These three equations express Hooke’s law in a general form for a triaxial state of stress. For application tensile stresses are considered positive quantities, and compressive stresses are negative. If a resulting normal strain is positive, it indicates that the material elongates, whereas a negative normal strain indicates the material contracts. If we now apply a shear stress to the element, Fig. 10–19a, experimental observations indicate that the material will deform only due to a shear strain ; that is, will not cause other strains in the material. Likewise, and will only cause shear strains and Figs. 10–19b and 10–19c, and so Hooke’s law for shear stress and shear strain can be written as