The FPSTIF field is used to define the response of the composite after a fiber failure when instantaneous degradation is used. Specifically, it is the ratio of the failed fiber constituent moduli to the unfailed fiber constituent moduli after fiber constituent failure occurs. For unidirectional materials, a value of 0.01 would specify that after a fiber failure occurs at a particular integration point, all six of the fiber constituent moduli ( , , , , , ) are reduced to 1% of the original undamaged fiber constituent moduli at the integration point in question. For woven materials, a value of 0.01 would specify that after a fiber failure occurs at a particular integration point, three of the fiber constituent moduli ( , , ) are reduced to 1% of the original undamaged fiber constituent moduli at the integration point in question. The fiber post-failure stiffness value must be greater than 0, and less than or equal to 1.
For woven composites, the PFTYPE field must be set to a value of 2 if the fiber post-failure stiffness is specified. If the PFTYPE field is set to a value of 1, the FPSTIF field is ignored.
The value of the FPSTIF field can have a pronounced effect on the predicted progressive failure response of a multilayer composite structure since this constant is largely responsible for the rate at which local loads are redistributed after a localized fiber constituent failure occurs. Consequently, as the value of FPSTIF is reduced from 1.0 toward 0.0, a local fiber failure is more likely to precipitate a cascade of localized fiber failures. Depending upon the magnitude of the fiber failure cascade, the result may be discernible as a noticeable softening of the overall structural response, or it may cascade without arresting and result in a global structural failure.
The FDE field defines the response of the composite after a fiber failure when energy-based degradation is used. Specifically, this value represents the total energy dissipated by the composite before and after a fiber failure.
If you request Energy-Based Degradation, this value represents the total energy of the composite before and after a fiber failure event. After a fiber failure event occurs, , , and are degraded linearly according to Appendix A.7 for post-fiber failure. As the composite strain increases beyond that at initial fiber failure, the composite properties are reduced according to the input fiber degradation energy, the composite strain at failure, the composite stress at failure, and the element volume. For more information on the fiber degradation energy, refer to the Theory Manual.
If the matrix constituent has failed prior to fiber failure, , , and are degraded according to the matrix degradation energy, otherwise , , and are degraded according to the fiber degradation energy.