Appendix A.4 - Argument #4: Principal Material Coordinate System

Helius PFA expresses constitutive relations and computes stress in the principal material coordinate system of the composite material. For unidirectional microstructures, the default principal material coordinate system is oriented with the '1' direction aligned with the fiber direction, while the '2' and '3' directions lie in the composite material's plane of transverse isotropy. However, in situations where it adds convenience or simplicity to the model creation process, you may change the orientation of the principal material coordinate system so that the '2' direction is aligned with the fiber direction, while the '1' and '3' directions lie in the composite material's plane of transverse isotropy.

For woven microstructures, the default principal material coordinate system is oriented with the '1' direction aligned with the fill tow direction, while the '2' direction corresponds to the warp tow direction, and the '3' direction corresponds with the out-of-plane direction. However, in situations where it adds convenience or simplicity to the model creation process, you may change the orientation of the principal material coordinate system so that the '2' direction is aligned with the fill tow direction, while the '1' direction corresponds to the warp tow direction. Additionally, you may change the orientation of the principal material coordinate system so that the '3' direction is aligned with the fill tow direction while the '2' direction corresponds to the warp tow direction.

The 4th argument is used to specify the orientation of the principal material coordinate system that will be used. The numerical value (1 or 2 for unidirectional materials and 1, 2, or 3 for woven materials) of the 4th argument specifies which of the principal material coordinate axes will be aligned with the fiber direction (for unidirectional composites) or fill tow direction (for woven composites). The availability of these alternatives provides you with more flexibility in specifying the orientation of the material plies within a section definition.

You should be aware that ANSYS outputs the composite average state of stress and strain in the coordinate system specified by the second argument; however, the constituent average states of stress and strain (stored in SVAR7, SVAR8, ..., SVAR90) are always output in Helius PFA's default principal material coordinate system. As an example, if the second argument is specified as 2, all composite average stress and strain states will be output in the local system you defined, with the local 2 direction corresponding to either the longitudinal axis of the fibers for unidirectional materials, or the fill axis for woven materials. However, all constituent average stress and strain states will be reported in the default principal coordinate system of the unidirectional or woven composite material.

Consider the following HELIUSPFA command that appears in an ANSYS input file representing a unidirectional microstructure:

HELIUSPFA, 9, 7, 5, 2, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0.1, 0.01
Note: The forth argument is assigned a value of 2. Therefore, this particular material will use a principal material coordinate system where the '2' axis is aligned with the reinforcing fibers, and the '1' and '3' axes lie in the composite material's plane of transverse isotropy.