Fatigue crack growth from a cracked elastic particle into a ductile matrix

Abstract

Fatigue crack growth from a cracked elastic particle into a ductile matrix Groh, S.; Olarnrithinun, S.; Curtin, W. A.; Needleman, A.; Deshpande, V. S.; Van der Giessen, E. The monotonic and cyclic crack growth rate of cracks is strongly influenced by the microstructure. Here, the growth of cracks emanating from pre-cracked micron-scale elastic particles and growing into single crystals is investigated, with a focus on the effects of (i) plastic confinement due to the elastic particle and (ii) elastic modulus mismatch between the reinforcement and matrix phases. Due to the small sizes of the particles and cracks, plasticity in the ductile crystal is modelled using a 2D discrete dislocation plasticity framework wherein dislocations are modelled as line singularities in an isotropic elastic isotropic material. Crack growth is modelled using a cohesive surface. Calculations reveal a threshold for fatigue crack growth and a transition to Paris power-law behavior, both depending on the existence of the elastic particle and the modulus mismatch. For a matched-modulus particle, the threshold is reduced by 25% and is attributed to slip blockage by the particle. For a high-modulus particle, the threshold is reduced by 50% due to the enhanced stress intensity factor caused by elastic mismatch and due to some slip blockage. However, crack growth halts after some amount of crack advance due to the decreasing effect of elastic mismatch and slip blocking as the crack moves away from the particle. The broad results here are compared with experimental observations in the literature, and are consistent in a number of respects. These results show that fatigue crack growth from micron-scale particles is strongly influenced by plasticity size effects, elastic mismatch, and particle constraints on plastic flow, all of which are captured within a discrete dislocation plasticity framework.

Mechanical, thermal, and physical properties of Mg–Ca compounds in the framework of the modified embedded-atom method

“…After a transition regime, the velocity saturates to about 2 nm/ps independently of the applied temperature. In addition to being in agreement with the scaling predicted by the phonon damping theory (Hirth and Lothe, 1992), the behavior of the edge dislocation velocity as a function of the applied shear stress predicted using the Ca MEAM potential is also in good agreement to the one predicted in materials crystallized in fcc crystal structure using semi-empirical potentials (Olmsted et al, 2005;, and classically used to described the dislocation motion in the framework of dislocation dynamics (Groh et al, 2003(Groh et al, , 2008Liu and Groh, 2013). Finally, using a least squares procedure, the velocity data reported in Fig.…”

Failure of metals II: Fatigue

“…By and large, bottom-up approaches to crack formation at the scale of nucleant grains/phases do not attempt full field polycrystalline solutions but are rather based on apparent Schmid factors that do not consider intergranular stress and strain field interactions [74,78] or realistic dislocation substructures (cf. [81,82]). Since inelastic strain at the grain level under HCF conditions is highly dependent on local stress, the estimates of grain-to-grain interactions are deemed relevant.…”

Hybrid discrete dislocation models for fatigue crack growth

“…Here, we briefly summarize the results from a series of crack growth analyses under cyclic loading conditions using discrete dislocation plasticity [16][17][18]13,20]. Plastic deformation is described through the motion of large numbers of discrete dislocations, which are treated as singularities in an isotropic elastic solid.…”

“…A brief overview of the theoretical framework is presented; background and further descriptions are given in [16][17][18]13,20] and references cited therein. Initially, the crystal is assumed to be free of mobile dislocations, but to contain a random distribution of dislocation sources and point obstacles.…”