The bumpers of cars have to be tough and light. Compare the use of aluminium and of polypropylene for car bumpers. First read the approximate toughness, \(G_{1 c}\) for the two materials from the chart of Fig. 3.7. Then divide each of these by the density of the material (read from any of the charts with density as an axis) to get a measure of toughness per unit mass. What conclusions can you draw?

Fig. 3.7

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1000 Fracture toughness - modulus 100 Design guidelines Fracture toughness, Kc (MPa.m1/2) 10 Metals Zinc alloys Composites Al alloys, Mg alloys Natural materials Lead alloys Leather PP PC Wood ABS- KJE Polymers KJE Elastomers EVA Polyurethane Silicone Ionomers elastomers Buty PTFE rubber 0.1 0.01 Prietaire polymer foams 0.001 Cork 0.01 0.1 Toughness G = kJ/m Cu alloys Ni alloys Steels Ti alloys OW alloys 100 10 Cast icens 0.1 Diamond SIGN SIC WC 0001 GFRP CFRP Brick Al2O3 Silicon 0.001 Stone Soda Saica Epoxies- Concrete Technical ceramics Nontechnical ceramics Foams Rigid polymer foams Lower limit for K MFA 15 10 100 1000 Young's modulus, E (GPa)