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 product's 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 2nd user material constant 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 2nd user material constant specifies which of the principal material coordinate axes will be aligned with the fiber direction (for unidirectional composites) or the fill tow direction (for woven composites). The availability of alternative orientations for the principal material coordinate system provides you with more flexibility in specifying the orientation of the material plies within a section definition.
You should be aware that MSC Nastran outputs the composite average state of stress and strain in the coordinate system specified by the second user material constant; however, the constituent average states of stress and strain (stored in SV8, SV9, ..., SV91) are always output in Helius PFA's default principal material coordinate system. As an example, if the second user material constant is specified as 2, all composite average stress and strain states will be output in the local system you define, 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 MATUDS entry that appears in a bulk data file representing a unidirectional microstructure.
MATUDS, 9001, MATUSR, hpfa, UMAT , REAL, 1, 2, 1, 0, 0, 0, 0, , , 0, 0, 0, 0, 0.1, 0.01, 0, , , 0, 0,
Note, the second user material constant 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.