Figure Of Merit
The ratio of modulus of elasticity to density (E/ ρ ) can be regarded as figure of merit of a material.
With E = 2.05 × 105 Mn/m2 and ρ = 0.78 × 104 kg/m3, steel has figure of merit E/ ρ = 26 × 106 N . m/kg .
It is coincidental that aluminium, magnesium, wood and glass have E/ ρ ratios which are very closely equal to that of steel. Plastics have E ratio much smaller but when glass fibres is used as reinforcing constituent in plastic, E/ ρ ratio of about 20 × 106 N . m/kg is achieved. Materials that have their figure of merit even higher than those of iron, steel, aluminium and wood are very brittle and not fit for use in structures. Fibres of boron, beryllium, carbon, etc. are the examples of materials that have (E / ρ) ratio in excess of 100 × 106 N . m/kg.