stress is a cause of strain or vice versa?

Let us try thought experiment. We clamp a steel bar in an extremely strong frame made of invar, which does not change size with temperature change. Now we heat or cool the steel bar. It cannot change its size, but it will experience compression or expansion stress. This example is not so very unreal, because continuous railway tracks experience just these effects. We can now say that the bar has not been strained, because its length is the same. Not so. Suppose we took an identical bar in free air and heated it, and then we compressed it in a vice until it was the same length as the invar frame, it would in every way be in the same condition as the bar in the invar frame, except that it has changed its length in the process.

The conclusion must be that strain is not to be measured by the actual change in dimension: it must be related to the change in dimension that would have occurred if the bar had not been constrained.

Approved