The UTM machine was designed and manufactured in the 1960's and it had been last calibrated in 1983. Together with the fact that there was very little documentation we had to use totally new equipment for data collection and processing. The load cell consisted of a proving ring with a full-bridge strain gage arrangement. The strain dynamometer also has a full-bridge strain gauge arrangement but on a cantilever beam. This section briefly covers the theory behind stress and strain and their measurement.
Stress is defined as force per unit area.
Strain is defined as the fractional change in length of the specimen. It shows that the curve linear within the elastic range of the material. Any strain within the elastic range is reversible if the stress is removed. When the stress exceeds the elastic limit the material starts to deform plastically. This phenomenon is known as yielding of the material and the point at which it starts yielding is called yield point.