Lessons · Engineering · strain: how much it stretches
Strain: how much it stretched, for its length
Strain is the change in length divided by the original length, ε = δ / L. It has no unit.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A strain gauge on the crane hook reads 800 microstrain and the inspector asks whether that is a lot. Without knowing what strain is, and what steel yields at, there is no answer, and the crane keeps lifting.
How to think about it
Put the stretch and the length in the same unit before dividing, and the answer is a pure number. Small numbers are normal: steel at working load is a few hundred microstrain, a few hundredths of a percent.
Worked example
ε = δ / LStretch over original length.
A 3 m tie rod stretches 1.5 mm under loadThe two lengths, in different units, which is the usual way they arrive.
ε = 1.5 mm / 3000 mm = 0.0005Same unit top and bottom, so the unit cancels.
That is 500 microstrain, or 0.05 %The three ways the same strain is written. A gauge reads microstrain; a spec may say percent.
Your turn
A 400 mm test piece grows by 0.2 mm. Write the strain.
ε = 0.2 / = 0.0005
Solve one, graded on the server
The trap
Dividing millimetres by metres. 1.5 mm over 3 m is not 0.5. Put both in the same unit and the answer is a pure number, as strain must be.