2009 Vol. 26, No. 3
2009, 26(3): 1-6.
Abstract:
A glass-fiber/epoxy composite truss with the triangle section and without joints was fabricated by manually guided filament winding and hand-tying with high-strength glass-fiber yarns. The design and fabrication of the mandrel，the methods of winding and hand-tying and the curing and demolding process were discussed. In order to evaluate the manufacturing quality，the diameter，fiber volume fraction and tensile properties of the ribs of the composite truss were measured. A three-point bending test was also conducted for the integral composite truss in order to evaluate the capability of resisting bending. The results show that the average error of the diameter is about 5.8%. The average value of fiber volume fraction reaches 64%. The average tensile strength and modulus are 747.5 MPa and 48.56 GPa，respectively，which are 9% and 15% higher than those of the samples prepared by compression molding process. It indicates that the manufacturing method of the manually guided filament winding cooperated with hand-tying can fabricate the composite truss with high quality. The results of the three-point bending test show that the ultra-lightweight (line density ≤1. 0 kg/m) composite truss has a good integral flexural stiffness (the buckling load is about 2000 N) .
A glass-fiber/epoxy composite truss with the triangle section and without joints was fabricated by manually guided filament winding and hand-tying with high-strength glass-fiber yarns. The design and fabrication of the mandrel，the methods of winding and hand-tying and the curing and demolding process were discussed. In order to evaluate the manufacturing quality，the diameter，fiber volume fraction and tensile properties of the ribs of the composite truss were measured. A three-point bending test was also conducted for the integral composite truss in order to evaluate the capability of resisting bending. The results show that the average error of the diameter is about 5.8%. The average value of fiber volume fraction reaches 64%. The average tensile strength and modulus are 747.5 MPa and 48.56 GPa，respectively，which are 9% and 15% higher than those of the samples prepared by compression molding process. It indicates that the manufacturing method of the manually guided filament winding cooperated with hand-tying can fabricate the composite truss with high quality. The results of the three-point bending test show that the ultra-lightweight (line density ≤1. 0 kg/m) composite truss has a good integral flexural stiffness (the buckling load is about 2000 N) .
2009, 26(3): 7-12.
Abstract:
Using the vacuum bag technique for the patches of boron fiber/epoxy (Bf/epoxy) composites，the center-cracked aluminum alloy plates with different thickness were repaired，and then the fatigue properties of the repaired specimens were tested. The fatigue life，crack growth rate and crack growth lag of the repaired specimens were investigated under fatigue loadings. The results show that the fatigue life of the center-cracked aluminum alloy plates is increased by repairing with Bf/epoxy patches，and the increment of fatigue life decreases with the increase of the plate thickness. The fatigue life of the repaired specimens with thicknesses of 1.76mm，5.20mm and 10.20mm is 22.30 times，12.84 times and 8.40 times that of those unrepaired specimens respectively. The repaired specimens with thickness of 1.76 mm can still bear fatigue loading when the aluminum alloy plate fractured completely，while the repaired specimens with thickness of 5.20mm and 10.20mm can not do so. Both crack growth rate and normalized crack length difference increase with the increase of the plate thickness.
Using the vacuum bag technique for the patches of boron fiber/epoxy (Bf/epoxy) composites，the center-cracked aluminum alloy plates with different thickness were repaired，and then the fatigue properties of the repaired specimens were tested. The fatigue life，crack growth rate and crack growth lag of the repaired specimens were investigated under fatigue loadings. The results show that the fatigue life of the center-cracked aluminum alloy plates is increased by repairing with Bf/epoxy patches，and the increment of fatigue life decreases with the increase of the plate thickness. The fatigue life of the repaired specimens with thicknesses of 1.76mm，5.20mm and 10.20mm is 22.30 times，12.84 times and 8.40 times that of those unrepaired specimens respectively. The repaired specimens with thickness of 1.76 mm can still bear fatigue loading when the aluminum alloy plate fractured completely，while the repaired specimens with thickness of 5.20mm and 10.20mm can not do so. Both crack growth rate and normalized crack length difference increase with the increase of the plate thickness.
2009, 26(3): 13-17.
Abstract:
The paper seeks to address an integrated technique for the processing parameter optimization，manufacturing and mechanical property-based experiments for the composite T-joint prepared by the resin transfer moulding (RTM) process. The numerical simulation for resin flow of RTM process was conducted to obtain the optimized injection modes of resin into mould and processing parameters. From the optimized results of RTM processing parameter，the composite T-joints were processed by using RTM technique for the tension and compression tests. The mechanical properties were determined and the failure mechanism was discussed from the experimental observation. The results show that the debonding and delamination occur along the interface of a triangular region of the T-joint subjected to tension loading，which are governed largely by the static tension properties of the resin. The bending delamination and fracture appear at the central cross-section of bottom plate of the T-joint subjected to compression loading，which are dependent on more of the ability of the fabric to support the fibre against tension. And the failure strength of compression is higher than that of tension.
The paper seeks to address an integrated technique for the processing parameter optimization，manufacturing and mechanical property-based experiments for the composite T-joint prepared by the resin transfer moulding (RTM) process. The numerical simulation for resin flow of RTM process was conducted to obtain the optimized injection modes of resin into mould and processing parameters. From the optimized results of RTM processing parameter，the composite T-joints were processed by using RTM technique for the tension and compression tests. The mechanical properties were determined and the failure mechanism was discussed from the experimental observation. The results show that the debonding and delamination occur along the interface of a triangular region of the T-joint subjected to tension loading，which are governed largely by the static tension properties of the resin. The bending delamination and fracture appear at the central cross-section of bottom plate of the T-joint subjected to compression loading，which are dependent on more of the ability of the fabric to support the fibre against tension. And the failure strength of compression is higher than that of tension.
2009, 26(3): 18-23.
Abstract:
The environmental effects of the hygrothermal and UV (ult raviolet) conditions on carbon/bismaleimide composites' static tensile and compressive properties were presented. Specimens with open holes，manufactured by T300/QY8911 laminates，were tested under the environments of normal，hygrothermal，UV and hygrothermal/UV.The moisture absorption tests results indicate that the balance moisture content of the specimens is around 0.6%. The static tests reveal that the tensile strength is hardly affected by the environments. The compressive strength and modulus of the laminates，dominated by the matrix，are reduced by the hygrothermal conditions. The compressive strength of specimen decreases by 6.8% and 20.3% on the condition of 75℃ and 105℃ after moisture absorption，respectively. However，the compressive st rength increases by about 10% after UV exposure (above 300 MJ/m2)，indicating their influence on the composite material is more complex，as interaction of the hygrothermal and UV conditions may exist.
The environmental effects of the hygrothermal and UV (ult raviolet) conditions on carbon/bismaleimide composites' static tensile and compressive properties were presented. Specimens with open holes，manufactured by T300/QY8911 laminates，were tested under the environments of normal，hygrothermal，UV and hygrothermal/UV.The moisture absorption tests results indicate that the balance moisture content of the specimens is around 0.6%. The static tests reveal that the tensile strength is hardly affected by the environments. The compressive strength and modulus of the laminates，dominated by the matrix，are reduced by the hygrothermal conditions. The compressive strength of specimen decreases by 6.8% and 20.3% on the condition of 75℃ and 105℃ after moisture absorption，respectively. However，the compressive st rength increases by about 10% after UV exposure (above 300 MJ/m2)，indicating their influence on the composite material is more complex，as interaction of the hygrothermal and UV conditions may exist.
2009, 26(3): 24-28.
Abstract:
Palygorskite/poly (sodium acrylate) (PAANa) superabsorbent composites were synthesized via aqueous solution polymerization with palygorskite and acrylate acid (AA) as raw materials. Swelling properties of palygorskite/PAANa superabsorbent composites were investigated. The superabsorbent composite was characterized by Fourier transform infrared spectroscopy (FTIR). The results show that the distilled water absorbency and physiological saline solution absorbency of 10% palygorskite/PAANa superabsorbent composites reach 1666g/g and 115g/g，respectively. In comparison to the distilled water absorbency and physiological saline solution absorbency of PAANa，724g/g and 58g/g，the performance of palygorskite/PAANa composites is improved largely. Palygorskite/PAANa composites have excellent water retention and repeat water absorbency properties. Palygorskite/PAANa composites water absorbency reduces along with the electrolyte solution ion intensity ascension，the reduction degree order being Al3+>Ca2+>Na+. Compared with sepiolite/PAANa，stilbite/PAANa，diatomite/PAANa，sericite/PAANa，kaolin/PAANa and montmorillonite/PAANa，the palygorskite/PAANa composites have super water absorbency；especially，the distilled water absorbency and physiological saline solution absorbency performance is enhanced largely.
Palygorskite/poly (sodium acrylate) (PAANa) superabsorbent composites were synthesized via aqueous solution polymerization with palygorskite and acrylate acid (AA) as raw materials. Swelling properties of palygorskite/PAANa superabsorbent composites were investigated. The superabsorbent composite was characterized by Fourier transform infrared spectroscopy (FTIR). The results show that the distilled water absorbency and physiological saline solution absorbency of 10% palygorskite/PAANa superabsorbent composites reach 1666g/g and 115g/g，respectively. In comparison to the distilled water absorbency and physiological saline solution absorbency of PAANa，724g/g and 58g/g，the performance of palygorskite/PAANa composites is improved largely. Palygorskite/PAANa composites have excellent water retention and repeat water absorbency properties. Palygorskite/PAANa composites water absorbency reduces along with the electrolyte solution ion intensity ascension，the reduction degree order being Al3+>Ca2+>Na+. Compared with sepiolite/PAANa，stilbite/PAANa，diatomite/PAANa，sericite/PAANa，kaolin/PAANa and montmorillonite/PAANa，the palygorskite/PAANa composites have super water absorbency；especially，the distilled water absorbency and physiological saline solution absorbency performance is enhanced largely.
2009, 26(3): 29-34.
Abstract:
Based on the modified experiments for picture-frame，the in-plain shear properties of the twill and satin tackified carbon fabrics were studied. The results show that the behavior of tackified woven fabrics is similar to that of the fabric. According to the testing results，the shear properties of tackified fabrics are analyzed from the concent ration of tackifier and the fabric architecture. The shear properties of fabrics become poorer and the wrinkling angle is bigger as the concent ration of tackifier is higher. Satin tackified fabrics are easier to form and the load is smaller，compared with tackified twill fabrics. The equation of variation of yarn width during shear loading is fitted f rom the microscopic observation. And a model is established to predict the wrinkling angle. The model is validated as the errors between computational and experimental results are less than 2°.
Based on the modified experiments for picture-frame，the in-plain shear properties of the twill and satin tackified carbon fabrics were studied. The results show that the behavior of tackified woven fabrics is similar to that of the fabric. According to the testing results，the shear properties of tackified fabrics are analyzed from the concent ration of tackifier and the fabric architecture. The shear properties of fabrics become poorer and the wrinkling angle is bigger as the concent ration of tackifier is higher. Satin tackified fabrics are easier to form and the load is smaller，compared with tackified twill fabrics. The equation of variation of yarn width during shear loading is fitted f rom the microscopic observation. And a model is established to predict the wrinkling angle. The model is validated as the errors between computational and experimental results are less than 2°.
2009, 26(3): 35-39.
Abstract:
Taking wollastonite as raw material，aluminium sulfate and water glass as coating modifiers，a new type nano-aluminium silicate-wollastonite composite was obtained by the chemical depositing method. By modifying this composite powder with amino-silane and filling it in PP，the polypropylene composite was obtained. The composite and modified powder were characterized by SEM，BET，grain size analyser，whiteness apparatus，XRD，EDX and FTIR. The mechanism of composite powder modified by amino-silane was discussed. The results show that the surface of wollastonite is coated with nano-aluminum silicate evenly，the whiteness is increased from 90.5 to 92.5，the specific surface area is increased from 1.41 m2/g to 4.78 m2/g，and the average grain size of aluminium silicate is 54 nm. Filling 40 wt% modified composite powder in PP，the tensile strength，bending st rength and heat distortion temperature of the material have been increased from 17.81 MPa to 21.97 MPa，from 23.72 MPa to 39.20 MPa and from 65.7℃ to 94.3℃，espectively.
Taking wollastonite as raw material，aluminium sulfate and water glass as coating modifiers，a new type nano-aluminium silicate-wollastonite composite was obtained by the chemical depositing method. By modifying this composite powder with amino-silane and filling it in PP，the polypropylene composite was obtained. The composite and modified powder were characterized by SEM，BET，grain size analyser，whiteness apparatus，XRD，EDX and FTIR. The mechanism of composite powder modified by amino-silane was discussed. The results show that the surface of wollastonite is coated with nano-aluminum silicate evenly，the whiteness is increased from 90.5 to 92.5，the specific surface area is increased from 1.41 m2/g to 4.78 m2/g，and the average grain size of aluminium silicate is 54 nm. Filling 40 wt% modified composite powder in PP，the tensile strength，bending st rength and heat distortion temperature of the material have been increased from 17.81 MPa to 21.97 MPa，from 23.72 MPa to 39.20 MPa and from 65.7℃ to 94.3℃，espectively.
2009, 26(3): 40-44.
Abstract:
To improve the interface adhesion between glass fiber (GF) and styrene acrylonit rile copolymer (SAN)，GF was treated firstly with silane coupling agent，then the surface modified with styrene (St) and acrylonitrile (AN) via in-situ emulsion polymerization. Three types of GF-g-SANs with different GF/St mass ratios were copolymerized and then filled in the melting SAN resin to prepare GF/SAN composites. The impact strength，morphology and dynamic mechanical property of the composites were characterized by impact test instrument，SEM and DMA，respectively. The Young‘s modulus of GF/SAN composite was calculated by the COX model. The results show that there is high interface adhesion between GF-g-SAN and SAN. The tough and rigid properties of the composites can be increased effectively with low content of short GF-g-SAN and high content of long GF-g-SAN，and the result predicted via the COX model is close to the experimental data.
To improve the interface adhesion between glass fiber (GF) and styrene acrylonit rile copolymer (SAN)，GF was treated firstly with silane coupling agent，then the surface modified with styrene (St) and acrylonitrile (AN) via in-situ emulsion polymerization. Three types of GF-g-SANs with different GF/St mass ratios were copolymerized and then filled in the melting SAN resin to prepare GF/SAN composites. The impact strength，morphology and dynamic mechanical property of the composites were characterized by impact test instrument，SEM and DMA，respectively. The Young‘s modulus of GF/SAN composite was calculated by the COX model. The results show that there is high interface adhesion between GF-g-SAN and SAN. The tough and rigid properties of the composites can be increased effectively with low content of short GF-g-SAN and high content of long GF-g-SAN，and the result predicted via the COX model is close to the experimental data.
2009, 26(3): 45-49.
Abstract:
Based on studying the technological procedure of polyurethane (PU)-nanometer calcium carbonate (nano-CaCO 3)/polyvinyl chloride (PVC) blending，and combining with PU reaction characteristics，nano-CaCO3 with its surface treated by stearic acid was dispersed in liquefied 4，4'-diphenylmethane diisocyanate (L-MDI) by ultrasonic wave irradiation technology and stirring operation，and then the PU-nano-CaCO3/PVC composite material was prepared by the one-step reactive ext rusion technology. The mechanical properties of PU-nano-CaCO3/PVC were experimentally studied. The optimum toughening effectiveness is acquired when the mass ratio of PU to nano-CaCO3 is 4∶1. There exists a synergism in toughening between nano-CaCO3 and PU，and nano-CaCO3 plays a function of reinforcement. When the mass ratio of PU to nano-CaCO3 to PVC is 20∶5∶75，the composite has excellent comprehensive properties with the impact strength of 58.3 kJ/m2 and tensile strength of 51.5 MPa.
Based on studying the technological procedure of polyurethane (PU)-nanometer calcium carbonate (nano-CaCO 3)/polyvinyl chloride (PVC) blending，and combining with PU reaction characteristics，nano-CaCO3 with its surface treated by stearic acid was dispersed in liquefied 4，4'-diphenylmethane diisocyanate (L-MDI) by ultrasonic wave irradiation technology and stirring operation，and then the PU-nano-CaCO3/PVC composite material was prepared by the one-step reactive ext rusion technology. The mechanical properties of PU-nano-CaCO3/PVC were experimentally studied. The optimum toughening effectiveness is acquired when the mass ratio of PU to nano-CaCO3 is 4∶1. There exists a synergism in toughening between nano-CaCO3 and PU，and nano-CaCO3 plays a function of reinforcement. When the mass ratio of PU to nano-CaCO3 to PVC is 20∶5∶75，the composite has excellent comprehensive properties with the impact strength of 58.3 kJ/m2 and tensile strength of 51.5 MPa.
2009, 26(3): 50-54.
Abstract:
To improve the mechanical properties of the short cut carbon fiber reinforced thermoplastic polyimide composites (CF/TPI)，the compound modification method of nitric acid oxidation and polyimide coating was adopted to modify the CF. The t reated and untreated CF were characterized by BET，AFM，SEM and TGA. The results show that the specific surface area of CF increases 144.2% after nitric acid oxidation，by increasing the roughness of CF. A PI layer is coated on the CF，and the CF thermal stability increases after coating PI. The tensile strength，elastic modulus，flexural strength and impact strength of the CF/TPI composites are improved by 11.34%，109.2%，18.78% and 74.15% respectively after compound modification of the carbon fiber.
To improve the mechanical properties of the short cut carbon fiber reinforced thermoplastic polyimide composites (CF/TPI)，the compound modification method of nitric acid oxidation and polyimide coating was adopted to modify the CF. The t reated and untreated CF were characterized by BET，AFM，SEM and TGA. The results show that the specific surface area of CF increases 144.2% after nitric acid oxidation，by increasing the roughness of CF. A PI layer is coated on the CF，and the CF thermal stability increases after coating PI. The tensile strength，elastic modulus，flexural strength and impact strength of the CF/TPI composites are improved by 11.34%，109.2%，18.78% and 74.15% respectively after compound modification of the carbon fiber.
2009, 26(3): 55-59.
Abstract:
Nano-SiO2 was firstly modified by coating with ethylene-methyl acrylate-glycidyl methacrylate (E-MA-GMA) elastomer through the direct solvent process，and the dispersion of nano-SiO2 in PBT and the effects of modified nano-SiO2 on the properties of PBT were investigated. FTIR，TEM and SEM results indicate that the epoxy group of E-MA-GMA can react with the hydroxyl groups of the silica surface to destroy the aggregation of SiO2 and reduce the interactions between the nano-SiO2 particles；SEM results show that the mass ratios of 10%～20% E-MA-GMA can improve the dispersion of nano-SiO2 in PBT and the interfacial adhesion between the two phases，and thus the flexural properties and tensile strength of nano-SiO2/PBT composite are increased. The incorporation of E-MA-GMA modified SiO2 particles increases the tensile strength，flexural strength and flexural modulus by 9%，14% and 20% higher than those of pure PBT respectively.
Nano-SiO2 was firstly modified by coating with ethylene-methyl acrylate-glycidyl methacrylate (E-MA-GMA) elastomer through the direct solvent process，and the dispersion of nano-SiO2 in PBT and the effects of modified nano-SiO2 on the properties of PBT were investigated. FTIR，TEM and SEM results indicate that the epoxy group of E-MA-GMA can react with the hydroxyl groups of the silica surface to destroy the aggregation of SiO2 and reduce the interactions between the nano-SiO2 particles；SEM results show that the mass ratios of 10%～20% E-MA-GMA can improve the dispersion of nano-SiO2 in PBT and the interfacial adhesion between the two phases，and thus the flexural properties and tensile strength of nano-SiO2/PBT composite are increased. The incorporation of E-MA-GMA modified SiO2 particles increases the tensile strength，flexural strength and flexural modulus by 9%，14% and 20% higher than those of pure PBT respectively.
2009, 26(3): 60-66.
Abstract:
Silyl-terminated polycaprolactone/ polydimethysiloxane (PCL-TESi/PDMS-TESi) was directly synthesized via —OH and —NCO nucleophilic addition reaction. A series of resin composites were prepared by blending epoxy and PCL-TESi/PDMS-TESi. The morphologies and the surface elements of the composite system were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. The surface tension of the composite system was obtained based on the contact angle by Owens-Wendt-Kaelble model. The water-resistance properties of the composites were also studied. The results show that the morphologies，surface tension and water resistance properties of the composites are quite dependent on the content of PCL-TESi/PDMS-TESi and the curing temperature. The surface tension decreases with increasing PCL-TESi/PDMS-TESi content because of the Si element migrating toward the surface to some extent. The compatibility of the two phases at 50℃ curing systems is better than that at room temperature curing systems，which results in that the composites cured at 50℃have higher surface tensions than those cured at room temperature. When the mass ratio of PCL-TESi/PDMS-ESi to the epoxy is 5∶5，the mass change of composite cured at room temperature is below 1% af ter soak in distilled water at 50℃ for 7 days.
Silyl-terminated polycaprolactone/ polydimethysiloxane (PCL-TESi/PDMS-TESi) was directly synthesized via —OH and —NCO nucleophilic addition reaction. A series of resin composites were prepared by blending epoxy and PCL-TESi/PDMS-TESi. The morphologies and the surface elements of the composite system were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. The surface tension of the composite system was obtained based on the contact angle by Owens-Wendt-Kaelble model. The water-resistance properties of the composites were also studied. The results show that the morphologies，surface tension and water resistance properties of the composites are quite dependent on the content of PCL-TESi/PDMS-TESi and the curing temperature. The surface tension decreases with increasing PCL-TESi/PDMS-TESi content because of the Si element migrating toward the surface to some extent. The compatibility of the two phases at 50℃ curing systems is better than that at room temperature curing systems，which results in that the composites cured at 50℃have higher surface tensions than those cured at room temperature. When the mass ratio of PCL-TESi/PDMS-ESi to the epoxy is 5∶5，the mass change of composite cured at room temperature is below 1% af ter soak in distilled water at 50℃ for 7 days.
2009, 26(3): 67-72.
Abstract:
Palladium-poly (methyl methacrylate-2-acrylamido-2-methylpropanesulfonic acid) (Pd-P(MMA-AMPS)) composite microspheres，containing amide group and sulfonic acid group，were prepared in the palladium (Ⅱ) chloride (PdCl2) solution without initiator，emulsifier and reductant by ultrasonic irradiation emulsifier-free emulsion polymerization. The effect of the mass ratio of monomers on the morphology of the composite microspheres was investigated，and the samples were characterized by XRD，TGA，FTIR，TEM and Zeta-sizer. The reaction mechanism of preparation in situ was also proposed. The results show that Pd2+ can be reduced to Pd0 particles and the monomers are polymerized simultaneously. There is interaction between carbonyl group in the composite copolymer and metallic palladium. The composite microspheres with diameter of 0.76μm and 20% of palladium mass fraction are successfully synthesized under the optimized experimental condition. The mono-disperse metallic palladium particles are coated by the polymer matrix effectively.
Palladium-poly (methyl methacrylate-2-acrylamido-2-methylpropanesulfonic acid) (Pd-P(MMA-AMPS)) composite microspheres，containing amide group and sulfonic acid group，were prepared in the palladium (Ⅱ) chloride (PdCl2) solution without initiator，emulsifier and reductant by ultrasonic irradiation emulsifier-free emulsion polymerization. The effect of the mass ratio of monomers on the morphology of the composite microspheres was investigated，and the samples were characterized by XRD，TGA，FTIR，TEM and Zeta-sizer. The reaction mechanism of preparation in situ was also proposed. The results show that Pd2+ can be reduced to Pd0 particles and the monomers are polymerized simultaneously. There is interaction between carbonyl group in the composite copolymer and metallic palladium. The composite microspheres with diameter of 0.76μm and 20% of palladium mass fraction are successfully synthesized under the optimized experimental condition. The mono-disperse metallic palladium particles are coated by the polymer matrix effectively.
2009, 26(3): 73-77.
Abstract:
The non-invasive longitudinal ultrasonic transmission technique was employed to in-situ monitor the curing behavior of the vinyl-ester resin in the molds with different thicknesses. The gel time，storage modulus and the curing reaction degree of resin were measured by the longitudinal ultrasonic wave technique. The results show that the ultrasonic transmission signals can reliably reflect the curing reactions of vinyl-ester resin. The gel time，changes of storage modulus and the reaction degree with the curing time can be in-situ achieved from the variations of velocities and at tenuation of longitudinal ultrasonic waves. The ultrasonic technique is a useful tool for non-invasive，in-situ and real time investigation of curing reaction processes of resin-based composites.
The non-invasive longitudinal ultrasonic transmission technique was employed to in-situ monitor the curing behavior of the vinyl-ester resin in the molds with different thicknesses. The gel time，storage modulus and the curing reaction degree of resin were measured by the longitudinal ultrasonic wave technique. The results show that the ultrasonic transmission signals can reliably reflect the curing reactions of vinyl-ester resin. The gel time，changes of storage modulus and the reaction degree with the curing time can be in-situ achieved from the variations of velocities and at tenuation of longitudinal ultrasonic waves. The ultrasonic technique is a useful tool for non-invasive，in-situ and real time investigation of curing reaction processes of resin-based composites.
2009, 26(3): 78-83.
Abstract:
The modified poly(p-phenylene benzoxazole) (PBO) containing polar hydroxyl groups in macromolecular chains was obtained by copolymerization from 1，3-diamino-4，6-dihydroxybenzenedihydroch1oride (DAR) and terephthalic acid (TPA) in poly (phosphoric acid) (PPA) solvent，with addition of certain amounts of 2，5-dihydroxyterephthalic acid (DHTA) to replace TPA. The obtained DHPBO-PPA dope was directly spun into fibers via a dry-jet wet-spinning process. The resultant polymers were characterized mainly by means of FTIR，dynamic contact angle analysis，etc. The interfacial shear strength (IFSS) was determined by the microbond pull-out test . The surface morphologies of the pull-out PBO and DHPBO fibers were observed by scanning elect ron microscopy. With the incorporation of 50 mol% hydroxyl polar groups，the contact angle on PBO fiber for water decreases from 71.4° to 50.7°，and the contact angle for ethanol decreases from 37.2° to 27.4°. The surface free-energy of DHPBO (20 % mol content of DHTA) fiber increases to 43.96 mJ/m2，which is 23.83% higher than that of PBO fiber. The IFSS between DHPBO (10% mol content of DHTA) fibers and epoxy resin is 18.87 MPa，which is 92.55% higher than that of PBO fibers.
The modified poly(p-phenylene benzoxazole) (PBO) containing polar hydroxyl groups in macromolecular chains was obtained by copolymerization from 1，3-diamino-4，6-dihydroxybenzenedihydroch1oride (DAR) and terephthalic acid (TPA) in poly (phosphoric acid) (PPA) solvent，with addition of certain amounts of 2，5-dihydroxyterephthalic acid (DHTA) to replace TPA. The obtained DHPBO-PPA dope was directly spun into fibers via a dry-jet wet-spinning process. The resultant polymers were characterized mainly by means of FTIR，dynamic contact angle analysis，etc. The interfacial shear strength (IFSS) was determined by the microbond pull-out test . The surface morphologies of the pull-out PBO and DHPBO fibers were observed by scanning elect ron microscopy. With the incorporation of 50 mol% hydroxyl polar groups，the contact angle on PBO fiber for water decreases from 71.4° to 50.7°，and the contact angle for ethanol decreases from 37.2° to 27.4°. The surface free-energy of DHPBO (20 % mol content of DHTA) fiber increases to 43.96 mJ/m2，which is 23.83% higher than that of PBO fiber. The IFSS between DHPBO (10% mol content of DHTA) fibers and epoxy resin is 18.87 MPa，which is 92.55% higher than that of PBO fibers.
2009, 26(3): 84-89.
Abstract:
The hygrothermal aging tests at 60℃，80℃and RH 90% on F-12 fiber were carried. The moisture absorption behavior law for long hygrothermal aging time was investigated. The effect of absorbed water on the chemical structure and the possible modes of water existing in F-12 fiber under the hygrothermal environment condition were analyzed by ATR-FTIR spectra. The possible moisture absorption mechanism was also discussed. The surface and fracture appearances were observed by SEM，and the deterioration of tensile strength was also studied. The results show that the moisture absorption behavior of F-12 fiber for long hygrothermal aging time belongs to a two-stage model，which accords with Fick‘s second law at the initial stage but deviates at the second stage，and does not reach balance after 2500 h. The molecular structure of F-12 fiber is not destroyed prominently by water. The surfaces of F-12 fiber become rough，on which some grooves appear after 2500 h. Multiple characters of brittleness and fibril split ting are observed around the fracture surface in the anaphase. The tensile strength decreases fluctuating but not continually.
The hygrothermal aging tests at 60℃，80℃and RH 90% on F-12 fiber were carried. The moisture absorption behavior law for long hygrothermal aging time was investigated. The effect of absorbed water on the chemical structure and the possible modes of water existing in F-12 fiber under the hygrothermal environment condition were analyzed by ATR-FTIR spectra. The possible moisture absorption mechanism was also discussed. The surface and fracture appearances were observed by SEM，and the deterioration of tensile strength was also studied. The results show that the moisture absorption behavior of F-12 fiber for long hygrothermal aging time belongs to a two-stage model，which accords with Fick‘s second law at the initial stage but deviates at the second stage，and does not reach balance after 2500 h. The molecular structure of F-12 fiber is not destroyed prominently by water. The surfaces of F-12 fiber become rough，on which some grooves appear after 2500 h. Multiple characters of brittleness and fibril split ting are observed around the fracture surface in the anaphase. The tensile strength decreases fluctuating but not continually.
2009, 26(3): 90-98.
Abstract:
Based on the t raditional co-extraction，an advanced multilayer gas assisted co-extraction precision molding technology (GACEPMT) was put forward. The research shows that GACEPMT can not only implement the automatic and accurate control of the product dimension，but also save energy. A comparative analysis on molding process and die swelling process of traditional co-extraction and gas assisted co-extrusion was implemented by the stable finite element numerical algorithm established. Based on the above analysis，the mechanism of eliminating the co-extrusion die swell was disclosed. The research results show that the multilayer co-extrusion die swell is caused by the viscoelastic melt secondary flow，and depends on the direction and strength of the melt secondary flow. The direction of the melt secondary flow depends on the sign of secondary normal stress difference，and the strength of the melt secondary flow is proportional to the secondary normal stress difference. The gas assisted segment of a gas assisted co-extrusion die can reduce and eliminate the melt first or secondary normal stress difference by wall slip of air cushion，which eliminates the melt secondary flow near the exit of die. The goal of eliminating the co-extrusion die swell was implemented by removing the melt secondary flow. So the GACEPMT can accurately control the shape and size of gas assisted co-extracting multilayer composite products and the shape and size of the gas assisted die are completely identical . Moreover，the results show that the extracting pressure can be reduced above 30% compared with traditional co-extraction.
Based on the t raditional co-extraction，an advanced multilayer gas assisted co-extraction precision molding technology (GACEPMT) was put forward. The research shows that GACEPMT can not only implement the automatic and accurate control of the product dimension，but also save energy. A comparative analysis on molding process and die swelling process of traditional co-extraction and gas assisted co-extrusion was implemented by the stable finite element numerical algorithm established. Based on the above analysis，the mechanism of eliminating the co-extrusion die swell was disclosed. The research results show that the multilayer co-extrusion die swell is caused by the viscoelastic melt secondary flow，and depends on the direction and strength of the melt secondary flow. The direction of the melt secondary flow depends on the sign of secondary normal stress difference，and the strength of the melt secondary flow is proportional to the secondary normal stress difference. The gas assisted segment of a gas assisted co-extrusion die can reduce and eliminate the melt first or secondary normal stress difference by wall slip of air cushion，which eliminates the melt secondary flow near the exit of die. The goal of eliminating the co-extrusion die swell was implemented by removing the melt secondary flow. So the GACEPMT can accurately control the shape and size of gas assisted co-extracting multilayer composite products and the shape and size of the gas assisted die are completely identical . Moreover，the results show that the extracting pressure can be reduced above 30% compared with traditional co-extraction.
2009, 26(3): 99-104.
Abstract:
Based on the time reversal theory，the active Lamb wave damage imaging method for composite structural delamination detection was researched. The theory basis of the time reversal process was analyzed，and the focusing process to the source was discussed. Depending on the character of Lamb wave propagation，the damage scattering signals were time reversed and reemitted to the damage，and then they will arrive at the damage synchronously so that the damage scattering signal caught again will be amplified. Details about the time reversal amplifying process were analyzed and given. Based on the focalization ability of time reversal theory to the wave origin，the wave propagation was rebuilt by an image method which displays the damage as the focusing point. Experiments on the glass fiber reinforced composite board indicate that the sensing data energy can be amplified by this method，and the location and range of the damage can be expressed well.
Based on the time reversal theory，the active Lamb wave damage imaging method for composite structural delamination detection was researched. The theory basis of the time reversal process was analyzed，and the focusing process to the source was discussed. Depending on the character of Lamb wave propagation，the damage scattering signals were time reversed and reemitted to the damage，and then they will arrive at the damage synchronously so that the damage scattering signal caught again will be amplified. Details about the time reversal amplifying process were analyzed and given. Based on the focalization ability of time reversal theory to the wave origin，the wave propagation was rebuilt by an image method which displays the damage as the focusing point. Experiments on the glass fiber reinforced composite board indicate that the sensing data energy can be amplified by this method，and the location and range of the damage can be expressed well.
2009, 26(3): 105-110.
Abstract:
Based on the theoretical analysis for the porosity，density and longitudinal velocity，the acoustic impedance for the carbon fiber unidirectional reinforced composite was measured using the ultrasonic immersion technique with the bottom echo reflection and a 5 MHz flat probe. The metallographic microscope was used to determine the actual porosity. An empirical equation was built for testing porosity with acoustic impedance based on the parameters calibration. The experiments for samples with porosity from 0.03% to 2.21% show that the porosity obtained from the acoustic impedance model basically accords with that from the metallographic method. According to this method，the composite porosity could be determined conveniently without measuring the material velocity and density. Influenced little by the morphology of the pores，the determination of the acoustic impedance has feasibility on nondestructive testing for composite porosity.
Based on the theoretical analysis for the porosity，density and longitudinal velocity，the acoustic impedance for the carbon fiber unidirectional reinforced composite was measured using the ultrasonic immersion technique with the bottom echo reflection and a 5 MHz flat probe. The metallographic microscope was used to determine the actual porosity. An empirical equation was built for testing porosity with acoustic impedance based on the parameters calibration. The experiments for samples with porosity from 0.03% to 2.21% show that the porosity obtained from the acoustic impedance model basically accords with that from the metallographic method. According to this method，the composite porosity could be determined conveniently without measuring the material velocity and density. Influenced little by the morphology of the pores，the determination of the acoustic impedance has feasibility on nondestructive testing for composite porosity.
2009, 26(3): 111-115.
Abstract:
Ti(C，N)-based cermets were prepared by vacuum liquid sintering and then heat-treated by double-glow plasma carburization. The microstructures，compositional distribution and phase constitution of the as-sintered and carburized cermets were analyzed by scanning electron microscope (SEM)，electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) technology，respectively. It is found that a surface zone enriched in titanium，molybdenum，tungsten，carbon and nitrogen，deficient in nickel is introduced by the double-glow plasma carburization. The high carbon activity in the surface region drives titanium，molybdenum and tungsten elements inside the substrate to diffuse outwards，and consequently，the nickel-rich binder is forced to transport inwards. After carburization，the hard phase enriches in the surface zone while the binder enriches in the near surface area. The surface hardness of the cermet is enhanced by the heat-treatment，while its transverse rapture strength (TRS) is hardly changed.
Ti(C，N)-based cermets were prepared by vacuum liquid sintering and then heat-treated by double-glow plasma carburization. The microstructures，compositional distribution and phase constitution of the as-sintered and carburized cermets were analyzed by scanning electron microscope (SEM)，electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) technology，respectively. It is found that a surface zone enriched in titanium，molybdenum，tungsten，carbon and nitrogen，deficient in nickel is introduced by the double-glow plasma carburization. The high carbon activity in the surface region drives titanium，molybdenum and tungsten elements inside the substrate to diffuse outwards，and consequently，the nickel-rich binder is forced to transport inwards. After carburization，the hard phase enriches in the surface zone while the binder enriches in the near surface area. The surface hardness of the cermet is enhanced by the heat-treatment，while its transverse rapture strength (TRS) is hardly changed.
2009, 26(3): 116-121.
Abstract:
The glass-coated amorphous FeCuNbVSiB，FeBSiCMn and CoNiFeSiB microwires with diameters 6.3～28.2μm，20.2～28.0μm and 14.0～35.2μm were synthesized by the Tayor-Ulitovsky method，respectively. The microstructure，morphology and magnetic properties of the microwires samples were studied by X-ray diffractometry (XRD)，scanning electron microscope (SEM) and vibrating sample magnetometer (VSM). The effects of different glass coating thicknesses and metal core diameters on the magnetic parameters of microwires samples were analyzed. The results show that the sequence influencing factors on the magnetic properties are the saturation magnetostriction coefficient，the composition and the dimension of the microwires in turn. With the increase of the microwires core diameter and glass coating thickness，the radial saturation fields of all the samples continually decrease，the coercive force of the Co-rich microwires decreases from 171 A/m to 63 A/m，but the coercive force of the FeCuNbVSiB and FeBSiCMn microwires decreases from 508 A/m and 390 A/m to 486 A/m and 278 A/m，then increases to 2570 A/m and 342 A/m，respectively.
The glass-coated amorphous FeCuNbVSiB，FeBSiCMn and CoNiFeSiB microwires with diameters 6.3～28.2μm，20.2～28.0μm and 14.0～35.2μm were synthesized by the Tayor-Ulitovsky method，respectively. The microstructure，morphology and magnetic properties of the microwires samples were studied by X-ray diffractometry (XRD)，scanning electron microscope (SEM) and vibrating sample magnetometer (VSM). The effects of different glass coating thicknesses and metal core diameters on the magnetic parameters of microwires samples were analyzed. The results show that the sequence influencing factors on the magnetic properties are the saturation magnetostriction coefficient，the composition and the dimension of the microwires in turn. With the increase of the microwires core diameter and glass coating thickness，the radial saturation fields of all the samples continually decrease，the coercive force of the Co-rich microwires decreases from 171 A/m to 63 A/m，but the coercive force of the FeCuNbVSiB and FeBSiCMn microwires decreases from 508 A/m and 390 A/m to 486 A/m and 278 A/m，then increases to 2570 A/m and 342 A/m，respectively.
2009, 26(3): 122-126.
Abstract:
Wood-ceramic composites which demonst rate some unique characteristics have at tracted scientists’ research at tention lately. A modified sol prepared through the methyl triethoxy silane (MTES) hydrolysis process was impregnated into the wood cells with vacuum-pressure cycles. The alternate pressure cycles overcame some barriers of slow and incomplete impregnation that the previous processing method presented. The results indicate that wood-ceramic composites made with the METS modified sol through the alternate pressure cycles exhibit improved water-repellency and plasticity，and the mechanical property is also enhanced.
Wood-ceramic composites which demonst rate some unique characteristics have at tracted scientists’ research at tention lately. A modified sol prepared through the methyl triethoxy silane (MTES) hydrolysis process was impregnated into the wood cells with vacuum-pressure cycles. The alternate pressure cycles overcame some barriers of slow and incomplete impregnation that the previous processing method presented. The results indicate that wood-ceramic composites made with the METS modified sol through the alternate pressure cycles exhibit improved water-repellency and plasticity，and the mechanical property is also enhanced.
2009, 26(3): 127-132.
Abstract:
The chitosan (CS)/apatite-wollastonite (AW) composite porous scaffold material was prepared with AW porous glass ceramic as matrix，being coated with CS. The composition，microstructure，mechanical and mineralizing properties of the composite materials were studied by means of FRIT，SEM，optical microscope and strength testing. The results show that the composite materials still have three dimensional and interconnective as well as uniformly distributed porous structure，the diameters of pores range from 100μm to 500μm，the porosity of the composite is about 80%，and the mechanical properties of the composite materials are increased greatly，the average compressive strength being 3.11 MPa，8.3 times that of the AW scaffold material. The in vitro simulated body fluid (SBF) soaking shows that the composite materials have high mineralization function，and indicates that they also have good bioactivity. This composite is a promising material for tissue engineering scaffold materials and non-load bearing bone implants.
The chitosan (CS)/apatite-wollastonite (AW) composite porous scaffold material was prepared with AW porous glass ceramic as matrix，being coated with CS. The composition，microstructure，mechanical and mineralizing properties of the composite materials were studied by means of FRIT，SEM，optical microscope and strength testing. The results show that the composite materials still have three dimensional and interconnective as well as uniformly distributed porous structure，the diameters of pores range from 100μm to 500μm，the porosity of the composite is about 80%，and the mechanical properties of the composite materials are increased greatly，the average compressive strength being 3.11 MPa，8.3 times that of the AW scaffold material. The in vitro simulated body fluid (SBF) soaking shows that the composite materials have high mineralization function，and indicates that they also have good bioactivity. This composite is a promising material for tissue engineering scaffold materials and non-load bearing bone implants.
2009, 26(3): 133-137.
Abstract:
A kind of nearly-zero-thermal-expansion 26 wt% ZrW2O8/ZrO2 ceramics was prepared by calcining the precursors synthesized by the chemical co-precipitation method at 1150℃ for 3.5 h. The effects of Al(NO3)3·9H2O addition on the phase composition，density and thermal expansion properties of the 26 wt% ZrW2O8/ZrO2 composites were investigated by X-ray diffraction，scanning electron microscopy and dilatometry. The results indicate that the composites with Al(NO3)3·9H2O added in the precursor contain only α-ZrW2O8 and m-ZrO2 phases，and a small amount of Al(NO3)3·9H2O additive can increase the density of the 26 wt% ZrW2O8/ZrO2 composites with slight effects on the thermal expansion properties. The relative density of the 26 wt% ZrW2O8/ZrO2 composites doped 2.21 wt% Al(NO3)3·9H2O can reach 98.67% of the theoretical density. The densification mechanism can be at tributed to the formation of the Al2(WO4)3 liquid phase，which accelerates the diffusion and promotes the sintering process and leads to the increasing of the density of the composites to some extent.
A kind of nearly-zero-thermal-expansion 26 wt% ZrW2O8/ZrO2 ceramics was prepared by calcining the precursors synthesized by the chemical co-precipitation method at 1150℃ for 3.5 h. The effects of Al(NO3)3·9H2O addition on the phase composition，density and thermal expansion properties of the 26 wt% ZrW2O8/ZrO2 composites were investigated by X-ray diffraction，scanning electron microscopy and dilatometry. The results indicate that the composites with Al(NO3)3·9H2O added in the precursor contain only α-ZrW2O8 and m-ZrO2 phases，and a small amount of Al(NO3)3·9H2O additive can increase the density of the 26 wt% ZrW2O8/ZrO2 composites with slight effects on the thermal expansion properties. The relative density of the 26 wt% ZrW2O8/ZrO2 composites doped 2.21 wt% Al(NO3)3·9H2O can reach 98.67% of the theoretical density. The densification mechanism can be at tributed to the formation of the Al2(WO4)3 liquid phase，which accelerates the diffusion and promotes the sintering process and leads to the increasing of the density of the composites to some extent.
2009, 26(3): 138-146.
Abstract:
By using the combustion synthesis under high gravity and adjusting the volume fraction of ZrO2，ZrO2(4Y)/Al2O3 composite ceramics with different compositions and micro-structural morphologies were prepared，and the correlations of the composition，microstructures and mechanical properties of the materials were investigated. The results of XRD，SEM and EDS show that as the volume fraction of ZrO2 is below 37%，the ceramic matrix grows from the melts to be the random-orientated rod-shaped colonies，of which the submicron-meter fibers of tetragonal ZrO2 are orderly embedded in the α- Al2O3. As the volume fraction of ZrO2 is above 40%，the ceramic matrix develops to be the tetragonal ZrO2 grains which are shaped like spheres. The results of mechanical properties indicate that the relative density of the ceramics decreases with increasing the volume fraction of ZrO2，the hardness and fracture toughness of the ceramics simultaneously have the maximum values as the volume fraction of ZrO2 is 33%，and the bending strength of the materials has the maximum value as the volume fraction of ZrO2 is 29%.
By using the combustion synthesis under high gravity and adjusting the volume fraction of ZrO2，ZrO2(4Y)/Al2O3 composite ceramics with different compositions and micro-structural morphologies were prepared，and the correlations of the composition，microstructures and mechanical properties of the materials were investigated. The results of XRD，SEM and EDS show that as the volume fraction of ZrO2 is below 37%，the ceramic matrix grows from the melts to be the random-orientated rod-shaped colonies，of which the submicron-meter fibers of tetragonal ZrO2 are orderly embedded in the α- Al2O3. As the volume fraction of ZrO2 is above 40%，the ceramic matrix develops to be the tetragonal ZrO2 grains which are shaped like spheres. The results of mechanical properties indicate that the relative density of the ceramics decreases with increasing the volume fraction of ZrO2，the hardness and fracture toughness of the ceramics simultaneously have the maximum values as the volume fraction of ZrO2 is 33%，and the bending strength of the materials has the maximum value as the volume fraction of ZrO2 is 29%.
2009, 26(3): 147-154.
Abstract:
The polyvinyl alcohol (PVA) short fiber reinforced fly ash-geopolymer composites (SFRGC) (width to height ratio = 12.5) were firstly manufactured by the single-screw extrusion technique. The dynamic responses were studied on the SFRGC with different fiber and fly ash contents by using a Radmana impact tester. The microstructure and impact failure mechanism were also explored by laser particle size analysis (LSA)，X-ray diffraction analysis (XRD) and scanning electron microscope (SEM). The results show that PVA fiber addition greatly increases the ductility of SFRGC，especially in the case of high volume fraction of fiber，resulting in a change of impact failure mode from the brittle pattern to a ductile one. A comparative fly ash addition also exhibited obvious influence on the impact behavior. SFRGC without or with low mass fraction of fly ash (≤10%) possess high impact strength，stiffness and toughness. However，the impact resistance is greatly reduced when too much (≥30%) fly ash was incorporated. In addition，the changes in the impact response of SFRGC af ter subjecting to 20 freeze-thaw cycles and 1 month of H2SO4 at tack were systematically investigated.
The polyvinyl alcohol (PVA) short fiber reinforced fly ash-geopolymer composites (SFRGC) (width to height ratio = 12.5) were firstly manufactured by the single-screw extrusion technique. The dynamic responses were studied on the SFRGC with different fiber and fly ash contents by using a Radmana impact tester. The microstructure and impact failure mechanism were also explored by laser particle size analysis (LSA)，X-ray diffraction analysis (XRD) and scanning electron microscope (SEM). The results show that PVA fiber addition greatly increases the ductility of SFRGC，especially in the case of high volume fraction of fiber，resulting in a change of impact failure mode from the brittle pattern to a ductile one. A comparative fly ash addition also exhibited obvious influence on the impact behavior. SFRGC without or with low mass fraction of fly ash (≤10%) possess high impact strength，stiffness and toughness. However，the impact resistance is greatly reduced when too much (≥30%) fly ash was incorporated. In addition，the changes in the impact response of SFRGC af ter subjecting to 20 freeze-thaw cycles and 1 month of H2SO4 at tack were systematically investigated.
2009, 26(3): 155-161.
Abstract:
C/C composites with PyC-TaC-PyC multi-interlayers (TaC-C/C) were prepared by the isothermal chemical vapor infiltration and impregnation and carbonization. Their ablation properties were tested with an oxyacetylene flame. Compared to the C/C composites，the ablation-resistance property of 3vol% TaC-C/C composite is not improved；14vol%TaC-C/C composite can withstand a long time in the oxyacetylene flame. After ablation for 20s in the oxyacetylene flame，the composites are mainly composed of C，TaC，(Ta，O) and Ta2O5 phases. The ablation morphologies of 3vol%TaC-C/C composite are mainly composed of the white ablation spot (5～20s) and the ablation concave (120s)，while the ablation morphologies of 14vol%TaC-C/C composites are composed of the ablation spot (5s)，a perfect oxide layer (20s) and the ablation concave (120s). The cross section of 14vol%TaC-C/C composites consists of a surface oxide layer，transition region and matrix region after ablation for 20s，which can protect carbon phases in composites fairly.
C/C composites with PyC-TaC-PyC multi-interlayers (TaC-C/C) were prepared by the isothermal chemical vapor infiltration and impregnation and carbonization. Their ablation properties were tested with an oxyacetylene flame. Compared to the C/C composites，the ablation-resistance property of 3vol% TaC-C/C composite is not improved；14vol%TaC-C/C composite can withstand a long time in the oxyacetylene flame. After ablation for 20s in the oxyacetylene flame，the composites are mainly composed of C，TaC，(Ta，O) and Ta2O5 phases. The ablation morphologies of 3vol%TaC-C/C composite are mainly composed of the white ablation spot (5～20s) and the ablation concave (120s)，while the ablation morphologies of 14vol%TaC-C/C composites are composed of the ablation spot (5s)，a perfect oxide layer (20s) and the ablation concave (120s). The cross section of 14vol%TaC-C/C composites consists of a surface oxide layer，transition region and matrix region after ablation for 20s，which can protect carbon phases in composites fairly.
2009, 26(3): 162-168.
Abstract:
The spatial network method is used to calculate the electromagnetic wave scattering characteristic of carbon fiber composite grid structures filled with absorbing foams. Along with the periodic characteristic of grid structures，using Floquet theory to analyze the periodic boundary condition，simplifying the analytical model to a cell，then the absorbing ability of different cell sizes was analyzed from the results. The numerical analysis result shows that the absorbing ability of the carbon fiber composite grid structure filled with foams is better than that of the structures without foams. The thickness and volume fraction of the filled foams are the major factors affecting the reflectivity of the grid structure.
The spatial network method is used to calculate the electromagnetic wave scattering characteristic of carbon fiber composite grid structures filled with absorbing foams. Along with the periodic characteristic of grid structures，using Floquet theory to analyze the periodic boundary condition，simplifying the analytical model to a cell，then the absorbing ability of different cell sizes was analyzed from the results. The numerical analysis result shows that the absorbing ability of the carbon fiber composite grid structure filled with foams is better than that of the structures without foams. The thickness and volume fraction of the filled foams are the major factors affecting the reflectivity of the grid structure.
2009, 26(3): 169-174.
Abstract:
A unit cell model based on meso-structure and fabrication procedure was presented for the in-plain coefficient of thermal conductivity for 2D braided C/SiC composites. In this unit cell model，the actual strand cross-section geometry was simplified and the void caused by the possible gap between two adjacent strands was taken into account. The influence of structure parameters on the 2D braided C/SiC composite's coefficient of thermal conductivity was predicted. The predicted results show that the composite's coefficient of thermal conductivity decreases with the increase of kinking distance，as well as，decreases with the increase of fiber volume fraction and void volume fraction. The test result shows that there is a good correlation between the predicted results and the experimental values.
A unit cell model based on meso-structure and fabrication procedure was presented for the in-plain coefficient of thermal conductivity for 2D braided C/SiC composites. In this unit cell model，the actual strand cross-section geometry was simplified and the void caused by the possible gap between two adjacent strands was taken into account. The influence of structure parameters on the 2D braided C/SiC composite's coefficient of thermal conductivity was predicted. The predicted results show that the composite's coefficient of thermal conductivity decreases with the increase of kinking distance，as well as，decreases with the increase of fiber volume fraction and void volume fraction. The test result shows that there is a good correlation between the predicted results and the experimental values.
2009, 26(3): 175-181.
Abstract:
Two kinds of 2D-C/SiC specimens with different interface strengths were chosen to perform a non-stress oxidation experiment at 700℃ in air. The SEM analysis shows that the oxidation mechanism of the material surface is reaction-controlled kinetics and the fibers become thin uniformly；the oxidation inside the material is diffusion-controlled，and the oxidized fibers exhibit a local notch or “neck-shrink” phenomenon. The oxidation results in the decrease of the modulus and strength by reducing the effective load bearing area. The specimens with strong interface have higher modulus but lower strength，while those with weak interface behave reversely and the fiber pullout is much longer. Based on the SEM analysis results，a micromechanical model was formulated for the initial modulus calculation. The simulation results are in good agreement with the experimental values.
Two kinds of 2D-C/SiC specimens with different interface strengths were chosen to perform a non-stress oxidation experiment at 700℃ in air. The SEM analysis shows that the oxidation mechanism of the material surface is reaction-controlled kinetics and the fibers become thin uniformly；the oxidation inside the material is diffusion-controlled，and the oxidized fibers exhibit a local notch or “neck-shrink” phenomenon. The oxidation results in the decrease of the modulus and strength by reducing the effective load bearing area. The specimens with strong interface have higher modulus but lower strength，while those with weak interface behave reversely and the fiber pullout is much longer. Based on the SEM analysis results，a micromechanical model was formulated for the initial modulus calculation. The simulation results are in good agreement with the experimental values.
2009, 26(3): 182-187.
Abstract:
In the manufacturing procedure of composite single-lap joints，some mesostructures of ten form at the overlap ends. Because there are severe strain/stress concent rations at the overlap ends，these mesostructures may have effects on the mechanical performance of the joints. The effects of mesostructures at the overlap ends，including resin fillets and composite bulges，on the mechanical response of adhesively bonded laminated composite single-lap joints under tensile load were investigated by experimental measurement and finite element analysis. By digital image correlation technique，the strain distributions at the overlap end were measured. A nonlinear finite element analysis based on the submodel technique was executed to analyze the functions of the mesostructures at the overlap ends. The results from experimental measurement are in reasonable agreement with the finite element analysis. The results from experimental measurements and finite element analysis indicate that resin fillets at the overlap ends could carry a portion of load so that the strain/stress concent rations at the overlap ends are reduced. The functions of composite bulges are related to their own structures，so their effects are different for different composite bulges.
In the manufacturing procedure of composite single-lap joints，some mesostructures of ten form at the overlap ends. Because there are severe strain/stress concent rations at the overlap ends，these mesostructures may have effects on the mechanical performance of the joints. The effects of mesostructures at the overlap ends，including resin fillets and composite bulges，on the mechanical response of adhesively bonded laminated composite single-lap joints under tensile load were investigated by experimental measurement and finite element analysis. By digital image correlation technique，the strain distributions at the overlap end were measured. A nonlinear finite element analysis based on the submodel technique was executed to analyze the functions of the mesostructures at the overlap ends. The results from experimental measurement are in reasonable agreement with the finite element analysis. The results from experimental measurements and finite element analysis indicate that resin fillets at the overlap ends could carry a portion of load so that the strain/stress concent rations at the overlap ends are reduced. The functions of composite bulges are related to their own structures，so their effects are different for different composite bulges.
2009, 26(3): 188-194.
Abstract:
For different distributions of the fiber bundles in matrix materials，a micromechanical model was developed to investigate the effective properties of the composites. The composites consist of a solid matrix and fiber bundles. It is assumed that the composite has a periodic microstructure and the contact between fibers in the fiber bundle is smooth. The stiffness coefficients，transverse Young‘s modulus，shear modulus and Poisson ratio of the composites are calculated by the two-scale expansion method. The numerical results are compared with the experimental data. It is indicated that the distributions of the fiber bundle can directly affect the effective properties of the composites.
For different distributions of the fiber bundles in matrix materials，a micromechanical model was developed to investigate the effective properties of the composites. The composites consist of a solid matrix and fiber bundles. It is assumed that the composite has a periodic microstructure and the contact between fibers in the fiber bundle is smooth. The stiffness coefficients，transverse Young‘s modulus，shear modulus and Poisson ratio of the composites are calculated by the two-scale expansion method. The numerical results are compared with the experimental data. It is indicated that the distributions of the fiber bundle can directly affect the effective properties of the composites.
2009, 26(3): 195-201.
Abstract:
Based on the idea of equivalent stiffness，the stiffened panels’ mechanical characteristics were simulated by setting the property parameters of laminates. An equivalent laminate modeling method was introduced for the global buckling analysis of stiffened panels with different sectional shapes and stringer distribution forms. The high accuracy model that describes a stiffened panel‘s real shape and its simplified equivalent stiffness model with alaminate property were compared using PATRAN/NASTRAN software. For common sectional shapes and stiffener distribution forms，their 1st order linear stability coefficients are in accordance with each other. Therefore it proves that the application of equivalent laminates to stiffened panels’ buckling analysis can satisfy the engineering requirements in accuracy，and can significantly raise the calculation efficiency.
Based on the idea of equivalent stiffness，the stiffened panels’ mechanical characteristics were simulated by setting the property parameters of laminates. An equivalent laminate modeling method was introduced for the global buckling analysis of stiffened panels with different sectional shapes and stringer distribution forms. The high accuracy model that describes a stiffened panel‘s real shape and its simplified equivalent stiffness model with alaminate property were compared using PATRAN/NASTRAN software. For common sectional shapes and stiffener distribution forms，their 1st order linear stability coefficients are in accordance with each other. Therefore it proves that the application of equivalent laminates to stiffened panels’ buckling analysis can satisfy the engineering requirements in accuracy，and can significantly raise the calculation efficiency.
2009, 26(3): 202-206.
Abstract:
Some static problems of hybrid laminated plates with composite and piezoelectricity can be expediently solved by taking the advantage of wavelet finite element methods. Based on the features of laminated structures，the scale function of B-spline wavelet on the interval (BSWI) was employed to discretize the domain in-plane of structure，and the BSWI element formula of Hamilton canonical equation for piezoelectric materials was established by employing the modified mixed H-R( Hellinger-Reissner) variational principle for piezoelectric materials. One of the main characteristics of the BSWI element is the analytical form in the direction with respect to thickness. For the solution of the specific problem to ensure the interlaminar continuity of the electricity and mechanics variables，the transfer matrix technique was used. Numerical examples show that the BSWI element presented in the paper is successful. Employing the approach to deriving the BSWI element formula for piezoelectric materials can be extended to establish the analogous BSWI element formula for magnetoelectroelastic materials.
Some static problems of hybrid laminated plates with composite and piezoelectricity can be expediently solved by taking the advantage of wavelet finite element methods. Based on the features of laminated structures，the scale function of B-spline wavelet on the interval (BSWI) was employed to discretize the domain in-plane of structure，and the BSWI element formula of Hamilton canonical equation for piezoelectric materials was established by employing the modified mixed H-R( Hellinger-Reissner) variational principle for piezoelectric materials. One of the main characteristics of the BSWI element is the analytical form in the direction with respect to thickness. For the solution of the specific problem to ensure the interlaminar continuity of the electricity and mechanics variables，the transfer matrix technique was used. Numerical examples show that the BSWI element presented in the paper is successful. Employing the approach to deriving the BSWI element formula for piezoelectric materials can be extended to establish the analogous BSWI element formula for magnetoelectroelastic materials.
2009, 26(3): 207-212.
Abstract:
An interfacial element derived from the combination of two rigid elements and three spring elements was developed to analyze the interfacial properties of composite laminates. The 3D stress distribution of a laminate was determined by means of a quasi-3D finite element mesh，in which each layer of the laminate is modeled by Mindlin plate elements，and the cohesion among the neighborhood layers is simulated by the interfacial elements，for which none of the additional nodes and degrees of freedom are needed. The in-plane stress can be determined from the plate elements，and the interlaminar stress can be confirmed from the spring force in interfacial elements. As an example，a rectangular multidirectional laminate under tension was analyzed. The 3D stress determined correlates well with that of a 3D solid element model. With the recommended interfacial elements, the delamination damage and its evolution can be easily simulated by the degradation of the corresponding spring stiffness.
An interfacial element derived from the combination of two rigid elements and three spring elements was developed to analyze the interfacial properties of composite laminates. The 3D stress distribution of a laminate was determined by means of a quasi-3D finite element mesh，in which each layer of the laminate is modeled by Mindlin plate elements，and the cohesion among the neighborhood layers is simulated by the interfacial elements，for which none of the additional nodes and degrees of freedom are needed. The in-plane stress can be determined from the plate elements，and the interlaminar stress can be confirmed from the spring force in interfacial elements. As an example，a rectangular multidirectional laminate under tension was analyzed. The 3D stress determined correlates well with that of a 3D solid element model. With the recommended interfacial elements, the delamination damage and its evolution can be easily simulated by the degradation of the corresponding spring stiffness.
2009, 26(3): 213-218.
Abstract:
The morphology，adhesive mechanism and adhesive characteristics on two different surfaces of Cybister (male) fore-foot‘s adhesive pads were studied. The SEM photos show that the adhesive padst ructure consists of three adhesive devices and many setae at the marginal side. Each adhesive device contains about 15 micro-adhesive discs like an insole，which form an array in parallel. Two adhesive mechanisms are vacuum adhesive force by the adhesive discs and the Van der Waals force by the setae. The normal and tangential adhesive forces of the adhesive pads were tested respectively on the elytra surface and the glass surface. The maximal normal force is 53.3 mN and the maximal tangential force is 213.5 mN. The results indicate that the tangential adhesive forces are 6～8 times larger than the normal ones on two different surfaces (dry or wet)，and the tangential adhesive forces on the wet surface are larger than those on the dry surface，while the results of the normal forces are contrary. The conclusions approve that the normal adhesive forces basically are equivalent to the Van der Waals forces and the tangential adhesive forces come into being because of the two adhesive mechanisms’ work. These works offer the theoretic references for designing the bionic robot‘s pads.
The morphology，adhesive mechanism and adhesive characteristics on two different surfaces of Cybister (male) fore-foot‘s adhesive pads were studied. The SEM photos show that the adhesive padst ructure consists of three adhesive devices and many setae at the marginal side. Each adhesive device contains about 15 micro-adhesive discs like an insole，which form an array in parallel. Two adhesive mechanisms are vacuum adhesive force by the adhesive discs and the Van der Waals force by the setae. The normal and tangential adhesive forces of the adhesive pads were tested respectively on the elytra surface and the glass surface. The maximal normal force is 53.3 mN and the maximal tangential force is 213.5 mN. The results indicate that the tangential adhesive forces are 6～8 times larger than the normal ones on two different surfaces (dry or wet)，and the tangential adhesive forces on the wet surface are larger than those on the dry surface，while the results of the normal forces are contrary. The conclusions approve that the normal adhesive forces basically are equivalent to the Van der Waals forces and the tangential adhesive forces come into being because of the two adhesive mechanisms’ work. These works offer the theoretic references for designing the bionic robot‘s pads.
