Brittle Failure in Polymers

Certain polymers (not all) are prone to cracking instead of yielding when stressed, especially at low temperatures. They have a low extensibility and low strength. They therefore need an impact modifier, which discourages crack growth. 

Amorphous polymers like polystyrene normally have very low impact strengths, whether the test specimens are notched or not. When struck, they fail by cracking, because their crack initiation and propagation stresses are both lower than the stress required to induce yielding. 

A second group of polymers is called pseudoductile, because cracks are difficult to start in them, but easy to propagate once started. They can be identified by the fact that their impact strength is high when measured using imnotched test specimens, but low with notched ones. They have a characteristic transition temperature at which they change from brittle to ductile behaviour. (This transition does not always correspond with the glass transition temperature.) 

Polyamides and polyethylene are pseudoductile. Problems with polyamide toughness arise at low temperatmes. 

A third group of thermoplastics including PMMA, acetal (polyformaldehyde) and PVC have rather similar crack initiation and yield stress values, and their behaviour is more difficult to predict, because failure can be either brittle or ductile, depending on the temperature and the strain rate. 

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