Yield stress, proof stress, toughness, Young's modulus, Hooke's law.

#### Hooke's Law

- "Stress is proportional to strain up to the
**PROPORTIONAL LIMIT**(1)" - This means that any increase in stress will bring about a proportional increase in strain up to the proportional limit.
- Written mathematically, the formula of Hooke's Law is:

\begin{align} E = \frac {\sigma}{\varepsilon} \end{align}

- $E$ is a constant. (
**Young's Modulus**[see below]) - $\sigma$ is stress (Pa)
- $\varepsilon$ is strain

#### Young's Modulus

- Young's Modulus is a measure of the
**stiffness**of a material. - It is also known as the
**Modulus of Stiffness/Elasticity** - This is the constant that is represented in Hooke's Law (see above)

#### Proportional Limit

- This is when an increase in stress
**no longer has a linear**relationship with an increase in strain. - Sometimes this is known as the
**elastic limit.**

#### Resilience

- This is the ability of a material to
**retain its shape**after being deformed. - It can be found through the area under the curve
**up to the proportional limit**.

#### Yield Stress

- This is the
**Stress Value**at the**Progressive Yield Point (3).** - This is when there is
**no relationship**between an increase in stress and an increase in strain. - Is a useful value as the UTS point occurs after some deformation where this only begins to deform.

#### Proof Stress

- An
**approximation**for the**Yield point/Elastic limit**for materials that don't have a definite one due to their structure. - To find the approximation, a
**tangent**is produced from the**Proportional Limit**. The points of intersection between the**graph**and the**tangent**defines the**Proof Stress**of the material. - The line may be moved slightly to accommodate for certain materials.
- Examples are
**0.1%/0.2% Proof Stress,**where the tangent is produced from a point slightly (0.001) to the right of the Proportional Limit.

#### Toughness

- "A material's ability to
**absorb energy**" - This is sometimes known as
**impact strength.** - This can be found by finding the
**Area**under the**ENTIRE curve.**

page revision: 2, last edited: 04 Oct 2011 01:21