Effect of Material Orthotropy on the Directions of Principal Stresses and Strains

This paper discusses theoretical and experimental studies of the difference in directions of principal stresses and strains in orthotropic materials. It has been found that, for orthotropic materials such as fiber glass and boron composites, the directions of principal stresses and principal strains do not coincide. The angular difference between the two directions is a function of material orthotropy (the ratio of moduli of elasticity in the principal directions of the material), filament orientation, and, for biaxially loaded specimens, a function of the ratio of stresses acting in the two orthogonal directions. For fiber glass composites subjected to uniaxial stresses, a maximum difference of about 16 deg exists between the directions of principal stresses and strains. This phenomenon is verified experimentally, and agreement exists between experimental and theoretical results. The importance of this phenomenon, as related to experimental work, is discussed briefly.