ESDU 72014 gives curves showing the magnitudes and directions of the principal stresses and principal strains as a function of the reading of strain gauge rosettes in equiangular or rectangular layout. The curves for stresses are for Poisson's ratio of either 0.29 (for steels) or 0.33 (for aluminium alloys), but the equations on which they are based are included for use with other values. Also given are the relationships between direct stresses, maximum shear stresses and their corresponding strains, and they apply to any material provided they do not interrelate stress and strain; where stress and strain are interrelated the equations apply to elastic isotropic materials. For strain gauges set at any angles, the Mohr's circle graphical method is described and illustrated to show how principal strains are obtained.
