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Heterogeneous tensile behavior of impact-modified polypropylene plates processed by injection moulding

Abstract : Due to the fast cooling of polymer on the mould surface, the injection moulding process is known to be responsible of property gradients through thickness. Nevertheless, the mechanical behaviour is usually characterized by considering an homogeneous polymer, even when tensile test specimens processed by injection moulding are used. In order to characterize the local three-dimensional behaviour, a new set up based on digital image correlation has been developed and applied to specimens machined from injection moulded plates. This experimental set up allows to measure the strain fields on both front and lateral sides of specimens with a good accuracy. In this study, two impact modified polypropylenes have been analyzed under tensile loading. These polymers are made of ethylene-propylene rubber nodules and lamellar talc particles embedded in a polypropylene matrix. They differ from the volumetric amount of talc (4% in material A, 0,5% in material B). Unlike neat polypropylene, these materials do not exhibit a necking phenomenon in tension. The axial strain and transverse strain through specimen width remain homogeneous along the gauge length during the tensile test. On the other hand, the transverse strains through thickness differ for both polymers. Polymer B exhibits large gradients of εzz strain between the centre and the plate surfaces while polymer A presents an homogeneous transverse deformation. As confirmed by microstructure investigation, these results reveal a large volumetric dilatation due to void growth for both polymers, but for polymer B, the large transverse gradient measured is due to a high volumetric dilatation gradient through plate thickness. The good thermal conductivity of talc could explain the different behaviour between the two polymers and the origin of volumetric dilatation gradient. In polymer B, the different cooling rates between plate core and surfaces induce large residual stresses which could create damage and initiate voids in plate core.
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Olivier de Almeida, Fabienne Touchard, Laurence Chocinski-Arnault. Heterogeneous tensile behavior of impact-modified polypropylene plates processed by injection moulding. PPS 26 -The Polymer Processing Society 26th Annual meeting, Jul 2010, Banff, Canada. 11 p. ⟨hal-01760276⟩



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