Engineering Rock Mechanics

Appendix A:

Stress and strain

a nu I ys is

Stress analysis

We do not talk about 'internal forces' when we are dealing with solid
bodies, we refer to stresses instead. The reason is simple. Consider a stack
of concrete blocks of different sizes supporting a heavy weight.

Heavy weight

W

This block supports W

The area it presents to the weight is 4ab

These blocks support W12 each

The area they present to the weight is 2ab each

These blocks support W/4 each

The area they present to the weight is ab each

As we can see, when the size of the solid body being considered changes,
so the force changes. But if we use stress, defined as

force

stress =-

area

then we can see that in the example above each block is subject to a stress
of W/hb-stress is independent of block size. Hence, if we were to divide
a solid body into 'elements', providing we work in terms of stresses the size
of the individual elements will not affect the stress values.