Nearly all modern technical materials are inhomogeneous from its micro- or nanostructure. Often the material properties are intentionally varied, e.g., to combine high hardness at the surface and good toughness in the interior in a cutting tool. Understanding the behavior of cracks in inhomogeneous materials is, therefore, very important. New ideas to this topic have been gained by the application of the concept of configurational forces.
Configurational forces are thermodynamic forces that act on all types of defects (vacancies, dislocations, grain boundaries, voids, cracks) in materials. The configurational force vector shows, in which direction a defect would like to move in order to minimize the total energy of the system. The higher the gain in total energy is, the higher the driving force on the defect becomes. If several defects are present in a material, they interact with each other. Therefore, material inhomogeneities influence strongly the driving force on a crack. This has been called the material inhomogeneity effect. The magnitude of the crack driving force determines whether a crack in a structure can grow or not.
The configurational forces concept allows us to quantify the crack driving force, the growth rate of a fatigue crack, and the crack growth direction in materials with inhomogeneity in Young’s modulus, yield stress, and/or strain hardening exponent. Also the influence of (thermal) residual stresses can be assessed. The configurational forces are evaluated by post-processing after a conventional finite element stress and strain analysis.