Papers by Author: Jia Lin Tsai

Abstract: This study investigated the functionalization and layer number effects on interfacial thermal conductivity (ITC) of graphene nanocomposites through molecular dynamics simulation. The functional groups grafted to the graphene surface were carboxyl and amine groups. The ITC between the graphene and the surrounding epoxy matrix was examined, and the effects of the functional groups and layer number on ITC were characterized through vibrational density of states (VDOS). It was revealed that the VDOS mismatch between the epoxy and the outermost layer of graphene was reduced by the functional groups. Thus, the functional groups can effectively improve the ITC between the graphene and epoxy matrix. Moreover, when the layer number of graphene increases, the ITC in nanocomposites increases correspondingly. This is attributed to the fact that the inner layers of graphene may interact with the epoxy matrix and contribute the interatomistic energy in the interface.

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: The research is aimed to investigate the influence of spherical nanoparticles on the fracture behavior of glass fiber/epoxy composites. Two different contents of silica nanoparticles, 10wt% and 20wt %, were introduced into the composite samples. Through a sol-gel technique, the silica particles with diameter of 25 nm were dispersed uniformly into the epoxy matrix. Subsequently, the silica epoxy mixtures were impregnated into the unidirectional glass fiber mat by means of a vacuum hand lay-up process to form the unidirectional glass fiber/epoxy laminate. During the fabrication, a porous film was inserted into the mid-plane of the laminate to generate the pre-crack. The Mode I fracture toughness of the composites with different nanoparticles contents were then determined form the double cantilever beam (DCB) specimens. Based on the experimental observations, it was found that the glass fiber/epoxy composites with silica nanoparticles exhibit superior fracture toughness than those that do not contain any silica particles. Scan Electronic Microscopy (SEM) observations on the failure surfaces indicated that the enhanced fracture toughness could be due to the improved interfacial bounding in conjunction with the nanoparticle debonding from the surrounding epoxy. In general, such failure mechanisms may complicate the fracture process, dissipating more fracture energy.

Abstract: This research aims to investigate strain rate effect on the out of plane shear strength of unidirectional fiber composites. Both glass/epoxy and graphite/epoxy composites were considered in this study. To demonstrate strain rate effect, composite brick specimens were fabricated and tested to failure in the transverse direction at strain ranges from 10-4/s to 700/s. Experimental observations reveal that the main failure mechanism of the specimens is the out of plane shear failure taking place on the plane oriented around 30 to 35 degree to the loading direction. The corresponding out-of-plane shear strength was obtained from the uniaxial failure stress through Mohr-Coulomb strength analysis. In addition, the associated shear strain rate on the failure plane was calculated through the coordinate transformation law. Results show that the out-plane shear strength increases with the increment of the shear train rates. A semi-logarithmic function expressed in terms of the normalized shear strain rate was employed to describe the rate dependence of the out-plane shear strength.

Abstract: This research is aimed to fabricate the nano and micron particle reinforced composites as well as to understand the particulate size effect on the mechanical behaviors of the composites. The stiffness, strength and fracture toughness were investigated in this study. Spherical alumina particles with diameters of 5 microns and 10-20 nano meters were dispersed respectively into the epoxy resin using the mechanical mixer followed by the sonication. To measure the stiffness and strength of the composites, coupon specimens were prepared and then tested in tension. On the other hand, the fracture toughness was evaluated by performing three point bending tests on the single edge notch bending specimens. Experimental results revealed that the Young’s modulus of composites basically is not affected by particulate size; while, the tensile strength of the composites containing nano particles is higher than that with micron particles. From the fracture tests, it was indicated that the composites containing nano particles possess superior fracture toughness than the composites with micron inclusions.

Abstract: This research focuses on the fabrication of glass fiber/epoxy organoclay nanocomposites as well as on the investigation of organoclay effect on transverse tensile strength and in-plane shear strength of the nanocomposites. To demonstrate the organoclay effect, three different loadings of organoclay were dispersed respectively in the epoxy resin using a mechanical mixer followed by sonication. The corresponding glass/epoxy nanocomposites were produced by impregnating dry glass fiber with organoclay epoxy compound via a vacuum hand lay-up procedure. For evaluating transverse tensile strengths, the unidirectional coupon specimens were prepared and tested in the transverse direction. Results indicate that with the increment of organoclay loadings, the glass/epoxy nanocomposites demonstrate higher transverse tensile strength. On the other hand, the in-plane shear strengths were measured from [± 45]s laminates. It is revealed that when the organoclay loadings increase, the in-plane shear strength of glass/epoxy nanocomposites also increases appropriately. Scanning Electron Microscopy (SEM) observations on the failure surfaces indicate that the increasing characteristics in transverse and in-plane failure stresses may be ascribed to the enhanced fiber/matrix bonding modified by the organoclay.

Abstract: This research aims to investigate strain rate effect on transverse compressive strength of unidirectional fiber composites. Both glass/epoxy and graphite/epoxy composites were taken into account in this study. To demonstrate strain rate effect, composite brick specimens were fabricated and tested to failure in the transverse direction at strain rate ranges from 10-4/s to 500/s. For strain rate less than 1/s, the experiments were conducted by a hydraulic MTS machine. However, the higher strain rate tests were performed using a Split Hopkinson Pressure Bar (SHPB). Experimental observations reveal that the transverse compressive strengths increase corresponding to the increment of the strain rates. A semi-logarithmic function was employed to describe the rate sensitivity of the transverse compressive strength. SEM photographic on the failure surfaces depicts that for glass/epoxy composites, the failure mechanism is mainly due to the matrix shear failure, however, for the graphite/epoxy composites, it becomes the fiber and epoxy interfacial debonding which could dramatically reduce the transverse compressive strengths of the fiber composites.

Abstract: This research is aimed to fabricate glass fiber/epoxy nanocomposites containing organoclay as well as to understand the organoclay effect on the in-plane shear strength of the nanocomposites. To demonstrate the organoclay effect, three different loadings of organoclay, were dispersed in the epoxy resin using mechanical mixer followed by sonication. The corresponding glass/epoxy nanocomposites were prepared by impregnating the organoclay epoxy mixture into the dry glass fiber through a vacuum hand lay-up process. Off-axis block glass/epoxy nanocomposites were tested in compression to produce in-plane shear failure. It is noted only the specimens showing in-plane shear failure mode were concerned in this study. Through coordinate transformation law, the uniaxial failure stresses were then converted to a plot of shear stress versus transverse normal stress from which the in-plane shear strength was obtained. Experimental results showed that the fiber/epoxy nanocomposite exhibit higher in-plane shear strength than the conventional composites. This increased property could be ascribed to the enhanced fiber/matrix adhesion promoted by the organoclay.