2007 Vol. 24, No. 2
2007, 24(2): 1-6.
Abstract:
Continuous fiber-reinforced ceramic matrix composites (CMC) are proved to be the most attractive materials in the development of high-tech fields, such as aeronautics, astronautics and so on. Among these composite materials, continuous fiber-reinforced silicon carbide ceramic matrix composites (CMC-SiC) have been deeply researched and widely developed. As an example, the strategic needs and application fields of CMC-SiC were introduced in the present paper, and the research status and the development trend of CMC-SiC were also analyzed in detail. At last, the opportunities and challenges faced by domestic researchers in this research field were discussed, as well as the strategies of sustainable development of advanced ceramic matrix composites.
Continuous fiber-reinforced ceramic matrix composites (CMC) are proved to be the most attractive materials in the development of high-tech fields, such as aeronautics, astronautics and so on. Among these composite materials, continuous fiber-reinforced silicon carbide ceramic matrix composites (CMC-SiC) have been deeply researched and widely developed. As an example, the strategic needs and application fields of CMC-SiC were introduced in the present paper, and the research status and the development trend of CMC-SiC were also analyzed in detail. At last, the opportunities and challenges faced by domestic researchers in this research field were discussed, as well as the strategies of sustainable development of advanced ceramic matrix composites.
2007, 24(2): 7-12.
Abstract:
A magnesium alloy matrix composite was fabricated by squeeze cast with the preform made of crystallized aluminum silicate short fibers as reinforcement and aluminum phosphate as binder. The reaction products were investigated by optical microscopic, XRD and SEM, and the chemical reaction at the interface between the short fiber and matrix was analysed by thermodynamic analysis. It is shown that the alumina silicate fiber can be chosen as the reinforcement of Mg matrix composites, and an ideal stronger interface was 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 addition, a brittle Mg2Si phase was precipitated at the interface through the reaction between amorphous SiO2 and Mg in the magnesium alloy matrix, which affects the mechanical properties of the magnesium alloy matrix composite.
A magnesium alloy matrix composite was fabricated by squeeze cast with the preform made of crystallized aluminum silicate short fibers as reinforcement and aluminum phosphate as binder. The reaction products were investigated by optical microscopic, XRD and SEM, and the chemical reaction at the interface between the short fiber and matrix was analysed by thermodynamic analysis. It is shown that the alumina silicate fiber can be chosen as the reinforcement of Mg matrix composites, and an ideal stronger interface was 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 addition, a brittle Mg2Si phase was precipitated at the interface through the reaction between amorphous SiO2 and Mg in the magnesium alloy matrix, which affects the mechanical properties of the magnesium alloy matrix composite.
2007, 24(2): 13-17.
Abstract:
In order to provide theoretic direction for interface design and control of WC/Fe composite, the dissolution thermodynamics of WC particles in the matrix was analysed for WC particles reinforced gray cast iron matrix composites, and the experimental techniques such as DTA, OM, SEM and XRD were used to investigate the interactions between the tungsten carbide (WC) particulates and the gray cast iron matrix, and the dissolution of the tungsten carbide particulates (WC) in the matrix was investigated. The results show that HT300 melting has a good wetting with WC particles, the surface of WC particles is dissolved obviously when the highest temperature of the system is 1450℃, and WC particles begin to be dissolved in the HT300 matrix as the temperature is about 1281℃. When the system temperature rises to a certain point, the resolution reaction of WC (WCW2C＋C) occurs and the diffusion of elements can accelerate the dissolution of WC particles.
In order to provide theoretic direction for interface design and control of WC/Fe composite, the dissolution thermodynamics of WC particles in the matrix was analysed for WC particles reinforced gray cast iron matrix composites, and the experimental techniques such as DTA, OM, SEM and XRD were used to investigate the interactions between the tungsten carbide (WC) particulates and the gray cast iron matrix, and the dissolution of the tungsten carbide particulates (WC) in the matrix was investigated. The results show that HT300 melting has a good wetting with WC particles, the surface of WC particles is dissolved obviously when the highest temperature of the system is 1450℃, and WC particles begin to be dissolved in the HT300 matrix as the temperature is about 1281℃. When the system temperature rises to a certain point, the resolution reaction of WC (WCW2C＋C) occurs and the diffusion of elements can accelerate the dissolution of WC particles.
2007, 24(2): 18-22.
Abstract:
The surface of flaky metal magnetic powders with diameter size of 5 μ m and thickness of 1 μ m was modified by SiO2. The appearance, structure and phase constitution of the powders before and after treatment were characterized by SEM, TEM and RA-IR. And the powders were randomly dispersed in paraffin at a mass ratio of 5 ∶ 1 for microwave electromagnetic parameters test and comparison in the frequency range of 2~18GHz. The results show that a high resistivity film is formed on the powder surface, and the real and imaginary parts of complex permittivity of the sample decrease by 20 and 7 in average but the corresponding complex permeability decreases slightly. The coatings with thickness of 1mm give a microwave absorption of 8dB with a band width of 7GHz (7.6~14.6GHz) and the microwave absorption of the modified flaky metal magnetic powders is improved.
The surface of flaky metal magnetic powders with diameter size of 5 μ m and thickness of 1 μ m was modified by SiO2. The appearance, structure and phase constitution of the powders before and after treatment were characterized by SEM, TEM and RA-IR. And the powders were randomly dispersed in paraffin at a mass ratio of 5 ∶ 1 for microwave electromagnetic parameters test and comparison in the frequency range of 2~18GHz. The results show that a high resistivity film is formed on the powder surface, and the real and imaginary parts of complex permittivity of the sample decrease by 20 and 7 in average but the corresponding complex permeability decreases slightly. The coatings with thickness of 1mm give a microwave absorption of 8dB with a band width of 7GHz (7.6~14.6GHz) and the microwave absorption of the modified flaky metal magnetic powders is improved.
2007, 24(2): 23-27.
Abstract:
In some processes of resin matrix composites, such as a hot pressing process and RTM, the resin flow is an important phenomenon which can be detected by resin pressure. Monitoring resin pressure can obtain a lot of significant information for the optimization of the processing parameters and the control of the quality of composite parts. In this paper, a resin pressure measuring system for hot pressing processes was established based on the principle of pressure transfer in liquid. The variation of resin pressure in the process of unidirectional glass fiber laminates was investigated. By using the measuring system, the results show that the accuracy and the dynamic respondence of the system can meet the requirements of resin pressure testing in hot pressing processes, which is valuable for the study of the resin flow behavior in the processing.
In some processes of resin matrix composites, such as a hot pressing process and RTM, the resin flow is an important phenomenon which can be detected by resin pressure. Monitoring resin pressure can obtain a lot of significant information for the optimization of the processing parameters and the control of the quality of composite parts. In this paper, a resin pressure measuring system for hot pressing processes was established based on the principle of pressure transfer in liquid. The variation of resin pressure in the process of unidirectional glass fiber laminates was investigated. By using the measuring system, the results show that the accuracy and the dynamic respondence of the system can meet the requirements of resin pressure testing in hot pressing processes, which is valuable for the study of the resin flow behavior in the processing.
2007, 24(2): 28-32.
Abstract:
The thermal conductive coating was obtained with the epoxy modified silicone resin as matrix, and the mixture of silicone nitride and alumina as the heat conductive fillers. Its mechanical properties, thermal conductivity, electrical insulation and thermal stability were studied. The results indicate that at the total 40％ mass fraction of fillers and preferable mass ratio of alumina to silicone nitride, the tensile strength, elongation at break and adhesion of the coating are 8.02MPa, 1.27％ and 572.2N · cm-2, respectively; and its thermal conductivity, dielectric constant, volume resistivity and surface resistivity are 1.25 W · (m · K)-1, 5.7, 3×1013Ω · cm and 4.3×1013Ω, respectively. In addition, the coating can be used for a long term at about 200℃. The results also demonstrate that the coating has higher heat conduction compared with the unfilled epoxy modified silicone resin based coating.
The thermal conductive coating was obtained with the epoxy modified silicone resin as matrix, and the mixture of silicone nitride and alumina as the heat conductive fillers. Its mechanical properties, thermal conductivity, electrical insulation and thermal stability were studied. The results indicate that at the total 40％ mass fraction of fillers and preferable mass ratio of alumina to silicone nitride, the tensile strength, elongation at break and adhesion of the coating are 8.02MPa, 1.27％ and 572.2N · cm-2, respectively; and its thermal conductivity, dielectric constant, volume resistivity and surface resistivity are 1.25 W · (m · K)-1, 5.7, 3×1013Ω · cm and 4.3×1013Ω, respectively. In addition, the coating can be used for a long term at about 200℃. The results also demonstrate that the coating has higher heat conduction compared with the unfilled epoxy modified silicone resin based coating.
2007, 24(2): 33-37.
Abstract:
Core-shell PB-g-PS impact modifiers with different mass ratios of polybutadiene (PB) to polystyrene (PS) were prepared with emulsion polymerization by grafting styrene onto polybutadiene latex. The mass ratio of polybutadiene to polystyrene was in a range from 30/70 to 90/10. Then these core-shell impact modifiers were blended with polystyrene to prepare the PB-g-PS/PS blends with a constant PB content of 20wt%. The grafting degrees of PB-g-PS copolymers were measured and the effects of different grafting degrees on the dispersion state and morphologies of rubber particles are discussed. It was generally accepted that polystyrene could only be toughened effectively by 1~3μm rubber particles. However, the experimental results of this paper show that PS can be actually toughened by sub-micron core-shell rubber particles. The optimal degree of grafting is 28.3% and the ratio of PB/PS in PB-g-PS copolymer is 70/30. The TEM photograph of PB-g-PS/PS (70/30) shows that the rubber particles disperse homogeneously in the PS matrix, and the izod notched impact strength attains 124.9J/m.
Core-shell PB-g-PS impact modifiers with different mass ratios of polybutadiene (PB) to polystyrene (PS) were prepared with emulsion polymerization by grafting styrene onto polybutadiene latex. The mass ratio of polybutadiene to polystyrene was in a range from 30/70 to 90/10. Then these core-shell impact modifiers were blended with polystyrene to prepare the PB-g-PS/PS blends with a constant PB content of 20wt%. The grafting degrees of PB-g-PS copolymers were measured and the effects of different grafting degrees on the dispersion state and morphologies of rubber particles are discussed. It was generally accepted that polystyrene could only be toughened effectively by 1~3μm rubber particles. However, the experimental results of this paper show that PS can be actually toughened by sub-micron core-shell rubber particles. The optimal degree of grafting is 28.3% and the ratio of PB/PS in PB-g-PS copolymer is 70/30. The TEM photograph of PB-g-PS/PS (70/30) shows that the rubber particles disperse homogeneously in the PS matrix, and the izod notched impact strength attains 124.9J/m.
2007, 24(2): 38-43.
Abstract:
Polyacrylonitrile (PAN)-based carbon fibers (CF) with nano-SiO2 modified emulsion sizing and unmo-dified emulsion sizing were prepared. The stability of both emulsions was studied by the static precipitation method and optical microscopy. Scanning electron microscope (SEM), X-ray energy spectrum tester (EDS), atomic force microscopy (AFM) and dynamic contact angle analysis (DCAA) were used to study the surface property changes of the CF without sizing and the CF with unmodified sizing and modified sizing. The interfacial adhesion properties between CF and the matrix resin were evaluated by the single fiber fragmentation test. The results show that the size of both unmodified and modified emulsion particles is little and both emulsion stabilities are better; moreover, the former is more excellent than the latter. The surface energy and toughness of CF are improved after sizing. Furthermore, the surface energy and toughness of the CF with modified sizing have maximum values. The interfacial shear strength (IFSS) value of the CF with modified sizing is 27.2% higher than that of the CF with unmodified sizing. The harmonic mean work of adhesion (W(h)a ) and geometrical mean work of adhesion (W(g)a ) between sizing modified carbon fibers and the matrix resin are also increased by 12.7% and 11.7%, respectively, compared to those of the CF with unmodified sizing.
Polyacrylonitrile (PAN)-based carbon fibers (CF) with nano-SiO2 modified emulsion sizing and unmo-dified emulsion sizing were prepared. The stability of both emulsions was studied by the static precipitation method and optical microscopy. Scanning electron microscope (SEM), X-ray energy spectrum tester (EDS), atomic force microscopy (AFM) and dynamic contact angle analysis (DCAA) were used to study the surface property changes of the CF without sizing and the CF with unmodified sizing and modified sizing. The interfacial adhesion properties between CF and the matrix resin were evaluated by the single fiber fragmentation test. The results show that the size of both unmodified and modified emulsion particles is little and both emulsion stabilities are better; moreover, the former is more excellent than the latter. The surface energy and toughness of CF are improved after sizing. Furthermore, the surface energy and toughness of the CF with modified sizing have maximum values. The interfacial shear strength (IFSS) value of the CF with modified sizing is 27.2% higher than that of the CF with unmodified sizing. The harmonic mean work of adhesion (W(h)a ) and geometrical mean work of adhesion (W(g)a ) between sizing modified carbon fibers and the matrix resin are also increased by 12.7% and 11.7%, respectively, compared to those of the CF with unmodified sizing.
2007, 24(2): 44-49.
Abstract:
A crosslinked nitrile-butadiene rubber (NBR)/hindered phenol composite was successfully prepared by mixing tetrakis (methylene-3-(3-5-ditert-butyl-4-hydroxy phenyl) propionyloxy) methane (AO-60) into NBR. The microstructure of the NBR/AO-60 composite was characterized by DSC, SEM, XRD and DMTA, and its thermal and dynamic mechanical properties were studied. The results show that the pure AO-60 is a crystalline material, but in the hindered phenol/NBR composites most of the AO-60 molecules are amorphous and form AO-60 domains. In the case of NBR/AO-60 composites, large numbers of hydrogen bonds are formed not only in the AO-60 molecules within AO-60-rich domains but also between NBR and AO-60 molecules. The dissociation of the intermolecular hydrogen bonds leads to the appearance of two novel transitions above the glass transition temperature of the matrix NBR. It was found that it is a good damping material because of the novel dynamic mechanical property of the NBR/AO-60 composites.
A crosslinked nitrile-butadiene rubber (NBR)/hindered phenol composite was successfully prepared by mixing tetrakis (methylene-3-(3-5-ditert-butyl-4-hydroxy phenyl) propionyloxy) methane (AO-60) into NBR. The microstructure of the NBR/AO-60 composite was characterized by DSC, SEM, XRD and DMTA, and its thermal and dynamic mechanical properties were studied. The results show that the pure AO-60 is a crystalline material, but in the hindered phenol/NBR composites most of the AO-60 molecules are amorphous and form AO-60 domains. In the case of NBR/AO-60 composites, large numbers of hydrogen bonds are formed not only in the AO-60 molecules within AO-60-rich domains but also between NBR and AO-60 molecules. The dissociation of the intermolecular hydrogen bonds leads to the appearance of two novel transitions above the glass transition temperature of the matrix NBR. It was found that it is a good damping material because of the novel dynamic mechanical property of the NBR/AO-60 composites.
2007, 24(2): 50-54.
Abstract:
Na-montmorillonite was organically modified by the cationic exchange method using 2, 4, 6-tris-(dimethylaminomethyl) phenol (DMP-30) as the modifier. The influence of the molar ratio of DMP-30 to HCl was stu-died on the organic treatment of Na-montmorillonite. The morphologies and structures of the as-obtained samples were characterized by SEM, TEM, IR and XRD. The results show that at a proper molar ratio (1 ∶ 2) of DMP-30 to HCl, the DMP-30 molecules are successfully intercalated into the layers of montmorillonite, and the space of organically modified montmorillonite layers increases from 1.34nm to 1.81nm. Nanocomposites were prepared by the in situ polymerization of epoxy resins in the presence of the organo-modified montmorillonite (OMMT). The structure of the nanocomposites was studied by XRD. The d001 peak of the organic montmorillonite disappeared, indicating that the organo-modified montmorillonite was completely exfoliated in the epoxy matrix.
Na-montmorillonite was organically modified by the cationic exchange method using 2, 4, 6-tris-(dimethylaminomethyl) phenol (DMP-30) as the modifier. The influence of the molar ratio of DMP-30 to HCl was stu-died on the organic treatment of Na-montmorillonite. The morphologies and structures of the as-obtained samples were characterized by SEM, TEM, IR and XRD. The results show that at a proper molar ratio (1 ∶ 2) of DMP-30 to HCl, the DMP-30 molecules are successfully intercalated into the layers of montmorillonite, and the space of organically modified montmorillonite layers increases from 1.34nm to 1.81nm. Nanocomposites were prepared by the in situ polymerization of epoxy resins in the presence of the organo-modified montmorillonite (OMMT). The structure of the nanocomposites was studied by XRD. The d001 peak of the organic montmorillonite disappeared, indicating that the organo-modified montmorillonite was completely exfoliated in the epoxy matrix.
2007, 24(2): 55-60.
Abstract:
The cure reactions of dicyclopentandiene bisphenol cyanate ester (DCPDCE) associated with an epoxy resin (2,2—bis (4—glycidyloxy phenyl) propane) were investigated using gel test, FTIR and DSC dynamic scanning at different stoichiometric ratios. It was observed that the epoxy resin not only catalyzed the trimerization of the cyanate ester which decreased the gel time of the EP/DCPDCE blends, but also diluted the cyanate ester which increased the gel time of the blends. When the weight fraction of EP in the EP/DCPDCE blend was 25wt%, the two effects of EP were balanced and the gel time of the blend was almost the same as that of the pure DCPDCE. When the blends were cured following the curing cycle, the reaction of producing triazine rings and oxazoline took place at the low temperature (160~180℃) step of the curing cycle, and the reaction of epoxy functions with the triazine rings former in the initial step as well as the isomerization of oxazoline to oxazolidinone took place at the high temperature (200~220℃) step. The structure of the final cured resin was related to the initial epoxy resin concentration of the blends.
The cure reactions of dicyclopentandiene bisphenol cyanate ester (DCPDCE) associated with an epoxy resin (2,2—bis (4—glycidyloxy phenyl) propane) were investigated using gel test, FTIR and DSC dynamic scanning at different stoichiometric ratios. It was observed that the epoxy resin not only catalyzed the trimerization of the cyanate ester which decreased the gel time of the EP/DCPDCE blends, but also diluted the cyanate ester which increased the gel time of the blends. When the weight fraction of EP in the EP/DCPDCE blend was 25wt%, the two effects of EP were balanced and the gel time of the blend was almost the same as that of the pure DCPDCE. When the blends were cured following the curing cycle, the reaction of producing triazine rings and oxazoline took place at the low temperature (160~180℃) step of the curing cycle, and the reaction of epoxy functions with the triazine rings former in the initial step as well as the isomerization of oxazoline to oxazolidinone took place at the high temperature (200~220℃) step. The structure of the final cured resin was related to the initial epoxy resin concentration of the blends.
2007, 24(2): 61-67.
Abstract:
The morphologies of pure poly (phenylene sulfide) (PPS) and glass, carbon, aramid fiber-reinforced poly (phenylene sulfide) composites crystallized isothermally from the melt were investigated by a polarized light microscope equipped with a hot-stage in real time. The spherulite growth rates of PPS were measured in the range of crystallization temperature from 235℃ to 265℃ as a function of crystallization time. The results show that the crystal growth rate decreases as the isothermal crystallization temperature increases, and the crystalline morphology of pure PPS varies from fine spherulite with sheaflike structure in part to bigger and perfect spherulite and then to fine and imperfect spherulite. The presence of fibers plays an important role in the morphologies of PPS which transformed from spherulite structure to the transcrystallization. And to what degree the transcrystallization induced depends on the kinds of the fibers. Among these fibers, glass fibers and aramid fibers possess the ability for inducing the transcrystallization of PPS while carbon fibers can not induce PPS to form the transcrystallization obviously.
The morphologies of pure poly (phenylene sulfide) (PPS) and glass, carbon, aramid fiber-reinforced poly (phenylene sulfide) composites crystallized isothermally from the melt were investigated by a polarized light microscope equipped with a hot-stage in real time. The spherulite growth rates of PPS were measured in the range of crystallization temperature from 235℃ to 265℃ as a function of crystallization time. The results show that the crystal growth rate decreases as the isothermal crystallization temperature increases, and the crystalline morphology of pure PPS varies from fine spherulite with sheaflike structure in part to bigger and perfect spherulite and then to fine and imperfect spherulite. The presence of fibers plays an important role in the morphologies of PPS which transformed from spherulite structure to the transcrystallization. And to what degree the transcrystallization induced depends on the kinds of the fibers. Among these fibers, glass fibers and aramid fibers possess the ability for inducing the transcrystallization of PPS while carbon fibers can not induce PPS to form the transcrystallization obviously.
2007, 24(2): 68-72.
Abstract:
Nano-ZnO/MC nylon 6 composites were prepared by the in situ polymerization method. The mechanical properties, morphology, structure and DSC, XRD analysis show that the inorganic particles disperse very well in the matrix and their diameters are smaller than 100nm. The tensile strength, break elongation, bend modulus and impact strength increase by 20%, 33%, 36% and 87% respectively compared with pure MC nylon. Nano-ZnO does not influence the characteristic peak of α-crystalline diffraction, and the crystal modality of nylon 6 is unchanged. The size of the grain is increased and the crystallinity is decreased for nano-ZnO adding. The dynamic XRD analysis shows that nano-ZnO increases the melting temperature of nano-ZnO/MC nylon 6 composites, while the crystalline morphology and relative crystalline degree are reversible among high and low temperatures.
Nano-ZnO/MC nylon 6 composites were prepared by the in situ polymerization method. The mechanical properties, morphology, structure and DSC, XRD analysis show that the inorganic particles disperse very well in the matrix and their diameters are smaller than 100nm. The tensile strength, break elongation, bend modulus and impact strength increase by 20%, 33%, 36% and 87% respectively compared with pure MC nylon. Nano-ZnO does not influence the characteristic peak of α-crystalline diffraction, and the crystal modality of nylon 6 is unchanged. The size of the grain is increased and the crystallinity is decreased for nano-ZnO adding. The dynamic XRD analysis shows that nano-ZnO increases the melting temperature of nano-ZnO/MC nylon 6 composites, while the crystalline morphology and relative crystalline degree are reversible among high and low temperatures.
2007, 24(2): 73-78.
Abstract:
The EPLEXOR 500 was used to investigate the dynamic viscoelasticities of T300/S-2 hybrid reinforced epoxy composites. The effects of the static load and dynamic load on the storage modulus, loss modulus and loss tangent of hybrid composites were studied. The effects of hybrid ratios and hybrid modes on dynamic viscoelasticities under different load levels were discussed and compared. The results show that the storage modulus of the composites with different hybrid ratios uniformly decreases with the increase of dynamic load and increases with the increase of static load. The loss modulus and loss tangent both decrease with the increase of the static or dynamic loads. The storage modulus of the hybrid composites under high load approximately conforms to the Composite Beam Theory. In the dynamic load sweep mode, the variation of loss tangent with the volume fraction of carbon fiber can still be described by the Mixture Rule, and the storage modulus of sandwich hybrid composites is much higher than that of interlaminar hybrid composites. In such sweep mode the loss tangent of sandwich hybrid compo-sites is also larger than that of interlaminar hybrid composites, but in the static load sweep mode, the case is reverse.
The EPLEXOR 500 was used to investigate the dynamic viscoelasticities of T300/S-2 hybrid reinforced epoxy composites. The effects of the static load and dynamic load on the storage modulus, loss modulus and loss tangent of hybrid composites were studied. The effects of hybrid ratios and hybrid modes on dynamic viscoelasticities under different load levels were discussed and compared. The results show that the storage modulus of the composites with different hybrid ratios uniformly decreases with the increase of dynamic load and increases with the increase of static load. The loss modulus and loss tangent both decrease with the increase of the static or dynamic loads. The storage modulus of the hybrid composites under high load approximately conforms to the Composite Beam Theory. In the dynamic load sweep mode, the variation of loss tangent with the volume fraction of carbon fiber can still be described by the Mixture Rule, and the storage modulus of sandwich hybrid composites is much higher than that of interlaminar hybrid composites. In such sweep mode the loss tangent of sandwich hybrid compo-sites is also larger than that of interlaminar hybrid composites, but in the static load sweep mode, the case is reverse.
2007, 24(2): 79-85.
Abstract:
Based on the special structural properties of natural fibril silicate attapulgite, silicate nano-fiber/rubber composites were successfully prepared by mechanically compounding, combined with the in-situ modification method. The structure and properties of the composites were studied, and the reinforcing mechanism of AT was discussed as well. Upon shear force during mixing as well as in-situ modification with silane coupling agent, attapulgite micron particles are separated into numerous nano-fibers with the length/diameter ratio of 10~30 into the rubber matrix. AT/rubber composites possess stress-strain behavior and distinct anisotropy similar to those for short fiber reinforced rubber composites (SFRC), higher modulus at higher temperatures and good processing property, resulted from nano-dispersing of AT and good rubber-AT interfacial bonding.
Based on the special structural properties of natural fibril silicate attapulgite, silicate nano-fiber/rubber composites were successfully prepared by mechanically compounding, combined with the in-situ modification method. The structure and properties of the composites were studied, and the reinforcing mechanism of AT was discussed as well. Upon shear force during mixing as well as in-situ modification with silane coupling agent, attapulgite micron particles are separated into numerous nano-fibers with the length/diameter ratio of 10~30 into the rubber matrix. AT/rubber composites possess stress-strain behavior and distinct anisotropy similar to those for short fiber reinforced rubber composites (SFRC), higher modulus at higher temperatures and good processing property, resulted from nano-dispersing of AT and good rubber-AT interfacial bonding.
2007, 24(2): 86-91.
Abstract:
Under precondition of high emissivity, the thickness of high emissivity ceramic coatings should be reduced to satisfy the requirement for mechanical property, which can also save raw materials and decrease the weight of the apparatus. So the relationship between emissivity and thickness of the coatings is important. In this paper, the coatings were prepared from Al2O3 sols and high emissivity fillers by spin and spray processes, respectively. The relationship between emissivity and thickness of coatings was examined, and the critical thickness of coatings was determined while the emissivity changes with the thickness rapidly. Based on Maxwell-Garnett theory, a new theoretical model about the relationship between emissivity and thickness of composite coatings is established. The emissivity values from the theoretical calculation are agree with the experimental data, which shows that the model can be used to predict the emissivity or the critical thickness of the composite coatings.
Under precondition of high emissivity, the thickness of high emissivity ceramic coatings should be reduced to satisfy the requirement for mechanical property, which can also save raw materials and decrease the weight of the apparatus. So the relationship between emissivity and thickness of the coatings is important. In this paper, the coatings were prepared from Al2O3 sols and high emissivity fillers by spin and spray processes, respectively. The relationship between emissivity and thickness of coatings was examined, and the critical thickness of coatings was determined while the emissivity changes with the thickness rapidly. Based on Maxwell-Garnett theory, a new theoretical model about the relationship between emissivity and thickness of composite coatings is established. The emissivity values from the theoretical calculation are agree with the experimental data, which shows that the model can be used to predict the emissivity or the critical thickness of the composite coatings.
2007, 24(2): 92-98.
Abstract:
The mechanical parameters and microstructures of the elytra were studied for four kinds of beetles (Potosia (Liocola) brevitarsis lewis, Cybister, Allomyrina dichotoma, Holotrichia trichophora). The elytras microstructures in cross-sections show that the elytra are composed of the compact dorse, the hollow section and the ventral face, the chitin fibre hollow-pillar structure connects the compact dorse and the ventral face, and the elytra couple each other with the concave-convex structure. The Youngs modulus and hardness of the elytra measured by the Nano Indenter SA2 (MTS) are 9.08, 8.21, 4.76, 6.00 GPa and 0.44, 0.48, 0.18, 0.18 GPa, respectively. Coupling forces between the elytra measured by a Multifunctional Test Machine are 46.43, 7.38, 11.34 mN/mm for Potosia (Liocola) brevitarsis lewis, Cybister and Allomyrina dichotoma. The research provides biomimetic information for light structure designs.
The mechanical parameters and microstructures of the elytra were studied for four kinds of beetles (Potosia (Liocola) brevitarsis lewis, Cybister, Allomyrina dichotoma, Holotrichia trichophora). The elytras microstructures in cross-sections show that the elytra are composed of the compact dorse, the hollow section and the ventral face, the chitin fibre hollow-pillar structure connects the compact dorse and the ventral face, and the elytra couple each other with the concave-convex structure. The Youngs modulus and hardness of the elytra measured by the Nano Indenter SA2 (MTS) are 9.08, 8.21, 4.76, 6.00 GPa and 0.44, 0.48, 0.18, 0.18 GPa, respectively. Coupling forces between the elytra measured by a Multifunctional Test Machine are 46.43, 7.38, 11.34 mN/mm for Potosia (Liocola) brevitarsis lewis, Cybister and Allomyrina dichotoma. The research provides biomimetic information for light structure designs.
2007, 24(2): 99-104.
Abstract:
The conception and the sensing principle of embedded-style carbon fiber cement paste piezoresistive sensors suitable for local compressive stress and strain monitoring of concrete structures were introduced. The influence of environmental temperature and humidity on the zero-output performance was analyzed. The temperature and humidity compensation methods were discussed based on the circuit design of sensors. The results show that the environmental temperature and humidity have great influences on the zero-output of carbon fiber cement paste piezoresistive sensors and their effects are about 6% and 200%, respectively. A compensation circuit of carbon fiber cement paste piezoresistive sensors is built up by a measurement-compensation dual form to eliminate the influence of noise signals made by temperature, humidity and polarization on zero-output.
The conception and the sensing principle of embedded-style carbon fiber cement paste piezoresistive sensors suitable for local compressive stress and strain monitoring of concrete structures were introduced. The influence of environmental temperature and humidity on the zero-output performance was analyzed. The temperature and humidity compensation methods were discussed based on the circuit design of sensors. The results show that the environmental temperature and humidity have great influences on the zero-output of carbon fiber cement paste piezoresistive sensors and their effects are about 6% and 200%, respectively. A compensation circuit of carbon fiber cement paste piezoresistive sensors is built up by a measurement-compensation dual form to eliminate the influence of noise signals made by temperature, humidity and polarization on zero-output.
2007, 24(2): 105-112.
Abstract:
The longitudinal and transverse tensile properties were tested for integrally-braided 5D carbon/phenolic composites carbonized at different temperatures in comparison to the virgin material. The tensile stress-strain curves were obtained and the principal mechanical properties (tensile strength, tensile modulus, failure strain, Poissons ratio) were determined. A phenomenological relationship between tensile properties and carbonization temperature was established. The fracture morphology and microstructure after tension were observed to further understand the deformation and the failure mechanism. Both longitudinal and transverse tensile strength and modulus show a transition from decrease to increase with temperature rise around a certain temperature point, and the magnitude of modulus change is relatively small. The failure strain exhibits monotonic gradual decrease. The property change is supposed to be correlated with the microstructural evolution of the phenolic matrix upon carbonization at different temperatures.
The longitudinal and transverse tensile properties were tested for integrally-braided 5D carbon/phenolic composites carbonized at different temperatures in comparison to the virgin material. The tensile stress-strain curves were obtained and the principal mechanical properties (tensile strength, tensile modulus, failure strain, Poissons ratio) were determined. A phenomenological relationship between tensile properties and carbonization temperature was established. The fracture morphology and microstructure after tension were observed to further understand the deformation and the failure mechanism. Both longitudinal and transverse tensile strength and modulus show a transition from decrease to increase with temperature rise around a certain temperature point, and the magnitude of modulus change is relatively small. The failure strain exhibits monotonic gradual decrease. The property change is supposed to be correlated with the microstructural evolution of the phenolic matrix upon carbonization at different temperatures.
2007, 24(2): 113-119.
Abstract:
The strain rate dependent behavior of T300 carbon/epoxy matrix composite in tension was studied by testing two kinds of laminates S and S at different rates. The tensile test was carried out on an MTS machine at the quasi-static strain rates of approximately 10-5~10-4 s-1 and the intermediate strain rates of about 100~101s-1. The tensile high strain rate test was conducted with the tensile split Hopkinson bar technique at strain rates of approximately 102~104 s-1. The specimens with identical geometry were used in all the tests. The dynamic stress-strain plot was obtained and compared with the quasi-static tensile test result. The results of the study indicate that Carbon/Epoxy composites are strain rate dependent materials. The stacking sequence has a significant effect on the material response. The tensile strength of the composites increases with increasing the strain rate, and the failure strain decreases when the strain rate increases. A constitutive model for characterizing the strain rate dependent behavior of T300/Epoxy composites was derived by simulating the experimental data.
The strain rate dependent behavior of T300 carbon/epoxy matrix composite in tension was studied by testing two kinds of laminates S and S at different rates. The tensile test was carried out on an MTS machine at the quasi-static strain rates of approximately 10-5~10-4 s-1 and the intermediate strain rates of about 100~101s-1. The tensile high strain rate test was conducted with the tensile split Hopkinson bar technique at strain rates of approximately 102~104 s-1. The specimens with identical geometry were used in all the tests. The dynamic stress-strain plot was obtained and compared with the quasi-static tensile test result. The results of the study indicate that Carbon/Epoxy composites are strain rate dependent materials. The stacking sequence has a significant effect on the material response. The tensile strength of the composites increases with increasing the strain rate, and the failure strain decreases when the strain rate increases. A constitutive model for characterizing the strain rate dependent behavior of T300/Epoxy composites was derived by simulating the experimental data.
2007, 24(2): 120-124.
Abstract:
The deformation of the composite unsymmetric laminates subjected to the thermal and chemical volume-tric shrinkage load was investigated by the three dimensional finite element methods during the curing process. The material mechanical properties and chemical volumetric shrinkage were expressed as the function of the temperature as well as the degree of cure. The relationship between the laminate curvature and the temperature and degree of cure was studied. The results of the numerical simulation show that the laminates curvature was closely related to the degree of cure at the moment when the cure process terminated; most of the process-induced deformation was generated in the cooling stage; the chemical volumetric shrinkage has slight influence on the laminate deformation and it mainly occurs at the first half of the second temperature dwelling.
The deformation of the composite unsymmetric laminates subjected to the thermal and chemical volume-tric shrinkage load was investigated by the three dimensional finite element methods during the curing process. The material mechanical properties and chemical volumetric shrinkage were expressed as the function of the temperature as well as the degree of cure. The relationship between the laminate curvature and the temperature and degree of cure was studied. The results of the numerical simulation show that the laminates curvature was closely related to the degree of cure at the moment when the cure process terminated; most of the process-induced deformation was generated in the cooling stage; the chemical volumetric shrinkage has slight influence on the laminate deformation and it mainly occurs at the first half of the second temperature dwelling.
2007, 24(2): 125-130.
Abstract:
A two dimensional finite elemental model is developed to simulate and analyze the compaction of the composite laminates during the autoclave processing. The model is based on the effective stress principle and Darcys law. The resin pressure is chosen as the additional degree of freedom of the nodes. The coupled equations are solved by the backward difference and direct approach because of its rapid convergence. The results of the numerical simulation show that, with the laminated thickness increasing, the thickness reduction ratio decreases, and the compaction implementing time and the resin distribution unsymmetry in the thickness direction increase. For the thicker laminate, compared with enhancing cure pressure, placing bleeder at both sides of the laminates improves the thickness reduction ratio significantly and gains a more uniform resin distribution.
A two dimensional finite elemental model is developed to simulate and analyze the compaction of the composite laminates during the autoclave processing. The model is based on the effective stress principle and Darcys law. The resin pressure is chosen as the additional degree of freedom of the nodes. The coupled equations are solved by the backward difference and direct approach because of its rapid convergence. The results of the numerical simulation show that, with the laminated thickness increasing, the thickness reduction ratio decreases, and the compaction implementing time and the resin distribution unsymmetry in the thickness direction increase. For the thicker laminate, compared with enhancing cure pressure, placing bleeder at both sides of the laminates improves the thickness reduction ratio significantly and gains a more uniform resin distribution.
2007, 24(2): 131-136.
Abstract:
The penetration and perforation of FRP laminates impacted by ogival-nosed projectiles at normal incidence were examined. Based on the assumption that the deformation is localized and that the mean pressure offered by the laminate targets to resist the projectiles can be decomposed into two parts: one part is a cohesive quasi-static resistive pressure due to the elastic-plastic deformation of the laminate materials and the other is a dynamic resistive pressure arising from velocity effects, a new formulation is developed by extending Wens model by assuming that the target resistance is no longer a constant, but a function of the penetration velocity. Equations are obtained for predicting the depth of penetration in the FRP laminate targets, the residual velocity and the ballistic limit in the case of perforation. It is shown that the theoretical predictions are in good correlation with available experimental data.
The penetration and perforation of FRP laminates impacted by ogival-nosed projectiles at normal incidence were examined. Based on the assumption that the deformation is localized and that the mean pressure offered by the laminate targets to resist the projectiles can be decomposed into two parts: one part is a cohesive quasi-static resistive pressure due to the elastic-plastic deformation of the laminate materials and the other is a dynamic resistive pressure arising from velocity effects, a new formulation is developed by extending Wens model by assuming that the target resistance is no longer a constant, but a function of the penetration velocity. Equations are obtained for predicting the depth of penetration in the FRP laminate targets, the residual velocity and the ballistic limit in the case of perforation. It is shown that the theoretical predictions are in good correlation with available experimental data.
2007, 24(2): 137-142.
Abstract:
The CFRP and GFRP laminates with a crack on a single side are loaded under tensile impact by the apparatus of Hopkinson bar.The curves of the load P(t) loaded on the specimens and the displacement δ(t) of the specimens were acquired according to the one-dimensional stress wave theory. The curves of dynamic stress intensity factor KⅠ (t) were set up according to the fracture toughness measuring theory and the stress changing σ(t) with time in the specimens. With the compliance changing rate method, the crack initiation time and the dynamic fracture toughness of CFRP and GFRP laminates were obtained at two kinds of strain rates separately. The experiment results show that the fracture toughness of CFRP and GFRP laminates increased with loading rates increasing.
The CFRP and GFRP laminates with a crack on a single side are loaded under tensile impact by the apparatus of Hopkinson bar.The curves of the load P(t) loaded on the specimens and the displacement δ(t) of the specimens were acquired according to the one-dimensional stress wave theory. The curves of dynamic stress intensity factor KⅠ (t) were set up according to the fracture toughness measuring theory and the stress changing σ(t) with time in the specimens. With the compliance changing rate method, the crack initiation time and the dynamic fracture toughness of CFRP and GFRP laminates were obtained at two kinds of strain rates separately. The experiment results show that the fracture toughness of CFRP and GFRP laminates increased with loading rates increasing.
2007, 24(2): 143-150.
Abstract:
The present work aims at developing a fiber optic based technique to detect delamination at the web/flange junction of a GFRP I-beam based on the frequency domain analysis of ultrasonic echoes. A PZT transmitter was employed to create the stress waves in the beam element, and an optical fiber interferometric sensor was attached to the surface of the beam to serve as an ultrasonic receiver. From the frequency spectrum of the sensor output corresponding to various positions of the PZT transmitter, the delamination location can be revealed. Based on the results from both theoretical and experimental studies, the feasibility of delamination detection with the proposed technique is demonstrated.
The present work aims at developing a fiber optic based technique to detect delamination at the web/flange junction of a GFRP I-beam based on the frequency domain analysis of ultrasonic echoes. A PZT transmitter was employed to create the stress waves in the beam element, and an optical fiber interferometric sensor was attached to the surface of the beam to serve as an ultrasonic receiver. From the frequency spectrum of the sensor output corresponding to various positions of the PZT transmitter, the delamination location can be revealed. Based on the results from both theoretical and experimental studies, the feasibility of delamination detection with the proposed technique is demonstrated.
2007, 24(2): 151-158.
Abstract:
Experimental studies and theoretical analysis on compressive properties of composite laminates with the edge rectangular delamination defect were carried out. The influence of laminate thickness (including stacking sequences), delamination location, shape, area and environment on the compressive strength was discussed. Both the delamination developing model and the soft inclusion model were used to estimate the compressive strength of laminates with delamination and to analyze the failure mechanism. The results show that medium thick laminates and thin laminates are sensitive to the edge delamination defect, while thick laminates are not. The defect location and shape have a certain effect on the laminate compressive strength. The hygrothermal environment not only changes the compressive mechanism of laminates with defect, but also affects the compressive strength of medium and thin laminates. The soft inclusion model is much more effective and can be used in engineering design analysis.
Experimental studies and theoretical analysis on compressive properties of composite laminates with the edge rectangular delamination defect were carried out. The influence of laminate thickness (including stacking sequences), delamination location, shape, area and environment on the compressive strength was discussed. Both the delamination developing model and the soft inclusion model were used to estimate the compressive strength of laminates with delamination and to analyze the failure mechanism. The results show that medium thick laminates and thin laminates are sensitive to the edge delamination defect, while thick laminates are not. The defect location and shape have a certain effect on the laminate compressive strength. The hygrothermal environment not only changes the compressive mechanism of laminates with defect, but also affects the compressive strength of medium and thin laminates. The soft inclusion model is much more effective and can be used in engineering design analysis.
2007, 24(2): 159-167.
Abstract:
A prediction method for the compression-compression fatigue life of laminated composites with impact damage was developed, which can predict the fatigue life of laminated composites with different ply parameters, geometry measures, and impact conditions based on mechanic properties and fatigue characteristics of unidirectional plies. The whole damage process of laminated composites under the impact loading and fatigue loading was analyzed for avoiding the errors induced by an impact damage hypothesis. The real impact damage status of composite laminates has been applied to analyze the fatigue life. Based on the progressive damage theory, the stress analysis was deduced. A 3-D progressive damage model for laminated composites with impact damage was established. A parametric modeling program was developed to predict the impact damage process and fatigue life of laminated compo-sites, which provides a better technical platform for impact resisting design andfatigue research of laminated composites.
A prediction method for the compression-compression fatigue life of laminated composites with impact damage was developed, which can predict the fatigue life of laminated composites with different ply parameters, geometry measures, and impact conditions based on mechanic properties and fatigue characteristics of unidirectional plies. The whole damage process of laminated composites under the impact loading and fatigue loading was analyzed for avoiding the errors induced by an impact damage hypothesis. The real impact damage status of composite laminates has been applied to analyze the fatigue life. Based on the progressive damage theory, the stress analysis was deduced. A 3-D progressive damage model for laminated composites with impact damage was established. A parametric modeling program was developed to predict the impact damage process and fatigue life of laminated compo-sites, which provides a better technical platform for impact resisting design andfatigue research of laminated composites.
2007, 24(2): 168-173.
Abstract:
Based on the proposed mechanical hypothesis and the analytical model of the grid stiffened composite panels, a new smeared stiffened theory was derived for analysing the global stability of grid-stiffened panels. The theory fully considered the effects of shell-stiffener interaction and influence of mid-plane shift. In this theory, the method is applicable for different types of grid stiffened elements. A general eigen function was deduced for computing global buckling load combined with a Rayleigh-Ritz method. The testing sample results were compared with those from the existing smeared stiffener theory, and they exhibited in good agreement. This proves the effectiveness of the proposed method.
Based on the proposed mechanical hypothesis and the analytical model of the grid stiffened composite panels, a new smeared stiffened theory was derived for analysing the global stability of grid-stiffened panels. The theory fully considered the effects of shell-stiffener interaction and influence of mid-plane shift. In this theory, the method is applicable for different types of grid stiffened elements. A general eigen function was deduced for computing global buckling load combined with a Rayleigh-Ritz method. The testing sample results were compared with those from the existing smeared stiffener theory, and they exhibited in good agreement. This proves the effectiveness of the proposed method.
2007, 24(2): 174-183.
Abstract:
The micromechanics Bridging Model constitutive theory of composite materials was incorporated with finite element software ABAQUS through a user subroutine UGENS to analyze the progressive failure process and ultimate strength of a composite wind turbine blade, which has a complicated three-dimensional geometry. Under the input of the constituent fiber and matrix properties, fiber volume fraction, and skin and rib lamination parameters such as lamination angle, thickness and number of laminas used, the software was able to estimate the overall load-carrying capacity of the wind turbine blade and to locate the position where the blade ultimate failure took place. Based on this, the composite wind blade structure can be effectively evaluated, and an optimization in structural design can be achieved easily. The thus developed method has been applied to the design and analysis of a 20kW composite wind turbine blade with an advanced structure, and favorable results with improved strength and stiffness and reduced weight have been obtained. This method is also well applicable to the ultimate failure analysis and strength design of other complicated composite structures.
The micromechanics Bridging Model constitutive theory of composite materials was incorporated with finite element software ABAQUS through a user subroutine UGENS to analyze the progressive failure process and ultimate strength of a composite wind turbine blade, which has a complicated three-dimensional geometry. Under the input of the constituent fiber and matrix properties, fiber volume fraction, and skin and rib lamination parameters such as lamination angle, thickness and number of laminas used, the software was able to estimate the overall load-carrying capacity of the wind turbine blade and to locate the position where the blade ultimate failure took place. Based on this, the composite wind blade structure can be effectively evaluated, and an optimization in structural design can be achieved easily. The thus developed method has been applied to the design and analysis of a 20kW composite wind turbine blade with an advanced structure, and favorable results with improved strength and stiffness and reduced weight have been obtained. This method is also well applicable to the ultimate failure analysis and strength design of other complicated composite structures.
2007, 24(2): 184-189.
Abstract:
The theories of mechanics of composite materials and poromechanics were used to develop a micromechanics model capable of simulating the effective elastic properties of Portland cement pastes. Using different length scales, four composite media could be identified for cement pastes and they were simulated using different modeling schemes: the C-S-H matrix was considered as a saturated porous medium; the hydration products were modeled using the Mori-Tanaka scheme; the effective elastic properties of the skeleton of cement paste were described using the three-phase model (or the generalized self-consistent scheme model); and finally the drained and undrained elastic moduli of cement pastes were simulated using the Mori-Tanaka scheme and the theory of poromechanics. The inputs of the proposed model are the intrinsic properties of the constituent components of cement pastes and can be easily obtained in the literature. The proposed model was used to predict the experimental results in the literature and the results demonstrated the effectiveness and accuracy of the model.
The theories of mechanics of composite materials and poromechanics were used to develop a micromechanics model capable of simulating the effective elastic properties of Portland cement pastes. Using different length scales, four composite media could be identified for cement pastes and they were simulated using different modeling schemes: the C-S-H matrix was considered as a saturated porous medium; the hydration products were modeled using the Mori-Tanaka scheme; the effective elastic properties of the skeleton of cement paste were described using the three-phase model (or the generalized self-consistent scheme model); and finally the drained and undrained elastic moduli of cement pastes were simulated using the Mori-Tanaka scheme and the theory of poromechanics. The inputs of the proposed model are the intrinsic properties of the constituent components of cement pastes and can be easily obtained in the literature. The proposed model was used to predict the experimental results in the literature and the results demonstrated the effectiveness and accuracy of the model.
