KKL (v2) model, which can be rewritten as a regular GD model with
$a(\alpha) = g(\alpha)$ and $w(\alpha) = 1-g(\alpha)$
where $g(\alpha) = 4(1-\alpha)^3 - 3 (1-\alpha)^4$
Karma, A., Kessler, D. A., and Levine, H. (2001).
Phase-field model of mode III dynamic fracture. Phys. Rev. Lett., 87(4):045501. doi:10.1103/PhysRevLett.87.045501