Fracture toughness, i.e. the resistance of a material to fracture, often plays a key role in the design process of materials. Premature failure of engineering components due to material property degradation can cause damage in the order of millions of Euros or people to lose their lives. Prime examples of such degradation mechanisms are hydrogen embrittlement, liquid metal embrittlement and stress corrosion cracking. Those mechanisms influence material properties at the atomic scale and often cause a reduction of grain boundary cohesion in polycrystalline materials, resulting in premature, intergranular fracture.
K-tests are a widely-used approach for atomistic simulations of fracture. In a nutshell, these K-tests are numerical fracture toughness tests in which only the region close to a stressed crack tip is simulated. The remaining, not explicitly modelled part of the material is replaced by boundary conditions based on the theory of linear elastic fracture mechanics (LEFM). K-tests are frequently employed for studying the fracture behaviour of monoclinic single crystals and special tilt grain boundaries. Also for the treatment of more general (up to triclinic) grain boundaries and single crystals mathematical frameworks like e.g. the $6^{\text{th}}$-order Stroh or Lekhnitskii formalism are available. These theories are elegant due to their analytical formulation but also predict spatial oscillations in the relevant field quantities that can lead to unphysical self-interpenetration of cracks for grain boundaries that do not possess at least monoclinic material symmetry. For this reason, typically simplified ($4^{\text{th}}$-order) versions of the mentioned approaches are employed. Such $4^{\text{th}}$-order theories are by default restricted to certain material symmetry classes, are therefore simpler and do not exhibit the mentioned oscillatory field components.
For the investigation of the aforementioned degradation mechanisms, such $4^{\text{th}}$-order approaches and their symmetry requirements are, however, too restrictive. Taking liquid metal embrittlement as an example, it is well-established that grain boundaries with a wide range of orientations, that are not necessarily aligned with the symmetry planes of the underlying crystal structure, are of major importance for a full understanding of this complex phenomenon. Grain boundaries with such general orientations require an up to triclinic description and therefore a $6^{\text{th}}$-order formalism.
Therefore, the present work aims to contribute to the development of an accurate, yet simple K-test framework for the simulation of generally oriented grain boundaries and single crystals. To this end, we first re-visit the $6^{\text{th}}$-order Stroh formalism and examine its applicability. We investigate in which cases the oscillations occur and quantify their influence on the relevant field quantities for a broad range of cubic crystals. Next, we examine the impact of the choice of LEFM approach ($6^{\text{th}}$- or $4^{\text{th}}$-order) on the K-test results. For this, Fe and Cu are considered as representative examples of body-centered cubic and face-centered cubic metals, respectively. Finally, we propose a K-test simulation strategy that includes crack tip tracing and is unambiguous regarding the determination of critical stress intensity factors of generally oriented grain boundaries and triclinic single crystals.