Stress and strain
Every stress produces a strain!
Stress
The previous
comment is true for elastic properties as well. A stress is a measure of the cause of the
deformation produced by a force:
Stress = Force per unit area the units for stress are Nm-2 or Pa.
Breaking stress
The maximum stress
that a material can stand before it breaks is called the breaking stress. There are two types of
breaking stress
(a) compressive breaking stress - the maximum squashing stress before
fracture
(b) tensile breaking stress - the maximum stretching stress before
fracture
Some examples of both of these are given in the following table
| Material |
|
Compressive breaking stress (MPa) |
|
Tensile breaking stress (MPa) |
| Steel |
|
552 |
|
400-800 |
| Rubber |
|
|
|
2.1 |
| Granite |
|
145 |
|
4.8 |
| Concrete |
|
21 |
|
2.1 |
| Oak |
|
59 |
|
117 |
| Porcelain |
|
552 |
|
55 |
| Bone (compact) |
|
170 |
|
120 |
| Nylon |
|
|
|
70 |
| Glass |
|
|
|
3.5-150 |
| Carbon fibre |
|
|
|
1000 |
| Cast iron |
|
|
|
30-140 |
Strain
Strain is a measure of the deformation produced by the
stress.
Strain = change in size/original size
Strain has no units as it is simply a ratio of two similar quantities
The
height of a granite column
The compressive breaking stress (F) of a material can be
used to work out the maximum height of a rock column that is possible on the surface of the
Earth.
Maximum pressure at the base = F =
rgh where
r is the
density of the rock.
Therefore for granite F = 145x10
6 and
r = 2500 kgm
-3 so h = 145x10
6/2500x9.81 = 5900
m
This is smaller than the height of many mountains but in a mountain the 'column' of
rock would be supported from the sides by the rest of the
mountain.
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