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/m2 or N m-2
Stress can also be measured in Pascals, Pa
A tensile stress will cause a tensile strain. Stress causes Strain
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
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).
