Sketch the stress-strain curve for a ductile metal and define the following terms: modulus of elasticity, yield point, resilience, ultimate tensile strength, fracture stress, plastic strain, elastic strain, elastic strain relaxation, elastic response, plastic response, strain hardening, and toughness; what is the relationship between engineering strain () and true strain (e)? derive a mathematical relationship between engineering strain  and true strain e, and sketch their variation with stress