8.9 Transformation of Strain; Mohr's Circle for Strain
Many problems encountered in engineering design involve a combination of axial, torsional, and bending loads applied to elastic bars. In such cases, the stresses may be computed as described in the preceding articles and the maximum normal or the maximum shear stress used a design structure is too complex analyzed in this manner, the stresses may have to be determined experimentally. Because stress is a mathematical abstraction, it cannot be measured directly. However, the stress-strain relationships defined by Hooke's law permit us to calculate stresses from strains, and strains can be measured. In this article, we derive the transformation equations for plane strain. If we know the strain components at a point associated with a given set of axes, these equations enable us to calculate the strain components with respect to any set of axes at that point. With this information, we are able to find the principal strains at the point. In this article, we also consider the conversion of strain measurements into stresses using Hooke's law.