Mechanical Properties of Materials

Elastic deformation is when there is no permanent deformation after a load is removed from a material, due to bonds being able to return to their normal length. • Strongly bonded materials require a greater force in order to pull apart and deform the bonds.

Plastic deformation is when there is a permanent deformation after a load is removed from a material, due to bonds being unable to return to their normal length. • Instead of stretching bonds, they are broken or reformed with new neighbours causing a large number of atoms to move relative to each other. • Stress is no longer proportional to strain. • Dislocations cause this plastic deformation. Mainly associated with metals. • Although the crystal structure doesn’t change, the shape of the material does.

A measure of the stiffness and bond strength of a material. • E = σ/ε (during elastic deformation) • N/mm2

The ratio of lateral to axial strains. • v = εL/ε • Dimensionless

Auxetic materials have a negative Poisson’s ratio, and so expand in all directions when under tension. • They are frequently used in memory foam beds.

Draw a parallel line through the yield point and measure the gradient of this line.

The yield strength is the point at which areas is no longer proportional to strain. • Above this level of stress, plastic deformation occurs. • It is a measure of resistance to plastic deformation. Metals: • Variable yield strength from low to high. Ceramics: • Hard to measure, as fractures usually occur before yield. Polymers: • Low yield strength.

The tensile strength is the maximum stress that a material can sustain. • It is the maximum stress shown on a stress-strain curve. • It occurs in metals once noticeable necking starts, and in polymers when the polymer backbone chains are aligned and about to break. Metals: • High tensile strength. Ceramics: • Medium tensile strength. Polymers: • Low tensile strength.

The ductility is the degree of plastic deformation before failure. • It can be found by drawing a line parallel to the Young’s modulus to find the plastic strain at failure of the material.

Toughness is the energy required to break a unit volume of a material. • It is a measure of the ability of a material to absorb energy up to fracture, or the resistance to fracture once a crack is present. • It can approximately be found by the area underneath a stress-strain curve. • Generally, ductile materials are tough, and brittle materials are not. • Jm-3 Metals: • Large toughness. Ceramics: • Low toughness. Polymers: • Low toughness.

Resilience is the capacity of a material to absorb energy during elastic deformation and recover energy during unloading. • It is the materials ability to store energy. • The modulus of resilience (U) is the strain energy per unit volume required to stress a material to the yield point. • Jm-3 • U = 1/2σyεy

Hardness is the resistance to localised plastic deformation (such as dents/scratches). • Large hardness means resistance to plastic deformation is great and so materials wear better.

• A scale of hardness devised by arranging 10 minerals so that one material could only scratch those below it. • Intervals of hardness are approximately equal except for between corundum and diamond.