The presentation focuses on the calculation of critical energy release rate for a given microstructure. By understanding the microstructure of a material and the strength of its components, we can determine its macroscopic fracture toughness. We will explore different scales, including the atomic scale of amorphous silica and periodic lattice structures, using numerical simulations. At each scale, we will demonstrate how local factors, such as micro-scale plasticity and the effect of microrotations, impact the effective fracture toughness. Our work shows that the critical energy release rate, which is a commonly used macroscopic measure of toughness, differs from the energy needed to create a new surface. To determine the elementary fracture properties used in phase-field simulations, we propose numerical homogenization techniques based on various microstructures.