50 Mechanical Behaviour of Plastics 12 10 8 z6 B cn 4 2 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Stmln (%) Fig. 2.7 Stress-strain curves for polypropylene Fig. 2.8 Stress-time curves for polypropylene