Strain
You will also be able to find the amount of stretch or elongation the specimen undergoes during tensile testing This can be expressed as an absolute measurement in the change in length or as a relative measurement called "strain". Strain itself can be expressed in two different ways, as "engineering strain" and "true strain". Engineering strain is probably the easiest and the most common expression of strain used. It is the ratio of the change in length to the original length, Engineering strain formula. Whereas, the true strain is similar but based on the instantaneous length of the specimen as the test progresses,where Li is the instantaneous length and L0 the initial length.