Module 3 - Mechanical properties of matter Flashcards
What is meant by stress
Force applied per unit cross-sectional area
What is meant by strain
Extension per unit length of material
What is the unit for strain
None
What is the equation for stress
σ = F/A
What is the unit for stress
Pascals
What is the equation for strain
ε = ΔL/L0
What is the relationship between stress and strain
- The greater the stress (force per unit area) the greater the strain (the extension per original length)
What is the proportionality between stress and strain and explain the constant
- σ α ε
- σ = kε
- This is a constant called the Young Modulus, which is a measure of a materials stiffness
What is the equation for Young Modulus in 2 main ways
- E = σ/ε
- E = FL/Ax
What is the unit for Young Modulus
Pascal, usually GPa
What is meant by stiffness
The extent to which a material under stress experiences strain
What is meant by the Young Modulus
The tensile stress per unit tensile strain
What is the gradient of a stress strain graph
The Young Modulus
What is the area under the stress strain graph within the limit of proportionality
- The energy density: The work done per unit volume to stretch a material
What is the UTS
The ultimate tensile strength, the maximum stress a material can withstand before fracture
In what region can you use the youngs modulus
The elastic limit
What is the Limit of Proportionality in terms of stress and strain
The value of the stress beyond which the stress is no longer proportional to the strain
Describe the plastic and elastic region in terms of stress
- Elastic: where the removal of stress will return the material to its original length
- Plastic: Removal of the stress will not return the object to its original length
What is the yield point
The point at which the strain increases with a small increase in stress
What is meant by a brittle material
A material that:
- Doesnt plastically deform
- low strain for large stress
Describe the stress/strain graph of a brittle material
Y = mx, where the loading and unloading curves are the same
Describe the stress strain graph for rubber
- Hysterisis loop
- No plastic deformation
What is meant by a ductile material
- Materials that have large plastic regions and so can be drawn into wire
What is a polymeric material
A material consisting of long chain molecules, that experience high strain before breaking
What is meant by a tough material
- A material that needs a large energy to break (greater area under stress strain graph)
What is meant by hard
A measure of the difficulty of scratching or denting a material
What is meant by a strong material
The extent to which a material resists breaking under stress
Describe the behaviour of rubber under stress
The stiffness of rubber gradually decreases as it is initially stretched. When the polymer chains have uncoiled fully, the rubber becomes stiffer.
