An \textsf{AT1} phase-field framework for quasi-static anti-plane shear fracture: Unifying $ξ$-based adaptivity and nonlinear strain energy density function

This work introduces a novel \textsf{AT1} phase-field framework for simulating quasi-static anti-plane shear fracture in geometrically linear elastic bodies. A key feature of this framework is the unification of $ξ$-based local mesh adaptivity -- where $ξ$ represents the characteristic length of the damage zone -- and an algebraically nonlinear strain energy density function. A modified Francfort-Marigo energy functional, together with its Ambrosio-Tortorelli-type regularization, is hereby proposed to address challenges within the framework of nonlinearly constituted materials. We dynamically optimize $ξ$ throughout the simulation, significantly enhancing the computational efficiency and accuracy of numerically approximating the local minimizers of the Ambrosio-Tortorelli (\textsf{AT1})-type phase-field model. The proposed regularization for the total energy functional comprises three distinct components: a nonlinear strain energy, an evolving surface energy, and a linear-type regularization term dependent on the length scale of the damage zone. Variational principles applied to this novel energy functional yield a coupled system of governing second-order quasilinear partial differential equations for the mechanics and phase-field variables. These equations are subsequently discretized using the conforming bilinear finite element method. The formulation is underpinned by four crucial parameters: two are integral to the nonlinear strain energy function, while the other two serve as penalty parameters. These penalty parameters are asymptotically calibrated and rigorously utilized in the numerical simulations. Our results demonstrate that this spatially adaptive approach leads to enhanced mesh adaptivity, ensuring the robust convergence of the numerical solution.