Mechanical properties

Review slide 14!!!

Tensile test

Specimen elongated to fracture at constant rate (along gauge length)

Elongation measured from

Crosshead displacement
Extensometer

Tensile tests more common because

Easier to perform
Stress & strain are negative in compression testing

Stress

Load/ cross-sectional area

Strain

Elongation/ original length

Young modulus, E

Slope of the stress-strain curve in the linear-elastic region (elongation of gauge length must be measured directly using an extensometer)

Tangent or secant modulus

Used when initial elastic portion of the stress-strain curve is not linear - e.g. grey cast iron, concrete, many polymers & composites

Poisson ratio

-lateral strain/tensile strain
0.17 < v < 0.46

Modulus decreases gradually as temperature increases

Higher temp increases potential energy of atoms
Mean interatomic distance increases (thermal expansion)
As interatomic spacing increases, slope becomes shallower so lower modulus

Beyond elastic limit

Plastic deformation, ductile
Fracture, brittle

Tensile strength

Maximum tensile stress carried before fracture

Yield strength

Stress required to produce a noticeable plastic strain
Easy to determine if material shows yield point e.g. carbon steels

Materials which show linear elastic behaviour - offset method -

0.2% proof stress/offset yield strength - stress required to produce permanent strain of 0.2%

Materials which show non-linear elastic behaviour

Stress to produce a given total (elastic & plastic) strain
Polymers - 1% yield stress
Composites - 0.5% yield stress

Ductility

Amount of plastic deformation a metal can undergo before fracture
%EL or %RA

Toughness

Measure of the ability of a material to absorb energy up to fracture

High toughness requires

High tensile strength
High ductility
(Large area under stress-strain curve)

Impact test (Charpy/ Izod)

Energy absorbed in fracturing the sample is determined from the difference in potential energy of the pendulum before and after fracture (change in mgh)

Hardness test

Measures resistance to scratching/ indentation
Non-destructive
Semi-quantitative

Rockwell vs Brinell, Vickers and Knoop

Depth vs size of indentation

Brittle ceramics - hardness test -

Hardness doesn’t correlate to tensile strength, tells us more about fracture toughness