2013 Vol. 30, No. 3
2013, 30(3): 1-6.
Abstract:
Calcium nano-montmorillonite(CaMMT)/ammonium polyphosphate(APP)compounds (CaMMT/APP) were prepared when different amounts of CaMMT were added at selected temperatures into the synthetic system of APP, and good dispersion of CaMMT and better thermal properties of the compounds were expected. The compunds were analysed by FTIR, XRD, TGA, 31P nuclear magnetic resonance (31P NMR), and their water solubility and pH were also measured. The completely exfoliated CaMMT is observed in the CaMMT/APP, indicating that CaMMT disperses well in APP. With the increasing content of CaMMT, the amount of APP with crystal form Ⅰ in the compunds increases, the degree of polymerization of APP decreases, and the initial degradation temperature of the compounds decreases, while the water solubilty of the compounds increases. However, higher contents of CaMMT help the compunds yield more residues at 800 ℃. With the same ammount of CaMMT, the higher the temperature, the more APP with crystal form Ⅱ with a higher degree of polymerization, and the water solubility of the compunds effectively decreases, and the thermostability of the compunds can be further enhanced. When CaMMT is added at 210℃, the relative mass of the residue of the compound is 29.69%, which is enhanced to 53.75% when CaMMT is added at 300 ℃.
Calcium nano-montmorillonite(CaMMT)/ammonium polyphosphate(APP)compounds (CaMMT/APP) were prepared when different amounts of CaMMT were added at selected temperatures into the synthetic system of APP, and good dispersion of CaMMT and better thermal properties of the compounds were expected. The compunds were analysed by FTIR, XRD, TGA, 31P nuclear magnetic resonance (31P NMR), and their water solubility and pH were also measured. The completely exfoliated CaMMT is observed in the CaMMT/APP, indicating that CaMMT disperses well in APP. With the increasing content of CaMMT, the amount of APP with crystal form Ⅰ in the compunds increases, the degree of polymerization of APP decreases, and the initial degradation temperature of the compounds decreases, while the water solubilty of the compounds increases. However, higher contents of CaMMT help the compunds yield more residues at 800 ℃. With the same ammount of CaMMT, the higher the temperature, the more APP with crystal form Ⅱ with a higher degree of polymerization, and the water solubility of the compunds effectively decreases, and the thermostability of the compunds can be further enhanced. When CaMMT is added at 210℃, the relative mass of the residue of the compound is 29.69%, which is enhanced to 53.75% when CaMMT is added at 300 ℃.
2013, 30(3): 7-13.
Abstract:
A new intumescent flame retardant (IFR) system, which was composed of a flame retardanted oligomer (PSPTR) and phenol formaldehyde resin (PF), was used to impart flame retardancy for ABS resin. The flammability properties of PSPTR-PF/ABS composites were investigated by limiting oxygen index (LOI) and vertical burning test (UL-94), respectively. The results show that the LOI value of PSPTR-PF/ABS composites reach 28.2, and UL-94 reached V-1 rating when the total addition mass fraction is 30%, in which the mass ratio of PSPTR to PF is 1:1. The thermal property and degradation processes of ABS and PSPTR-PF/ABS composites were studied by thermogravimetric analysis-infrared spectrometry (TG-IR) in detail. In addition, the morphology and structure of char layer were characterized by SEM, XRD and Raman spectra. The results demonstrate that PF can improve the compactness and graphite structure of intumescent char layer, which can increase the flame retardant performance of ABS eventually.
A new intumescent flame retardant (IFR) system, which was composed of a flame retardanted oligomer (PSPTR) and phenol formaldehyde resin (PF), was used to impart flame retardancy for ABS resin. The flammability properties of PSPTR-PF/ABS composites were investigated by limiting oxygen index (LOI) and vertical burning test (UL-94), respectively. The results show that the LOI value of PSPTR-PF/ABS composites reach 28.2, and UL-94 reached V-1 rating when the total addition mass fraction is 30%, in which the mass ratio of PSPTR to PF is 1:1. The thermal property and degradation processes of ABS and PSPTR-PF/ABS composites were studied by thermogravimetric analysis-infrared spectrometry (TG-IR) in detail. In addition, the morphology and structure of char layer were characterized by SEM, XRD and Raman spectra. The results demonstrate that PF can improve the compactness and graphite structure of intumescent char layer, which can increase the flame retardant performance of ABS eventually.
2013, 30(3): 14-20.
Abstract:
Polypropylene (PP) fibers surface was modified with acrylic acid by chemical grafting method. The effects of modified PP fibers on plastic shrinkage cracking, mechanical properties and bubbles structure of foamed cement were studied. The results show that modified PP fibers can improve bubbles structure, reduce plastic shrinkage cracking and refine plastic shrinkage cracks of the foamed cement, and it can increase flexural strength, compressive strength and flexural toughness. When the mass ratio of modified PP fibers to cement is 0.7%, the bubbles structure of foamed cement is improved significantly, and the plastic shrinkage cracking value decreases by 85.4%, while plastic shrinkage cracks whose width is less than 1 mm is as high as 73.1%. In the meantime, the flexural strength and compressive strength increase by 48.8% and 30.3%, respectively, and flexural toughness increase significantly. The surface groups of PP fibers before and after modification and the fracture surface micro-morphology and bubbles structure of foamed cement were observed by FTIR spectrometer, SEM and optical microscope. The action mechanisms of modified PP fibers were discussed.
Polypropylene (PP) fibers surface was modified with acrylic acid by chemical grafting method. The effects of modified PP fibers on plastic shrinkage cracking, mechanical properties and bubbles structure of foamed cement were studied. The results show that modified PP fibers can improve bubbles structure, reduce plastic shrinkage cracking and refine plastic shrinkage cracks of the foamed cement, and it can increase flexural strength, compressive strength and flexural toughness. When the mass ratio of modified PP fibers to cement is 0.7%, the bubbles structure of foamed cement is improved significantly, and the plastic shrinkage cracking value decreases by 85.4%, while plastic shrinkage cracks whose width is less than 1 mm is as high as 73.1%. In the meantime, the flexural strength and compressive strength increase by 48.8% and 30.3%, respectively, and flexural toughness increase significantly. The surface groups of PP fibers before and after modification and the fracture surface micro-morphology and bubbles structure of foamed cement were observed by FTIR spectrometer, SEM and optical microscope. The action mechanisms of modified PP fibers were discussed.
2013, 30(3): 21-27.
Abstract:
A series of SHPF/CPE-AO2246 ternary composites were prepared by using chlorinated polyethylene (CPE), 2, 2'-methylene-bis-(4-methyl-6-tert-butyl-phenol) (AO2246) and seven-hole hollow polyester fibers with 3D crimp(SHPF). The properties and microstructure of the composites were measured and analysed by means of dynamic mechanical analysis, SEM, SW230 absorption instrument and HD026NE electronic fabric tensile tester. The results show that a large number of AO2246 crystals coated on the fiber surface are formed as a result of crystallization induction of SHPF, accelerating the formation of network structure consisting of fiber and AO2246 crystals. The maximum modulus of SHPF/CPE-AO2246 composite with 20% mass ratio (based on the total mass of CPE and AO2246) of SHPF is 3 times as much as that of CPE-AO2246 without SHPF, and the loss factor of the composite decreases sharply due to larger increase of storage modulus of the composite. The area under the curves of loss modulus vs temperature, however, increases with increasing fiber content. This indicates that capability of energy consumption doesn't decline. The mechanical properties of composites obtain great improvement by adding SHPF and the formation of network structure with hollow structure gives ternary composites sound absorption property.
A series of SHPF/CPE-AO2246 ternary composites were prepared by using chlorinated polyethylene (CPE), 2, 2'-methylene-bis-(4-methyl-6-tert-butyl-phenol) (AO2246) and seven-hole hollow polyester fibers with 3D crimp(SHPF). The properties and microstructure of the composites were measured and analysed by means of dynamic mechanical analysis, SEM, SW230 absorption instrument and HD026NE electronic fabric tensile tester. The results show that a large number of AO2246 crystals coated on the fiber surface are formed as a result of crystallization induction of SHPF, accelerating the formation of network structure consisting of fiber and AO2246 crystals. The maximum modulus of SHPF/CPE-AO2246 composite with 20% mass ratio (based on the total mass of CPE and AO2246) of SHPF is 3 times as much as that of CPE-AO2246 without SHPF, and the loss factor of the composite decreases sharply due to larger increase of storage modulus of the composite. The area under the curves of loss modulus vs temperature, however, increases with increasing fiber content. This indicates that capability of energy consumption doesn't decline. The mechanical properties of composites obtain great improvement by adding SHPF and the formation of network structure with hollow structure gives ternary composites sound absorption property.
2013, 30(3): 28-34.
Abstract:
By use of non-crimp fabric(NCF), CYCOM® 890 RTM and PMI foam, the influence of pumping on sandwich panel interface quality and fiber volume fraction in VARI process were studied in this article, the types and causes of defects by the means of mechanical properties testing and microstructure analysis of the composite sandwich plate were analyzed and the process parameters were optimized. The results show that the pumping step is conducive to improve fiber volume fraction and mechanical properties for the composite sandwich structure. Pumping from 100 ℃ to 120 ℃ within 30 min in a stable technology, the interlaminar shear strength and flexural strength increase significantly.
By use of non-crimp fabric(NCF), CYCOM® 890 RTM and PMI foam, the influence of pumping on sandwich panel interface quality and fiber volume fraction in VARI process were studied in this article, the types and causes of defects by the means of mechanical properties testing and microstructure analysis of the composite sandwich plate were analyzed and the process parameters were optimized. The results show that the pumping step is conducive to improve fiber volume fraction and mechanical properties for the composite sandwich structure. Pumping from 100 ℃ to 120 ℃ within 30 min in a stable technology, the interlaminar shear strength and flexural strength increase significantly.
2013, 30(3): 35-38.
Abstract:
There was an interface when the RTM and prepreg process was co-cured. Interphase reaction of RTM and prepreg process was investigated by differential scanning calorimetry (DSC)and fourier transform-infrared (FTIR) spectrometer analysis. The size and concentration of free volume were tested by positron annihilation spectroscopy. As a result, the reaction peak of the RTM and prepreg process is 160 ℃, and reaction peak of co-curing system is 275.14 ℃. Heat release of RTM, prepreg and co-curing is 236.2 J/g, 193.9 J/g and 83.15 J/g, respectively. Free volume size and concentration of co-curing system are larger than RTM and prepreg process. Bending strength of co-curing system is 83.4 MPa, which is less than RTM (140.2 MPa) and prepreg process (105.4 MPa). Curing agents of RTM and prepreg process react each other and lead to a weak interphase.
There was an interface when the RTM and prepreg process was co-cured. Interphase reaction of RTM and prepreg process was investigated by differential scanning calorimetry (DSC)and fourier transform-infrared (FTIR) spectrometer analysis. The size and concentration of free volume were tested by positron annihilation spectroscopy. As a result, the reaction peak of the RTM and prepreg process is 160 ℃, and reaction peak of co-curing system is 275.14 ℃. Heat release of RTM, prepreg and co-curing is 236.2 J/g, 193.9 J/g and 83.15 J/g, respectively. Free volume size and concentration of co-curing system are larger than RTM and prepreg process. Bending strength of co-curing system is 83.4 MPa, which is less than RTM (140.2 MPa) and prepreg process (105.4 MPa). Curing agents of RTM and prepreg process react each other and lead to a weak interphase.
2013, 30(3): 39-44.
Abstract:
The nickel plating carbon fibers were modified by the coupling agent KH560. The surface morphology, structure, wettability of fiber, as well as the surface state and adhesive interaction of fiber/resin before and after the coupling treatment were characterized by SEM, FTIR, contact angle measurement and single fiber fragmentation test, respectively. The results show that the surface of nickel layer is flat, polar groups such as hydroxyl increases and the film on surface of nickel layer is intact after the coupling treatment of KH560. The wettability of nickle plating carbon fiber/resin is improved. There is no bubble on the interface of nickel plating carbon fiber/resin. The interface debonding on both sides of breakpoints decreases. The saturation breakpoints of single fiber increase from 46 to 65. The interfacial shear strength of nickel plating carbon fiber/resin increases from (34.87±2.23) MPa to (72.51±3.77) MPa.
The nickel plating carbon fibers were modified by the coupling agent KH560. The surface morphology, structure, wettability of fiber, as well as the surface state and adhesive interaction of fiber/resin before and after the coupling treatment were characterized by SEM, FTIR, contact angle measurement and single fiber fragmentation test, respectively. The results show that the surface of nickel layer is flat, polar groups such as hydroxyl increases and the film on surface of nickel layer is intact after the coupling treatment of KH560. The wettability of nickle plating carbon fiber/resin is improved. There is no bubble on the interface of nickel plating carbon fiber/resin. The interface debonding on both sides of breakpoints decreases. The saturation breakpoints of single fiber increase from 46 to 65. The interfacial shear strength of nickel plating carbon fiber/resin increases from (34.87±2.23) MPa to (72.51±3.77) MPa.
2013, 30(3): 45-50.
Abstract:
The different dosages of D101 polystyrene macroporous resin beads modified polyurethane/epoxy elastomer composites(D101/PU-EP) were prepared by the casting method, and their acoustic properties were studied. The bulk moduli and shear moduli of the D101/PU-EP elastomer composites were calculated from hollow spheres containing coating-foam composite modulus prediction model based on equivalent inclusion principle. And then combined with the calculation of the modulus of the acoustic model simulation results, the underwater acoustic performance of D101/PU-EP elastomer composite was theoretically simulated by transfer matrix method, and the effects of D101 content and gradient structure on the underwater acoustic properties of macroporous resin beads of the composites were obtained, and meanwhile the underwater acoustic properties of this composites were measured by underwater acoustic material sound pulsetube system(test in tube, water backing condition). The results show that D101 macroporous resin beads can improve the underwater acoustic absorption properties of the composites effectively, the average acoustic absorption coefficient of the composite with three-layer gradient structure(the average acoustic absorption coefficient of 10% content of D101 composites is 0.53, and maximum is 0.64) is higher than that of the single-layer structure composite(the average value is 0.46, and maximum is 0.52) containing the same contents of D101, and the numerical examples show underwater acoustic properties experimental results of D101/PU-EP elastomer composites are basically consistent with the acoustic model simulation results.
The different dosages of D101 polystyrene macroporous resin beads modified polyurethane/epoxy elastomer composites(D101/PU-EP) were prepared by the casting method, and their acoustic properties were studied. The bulk moduli and shear moduli of the D101/PU-EP elastomer composites were calculated from hollow spheres containing coating-foam composite modulus prediction model based on equivalent inclusion principle. And then combined with the calculation of the modulus of the acoustic model simulation results, the underwater acoustic performance of D101/PU-EP elastomer composite was theoretically simulated by transfer matrix method, and the effects of D101 content and gradient structure on the underwater acoustic properties of macroporous resin beads of the composites were obtained, and meanwhile the underwater acoustic properties of this composites were measured by underwater acoustic material sound pulsetube system(test in tube, water backing condition). The results show that D101 macroporous resin beads can improve the underwater acoustic absorption properties of the composites effectively, the average acoustic absorption coefficient of the composite with three-layer gradient structure(the average acoustic absorption coefficient of 10% content of D101 composites is 0.53, and maximum is 0.64) is higher than that of the single-layer structure composite(the average value is 0.46, and maximum is 0.52) containing the same contents of D101, and the numerical examples show underwater acoustic properties experimental results of D101/PU-EP elastomer composites are basically consistent with the acoustic model simulation results.
2013, 30(3): 51-55.
Abstract:
An equation of mechanical sensitivity of pressure-sensitive sensor based on conductive rubber filled by carbon black (CB) was given by general effective media theory. According the equation, mechanical sensitivity was studied by the strain effect, the piezoresistive effect and major parameters of the composite. It concludes that the volume fraction of CB has a great impact on mechanical sensitivity. When the volume fraction of CB is close to the critical volume fraction, the scope of sensitivity is 0.1-11.5 MPa-1, and the piezoresistive effect is the main sensitive mechanism. When the volume fraction of CB is within the percolation zone, the scope of sensitivity is 0.2-3.6 MPa-1 , and sensitive mechanism is related to the pressure, i. e. when the pressure is kept at a low value, the piezoresistive effect plays a main role, whereas the pressure is kept at a high value, the strain effect would play a main role. When the volume fraction of CB is within the conductive zone, the scope of sensitivity is 0.3-1.7 MPa-1, and the strain effect is the main sensitive mechanism.
An equation of mechanical sensitivity of pressure-sensitive sensor based on conductive rubber filled by carbon black (CB) was given by general effective media theory. According the equation, mechanical sensitivity was studied by the strain effect, the piezoresistive effect and major parameters of the composite. It concludes that the volume fraction of CB has a great impact on mechanical sensitivity. When the volume fraction of CB is close to the critical volume fraction, the scope of sensitivity is 0.1-11.5 MPa-1, and the piezoresistive effect is the main sensitive mechanism. When the volume fraction of CB is within the percolation zone, the scope of sensitivity is 0.2-3.6 MPa-1 , and sensitive mechanism is related to the pressure, i. e. when the pressure is kept at a low value, the piezoresistive effect plays a main role, whereas the pressure is kept at a high value, the strain effect would play a main role. When the volume fraction of CB is within the conductive zone, the scope of sensitivity is 0.3-1.7 MPa-1, and the strain effect is the main sensitive mechanism.
2013, 30(3): 56-62.
Abstract:
The irradiation effects of 976 nm continuous wave laser on carbon fiber reinforced E-51 resin matrix composite were studied experimentally, with a 0.4 Mach(1 Mach=340 m/s) tangential airflow, a 0.4 Mach tangential nitrogen flow and no gas flow on the target surface, respectively. The ablation laws of the materials were obtained for various laser power densities. Experimental results show that, when the pyrolysis happens severely, the solid particles flowing out of the interface with the pyrolysis gas, will shield the incident laser. The tangential gas flow, on one hand, can weaken the shield effect of the solid particles outflow the interface, which is helpful to the ablation in the irradiated area. On the other hand, the tangential gas flow can cool the target, which is adverse to the ablation. Comparing with the nitrogen gas flow, airflow is of advantage to the combustion of the production, which can heat the downstream area obviously. The loading of the tangential airflow will promote the diffusion of the oxygen to the target surface, which results in the oxidative ablation of the carbon fiber at a low temperature. Under three different gas flow statuses, the utilization ratio of the laser beam decreases with the increasing incident power density, for the power density in the range of 100～800 W/cm2.
The irradiation effects of 976 nm continuous wave laser on carbon fiber reinforced E-51 resin matrix composite were studied experimentally, with a 0.4 Mach(1 Mach=340 m/s) tangential airflow, a 0.4 Mach tangential nitrogen flow and no gas flow on the target surface, respectively. The ablation laws of the materials were obtained for various laser power densities. Experimental results show that, when the pyrolysis happens severely, the solid particles flowing out of the interface with the pyrolysis gas, will shield the incident laser. The tangential gas flow, on one hand, can weaken the shield effect of the solid particles outflow the interface, which is helpful to the ablation in the irradiated area. On the other hand, the tangential gas flow can cool the target, which is adverse to the ablation. Comparing with the nitrogen gas flow, airflow is of advantage to the combustion of the production, which can heat the downstream area obviously. The loading of the tangential airflow will promote the diffusion of the oxygen to the target surface, which results in the oxidative ablation of the carbon fiber at a low temperature. Under three different gas flow statuses, the utilization ratio of the laser beam decreases with the increasing incident power density, for the power density in the range of 100～800 W/cm2.
2013, 30(3): 63-69.
Abstract:
Carbon black (CB)/polycarbonate (PC) conductive composites was synthesized by in-situ polymerization method, during which CB could be dispersed more easily and more uniformly in PC with lower viscosity. Moreover, the catalyst loaded on CB also plays the role of the coupling agent to help CB better dispersing. Compared with the conventional melt blending method, CB/PC conductive composites synthesized by in-situ polymerization method have lower percolation threshold, and the volume resistivity of the composite with 4.32% mass fraction of CB is 1.56×106 Ω·mm. SEM shows that CB in the samples synthesized by in-situ method is mixed more completely, and forms conductive network more effectively.The logarithm of the positive temperature coefficient (PTC) of the CB/PC sample obtained by in-situ method is 4.69. CB/PC conductive composites have the potential as self-temperature control material.
Carbon black (CB)/polycarbonate (PC) conductive composites was synthesized by in-situ polymerization method, during which CB could be dispersed more easily and more uniformly in PC with lower viscosity. Moreover, the catalyst loaded on CB also plays the role of the coupling agent to help CB better dispersing. Compared with the conventional melt blending method, CB/PC conductive composites synthesized by in-situ polymerization method have lower percolation threshold, and the volume resistivity of the composite with 4.32% mass fraction of CB is 1.56×106 Ω·mm. SEM shows that CB in the samples synthesized by in-situ method is mixed more completely, and forms conductive network more effectively.The logarithm of the positive temperature coefficient (PTC) of the CB/PC sample obtained by in-situ method is 4.69. CB/PC conductive composites have the potential as self-temperature control material.
2013, 30(3): 70-75.
Abstract:
In order to adapt the unusual operating conditions such as high rotating speed, excessive load and insufficient lubrication and so on, the carbon fabric was applied to wet friction composites for its excellent tribology performance, self-lubricating and low density. Three kinds of carbon fabric/resin composites were developed with 1 K, 3 K and 6 K carbon fabric as reinforcement, respectively, and the wet tribology performance of the carbon fabric/resin composites were investigated. The experimental results show that the instantaneous braking stability and the dynamic friction coefficient decreases, but the wear resistance enhances with the increase of monofilaments in fiber bundle. The wear rates of all composites are less than 1.10×10-5 mm3/J, indicating excellent wear resistance of the composites. The wear rates of couple plates are small too, which is only about 0.40×10-5 mm3/J. The breakage and removal of carbon fibers along with resin peeling off from the surface appear to be the dominant wear mechanism, but no large size of wear debris and "third body" abrasive particles embedded into the matrix were formed which effectively avoids severe abrasion of the friction materials and the couple plates.
In order to adapt the unusual operating conditions such as high rotating speed, excessive load and insufficient lubrication and so on, the carbon fabric was applied to wet friction composites for its excellent tribology performance, self-lubricating and low density. Three kinds of carbon fabric/resin composites were developed with 1 K, 3 K and 6 K carbon fabric as reinforcement, respectively, and the wet tribology performance of the carbon fabric/resin composites were investigated. The experimental results show that the instantaneous braking stability and the dynamic friction coefficient decreases, but the wear resistance enhances with the increase of monofilaments in fiber bundle. The wear rates of all composites are less than 1.10×10-5 mm3/J, indicating excellent wear resistance of the composites. The wear rates of couple plates are small too, which is only about 0.40×10-5 mm3/J. The breakage and removal of carbon fibers along with resin peeling off from the surface appear to be the dominant wear mechanism, but no large size of wear debris and "third body" abrasive particles embedded into the matrix were formed which effectively avoids severe abrasion of the friction materials and the couple plates.
2013, 30(3): 76-81.
Abstract:
To improve the porosity and mineralization of chitosan scaffolds, the woodpile architecture of the scaffolds were constructed, which were then modified by surface phosphorylation(PCSW) and biomineralization(BMCW). FTIR results show that phosphate species are conjugated to chitosan molecules. XRD results demonstrate that phosphate crystals form after 24 h of biomineralization, and highly crystallized hydroxyapatites(HA) have been detected after 48 h. SEM show that layers of dense HA crystals deposite on both of the inner and outer surfaces of the scaffolds. The compressive strength and modulus of the BMCW after biomineralization 48 h are (0.54±0.005) MPa and (5.47±0.65) MPa, respectively, indicating that BMCW can be used as a promising material for non-load bearing bone repairs.
To improve the porosity and mineralization of chitosan scaffolds, the woodpile architecture of the scaffolds were constructed, which were then modified by surface phosphorylation(PCSW) and biomineralization(BMCW). FTIR results show that phosphate species are conjugated to chitosan molecules. XRD results demonstrate that phosphate crystals form after 24 h of biomineralization, and highly crystallized hydroxyapatites(HA) have been detected after 48 h. SEM show that layers of dense HA crystals deposite on both of the inner and outer surfaces of the scaffolds. The compressive strength and modulus of the BMCW after biomineralization 48 h are (0.54±0.005) MPa and (5.47±0.65) MPa, respectively, indicating that BMCW can be used as a promising material for non-load bearing bone repairs.
2013, 30(3): 82-86.
Abstract:
A novel bamboo bundle laminated composite(CBLC) with a corrugated structure was developed. Three types of stacking sequences including type Ⅰ (0°)6, type Ⅱ (0/90°)3 and type Ⅲ (90°)6 were designed. The effect of stacking sequence on the properties of tension, biaxial bending and 3D compression were investigated, meanwhile, the strain fields under bending load were characterized by digital spackle correlation method (DSCM). It has been found that the behaviors of tension, bending and compression are effected significantly by stacking sequence of composites with type Ⅰ > type Ⅱ > type Ⅲ for the stretching and vertical bending tests, and type Ⅱ > type Ⅲ > type Ⅰ for the lateral bending performance. The fracture mechanisms under tensile process are dissimilar, which are controlled by stacking sequences. Type Ⅰ shows a ductile fracture model, type Ⅱ displays a gradual delamination and type Ⅲ illustrates a brittle failure. Strain-fields in x and y direction mainly concentrate at the outermost layer near the bottom of corrugated wave connection with Exx < Eyy. Multiple comparisons show that the compressive properties of CBLC are significantly different in the three dimensions with y > x > z. The stacking sequences have a significant effect on the compression loading and strength, except for compression modulus.
A novel bamboo bundle laminated composite(CBLC) with a corrugated structure was developed. Three types of stacking sequences including type Ⅰ (0°)6, type Ⅱ (0/90°)3 and type Ⅲ (90°)6 were designed. The effect of stacking sequence on the properties of tension, biaxial bending and 3D compression were investigated, meanwhile, the strain fields under bending load were characterized by digital spackle correlation method (DSCM). It has been found that the behaviors of tension, bending and compression are effected significantly by stacking sequence of composites with type Ⅰ > type Ⅱ > type Ⅲ for the stretching and vertical bending tests, and type Ⅱ > type Ⅲ > type Ⅰ for the lateral bending performance. The fracture mechanisms under tensile process are dissimilar, which are controlled by stacking sequences. Type Ⅰ shows a ductile fracture model, type Ⅱ displays a gradual delamination and type Ⅲ illustrates a brittle failure. Strain-fields in x and y direction mainly concentrate at the outermost layer near the bottom of corrugated wave connection with Exx < Eyy. Multiple comparisons show that the compressive properties of CBLC are significantly different in the three dimensions with y > x > z. The stacking sequences have a significant effect on the compression loading and strength, except for compression modulus.
2013, 30(3): 87-92.
Abstract:
The tensile fracture of the 3D C/C braided composite structures was studied through the static tensile test and finite element simulation analysis. Three modeling structures including circularly holed, double-edge-semicircle and double- edge-V-shaped specimens were considered for the investigation. The results show that the three open structures in the tensile failure have a strong linear stress-strain relationship and locality stress concentration. At the fixing width of the C/C specimens, the tensile notched strength is not sensitive to size, but it is sensitive to the radius of the notch-tips. The linear elastic failure mechanism can not be applied to the damaged zone of the material, which suggest that a large-scale stress relaxation should exist. The size of the damaged zone increases when the radius of the notch-tips increases, while the size is insensitive to the location of the openings.
The tensile fracture of the 3D C/C braided composite structures was studied through the static tensile test and finite element simulation analysis. Three modeling structures including circularly holed, double-edge-semicircle and double- edge-V-shaped specimens were considered for the investigation. The results show that the three open structures in the tensile failure have a strong linear stress-strain relationship and locality stress concentration. At the fixing width of the C/C specimens, the tensile notched strength is not sensitive to size, but it is sensitive to the radius of the notch-tips. The linear elastic failure mechanism can not be applied to the damaged zone of the material, which suggest that a large-scale stress relaxation should exist. The size of the damaged zone increases when the radius of the notch-tips increases, while the size is insensitive to the location of the openings.
2013, 30(3): 93-99.
Abstract:
The thermal conductivity enhanced SiO2-Al2O3/PEG form-stable phase change composite was synthesized through the ultrasound assisted sol-gel method with aluminum isopropanol (AIP) and tetraethoxysilane as the sol precursor, and polyethylene glycol (PEG) as the phase change component. FTIR and XRD were used to characterize the structure and measure the crystallization properties of the SiO2-Al2O3/PEG. The thermal performance and thermal stability were analyzed by DSC and TGA. The form-stable performance was fixed by microscopic heating stage and digital photo technology. The thermal conductivity of the SiO2-Al2O3/PEG were measured by the thermal conductivity meter. The results show that the phase change enthalpy of the product reaches above 100 J/g, and the material has a good thermal stability within 300 ℃. When the mass ratio of Al to Si is 0.2:1, the thermal conductivity of the composite reaches 0.414 W/(m·K), which increases by 39.3%. Besides, it has an excellent form-stable performance.
The thermal conductivity enhanced SiO2-Al2O3/PEG form-stable phase change composite was synthesized through the ultrasound assisted sol-gel method with aluminum isopropanol (AIP) and tetraethoxysilane as the sol precursor, and polyethylene glycol (PEG) as the phase change component. FTIR and XRD were used to characterize the structure and measure the crystallization properties of the SiO2-Al2O3/PEG. The thermal performance and thermal stability were analyzed by DSC and TGA. The form-stable performance was fixed by microscopic heating stage and digital photo technology. The thermal conductivity of the SiO2-Al2O3/PEG were measured by the thermal conductivity meter. The results show that the phase change enthalpy of the product reaches above 100 J/g, and the material has a good thermal stability within 300 ℃. When the mass ratio of Al to Si is 0.2:1, the thermal conductivity of the composite reaches 0.414 W/(m·K), which increases by 39.3%. Besides, it has an excellent form-stable performance.
2013, 30(3): 100-106.
Abstract:
Two dimensional C/SiC composite coated with Si-B-C ceramic was prepared via chemical vapor deposition (CVD). Properties and structure evolution and self-healing mechanisms of the C/SiC composite were studied after oxidation for 10 h during 700-1200 ℃. At the same time, the evolution of morphologies, composition and phase for Si-B-C ceramic were also investigated. The experimental results show that the oxidation of Si-B-C ceramic accelerates with the temperature increasing, however, the oxidation scale is shallow and no more than 7 μm. With the temperature increasing, viscosity of borosilicate glass oxidized from Si-B-C ceramic reduces but volatilization accelerates. When the temperature increases to 1200 ℃, SiO2 crystallizes from borosilicate glass. C/SiC composite coated with Si-B-C ceramic shows an excellent oxidation resistance. Mass loss increases with temperature increasing, which is only 0.47% after oxidation for 10 h at 1200 ℃. Furthermore, the strength retention ratio is 91.6% at 1000 ℃, higher than that at other temperatures. The main mechanisms for excellent oxidation resistance of C/SiC composite is that borosilicate glass oxidized from Si-B-C ceramic can seal cracks in composite effectively.
Two dimensional C/SiC composite coated with Si-B-C ceramic was prepared via chemical vapor deposition (CVD). Properties and structure evolution and self-healing mechanisms of the C/SiC composite were studied after oxidation for 10 h during 700-1200 ℃. At the same time, the evolution of morphologies, composition and phase for Si-B-C ceramic were also investigated. The experimental results show that the oxidation of Si-B-C ceramic accelerates with the temperature increasing, however, the oxidation scale is shallow and no more than 7 μm. With the temperature increasing, viscosity of borosilicate glass oxidized from Si-B-C ceramic reduces but volatilization accelerates. When the temperature increases to 1200 ℃, SiO2 crystallizes from borosilicate glass. C/SiC composite coated with Si-B-C ceramic shows an excellent oxidation resistance. Mass loss increases with temperature increasing, which is only 0.47% after oxidation for 10 h at 1200 ℃. Furthermore, the strength retention ratio is 91.6% at 1000 ℃, higher than that at other temperatures. The main mechanisms for excellent oxidation resistance of C/SiC composite is that borosilicate glass oxidized from Si-B-C ceramic can seal cracks in composite effectively.
2013, 30(3): 107-113.
Abstract:
Nanoscale ZnO composite powders doped with the rare earth Pr2O3 were synthesized by solid-state reaction at low temperature. The high-voltage ZnO varistors were prepared through the sintering of the above-mentioned powders at different temperature. The powders and high-voltage ZnO varistors were characterized by XRD, specific surface analysis, scanning electron microscope, TEM and so on. As a comparison, the ZnO varistors indoped by Pr2O3 were studied, and the mechanism of Pr2O3 doping was discussed in detail. The results show that the doped Pr2O3 tends to segregate at grain boundaries of ZnO varistors, and can activate the ZnO grain boundaries and promote the growth of crystal at low sintering temperature (1030-1130 ℃). Furthermore, the Pr2O3 doping leads to the formation of texture among the grain boundaries of ZnO varistors prepared at 1080 ℃, and make the doped-Pr2O ZnO varistors more uniform and solid than that of the undoped-Pr2O3 ZnO varistors, which are helpful to the improvement of the grain boundaries of ZnO varistors, and enhancement of the electronic properties of ZnO varistors.The high-voltage ZnO varistor of doped Pr2O3 with the best electronic properties was prepared at 1080 ℃, which has a potential gradient of 864.39 V/mm, a nonlinear coefficient of 28.75 and a leakage current of 35 μA.
Nanoscale ZnO composite powders doped with the rare earth Pr2O3 were synthesized by solid-state reaction at low temperature. The high-voltage ZnO varistors were prepared through the sintering of the above-mentioned powders at different temperature. The powders and high-voltage ZnO varistors were characterized by XRD, specific surface analysis, scanning electron microscope, TEM and so on. As a comparison, the ZnO varistors indoped by Pr2O3 were studied, and the mechanism of Pr2O3 doping was discussed in detail. The results show that the doped Pr2O3 tends to segregate at grain boundaries of ZnO varistors, and can activate the ZnO grain boundaries and promote the growth of crystal at low sintering temperature (1030-1130 ℃). Furthermore, the Pr2O3 doping leads to the formation of texture among the grain boundaries of ZnO varistors prepared at 1080 ℃, and make the doped-Pr2O ZnO varistors more uniform and solid than that of the undoped-Pr2O3 ZnO varistors, which are helpful to the improvement of the grain boundaries of ZnO varistors, and enhancement of the electronic properties of ZnO varistors.The high-voltage ZnO varistor of doped Pr2O3 with the best electronic properties was prepared at 1080 ℃, which has a potential gradient of 864.39 V/mm, a nonlinear coefficient of 28.75 and a leakage current of 35 μA.
2013, 30(3): 114-119.
Abstract:
A kind of Al2O3-TiC/Al2O3-TiC-CaF2 laminated composite ceramic layered stack by Al2O3-TiC ceramic and self-lubricating Al2O3-TiC-CaF2 composite was prepared by vacuum hot pressing sintering. The friction and wear behaviors and wear mechanism of laminated composite were researched though the ring-disc friction and wear tester in different load conditions and rotation speeds, the worn morphology was observed by SEM and the elements were investigated by EDS. Results show that the friction coefficient and wear rate decline when the rotation speed increases at the same load conditions, and also decrease with the increase of the load with the same rotating speed. The main wear mechanisms of Al2O3-TiC/Al2O3-TiC-CaF2 laminated composite are abrasive wear and adhesive wear.
A kind of Al2O3-TiC/Al2O3-TiC-CaF2 laminated composite ceramic layered stack by Al2O3-TiC ceramic and self-lubricating Al2O3-TiC-CaF2 composite was prepared by vacuum hot pressing sintering. The friction and wear behaviors and wear mechanism of laminated composite were researched though the ring-disc friction and wear tester in different load conditions and rotation speeds, the worn morphology was observed by SEM and the elements were investigated by EDS. Results show that the friction coefficient and wear rate decline when the rotation speed increases at the same load conditions, and also decrease with the increase of the load with the same rotating speed. The main wear mechanisms of Al2O3-TiC/Al2O3-TiC-CaF2 laminated composite are abrasive wear and adhesive wear.
2013, 30(3): 120-124.
Abstract:
Nanodiamond-vitrified bond composite powder was prepared by polyacrylamide gel(P-G)method. The powder was pressed and sintered at low temperature, and the nanodiamond-vitrified bond composite sample was obtained. Contrast with this method, vitrified bond was also prepared by melting method, then mixed with nanodiamond and sintered at a proper temperature. The phase constitution and microstructure of sintered samples were characterized by X-ray diffraction and field emission scanning electron microscopy. The flexural strength was measured by three-point bending method. The volume density and porosity were tested according to Archimedes' principle. The results show that nanodiamond-vitrified bond composite powder can be obtained by P-G method, and the flexural strength, volume density and porosity of samples sintered at 800 ℃/2 h are 60.41 MPa, 1.81 g/cm3 and 15.67%, respectively. When nanodiamond powder is mixed with vitrified bond matrix mechanically, the flexural strength, volume density and porosity of sintered samples are 46.48 MPa, 2.23 g/cm3 and 11.75%, respectively, and the agglomerate of nanodiamond is clearly observed. P-G method can be used as a new technology for preparing nanodiamond-vitrified composite as ultrafine grinding tools.
Nanodiamond-vitrified bond composite powder was prepared by polyacrylamide gel(P-G)method. The powder was pressed and sintered at low temperature, and the nanodiamond-vitrified bond composite sample was obtained. Contrast with this method, vitrified bond was also prepared by melting method, then mixed with nanodiamond and sintered at a proper temperature. The phase constitution and microstructure of sintered samples were characterized by X-ray diffraction and field emission scanning electron microscopy. The flexural strength was measured by three-point bending method. The volume density and porosity were tested according to Archimedes' principle. The results show that nanodiamond-vitrified bond composite powder can be obtained by P-G method, and the flexural strength, volume density and porosity of samples sintered at 800 ℃/2 h are 60.41 MPa, 1.81 g/cm3 and 15.67%, respectively. When nanodiamond powder is mixed with vitrified bond matrix mechanically, the flexural strength, volume density and porosity of sintered samples are 46.48 MPa, 2.23 g/cm3 and 11.75%, respectively, and the agglomerate of nanodiamond is clearly observed. P-G method can be used as a new technology for preparing nanodiamond-vitrified composite as ultrafine grinding tools.
2013, 30(3): 125-132.
Abstract:
BCx was prepared on SiC by chemical vapor deposition (CVD) method. Oxidation behavior of BCx was investigated and the kinetic parameters of BCx oxidation were obtained under static air. The self-healing time of the cracks pre-prepared in the layered BCx/SiC structure was predicted. SEM was used to observe the self-healing of the cracks. The results show that the predicted time is in good agreement with the experimental results.
BCx was prepared on SiC by chemical vapor deposition (CVD) method. Oxidation behavior of BCx was investigated and the kinetic parameters of BCx oxidation were obtained under static air. The self-healing time of the cracks pre-prepared in the layered BCx/SiC structure was predicted. SEM was used to observe the self-healing of the cracks. The results show that the predicted time is in good agreement with the experimental results.
2013, 30(3): 133-141.
Abstract:
The high temperature mechanical properties of the nano-SiO2 steel fiber reinforced concrete(NSFC), the steel fiber reinforced concrete(SFRC) and the normal concrete(NC) were investigated, such as the compressive strength, splitting tensile strength, flexural strength, and so on. SEM and XRD were used to analyze micro-structure and interfacial transition zone of steel fiber at different temperatures. The results show that the compressive strength, splitting tensile strength and flexural strength of NSFC are better than that of SFRC and NC in the test temperatures, and reach maximum at 400 ℃. Compared with NC, the compressive strength, splitting tensile strength and flexural strength of NSFC increase by 27.01%, 63.28% and 54.12%, respectively, at room temperature, and by 35.09%, 84.62% and 87.23%, respectively, at 400 ℃. SEM and XRD analysis shows that there is a permeable diffusion layer on the steel fiber surface because of solid state reaction in the interfacial transition zone of steel fiber and concrete. This permeable diffusion layer is white, bright, serrated, and mainly consists of FeSi2 and the complex hydrated calcium silicate. The compounds of this layer change the interfacial transition zone structure, enhance bonding capacity of the steel fiber and matrix, and increase the high temperature mechanical properties of concrete.
The high temperature mechanical properties of the nano-SiO2 steel fiber reinforced concrete(NSFC), the steel fiber reinforced concrete(SFRC) and the normal concrete(NC) were investigated, such as the compressive strength, splitting tensile strength, flexural strength, and so on. SEM and XRD were used to analyze micro-structure and interfacial transition zone of steel fiber at different temperatures. The results show that the compressive strength, splitting tensile strength and flexural strength of NSFC are better than that of SFRC and NC in the test temperatures, and reach maximum at 400 ℃. Compared with NC, the compressive strength, splitting tensile strength and flexural strength of NSFC increase by 27.01%, 63.28% and 54.12%, respectively, at room temperature, and by 35.09%, 84.62% and 87.23%, respectively, at 400 ℃. SEM and XRD analysis shows that there is a permeable diffusion layer on the steel fiber surface because of solid state reaction in the interfacial transition zone of steel fiber and concrete. This permeable diffusion layer is white, bright, serrated, and mainly consists of FeSi2 and the complex hydrated calcium silicate. The compounds of this layer change the interfacial transition zone structure, enhance bonding capacity of the steel fiber and matrix, and increase the high temperature mechanical properties of concrete.
2013, 30(3): 142-148.
Abstract:
9CrWTi-0.35%Y2O3 oxide dispersion strengthened ferritic-martensitic steels were prepared by mechanically alloying (MA) and spark plasma sintering (SPS). SPS temperature and displacement measurement, OM, FE-SEM, TEM and EDX were employed to characterize the shrink curve, the microstructures and chemical composition evolution before and after heat treatment. The tensile properties at room temperature were also investigated by electronic tensile test. The result shows that a liquid phase sintering feature is exhibited in 9CrWTi/FMs and 9CrTWi-0.35%Y2O3/FMs. The density and tensile strength of 9CrWTi-0.35%Y2O3/FMs increase with temperature and pressure, while porosity and grain size decrease and total elongation is only about 2%. Dispersoids about 5-20 nm enriched in Y—Ti—O are uniformly distributed in the matrix. The microstructures are of equiaxed ferritic, slender lath martensite, and mainly equiaxed and residual ferritic SPS, after 10%NaCl water solution quenching and tempering at 750 ℃, respectively. The 9CrWTi-0.35%Y2O3/FMs by SPS exhibits ultimate tensile strength and yield strengthen of 1554 MPa and 1430 MPa, and a total elongation to fracture of 1.8%, while after salt water quenching and tempering, which successively change to 1198 MPa, 1006 MPa and 12.8%, respectively.
9CrWTi-0.35%Y2O3 oxide dispersion strengthened ferritic-martensitic steels were prepared by mechanically alloying (MA) and spark plasma sintering (SPS). SPS temperature and displacement measurement, OM, FE-SEM, TEM and EDX were employed to characterize the shrink curve, the microstructures and chemical composition evolution before and after heat treatment. The tensile properties at room temperature were also investigated by electronic tensile test. The result shows that a liquid phase sintering feature is exhibited in 9CrWTi/FMs and 9CrTWi-0.35%Y2O3/FMs. The density and tensile strength of 9CrWTi-0.35%Y2O3/FMs increase with temperature and pressure, while porosity and grain size decrease and total elongation is only about 2%. Dispersoids about 5-20 nm enriched in Y—Ti—O are uniformly distributed in the matrix. The microstructures are of equiaxed ferritic, slender lath martensite, and mainly equiaxed and residual ferritic SPS, after 10%NaCl water solution quenching and tempering at 750 ℃, respectively. The 9CrWTi-0.35%Y2O3/FMs by SPS exhibits ultimate tensile strength and yield strengthen of 1554 MPa and 1430 MPa, and a total elongation to fracture of 1.8%, while after salt water quenching and tempering, which successively change to 1198 MPa, 1006 MPa and 12.8%, respectively.
2013, 30(3): 149-153.
Abstract:
Based on the propagation characteristics of microwave and the microwave sensitivity to the interface, the phase difference at the interface of thermal barrier coating(TBC) and waveguide probe was used to characterize the long crack on the metal substrate surface under TBC using computer simulation technology-microwave studio (CST-MWS). The influences of top coating thickness and crack direction were studied. Besides, microwave evaluation of cracks with different width and the crack length direction parrallel to the long brim of the rectangular waveguide probe were studied when the thickness of TBC is 400 μm. The results show that the top coating thickness affects the sensitive frequency, and that the carck direction influences the evaluation sensitive, and the evaluation sensitive is the minimum when the angle between the rack length direction and the long brim of the rectangular waveguide probe is 50°-55°. It is also observed that the crack can not be detected when the crack width is less than 8 μm, and the effect of detection is not obvious when the crack width is 10-30 μm, and the wider the cracks, the greater the phase difference of the reflection coefficient when crack width is in the range of 30 μm-1 mm. Therefore, it is feasible to evaluate the crack on the metal substrate surface using microwave non-destruction evaluation.
Based on the propagation characteristics of microwave and the microwave sensitivity to the interface, the phase difference at the interface of thermal barrier coating(TBC) and waveguide probe was used to characterize the long crack on the metal substrate surface under TBC using computer simulation technology-microwave studio (CST-MWS). The influences of top coating thickness and crack direction were studied. Besides, microwave evaluation of cracks with different width and the crack length direction parrallel to the long brim of the rectangular waveguide probe were studied when the thickness of TBC is 400 μm. The results show that the top coating thickness affects the sensitive frequency, and that the carck direction influences the evaluation sensitive, and the evaluation sensitive is the minimum when the angle between the rack length direction and the long brim of the rectangular waveguide probe is 50°-55°. It is also observed that the crack can not be detected when the crack width is less than 8 μm, and the effect of detection is not obvious when the crack width is 10-30 μm, and the wider the cracks, the greater the phase difference of the reflection coefficient when crack width is in the range of 30 μm-1 mm. Therefore, it is feasible to evaluate the crack on the metal substrate surface using microwave non-destruction evaluation.
2013, 30(3): 154-159.
Abstract:
The influence of varying anodizing voltage and time on the tensile strength and interlaminar shear strength(ILSS) of glass fiber reinforced aluminum (GLARE) laminates was investigated. The topographic information of Al2O3 anodic film and fracture surface on laminates was obtained by SEM. The effect of aluminum/resin adhesion on the mechanical properties of GLARE laminates was discussed. The results show that tensile strength and interlaminar shear strength of laminates increase with prolonging the time of anodizing at 20 V, and reach maximum at 20 min, then decrease with extending the anodic time. The tensile strength and interlaminar shear strength of laminates increase with raising the voltage of anodizing at 20 min, and reach maximum at 20 V, then decline with increasing the anodic voltage.
The influence of varying anodizing voltage and time on the tensile strength and interlaminar shear strength(ILSS) of glass fiber reinforced aluminum (GLARE) laminates was investigated. The topographic information of Al2O3 anodic film and fracture surface on laminates was obtained by SEM. The effect of aluminum/resin adhesion on the mechanical properties of GLARE laminates was discussed. The results show that tensile strength and interlaminar shear strength of laminates increase with prolonging the time of anodizing at 20 V, and reach maximum at 20 min, then decrease with extending the anodic time. The tensile strength and interlaminar shear strength of laminates increase with raising the voltage of anodizing at 20 min, and reach maximum at 20 V, then decline with increasing the anodic voltage.
2013, 30(3): 160-167.
Abstract:
Based on the meso-structure of three-dimensional and full five-directional(Q5D) braided composites, the parameric finite element model of three-dimensional and full five-directional braided composites was established. By using the reasonable boundary condition, the elastic constants, thermal conductivity coefficient and thermal expansion coefficient were calculated, which were in a good agreement with experimental results. Furthermore, the changing characteristics of elastic property and thermo-physical properties of the braided composites with the braided angle and volume fraction of fibers were described, and the calculation results were compared with the corresponding results of three-dimensional four-directional(4D) and five-directional(5D) braided composites. The results show that the mechanical properties, longitudinal thermal conductivity and the design of zero inflated structure of three-dimensional and full five-directional braided composites are better, providing a basis for the analysis of strength problem and the coupling problem of thermo-mechanics.
Based on the meso-structure of three-dimensional and full five-directional(Q5D) braided composites, the parameric finite element model of three-dimensional and full five-directional braided composites was established. By using the reasonable boundary condition, the elastic constants, thermal conductivity coefficient and thermal expansion coefficient were calculated, which were in a good agreement with experimental results. Furthermore, the changing characteristics of elastic property and thermo-physical properties of the braided composites with the braided angle and volume fraction of fibers were described, and the calculation results were compared with the corresponding results of three-dimensional four-directional(4D) and five-directional(5D) braided composites. The results show that the mechanical properties, longitudinal thermal conductivity and the design of zero inflated structure of three-dimensional and full five-directional braided composites are better, providing a basis for the analysis of strength problem and the coupling problem of thermo-mechanics.
2013, 30(3): 168-176.
Abstract:
Graded extended finite element was proposed for fracture characteristic analysis in graded composites whose varying properties were predicted by micromechanics method. The spatially varying stiffness matrix was calculated through the linear interpolation of displacement field and a continuous gradient finite element model was established. The superiority of graded extended finite element method (XFEM) was verified through comparing the results of graded element, homogeneous element and relevant literatures. Furthermore, the influence of material properties on stress intensity factor (SIF) was discussed in detail. The results of graded XFEM can converge to the exact solutions quickly as increasing mesh density, whereas the calculation error of homogeneous XFEM cannot vanish as increasing mesh density, and it rises with the increase of crack length and property gradient. The increase of property gradient and thickness ratio of coating and substrate enlarge the SIF in coating graded material. The increase of crack length and the decrease of thickness ratio of connecting layer and substrate lead to the raise of SIF in connecting graded material.
Graded extended finite element was proposed for fracture characteristic analysis in graded composites whose varying properties were predicted by micromechanics method. The spatially varying stiffness matrix was calculated through the linear interpolation of displacement field and a continuous gradient finite element model was established. The superiority of graded extended finite element method (XFEM) was verified through comparing the results of graded element, homogeneous element and relevant literatures. Furthermore, the influence of material properties on stress intensity factor (SIF) was discussed in detail. The results of graded XFEM can converge to the exact solutions quickly as increasing mesh density, whereas the calculation error of homogeneous XFEM cannot vanish as increasing mesh density, and it rises with the increase of crack length and property gradient. The increase of property gradient and thickness ratio of coating and substrate enlarge the SIF in coating graded material. The increase of crack length and the decrease of thickness ratio of connecting layer and substrate lead to the raise of SIF in connecting graded material.
2013, 30(3): 177-183.
Abstract:
Three different repair methods were adopted in the paper to restore the mechanical performance of composite honeycomb sandwich panels with different initial damages, and an experimental study was conducted to compare the edgewise compressive capacities of repaired specimens. Test results demonstrate that all these repair methods can recover the edgewise compressive strength of specimens effectively, and the strength recoveries are all above 79% of the intact panel level. In order to analyze experimental results, k-sample Anderson-Darling test was introduced to investigate the influence of process parameters on edgewise compressive strength of repaired specimens. Test results show that middle-temperature cure can hardly affect the edgewise compressive performance of repaired specimens. However, the edgewise compressive strength is higher for specimens cured by autoclave, but the improve extent is influenced by the effective height of the repair region. Besides, unsymmetrical repair structures should be adopted cautiously to avoid the mechanical performance degradation induced by the additional bending moment.
Three different repair methods were adopted in the paper to restore the mechanical performance of composite honeycomb sandwich panels with different initial damages, and an experimental study was conducted to compare the edgewise compressive capacities of repaired specimens. Test results demonstrate that all these repair methods can recover the edgewise compressive strength of specimens effectively, and the strength recoveries are all above 79% of the intact panel level. In order to analyze experimental results, k-sample Anderson-Darling test was introduced to investigate the influence of process parameters on edgewise compressive strength of repaired specimens. Test results show that middle-temperature cure can hardly affect the edgewise compressive performance of repaired specimens. However, the edgewise compressive strength is higher for specimens cured by autoclave, but the improve extent is influenced by the effective height of the repair region. Besides, unsymmetrical repair structures should be adopted cautiously to avoid the mechanical performance degradation induced by the additional bending moment.
2013, 30(3): 184-190.
Abstract:
To meet the engineering application requirements of establishing nonlinear constitutive theory for fiber reinforced composites with different properties in tension and compression, this paper presented a generalized Hill yield criterion, which combines Drucker-Prager yield criterion considering the effect of hydrostatic pressure and Hill yield criterion suitable for anisotropic materials. Based on this theory, Sun and Chen's one-parameter plasticity model was extended to a unified nonlinear constitutive model incorporating the difference between the yield strength in tension and compression. The experimental results show that, the proposed model can be used to accurately predict the off-axis tensile and compressive nonlinear response for carbon fiber/epoxy (IM600/Q133) composites.
To meet the engineering application requirements of establishing nonlinear constitutive theory for fiber reinforced composites with different properties in tension and compression, this paper presented a generalized Hill yield criterion, which combines Drucker-Prager yield criterion considering the effect of hydrostatic pressure and Hill yield criterion suitable for anisotropic materials. Based on this theory, Sun and Chen's one-parameter plasticity model was extended to a unified nonlinear constitutive model incorporating the difference between the yield strength in tension and compression. The experimental results show that, the proposed model can be used to accurately predict the off-axis tensile and compressive nonlinear response for carbon fiber/epoxy (IM600/Q133) composites.
2013, 30(3): 191-197.
Abstract:
For optimizing damage resistance of composite laminates under impact, a method of optimization based on solid-shell coupling finite element(FE) model was presented. Solid element was used to simulate the impact region, and shell element was used in the other region of the laminates in the model. They were connected by coupling constraint. A damage variable related to the number of fiber orientations was developed in the application of progressive damage material constitutive relationship. Optimization was implemented by genetic algorithm. Effectiveness of the damage resistance optimization method was demonstrated by classical examples. Ply stacking method of composite wing-box panel aiming at impact damage resistance was optimized. Results indicate that the method of genetic algorithm optimization based on solid-shell coupling model is highly efficient and well convergent. The performance of laminates against impact loading was improved.
For optimizing damage resistance of composite laminates under impact, a method of optimization based on solid-shell coupling finite element(FE) model was presented. Solid element was used to simulate the impact region, and shell element was used in the other region of the laminates in the model. They were connected by coupling constraint. A damage variable related to the number of fiber orientations was developed in the application of progressive damage material constitutive relationship. Optimization was implemented by genetic algorithm. Effectiveness of the damage resistance optimization method was demonstrated by classical examples. Ply stacking method of composite wing-box panel aiming at impact damage resistance was optimized. Results indicate that the method of genetic algorithm optimization based on solid-shell coupling model is highly efficient and well convergent. The performance of laminates against impact loading was improved.
2013, 30(3): 198-204.
Abstract:
This paper developed an integral method with the integration path of warp direction to predict the macro elastic property in warp direction of 2.5D woven composites. The evaluated elastic modulus was compared with experimental results and classical volume average method. It shows that the predicted value is much better, and the predicted method is more effective. The distribution of elastic property during woven cyclic process in the warp direction of 2.5D woven composites was investigated, and some experiments were done with similar woven structures of 2.5D woven composite. It shows that the smaller bending angle of warp and the additional straight warp can improve the elastic property in the warp direction of 2.5D woven composites, and the nonlinearity of stress-strain curve can also be improved.
This paper developed an integral method with the integration path of warp direction to predict the macro elastic property in warp direction of 2.5D woven composites. The evaluated elastic modulus was compared with experimental results and classical volume average method. It shows that the predicted value is much better, and the predicted method is more effective. The distribution of elastic property during woven cyclic process in the warp direction of 2.5D woven composites was investigated, and some experiments were done with similar woven structures of 2.5D woven composite. It shows that the smaller bending angle of warp and the additional straight warp can improve the elastic property in the warp direction of 2.5D woven composites, and the nonlinearity of stress-strain curve can also be improved.
2013, 30(3): 205-210.
Abstract:
The stress concentration, inhere relationship between geometrical structure parameters and fracture forms in C/SiC riveting joints were investigated by FEM and experimental validation. The results show that the cone angle has little influence on the stress concentration, but it has important influence on stress value. The suitable cone angle range is 2°-10°. The critical radius of C/SiC rivet is 2.2016 mm by simulations. The rivet will break when the radius of C/SiC rivet is smaller than 2.2016 mm. Otherwise, the rivet will be pulled out of SiC. The critical radius of C/SiC rivet is 2.25 mm by experiment. The error between simulation result and experimental result is 2.2%, which validates the established FEM models for analyzing the stress concentration and fracture forms and the stimulation results.
The stress concentration, inhere relationship between geometrical structure parameters and fracture forms in C/SiC riveting joints were investigated by FEM and experimental validation. The results show that the cone angle has little influence on the stress concentration, but it has important influence on stress value. The suitable cone angle range is 2°-10°. The critical radius of C/SiC rivet is 2.2016 mm by simulations. The rivet will break when the radius of C/SiC rivet is smaller than 2.2016 mm. Otherwise, the rivet will be pulled out of SiC. The critical radius of C/SiC rivet is 2.25 mm by experiment. The error between simulation result and experimental result is 2.2%, which validates the established FEM models for analyzing the stress concentration and fracture forms and the stimulation results.
2013, 30(3): 211-219.
Abstract:
The simulation model was established for the damage analysis of laminated polymer matrix composites under high velocity oblique impact by 3D visco-elastic constitutive equations. The cohesive elements were involved between layers to simulate delaminations and the 3D-Hashin failure criteria were used to predict the in-plane damage. Furthermore, the failure process and damage characters of composite laminates under high velocity oblique impact were studied by the non-linear finite element method and stiffness reduction schemes. It is found that the delamination, matrix crack and fiber breakage are the main damage modes of composite laminates. The residual velocity reduces and damage area changes markedly within a certain range of entering angles when entering angle increases and impact velocity keeps unchanged. The residual velocity increases and damage area changes markedly within a certain range of entering angles when entering angle keeps unchanged and impact velocity increases.
The simulation model was established for the damage analysis of laminated polymer matrix composites under high velocity oblique impact by 3D visco-elastic constitutive equations. The cohesive elements were involved between layers to simulate delaminations and the 3D-Hashin failure criteria were used to predict the in-plane damage. Furthermore, the failure process and damage characters of composite laminates under high velocity oblique impact were studied by the non-linear finite element method and stiffness reduction schemes. It is found that the delamination, matrix crack and fiber breakage are the main damage modes of composite laminates. The residual velocity reduces and damage area changes markedly within a certain range of entering angles when entering angle increases and impact velocity keeps unchanged. The residual velocity increases and damage area changes markedly within a certain range of entering angles when entering angle keeps unchanged and impact velocity increases.
2013, 30(3): 220-224.
Abstract:
The model of ceramic particles uniform distributed 0-3 piezoelectric ceramic/polymer composites was built by MATLAB software and simulated by the finite element analysis software ANSYS, and the internal stress distribution and charge distribution of the piezoelectric composites in static load were studied. The changes of the maximum voltage produced by piezoelectric ceramic in the composites with the changes of the concentration of the piezoelectric ceramic particle and the static load were also investigated. It shows that when the piezoelectric composites is in static load, the stress of the piezoelectric phase recieved is far greater than that of polymer phase. The edges of the piezoelectric phase recieve the highest stress and the most charge is produced. With the increase of the piezoelectric ceramic volume fraction in the composites, the maximum voltage of the piezoelectric ceramic/polymer composites increases. When the piezoelectric ceramic volume fraction is up to 30%, the maximum voltage is as high as 2.86×10-5 V. With the increasing of the static load, the maximum voltage produced by piezoelectric composites increases linearly, and the damping effect is more obvious. The results show that the damping effect is in a good agreement with the published literature.
The model of ceramic particles uniform distributed 0-3 piezoelectric ceramic/polymer composites was built by MATLAB software and simulated by the finite element analysis software ANSYS, and the internal stress distribution and charge distribution of the piezoelectric composites in static load were studied. The changes of the maximum voltage produced by piezoelectric ceramic in the composites with the changes of the concentration of the piezoelectric ceramic particle and the static load were also investigated. It shows that when the piezoelectric composites is in static load, the stress of the piezoelectric phase recieved is far greater than that of polymer phase. The edges of the piezoelectric phase recieve the highest stress and the most charge is produced. With the increase of the piezoelectric ceramic volume fraction in the composites, the maximum voltage of the piezoelectric ceramic/polymer composites increases. When the piezoelectric ceramic volume fraction is up to 30%, the maximum voltage is as high as 2.86×10-5 V. With the increasing of the static load, the maximum voltage produced by piezoelectric composites increases linearly, and the damping effect is more obvious. The results show that the damping effect is in a good agreement with the published literature.
2013, 30(3): 225-232.
Abstract:
Quasi-static tensile experiments were performed on the epoxy-based syntactic foams filled by hollow ceramic microspheres with different volume fractions in this study. The effects of microspheres volume fraction on the Young's modulus and Poisson's ratio of syntactic foams were investigated. Microstructure based three-dimensional unit cell models were constructed to capture the stress/strain fields in mesoscale. Cohesive elements were introduced to the FE model to simulate the mechanical behavior of the interface between the microspheres and the matrix material. To verify the validity of the proposed model considering the interface effect, numerical results and two classical analytical methods were compared with the experimental data. It can be found that the ideal interfacial bonding hypothesis based FE model and the classical analytical methods seriously overestimates the experimentally determined Young's modulus and Poisson's ratio. The elastic properties of syntactic foams heavily depend on the mechanical properties of the interface, and only the results obtained by the FE model considering the interfacial effect are much closer to the experimental values and thus provide more accurate predictions.
Quasi-static tensile experiments were performed on the epoxy-based syntactic foams filled by hollow ceramic microspheres with different volume fractions in this study. The effects of microspheres volume fraction on the Young's modulus and Poisson's ratio of syntactic foams were investigated. Microstructure based three-dimensional unit cell models were constructed to capture the stress/strain fields in mesoscale. Cohesive elements were introduced to the FE model to simulate the mechanical behavior of the interface between the microspheres and the matrix material. To verify the validity of the proposed model considering the interface effect, numerical results and two classical analytical methods were compared with the experimental data. It can be found that the ideal interfacial bonding hypothesis based FE model and the classical analytical methods seriously overestimates the experimentally determined Young's modulus and Poisson's ratio. The elastic properties of syntactic foams heavily depend on the mechanical properties of the interface, and only the results obtained by the FE model considering the interfacial effect are much closer to the experimental values and thus provide more accurate predictions.
2013, 30(3): 233-239.
Abstract:
RSA (random sequential adsorption) method was used to generate the representative volume element models of syntactic foams. Finite element software ANSYS was adopted to analyze the thermal expansion of syntactic foams. The relationships of thermal expansion coefficient of HGMs/resin and hollow glass microballoons (HGMs) filling ratio, coefficient of thermal expansion and HGMs wall-thickness were obtained. The two relation curves were fitted to linear function, and exponential function respectively, with the purpose of analyzing the sensitivity of the HGMs filling volume fraction and wall-thickness to the coefficient of thermal expansion. The stress fields and displacement fields caused by temperature load were also obtained. The results show that both of the increase of HGMs filling volume fraction and wall-thickness can reduce the coefficient of thermal expansion of HGMs/resin syntactic foams, and HGMs filling volume fraction shows the higher sensitivity than that of wall-thickness.
RSA (random sequential adsorption) method was used to generate the representative volume element models of syntactic foams. Finite element software ANSYS was adopted to analyze the thermal expansion of syntactic foams. The relationships of thermal expansion coefficient of HGMs/resin and hollow glass microballoons (HGMs) filling ratio, coefficient of thermal expansion and HGMs wall-thickness were obtained. The two relation curves were fitted to linear function, and exponential function respectively, with the purpose of analyzing the sensitivity of the HGMs filling volume fraction and wall-thickness to the coefficient of thermal expansion. The stress fields and displacement fields caused by temperature load were also obtained. The results show that both of the increase of HGMs filling volume fraction and wall-thickness can reduce the coefficient of thermal expansion of HGMs/resin syntactic foams, and HGMs filling volume fraction shows the higher sensitivity than that of wall-thickness.
2013, 30(3): 240-246.
Abstract:
A carbon fiber/polymer smart layer able to sense dynamic strain was developed to perform structural strain modal diagnosis. The sensitivity of such a smart material to dynamic load at low frequency was revealed by the tension tests conducted with different frequencies, and the influencing factors on the error of the dynamic response were analyzed theoretically. The smart layer was continuously laid on a cantilever beam and took place of traditional strain gauges to test its strain modals. The test results indicate that the smart layer is able to exactly present the natural frequencies and the strain modal shapes of the first three orders for the beam. The test was conducted again on the beam with added mass. It is found that the added mass leads to the decrease of the natural frequencies and induces mutations on the smart layer characterized strain modal just in the position where the mass added. The results demonstrate the ability of the smart layer characterized strain modal to identify the change of the structural physical parameters. Good agreement was achieved between the results attained with the smart layer and the strain gauges. In addition, based on the ability of covering in smart layer, the information about the structural strain modal is fully captured. Furthermore, the change of the structural physical parameters is located by the tests with added measured points in the suspected area.
A carbon fiber/polymer smart layer able to sense dynamic strain was developed to perform structural strain modal diagnosis. The sensitivity of such a smart material to dynamic load at low frequency was revealed by the tension tests conducted with different frequencies, and the influencing factors on the error of the dynamic response were analyzed theoretically. The smart layer was continuously laid on a cantilever beam and took place of traditional strain gauges to test its strain modals. The test results indicate that the smart layer is able to exactly present the natural frequencies and the strain modal shapes of the first three orders for the beam. The test was conducted again on the beam with added mass. It is found that the added mass leads to the decrease of the natural frequencies and induces mutations on the smart layer characterized strain modal just in the position where the mass added. The results demonstrate the ability of the smart layer characterized strain modal to identify the change of the structural physical parameters. Good agreement was achieved between the results attained with the smart layer and the strain gauges. In addition, based on the ability of covering in smart layer, the information about the structural strain modal is fully captured. Furthermore, the change of the structural physical parameters is located by the tests with added measured points in the suspected area.
2013, 30(3): 247-252.
Abstract:
The damage modes and failure mechanism were released through compressive experiment on stitched composites with impact damage. The stress field of stitched composite plate with impact damage, which was modeled by a hole, was calculated by hybrid stress elements. The residual compressive strength was predicted by the point stress criterion based on characteristic curve. The results indicate that the impact damage is approximate to an round shape, and the compressive damage type is different in stitched and unstitched laminate. The compression strength can be simulate by equivalent hole method, and the character distance and damage area are main influence factors of calculated results.
The damage modes and failure mechanism were released through compressive experiment on stitched composites with impact damage. The stress field of stitched composite plate with impact damage, which was modeled by a hole, was calculated by hybrid stress elements. The residual compressive strength was predicted by the point stress criterion based on characteristic curve. The results indicate that the impact damage is approximate to an round shape, and the compressive damage type is different in stitched and unstitched laminate. The compression strength can be simulate by equivalent hole method, and the character distance and damage area are main influence factors of calculated results.
