ELASTIC-MATERIALS WITH COINCIDENT PRINCIPAL STRESS AND STRAIN AXES

In this note, a representation is derived for a Cauchy stress response function that is necessary and sufficient in order that a simple mate rial without memory be such that the principal axes of strain and stre ss always coincide. It is found that the material need not be isotropi c. However, if the material is in addition hyperelastic, so that the w ork done in any cyclic motion is zero, it is shown that the material i s necessarily isotropic.