DEFROMATION OF SOLIDS
stress: F acting on unit cross-sectional area, A
stress = F/A [Pa/Nm^-2]
-increase length > tension, tensile stress
-decrease length > compression, compressive stress
strain: extension per unit length
strain = e/l [e: extension, l: original length]

general stress-strain graph of ductile material (OPEAD) (ductile- long deformn before break, ie large plastic region)

elastic deformn:
-OP st. line through (0,0) => strain directly proportional to stress [e/l ∝ F/A => F ∝ e (l, A: const for given wire)] [P: proportional limit]
-OP > E (elastic limit): strain not proportional to stress (non-linear graph), but still returns to original length when stress removed
plastic/ permanent deformn:
-occurs beyond E; when stress removed > permanent extension
-small increase in stress > large increase in strain
-if load removed at A, recovery of wire along AO' (almost // to OP) > OO' => permanent extension; stress reapplied O'AD
at D: more than 1 'waists' form (material becomes very thin) => ductile fra