Chapter 9. Measurement of Strain
A particular case of length measurement is the determination of strains, that is, the small changes in the dimensions of solid bodies as they are subjected to forces. The emphasis on such measurements comes from the importance of knowing whether a structure is strong enough for its purpose or whether it could fail in use.
The interrelation between stress (the force per unit area) and strain (the fractional change in dimension) is a complex one, involving in general three dimensions, particularly if the material concerned is not isotropic, that is, does not have the same properties in all directions. A simple stress/strain concept is of a uniform bar stretched lengthwise, for which Young's ...