Progress in Micromechanical Research of Fracture of Composite Materials

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Authors: Hideki Sekine, Shoji Kamiya
Abstract: The fiber bridging effect, which affects the crack extension resistance in fibrous composites, is discussed in the case of in-plane-shear mode crack extension parallel to fibers in unidirectional fiber-reinforced composites. We first make a model of the bridging of a single fiber and estimate the force acting on the crack surface through a bridging fiber. Then, introducing the stochastic process of fiber breakage, we obtain the quantitative relationship between the relative crack surface displacement and the equivalent cohesive stress which is the probabilistic expectation of forces acting on the crack surface through a large number of bridging fibers. We numerically simulate the crack extension behavior with the equivalent cohesive stress acting on the crack surface. Then the simulated results are consistent with the experimental results. We finally conclude that the in-plane-shear mode crack extension is greatly affected by the stochastic process of fiber breakage.
Authors: H. Suzuki, Hideki Sekine
Abstract: A probabilistic fracture model is introduced to clarify the influence of the fiber bundle-matrix interfacial condition on the fracture energy and fracture behavior of short-fiber-reinforced SMC composites. In this paper, we focus on the study of the influences of two parameters of the interfacial condition, i.e., the debond stress and the constant which governs the frictional forces acting on the debonding interfaces between fiber bundles and matrix in a debonding process, and then the influences of these parameters on the fracture energy and load-displacement curve are elucidated.
Authors: Makoto Katagiri, Akihiko Kumaki, Yoshito Izumi, H. Suzuki, Hideki Sekine
Abstract: By use of a probabilistic fracture model, a numerical simulation method for deformation and fracture behavior of whisker reinforced ceramics is developed first. A crack in whisker reinforced ceramics is regarded as the crack with a cohesive stress acting on the crack surface, and then the tension-softening relation is derived on the basis of a micromechanical study. After the numerical simulation method is constructed by incorporating the tension-softening relation in an FEM scheme, we simulate the load-load point displacement relationship for an edge-cracked bend specimen of a SiC whisker/alumina composite. The fracture toughness determined from the simulated maximum load is consistent with that obtained from experiment.
Authors: H. Suzuki, S. Kinugawa, Hideki Sekine
Abstract: On the basis of a micromechanical study, a method for evaluating load carrying capacity of notched CFRP laminates is developed. The damage zone at a notch tip in CFRP laminates is modeled as a fictitious crack with a cohesive stress acting on the crack surface. Then, applying the Weibull weakest link theory to the strength of surviving fiber bundles on the crack surface, we derive the relationship between the cohesive stress and the crack opening displacement, i.e., the tension-softening relation. By incorporating it in a BEM scheme, the load-displacement relationship is simulated. The simulated result for notched CPRP laminates is compared with experimental ones, and it is found that the simulated and experimental results of load carrying capacity are consistent.
Authors: Shoji Kamiya, Hideki Sekine
Abstract: Apparent fracture strength of notched fiber-reinforced composite laminates depends on the notch tip radius even if it is evaluated in terms of the local parameters such as the stresses at a notch tip or the stress intensity factors. Although numbers of phenomenological explanations have been made, this phenomenon has not yet been physically clear enough. In order to elucidate its key mechanism, our interest is here focused on the interlaminar crack extension from a notch tip in cross-ply laminates subjected to mode-I loading. We find a stochastically expected upper bound of interlaminar crack length due to the probabilistic breakage process of fibers in load-bearing laminas inside the delaminated zone. This upper bound, i.e., the critical length of interlaminar crack, is inherent to the laminate and corresponds to its notched strength. The well-known variation in apparent fracture strength of notched fiber-reinforced composite laminates with respect to the notch tip radius is clearly explained as the scale effect of this constant critical length in different displacement distributions ahead of notch tips of different radii.
Authors: Hideki Sekine, K. Yamada
Abstract: This paper concerns a micromechanical study of the tensile strength deterioration of short-glass-fiber reinforced thermoplastics by addition of a slight amount of inorganic agent. Tensile tests were conducted using short E-glass fiber reinforced polyamide 6 with a slight amount of the inorganic addition agents TiO2, ZnO and ZnS. It is found by the tensile tests that the tensile strength decreases with increasing the hardness of the inorganic addition agents, and scarcely depends on the amount of the inorganic addition agents. After measuring the pull-out length of glass fibers on the fracture surfaces of test specimens, the variation of the scale parameter of Weibull moduli is estimated from the cumulative probability of pull-out length. Finally, the tensile strength deterioration is numerically predicted using the data. The predicted values of tensile strength are consistent with the experimental ones.
Authors: T. Okabe, M Nishikawa, Nobuo Takeda, Hideki Sekine
Abstract: This paper examines the stress distribution around a fiber break in alumina-fiber reinforced aluminum matrix (Al2O3/Al) composites using finite element analysis and predicts the tensile strength using tensile failure simulations. In particular, we discuss the effect of the matrix hardening on the tensile failure of the Al2O3/Al composites. First, we clarify the differences in the stress distribution around a fiber break between an elastic-perfect plastic matrix and an elastic-plastic hardening matrix using finite element analysis. Second, the procedure for simulating fiber damage evolution in the Al2O3/Al composites is presented. The simulation incorporates the analytical solution for the axial fiber stress distribution of a broken fiber in the spring element model for the stress analysis of the whole composite. Finally, we conduct Monte Carlo simulations of fiber damage evolution to predict the tensile strength of the Al2O3/Al composites, and discuss the effect of matrix hardening on the tensile strength of the Al2O3/Al composites. Coupled with size-scaling analysis, the simulated results express the size effect on the strength of the composites, which is seen in experimental results.

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