Key Engineering Materials Vols. 353-358

Abstract: Recently, applications of integrated large composite structures have been attempted to many structures of vehicles. In order to improve the cost performance and reliability, it is necessary to judge the structural integrity of composite structures. Fracture simulation techniques using FEM have been developed for the purpose. Since a number of iterations of finite element analysis are required in the fracture simulation, the simulation techniques consume many memory resources and much calculation time. In this study, a personal computer cluster (PC cluster) and the domain decomposition method were incorporated into a fracture simulation system. Calculations using a Windows PC cluster were carried out to confirm the efficiency of the proposed simulation system. As a result, it is concluded that adopting the domain decomposition method and the computer cluster is remarkably efficient to reduce calculation time.

Abstract: Temperature induced change, and stress induced change as well, in intrinsic timescale were investigated by nonlinear creep tests on poly(methyl methacrylate). With four different experimental temperatures, from 14 to 26 degrees centigrade, time-dependent axial elongations of the specimen were measured at seven different stress levels, from 14 MPa to 30 MPa, and modeled according to the concept of time-temperature-stress equivalence. The test duration was only 4000 seconds. The corresponding temperature shift factors, stress shift factors and temperature-stress shift factors were obtained according to the time-temperature superposition principle (TTSP), the time-stress superposition principle (TSSP) and the time-temperature-stress superposition principle (TTSSP). The master creep compliance curve up to about two-year at a reference temperature 14 degrees centigrade and a reference stress 14 MPa was constructed by shifting the creep curves horizontally along the logarithmic time axis using shift factors. It is shown that TTSSP provides an effective accelerated test technique in the laboratory, the results obtained from a short-term creep test of PMMA specimen at high temperature and stress level can be used to construct the master creep compliance curve for prediction of the long-term mechanical properties at relatively lower temperature and stress level.

Abstract: An experimental study of microstructure and mechanical properties in the Al2O3 particulate reinforced 6061 Aluminum composites has been used to determine the effect of extrusion and particle volume fraction (20, 26, 30, 40, 50, 60%Vf) in deformed metal matrix composites. The microstructure of Al2O3 /6061Al composite before and after hot extrusion is investigated by TEM and SEM. Results show that dislocation and subgrain generated after hot extrusion as well as the particle distribution of composite become more uniform with extrusion ratio of 10:1. The ultimate strength, yield strength and elongation of the composite also increase after hot extrusion. Dispersion strengthening and subgrain boundary strengthening is discussed and also the effect of precipitate introduced by heat treatment both after casting and after extrusion. The yield stress (0.2% offset) of the composites has been calculated and predicted using a standard dislocation hardening model. Whilst the correlation between this and the measured value of yield stress obtained in previous experimental test is reasonable.

Abstract: This study was performed to evaluate the effects of micropowders content on the compressive strength of cement-based composite pastes. Granular ground blast slag (GGBS), grade I fly ash (FAI), ultra fine fly ash (UFA) and silica fume (SF) were selected as the micropowders. The compressive strength was tested after 3d, 7d and 28d curing. Results show that the paste containing both UFA and SF has the highest packing density and corresponding excellent compressive strength. The higher the packing density is, the better the compacting and filling effect of micropowders will be, which contributes to developing higher compressive strength of cement-based pastes.

Abstract: The rheological parameters of cement pastes were investigated by varying the type and content of micropowders and the ratio of water to binder. Compressible packing model was used to calculate the packing density and to evaluate the influence of micropowders gradation on the rheological properties of fresh cement pastes. Results indicate that the higher the packing density is, the lower the yielding shear stress and plastic viscosity will be. When the ratio of water to binder is less than 0.20, the cement paste with 15% UFA and 15% SF has highest packing density and lowest yielding shear stress and plastic viscosity, which is beneficial to the workability of ultra-high performance concrete.

Abstract: SiCp/Cu composites were fabricated at 700 oC and under 40MPa for 10min in vacuum by electric conduction sintering. The mechanical properties and microstructure of the composites were studied. The porosity of the composites increases when the volume fraction of SiCp exceeds 35%. The flexural strength of the composites decreases with increasing fraction of SiCp. The composites with 10 vol.% SiCp exhibited ductile fracture behavior, while composites with the volume fraction of SiCp in the range of 20–65% exhibited brittle fracture behavior. The composites with 35 vol.% SiCp showed the highest hardness.

Abstract: The tension-tension fatigue tests for SiC/SiC composites were performed under the conditions that the maximum load Pmax was 80-90% to the fracture load of the tensile tests and the stress ratio was Rσ = 0.5. The composites exhibited a width in stress-strain hysteresis loop under one load cycling. In some cases the mean strain εmean gradually increase with increasing in number of cycles. These variations would reflect the developments of the fatigue damage at the fiber/matrix interface during the cyclic loading process. The pull-out lengths of the fibers for the fatigued- and not fatigued-specimens were measured through the SEM observations after the tensile test. In all materials, the average pull-out length of fibers in fatigued material was larger than in not fatigued material because the cyclic loading affected on the fiber/matrix interfacial strength.

Abstract: The solvent debinding processes of wax-based binders in powder injection molded Ti(C,N)-based cermets were studied. Effects of debinding temperature, debinding time, sample thickness, solvent types and solvent concentration on binder removal were investigated. Also, the dimension change of samples during the solvent debinding process was studied. It can be found that upwards of 70% paraffin waxes were removed and open pore channels were formed basically when the samples were immersed in heptane for 6 h at 50 °C.

Abstract: It is attractive to use multi-wall carbon nanotubes (MWNTs), which has a high modulus and strength, to enhance the properties of metal matrix composites by hybrid strategy. In this paper, novel aluminum borate whisker (ABOw) and MWNTs hybrid composites were prepared by squeeze cast technique and the properties were investigated. The results show that hybrid preforms may be made by co-deposition of ABOw and MWNTs in a solution by wet method. MWNTs separate the ABOw from whisker-to-whisker contact and decrease the compressive deformation of the hybrid preforms during squeeze cast processing. MWNTs distribute along mainly in the grains of aluminum matrix, making the mechanical properties of the hybrid composites higher than singularly reinforced composite.

Abstract: Tensile strength is an important material property and usually can be determined experimentally. The strain rate dependent behavior of T300 carbon/epoxy matrix composite was characterized over a wide strain rate range (10×10-5 s-1to10×104s-1). The low to moderate strain rate experiments were carried out on a MTS machine, while the high strain rate experiment was conducted with a split Hopkinson tensile bar. A rate dependent model was introduced to simulate the material response. Two kinds of stacking sequence of composite specimens [(45/-45)4]s and [(0/45/90/-45)2]s were tested at different strain rates, and the results were used to determine parameters of the model. The predictions of the model showed to agree fairly well with the experimental results. The tensile strength and initial elastic modulus of the composites increase when the strain rate increases.