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Abstract: Fracture of solid materials is a complex multiscale process that essentially occurs due to macroscopic propagation of cracks formed by hierarchical evolution of atomic scale breaking of interatomic bonds. Because it depends only on the initial crack size and the applied stress, the critical stress intensity factor (Kc ) is commonly used as the single parameter to predict whether a mechanically stressed brittle material will fracture by crack propagation or not. In this study, we have shown that when a crack is highly constrained in a nanoscale brittle solid, Kc becomes gage-length dependent, hence, it may not be used as the single parameter for fracture prediction. Classical molecular dynamics simulations have been used to conduct fracture simulations of center-cracked brittle single crystal sodium chloride nanosolids. Infinitely rigid end tabs were modeled to generate boundary constraints on the specimen. The degree of constraints was varied by varying the gage length between the tabs. Our study reveals that for a given crack length, the fracture strain increases as the gage length decreases. Such gage-length dependency of failure strains suggests that the critical stress intensity factor, a measure of material fracture toughness, can be affected by boundary constraints. We have also measured the K-dominance zone size for different degree of constraints and found the K-dominance zone decreases as gage length decreases. It is thus inferred that Kc alone cannot fully capture the fracture force. [Copyright &y& Elsevier]

K-dominance of static crack tip in functionally gradient materials (FGMs) with a crack oriented along the direction of the elastic gradient is studied through coherent gradient sensing (CGS), digital speckle correlation method (DSCM) and finite element method (FEM). In the direction of crack propagation, the shear modulus has a linear variation with constant mass density and Poisson's ratio. First, the CGS and DSCM governing equations related to the measurements and the elastic solutions at mode I crack in FGMs are obtained in terms of the stress intensity factor, material constants and graded index. Secondly, two kinds of FGMs specimens and one homogenous specimen are prepared to observe the influences of the property variation on the K-dominance. Then, CGS and DSCM experiments using three-point-bending of FGMs and homogenous beams are performed. Thirdly, based on the results of the experiments, the stress intensity factors of three kinds of specimens are calculated by CGS and DSCM. Meanwhile, the stress intensity factors are obtained by FEM. Finally, comparing the results from CGS, DSCM and FEM, the K-dominance of mode-I static crack tip in FGMs is discussed in detail. [ABSTRACT FROM AUTHOR]

Abstract: This work describes the effect of material gradation (parallel to the crack plane) on stress intensity factors and K-dominance, i.e. the dominance of the singular region, of fracture specimens; SE(T), SE(B) and C(T). The extent of K-dominance is investigated by comparing the actual stress field with the Williams’ asymptotic stress field. Linear-elastic finite element analyses are performed using graded elements which incorporate graded material properties at the element level. Material gradation and crack geometry are systematically varied to perform the parametric study. Results reveal that the effect of material gradation on K I is most pronounced when a short crack is located on the stiffer side of the fracture specimen. For a given specimen and crack geometry, the extent of K-dominance yields a curve with a peak point at a certain material gradation. Results of the present study provide valuable insight into the K-dominance of FGMs. [Copyright &y& Elsevier]