms could be applied, but have also shown that they will have difficulty predicting correct behaviour, because none of them directly incorporates the morphology and temperature predictions. In order to come up with improved fatigue algorithms for plastics, we cannot just try and turn the knobs on the available fatigue algorithms for metals. These have clearly been devised to match metal response, and lack the characteristic behaviour observable in plastics. Fatigue algorithms are clever methods to attempt and match experimental results borne from pure phenomenological attempts of capturing certain experimental results into an equation, and some behavioural argumentation to extend the curve to multiaxiality and non-linear behaviour. For both SN and eN algorithms it is assumed that nothing happens until a certain fatigue threshold is reached, which is not the case for plastics, since visco-elasticity/plasticity is always present. It is also assumed that although the order of loading is important, the speed of loading up until a reasonable limit is not too influential for the fatigue behaviour. This is yet another aspect which is clearly not the case for plastics, as we have demonstrated through the numerical experiment involving the temperature effect. Temperature dependence in SN and eN algorithms are typically handled by taking the maximum temperature over the life span, and use that as a conservative estimate in obtaining life. In plastics this would nearly always lead to low cycle fatigue results, and we could not rely on ambient measurements of temperature in the first place, an analysis would have to be made. In order to improve on this, the following points need to be considered.  The injection moulding process needs to be simulated, prior to the FEA analysis to obtain a distribution of properties. This would provide the analyst with the skin effect, and the distribution of crystallinity. This would also justify the use of an energy-based approach, as this can describe the accumulation of actual damage.  If we do not wish to make the whole fatigue analysis FEA-bound, that is, needing to perform a full thermomechanical FEA for each loading cycle, we need a simplified way to deal with the energy being released when going through the cycles. Some attempts in this direction have been made [Manson&Hertsberg]. However, they assume adiabatic conditions, which would lead to unrealistic high local heating, as they point out themselves. One would need a simplified equation to take into account both local heating, as well as diffusion and convection to the environment.  The non-linear visco-elastic behaviour of the plastic would need to be taken into account using a methodology similar to a Ramberg-Osgood law, which in itself is not sufficient to capture the behaviour in a fatigue analysis. The answer to an efficient solution to the problem fatigue of plastics is not yet out there, but if we take into account the understanding of the behaviour of plastics as discussed, we can at least make efficient stabs at the structure of a potentially successful solution, rather than trying to modify phenomenological models originally intended for a different class of materials. R EFERENCES [1] Doi, M., Introduction to Polymer Physics, Oxford University Press, (1996). [2] Doi, M., Edwards, F.S., The Theory of Polymer Dynamics, Oxford University Press, (1986). [3] Bird, R., Armstrong, R., Hassager, O., Dynamics of Polymeric Liquids - Fluid Mechanics, John Wiley and Sons, (1987). [4] Bird, R., Armstrong, R., Hassager, O., Dynamics of Polymeric Liquids - Kinetic Theory, John Wiley and Sons, (1987). [5] Elias, H-G., An Introduction to Plastics, Wiley VCH Verlag, (1997). [6] Ferry, J., Viscoelastic Properties of Polymers, John Wiley & Sons, (1980).