A biscuit is stiff but weak, steel is stiff but strong, nylon is flexible (low E) and strong, raspberry jelly is flexible (low E)
(p 50) the slope of a stress-strain diagram measures the elastic stiffness or floppiness of a given solid. This is also known as Young’s modulus of elasticity or “elastic modulus”.
Stiffness = E = stress / strain [SI: MN/m^2]
The stiffness of a material is the stress for which the length of the material would double (if the material doesn’t break by then).
Stiff (high E) vs Flexible (low E)
(p 49) How far the atoms at any point in a solid are being pulled apart – that is by what proportion the bonds between the atoms are stretched.
Strain = (increase of length) / (original length) [no measurement units – sometimes it is conveyed as %]
(p 56) The stress required to break a piece of the material itself. A lot of times we are concerned with the “ultimate tensile stress” which is the “tensile strength” of a material determined by breaking a small test-piece in a testing machine.
Strong (withstands high stress) vs Weak (breaks at low stress)
(p 46) A measure of how hard the atoms in a material are being pushed together or pulled apart as a result of external forces. Unlike the concept of pressure stress in a solid is often a directional or one-dimensional affair (for most discussions).
Stress = (load or force applied) / (cross-sectional area) [SI: MN/ m^2]
1 Meganewton ~ 100 tons force