Question: A plastic bag is manufactured from polyethylene. This material is char

Question

Question: A plastic bag is manufactured from polyethylene. This material is characterized by being able to strain harden. This hardening can be explained by: a) Due to the deformation a major increase of Tg is observed. b) The deformation melts the crystallites in the material and thereby allows the chains to arrange themselves in bundles parallel to the direction of deformation c) The degree of crystallinity increases significantly due to the deformation which in turn significantly increases the modulus d) The chains in the material are eventually aligned with the direction of deformation. This is mediated through the influence of the strain on the crystallite microstructure in the material

Explanation / Answer

Strain hardening is observed as a strengthening of a material during large strain deformation. It is caused by large scale orientation of chain molecules and lamellar crystals. This phenomenon is quite often observed when plastic materials are stretched beyond their yield point. Polymers that exhibit greater strain hardening are often tougher and undergo ductile rather than brittle failure.

The degree of strain hardening depends on both intrinsic and extrinsic factors. Important intrinsic factors include molecular structure, cohesive energy, molecular weight (distribution), and morphology (degree of crystallinity and branching). Important extrinsic factors are strain rate, temperature, stress state, and specimen geometry. The morphology is one of the most important factors which also depends on the processing conditions.

A ductile amorphous material shows a characteristic yield point. At this point, the material starts to undergo plastic deformation. Materials with high plasticity show strong necking and cold drawing, that is, beyond the yield point, the cross section in the necking region will steadily reduce until the material breaks abruptly at rather large elongation. The necking is often followed by strain hardening because the chain molecules tend to orient and allign in the direction of the load which increases the strength and stiffness of the plastic in stretch direction. The opposite of strain hardening is strain softening which is a less common phenomenon. Miehe et al. postulated that true strain softening is the result of localized shear band formation which produces local voids which in turn weaken the polymer.

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