Use an energy-based material degradation scheme for progressive failure.
Helius PFA's default method of imposing material degradation for intra-laminar material failure has been to instantaneously degrade the stiffness of each constituent material as shown in an earlier graphic (Damage States in Unidirectional Composite Materials). This method of instantaneous (or very rapid) degradation works well for relatively coarse meshes. However, if this same scheme is applied to very refined meshes, damage is predicted to evolve too rapidly, leading to premature global failure prediction. Generally speaking, the predicted progressive failure response exhibits considerable mesh sensitivity, with coarse meshes predicting global failure loads that are too high and refined meshes predicting global failure loads that are too low.
To mitigate this mesh sensitivity, later versions of Helius PFA provide an optional energy-based material degradation scheme. This energy-based material degradation scheme differs from the product's original material degradation scheme in two ways. First, in the energy-based degradation scheme, the rate of stiffness degradation is finite, not instantaneous. The finite rate that stiffness is decreased is directly related to the size of the element in which the material failure occurs. Generally speaking, as the size of the element decreases, the rate of stiffness degradation decreases. Conversely, as the size of the element increases, the rate of stiffness degradation increases. In contrast, in the product's original degradation scheme, the material degradation is instantaneous. Second, in the new energy-based degradation scheme, the final fully-degraded stiffness of the failed material is zero. In the original degradation scheme, the final fully-degraded stiffness of the failed material is not zero, but is a finite user-specified value.
Regardless of the stiffness degradation scheme employed in an analysis, you must supply a single material constant for each constituent material (fiber and matrix). However, the interpretation of these constants is different depending on the chosen degradation scheme. In the energy-based material degradation scheme, the two material degradation constants are interpreted as the total energy densities dissipated by the composite material due to failure of each constituent material. In the original default degradation scheme, the two material degradation constants are interpreted as dimensionless ratios that define the final fully degraded stiffness of each constituent relative to the original undamaged stiffness of each constituent.