**Deforming Solids **

Forces can be used to change the speed, direction and shape of an object. This section of Physics looks at using forces to change of shape of a solid object, either temporarily or permanently.

If a pair of forces are used to *squash* a material we say that they are *compressive* forces.

If a pair of forces is used to *stretch* a material we say that they are *tensile* forces.

**Tensile Stress, σ **

Tensile stress is defined as the force applied per unit cross-sectional area (which is the same as pressure).

This is represented by the equations:

The largest tensile stress that can be applied to a material before it breaks is called the ultimate tensile stress (UTS). Nylon has an UTS of 85 MPa whilst Stainless steel has a value of 600 MPa and Kevlar a massive 3100 MPa

**Stress is measured in Newtons per metre squared, N/m ^{2} or N m^{-2}**

**Stress can also be measured in Pascals, Pa**

A tensile stress will cause a tensile strain. *Stress causes Strain*

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**Tensile Strain, ε**

Tensile strain is a measure of how the extension of a material compares to the original, unstretched length.

This is represented by the equations:

Steel wire will undergo a strain of 0.01 before it breaks. This means it will stretch by 1% of its original length then break. Spider silk has a breaking strain of between 0.15 and 0.30, stretching by 30% before breaking

**Strain has no units, it is a ratio of two lengths**

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**Stress-Strain Graphs**

A stress-strain graph is very useful for comparing different materials.

Here we can see how the strain of two materials, **a** and **b**, changes when a stress is applied.

If we look at the dotted lines we can see that the same amount of stress causes a bigger strain in **b** than in **a**. This means that **b** will increase in length more than **a** (compared to their original lengths).