Materials - Tensile strength

Classes of materials

metals and alloys, ceramics and glasses, polymers, composites, natural materials, semiconductors, nano materials

Forces can cause materials

To deform (change shape), to fail (break)

Mechanical properties that describe how materials behave under load

Stiffness, strength, toughness

Processes of material degradation overtime

Wear, creep, fatigue

tensile force

Pulling force applied at a right angle to stretch an object

Tensile test

Measures the response of a material to a slowly applied uniaxial force. The yield strength, tensile strength, modulus of elasticity, and ductility are obtained.

Tensile test description

Take a sample of a material. Pull on the ends to stretch it. Measure force (f). Measure stretch amount (elongation) (🔺L=L1-L2)
Tensile test -> force vs elongation

Tensile specimen

Any size and shape, cross section same throughout (parallel lines)

Stress

Stress = Force / cross sectional area

Strain

Strain = Elongation / initial length

Shear stress

stress that occurs when forces act in parallel but opposite directions, pushing parts of a solid in opposite directions, causes sliding

Stress curve shape

Starts with gradual stretching of material, eventually ends with the specimens failure (marked with X)

Yield point

The point that separates the elastic and plastic regions of a stress/strain graph

Yield point shows

Yield stress or strength of a material

In the elastic region of a stress/ strain curve

Strain is proportional to stress. Follows hooked law. Behaves like a spring

If you remove the stress on a material in the elastic region

The strain goes back to zero

Hooked law formula

Sigma = E (Youngs modulus) x e

E (Young's Modulus/ elastic modulus)

Is equal to the slope of the stress/strain graph

Stiffness of a material

Indicates it's ability to return to its original shape or form after an applied load is removed

Axial stiffness formula

k = E x A (cross sectional area)/L(length of element)

Hydrostatic pressure

Occurs when a solid is subjected to equal compression on all sides eg. Deep in the earth's crust, ocean

Dilatation

The strain due to pressure is a change in volume

E is measured

By applying a tensile stress to the material

Most values have a equal value of E in both

Compression and tension

Poisson's ratio

Negative of lateral strain to tensile strain

Energy hysteresis

Energy lost in non-linear elastic unloading, equal to the area between the loading and unloading lines of a stress/strain graph

Lateral strain

Inward strain in the two directions perpendicular to the tensile direction

Elastic energy

Area under the line in a stress/strain graph

Buckling

When a long thin structure is loaded in compression and an abrupt shape change occurs

Bending is a form of deformation known as

Deflection

In a bent beam where is the strain maximum and zero

the strain is Maximum at the surfaces and zero at the centre (neutral axis)

Types of bending

Cantilever, three-point, four-point