2008 Vol. 25, No. 5
2008, 25(5): 1-7.
Abstract:
The tungsten based composites, co-strengthened by La2O3and TiC with different contents, were prepared by the powder metallurgy method. The structure and mechanical properties at room temperature of La2O3-TiC/W composites were investigated. The results show that addition of La2O3 and TiC improves the mechanical properties of tungsten composites. The co-strengthening effect of La2O3 and TiC is better than that of La2O3 or TiC. Density and relative density of La2O3-TiC/W composites decrease with increasing the TiC content, which influences the hardness and elastic modulus of the composites. La2O3 with appropriate content is beneficial to improvement of relative density. The flexural strength has a maximum of 901MPa with composition of 1% La2O3-5% TiC/W, and fracture toughness has a maximum of 10.07 MPa·m1/2 with composition of 1% La2O3-10% TiC/W. In this experiment, the La2O3-TiC/W composite has optimal mechanical properties with addition of 1wt% La2O3and 5wt%TiC. The strengthening mechanisms of La2O3-TiC/W composite are fine grain strengthening and load transferring.The toughening mechanisms are fine grain toughening, crack deflection and microbridging.
The tungsten based composites, co-strengthened by La2O3and TiC with different contents, were prepared by the powder metallurgy method. The structure and mechanical properties at room temperature of La2O3-TiC/W composites were investigated. The results show that addition of La2O3 and TiC improves the mechanical properties of tungsten composites. The co-strengthening effect of La2O3 and TiC is better than that of La2O3 or TiC. Density and relative density of La2O3-TiC/W composites decrease with increasing the TiC content, which influences the hardness and elastic modulus of the composites. La2O3 with appropriate content is beneficial to improvement of relative density. The flexural strength has a maximum of 901MPa with composition of 1% La2O3-5% TiC/W, and fracture toughness has a maximum of 10.07 MPa·m1/2 with composition of 1% La2O3-10% TiC/W. In this experiment, the La2O3-TiC/W composite has optimal mechanical properties with addition of 1wt% La2O3and 5wt%TiC. The strengthening mechanisms of La2O3-TiC/W composite are fine grain strengthening and load transferring.The toughening mechanisms are fine grain toughening, crack deflection and microbridging.
2008, 25(5): 8-13.
Abstract:
The microstructure evolution and mechanical properties of Sn-3.5Ag and its new composite solder containing three different nano-sized polyhedral oligomeric silsesquioxanes (POSS) particulates with the best mass fractions during isothermal aging were studied. All of the above four solder joints were aged at three different temperatures (125℃，150℃，175℃) up to 1000 h and the intermetallic compound growth was measured. The interfacial layers between the solder and the copper substrate were examined using the optical and scanning electronic microscopy. The growth kinetics of intermetallic interfacial layers formed between the solder and the copper substrate was characterized. The growth rate of the intermetallic layers of the composite solders is slower than that of Sn-3.5Ag solder. The activation energies of the composite solders are 80，97 and 77kJ/mol, respectively，and are higher than those of Sn-3.5Ag solder. Besides，the shear strength of the composite solder joints is higher and not so sensitive to the different aging processes.
The microstructure evolution and mechanical properties of Sn-3.5Ag and its new composite solder containing three different nano-sized polyhedral oligomeric silsesquioxanes (POSS) particulates with the best mass fractions during isothermal aging were studied. All of the above four solder joints were aged at three different temperatures (125℃，150℃，175℃) up to 1000 h and the intermetallic compound growth was measured. The interfacial layers between the solder and the copper substrate were examined using the optical and scanning electronic microscopy. The growth kinetics of intermetallic interfacial layers formed between the solder and the copper substrate was characterized. The growth rate of the intermetallic layers of the composite solders is slower than that of Sn-3.5Ag solder. The activation energies of the composite solders are 80，97 and 77kJ/mol, respectively，and are higher than those of Sn-3.5Ag solder. Besides，the shear strength of the composite solder joints is higher and not so sensitive to the different aging processes.
2008, 25(5): 14-18.
Abstract:
The spherical nickel nanoparticles (an average diameter of 70 nm) were prepared by the wire electrical explosion method，Fe3O4 was coated on nickel to form the complete coverage by the co-precipitation method，and the silane coupling agent 3-mercaptopropyltrimethoxysilane (HS-(CH2)3Si(OCH3)3，MPTS) was used as a primer to improve the nickel surface.The as-synthesized Fe3O4/Ni composite particles were uniformly dispersed into the phenolic cement， then the mixture was pasted on the metal plate with the area of 200 mm×200 mm as the test board.The test of microwave absorption was carried out by the radar-absorbing materials (RAM) reflectivity far field radar cross-section (RCS) method. The results show that as a microwave absorber，the properties of the Fe3O4/Ni nanoparticles with core-shell structure are superior to those of Ni nanoparticles or Fe3O4 nanoparticles at the same mass quality ratios. Furthermore，with the content of Ni increasing，the microwave absorption performances of the as synthesized Fe3O4 /Ni nanoparticles are enhanced. At the same time，the absorption frequency ranges will shift to the high frequency sections when the contents of Fe3O4 increase.
The spherical nickel nanoparticles (an average diameter of 70 nm) were prepared by the wire electrical explosion method，Fe3O4 was coated on nickel to form the complete coverage by the co-precipitation method，and the silane coupling agent 3-mercaptopropyltrimethoxysilane (HS-(CH2)3Si(OCH3)3，MPTS) was used as a primer to improve the nickel surface.The as-synthesized Fe3O4/Ni composite particles were uniformly dispersed into the phenolic cement， then the mixture was pasted on the metal plate with the area of 200 mm×200 mm as the test board.The test of microwave absorption was carried out by the radar-absorbing materials (RAM) reflectivity far field radar cross-section (RCS) method. The results show that as a microwave absorber，the properties of the Fe3O4/Ni nanoparticles with core-shell structure are superior to those of Ni nanoparticles or Fe3O4 nanoparticles at the same mass quality ratios. Furthermore，with the content of Ni increasing，the microwave absorption performances of the as synthesized Fe3O4 /Ni nanoparticles are enhanced. At the same time，the absorption frequency ranges will shift to the high frequency sections when the contents of Fe3O4 increase.
2008, 25(5): 19-24.
Abstract:
In order to investigate the non-linear conductivity characteristic of the silicon carbide (SiC)/low density polyethylene (LDPE) composites，combined with scanning electron microscopic observation and X-ray diffraction，the DC current-voltage characteristic of LDPE/SiC composites with different types，contents，crystal forms and sizes of SiC was studied. The experimental results show that the increase of the SiC content will cause the increase of the conductivity of SiC/LDPE composite and the decrease of the critical electric field under which the nonlinearity coefficient of conductivity changed；under the same electric field and with the same content of SiC，the conductivities of the composites filled with β-crystal SiC are higher than those filled with α-crystal SiC for nano-meter SiC green and black SiC，and the nonlinearities of the composites filled with β-crystal SiC are stronger when the filling content of the black α-crystal SiC up to 37.5wt%，the increase of the size of SiC filler can cause the decrease of the critical electric field at which the relationship curve in loglog coordination presents the turn point.
In order to investigate the non-linear conductivity characteristic of the silicon carbide (SiC)/low density polyethylene (LDPE) composites，combined with scanning electron microscopic observation and X-ray diffraction，the DC current-voltage characteristic of LDPE/SiC composites with different types，contents，crystal forms and sizes of SiC was studied. The experimental results show that the increase of the SiC content will cause the increase of the conductivity of SiC/LDPE composite and the decrease of the critical electric field under which the nonlinearity coefficient of conductivity changed；under the same electric field and with the same content of SiC，the conductivities of the composites filled with β-crystal SiC are higher than those filled with α-crystal SiC for nano-meter SiC green and black SiC，and the nonlinearities of the composites filled with β-crystal SiC are stronger when the filling content of the black α-crystal SiC up to 37.5wt%，the increase of the size of SiC filler can cause the decrease of the critical electric field at which the relationship curve in loglog coordination presents the turn point.
2008, 25(5): 25-32.
Abstract:
A novel approach for toughening the carbon fiber/epoxy composite using electrospun polysulfone (PSF) nanofibers was performed．As-received nanofibers with random orientation were directly electrospun onto the layers of carbon fiber/epoxy prepregs to toughen the composites．The ratio of different solvents and the concentration of PSF solutions， which affected the diameter and distribution of PSF nanofibers，were discussed．The mode Ⅱ interlaminar fracture toughness (GⅡC) of the nanofibers toughened composites was tested，compared to those of PSF films toughened composites. GⅡC increased 54%，130% and 177% by loading 1%，3% and 5% mass fraction of nanofibers，which was higher than those of the same content PSF films toughened composites．The SEM images show that PSF particles distributed through the whole composite interleaves and exhibited random alignment. The ILSS of nanofibers and PSF films toughened composites decreases，while the reductions of ILSS for PSF films toughened composites are larger．DMTA tests reveal good compatibility between nanofibers and epoxy matrix，compared to PSF film toughened composites.
A novel approach for toughening the carbon fiber/epoxy composite using electrospun polysulfone (PSF) nanofibers was performed．As-received nanofibers with random orientation were directly electrospun onto the layers of carbon fiber/epoxy prepregs to toughen the composites．The ratio of different solvents and the concentration of PSF solutions， which affected the diameter and distribution of PSF nanofibers，were discussed．The mode Ⅱ interlaminar fracture toughness (GⅡC) of the nanofibers toughened composites was tested，compared to those of PSF films toughened composites. GⅡC increased 54%，130% and 177% by loading 1%，3% and 5% mass fraction of nanofibers，which was higher than those of the same content PSF films toughened composites．The SEM images show that PSF particles distributed through the whole composite interleaves and exhibited random alignment. The ILSS of nanofibers and PSF films toughened composites decreases，while the reductions of ILSS for PSF films toughened composites are larger．DMTA tests reveal good compatibility between nanofibers and epoxy matrix，compared to PSF film toughened composites.
2008, 25(5): 33-38.
Abstract:
The effect of fillers on the volume shrinkage of the unsaturated polyester (UP) was studied. And the mechanism of CaCO3 fillers influence on the volume shrinkage of UP was investigated by SEM，DSC，TG and swelling test. With the increase of the fillers in the CaCO3/UP composites，the volume shrinkage ratio of the composites is improved at a lower filler amount，and then decreased at higher filler amounts. Compared with the composites obtained with the CaCO3 fillers untreated with coupling agent，the composites obtained with the CaCO3 fillers treated with coupling agent had a smaller volume shrinkage ratio. It was found that the CaCO3 fillers influence the volume shrinkage of the composites by the following ways：in the composites the resin is replaced by fillers and the form of cavity between the interface of the resin and the fillers is beneficial to the decrease in volume shrinkage；the decrease of volatility of styrene by the fillers causes the decrease of the crosslinking density of the composites，resulting in increase of the volume shrinkage. The improvement of interface action between the fillers and the resin，and the decrease of volatility of styrene as a result of coupling agent application result in the decrease of crosslinking density，leading to the decrease of the volume shrinkage.
The effect of fillers on the volume shrinkage of the unsaturated polyester (UP) was studied. And the mechanism of CaCO3 fillers influence on the volume shrinkage of UP was investigated by SEM，DSC，TG and swelling test. With the increase of the fillers in the CaCO3/UP composites，the volume shrinkage ratio of the composites is improved at a lower filler amount，and then decreased at higher filler amounts. Compared with the composites obtained with the CaCO3 fillers untreated with coupling agent，the composites obtained with the CaCO3 fillers treated with coupling agent had a smaller volume shrinkage ratio. It was found that the CaCO3 fillers influence the volume shrinkage of the composites by the following ways：in the composites the resin is replaced by fillers and the form of cavity between the interface of the resin and the fillers is beneficial to the decrease in volume shrinkage；the decrease of volatility of styrene by the fillers causes the decrease of the crosslinking density of the composites，resulting in increase of the volume shrinkage. The improvement of interface action between the fillers and the resin，and the decrease of volatility of styrene as a result of coupling agent application result in the decrease of crosslinking density，leading to the decrease of the volume shrinkage.
2008, 25(5): 39-44.
Abstract:
Epoxy resin (ER) composites filled with aligned carbon coated nickel (Ni-C) nanoparticle were prepared by use of magnetic field assisted curing process，and the electrical properties were tested. The higher the content of nanoparticles，the greater the magnitude of the magnetic field applied，the lower the resistivity of the composites obtained. I-U characteristic of the composites is tested nonlinear，indicating that the electrical conduction depends on the tunneling transport of electron cross the adjacent Ni-C nanoparticles. The resistivity of the composites exhibits negative temperature coefficient (NTC) effect above the room temperature，and can be adjusted by varying the filler content and the magnitude of magnetic field. A modified quantum tunneling model was developed to understand the origin of NTC effect in the composites， which shows that the NTC effect is attributed to the dominant thermal activation tunneling of electron，and to the low thermal expansivity of the epoxy resin matrix.
Epoxy resin (ER) composites filled with aligned carbon coated nickel (Ni-C) nanoparticle were prepared by use of magnetic field assisted curing process，and the electrical properties were tested. The higher the content of nanoparticles，the greater the magnitude of the magnetic field applied，the lower the resistivity of the composites obtained. I-U characteristic of the composites is tested nonlinear，indicating that the electrical conduction depends on the tunneling transport of electron cross the adjacent Ni-C nanoparticles. The resistivity of the composites exhibits negative temperature coefficient (NTC) effect above the room temperature，and can be adjusted by varying the filler content and the magnitude of magnetic field. A modified quantum tunneling model was developed to understand the origin of NTC effect in the composites， which shows that the NTC effect is attributed to the dominant thermal activation tunneling of electron，and to the low thermal expansivity of the epoxy resin matrix.
2008, 25(5): 45-50.
Abstract:
The effects of Methacryl-POSS coating on the interfacial properties of carbon fiber/polyarylacetylene (CF/PAA) composites were studied by means of micro-debonding measurement，three-point bend test，scanning electron microscopy (SEM)，force modulation mode atomic force microscopy (AFM)，and dynamic mechanical thermal analysis (DMTA). Wilhelmy method was used to study the wettability of PAA resin on the carbon fiber surface before and after treatment. The degree of surface roughness and the wettability of carbon fiber surface are both increased after POSS coating treatment. Compared with untreated CF/PAA composite，the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of POSS coating treated composites are increased by 36% and 50%，respectively；the glass transition temperature of the CF/PAA composites modified with POSS coating is increased by 12℃ and the loss tangent (tan δ) is decreased by 53%. The interfacial bonding of CF/PAA composites can be improved effectively by using POSS coating.
The effects of Methacryl-POSS coating on the interfacial properties of carbon fiber/polyarylacetylene (CF/PAA) composites were studied by means of micro-debonding measurement，three-point bend test，scanning electron microscopy (SEM)，force modulation mode atomic force microscopy (AFM)，and dynamic mechanical thermal analysis (DMTA). Wilhelmy method was used to study the wettability of PAA resin on the carbon fiber surface before and after treatment. The degree of surface roughness and the wettability of carbon fiber surface are both increased after POSS coating treatment. Compared with untreated CF/PAA composite，the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of POSS coating treated composites are increased by 36% and 50%，respectively；the glass transition temperature of the CF/PAA composites modified with POSS coating is increased by 12℃ and the loss tangent (tan δ) is decreased by 53%. The interfacial bonding of CF/PAA composites can be improved effectively by using POSS coating.
2008, 25(5): 51-56.
Abstract:
The carbon nanotubes (CNTs)/styrene-butadiene rubber (SBR) powder composites filled with different contents of CNTs were prepared by the spray drying method. The dispersion of CNTs in the matrix was investigated. Electrical and dielectric properties of the composites were tested and the corresponding theoretical analysis was discussed. The results show that CNTs are well dispersed in the matrix，which is beneficial for the reinforcement and modification of CNTs on the rubber matrix. Compared with the pure SBR and composites filled with CB，the composites filled with CNTs present a higher dielectric constant and lower dielectric loss under frequencies of 8~18GHz. The electrical conductivity of the composites filled with CNTs is gradually improved with the augment of filled CNTs. Compared with that of pure SBR and composites filled with 60 phr CB，the electrical conductivity of the composites filled with 60 phr CNTs is enhanced by about 10 orders of the magnitudes. There exist contemporary tunnel and percolation conductive mechanism in the CNTs/SBR composites. Powder SBR composites modified with CNTs could be a new type of nanocomposites with excellent comprehensive properties and will be potentially applied in the fields of antistatic rubber，electromagnetic shield, dielectric materials and so on.
The carbon nanotubes (CNTs)/styrene-butadiene rubber (SBR) powder composites filled with different contents of CNTs were prepared by the spray drying method. The dispersion of CNTs in the matrix was investigated. Electrical and dielectric properties of the composites were tested and the corresponding theoretical analysis was discussed. The results show that CNTs are well dispersed in the matrix，which is beneficial for the reinforcement and modification of CNTs on the rubber matrix. Compared with the pure SBR and composites filled with CB，the composites filled with CNTs present a higher dielectric constant and lower dielectric loss under frequencies of 8~18GHz. The electrical conductivity of the composites filled with CNTs is gradually improved with the augment of filled CNTs. Compared with that of pure SBR and composites filled with 60 phr CB，the electrical conductivity of the composites filled with 60 phr CNTs is enhanced by about 10 orders of the magnitudes. There exist contemporary tunnel and percolation conductive mechanism in the CNTs/SBR composites. Powder SBR composites modified with CNTs could be a new type of nanocomposites with excellent comprehensive properties and will be potentially applied in the fields of antistatic rubber，electromagnetic shield, dielectric materials and so on.
2008, 25(5): 57-63.
Abstract:
The blend resin with different proportions of benzoxazine/phenolic was investigated on the kinetics of reaction，thermal degradation kinetics and ablative property. The resin composite was prepared by impregnating glass cloth. The high temperature and normal temperature mechanical properties，the thermal properties and the ablative properties of the composite were investigated. The results show that the blend resin composite has excellent mechanical properties at high temperature and normal temperature and its thermal properties meet the needs of ablative materials. This blend resin composite can be used as the ablative composite.
The blend resin with different proportions of benzoxazine/phenolic was investigated on the kinetics of reaction，thermal degradation kinetics and ablative property. The resin composite was prepared by impregnating glass cloth. The high temperature and normal temperature mechanical properties，the thermal properties and the ablative properties of the composite were investigated. The results show that the blend resin composite has excellent mechanical properties at high temperature and normal temperature and its thermal properties meet the needs of ablative materials. This blend resin composite can be used as the ablative composite.
2008, 25(5): 64-68.
Abstract:
The short cut carbon fibers (CF) were treated by air oxidation and nitric acid oxidation，and the mechanical properties of CF reinforced thermoplastic polyimide(TPI) resin matrix composites (CF/TPI) were studied.The results of Boehm titration method show that the acidic functional groups on the treated surface of CF increase as the treating time extends，and the nitric acid oxidation is better than the air oxidation. Compared with the untreated composite，the tensile strength and flexural strength are improved by 10% and 14%，when the treatment time of nitric acid is 20 min. X-ray photoelectron spectroscopy (XPS) indicates that the acidic oxygen functional groups on CF increase by 35.89 %. The atomic force microscopy (AFM ) images show that the nitric acid is capable of increasing the roughness of CF. The scanning electron microscopy(SEM) photograph of the fracture surface reveals that there is interface bonding between treated CF and TPI .
The short cut carbon fibers (CF) were treated by air oxidation and nitric acid oxidation，and the mechanical properties of CF reinforced thermoplastic polyimide(TPI) resin matrix composites (CF/TPI) were studied.The results of Boehm titration method show that the acidic functional groups on the treated surface of CF increase as the treating time extends，and the nitric acid oxidation is better than the air oxidation. Compared with the untreated composite，the tensile strength and flexural strength are improved by 10% and 14%，when the treatment time of nitric acid is 20 min. X-ray photoelectron spectroscopy (XPS) indicates that the acidic oxygen functional groups on CF increase by 35.89 %. The atomic force microscopy (AFM ) images show that the nitric acid is capable of increasing the roughness of CF. The scanning electron microscopy(SEM) photograph of the fracture surface reveals that there is interface bonding between treated CF and TPI .
2008, 25(5): 69-73.
Abstract:
PTW (Potassium titanate whiskers) reinforced monomer casting（MC）nylon composites were prepared by anionic polymerization of caprolactam for improving the ability of train plates’ gasket affected by absorbing moisture to get a high and steady friction coefficient beside better toughness and intensity，and treated by boiled water. The tribological properties of the composites were investigated by a ring-blank wear tester under dry condition. Compared with the samples treated by boiled water，the friction coefficient of the untreated sample becomes instable because of the moisture absorbing ability，while the treated one could get stability in a short time. The main wear mechanism of the pure nylon is adhesive，and the PTW reinforced MC nylon reduces the adhesive wear obviously. The total mass wear ratio of the pure sample is higher than that of the reinforced one. The moisture absorbing ability cannot affect the wear mechanism obviously. The PTW reinforced monomer casting nylon treated by boiled water could improve the tribological capability.
PTW (Potassium titanate whiskers) reinforced monomer casting（MC）nylon composites were prepared by anionic polymerization of caprolactam for improving the ability of train plates’ gasket affected by absorbing moisture to get a high and steady friction coefficient beside better toughness and intensity，and treated by boiled water. The tribological properties of the composites were investigated by a ring-blank wear tester under dry condition. Compared with the samples treated by boiled water，the friction coefficient of the untreated sample becomes instable because of the moisture absorbing ability，while the treated one could get stability in a short time. The main wear mechanism of the pure nylon is adhesive，and the PTW reinforced MC nylon reduces the adhesive wear obviously. The total mass wear ratio of the pure sample is higher than that of the reinforced one. The moisture absorbing ability cannot affect the wear mechanism obviously. The PTW reinforced monomer casting nylon treated by boiled water could improve the tribological capability.
2008, 25(5): 74-78.
Abstract:
The carbon fiber precursors were prepared from liquefied wood in phenol by adding hexamethylenetetramine and soaking in the solution containing hydrochloric acid and formaldehyde as the main components. The carbon fibers were obtained from the precursors by the direct carbonization. The influence of the carbonization condition on the mechanical properties and yield of the carbon fibers was investigated by the paper frame method. The results show that the mechanical properties of the carbon fibers increase obviously with increasing carbonization temperature and time，but decrease with increasing the heating rate. It is also found that the higher the wood/phenol ratio，the higher the increasing amplitude of mechanical properties，and the smaller the dimension shrinkage. Carbon fibers from liquefied wood with tensile strength of 1.7 GPa，tensile modulus of 159 GPa and yield of 60% were reached under optimal carbonization conditions.
The carbon fiber precursors were prepared from liquefied wood in phenol by adding hexamethylenetetramine and soaking in the solution containing hydrochloric acid and formaldehyde as the main components. The carbon fibers were obtained from the precursors by the direct carbonization. The influence of the carbonization condition on the mechanical properties and yield of the carbon fibers was investigated by the paper frame method. The results show that the mechanical properties of the carbon fibers increase obviously with increasing carbonization temperature and time，but decrease with increasing the heating rate. It is also found that the higher the wood/phenol ratio，the higher the increasing amplitude of mechanical properties，and the smaller the dimension shrinkage. Carbon fibers from liquefied wood with tensile strength of 1.7 GPa，tensile modulus of 159 GPa and yield of 60% were reached under optimal carbonization conditions.
2008, 25(5): 79-84.
Abstract:
The total hemisphere emissivity (εH) at 90℃ and the normal spectral reflectance at room temperature of 3D C/SiC composites were measured with the steady state calorimeter method and FTIR，respectively. The effects of the surface morphology，thickness of SiC coating and high temperature oxidation on the thermal radiation properties of 3D C/SiC were investigated. The results show that 3D C/SiC composites have excellent thermal radiation properties and the total hemisphere emissivity attains 0.83. With increasing the SiC coating thickness，the total hemisphere emissivity of 3D C/SiC composites decreases first and increases then. After oxidation，the thermal radiation properties of 3D C/SiC with three character surface remarkably increase.
The total hemisphere emissivity (εH) at 90℃ and the normal spectral reflectance at room temperature of 3D C/SiC composites were measured with the steady state calorimeter method and FTIR，respectively. The effects of the surface morphology，thickness of SiC coating and high temperature oxidation on the thermal radiation properties of 3D C/SiC were investigated. The results show that 3D C/SiC composites have excellent thermal radiation properties and the total hemisphere emissivity attains 0.83. With increasing the SiC coating thickness，the total hemisphere emissivity of 3D C/SiC composites decreases first and increases then. After oxidation，the thermal radiation properties of 3D C/SiC with three character surface remarkably increase.
2008, 25(5): 85-90.
Abstract:
The 2D C/SiC plate seeded with the defects was manufactured by the chemical vapor infiltration (CVI) process. The C/SiC plate with defects was examined using three Nondestructive Testing (NDT) technologies: thermography，X-ray radiography and computed tomography. The flexural properties of the sample with the defect were also investigated. The results show that X-ray radiographic technology is applicable to examine the defects with different densities，thermographic technology can be applicable to test the defects with poor thermal diffusivity such as the pores and delaminations，and industrial computed tomographic technology is applicable to detect the exact localization，in particular the visualization of the depth and thickness of the defects. It is found that the 2D C/SiC samples can easily crack and delaminate in the interface between matrix and defect under loading. With increasing the defect length，the flexural strength of the samples and the crack propagation energy decrease.
The 2D C/SiC plate seeded with the defects was manufactured by the chemical vapor infiltration (CVI) process. The C/SiC plate with defects was examined using three Nondestructive Testing (NDT) technologies: thermography，X-ray radiography and computed tomography. The flexural properties of the sample with the defect were also investigated. The results show that X-ray radiographic technology is applicable to examine the defects with different densities，thermographic technology can be applicable to test the defects with poor thermal diffusivity such as the pores and delaminations，and industrial computed tomographic technology is applicable to detect the exact localization，in particular the visualization of the depth and thickness of the defects. It is found that the 2D C/SiC samples can easily crack and delaminate in the interface between matrix and defect under loading. With increasing the defect length，the flexural strength of the samples and the crack propagation energy decrease.
2008, 25(5): 91-97.
Abstract:
Using needled carbon fiber felt as the preform，the fiber coating with multilayered (PyC/SiC/TaC) fabricating by chemical vapor infiltration was used to modify C/C composites. The novel C/C composites were fabricated by chemical vapor infiltration and impregnation of thermosetting resin and carbonization. The microstructures and mechanical properties of C/C composites were investigated. The results show that the multilayered (PyC/SiC/TaC) coating enwraps around the carbon fiber evenly and compactly and forms cannular microstructure in C/C composites; both the strength and toughness of modified composites increase apparently. The average flexural strength of the composites reaches 522MPa，the maximum fracture displacement reaches 1.19mm，and the fracture mode shows brittle fracture. After high temperature treating (HTT) at 2000℃，the flexural strength of the composites decreases，while the maximum fracture displacement increases，and the fracture mode of the composites shows "pseudo plastic" fracture.
Using needled carbon fiber felt as the preform，the fiber coating with multilayered (PyC/SiC/TaC) fabricating by chemical vapor infiltration was used to modify C/C composites. The novel C/C composites were fabricated by chemical vapor infiltration and impregnation of thermosetting resin and carbonization. The microstructures and mechanical properties of C/C composites were investigated. The results show that the multilayered (PyC/SiC/TaC) coating enwraps around the carbon fiber evenly and compactly and forms cannular microstructure in C/C composites; both the strength and toughness of modified composites increase apparently. The average flexural strength of the composites reaches 522MPa，the maximum fracture displacement reaches 1.19mm，and the fracture mode shows brittle fracture. After high temperature treating (HTT) at 2000℃，the flexural strength of the composites decreases，while the maximum fracture displacement increases，and the fracture mode of the composites shows "pseudo plastic" fracture.
2008, 25(5): 98-103.
Abstract:
Mori-Tanaka method and the modified shear-lag model were combined to develop formulae to predict the modulus and to give the stress field of unidirectional short fiber reinforced high density foam plastics. The finite element models accounting for different cases were also built to analyze the stress distribution of the fibers and matrix. The research results show that the theoretical prediction agrees well with the solution of the finite element analysis. The modified shear-lag model can be used to explain the stress transfer mechanism of the material. It is found that the axial stress of fibers and interfacial shear stress increase with increasing the volume fraction of void，which would make fibers easier to debond and break. The situations，fiber debonded at the end or penetrated void，lead to local stress concentration and have small influence on the whole stress distribution of the material.
Mori-Tanaka method and the modified shear-lag model were combined to develop formulae to predict the modulus and to give the stress field of unidirectional short fiber reinforced high density foam plastics. The finite element models accounting for different cases were also built to analyze the stress distribution of the fibers and matrix. The research results show that the theoretical prediction agrees well with the solution of the finite element analysis. The modified shear-lag model can be used to explain the stress transfer mechanism of the material. It is found that the axial stress of fibers and interfacial shear stress increase with increasing the volume fraction of void，which would make fibers easier to debond and break. The situations，fiber debonded at the end or penetrated void，lead to local stress concentration and have small influence on the whole stress distribution of the material.
2008, 25(5): 104-113.
Abstract:
9 two spams partially prestressed concrete continuous beams with unbonded CFRP tendon were fabricated，which were tested with two point load applied at each mid span. During the test，the mechanical behaviors at the stage of cracks，yielding of the non prestressed steel bar both in the critical section of intermediate support and mid span and ultimate limit state were investigated. The law of stress increment in the unbonded CFRP tendon was obtained in the design state and in the ultimate state. The calculation formula of equivalent length of equivalent plastic hinge zone of continuous beams is presented based on the test data for both states. The calculation formulas of the moment modification coefficient with the global reinforcement index of intermediate support as variable and with the plastic rotation of intermediate support as variable are presented respectively.
9 two spams partially prestressed concrete continuous beams with unbonded CFRP tendon were fabricated，which were tested with two point load applied at each mid span. During the test，the mechanical behaviors at the stage of cracks，yielding of the non prestressed steel bar both in the critical section of intermediate support and mid span and ultimate limit state were investigated. The law of stress increment in the unbonded CFRP tendon was obtained in the design state and in the ultimate state. The calculation formula of equivalent length of equivalent plastic hinge zone of continuous beams is presented based on the test data for both states. The calculation formulas of the moment modification coefficient with the global reinforcement index of intermediate support as variable and with the plastic rotation of intermediate support as variable are presented respectively.
2008, 25(5): 114-119.
Abstract:
The pultrusion of the carbon fiber reinforced plastics (CFRP) is a dynamic process. The degree of cure and temperature are coupled during this process. According to the curing kinetics and the heat transfer theory，the models of unsteady temperature filed and curing were established. The kinetic parameters used to predict the temperature profile were obtained from differential scanning calorimetry (DSC). The finite element method and finite difference method were combined，and the indirect decoupling method based on ANSYS was adopted to simulate the temperature profile in FRP during pultrusion. The fiber Bragg grating (FBG) was encapsulated in an aluminum capillary in order to exclude the non-temperature influence. The feature and the temperature sensitivity coefficient were acquired through experiment to guarantee the accuracy of the real time temperature detecting. The simulative result is in good agreement with the experimental one. The simulative method makes convenience for the optimization of technology parameters of pultrusion and takes the place of the traditional trial-and-error method.
The pultrusion of the carbon fiber reinforced plastics (CFRP) is a dynamic process. The degree of cure and temperature are coupled during this process. According to the curing kinetics and the heat transfer theory，the models of unsteady temperature filed and curing were established. The kinetic parameters used to predict the temperature profile were obtained from differential scanning calorimetry (DSC). The finite element method and finite difference method were combined，and the indirect decoupling method based on ANSYS was adopted to simulate the temperature profile in FRP during pultrusion. The fiber Bragg grating (FBG) was encapsulated in an aluminum capillary in order to exclude the non-temperature influence. The feature and the temperature sensitivity coefficient were acquired through experiment to guarantee the accuracy of the real time temperature detecting. The simulative result is in good agreement with the experimental one. The simulative method makes convenience for the optimization of technology parameters of pultrusion and takes the place of the traditional trial-and-error method.
2008, 25(5): 120-124.
Abstract:
The stress field，saturated stress，stress transfer efficiency and effective length in carbon nanotubes (CNTs) reinforced polymer composites were analyzed based on the three-phase concentric cylindrical shell model along with shear-lag arguments. The thickness for interphase is comparable with the diameter of CNTs(0.1~100nm)，in the CNTs reinforced polymer composites. The interphase region should be included in the three-phase concentric cylindrical shell model to analyze the stress in CNTs reinforced polymer composites. The stress transfer efficiency and the CNTs saturated stress decrease monotonically with increasing the interphase thickness，but the converse is true for the CNTs effective length. The aspect ratio of CNTs influences the stress transfer efficiency and effective length in the CNTs reinforced polymer composites significantly when the aspect ratio is small.
The stress field，saturated stress，stress transfer efficiency and effective length in carbon nanotubes (CNTs) reinforced polymer composites were analyzed based on the three-phase concentric cylindrical shell model along with shear-lag arguments. The thickness for interphase is comparable with the diameter of CNTs(0.1~100nm)，in the CNTs reinforced polymer composites. The interphase region should be included in the three-phase concentric cylindrical shell model to analyze the stress in CNTs reinforced polymer composites. The stress transfer efficiency and the CNTs saturated stress decrease monotonically with increasing the interphase thickness，but the converse is true for the CNTs effective length. The aspect ratio of CNTs influences the stress transfer efficiency and effective length in the CNTs reinforced polymer composites significantly when the aspect ratio is small.
2008, 25(5): 125-133.
Abstract:
An experimental study on the impact resistance and compressive strength of damaged laminates was carried out by using about 800 specimens of 14 composite systems. It has been found that there is a common knee point for the impact energy-dent depth curve and the dent depth-compressive strength curve for the same type of composite laminates. The overlapped area of the inner delaminations does not increase and its compressive strength does not reduce generally after the knee point occurs. Some fiber breakage at the impact location on the surface could be visible after the knee point occurs. The damage tolerance behavior characterized by the traditional CAI method may get the opposite conclusion obtained from the structural verification test results. Therefore，a characterization system for damage tolerance and damage resistance behaviors of composite systems is proposed by using the knee point features of the capability of composite laminates to withstand impact (including damage resistance and damage tolerance)，i.e. the damage resistance and damage tolerance behavior is characterized by means of the maximum contact force，Fmaxand the compressive failure threshold value (strength or failure strain) (CAIT) of the specimens with quasi-isotropic stacking sequence respectively.
An experimental study on the impact resistance and compressive strength of damaged laminates was carried out by using about 800 specimens of 14 composite systems. It has been found that there is a common knee point for the impact energy-dent depth curve and the dent depth-compressive strength curve for the same type of composite laminates. The overlapped area of the inner delaminations does not increase and its compressive strength does not reduce generally after the knee point occurs. Some fiber breakage at the impact location on the surface could be visible after the knee point occurs. The damage tolerance behavior characterized by the traditional CAI method may get the opposite conclusion obtained from the structural verification test results. Therefore，a characterization system for damage tolerance and damage resistance behaviors of composite systems is proposed by using the knee point features of the capability of composite laminates to withstand impact (including damage resistance and damage tolerance)，i.e. the damage resistance and damage tolerance behavior is characterized by means of the maximum contact force，Fmaxand the compressive failure threshold value (strength or failure strain) (CAIT) of the specimens with quasi-isotropic stacking sequence respectively.
2008, 25(5): 134-140.
Abstract:
An optimization technology was adopted to reconstruct the loads applied on the advanced composite grids structure （AGS）. Based on the forward response model built in literature [11]，a figure of merit J was defined to measure the difference between the calculated response and the measured response. Thus，the inverse problem was converted to searching for inputs that satisfy the minimization of the defined J. Smoothing arithmetic was adopted to the solving and power gradient cloud together with the extreme point of figure of merit J was utilized to guide the searching for the load location. Therefore，a set of methods of load reconstruction are put forward，including location determination and load history recovering. The results of the numerical example and the physical experiment indicate that the methods proposed in this paper run stably and accurately，which provides the necessary techniques for engineering application of AGS.
An optimization technology was adopted to reconstruct the loads applied on the advanced composite grids structure （AGS）. Based on the forward response model built in literature [11]，a figure of merit J was defined to measure the difference between the calculated response and the measured response. Thus，the inverse problem was converted to searching for inputs that satisfy the minimization of the defined J. Smoothing arithmetic was adopted to the solving and power gradient cloud together with the extreme point of figure of merit J was utilized to guide the searching for the load location. Therefore，a set of methods of load reconstruction are put forward，including location determination and load history recovering. The results of the numerical example and the physical experiment indicate that the methods proposed in this paper run stably and accurately，which provides the necessary techniques for engineering application of AGS.
2008, 25(5): 141-146.
Abstract:
An analytical method is presented for predicting the elastic modulus of concrete with the inhomogeneous interfacial transition zone (ITZ). Based on the characteristics of distribution of cement grains in the ITZ layer，the local water/cement ratio and porosity at any point in the ITZ layer were given. By dividing the inhomogeneous ITZ into a series of concentric spherical shell elements，the elastic moduli of each spherical shell element and the cement paste matrix were determined through the inverse method. The three-phase concrete was decomposed into a series of two-phase composite substructures. The elastic modulus of concrete was derived by applying the solution for a two-phase composite sphere model. The validity of the method presented in this paper was verified with two sets of experimental results reported in the literature. It demonstrates that，for a given aggregate volume fraction，the elastic modulus of concrete decreases with the increase of the maximum cement diameter and water/cement ratio，but increases with the increase of the maximum aggregate diameter. The aggregate gradation also has a certain influence on the elastic modulus of concrete.
An analytical method is presented for predicting the elastic modulus of concrete with the inhomogeneous interfacial transition zone (ITZ). Based on the characteristics of distribution of cement grains in the ITZ layer，the local water/cement ratio and porosity at any point in the ITZ layer were given. By dividing the inhomogeneous ITZ into a series of concentric spherical shell elements，the elastic moduli of each spherical shell element and the cement paste matrix were determined through the inverse method. The three-phase concrete was decomposed into a series of two-phase composite substructures. The elastic modulus of concrete was derived by applying the solution for a two-phase composite sphere model. The validity of the method presented in this paper was verified with two sets of experimental results reported in the literature. It demonstrates that，for a given aggregate volume fraction，the elastic modulus of concrete decreases with the increase of the maximum cement diameter and water/cement ratio，but increases with the increase of the maximum aggregate diameter. The aggregate gradation also has a certain influence on the elastic modulus of concrete.
2008, 25(5): 147-150.
Abstract:
The crack-bridge behavior of inclined steel fibers in the cementitious composites was researched in the meso-scale to analyze the mechanical behavior of random distribution fiber reinforced concrete which carries the pull out force. The theoretical solution of a single fiber distributed randomly was integrated in the 3D space to describe the change course of the pull out fiber concrete. The function relation of the pull out force，pull out work value with crack open displacement and fiber inclined angle was presented. Increasing the inclination angle of fiber will depress the crack-bridge force. The theoretical values of the pull out force and pull out work are consistent with the experimental values.
The crack-bridge behavior of inclined steel fibers in the cementitious composites was researched in the meso-scale to analyze the mechanical behavior of random distribution fiber reinforced concrete which carries the pull out force. The theoretical solution of a single fiber distributed randomly was integrated in the 3D space to describe the change course of the pull out fiber concrete. The function relation of the pull out force，pull out work value with crack open displacement and fiber inclined angle was presented. Increasing the inclination angle of fiber will depress the crack-bridge force. The theoretical values of the pull out force and pull out work are consistent with the experimental values.
2008, 25(5): 151-156.
Abstract:
A coupled model of residual stiffness-residual strength was presented under the assumption that the strength and stiffness of the laminates are degraded in fatigue process due to the same damage modes at the same time. A relationship between the damage defined by residual stiffness and that defined by residual strength was set up. The coupled model was validated by the experimental results of three kinds of laminates with different stacking sequences. The results show that the residual stiffness model and residual strength model presented in this paper are applicable to describe the stiffness and strength degradation rules. The strength degradation rule can be easily determined by only a few specimens based on this coupled model. And the parameter in the strength degradation model can be regarded as a material constant.
A coupled model of residual stiffness-residual strength was presented under the assumption that the strength and stiffness of the laminates are degraded in fatigue process due to the same damage modes at the same time. A relationship between the damage defined by residual stiffness and that defined by residual strength was set up. The coupled model was validated by the experimental results of three kinds of laminates with different stacking sequences. The results show that the residual stiffness model and residual strength model presented in this paper are applicable to describe the stiffness and strength degradation rules. The strength degradation rule can be easily determined by only a few specimens based on this coupled model. And the parameter in the strength degradation model can be regarded as a material constant.
2008, 25(5): 157-162.
Abstract:
Based on Mori-Tanaka method and the equivalent inclusion theory，the bridging matrix which is used to define the relationship between mean stress tensors in resin and fiber of fiber-reinforcement composites can be achieved in this paper，as well as the explicit formula of matrix components. Considering the influence of factors such as practical engineering，symmetry of compliance matrix，physical performance of resin and micro-geometrical characteristics，the expression of components is simplified reasonably. Corrected parameters，which are the functions of the equivalent plastic strain of the matrix，are introduced and the mean stress distributions of fibers and resin are determined accordingly. The numerical results indicate that the theoretical values of the method presented in this paper are well fit with experimental results.
Based on Mori-Tanaka method and the equivalent inclusion theory，the bridging matrix which is used to define the relationship between mean stress tensors in resin and fiber of fiber-reinforcement composites can be achieved in this paper，as well as the explicit formula of matrix components. Considering the influence of factors such as practical engineering，symmetry of compliance matrix，physical performance of resin and micro-geometrical characteristics，the expression of components is simplified reasonably. Corrected parameters，which are the functions of the equivalent plastic strain of the matrix，are introduced and the mean stress distributions of fibers and resin are determined accordingly. The numerical results indicate that the theoretical values of the method presented in this paper are well fit with experimental results.
2008, 25(5): 163-168.
Abstract:
The tensile properties of two kinds of plane woven composite laminates after low velocity impact were studied by experiment. And an FEM model was established to simulate the damage growth in the laminates. In this model，the low velocity impact damage area was equivalent to a soft zone with a regular shape，both different damage criterions and modulus attenuation rules were used for two different ply forms. The results of the FEM model show good agreement with the experimental data. It means that this model can be used to predict the damage growth behavior and residual tensile strength of plane woven laminates with impact damage. The impacted plane woven composite laminates with different ply forms possess different damage propagation ways. The tensile strength after low velocity impact of plane woven laminates decreases over 50%. These results should be considered in designing the composite structure.
The tensile properties of two kinds of plane woven composite laminates after low velocity impact were studied by experiment. And an FEM model was established to simulate the damage growth in the laminates. In this model，the low velocity impact damage area was equivalent to a soft zone with a regular shape，both different damage criterions and modulus attenuation rules were used for two different ply forms. The results of the FEM model show good agreement with the experimental data. It means that this model can be used to predict the damage growth behavior and residual tensile strength of plane woven laminates with impact damage. The impacted plane woven composite laminates with different ply forms possess different damage propagation ways. The tensile strength after low velocity impact of plane woven laminates decreases over 50%. These results should be considered in designing the composite structure.
2008, 25(5): 169-174.
Abstract:
Based on the theory of Fourier series，a model for predicting the stiffness of filament wound composite was provided according to the characteristic of micro-structure of filament-wound composite. In this model，the stiffness of representative volume element (RVE) was treated as a stiffness field; the function of this field was described by 2D Fourier series expansion and the expressions were deduced. The result of the example shows that the model can predict the macro-stiffness of filament-wound composite, also can reflect the effect of internal micro-structure of filament wound composite on the stiffness characteristic of filament wound composite.
Based on the theory of Fourier series，a model for predicting the stiffness of filament wound composite was provided according to the characteristic of micro-structure of filament-wound composite. In this model，the stiffness of representative volume element (RVE) was treated as a stiffness field; the function of this field was described by 2D Fourier series expansion and the expressions were deduced. The result of the example shows that the model can predict the macro-stiffness of filament-wound composite, also can reflect the effect of internal micro-structure of filament wound composite on the stiffness characteristic of filament wound composite.
2008, 25(5): 175-180.
Abstract:
A surrogate-based inverse procedure is presented for characterizing the material property of functionally graded material（FGM）. A forward model was built by the finite element software and the dynamic displacement responses excited by the load was obtained. A response surface model was established based on the responses of the sample points selected by employing design of experiment， the substitute for the finite element software as the forward operator and the genetic algorithm was employed as the inverse operator. The maximums of the displacement responses with different levels of noise were used as the inputs for the whole procedure，and the material volume fraction parameters were finally determined. The dynamic displacement responses excited by the single sin load in an actual SiC C functionally graded beam were applied to determine the volume fractions of the SiC-C beam through the genetic algorithm. The numeric example demonstrates the efficiency of the presented method. The surrogate-based inverse method improves the calculating efficiency by avoiding repetitious forward calculations.
A surrogate-based inverse procedure is presented for characterizing the material property of functionally graded material（FGM）. A forward model was built by the finite element software and the dynamic displacement responses excited by the load was obtained. A response surface model was established based on the responses of the sample points selected by employing design of experiment， the substitute for the finite element software as the forward operator and the genetic algorithm was employed as the inverse operator. The maximums of the displacement responses with different levels of noise were used as the inputs for the whole procedure，and the material volume fraction parameters were finally determined. The dynamic displacement responses excited by the single sin load in an actual SiC C functionally graded beam were applied to determine the volume fractions of the SiC-C beam through the genetic algorithm. The numeric example demonstrates the efficiency of the presented method. The surrogate-based inverse method improves the calculating efficiency by avoiding repetitious forward calculations.
2008, 25(5): 181-188.
Abstract:
By adopting a piecewise impact force model and the First Shearing Deformation Theory (FSDT), the dynamic behavior of composite laminates with simply supported boundary was analyzed under low velocity impact in the geometric center by a steel ball，including the evolution rules of the impact force and contact time with relations to impact velocity，and vibration response of laminates，stress-wave propagation，as well as surface dent. The delamination area of the laminates was calculated and analyzed by adopting hypothesis of no thickness interface with a simplified version of Tsai-Wu‘s Damage Criteria, and the relation between size of delamination area and initial velocity of steel ball is also presented. The study shows that the impact force is in proportion to the initial velocity of the steel ball. And the phase velocity of stress-wave propagation is the same in each layer of composite laminates for a given direction. The velocity of tensional stress-wave propagation in the direction of fiber concentration is faster than that in the other directions，while it is on the contrary for shearing stress-wave propagation. The delamination of composite laminates occurs even under low velocity impact. The damage area expands and its shape changes with increasing the impact velocity.
By adopting a piecewise impact force model and the First Shearing Deformation Theory (FSDT), the dynamic behavior of composite laminates with simply supported boundary was analyzed under low velocity impact in the geometric center by a steel ball，including the evolution rules of the impact force and contact time with relations to impact velocity，and vibration response of laminates，stress-wave propagation，as well as surface dent. The delamination area of the laminates was calculated and analyzed by adopting hypothesis of no thickness interface with a simplified version of Tsai-Wu‘s Damage Criteria, and the relation between size of delamination area and initial velocity of steel ball is also presented. The study shows that the impact force is in proportion to the initial velocity of the steel ball. And the phase velocity of stress-wave propagation is the same in each layer of composite laminates for a given direction. The velocity of tensional stress-wave propagation in the direction of fiber concentration is faster than that in the other directions，while it is on the contrary for shearing stress-wave propagation. The delamination of composite laminates occurs even under low velocity impact. The damage area expands and its shape changes with increasing the impact velocity.
