CAE-Companion-2018-2019

Modeling of Materials & Connections WISSEN CAE

strain-rates. This, however, is hard to achieve experimentally. It is therefore useful to parameterize the material response e.g. by a simplified G’Sell-Jonas law

Figure 4: Typical stress-strain relation of a foam under compression for different densities and its mathematical parameterisation. Plastics Thermoplastics are polymers with non-crosslinked chain molecules in amorphous or crystalline structure. They turn to liquids when heated and freeze to very glassy states when cooled sufficiently. The same effect occurs if we increase and decrease the strain rate respectively. At small strains, thermo- plastics behave viscoelastic, i.e. they have a certain strain rate dependency but (almost) no permanent deformation. At large strains, thermoplastics can be described in a pretty good approximation by viscoplasticity, i.e. strain rate depen- dency below the yield surface is neglected. Effects like „ „ increase of volume during plastic flow (crazing), „ „ different yield stress under tension/compression/shear/ biaxial tension and „ „ decrease of Young’s modulus for increasing strain (damage) also need to be considered to obtain a reasonable material formulation for thermoplastics.

fromwhich the stress-strain curves at constant strain-rates may be computed. However there are, similar to metals, a lot of open questions due to the influence of the process chain. The influence of fibre orientation and weld lines that lead to a much more brittle behavior (Figure 6,7) can be considered by coupling of the analysis of injection moulding and structural analysis (“integrative simulations”) which are topics of ongo- ing investigations.

Figure 6: Fill study of a specimen enforcing weld lines

Figure 7: Influence of weld lines during tensile test

Figure 5: Parameterization of dynamic stress-strain curves. Also for plastic materials, modern material laws allow for a direct input of the stress-strain relation, but only at constant

CAEWissen by courtesy of the Institute for Mechanics andMaterials at the THMittelhessen, Giessen in collaboration with the Department of Mechanics & Simulation at the German Institute for Polymers (DKI), Darmstadt.

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