<P> In material science and solid mechanics, orthotropic materials have material properties that differ along three mutually - orthogonal twofold axes of rotational symmetry . They are a subset of anisotropic materials, because their properties change when measured from different directions . </P> <P> A familiar example of an orthotropic material is wood . In wood, one can define three mutually perpendicular directions at each point in which the properties are different . These are the axial direction (along the grain), the radial direction, and the circumferential direction . Because the preferred coordinate system is cylindrical - polar, this type of orthotropy is also called polar orthotropy . Mechanical properties, such as strength and stiffness, measured axially (along the grain) are typically better than those measured in the radial and circumferential directions (across the grain). These directional differences in strength can be quantified with Hankinson's equation . </P> <P> Another example of an orthotropic material is sheet metal formed by squeezing thick sections of metal between heavy rollers . This flattens and stretches its grain structure . As a result, the material becomes anisotropic--its properties differ between the direction it was rolled in and each of the two transverse directions . </P>

If a material has three mutually perpendicular planes of elastic symmetry
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