Issue 20

R. Brighenti et alii, Frattura ed Integrità Strutturale, 20 (2012) 6-16; DOI: 10.3221/IGF-ESIS.20.01 7 happens to be comparable with the crack size, leading to the violation of the small-scale yielding hypothesis. The effects of the material microstructure on the crack growth at small scale [1] can be modelled by taking into account the non- uniform stress field induced by embedded inohomogeneities. Even for a uniform remote stress applied to the structural component, an oscillating stress field might develop at the microscale. In the present paper, by using both the solution of a homogeneous elastic infinite plane with a circular elastic inclusion and the superposition principle, the stress field for a regular arrangement of inclusions is determined, and the corresponding mixed mode Stress Intensity Factors (SIFs) are computed. Crack paths are evaluated by applying both the maximum principal stress criterion (the Sih criterion [2, 3]) and the minimum plastic zone extension criterion (the R- criterion [4, 5]). The trajectory described by the crack tip is computed through an incremental method, where the Mode I and Mode II SIFs of the kinked crack are approximately evaluated as a function of the SIFs related to a projected straight crack. Finally, some examples related to metallic alloys are examined. It is shown that small-scale fluctuations of the stress field heavily affect the crack path for short cracks while, after reaching a transition point during the crack propagation process, such an influence disappears for sufficiently long cracks. M ICROSTRESS FIELD INDUCED BY MATERIAL INHOMOGENEITIES tructural materials always present heterogeneity features due to either the composite nature of the materials (e.g. composite materials characterised by a matrix and a reinforcing phase; concrete-like materials having a cement- based paste with dispersion of aggregates of different sizes) or unavoidable inhomogeneities (e.g. metallic alloys composed by a base material and secondary inclusions), see Fig. 1. Due to such inhomogeneity characteristics, the stress field in the material at microscopic level might be non-uniform and multiaxial even if a uniaxial uniform remote stress is applied. A local fluctuation of the microstress field can play a crucial role in the crack path assessment for cracks having length comparable with a characteristic material length. (a) (b) Figure 1 : (a) Micrograph of pure iron with ferrite inclusions and crystals having a polygonal shape. (b) Typical concrete material with aggregates, cement paste and voids. The modelling rationale here adopted to describe the inhomogeneities contained in the material is based on a periodic distribution of spherical particles embedded in the base material. By considering, for the sake of simplicity, a single inclusion of radius embedded in an infinite plane under remote uniform stress (Fig. 2), the elastic stress field can be determined by applying the superposition principle together with the Kirsch solution [6]. The resulting stress field, , , x y xy    , is uniform within the inclusion, and can be expressed as a fraction of the remote applied stress 0 y  : 0 0 , , 0 x y xy x y y y k k           (1) On the other hand, the stress field , , x y xy    under plane stress condition in the region around the inclusion (see point P in Fig. 2) can be expressed as follows [6]: S