This is a brittle fracture criterion. The material cracks once the damage parameter, $D$, has evolved from 0 to 1. The damage is defined as:

$D = \displaystyle{ \frac{1}{t_s} \int_0^{t} H(\bar{\sigma}_1 - \sigma_s) \cdot (\bar{\sigma}_1 / \sigma_s )^{\alpha_s} } \mathrm{d}t$

$\bar{\sigma}_1$ is defined as:

$\bar{\sigma}_1 = \sigma_1^{dev} - (1-\beta_s) \cdot p $

where $\sigma_1^{dev}$ is the maximum deviatoric principal stress and $p$ is the pressure. With $\beta_s = 0$, $\bar{\sigma}_1$ equals the maximum principal stress. Note that the damage only grows if $\bar{\sigma}_1 \geq \sigma_s.$

Crack propagation is controlled by a stress intensity criterion (if having node splitting activated). The stress intensity $K_I$ is estimated for the integration points surrounding the crack tip. The crack will propagate if $K_I > K_c$ (Modus I crack).