The Volume reference temperature Tref is the temperature at which there are no thermal strains in the fibers. As a default, the value is obtained from a Common model input. You can also select User defined to enter a value or expression for the temperature locally.

The fiber Temperature T is by default obtained from a Common model input. You can also select an existing temperature variable from a heat transfer interface, if any temperature variables exist, or manually enter a value or expression by selecting User defined.

From the Material list, select the Boundary material (the default) or any other material to define the fiber’s thermal expansion properties. In most cases, you would use the boundary material for the base material, and additional Material nodes without boundary selection as the fiber material.

Select an Input type to specify how the thermal strains in the fibers should be modeled. The default is Secant coefficient of thermal expansion, in which case the thermal strain in the fibers is given by

Here, the secant coefficient of thermal expansion α can be temperature-dependent.

When Input type is Tangent coefficient of thermal expansion, the thermal strain in the fibers is given by

where αt is the tangential coefficient of thermal expansion.

When Input type is Thermal strain, enter explicitly the thermal strain in the fibers dL as function of temperature.

In all three cases, the default is to take values From material. When entering data as User defined, enter the coefficient of the thermal expansion or the thermal strain in the fiber direction a, since thermal expansion is considered in the fiber direction only.

In all cases, the contribution to the thermal strain tensor is defined from the thermal strain in the fibers εth,fib, and the fiber direction a as

When Approximation in the parent Fiber node is set to Beam, or when it is set to Wire and the Fiber placement is set to Volume distribution, you can take local thermal bending of the fiber into account.

From the list, select Temperature difference in thickness direction or Temperature gradient in thickness direction. In either case, the values given refer to the thickness of the base material. Typically, you would thus enter the same data as in the Thermal Expansion node of the base material.

When Temperature difference in thickness direction is selected, enter the temperature difference ΔTz. This is the temperature difference between the top and bottom surfaces.

When Temperature gradient in thickness direction is selected, enter the temperature gradient T’ in the direction from the bottom surface to the top surface.

Physics tab with Fiber node selected in the model tree: