A theoretical model to describe the influence of material microstructure on fatigue crack propagation

The evaluation of crack initiation, short crack growth as well as crack path at microscopic scale [1] are crucial issues for the safety assessment of macroscopically fracture-free structural components. As a matter of fact, the fatigue life of the component is mainly consumed in the early stage crack propagation. At the microscale, structural materials (ranging from brittle to ductile ones) are characterized by heterogeneous mechanical properties. Such inhomogeneities, which tend to yield local stress field fluctuations, might heavily affect crack initiation and short crack propagation [2]. In the present paper, the crack propagation at the material microscale is modelled by taking into account the spatial variability of mechanical characteristics of the material as well as the local stress field disturbance induced by properly-defined equivalent inclusions. Note that even in the simple case of an elastic material under uniaxial remote applied stress, the local stress field is multiaxial and highly non-uniform [3]. By adopting several crack extension criteria under mixed mode (such as the maximum principal stress criterion [4], the R-criterion based on the minimum extension of the core plastic zone [5] , etc.) the short crack path is determined, and a predictive law for fatigue crack growth rate is proposed. It is shown a strong microstructure dependence of the crack path arises in the short-crack regime, while the microstructure of the material does not influence the crack propagation for sufficiently long cracks. References [1] Suresh S. Crack deflection: implications for the growth of long and short fatigue cracks. Metallurgical Trans 1985 14A: 2375-2385. [2] Kitagawa H, Yuuki R, Ohira T. Crack-morphological aspects in fracture mechanics. Engng Fract Mechs 1975; 7:515-529. [3] R. Brighenti, A. Carpinteri, A. Spagnoli. Crack path dependence on inhomogeneities of material microstucture. Submitted to the Fracture and Structural Integrity journal, 2011. [4] Sih, G.C., (1974) Strain-energy-density factor applied to mixed mode crack problems. Int. J. Fracture 10, pp. 305-321. [5] Shafique, M.A.K., Marwan, K.K., (2004) A new criterion for mixed mode fracture initiation based on the crack tip plastic core region, Int. J Plasticity 20, pp. 55-84.