What is a strain gage?
Strain is the amount of stretch imposed on a material, expressed as a change from its original length. Typically, a pair of marks, called “gage marks”, are placed on the test specimen prior to the test. As load is applied, the change in gage length is noted. Strain is most easily pictured by imagining a rubber band. Imagine a relaxed rubber band, with two ballpoint pen marks on it, spaced 1 cm apart. If the rubber band is stretched so that the marks become 2 cm apart, then we say that the material has achieved 100% strain. With elastomers such as rubber, large strains, on the order of 500% or even more, can be achieved before breakage occurs. In most, less elastic, engineering materials, strains are much smaller, typically below about 3%. Percentage of original gage length may be used, but often the strains are so low that another system is used: microstrain. If you express the strain as a fraction of the original gage length, so that 100% strain is 1, then multiply the result by 1 mil
