The effect of assuming the principal directions in neutron diffraction measurement of stress tensors

In general, the components of stress in any three perpendicular directions can be determined by measuring the strains in those three directions. There is, however, no way to tell from the data if these are the principal stresses. The magnitude of the principal stresses will depend on the magnitude of the shear stresses that were neglected. It is shown that selecting the incorrect directions for the principal stresses leads to an error not in the components of stress measured but in the assumption that they are the principal stresses.