Behavior of cracks in inhomogeneous materials

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.

 

 

Bimaterial specimen consisting of hard steel and soft ARMCO iron with 30° inclined interface. A configurational force ftip appears at the crack tip, and a series of configurational forces fΣ are induced at the interface.

 

 

 

 

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.

Design concepts for damage resistant materials

Development of new fracture resistant materials and components

 

Important publications

O. Kolednik, The yield-stress gradient effect in inhomogeneous materials. International Journal of Solids and Structures 37 (2000) 781-808.

N.K. Simha, F.D. Fischer, O. Kolednik, C.R. Chen, Inhomogeneity effects on the crack driving force in elastic and elastic-plastic materials. Journal of the Mechanics and Physics of Solids 51 (2003) 209-240.

N. K. Simha, F. D. Fischer, O. Kolednik, J. Predan, G. X. Shan, Crack tip shielding or anti-shielding due to smooth and discontinuous material inhomogeneities. International Journal of Fracture 135 (2005) 73-93.

O. Kolednik, J. Predan, N. Gubeljak, F.D. Fischer, Modeling fatigue crack growth in a bimaterial specimen with the configurational forces model. Materials Science and Engineering A519 (2009) 172-183.

O. Kolednik, J. Predan, F.D. Fischer, Cracks in inhomogeneous materials: Comprehensive assessment using the configurational forces concept. Engineering Fracture Mechanics 77 (2010) 3611-3624.

F.D. Fischer, J. Predan, P. Fratzl, O. Kolednik, Semi-analytical approaches to assess the crack driving force in periodically heterogeneous elastic materials. International Journal of Fracture 173 (2012) 57-70.

F.D. Fischer, J. Predan, R. Müller, O. Kolednik, On problems with the determination of the fracture resistance for materials with spatial variations of the Young’s modulus. International Journal of Fracture 190 (2014) 23−38.

 

 

Part of this research has been funded by the Austrian COMET Competence Center Programme via the COMET K2 Center for Materials, Processing and Product Engineering in Leoben. Cooperation partners Institute of Mechanics, Montanuniversität Leoben and University of Maribor.