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Compression fatigue and 3D morphologies of EVA based foam : a study using X-rays tomographyThursday (08.10.2020) 15:00 - 15:20
Owing to their excellent specific mechanical properties, i.e., their capacity to absorb shocks and to release the related stored energy, ethylene-vinyl acetate foams (EVA) with closed or open cells are widely used for midsoles of running shoes: they fulfil the compromise between the runners’ performances, their comfort and the mitigation of their injuries. Such a compromise must be valid for the lifetime of running shoes, up to approximately 1000 km, where the midsoles are gradually damaged and subjected to interrupted cyclic loadings mainly composed of shear and compression deformation modes. To increase their durability, it is of primary importance to understand the fatigue of EVA foams, i.e., the correlation between the evolution of (i) their mechanical properties and (ii) their cellular structures upon cycling. The link between points (i) and (ii) still remains unclear and restrains the optimisation of the foam formulation and processing. This is mainly due to the difficulty to analyse the cells deformation mechanisms on a same sample upon fatigue, e.g., by using standard optical or SEM microscopy which require the sample destruction. To circumvent this difficulty, we performed cyclic compression tests on EVA foams (up to a prescribed representative strain, at 1Hz) and the 3D cellular structure of each sample was analysed at various stages of the fatigue tests using non-destructive X-ray microtomography. At the sample scale, we found that samples exhibited a rapid consolidation with a decrease in stress levels during cycling, an increase in the residual strain (at zero stress) and with a steady state stress-strain response above 1000 cycles (15 mins of running time). Stopping the tests at that time lead to a recovery of the residual strain practically up to zero and the 3D images give evidence that the cellular structures of the samples remained practically unchanged. Repeating this procedure yielded to a gradual plastic residual strain, i.e., a non-zero residual strain after representative waiting time (24 h). Corresponding 3D images emphasised gradual irreversible collapses of cell walls (and thus an anisotropic decrease of pores). The cell wall plastic deformation occurred without tears but rather with irreversible bending/buckling deformation modes due to a gradual creeping strain induced during cycling.