Advanced Materials Research Vols. 139-141

Abstract: The mechanical behavior of polymer–matrix composites uniaxially reinforced with carbon or glass fibers subjected to compression/tension perpendicular to the fibers was studied using computational micromechanics. This is carried out using the finite element simulation of a representative volume element of the microstructure idealized as a random dispersion of parallel fibers embedded in the polymeric matrix. Two different interface strength values were chosen to explore the limiting cases of composites with strong or weak interfaces, and the actual failure mechanisms (plastic deformation of the matrix and interface decohesion) are included in the simulations through the corresponding constitutive models. Composites with either perfect or weak fiber/matrix interfaces (the latter introduced through cohesive elements) were studied to assess the influence of interface strength on the composite behavior. It was found that the composite properties under transverse compression/tension were mainly controlled by interface strength and the matrix yield strength in uniaxial compression/tension.

Abstract: The well-dispersed core-shell nanospheres (TiO2-g-PS) were synthesized via nano-TiO2 grafting polymerization and were used to prepare PS/TiO2 nanocomposites by melt blending. The tensile and thermal properties of the nanocomposites were measured by means of tensile test and thermogravimetric analysis (TGA). The fracture surface of the tensile test specimens was also explored through scanning electron microscopy (SEM). The reasults show that the tensile performance and thermal stability of the PS/TiO2 nanocomposites depend on the loading and state of the TiO2 in the PS matrix. The optimal properties of nanocomposites are achieved at 1.0wt% of TiO2-g-PS nanoparticles and the tensile strength is increased up to 43.6% comparing to the pure PS. The onset temperature at which 10% mass is lost is increased 10oC. The dispersion of the particles and interfacial adhesion lead to enhance these properties.

Abstract: The purpose of this study is to investigate the processibility of 6421 bismaleimide (BMI) resin for resin transfer molding (RTM) process and the mechanical performance of 6421 BMI composites reinforced by carbon fiber three-dimension (3D) and five-direction braided preforms. The thermal properties of 6421 BMI resin were investigated using differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). According to these data, the processing parameters and curing rule of BMI for RTM were derived. The flexural test results showed that 3D and five-direction braided composites had higher flexural modulus than that of laminated composites while the flexural strength was lower than that of the laminated composites. The impact strength of 3D and five-direction braided composites is 30.1% more than that of laminated composites. The results of mechanical properties tests indicate that carbon fiber 3D and five-direction braided/BMI composites are suitable for application.

Abstract: The tribological properties of PEEK composites filled with CaCO3 whisker in various content of 0-30%(weight percent) were investigated.The composite specimens were prepared by compression molding. Tribological testing of composites in water lubrication against carbon steel ring was carried out on a MM200 block-on-ring apparatus.Data on neat PEEK were also included for comparison. The morphologies of wear traces were observed by SEM.It was observed that inclusion of CaCO3 whisker significantly improved the wear resistance and reduced the friction coefficient of PEEK composites under water lubricated condition. With an increase in CaCO3 whisker contents, the specific wear rate of composites decreased and showed minima for 20% CaCO3 content as 5.2% of that for PEEK. In terms of friction applications under water lubrication, the optimal content of CaCO3 whisker in PEEK composites should be recommended as 15% to 20%.The SEM observation of the wear traces indicated that the transfer film is not the main reason for the enhancement of wear resistance of the composites in water lubrication.CaCO3 whisker contributed to increase the load-carrying capacity and abate the scuffing and adhesion of PEEK composites, finally led to a significant improvement of the wear resistance.

Abstract: Cu-based bulk metallic glasses (BMGs) with 5 mm in diameter were synthesized by copper mold casting. The effects of tin (Sn) addition on glass-forming ability (GFA), thermal stability of BMGs were investigated. It was found the addition of 4 at. % Sn is effective for an increase in GFA. The crystallization behaviors of Cu-based bulk metallic glasses during continuous heating are investigated mainly by differential scanning calorimeter(DSC).The results show that the characteristic temperatures(Tg,Tx and Tp) shifted to high temperature region with increasing of heating rates. The activation energies Eg, Ex and Ep, corresponding to characteristic temperatures Tg, Tx, and Tp, respectively, were obtained from Kissinger and Ozawa equations. The calculated activation energies agree well with Kissinger or Ozawa equations due to the almost invariability of the crystallization volume fraction at the characteristic temperatures for different heating rates.

Abstract: This paper presents a boundary element-free computational method for the fracture analysis of 2-D anisotropic bodies. The study starts from a derived traction boundary integral equation (BIE) in which the boundary conditions of both upper and lower crack surfaces are incorporated into and only the Cauchy singular kernal is involved. The boundary element-free method is achieved by combining this new BIE and the moving least-squares (MLS) approximation. The new BIE introduces two new variables: the displace density and The dislocation density. For each crack, the dislocation density is first expressed as the product of the characteristic term and unknown weight function, and the unknown weight function is approximated with the MLS approximation. The stress intensity factors (SIFs) can be calculated from the the weight function. The examples of the straight and circular-arc cracks are computed, and the convergence and efficiency are discussed.

Abstract: AZ91 magnesium matrix composite was fabricated by squeeze infiltration process. The aluminum silicate short fibers were used as reinforcement, and aluminum phosphate was adopted as binder, fabricating process was improved as well as reducing fabricating cost effectively. The optimum technique of squeeze casting, that was, preform-body temperature of 6600C, mould temperature of 5600C, pouring temperature of 7600C and pressure of 30~50MPa. The reaction products were investigated by optical microscopic, XRD and SEM. The results show that an ideal stronger interface is formed by the chemical reaction between magnesium alloy matrix and aluminum phosphate binder due to the produce of MgO particles and a little MgAl2O4 particles in the interface. AZ91 magnesium matrix composite is well-organized, and its grain size is significantly smaller than that of the matrix. The structure of AZ91 magnesium matrix composite is uniform without casting defects such as shrinkages and inclusions.

Abstract: In this experiment, the micro copper powders with high temperature of oxidation resistance were prepared by coating with 2 wt.% of silane coupling agent(KH560) solution in order to protect them from oxidation in isotropically conductive adhesives, and X-ray diffraction analysis(XRD), thermo gravimetric analysis(TGA) and accelerated aging test were applied to analyze the anti-oxidant property of the samples before and after they were treated. Experimental results showed that the polymer of silane coupling agent on the surfaces of copper powders had high thermal stability exposed under high temperature and was very effective in preventing the copper powders from oxidation in isotropically conductive adhesives. Besides, the epoxy conductive adhesive sample with plated copper as electrically conductive fillers still had good conductive property even exposed at 160 degrees with its bulk resistivity was 8.75×10-3Ω•cm.

Abstract: Sliding wear was investigated for ZrO2 ceramics against M50 bearing steel under vacuum condition. The research was studied on the ball-disk machine under different loads and steady sliding velocity. The worn surfaces of tested samples were examined using optical microscope, scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The results showed that the friction coefficient of ZrO2 was decided by the applied load, the critical load of ZrO2 was 10N applied load under vacuum condition, the specific wear rate of ZrO2 drop rapidly with exceeded applied critical loads. The ZrO2 ceramics materials have transferred on the M50 bearing steel surface under high vacuum condition. The dominant wear mechanisms of ZrO2 ceramics were mico-adhesion, tribochemical wear, brittle scraps and micro-furrow wear under vacuum condition.

Abstract: Three weaves (plain, twill 1/3 and satin 8/3) of hybrid kevlar/PTFE fabric composites were selected for investigating the influence of weaves on the tribological properties. Pin-on-flat linear reciprocating wear studies were done on the three kinds of composites on a CETR Tester under pressure 20 MPa and frequency 13 Hz. The relationship between the tensile strength and the tribolog- ical performance of fabric composites was also examined. It was observed that though the friction coefficient of satin 8/3 was the lowest, the specific wear rate was also the highest. Plain weave, however, performed worse in friction-reduction while resulting best in antiwear performance. The tensile strength does not have an influence on the wear performance. So the different tribological performance of the three weaves of fabric composites may attribute to the different PTFE proportion in the fabric surface and adhesive wear. A model of wear process of fabric composites was proposed and supported by the Confocal Laser Scanning Microscope (CLSM) studies.