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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.13, #6)
Non-Local Theory Solution for an Anti-Plane Shear Permeable Crack in Functionally Graded Piezoelectric Materials by Zhen-Gong Zhou; Biao Wang (pp. 345-367).
In this paper, the non-local theory solution of a Griffith crack in functionally graded piezoelectric materials under the anti-plane shear loading is obtained for the permeable electric boundary conditions, in which the material properties vary exponentially with coordinate parallel to the crack. The present problem can be solved by using the Fourier transform and the technique of dual integral equation, in which the unknown variable is the jump of displacement across the crack surfaces, not the dislocation density function. To solve the dual integral equations, the jump of the displacement across the crack surfaces is directly expanded in a series of Jacobi polynomials. From the solution of the present paper, it is found that no stress and electric displacement singularities are present near the crack tips. The stress fields are finite near the crack tips, thus allows us to use the maximum stress as a fracture criterion. The finite stresses and the electric displacements at the crack tips depend on the crack length, the functionally graded parameter and the lattice parameter of the materials, respectively. On the other hand, the angular variations of the strain energy density function are examined to associate their stationary value with locations of possible fracture initiation.
Keywords: crack; functionally graded piezoelectric materials; mechanics of solids; non-local theory
Temperature Effects on the Impact Behavior of Fiberglass and Fiberglass/Kevlar Sandwich Composites by Aaron Halvorsen; Amin Salehi-Khojn; Mohammad Mahinfalah; Reza Nakhaei-Jazar (pp. 369-383).
Impact tests were performed on sandwich composites with Fiberglass and Fiberglass/Kevlar face sheets subjected to varied temperatures. A number of specimens were tested at −50 to 120 °C temperature range and at 20, 30, and 45 J low velocity energy levels. Impact properties of the sandwich composites that were evaluated include maximum normal and shear stresses, maximum energy absorption, non-dimensional parameters (AEMP, PI, and RD), and compression after impact strength. Composite specimens tested have a urethane foam filled honeycomb center sandwiched between a variation of four layered Fiberglass and Kevlar/Fiberglass face sheets in a thermoset polymer epoxy matrix. Results showed that the impact performance of these sandwich composites changed over the range of temperature considered and with the addition of a Kevlar layer.
Keywords: temperature; impact behavior; compression; hybrid
Machinability of Metal Matrix Composites Reinforced by 3-D Network Structure by Shouren Wang; Haoran Geng; Bo Song; Yingzi Wang (pp. 385-395).
Metal matrix composites reinforced by three-dimensional (3-D) continuous network structure reinforcement (3DCNRMMC) are difficult to machine due to serious tool wear and poor surface roughness caused by the brittle and hard reinforcement which interpenetrate into ductile matrix. In order to achieve the approach of low cost of 3DCNRMMC, the machinability of it needs to be understood. The influences of three cutting parameters and volume fraction of reinforcement on cutting force were analyzed in detail. The results indicate that: (1) Due to the brittle phase(s) introduced into ductile matrix of composites, there is a large fluctuation of cutting force causing deterioration of machinability. The fluctuation ranges of cutting forces, initially increase rapidly with the increase of volume fraction of reinforcement and then decrease finally, are largest at the range of the volume fraction of 55–65%; (2) The influence of cutting parameters on cutting force is obvious. With the increases of cutting speed, cutting force decreases gradually unless cutting speed exceeds the value of 209 m/min. Cutting forces increase with increasing feed rate and depth of cut; (3) Owing to the large fluctuation of cutting force, there were some cratered surfaces caused by Si3N4 reinforcement pulling-out and flaking-off. Some brittle phase protruding from the machined surface caused the deterioration of machined surface.
Keywords: metal matrix composites; machinability; cutting force; machined surface topography
Oxidation Protective Multilayer CVD SiC Coatings Modified by a Graphitic B-C Interlayer for 3D C/SiC Composite by Shoujun Wu; Laifei Cheng; Wenbin Yang; Yongsheng Liu; Litong Zhang; Yongdong Xu (pp. 397-406).
A layered graphitic CVD B-C coating was introduced between two CVD SiC coating layers. Microstructure and chemical characterization of the CVD B-C and the hybrid SiC/B-C/SiC multilayer coating was performed using SEM, EDS, XPS and XRD. Oxidation protection ability of the coating for the C/SiC composite was studied using a thermogravimetric analyzer (TGA) in the isothermal mode and by measuring residual flexural strength. The layered graphitic CVD B-C coating middle layer reduced the maximum crack width in the CVD SiC coating. The hybrid SiC/B-C/SiC multilayer coating provided a better oxidation protection for C/SiC composite than a three layer CVD SiC coating due to coating crack control and sealing effects at temperatures up to 1,300°C for 900 min.
Keywords: CVD SiC; CVD B-C; C/SiC composite; oxidation
Experimental Investigations on Deformation and Fracture Behavior of Glass Sphere Filled Epoxy Functionally Graded Materials by X. F. Yao; T. C. Xiong; W. Xu; H. Y. Yeh (pp. 407-420).
In this paper, deformation and fracture behavior of glass sphere filled epoxy functionally graded materials (FGM) are numerically evaluated and experimentally studied. The fabrication of the FGM is described in detail, and the spatial gradation of elastic modulus and the microscopic structure in FGM are measured and analyzed. The deformation and fracture characterization of the FGM specimen with a crack oriented along the direction of the elastic gradient under three point bend are studied by the experimental and the finite element method. The influences of crack location at both the stiff and the compliant sides of the FGM specimen on crack initiation, deformation field and stress intensity factor are analyzed. The results are: (a) The neutral-axis in the FGM specimen under three-point-bending will shift toward the stiffer side; (b) The initial fracture load increases with the increase of elastic modulus at the crack tip; (c) The elastic gradients shield a crack on the compliant side and lower the stress intensity factor when compared to the one with crack on the stiff side. These results will be useful for better design and reliable evaluation of FGM.
Keywords: functionally graded material; fracture and deformation; stress intensity factor; digital speckle correlation method (DSCM); strain gauge