Materials

Kerboodle textbook chapter 6

What is plastic deformation?

An irreversible change in the shape of an object due to a compressive or tensile force - removal of the stress or force produces permanent deformation

What is elastic deformation?

• A reversible change in the shape of an object due to a compressive or tensile force

• If you remove the stress or force the object will return the to its original shape and size (there is no permanent strain)

What does elastic mean?

When a material will return to it's original length when the load is removed

Describe elastic behaviour

The material has exceeded its elastic limit and it is now permanently deformed. It will no longer return to its original length.

What is the elastic limit?

The value of stress or force beyond which elastic deformation becomes plastic deformation, and the material / object will no longer return to its original shape and size when the stress or force is removed

State Hooke’s law

The force applied is directly proportional to the extension of the spring given the limit of proportionality is not exceeded.

What is the force constant?

• The force diveded by extension (or compression) for an object obeying Hooke's law

• It is called constant of proportionality k in Hooke's law, measured in Nm^-1

What is the the limit of proportionality?

The value of stress or force beyond which stress is no longer directly proportional to strain

What is a yield point?

A point on a stress-strain graph beyond which the deformation is no longer entirely elastic

Define ultimate tensile strength (UTS)

The maximum tensile stress that a material can withstand before it breaks

Define stiffness

The ability of an object to resist deformation

What is a polymeric material?

• A material comprising of long-chain moleules (polymers) that does not obey Hooke’s law

• May show large strains

• Polyethene and Rubber are examples.

What does brittle mean?

When a material shows very little plastic deformation before it brakes and deforms very little (if at all) under high stress

What does ductile mean?

When the material has a large plastic region in a stress-strain graph, so can be drawn into wires. This means the material undergoes a large plastic deformation before it breaks.

Define stress

The force per unit cross sectional area, measured in pascals (Pa)

Define strain

The ratio of extension to original length.

What is the unit of strain?

Strain does not have a unit as it is a ratio.

Define young’s modulus

The ratio of tensile stress to tensile strain when these quantities are directly proportional to each other

What is the unit of young’s modulus?

Pa or Nm^-1

What is the equation for young’s modulus?

E = σ/ε

Young’s modulus = tensile stress / tensile strain

What is the eqaution for tensile stress?

σ = F/A

Tensile stress = force / cross-sectional area

What is the equation for tensile strain?

ε = x/L

Tensile strain = extension / original length

What is a compressive force?

Two or more forces together that reduce the length or volume of an object

What is compressive deformation?

A change in the shape of an object due to compressive forces

What is compression?

The decrease in length of an object when a compressive force is exerted on it

What is are tensile forces?

Equal and opposite forces acting on a material to stretch it

What is tensile deformation?

A change in the shape of an obiect due to tensile forces.

What is a hysteresis loop?

• A loop-shaped plot on a stress-strain or force-extension graph.

• Obtained when, for example, loading and unloading a material produce different deformations

What does the area of a hysteris loop represent on a force-extension graph?

The energy absorbed or released by the material

What does the area of a hysteris loop represent on a stress-strain graph?

The energy absorbed or released per unit volume

What does the area under a loading curve on a force-extension graph represent?

The energy absorbed by the object during loading

What does the area under a unloading curve on a force-extension graph represent?

The energy released by the object during unloading

What does the area under a loading curve on a stress-strain graph represent?

The energy absorbed per unit volume by the object during loading

What does the area under a unloading curve on a stress-strain graph represent?

The energy released per unit volume by the object during unloading