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Subsubsection 4.1.1.1: Transversely Isotropic Materials Up Subsection 4.1.1: Specifying Fiber Orientation or Material Axes Subsection 4.1.2: Uncoupled Materials

#### 4.1.1.2 Orthotropic Materials

For orthotropic materials, the user needs to specify two fiber directions and . From these FEBio will generate an orthonormal set of material axes vectors as follows: The vectors and are defined using the mat_axis element. This element takes a type attribute, which can take on the following values:
 Value Description local Use local element numbering (1) vector Specify the vectors and directly. (2) angles Specify the angles and [deg]. (3)
1. When specifying local as the material axis type, the value is interpreted as a list of three local element node numbers. When specifying zero for all three, the default (1,2,4) is used.
<mat_axis type="local">0,0,0</mat_axis>

2. When using the vector type, you need to define the two generator vectors and . These are specified as child elements of the mat_axis element:
<mat_axis type="vector">
<a>1,0,0</a>
<d>0,1,0</d>
</mat_axis>

3. When using the angle type, you need to define the two angles and in degrees. These are specified as child elements of the mat_axis element:
<mat_axis type="angles">
<theta>0</theta>
<phi>90</phi>
</mat_axis>

The material axes are related to the global Cartesian basis (or local element axes) via

Subsubsection 4.1.1.1: Transversely Isotropic Materials Up Subsection 4.1.1: Specifying Fiber Orientation or Material Axes Subsection 4.1.2: Uncoupled Materials