Toughness and endurance

The phenomena of ductile and brittle fracture and of ultimate stress and elongation have been discussed in Chap. 13. 

The ultimate stress is very much time-dependent, as may be understood from the viscoelastic behaviour of polymers. At very high velocities there is, even in ductile materials, a change from ductile to brittle fracture. 

Impact strength is the resistance to breakage under high-velocity impact conditions. This property is of great practical importance, but extremely difficult to define in scientific terms. 

Many impact tests measure the energy required to break a standard sample under certain specified conditions. The most widely used tests are the lzod test (pendulum-type instrument with notched sample, which is struck on the free end), the Charpy test (pendulum-type instrument with sample supported at the two ends and struck in the middle), the falling-weight test (standard ball dropped from known height), and the high-speed stress-strain test. 

The impact strength is temperature-dependent near the glass temperature the impact strength of glassy polymers increases dramatically with temperature. Secondary transitions play an important role-, a polymer with a strong low-temperature secondary transition in the glassy state is nearly always much tougher than a polymer which has no such transition. 

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PC/SMA (polycarbonate with styrene-maleic anhydride alloy) exhibits toughness and endurance at high temperatures. 

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