Appendix A.12 - Argument #15: Fiber Post-Failure Stiffness Fraction

The 15th argument is a fraction used to define the damaged elastic moduli of the fiber constituent after fiber constituent failure occurs. Specifically, it is the ratio of the failed fiber constituent moduli to the unfailed fiber constituent moduli. For example, 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. 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, i.e., all six fiber constituent moduli are reduced by the same percentage, while the fiber constituent Poisson ratios remain unchanged.

Note: For woven composites, if the fiber post-failure stiffness is specified, the 5th argument (progressive failure analysis) must be set to a value of 2. If the 5th argument is set to a value of 1, the 15th argument will be ignored.

The value of the 15th argument can have a pronounced effect on the predicted progressive failure response of a multilayer composite structure. 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 the 15th argument 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.