Appendix A.6 - Fiber Post-Failure Stiffness (FPSTIF) / Fiber Degradation Energy (FDE)

FPSTIF

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 ( e11 supf , e22 supf , e33 supf , g12 supf , g13 supf , g23 supf ) 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 ( e11 supf , g12 supf , g13 supf ) 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.

Note: In response to a fiber constituent failure, the current implementation of Helius PFA imposes an isotropic degradation of the fiber properties for unidirectional materials and an orthotropic degradation of the fiber properties for woven materials.

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.

FDE

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, e11 supf , g12 supf , and g13 supf 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.

Note: After fiber failure, the matrix is assumed to have failed (whether it has or not), and the properties of the constituents are no longer needed to compute failure. Therefore, the properties of the constituents are not updated after a fiber failure.

If the matrix constituent has failed prior to fiber failure, e22 supf , e33 supf , and g23 supf are degraded according to the matrix degradation energy, otherwise , , and are degraded according to the fiber degradation energy.

Note: If the fiber degradation energy is specified as too low, the properties of the composite will be instantaneously reduced (instead of gradually) to near zero when the fiber failure criterion exceeds 1.0. Refer to Appendix A.7 and the Theory Manual for more information.