2004 Vol. 21, No. 5
2004, 21(5): 1-6.
Abstract:
Titanium matrix composites reinforced with different mole ratios between TiB and TiC were fabricated by common casting technique utilizing the self-propagation high-temperature synthesis reactions between titanium and B4 C and graphite. The mechanical properties of the titanium matrix composites at high temperature were tested. The results show that tensile properties of the titanium matrix composites at high temperature are improved obviously due to the in situ synthesized reinforcements. Fracture mechanism is related to temperature. When the temperature is low, the fracture of reinforcement makes the composite failure. As temperature increases, the debonding between matrix alloy and reinforcement TiB conduces to the composites failure. Because the crack is likely to nucleate and grow on the end of TiB whiskers which makes the composites to failure, the addition of graphite forms more TiC particles and improve the mechanical properties of titanium matrix composites at high temperature.
Titanium matrix composites reinforced with different mole ratios between TiB and TiC were fabricated by common casting technique utilizing the self-propagation high-temperature synthesis reactions between titanium and B4 C and graphite. The mechanical properties of the titanium matrix composites at high temperature were tested. The results show that tensile properties of the titanium matrix composites at high temperature are improved obviously due to the in situ synthesized reinforcements. Fracture mechanism is related to temperature. When the temperature is low, the fracture of reinforcement makes the composite failure. As temperature increases, the debonding between matrix alloy and reinforcement TiB conduces to the composites failure. Because the crack is likely to nucleate and grow on the end of TiB whiskers which makes the composites to failure, the addition of graphite forms more TiC particles and improve the mechanical properties of titanium matrix composites at high temperature.
2004, 21(5): 7-10.
Abstract:
The constant stress creep behavior at 350 ℃ was investigated for Al2O3 fiber reinforced Al based composite fabricated by squeeze casting. During the interrupted creep process, the variations of microstructure were observed by means of SEM. The fracture of fibers, the debonding of fiber and matrix, and the deformation of matrix were found in creep of composite materials. On the basis of the changes of microstructure in three phases of creep, the creep behavior of the composite was analyzed. It is suggested that the glide and climb of dislocation around fibers contribute to creep of the composite.The authors propound a rupture mechanism of short fiber reinforced MMCs, which succeeds in interpreting the three stages of creep for composites.
The constant stress creep behavior at 350 ℃ was investigated for Al2O3 fiber reinforced Al based composite fabricated by squeeze casting. During the interrupted creep process, the variations of microstructure were observed by means of SEM. The fracture of fibers, the debonding of fiber and matrix, and the deformation of matrix were found in creep of composite materials. On the basis of the changes of microstructure in three phases of creep, the creep behavior of the composite was analyzed. It is suggested that the glide and climb of dislocation around fibers contribute to creep of the composite.The authors propound a rupture mechanism of short fiber reinforced MMCs, which succeeds in interpreting the three stages of creep for composites.
2004, 21(5): 11-15.
Abstract:
Pressureless melt infiltration was used for preparing high TiC content TiC/NiAl composites. Effects of the infiltration temperature and time on the microstructure, hardness and fracture toughness of the composites were studied. XRD and EDS/TEM were used to analyze the phase identification and the solubility of NiAl and TiC in each other. The results show that pressureless melt infiltration is an effective method to fabricate dense TiC/NiAl composites. The infiltration time can greatly decrease when the infiltration temperature suitably increases. No obvious effect of the infiltration temperature and time on the hardness and the fracture toughness of the composites was found. NiAl and TiC were well bonded, and they are the only two phases in the composites after infiltration. TiC partially dissolved into NiAl phase, and the solubility increased with increasing infiltration time. Little Al and a trace of Ni were detected at the outer layer of TiC.
Pressureless melt infiltration was used for preparing high TiC content TiC/NiAl composites. Effects of the infiltration temperature and time on the microstructure, hardness and fracture toughness of the composites were studied. XRD and EDS/TEM were used to analyze the phase identification and the solubility of NiAl and TiC in each other. The results show that pressureless melt infiltration is an effective method to fabricate dense TiC/NiAl composites. The infiltration time can greatly decrease when the infiltration temperature suitably increases. No obvious effect of the infiltration temperature and time on the hardness and the fracture toughness of the composites was found. NiAl and TiC were well bonded, and they are the only two phases in the composites after infiltration. TiC partially dissolved into NiAl phase, and the solubility increased with increasing infiltration time. Little Al and a trace of Ni were detected at the outer layer of TiC.
2004, 21(5): 16-21.
Abstract:
25 mm-diameter polycrystalline diamond composites of typical interface structures were sintered in situ under 5.8 GPa at 1500℃ for ３ min by use of different thick cobalt foils as medial layer and different diamond grain sizes. The linear distribution of Co K α (EDW) and characteristic X-ray image of cobalt near the interface was examined by SEM. The bonding strength of the PDC interface was measured by shear test and the heat resistance was measured by the numbers of quench and compression tests. The bonding mechanism of the interface between cemented tungsten carbide and polycrystalline diamond was discussed. The results show that the bonding strength of the PDC interface increased firstly from 2.67 GPa to 2.82 GPa then decreased to 1.47 GPa and the heat resistance decreased from 15, 11 to 5 times separately as the interface cobalt changed from even distribution to high content of cobalt then to cobalt medial layer, compared to the small grain size(10 μm) interface(2.67 GPa, 15 times) the bonding strength and the heat resistance of the coarse grain size(80 μm) interface(0.52 GPa, 7 times) decreased greatly. These indicated that the properties of the PDC interface were depended not only on the content and existential state of the cobalt but also on the grain size of diamond at the interface.
25 mm-diameter polycrystalline diamond composites of typical interface structures were sintered in situ under 5.8 GPa at 1500℃ for ３ min by use of different thick cobalt foils as medial layer and different diamond grain sizes. The linear distribution of Co K α (EDW) and characteristic X-ray image of cobalt near the interface was examined by SEM. The bonding strength of the PDC interface was measured by shear test and the heat resistance was measured by the numbers of quench and compression tests. The bonding mechanism of the interface between cemented tungsten carbide and polycrystalline diamond was discussed. The results show that the bonding strength of the PDC interface increased firstly from 2.67 GPa to 2.82 GPa then decreased to 1.47 GPa and the heat resistance decreased from 15, 11 to 5 times separately as the interface cobalt changed from even distribution to high content of cobalt then to cobalt medial layer, compared to the small grain size(10 μm) interface(2.67 GPa, 15 times) the bonding strength and the heat resistance of the coarse grain size(80 μm) interface(0.52 GPa, 7 times) decreased greatly. These indicated that the properties of the PDC interface were depended not only on the content and existential state of the cobalt but also on the grain size of diamond at the interface.
2004, 21(5): 22-27.
Abstract:
The composite material was prepared by mixing Pb (Ni1/3Nb2/3) O3-PbTiO3-based ferroelectrics and NiCuZn ferrite material. The phase structure and dielectric properties of the composite were investigated. The dielectric constant of composite material simulated by amending the logarithm equation and the amending coefficient were estimated. The results show that there are no other phases in the composite except dielectric phases and ferrite phase, indicating the two materials chosen in the present study are chemically compatible. It is found that there is no conventional dielectric peak appearing in the plot of dielectric constant versus temperature. The ferroelectric phase plays a main role in the dielectric constant at low temperature range from -80℃ to 20℃. The dielectric constant and the dissipation factor of composite material continuously increase with increasing temperature, which should be attributed to the leakance induced by the ferrite phase. The dielectric constant of composite material simulated was in agreement with experimental results.
The composite material was prepared by mixing Pb (Ni1/3Nb2/3) O3-PbTiO3-based ferroelectrics and NiCuZn ferrite material. The phase structure and dielectric properties of the composite were investigated. The dielectric constant of composite material simulated by amending the logarithm equation and the amending coefficient were estimated. The results show that there are no other phases in the composite except dielectric phases and ferrite phase, indicating the two materials chosen in the present study are chemically compatible. It is found that there is no conventional dielectric peak appearing in the plot of dielectric constant versus temperature. The ferroelectric phase plays a main role in the dielectric constant at low temperature range from -80℃ to 20℃. The dielectric constant and the dissipation factor of composite material continuously increase with increasing temperature, which should be attributed to the leakance induced by the ferrite phase. The dielectric constant of composite material simulated was in agreement with experimental results.
2004, 21(5): 28-34.
Abstract:
TiB2-40Cu ceramic-metal composite with the relative density of about 90% was prepared by combustion synthesis. The elastic-plastic deformation behavior of high temperature compression was studied in order to prove that TiB2-40Cu has certain plasticity in high temperature. On the basis of high temperature compression, the two-time hot pressing sintering was conducted at 1050℃,1090℃ and 1150℃ . The deformation behavior and the variation of microstructure and mechanical properties were investigated in detail. The results show that the relative density and the flexural strength increase greatly after two-time hot pressing. The relative density reaches 605.5 MPa and the flexural strength reaches 96% at 1090℃,and increases 12% and 6% respectively. Macroscopical and microcosmic densification mechanisms were studied. The results show that densification processes are affected by multiplicate mechanisms together.
TiB2-40Cu ceramic-metal composite with the relative density of about 90% was prepared by combustion synthesis. The elastic-plastic deformation behavior of high temperature compression was studied in order to prove that TiB2-40Cu has certain plasticity in high temperature. On the basis of high temperature compression, the two-time hot pressing sintering was conducted at 1050℃,1090℃ and 1150℃ . The deformation behavior and the variation of microstructure and mechanical properties were investigated in detail. The results show that the relative density and the flexural strength increase greatly after two-time hot pressing. The relative density reaches 605.5 MPa and the flexural strength reaches 96% at 1090℃,and increases 12% and 6% respectively. Macroscopical and microcosmic densification mechanisms were studied. The results show that densification processes are affected by multiplicate mechanisms together.
2004, 21(5): 35-41.
Abstract:
The electrical properties of carbon black (CB )/ thermoplastic vulcanizates (TPV) (EPDM/PP type) were investigated in details and their singularity was found. The crosslinked rubber particles in TPV can cause both exclusive effect for carbon black particles which is positive to current conduct and barrier effect which is negtive to current conduct. At the low content of rubber phase, with the increase of rubber phase the resistance of electricity of composite decreases, while at the high content of rubber phase, the higher the content of rubber phase, the higher the resistance of electricity of composite, the percolation behavior of composites with loading of carbon black is weakened apparently by rubber particles. When annealing the composites in the melt state, the resistance-time dependence of composites is strongly affected by the pressure of mold-annealing. At a certain time of annealing, the higher the pressure is, the higher the resistance is. When the pressure reaches a certain value, the resistance of composites basically has nothing to do with the annealing time. This is assumed that the restoration of deformation of compressed rubber particles in TPV would destroy the network of carbon black to some extent, which counters the positive effect of carbon black agglomerating during annealing. CB/TPV composite exhibits the NTC behavior even though the temperature is in the melting region of composite, which is attributed to the exclusive effect brought by the thermal expansion of rubber particles. The stability of resistance of composite in the melt state is ascribed to the competition between carbon black agglomerating (which is helpful to conduct) and rubber particles aggregating (which is harmful to conduct).
The electrical properties of carbon black (CB )/ thermoplastic vulcanizates (TPV) (EPDM/PP type) were investigated in details and their singularity was found. The crosslinked rubber particles in TPV can cause both exclusive effect for carbon black particles which is positive to current conduct and barrier effect which is negtive to current conduct. At the low content of rubber phase, with the increase of rubber phase the resistance of electricity of composite decreases, while at the high content of rubber phase, the higher the content of rubber phase, the higher the resistance of electricity of composite, the percolation behavior of composites with loading of carbon black is weakened apparently by rubber particles. When annealing the composites in the melt state, the resistance-time dependence of composites is strongly affected by the pressure of mold-annealing. At a certain time of annealing, the higher the pressure is, the higher the resistance is. When the pressure reaches a certain value, the resistance of composites basically has nothing to do with the annealing time. This is assumed that the restoration of deformation of compressed rubber particles in TPV would destroy the network of carbon black to some extent, which counters the positive effect of carbon black agglomerating during annealing. CB/TPV composite exhibits the NTC behavior even though the temperature is in the melting region of composite, which is attributed to the exclusive effect brought by the thermal expansion of rubber particles. The stability of resistance of composite in the melt state is ascribed to the competition between carbon black agglomerating (which is helpful to conduct) and rubber particles aggregating (which is harmful to conduct).
2004, 21(5): 42-47.
Abstract:
The tribological properties of polyacrylate coatings modified by nanometer SiO2 particles were investigated under water lubrication. The interfacial coalescence between nano-SiO2 and acrylic resin polymer was characterized by FTIR spectra. SEM was used to characterize the surface abrasion, and the wear mechanism was discussed by assistance of FTIR and abrasion experiments. The results show that chemical bonds exist between SiO2 and polyacrylate, and the addition of nanometer SiO2 particles helps to form a lubricating membrane and absorbed water layer under water lubrication, which improves the friction-reduction performance. When the amount of nano-silica is low, the main wear mechanisms of coatings are tribocorrosion abrasion and grain abrasion. As the coating contains 5wt% nanometer SiO2, the favorable wear-resistance of the coating is obtained, which provides an integrated lubricating membrane and absorbed water layer.
The tribological properties of polyacrylate coatings modified by nanometer SiO2 particles were investigated under water lubrication. The interfacial coalescence between nano-SiO2 and acrylic resin polymer was characterized by FTIR spectra. SEM was used to characterize the surface abrasion, and the wear mechanism was discussed by assistance of FTIR and abrasion experiments. The results show that chemical bonds exist between SiO2 and polyacrylate, and the addition of nanometer SiO2 particles helps to form a lubricating membrane and absorbed water layer under water lubrication, which improves the friction-reduction performance. When the amount of nano-silica is low, the main wear mechanisms of coatings are tribocorrosion abrasion and grain abrasion. As the coating contains 5wt% nanometer SiO2, the favorable wear-resistance of the coating is obtained, which provides an integrated lubricating membrane and absorbed water layer.
2004, 21(5): 48-51.
Abstract:
The multi-walled carbon nanotubes/resin epoxy composite was prepared and the composition and operating conditions were obtained. The transmission electron microscopy (TEM) was used to observe carbon nanotubes and the carbon nanotubes/resin epoxy composite. Moreover, the mechanical properties of the carbon nanotubes/resin epoxy composite were measured by the experiments of tension and compression. The results show that carbon nanotubes could obviously improve the mechanical properties of the resin epoxy bulk material.
The multi-walled carbon nanotubes/resin epoxy composite was prepared and the composition and operating conditions were obtained. The transmission electron microscopy (TEM) was used to observe carbon nanotubes and the carbon nanotubes/resin epoxy composite. Moreover, the mechanical properties of the carbon nanotubes/resin epoxy composite were measured by the experiments of tension and compression. The results show that carbon nanotubes could obviously improve the mechanical properties of the resin epoxy bulk material.
2004, 21(5): 52-56.
Abstract:
The reinforced effects of two kinds of diabase were compared. The dosage of the fiber and the types of coupling agents were studied which influence the mechanical properties. SEM was used to analyze and test the structure of the diabase/PVC composites. Conclusions are as follows: the reinforced effect of No.1 diabase is preferable. With the addition of diabase, the strength of the composite first increases and then decreases.The results show that the mixed coupling agent is the best choice.
The reinforced effects of two kinds of diabase were compared. The dosage of the fiber and the types of coupling agents were studied which influence the mechanical properties. SEM was used to analyze and test the structure of the diabase/PVC composites. Conclusions are as follows: the reinforced effect of No.1 diabase is preferable. With the addition of diabase, the strength of the composite first increases and then decreases.The results show that the mixed coupling agent is the best choice.
2004, 21(5): 57-61.
Abstract:
Three-dimensionally braided carbon fiber reinforced SiC(3D-B Cf/SiC) composites were fabricated through 8 cycles of vacuum infiltration of polycarbosilane(PCS)/divinylbenzene(DVB) and subsequent pyrolysis under an inert atmosphere. The effects of pyrolysis processes on the microstructure and mechanical properties of Cf/SiC composites were investigated. The results show that increasing heating rates can increase the density and result in better interfacial bonding leading to better properties of Cf/SiC composites. At the same time, one cycle of 1600℃ heat treatment could also increase the density and mechanical properties of Cf/SiC composites. It was found that the flexural strength of the Cf/SiC composites after pyrolysis by 15℃/min and one cycle of 1600°C heat treatment reached 556.7 MPa.
Three-dimensionally braided carbon fiber reinforced SiC(3D-B Cf/SiC) composites were fabricated through 8 cycles of vacuum infiltration of polycarbosilane(PCS)/divinylbenzene(DVB) and subsequent pyrolysis under an inert atmosphere. The effects of pyrolysis processes on the microstructure and mechanical properties of Cf/SiC composites were investigated. The results show that increasing heating rates can increase the density and result in better interfacial bonding leading to better properties of Cf/SiC composites. At the same time, one cycle of 1600℃ heat treatment could also increase the density and mechanical properties of Cf/SiC composites. It was found that the flexural strength of the Cf/SiC composites after pyrolysis by 15℃/min and one cycle of 1600°C heat treatment reached 556.7 MPa.
2004, 21(5): 62-67.
Abstract:
Carbon nanotubes (CNTs)/PBO composites were synthesized by in-situ polymerization. Further more,the polymerization mechanism was discussed through some analyses contrasting with PBO and properties of the composites were studied also with TG and intensity measured. Analyses of samples from PBO and CNTs/PBO composites during polymerization in the same situation show that active groups on the carbon nanotube surface influence the polymerization greatly. Decreasing their addition and putting off their addition time could improve the polymerizing condition. It is found that CNTs/PBO composites have super thermal resistance like PBO and their tensile properties of fiber are enhanced by 40%~70% compared with PBO fiber produced in the same way.
Carbon nanotubes (CNTs)/PBO composites were synthesized by in-situ polymerization. Further more,the polymerization mechanism was discussed through some analyses contrasting with PBO and properties of the composites were studied also with TG and intensity measured. Analyses of samples from PBO and CNTs/PBO composites during polymerization in the same situation show that active groups on the carbon nanotube surface influence the polymerization greatly. Decreasing their addition and putting off their addition time could improve the polymerizing condition. It is found that CNTs/PBO composites have super thermal resistance like PBO and their tensile properties of fiber are enhanced by 40%~70% compared with PBO fiber produced in the same way.
2004, 21(5): 68-73.
Abstract:
An L9 experimental matrix design based on Taguchi method was conducted to optimize the tensile strength of injection-molded glass fiber-reinforced PA66 (Zytel 70G33L) with and without weld lines. Based on a standard analysis of variance (ANOVA), it has been found that the relative significance of each factor on the weld line strength is arranged in decreasing order of melt temperature (F=70.98%), packing pressure (F=14.12%), injection velocity (F=13.14%) and injection pressure (F=0.82%). Under the optimum factors levels, the predicted optimum tensile strength is 156.44MPa and in good agreement with the confirmative experimental result 164.987MPa, while the weld lines are strengthened approximately by 20%~50%.
An L9 experimental matrix design based on Taguchi method was conducted to optimize the tensile strength of injection-molded glass fiber-reinforced PA66 (Zytel 70G33L) with and without weld lines. Based on a standard analysis of variance (ANOVA), it has been found that the relative significance of each factor on the weld line strength is arranged in decreasing order of melt temperature (F=70.98%), packing pressure (F=14.12%), injection velocity (F=13.14%) and injection pressure (F=0.82%). Under the optimum factors levels, the predicted optimum tensile strength is 156.44MPa and in good agreement with the confirmative experimental result 164.987MPa, while the weld lines are strengthened approximately by 20%~50%.
2004, 21(5): 74-78.
Abstract:
An innovative technology is presented, by which Al/Al2O3 composites could be synthesized with the reaction between silica glass and aluminum melts, because silica glass has a series of specialities, such as easy to be net-shaped, highly dense, and isotropic. This method could overcome the limitation of granular reactants, which were only used in the course of studying and synthesizing Al/Al2O3 composites up to now. The products are compact, homogeneous and network Al/Al2O3 ceramic matrix composite structures, in which Al and Al2O3 are mutually interpenetrated. The volume fraction of aluminum in Al/Al2O3 would increase with the increase of reactive temperature. Since it is found that there are some aluminum phases encompassed by Al2O3 particles existing in the Al/Al2O3 composites, the microstructure evolution of network Al/Al2O3 is confirmed to be a sintering process. Compared with that of technologies synthesizing Al/Al2O3, the process has the lower reactive temperature of 1000℃, faster reactive rate, and higher values of fracture toughness and bending strength.
An innovative technology is presented, by which Al/Al2O3 composites could be synthesized with the reaction between silica glass and aluminum melts, because silica glass has a series of specialities, such as easy to be net-shaped, highly dense, and isotropic. This method could overcome the limitation of granular reactants, which were only used in the course of studying and synthesizing Al/Al2O3 composites up to now. The products are compact, homogeneous and network Al/Al2O3 ceramic matrix composite structures, in which Al and Al2O3 are mutually interpenetrated. The volume fraction of aluminum in Al/Al2O3 would increase with the increase of reactive temperature. Since it is found that there are some aluminum phases encompassed by Al2O3 particles existing in the Al/Al2O3 composites, the microstructure evolution of network Al/Al2O3 is confirmed to be a sintering process. Compared with that of technologies synthesizing Al/Al2O3, the process has the lower reactive temperature of 1000℃, faster reactive rate, and higher values of fracture toughness and bending strength.
2004, 21(5): 79-82.
Abstract:
The precursor, Polyalumicarbosilane (PACS) of super-high temperature resistance SiC ( Al) fibers was synthesized by the reaction of polysilocarbosilane, or PSCS, with Al ( AcAc)3. The process to prepare SiC ( Al) fibers by PACS is in four steps: (1)Melt-spinning of PACS into "green fibers", (2) Curing the fibers to make fibers infusible, (3) Pyrolysis of the cured fibers at 1300℃ in inert atmosphere, (4) Sintering of the pyrolyzed fibers. The composition of SiC ( Al) fibers is described in the formula Si1C1.15O0.026Al0.013, in which there are mainly β-SiC grains in size of 95nm, 0.87wt% aluminum, small amount of α -SiC, SiCxOy phase and free carbon. The content of SiCxOy phase and free carbon is much lower than that of Nicalon fibers. The content of oxygen and silicon on the surface of the fibers is some higher than inside; while Si, C, O and Al are uniformly distributed inside the fibers. The average tensile strength of SiC ( Al) fibers is 2.3 GPa, with the average diameter 13 μm. The initial tensile strength of the fibers remains 95% after the treatment at 1400℃ for 1h in argon. After sintering at 1800℃ for 1h in argon, SiC ( Al) fibers remain the initial tensile strength by 71%. The high temperature stability of the fibers is better than that of Nicalon, but inferior to that of Tyranno SA.
The precursor, Polyalumicarbosilane (PACS) of super-high temperature resistance SiC ( Al) fibers was synthesized by the reaction of polysilocarbosilane, or PSCS, with Al ( AcAc)3. The process to prepare SiC ( Al) fibers by PACS is in four steps: (1)Melt-spinning of PACS into "green fibers", (2) Curing the fibers to make fibers infusible, (3) Pyrolysis of the cured fibers at 1300℃ in inert atmosphere, (4) Sintering of the pyrolyzed fibers. The composition of SiC ( Al) fibers is described in the formula Si1C1.15O0.026Al0.013, in which there are mainly β-SiC grains in size of 95nm, 0.87wt% aluminum, small amount of α -SiC, SiCxOy phase and free carbon. The content of SiCxOy phase and free carbon is much lower than that of Nicalon fibers. The content of oxygen and silicon on the surface of the fibers is some higher than inside; while Si, C, O and Al are uniformly distributed inside the fibers. The average tensile strength of SiC ( Al) fibers is 2.3 GPa, with the average diameter 13 μm. The initial tensile strength of the fibers remains 95% after the treatment at 1400℃ for 1h in argon. After sintering at 1800℃ for 1h in argon, SiC ( Al) fibers remain the initial tensile strength by 71%. The high temperature stability of the fibers is better than that of Nicalon, but inferior to that of Tyranno SA.
2004, 21(5): 83-87.
Abstract:
The porous Si3N4 ceramics with a high strength, uniform structure and a relative high porosity were obtained by adding a little amount of nano carbon in slurry by gelcasting and two step sintering. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy dispersive X-ray spectroscopy (EDS), Archimedes and three-point bending methods were employed to characterize and analyze the microstructures and basic mechanical performances of porous Si3N4 ceramics.The results show that the strength of sintered porous Si3N4 ceramics reaches 100 MPa and its porosity is more than 60% under proper technical conditions. SEM shows that the pores are formed by pillar β-Si3N4, and XRD and EDS show that SiC were formed. The pillar β-Si3N4 structure, uniform porous distribution and formative SiC particles are the main reasons for good mechanical performances.
The porous Si3N4 ceramics with a high strength, uniform structure and a relative high porosity were obtained by adding a little amount of nano carbon in slurry by gelcasting and two step sintering. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy dispersive X-ray spectroscopy (EDS), Archimedes and three-point bending methods were employed to characterize and analyze the microstructures and basic mechanical performances of porous Si3N4 ceramics.The results show that the strength of sintered porous Si3N4 ceramics reaches 100 MPa and its porosity is more than 60% under proper technical conditions. SEM shows that the pores are formed by pillar β-Si3N4, and XRD and EDS show that SiC were formed. The pillar β-Si3N4 structure, uniform porous distribution and formative SiC particles are the main reasons for good mechanical performances.
2004, 21(5): 88-92.
Abstract:
In order to develop the light-weight biomimetics composite structures, two kinds of beetle fore wings beetles, Allomyrina dichotoma and Prosopocoilus inclinatus, were selected as the biomimetics objectives. Based on the microscope observation, the structure characterizations and the optimum design mentality in these two beetle fore wings were inspected. As a result: (1)Both beetle fore-wings are of sandwich plate structure with a central void layer, where there exist many distributed trabeculae. Thus the beetle fore wings are one of typical light-weight composite frame structures. (2)The structure of allomyrina dichotoma fore-wing is disposable and shows an economical design way, while the structure of prosopocoilus inclinatus shows a strong and durable design way.
In order to develop the light-weight biomimetics composite structures, two kinds of beetle fore wings beetles, Allomyrina dichotoma and Prosopocoilus inclinatus, were selected as the biomimetics objectives. Based on the microscope observation, the structure characterizations and the optimum design mentality in these two beetle fore wings were inspected. As a result: (1)Both beetle fore-wings are of sandwich plate structure with a central void layer, where there exist many distributed trabeculae. Thus the beetle fore wings are one of typical light-weight composite frame structures. (2)The structure of allomyrina dichotoma fore-wing is disposable and shows an economical design way, while the structure of prosopocoilus inclinatus shows a strong and durable design way.
2004, 21(5): 93-97.
Abstract:
Back reflection characteristics of laser were researched for the infrared camouflage coating and the microwave absorbing coating. The results show that the back reflection of laser decreases with the increasing of the emissivity of coating. Considering the application to a tactical target, the laser and infrared camouflage can be compatible by the reasonable design of the coating pattern. Considering the camouflage principle and material design, the laser and microwave absorption can be compatible by making a microporous surface on the material, which makes back reflection of laser decrease sharply and microwave absorption change hardly.
Back reflection characteristics of laser were researched for the infrared camouflage coating and the microwave absorbing coating. The results show that the back reflection of laser decreases with the increasing of the emissivity of coating. Considering the application to a tactical target, the laser and infrared camouflage can be compatible by the reasonable design of the coating pattern. Considering the camouflage principle and material design, the laser and microwave absorption can be compatible by making a microporous surface on the material, which makes back reflection of laser decrease sharply and microwave absorption change hardly.
2004, 21(5): 98-102.
Abstract:
The mechanical properties and microstructure of hydroxypapatite/carbon nano-tube composite were investigated. The phase constitution was studied by XRD and the microstructure is observed by SEM. The bending strength and fracture toughness of the composite will be improved when different content of carbon nano-tube is added to hydroxyapatite. When the composite was sintered at a higher temperature, part hydroxyapatite will decompose sometimes which will result in the produce of Ca3 ( PO4)2 and Ca2P2O7. At the same time, part carbon nano-tube will decompose or change from one crystal to another. Carbon nano-tube can decrease the sintering temperature of hydroxyapatite, thus, the sintering temperature of the composite should be control led below 1100℃ and the fine crystal material will be obtained.
The mechanical properties and microstructure of hydroxypapatite/carbon nano-tube composite were investigated. The phase constitution was studied by XRD and the microstructure is observed by SEM. The bending strength and fracture toughness of the composite will be improved when different content of carbon nano-tube is added to hydroxyapatite. When the composite was sintered at a higher temperature, part hydroxyapatite will decompose sometimes which will result in the produce of Ca3 ( PO4)2 and Ca2P2O7. At the same time, part carbon nano-tube will decompose or change from one crystal to another. Carbon nano-tube can decrease the sintering temperature of hydroxyapatite, thus, the sintering temperature of the composite should be control led below 1100℃ and the fine crystal material will be obtained.
2004, 21(5): 103-109.
Abstract:
The porous materials and organic phase-changing materials were selected to fabricate a granulated phase-changing composite for energy storage. The energy-storing performance, durability and heat insulation of this composite were studied. The results show that organic phase-changing materials can penetrate into the porous space of pore diameters from sub-micrometers to several hundreds micrometers, and occupy major part of the porous space. The resulting composite exhibits outstanding energy-storing performance and good durability. The phase-changing behavior is influenced by the volume fraction of organic phase-changing materials and the porous structure. The energy saving performance of the composite for building is superior to that of expanded perlite.
The porous materials and organic phase-changing materials were selected to fabricate a granulated phase-changing composite for energy storage. The energy-storing performance, durability and heat insulation of this composite were studied. The results show that organic phase-changing materials can penetrate into the porous space of pore diameters from sub-micrometers to several hundreds micrometers, and occupy major part of the porous space. The resulting composite exhibits outstanding energy-storing performance and good durability. The phase-changing behavior is influenced by the volume fraction of organic phase-changing materials and the porous structure. The energy saving performance of the composite for building is superior to that of expanded perlite.
2004, 21(5): 110-115.
Abstract:
C/C composites using mesocarbon microbeads (MCMB) as matrix and pitch based carbon fibers (CF) as reinforcement were prepared through simple oxidation treatment,mechanical blend,hot press molding and carbonization. The influences of oxidation treatments of MCMB and carbon fiber on the properties of C/C composites such as bulk density,volume shrinkage and bending strength were investigated in detail. It was found that the bulk density and volume shrinkage of C/C composites declined after adding CF, while the bending strength increased when CF was oxidized under appropriate conditions. High strength C/C composite with a density of 1.64 g/cm3 and bending strength of 72.0 MPa was obtained by hot press molding and carbonization at 1000℃ for 1h from MCMB oxidized at 250℃ in air for 60min and CF treated in nitric acid for 10h. This novel and simple approach constituted a more practical method to prepare high performance C/C composites than the present ones.
C/C composites using mesocarbon microbeads (MCMB) as matrix and pitch based carbon fibers (CF) as reinforcement were prepared through simple oxidation treatment,mechanical blend,hot press molding and carbonization. The influences of oxidation treatments of MCMB and carbon fiber on the properties of C/C composites such as bulk density,volume shrinkage and bending strength were investigated in detail. It was found that the bulk density and volume shrinkage of C/C composites declined after adding CF, while the bending strength increased when CF was oxidized under appropriate conditions. High strength C/C composite with a density of 1.64 g/cm3 and bending strength of 72.0 MPa was obtained by hot press molding and carbonization at 1000℃ for 1h from MCMB oxidized at 250℃ in air for 60min and CF treated in nitric acid for 10h. This novel and simple approach constituted a more practical method to prepare high performance C/C composites than the present ones.
2004, 21(5): 116-121.
Abstract:
The process procedure of composites has decisive effects on the mechanical strength of the end product, but the mechanics is rather complicated for the process. A set of cure pressures were applied to produce various void contents. Qualitative and quantitative characterization of porosity in terms of void distribution, shape, size and volume fraction was carried out by ultrasonic C-scan, microscopic analysis and acid digestion methods. Effects of porosity on the interlaminar shear strength, flexural strength and tensile strength for the laminates were discussed and the critical value of void content at which the strength began to decrease was also obtained. The results show that the effects of porosity on the strength are mainly attributed to the void content, void distribution, size and shape, and the void content has little connection with the generation method. The investigation of this paper correlates the process parameter, porosity and performance, and embodies the integrated characteristics of design, manufacture and performance of composites.
The process procedure of composites has decisive effects on the mechanical strength of the end product, but the mechanics is rather complicated for the process. A set of cure pressures were applied to produce various void contents. Qualitative and quantitative characterization of porosity in terms of void distribution, shape, size and volume fraction was carried out by ultrasonic C-scan, microscopic analysis and acid digestion methods. Effects of porosity on the interlaminar shear strength, flexural strength and tensile strength for the laminates were discussed and the critical value of void content at which the strength began to decrease was also obtained. The results show that the effects of porosity on the strength are mainly attributed to the void content, void distribution, size and shape, and the void content has little connection with the generation method. The investigation of this paper correlates the process parameter, porosity and performance, and embodies the integrated characteristics of design, manufacture and performance of composites.
2004, 21(5): 122-127.
Abstract:
The developments of temperature distributions were studied for thick polymeric matrix composites. The finite element formulation of the transient heat transfer problem was carried out for the composites based on the heat transfer differential equations with non-linear internal heat sources. The finite element analysis software, which was based on the general finite element software package, was developed for numerical simulation of the entire composite cure process. Two kinds of thick polymeric matrix composites cure processes were analyzed by the finite element software. It was found that the cure process which was used to cure common composites was unfit for thick composites.
The developments of temperature distributions were studied for thick polymeric matrix composites. The finite element formulation of the transient heat transfer problem was carried out for the composites based on the heat transfer differential equations with non-linear internal heat sources. The finite element analysis software, which was based on the general finite element software package, was developed for numerical simulation of the entire composite cure process. Two kinds of thick polymeric matrix composites cure processes were analyzed by the finite element software. It was found that the cure process which was used to cure common composites was unfit for thick composites.
2004, 21(5): 128-133.
Abstract:
Based on the quasi-static and rapid loading test results of short cylindrical concrete specimens wrapped with fiber reinforced plastic (FRP) under three different constraint ratios(ξ), the multi-hitting test and quasi-static loading test after multi-hitting were conducted. The test results show that FRP confined concrete is a kind of material which can well resist multi-hitting. The stiffness of FRP confined concrete decreases along with increasing hitting times and also gradually converges. The final residual stiffness decreases along with ξ and η(ratio between maximum hitting stress and strength of FRP confined concrete) increase. For FRP confined concrete damaged by multi-hitting, compared with intact specimens, the elastic modulus decreases, while the strength is nearly unchanged and the ultimate strain increases. Ductility of FRP weak-confined concrete damaged by multi hitting obviously increases. The slope of linear strengthening of the stress-strain curve of FRP well-confined concrete damaged by multi-hitting increases along with the damage degree increasing and the final stress-strain curve of FRP well-confined concrete gradually approaches to linear elastic. This work provides the basis for further research on application of FRP confined concrete to defending structures.
Based on the quasi-static and rapid loading test results of short cylindrical concrete specimens wrapped with fiber reinforced plastic (FRP) under three different constraint ratios(ξ), the multi-hitting test and quasi-static loading test after multi-hitting were conducted. The test results show that FRP confined concrete is a kind of material which can well resist multi-hitting. The stiffness of FRP confined concrete decreases along with increasing hitting times and also gradually converges. The final residual stiffness decreases along with ξ and η(ratio between maximum hitting stress and strength of FRP confined concrete) increase. For FRP confined concrete damaged by multi-hitting, compared with intact specimens, the elastic modulus decreases, while the strength is nearly unchanged and the ultimate strain increases. Ductility of FRP weak-confined concrete damaged by multi hitting obviously increases. The slope of linear strengthening of the stress-strain curve of FRP well-confined concrete damaged by multi-hitting increases along with the damage degree increasing and the final stress-strain curve of FRP well-confined concrete gradually approaches to linear elastic. This work provides the basis for further research on application of FRP confined concrete to defending structures.
2004, 21(5): 134-139.
Abstract:
The cure simulation is essentially to solve the coupled equations involved in the transient heat transfer and the cure kinetics of the resin. In this paper, the internal heat generation caused by the exothermic cure reaction was treated as the nodal lumped load with the help of the control volume technique. The finite element method (FEM) was employed to solve the pertinent equations. The numerical example of a thick plank solved with the two dimensional FEM was compared with other researchers and good agreement was obtained. A three dimensional simulation for a cylinder with a hole was presented. The results show that the present procedure is numerically stable and reliable when the complex shape problems are involved.
The cure simulation is essentially to solve the coupled equations involved in the transient heat transfer and the cure kinetics of the resin. In this paper, the internal heat generation caused by the exothermic cure reaction was treated as the nodal lumped load with the help of the control volume technique. The finite element method (FEM) was employed to solve the pertinent equations. The numerical example of a thick plank solved with the two dimensional FEM was compared with other researchers and good agreement was obtained. A three dimensional simulation for a cylinder with a hole was presented. The results show that the present procedure is numerically stable and reliable when the complex shape problems are involved.
2004, 21(5): 140-145.
Abstract:
The need to characterize damage resistance and damage tolerance behaviour of composite laminates was proposed by the design requirements and usage experience. Based on the experimental study, the authors pointed out that traditional CAI (by NASA standard) could not guide development of materials and structural design correctly. In the paper, it was proposed that the damage resistance behaviour could be characterized by the maximum contact force Fmax of the contact force-dent depth curve of typical laminates and the damage tolerance behaviour could be characterized by the threshold CAIT(Compression failure strain After Impact Threshold) of the dent depth -compressive failure strain curve of typical laminates. In the meanwhile the suggestion for their measurement methods was proposed as well.
The need to characterize damage resistance and damage tolerance behaviour of composite laminates was proposed by the design requirements and usage experience. Based on the experimental study, the authors pointed out that traditional CAI (by NASA standard) could not guide development of materials and structural design correctly. In the paper, it was proposed that the damage resistance behaviour could be characterized by the maximum contact force Fmax of the contact force-dent depth curve of typical laminates and the damage tolerance behaviour could be characterized by the threshold CAIT(Compression failure strain After Impact Threshold) of the dent depth -compressive failure strain curve of typical laminates. In the meanwhile the suggestion for their measurement methods was proposed as well.
2004, 21(5): 146-152.
Abstract:
Based on the similarity of integration area of random inclusions, similar sub-domain boundary element method scheme has been presented for the first time in this paper. Then, the 2D solid with randomly distributed inclusions can be reduced to a multiply connected domain of the matrix with inner boundary conditions. In this way, computational efficiency is enhanced significantly comparing to the conventional multi-domain approach of the FEM or BEM. As numerical examples, plenty of numerical computation for the solid with randomly distributed circular or elliptic inclusions is performed using the similar sub-domain BEM scheme. Furthermore, the interface between the matrix and inclusion can be not only ideal interface, but also the interface with interphase layers. The above-mentioned computation provides reliable numerical simulation methods for the investigation of the macroscopically effective properties of the corresponding fiber-reinforced composites.
Based on the similarity of integration area of random inclusions, similar sub-domain boundary element method scheme has been presented for the first time in this paper. Then, the 2D solid with randomly distributed inclusions can be reduced to a multiply connected domain of the matrix with inner boundary conditions. In this way, computational efficiency is enhanced significantly comparing to the conventional multi-domain approach of the FEM or BEM. As numerical examples, plenty of numerical computation for the solid with randomly distributed circular or elliptic inclusions is performed using the similar sub-domain BEM scheme. Furthermore, the interface between the matrix and inclusion can be not only ideal interface, but also the interface with interphase layers. The above-mentioned computation provides reliable numerical simulation methods for the investigation of the macroscopically effective properties of the corresponding fiber-reinforced composites.
2004, 21(5): 153-158.
Abstract:
An iterative scheme to utilize the pretension process for the band wound vessels with a liner was proposed. Combined with the real-fictitious finite element analysis strategy developed by the authors or some simplified analytical equations, the typical cylinder vessels wound with steel band or carbon fiber were analyzed. Numerical results show that a pretension process can be obtained ensuring the tensions in each wound layer to get a respectable value and the provided iterative scheme gives a much suitable pretension process for the engineering practice.
An iterative scheme to utilize the pretension process for the band wound vessels with a liner was proposed. Combined with the real-fictitious finite element analysis strategy developed by the authors or some simplified analytical equations, the typical cylinder vessels wound with steel band or carbon fiber were analyzed. Numerical results show that a pretension process can be obtained ensuring the tensions in each wound layer to get a respectable value and the provided iterative scheme gives a much suitable pretension process for the engineering practice.
2004, 21(5): 159-164.
Abstract:
Non-geodesic trajectory mandrel rotation and motion equations of the eye for fiber reinforced composite pressure vessels were derived by the theory of differential geometry. Practical winding parameters such as winding angle and thickness can be applied directly to stress analysis. The incremental constitutive model of laminate and piecewise linear methods were used to simulate the process of laminate damage. A new stiffness degradation model and test methods were introduced. An experimental method which determined the factor of stiffness degradation was also demonstrated . Numerical analysis for filament wound pressure vessels was finished by using this model. The results show that flexural stiffness degradation is the main factor which affects the axial displacement after the initial damage occurs.
Non-geodesic trajectory mandrel rotation and motion equations of the eye for fiber reinforced composite pressure vessels were derived by the theory of differential geometry. Practical winding parameters such as winding angle and thickness can be applied directly to stress analysis. The incremental constitutive model of laminate and piecewise linear methods were used to simulate the process of laminate damage. A new stiffness degradation model and test methods were introduced. An experimental method which determined the factor of stiffness degradation was also demonstrated . Numerical analysis for filament wound pressure vessels was finished by using this model. The results show that flexural stiffness degradation is the main factor which affects the axial displacement after the initial damage occurs.
2004, 21(5): 165-169.
Abstract:
Rayleigh surface wave propagation for multiplayer anisotropic materials is studied. Both displacements and stresses are combined into a mixed state vector, from the system of Lagrange to that of Hamilton and so that the orders of differential equations are reduced from two to one. The eigenvalues of wave equations are solved by the precise integration and extended Wittrick-Williams algorithm. The numerical examples have proved the efficiency and precision of this method in anisotropic and isotropic materials.
Rayleigh surface wave propagation for multiplayer anisotropic materials is studied. Both displacements and stresses are combined into a mixed state vector, from the system of Lagrange to that of Hamilton and so that the orders of differential equations are reduced from two to one. The eigenvalues of wave equations are solved by the precise integration and extended Wittrick-Williams algorithm. The numerical examples have proved the efficiency and precision of this method in anisotropic and isotropic materials.
