Advanced Materials Research Vols. 123-125

Abstract: Due to the high specific strength, high specific modulus and good fatigue characteristics, carbon/epoxy composite bicycle frame can have superior performance to aluminum bicycle. As the strength of composite frame varies according to the number of layers and stacking sequence, it is necessary to determine the appropriate number of layers and stacking sequence for designing light weight composite frame. The frame is divided into six parts, i.e. head tube, seat tube, top tube, bottom tube, seat stay, and chain stay parts. Since different stress levels are generated among the different parts, different numbers of layer with different stacking sequences are considered for each part. Three different loading conditions, i.e. horizontal loading, vertical loading, and pedal loading, are considered in the analysis. The superb stacking sequence and number of layers of the composite frame with the lowest failure index were determined which satisfy the three different loading conditions.

Abstract: Resin transfer molding is a popular process to fabricate polymer composites reinforced with a large amount of glass or carbon fibers. In general, fiber reinforcements are put in a mold, and a liquid resin such as epoxy resin is injected into the mold after preheating. For successful production of polymer composites via a resin transfer molding process, the filling and curing stages of the liquid resin as well as the mold design should be optimized. Recently, polymer composites reinforced with nanoparticles are attracting attention of researchers in academia and industries because efficient reinforcement can be achieved by small loading of nanoparticles such as carbon nanotubes and exfoliated clays. In this work, as an effort to develop light weight automotive parts, graphenes were investigated as a nano size reinforcement of epoxy resin for resin transfer molding. Graphenes were prepared from graphites by microwave irradiation. Addition of graphenes to bisphenol A based epoxy resins such as YD-128 from Kukdo Chemical results in an increase in viscosity and shear thinning behavior, affecting the filling process. The curing of epoxy resins is also affected by graphenes. In order to develop a model for simulation of the filling and curing of epoxy resins containing different amounts of graphenes in the resin transfer molding, FLUENT and MATLAB have been used in this study, which are a finite element based computational fluid dynamics analysis tool and a general purpose numerical analysis tool, respectively. The effects of graphenes on the mold filling pattern and curing profile are discussed for the resin transfer molding of bisphenol A based epoxy resins.

Abstract: The simulating software ANSYS is applied to carry out the coupling analysis of the 1-3 cement based piezoelectric composites. The interior displacement and strain distribution were obtained and discussed. The influences of material parameters such as thickness, relative dielectric constant, Poisson’s ratio and Yang’s modulus on the composites were analyzed based on the FEM model. The results indicate that active strain and displacement of the 1-3 cement based piezoelectric composites is mainly concentrated on the piezoelectric ceramic rods. The performance of the 1-3 cement based piezoelectric composites can be improved efficiently by increasing the thickness, Poisson's ratio, and reducing the Yang's modulus, the relative dielectric constant.

Abstract: Resin-rich zones are a common phenomenon in liquid composite molding processes. These resin-rich zones cause unwanted residual stress and deformation, and part-to-part variation, and thus they need to be studied in the design of composite structures. An experimental study on the formation of resin-rich zones in angled composite parts is presented in this paper. Two open-channel mold sets were designed and fabricated. Fiber preforms were loaded into these molds and the gaps formed were visually inspected by a microscope. The influences of corner radius, fiber volume fraction, enclosed angle, and stacking sequence were investigated, and significant factors affecting gap thickness were identified by Design of Experiments (DOE). It can be concluded from the experimental results that: 1) Fiber volume fraction is the most significant factor affecting gap thickness. Gap thickness is inversely proportional to fiber volume fraction; 2) Gap thickness is inversely proportional to radius; 3) The gap thickness of unidirectional preforms is larger than that of the cross-ply preforms.

Abstract: The flow stresses of AZ41M and ZK 60 wrought magnesium alloys under the deformation conditions of twin rolling casting and hot compression (TRC-HC) at different temperature and strain rates were studied. The deformation behavior and failure mechanism of them were discussed. The microstructure evolutions were analyzed by OM and EBSD technique. The results indicated that AZ41M and ZK 60 have different strain-stress curve under the same conditions. Working hardening results in occurrence of cracks in or around the shear bands. The recrystallized, equiaxed and fined grains in shear bands attribute to recovery and recrystallization, grains refinement causes local working hardening as well as decreases of crack tip driving forces. The stresses concentrate in shear bands causing cracks initiation and propagation. Casting defects to be the nucleus of cracks is another failure mode. With the increases of strains, dislocations rearrange forming sub-grains, the low angle grain boundaries (LAGBs) continuously evolved into high angle grain boundaries (HAGBs).

Abstract: The purpose of this paper is the application of Method of Fundamental Solutions (MFS) to the torsion problem of hollow rods made with functionally graded materials. This method belongs to so-called meshless methods. The proposal of the paper is to solve the problem by numerical procedure, which is proper combinations of the Method of Fundamental Solutions, the approximation by Radial Basis Functions (RBF) and Homotopy Analysis Method. The numerical experiment has been performed for the bar with circular cross-section.

Abstract: The present paper investigates interactions between a main crack and a surrounding layer of crazing patterns. Analysis of the stress field distribution as well as the energy induced during these interactions is based on the resolution of some equations along with appropriate boundary conditions and the use of a numerical approach. The effect of amplification and shielding on the resulting stress field is shown through a study of mode I Stress Intensity Factor (SIF). Besides, to quantify the effects of this damage on the main crack, it is shown that the Energy Release Rate (ERR) is defined as being a superposition of the energy released due to the linear propagation of the main crack as well as the one due to the translational change in the growth of the damage. It is also proven that crazes closer to the main crack dominate the resulting interaction effect and reflect an anti-shielding of the damage while a reduction constitutes a material toughness.

Abstract: This research aims to investigate the effects of particle size, volume fraction and dispersion on the tensile strengths of particulate nanocomposites with an embedded crack. The finite element micromechanical model in conjunction with linear elastic fracture mechanics (LEFM) was used to study the particle effect on the fracture behavior of nanocomposites. Results indicated that the tensile strength of particulate composites increases when the particle size is decreasing, however it can be deduced dramatically by the local aggregation of particles. The simulation results are in good agreement with the experimental observation. In addition, the predictions show that the tensile strength decreases with the increase of volume fraction of nanoparticles. So far, no consistent experimental data can be found to validate the above results and thus further study in this issue is required.

Abstract: In the present paper, we concentrate on the heterogeneous cement mortars and we treat them as Cosserat-based media. The autogenous shrinkage phenomenon at early age (from 1 up to 3 days after mixing) has been analyzed by means of Cosserat theory. The characteristic length scale parameter Lc in this theory helps us to change the size specimen from macro-scale to micro-scale using the theoretical size effect aspects. This methodology is also capable of treating cracks initiation and their appearance in the cementitious matrix surrounding the sand-inclusions, which should occurred inside of the Representative Volume Elementary (RVE) of mortar subjected to self-desiccation shrinkage during hydration at early age. By taking advantage of the Nonlinear Finite Element Analysis (NFEA), the numerical experiments have been performed. The numerical outcomes are well agreed with the experimental observations coming from Scanning Electronic Microscopy (SEM) images. It concludes that the inclusions create not only a hygro stress concentration around the grains but also the number of inclusions should influence the network in cementitous matrix.

Abstract: In the present study, we used computed tomography (CT) images of aluminum foam material obtained by an industrial volume CT system for a numerical analysis of the compressive behavior of the material. A three-dimensional model of aluminum foam was developed by stacking the CT images. The simulated and experimental compressive behaviors were compared. In particular, the simulated compressive behaviors were compared with those of experiments using various strain rates in order to verify the accuracy of the three-dimensional model. The results showed that the simulated compressive stress-strain curve displayed a similar tendency to that of the experiment, but had a higher compressive strength. The results also showed that the simulated compressive deformation pattern displayed a similar tendency to those of the experiments for all strain rates, and the deformation was significantly affected by the simulation of pore shape and size.