Definition of tough and brittle behaviour in polymers

An Introduction to the Mechanical Properties of Solid Polymers I. M. Ward and J. Sweeney 2004 John Wiley Sons, Ltd ISBN 0471 49625 1 (HB) 0471 49626 X (PB) 

The distinction between brittle and ductile failure is also manifested in two other ways (1) the energy dissipated in fracture and (2) the nature of the fracture surface. The energy dissipated is an important consideration for practical applications and forms the basis of the Charpy and Izod impact tests (discussed in Section 12.8 below). At the testing speeds under which the practical impact tests are conducted it is difficult to determine the stress-strain curve, so impact strengths are customarily quoted in terms of the fracture energy for a standard specimen. 

The appearance of the fracture surface also can be an indication of the distinction between brittle and ductile failure, although the present state of knowledge concerning the crack propagation is not sufficiently extensive to make this distinction more than empirical. 

For polymers, the situation is clearly more complicated than that for the brittle-ductile transition in metals, as there are in general four regions of behaviour and not two. It is of considerable value to discuss the factors that influence the brittle-ductile transition, and then to consider further factors that are involved in the observation of necking and cold-drawing. 

SO impact strengths are customarily quoted in terms of the fracture energy for a standard 

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