Search

Molecular Dynamics (MD) simulations are a viable alternative to experimental methods to obtain mechanical properties of EPON 862-DETDA-SWCNTcomposites. This paper investigates the effect of SWCNT carbon vacancy defects on the Young’s modulus of the EPON 862-DETDA-SWCNT composite using MD simulations performed via Accelrys. For a composite with 7-12 weight% SWCNT, 2 carbon vacancy defects in the SWCNT is found to reduce the Young’s Modulus by 13-18%, while 4 carbon vacancy defects in the SWCNT reduced the Young’s Modulus of the composite by 21-30%. This clearly indicates that carbon vacancy defects are one potential cause of disparity, and lower Young’s modulus values of Epoxy-SWCNT composites cited in the literature.

The present study focuses on the prediction of mechanical properties of single-walled carbon nanotubes (SWCNT) reinforced epoxy resin (DGEBF) cross-linked with curing agent W (DETDA). The MD models of the reinforced epoxy were built using the amorphous module of Material Studio (Accelrys Inc.). The COMPASS force field was used in the simulations. The amorphous structure was achieved by using periodic boundary conditions and then subjecting to an energy minimization using an ensemble of the constant-volume and temperature (NVT). The structures were equilibrated for 100 picoseconds (ps) and then followed by MD equilibrations at room temperature for another 200 ps. Since at room temperature most of the atoms are in static mode, the atoms were excited using simulation temperatures above the glass transition temperatures. In an attempt of finding global energy minimum, simulated annealing runs were then carried out starting at elevated temperatures at atmospheric pressure using the ensembles of the constant number of particles, constant-pressure and constant temperature (NPT). The molecular structure temperature was then gradually lowered to a room temperature. Each subsequent simulation was started from the final configuration obtained at the preceding temperature. Density of the epoxy at each temperature was calculated from the average specific volume and glass transition temperature (Tg) was estimated based on the discontinuity in the slope of the density-temperature plot. The amorphous structures obtained at room temperature were analyzed to determine the fundamental mechanical properties of the SWCNT reinforced EPON 862-W. Calculations of fundamental mechanical material properties of single-walled carbon nanotube (CNT) were performed using molecular dynamics simulations via Material Studio. A simple but effective technique of extrapolation was adopted to compensate for the problem of CNT distortion because of smaller lattice sizes. Property calculations were performed at each density value and extrapolated to the actual value of density equal to 1.9 gm/cm 3 . A similar extrapolation technique was employed to overcome the issue of achieving exact theoretical

The present study). A quad oligomer of DEGBF cross-linked with DETDA was used for the calculations. A MD model of the epoxy was built using the amorphous module of Material Studio (Accelrys Inc.). The Polymer Consistent Force Field (PCFF) was used in the simulation. The amorphous structure was achieved by giving periodic boundary conditions and then subjected to an energy minimization using the ensemble of the constant-volume and temperature (NVT). The structure was equilibrated for 100 picoseconds (ps) followed by a MD equilibration at room temperature for another 200 ps. Since at room temperature most of the atoms are in static mode, the atoms were excited using simulation temperatures above the glass transition temperatures. In an attempt of finding the global energy minimum, simulated annealing runs were carried out starting at elevated temperatures and atmospheric pressure using the ensembles of the constant number of particles, constant-pressure and constant temperature (NPT). The temperature was gradually lowered to room temperature level. Each subsequent simulation was started from the final configuration obtained at the preceding temperature. Density of the epoxy at each temperature was calculated from the average specific volume and glass transition temperature (Tg) was estimated based on the discontinuity in the slope of the density-temperature plot. The amorphous structure obtained at room temperature was analyzed to determine the fundamental mechanical properties of EPON 862-W.

The macro-level material behavior depends on the interactions at the fundamental molecular level of the constituent materials. The different constituent phase material interactions in alumina particulate hybrid composites are modeled based on their molecular configurations to provide an insight into the material interactions at these lower length scales. Functionalization improves the bonding between the different material phase constituents. The interface interaction between the inorganic alumina, functionalized alumina (silane-alumina) and organic epoxy molecular model configurations are investigated and analyzed using molecular dynamics simulations. The studies indicate that silane functionalized inorganic alumina has higher affinity to the organic epoxy polymer compared to the untreated alumina based on the computed interaction interface energies obtained from the molecular dynamics simulations. This improved molecular adhesion and affinity of silane-alumina and epoxy-silane could potentially be attributed to the increase in fracture toughness values with functionalization observed in our experimental studies on the mode-I fracture behavior of the alumina nanoparticulate epoxy glass hybrid composite laminates.

[ABST RACT ] App ro ximately 70% of structural failures are due to fatigue. The fatigue mechanism in composites is much more complex than that of metals. The damage may be in one or more forms, such as a failure in a fiber -matrix interface, matrix cracking, delami nation (s), and /or fiber breakage. Matrix cracking and delamination both reduce stored energy and stiffness. Damage from these mechanisms can be detected very early in the fatigue life of a composite. This damage produces a reduction of elastic properties s uch as stiffness. There is always a correlation between damage and stiffness reduction. Fatigue in multidirectional laminates is a well understood phenomenon. It may be said that the damage process of multi -directional laminates has been clarified, at lea st qualitatively, if not quantitatively. Even though fatigue behavior is similar in woven composites, it is incorrect to apply the same concepts of the damage mechanism of multi -directional laminates to woven composites. This is because each ply in woven c omposites is itself bi -directionally reinforced by fiber bundles in the different directions. Recently, the use of woven composites for aerospace and other industrial applications has grown exponentially. These composites are typically manufactured using the low cost vacuum assisted resin transfer molding process (VARTM). With the large variations in fiber volume fraction, weave angle, thickness variations , an accurate estimation of the fatigue life prediction curve become s a challenging task. It is common knowledge that the ultimate strength of composite materials is dependent on the fiber volume fraction, thickness of the laminate and the warp weft angle. Most of the fatigue analyses today are based upon the average ultimate tensile strengths conducted p rior to fatigue testing . Variations in the se ultimate strengths of the composites can lead to extensive scatter in the fatigue data. This paper presents an innovative technique for the accurate prediction of the fatigue life in woven and braided composites . The technique uses a mu lti -variant analysis in conjunction with experimental strength data. This procedure first predicts an accurate tensile/compressive strength of a specimen and then uses these results to develop the fatigue life setup criteria . Resul ts indicate that this technique produces minimal scatter in the fatigue data, accurately predicts the tensile/compressive strengths as a function of the weave angle, the fiber volume fraction and the thickness of the specimen. This paper illustrates the ap plication of this newly developed technique in the estimation of the fatigue life in woven and braided composites.

Braided composites have good properties in mutually orthogonal directions; more balanced properties than unidirectional laminates and have better impact resistance. These composites are being considered as a primary load carrying components in place of conventional laminated composites. They are being manufactured by using new process such as Vacuum Assisted Resin Transfer Molding (VARTM). This new process is a low cost, affordable and suitable for high volume manufacturing environment. In VARTM process, the flow of resin occurs in-plane as well as in the transverse directions to the preform. The permeability of the preform, fiber architecture and fabric crimp has an influence on the wetting of the fabric. This paper addresses a detailed study of VARTM manufactured 2x2 biaxial braided composites. These composites are fabricated using AS4 carbon fibers and vinyl ester resin system components. These braided composites are being evaluated for structural applications. To assess the feasibility of this material manufactured through VARTM, it is very important to understand the tensile behavior of these composite materials. Numbers of tests are performed to evaluate the basic properties such as modulus, ultimate tensile strength, and Poisson's ratio of VARTM manufactured braided composites. This paper also presents, a finite element model for the 2 x 2 biaxial braided composites to predict the mechanical properties.

One of the driving forces for the increasing structural application of textile composites is the potential to tailor the material microstructure to meet specific thermomechanical requirements. In order to fully exploit this potential, thorough understanding of the effect of various factors on the behavior of the material is essential. This paper focuses on understanding the effect of braid angle and tow waviness on the effective moduli of 2x2 biaxial braided composites. To achieve this goal, full three-dimensional finite element micromechanics models were developed first. Then extensive parametric study was conducted for typical glass fiber/epoxy (S2/SC-15) and carbon fiber/ epoxy (AS4/411-350) material systems. The effects of tow waviness and braid angle on the effective engineering properties were determined for a wide range of braid angles and waviness ratios. Equivalent laminated materials with angle plies and a resin layer were also used to produce reference values for comparison. The results are presented and discussed. The transverse properties of the 2x2 biaxial braids are most sensitive to the change of braid angle and waviness ratio. The in- plane properties of the braid can be well predicted by the classical laminate theory for a range of small waviness for the carbon fiber/epoxy material.

Woven textile composites are replacing multidirectional laminates primarily because of their properties in mutually orthogonal directions as well as more balanced properties and better impact resistance. One of the affordable low cost woven composites is manufactured by using graphite fibers and SC-15 epoxy resin through resin infusion or resin transfer molding processes. Before these composites can be used in aircraft as primary load carrying structures, it is essential to investigate behavior of these laminates subjected to low velocity impact loading. This research addresses the progressive damage and deformation mechanics of thin and thick woven composites subjected to low velocity impact loading.

The present study provides the performance evaluation of 2x2 twill woven composite (S2-Glass and C-50 resin system) material for Integral Armor applications. The laminates were fabricated by two low cost processes: RTM (Resin Transfer Molding) and VARIM or RI (Vacuum Assisted Resin Infusion Molding). These components are expected to be under fatigue loading. Fatigue behavior of the twill woven laminate is presented. Tension-Compression (R = -1) fatigue experiments were performed for both RI and RTM panels. All the fatigue tests were performed at 1 Hz frequency. S-N diagram and stiffness degradation over the fatigue life of the specimen was obtained. The fatigue tests indicated that with the same volume of fibers and with more resin, RTM process improved the fatigue performance over RI process. The ratios of logarithm of fatigue life cycles of RTM and RI is almost constant. Therefore on the absolute scale the ratio keeps increasing at higher fatigue life or at lower fatigue loads. However, with the same volume of fibers and resin, the fatigue performance of RTM and RI processed panels were comparable. It was also observed that both RTM and RI exhibited classical three-stage degradation in stiffness.

Monolithic ceramics are highly sensitive to process and service related flaws, making them inherently brittle. Due to their low toughness, these materials fail catastrophically. Continuous fiber-reinforced composites usually provide a significant amount of toughness as well as avoid catastrophic failure. The interface between the fiber and the matrix can significantly affect the mechanical properties of the composite. A very strong interface yields brittle fracture as the crack propagates through the matrix and fiber. In contrast a very weak interface between the fiber and matrix is incapable of transferring the load from the matrix to the fiber and hence the properties of the fiber are not used effectively and the mechanical properties are similar to that of a porous material. Therefore the design and optimization of the interface properties are of vital importance for fabricating ceramic composites with superior mechanical performance. Before these coated ceramic composites can be used in primary structural applications, it is necessary to obtain the fundamental properties of these materials. This paper presents a study on the effects of fiber coating thickness and fiber volume fraction on the transverse mechanical properties of coated ceramic composites.

In this paper a systematic investigation was carried out on the microstructural evolution of a sintered Si3N4 ceramics associated with high temperature soaking in air. The soaking was conducted at 1275 °C for various periods of time, ranging from 16 to 40 hours. High resolution transmission electron microscopy (HRTEM) and conventional TEM were used to study the microstructures of the virgin (as received) and soaked samples. It was found that the Si3N4 ceramics has been severely attacked even inside the material, most probably by oxygen. The originally distinctly shaped Si3N4 grain boundaries and triple junctions became more rounded and the apparent volume fraction of the amorphous phases was increased due to high temperature thermal soaking. Studies on samples prepared with the electron transparent region deliberately located in various specific regions showed that in general the thickness of the amorphous grain boundary phase expanded due to soaking. The clear-cut atomic steps or the faceted Si3N4 grains along the grain boundaries were smeared out. All these changes could be explained on the basis of oxygen attacks through fast path diffusion of oxygen species along the grain boundary phase, surface diffusion in the micropores or even through volume diffusion in the Si3N4 grains along specific orientation. This may contribute to a better understanding of the high temperature behavior of this material in oxidizing atmosphere.

The development of a silicon carbide-type fiber from an organometallic precursor has led to a major resurgence of interest in fiber-reinforced ceramic matrix composites. By combining this high strength fiber with a variety of ceramic matrices it has been possible to achieve tough composites offering significant potential advantages over monolithic ceramics and carbon-carbon for high temperature applications. A continuous-fiber ceramic matrix composite (CFCC) typical of materials proposed for such industrial applications as power generation, heat recovery and chemical production as well as biomedical and environmental applications was tested in uniaxial tension using a universal test machine. Test parameters investigated included: test mode (load versus displacement), test rate (0.003 mm/s, 0.03 mm/s, 50 N/s and 500 N/s), specimen geometry (straight-sided versus reducedgauge section) and type of specimen volume (long/thin versus short/fat). Typical properties include an average elastic modulus 140 ± 10 GPa, an average proportional limit stress of 45 ± 20 MPa, an average ultimate tensile strength of 210 ± 20 MPa and an average modulus of toughness of 760 ± 200 kJ/m.