Mechanical properties of materials Flashcards
Define stress
It is defined as the force applied normal to the face of the cross sectional area
Define strain
it is defined as the change in length in a given direction divided by the original length
Stress v strain graph for metals
Stress v strain graph for ceramics
Define young’s modulus
Measurement of stiffness of an elastic material
Equation for Bulk modulus
Define poisson’s ratio
It describes the contraction of a material in the perpendicular direction to the tensile stress
The equation for Poisson’s ratio
Define shear modulus
Ratio of shear stress to shear strain(different for different materials)
Equation for shear modulus
Liquids cannot support shear stress, G = 0
3 modes of polymer mechanical behaviour
A-Brittle polymer
- fracture during elastic deformation
B-Solid polymer
- with a yield point and regions of plastic deformation
C-Elastomer
- totally elastic and large recoverable strains at low stress
Describe necking in polymer mechanical behaviour
- chains aligned parallel to elongation direction localised strengthening
- resistance to continued deformation the neck propagates along the entire gauge length prior to fracture
Describe elastic deformation in semi-crystalline materials(polymer mechanical behaviour)
initial elongation of amorphous tie molecules, followed by a small reversible increase in the lamellar crystallite thickness
Define plastic deformation in semi-crystalline materials(polymer mechanical behaviour)
tilting of lamellar chains, separation of lamellar crystallites, orientation with the tensile axis
