2010 Vol. 27, No. 4
2010, 27(4): 1-8.
Abstract:
he effect of high-permeable distribution medium（DM）on resin flow behavior in the vacuum infusion molding process was studied through visualization flow experiments. The results show that DM is able to largely reduce the mold filling time in vacuum infusion molding process ， and mold filling time is linearly reduced with the scale of DM. The flow-enhancement effect of DM decreases with the thickness of fiber preforms. The difference of resin flow front between top and bottom of fiber preforms linearly increases with the thickness of fiber preforms. The flow-enhancement mechanism of DM takes markedly on thickness effect. This thickness effect is important for vacuum infusion molding processing parameter optimization and quality control.
he effect of high-permeable distribution medium（DM）on resin flow behavior in the vacuum infusion molding process was studied through visualization flow experiments. The results show that DM is able to largely reduce the mold filling time in vacuum infusion molding process ， and mold filling time is linearly reduced with the scale of DM. The flow-enhancement effect of DM decreases with the thickness of fiber preforms. The difference of resin flow front between top and bottom of fiber preforms linearly increases with the thickness of fiber preforms. The flow-enhancement mechanism of DM takes markedly on thickness effect. This thickness effect is important for vacuum infusion molding processing parameter optimization and quality control.
2010, 27(4): 9-14.
Abstract:
Quaternary composites consisting of two different types of filler (carbon black and carbon fiber) and binary polymer (polyethylene and polypropylene) were prepared by melt blending. The microstructure of the conductive networks and conductivity properties of the composite were investigated. The results show that the double---percolated networks, as well as the mutual bridging between carbon fiber and conductive high density polyethylene (HDPE) phase region reduce the percolation threshold and improve the conductivity. The volume resistivity of quaternary composites decreases by 1～5 orders of magnitude at the same filler content compared to CB/HDPEPP and CBCF/HDPE ternary system. Meanwhile, the negative temperature coefficient effect is inhibited and the stability of the electrical properties is raised, due to the existence of the double---percolated networks. Compared with CBCF/HDPE ternary system, the NTC effect of CBCF/HDPEPP system decreases from 2 orders of magnitude to 0.6 orders of magnitude, while the characteristic relaxation time increases to 2370s from 951s.
Quaternary composites consisting of two different types of filler (carbon black and carbon fiber) and binary polymer (polyethylene and polypropylene) were prepared by melt blending. The microstructure of the conductive networks and conductivity properties of the composite were investigated. The results show that the double---percolated networks, as well as the mutual bridging between carbon fiber and conductive high density polyethylene (HDPE) phase region reduce the percolation threshold and improve the conductivity. The volume resistivity of quaternary composites decreases by 1～5 orders of magnitude at the same filler content compared to CB/HDPEPP and CBCF/HDPE ternary system. Meanwhile, the negative temperature coefficient effect is inhibited and the stability of the electrical properties is raised, due to the existence of the double---percolated networks. Compared with CBCF/HDPE ternary system, the NTC effect of CBCF/HDPEPP system decreases from 2 orders of magnitude to 0.6 orders of magnitude, while the characteristic relaxation time increases to 2370s from 951s.
2010, 27(4): 15-20.
Abstract:
The microstructure of 3D needling felts is beneficial for the wave absorption. Wave absorbing composites with high temperature resistance could be formed by using carbon and boron nitride matrix with 3D needling carbon felts as reinforcement. In this paper, porous 3D needling carbon/carbon composites (C/C) were fabricated by precursor infiltration pyrolysis (PIP), then C/C--BN composites were fabricated by introducing BN into the porous C/C by using chemical vapor infiltration (CVI). The effects of CVI time on the microstructures, mechanical properties and dielectric properties of 3D needling C/C--BN were studied. With the increase of CVI time, the density and flexural strength of C/C--BN increase, while the porosity decreases. When CVI time reaches 160h, C/C--BN attains a density of 1.43g/cm3, a total porosity of 25%, and a flexural strength of 82MPa. Because of the decrease of porosity with the increase of CVI time, the dielectric constant of the C/C--BN composite increases and the dielectric loss decreases.
The microstructure of 3D needling felts is beneficial for the wave absorption. Wave absorbing composites with high temperature resistance could be formed by using carbon and boron nitride matrix with 3D needling carbon felts as reinforcement. In this paper, porous 3D needling carbon/carbon composites (C/C) were fabricated by precursor infiltration pyrolysis (PIP), then C/C--BN composites were fabricated by introducing BN into the porous C/C by using chemical vapor infiltration (CVI). The effects of CVI time on the microstructures, mechanical properties and dielectric properties of 3D needling C/C--BN were studied. With the increase of CVI time, the density and flexural strength of C/C--BN increase, while the porosity decreases. When CVI time reaches 160h, C/C--BN attains a density of 1.43g/cm3, a total porosity of 25%, and a flexural strength of 82MPa. Because of the decrease of porosity with the increase of CVI time, the dielectric constant of the C/C--BN composite increases and the dielectric loss decreases.
2010, 27(4): 21-25.
Abstract:
NiFe2O4/T-ZnOW composites were prepaired by encapsulating T-ZnOW with an NiFe2O4 film through ferrite electrodeless plating. The composites were characterized by X-ray analyses (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). The magnetic properties were measured by vibrating sample magnetometer (VSM). The results indicate that the surface of T-ZnOW is covered with a compact layer of NiFe2O4, and the coating on the needlepoint is thinner than that on the root of T-ZnOW The composites exhibit excellent ferromagnetic behavior. The saturation magnetization and coercive force increase with the annealing temperature, and reach the peak at 800℃.
NiFe2O4/T-ZnOW composites were prepaired by encapsulating T-ZnOW with an NiFe2O4 film through ferrite electrodeless plating. The composites were characterized by X-ray analyses (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). The magnetic properties were measured by vibrating sample magnetometer (VSM). The results indicate that the surface of T-ZnOW is covered with a compact layer of NiFe2O4, and the coating on the needlepoint is thinner than that on the root of T-ZnOW The composites exhibit excellent ferromagnetic behavior. The saturation magnetization and coercive force increase with the annealing temperature, and reach the peak at 800℃.
2010, 27(4): 26-30.
Abstract:
A novel degradable α-tricalcium phosphate/multi-(amino acid) copolymer composite (α---TCP/MAACP) was prepared by the in situ copolymerization method and characterized by FTIR, XRD, SEM and in vitro degradation experiments in simulated body fluid. The results exhibit that the composite is composed of an amide copolymer and α-TCP. The composite shows a fast degradation in the primary period and 6.8% of mass loss is observed in the first week. A total of 20.1% mass is lost after 12 weeks immersion. No strong acidic degradation products are produced and the pH of soaking medium is in a range of 6.6～7.0 during the immersion period. The water absorption rate of the composite is about 11% and no significant change occurs during the soaking time. SEM analysis shows that the surface becomes smooth after 2 weeks immersion and some precipitation is observed. The precipitation increases with soaking time and covers the most part of surface at the end of 12 weeks. XRD determination shows that the precipitation is poorly crystallized hydroxyapatite, indicating the composite is bioactive. After 12 weeks soaking, compared to that of before soaking, the crystallinity of surface decreases but no significant change for that of cross section is observed, indicating that the composite may be a surface erosion material. Collectively, this composite can be potential bone substitutes for bone repair and tissue engineering.
A novel degradable α-tricalcium phosphate/multi-(amino acid) copolymer composite (α---TCP/MAACP) was prepared by the in situ copolymerization method and characterized by FTIR, XRD, SEM and in vitro degradation experiments in simulated body fluid. The results exhibit that the composite is composed of an amide copolymer and α-TCP. The composite shows a fast degradation in the primary period and 6.8% of mass loss is observed in the first week. A total of 20.1% mass is lost after 12 weeks immersion. No strong acidic degradation products are produced and the pH of soaking medium is in a range of 6.6～7.0 during the immersion period. The water absorption rate of the composite is about 11% and no significant change occurs during the soaking time. SEM analysis shows that the surface becomes smooth after 2 weeks immersion and some precipitation is observed. The precipitation increases with soaking time and covers the most part of surface at the end of 12 weeks. XRD determination shows that the precipitation is poorly crystallized hydroxyapatite, indicating the composite is bioactive. After 12 weeks soaking, compared to that of before soaking, the crystallinity of surface decreases but no significant change for that of cross section is observed, indicating that the composite may be a surface erosion material. Collectively, this composite can be potential bone substitutes for bone repair and tissue engineering.
2010, 27(4): 31-37.
Abstract:
Nano-SiC-ZrO2/MoSi2 composite ceramics were prepared by hot-pressed sintering. The microstructures and properties were investigated using X-ray diffractometer, scanning electron microscope and mechanical properties testing. The results show that the addition of nano-ZrO2 and nano-SiC particles can refines the grain of MoSi2 matrix effectively. The SiC-ZrO2 synergism is advantageous to enhance the bending strength and fracture toughness of the MoSi2 matrix ceramic, and the effect of nano-SiC on refining and strengthening the MoSi2 matrix is better than that of nano-ZrO2 in the synergistic phase. The bending strength and fracture toughness of 20vol%SiC+10vol%ZrO2+MoSi2 improve 280% and 140% respectively compared with the MoSi2 matrix. Different dislocation configurations are found around the particles in the ceramic matrix. ZrO2 produces dislocation in the matrix depending on the volume effect caused by its own phase transitions. The second particles pining dislocation include SiC particles and non-phase-transformed ZrO2 particles. The pining effect of dispersed phases on the dislocation in the composite ceramics is obvious, especially the intragranular type SiC and ZrO2 particles.
Nano-SiC-ZrO2/MoSi2 composite ceramics were prepared by hot-pressed sintering. The microstructures and properties were investigated using X-ray diffractometer, scanning electron microscope and mechanical properties testing. The results show that the addition of nano-ZrO2 and nano-SiC particles can refines the grain of MoSi2 matrix effectively. The SiC-ZrO2 synergism is advantageous to enhance the bending strength and fracture toughness of the MoSi2 matrix ceramic, and the effect of nano-SiC on refining and strengthening the MoSi2 matrix is better than that of nano-ZrO2 in the synergistic phase. The bending strength and fracture toughness of 20vol%SiC+10vol%ZrO2+MoSi2 improve 280% and 140% respectively compared with the MoSi2 matrix. Different dislocation configurations are found around the particles in the ceramic matrix. ZrO2 produces dislocation in the matrix depending on the volume effect caused by its own phase transitions. The second particles pining dislocation include SiC particles and non-phase-transformed ZrO2 particles. The pining effect of dispersed phases on the dislocation in the composite ceramics is obvious, especially the intragranular type SiC and ZrO2 particles.
2010, 27(4): 38-44.
Abstract:
Based on the main system of Ti-B4C-C (sucrouse as the precursor of carbon) in mole ratio and with the additives of 5wt%Al, 5wt%Al＋20wt%Al-Ni and 5wt%Al＋20wt%Cu-Ni, the Ti(C,N)-TiB2 multi-phased ceramic preforms were prepared by self-reactive spray forming technology. And the structure and mechanical performances of the self-reactive spray formed preforms in different systems were contrastively studied. The results show that the four kinds of preforms have the similar structural character, which is composed of the main phases of continuous matrix phase Ti(Cx, Ny), dispersed micro/nanometric rodlike grains TiB2, some by-product phases and a few of pores. The preforms of Ti-B4C-C-Al system have the highest hardness. The preforms of Ti-B4C-C-Al ＋Cu-Ni system have the lowest density and flexural strength. The preforms of Ti-B4C-C-Al ＋Al-Ni system have the highest density, flexural and fracture toughness, which is due to the crack deflection and pinning and micro-cracks toughening mechanism.
Based on the main system of Ti-B4C-C (sucrouse as the precursor of carbon) in mole ratio and with the additives of 5wt%Al, 5wt%Al＋20wt%Al-Ni and 5wt%Al＋20wt%Cu-Ni, the Ti(C,N)-TiB2 multi-phased ceramic preforms were prepared by self-reactive spray forming technology. And the structure and mechanical performances of the self-reactive spray formed preforms in different systems were contrastively studied. The results show that the four kinds of preforms have the similar structural character, which is composed of the main phases of continuous matrix phase Ti(Cx, Ny), dispersed micro/nanometric rodlike grains TiB2, some by-product phases and a few of pores. The preforms of Ti-B4C-C-Al system have the highest hardness. The preforms of Ti-B4C-C-Al ＋Cu-Ni system have the lowest density and flexural strength. The preforms of Ti-B4C-C-Al ＋Al-Ni system have the highest density, flexural and fracture toughness, which is due to the crack deflection and pinning and micro-cracks toughening mechanism.
2010, 27(4): 45-51.
Abstract:
Organic---attapulgite (OAT) was obtained by pretreating attapulgite, then the nano---OAT/polyactide (PLA)-x ( x=1wt%、 3wt%、 5wt%) composites were prepared respectively by in situ polymerization. The composites were characterized by infrared absorption spectroscopy (IR), scanning electronic microscope (SEM) and X-Ray diffraction (XRD). SEM results show equably uniform dispersion and good interfacial compatibility between PLA and OAT. The mechanical capability and thermal analysis (TGA ＆ DSC) of the composies were measured. Compared with the pure PLA, the tensile strength and elastic modulus of the OAT/PLA3 are improved by 98.6% and 130.0% respectively. The thermal stability of composites is markedly improved. It also shows that the degradation velocity of OAT/PLA-x is obviously speeded up.
Organic---attapulgite (OAT) was obtained by pretreating attapulgite, then the nano---OAT/polyactide (PLA)-x ( x=1wt%、 3wt%、 5wt%) composites were prepared respectively by in situ polymerization. The composites were characterized by infrared absorption spectroscopy (IR), scanning electronic microscope (SEM) and X-Ray diffraction (XRD). SEM results show equably uniform dispersion and good interfacial compatibility between PLA and OAT. The mechanical capability and thermal analysis (TGA ＆ DSC) of the composies were measured. Compared with the pure PLA, the tensile strength and elastic modulus of the OAT/PLA3 are improved by 98.6% and 130.0% respectively. The thermal stability of composites is markedly improved. It also shows that the degradation velocity of OAT/PLA-x is obviously speeded up.
2010, 27(4): 52-58.
Abstract:
The isothermal and continuous heating cure processes of a carbon fiber (CF)/bismaleimide (BMI) prepreg were investigated by DMA. Based on the change of dynamic mechanical properties, the criteria for determining the softening, melting, gelation, vitrification transitions and cure reaction points of the prepreg were established, which were confirmed by the measurements of micropolariscope and DSC. The effects of temperature, heating rate and frequency on the transition and reaction points were studied. Maximum mechanical conversions and reaction conversions of isothermal cure processes were measured. The results show that DMA can well detect various transitions and reaction points of the CF/BMI prepreg. Due to the difference in cure reaction mechanism, the variations of the ultimate mechanical and reaction conversions with the isothermal temperature are different below and above 180℃.
The isothermal and continuous heating cure processes of a carbon fiber (CF)/bismaleimide (BMI) prepreg were investigated by DMA. Based on the change of dynamic mechanical properties, the criteria for determining the softening, melting, gelation, vitrification transitions and cure reaction points of the prepreg were established, which were confirmed by the measurements of micropolariscope and DSC. The effects of temperature, heating rate and frequency on the transition and reaction points were studied. Maximum mechanical conversions and reaction conversions of isothermal cure processes were measured. The results show that DMA can well detect various transitions and reaction points of the CF/BMI prepreg. Due to the difference in cure reaction mechanism, the variations of the ultimate mechanical and reaction conversions with the isothermal temperature are different below and above 180℃.
2010, 27(4): 59-63.
Abstract:
The single fiber fragmentation test (SFFT) was used to study the interfacial shear strength (IFSS) of the high---performance organic fibers polybenzoxazole (PBO), aramid fiber Twaron and ultra high molecular weight polyethylene (UHMWPE) fiber/tough epoxy by statistics on the filament’s saturated break points and analysis of the breakpoint morphology. The results were compared with the interlaminar shear strength (ILSS) of unidirectional composites in the interply shear test. By means of XPS and SEM, the effect of the filament’s physical and chemical surface characteristics on the IFSS of fiber/matrix was also studied. The results show that the IFSS of Twaron/epoxy is higher than that of PBO/epoxy, and the strength of UHMWPE/epoxy is too low to be measured using SFFT. And both the IFSS and the ILSS are consistent for all three kinds of organic fibers/epoxy. Fibers with higher surface chemical activity have higher IFSS.
The single fiber fragmentation test (SFFT) was used to study the interfacial shear strength (IFSS) of the high---performance organic fibers polybenzoxazole (PBO), aramid fiber Twaron and ultra high molecular weight polyethylene (UHMWPE) fiber/tough epoxy by statistics on the filament’s saturated break points and analysis of the breakpoint morphology. The results were compared with the interlaminar shear strength (ILSS) of unidirectional composites in the interply shear test. By means of XPS and SEM, the effect of the filament’s physical and chemical surface characteristics on the IFSS of fiber/matrix was also studied. The results show that the IFSS of Twaron/epoxy is higher than that of PBO/epoxy, and the strength of UHMWPE/epoxy is too low to be measured using SFFT. And both the IFSS and the ILSS are consistent for all three kinds of organic fibers/epoxy. Fibers with higher surface chemical activity have higher IFSS.
2010, 27(4): 64-69.
Abstract:
High energy and low energy ultrasonic waves were used to modify a kind of multi---walled carbon nanotubes (MWCNTs) /epoxy resin. By means of rheometer, DSC, FTIR, XPS, XRD and TEM, the physical and chemical characteristics of MWCNTs treated by ultrasonic waves with high and low energies were studied. The dispersion and compatibility of MWCNTs in epoxy resin were also analyzed. Moreover, the thermal stability and flexural property of MWCNTs/epoxy resin were measured. The experimental results show that the modified extent of MWCNTs is different with different ultrasonic wave energy. MWCNTs modified with high energy ultrasonic wave have better dispersive degree and dispersive efficiency in the resin, and the surface activity of MWCNTs is increased, which is not the case for the condition of low energy ultrasonic wave treatment. In addition, the flexural modulus and glass transition temperature of epoxy resin can be enhanced by adding a small amount of MWCNTs modified with high energy ultrasonic wave, indicating that there are strong interfaces between MWCNTs and resin matrix.
High energy and low energy ultrasonic waves were used to modify a kind of multi---walled carbon nanotubes (MWCNTs) /epoxy resin. By means of rheometer, DSC, FTIR, XPS, XRD and TEM, the physical and chemical characteristics of MWCNTs treated by ultrasonic waves with high and low energies were studied. The dispersion and compatibility of MWCNTs in epoxy resin were also analyzed. Moreover, the thermal stability and flexural property of MWCNTs/epoxy resin were measured. The experimental results show that the modified extent of MWCNTs is different with different ultrasonic wave energy. MWCNTs modified with high energy ultrasonic wave have better dispersive degree and dispersive efficiency in the resin, and the surface activity of MWCNTs is increased, which is not the case for the condition of low energy ultrasonic wave treatment. In addition, the flexural modulus and glass transition temperature of epoxy resin can be enhanced by adding a small amount of MWCNTs modified with high energy ultrasonic wave, indicating that there are strong interfaces between MWCNTs and resin matrix.
2010, 27(4): 70-74.
Abstract:
Based on considerable non-destructive identification data for composite components by autoclave process, sub-cluster statistics theory was applied in this paper to investigate the relationship between the structure form of composite components and manufacturing quality. A sub-cluster model about delamination area was set up. The process quality of composite components by autoclave process was evaluated on the basis of the sub-cluster parameters reflecting the tendency of delamination area. The results show that in the statistical range of composite components applied in aircrafts, the small delamination area is in a dominant position in the delamination area distribution of composite components. Moreover, the complex composite structures (such as I-shaped laminatess), the thinner (1~2 mm) laminates and the thicker (>5 mm) laminates increase the tendency of the large delamination area appearance and the dispersibility of delamination area, and thus lead to a worse manufacturing quality of composite components.
Based on considerable non-destructive identification data for composite components by autoclave process, sub-cluster statistics theory was applied in this paper to investigate the relationship between the structure form of composite components and manufacturing quality. A sub-cluster model about delamination area was set up. The process quality of composite components by autoclave process was evaluated on the basis of the sub-cluster parameters reflecting the tendency of delamination area. The results show that in the statistical range of composite components applied in aircrafts, the small delamination area is in a dominant position in the delamination area distribution of composite components. Moreover, the complex composite structures (such as I-shaped laminatess), the thinner (1~2 mm) laminates and the thicker (>5 mm) laminates increase the tendency of the large delamination area appearance and the dispersibility of delamination area, and thus lead to a worse manufacturing quality of composite components.
2010, 27(4): 75-80.
Abstract:
The formation mechanism of crack defects in resin---rich areas was studied for the quartz fiber reinforced polyarylacetylene(PAA) composite. The curing characteristic and shrinkage of PAA resin were evaluated by means of differential scanning calorimetry, infrared ray, three---point bending and micrograph. Moreover, the influences of mechanical performances of PAA resin, preform structure and fiber volume fraction on formation of crack defects were investigated. The results indicate that large curing shrinkage and brittleness of PAA resin are the main reasons for crack defects, and the crack defects can be reduced by increasing the fiber volume fraction and reducing the resin---rich area. Besides, resin modification by phenolic resin was proved to be an effective method to reduce crack defects.
The formation mechanism of crack defects in resin---rich areas was studied for the quartz fiber reinforced polyarylacetylene(PAA) composite. The curing characteristic and shrinkage of PAA resin were evaluated by means of differential scanning calorimetry, infrared ray, three---point bending and micrograph. Moreover, the influences of mechanical performances of PAA resin, preform structure and fiber volume fraction on formation of crack defects were investigated. The results indicate that large curing shrinkage and brittleness of PAA resin are the main reasons for crack defects, and the crack defects can be reduced by increasing the fiber volume fraction and reducing the resin---rich area. Besides, resin modification by phenolic resin was proved to be an effective method to reduce crack defects.
2010, 27(4): 81-86.
Abstract:
Three-dimensional (3D) spacer fabric composite is a new light-weight sandwich structure, the reinforcement of which is integrally woven with a hollow core. A vacuum assisted resin transfer molding (VARTM) process was used for the fabrication of 3D spacer fabric composites with phenolic matrix. The key process problems, such as infusion process, fabric thickness recovery and resin distribution, were investigated systematically, and the influence factors were discussed. The results indicate that higher fiber volume fraction can be achieved for thicker 3D spacer fabric composite. The resin impregnation velocity in the weft direction is larger than that in the warp direction. And heat treatment enhances thickness recovery properties effectively. Besides, high permeability media（HPM） and low viscosity are helpful to resin distribution.
Three-dimensional (3D) spacer fabric composite is a new light-weight sandwich structure, the reinforcement of which is integrally woven with a hollow core. A vacuum assisted resin transfer molding (VARTM) process was used for the fabrication of 3D spacer fabric composites with phenolic matrix. The key process problems, such as infusion process, fabric thickness recovery and resin distribution, were investigated systematically, and the influence factors were discussed. The results indicate that higher fiber volume fraction can be achieved for thicker 3D spacer fabric composite. The resin impregnation velocity in the weft direction is larger than that in the warp direction. And heat treatment enhances thickness recovery properties effectively. Besides, high permeability media（HPM） and low viscosity are helpful to resin distribution.
2010, 27(4): 87-93.
Abstract:
The mixed reinforcement combining bias stitching and Z---pin inserting was used to prepare foam core sandwich structures. The prepared sandwich structures were tested under flatwise compressive, flatwise tensile and shear loads, and the mechanical properties and damage modes were also studied. The results indicate that the compressive strength and modulus are improved with increasing Z---pin inserting, while the tensile strength, modulus and shear strength are improved with increasing bias stitching. However, there is no significant distinction in the shear modulus between different specimens.
The mixed reinforcement combining bias stitching and Z---pin inserting was used to prepare foam core sandwich structures. The prepared sandwich structures were tested under flatwise compressive, flatwise tensile and shear loads, and the mechanical properties and damage modes were also studied. The results indicate that the compressive strength and modulus are improved with increasing Z---pin inserting, while the tensile strength, modulus and shear strength are improved with increasing bias stitching. However, there is no significant distinction in the shear modulus between different specimens.
2010, 27(4): 94-99.
Abstract:
For the carbon fiber satin fabric/epoxy 914 prepreg, a resin pressure online measuring system for hot pressing process was used to measure the value and distribution of resin pressure. Additionally, the effects of vacuum and autoclave pressure on resin pressure were studied. Moreover, the influence of vacuum and autoclave pressure on void formation was investigated using micrographs. The experimental results show that the online measuring system applied can quantitatively analyze the influence of vacuum in plies and resin pressure evolution. The resin bears most of autoclave pressure and distributes uniformly in thickness and in-plane directions during autoclave zero-bleeding process. Vacuum can exhaust entrapped air through gas channels in prepreg stacks, and the effect depends on resin rheological property and compaction of prepreg stacks. In addition, the results of void analysis are consistent with the measurements of resin pressure under different pressure conditions.
For the carbon fiber satin fabric/epoxy 914 prepreg, a resin pressure online measuring system for hot pressing process was used to measure the value and distribution of resin pressure. Additionally, the effects of vacuum and autoclave pressure on resin pressure were studied. Moreover, the influence of vacuum and autoclave pressure on void formation was investigated using micrographs. The experimental results show that the online measuring system applied can quantitatively analyze the influence of vacuum in plies and resin pressure evolution. The resin bears most of autoclave pressure and distributes uniformly in thickness and in-plane directions during autoclave zero-bleeding process. Vacuum can exhaust entrapped air through gas channels in prepreg stacks, and the effect depends on resin rheological property and compaction of prepreg stacks. In addition, the results of void analysis are consistent with the measurements of resin pressure under different pressure conditions.
2010, 27(4): 100-105.
Abstract:
To enhance the interfacial adhesion between silica fibers(SF) and the polyarylacetylene matrix（PAA）, the modification phenolic resin was used as the surface treatment agent for SF. The influence of treatment technology on the silica fibers reinforced polyarylacetylene composites (SF/PAA) mechanical property and dielectric property was investigated by properties testing. The composition and functional group of the silica fiber surface were examined by X---ray photoelectron spectroscopy (XPS). The fracture surface of the composites was observed by scanning electron microscopy (SEM). The results of the properties testing indicate that mechanical property and dielectric property of the composites treated by modification phenolic resin are increased greatly. From XPS analysis, it can be seen that the interface of silica fiber has been grafted with the modification phenolic resin. Compared the SEM photographs of fracture surfaces of the composites, it shows that the interfacial adhesion of the composites treated by the modification phenolic resin is improved greatly.
To enhance the interfacial adhesion between silica fibers(SF) and the polyarylacetylene matrix（PAA）, the modification phenolic resin was used as the surface treatment agent for SF. The influence of treatment technology on the silica fibers reinforced polyarylacetylene composites (SF/PAA) mechanical property and dielectric property was investigated by properties testing. The composition and functional group of the silica fiber surface were examined by X---ray photoelectron spectroscopy (XPS). The fracture surface of the composites was observed by scanning electron microscopy (SEM). The results of the properties testing indicate that mechanical property and dielectric property of the composites treated by modification phenolic resin are increased greatly. From XPS analysis, it can be seen that the interface of silica fiber has been grafted with the modification phenolic resin. Compared the SEM photographs of fracture surfaces of the composites, it shows that the interfacial adhesion of the composites treated by the modification phenolic resin is improved greatly.
2010, 27(4): 106-110.
Abstract:
To improve the atomic oxygen (AO) erosion resistance of the silicone coating, hybrid coating was prepared on the polyimide substrate by aminopropylsilsesquioxane(APOSS) cross-linked epoxy silicone resin. The surface morphology, chemical composition and chemical structure of the hybrid coating were analyzed before and after AO exposure in a ground atomic oxygen simulated facility. The results show that APOSS can prevent the micro-cracks in silicone coating and avoid the “undercutting” phenomenon, and the mass loss of materials decreases obviously. During AO exposure, O and Si elements in APOSS changed slowly from the low-energy state to the high binding energy state (oxidation state), while a protective layer of SiO2 formed on the coating surface and prevented the further AO erosion of the materials.
To improve the atomic oxygen (AO) erosion resistance of the silicone coating, hybrid coating was prepared on the polyimide substrate by aminopropylsilsesquioxane(APOSS) cross-linked epoxy silicone resin. The surface morphology, chemical composition and chemical structure of the hybrid coating were analyzed before and after AO exposure in a ground atomic oxygen simulated facility. The results show that APOSS can prevent the micro-cracks in silicone coating and avoid the “undercutting” phenomenon, and the mass loss of materials decreases obviously. During AO exposure, O and Si elements in APOSS changed slowly from the low-energy state to the high binding energy state (oxidation state), while a protective layer of SiO2 formed on the coating surface and prevented the further AO erosion of the materials.
2010, 27(4): 111-117.
Abstract:
Reactive core-shell modifiers of ABS-g-MA and ABS-g-AA were synthesized to toughen PA6. The difference between the modifiers lays in the reactive monomers on the shell, which would induce different interface properties between the PA6 blends. So the brittle-ductile transition and fracture behavior of the blends were studied when the other parameters were the same. Molau test and torque results indicate ABS-g-MA/PA6 shows superior interface strength. TEM shows ABS-g-MA and ABS-g-AA modifiers disperse in PA6 matrix uniformly. Mechanical test shows brittle-ductile transition takes place between 25%~30% modifiers mass fraction for ABS-g-MA/PA6 blends and impact strength more than 900 J/m is achieved, however, brittle-ductile transition takes place between 30%~35% modifiers mass fraction for ABS-g-AA/PA6 blends. Fracture mechanisms display cavitation of rubber particles and shear yielding of matrix are the major deformation mechanisms for ABS-g-MA/PA6 blends, however, no cavitation can be found and shear yielding is indistinctive for ABS-g-AA/PA6 blends. Vu-Khanh test shows ABS-g-MA/PA6 blends have much higher Gi value and toughness due to the higher interface strength.
Reactive core-shell modifiers of ABS-g-MA and ABS-g-AA were synthesized to toughen PA6. The difference between the modifiers lays in the reactive monomers on the shell, which would induce different interface properties between the PA6 blends. So the brittle-ductile transition and fracture behavior of the blends were studied when the other parameters were the same. Molau test and torque results indicate ABS-g-MA/PA6 shows superior interface strength. TEM shows ABS-g-MA and ABS-g-AA modifiers disperse in PA6 matrix uniformly. Mechanical test shows brittle-ductile transition takes place between 25%~30% modifiers mass fraction for ABS-g-MA/PA6 blends and impact strength more than 900 J/m is achieved, however, brittle-ductile transition takes place between 30%~35% modifiers mass fraction for ABS-g-AA/PA6 blends. Fracture mechanisms display cavitation of rubber particles and shear yielding of matrix are the major deformation mechanisms for ABS-g-MA/PA6 blends, however, no cavitation can be found and shear yielding is indistinctive for ABS-g-AA/PA6 blends. Vu-Khanh test shows ABS-g-MA/PA6 blends have much higher Gi value and toughness due to the higher interface strength.
2010, 27(4): 118-123.
Abstract:
A series of semirigid interpenetrating polymer networks (IPN) foams, based on polyurethane （PU） and epoxy resin（ER）, with different densities were prepared by a simultaneous polymerization technique. Compression and tension properties of these foams were measured. Over the range of densities examined, the compression moduli and yield stress of these IPN foams could be described by a power---law relationship with respect to density respectively. The tensile moduli and break strength of these foams were also found to exhibit the similar relationships with respect to density of the foams. Therefore, the change of mechanical properties along with the density of the foams could be well predicted using these models. Experimental results reveal that the IPN foams possess both better ductility and higher tensile strength, and the energy absorption in tension test is higher compared with that in compression test at the same strain.
A series of semirigid interpenetrating polymer networks (IPN) foams, based on polyurethane （PU） and epoxy resin（ER）, with different densities were prepared by a simultaneous polymerization technique. Compression and tension properties of these foams were measured. Over the range of densities examined, the compression moduli and yield stress of these IPN foams could be described by a power---law relationship with respect to density respectively. The tensile moduli and break strength of these foams were also found to exhibit the similar relationships with respect to density of the foams. Therefore, the change of mechanical properties along with the density of the foams could be well predicted using these models. Experimental results reveal that the IPN foams possess both better ductility and higher tensile strength, and the energy absorption in tension test is higher compared with that in compression test at the same strain.
2010, 27(4): 124-130.
Abstract:
In this paper, an analytical approach was proposed for solving the buckling of functionally gradient material (FGM) Euler-Bernoulli and Timoshenko beams with cracks. The discontinuity of rotation caused by the cracks was simulated by means of the rotational spring model. The governing differential equations for buckling of an FGM beam were established and their solutions were found firstly. The recurrence formula of solution using the transfer matrix method was developed in the current research. Then the eigenvalue equations for buckling of an FGM beam can be conveniently obtained from a third-order determinant. A comprehensive parametric study is conducted to investigate the influences of the locations and number of cracks, shear deformation, material properties, slenderness ratio and various end supports on the critical buckling loads of cracked FGM beams. Numerical examples show that the developed method can simply, exactly and effectively solve the buckling of cracked FGM beams with various conditions.
In this paper, an analytical approach was proposed for solving the buckling of functionally gradient material (FGM) Euler-Bernoulli and Timoshenko beams with cracks. The discontinuity of rotation caused by the cracks was simulated by means of the rotational spring model. The governing differential equations for buckling of an FGM beam were established and their solutions were found firstly. The recurrence formula of solution using the transfer matrix method was developed in the current research. Then the eigenvalue equations for buckling of an FGM beam can be conveniently obtained from a third-order determinant. A comprehensive parametric study is conducted to investigate the influences of the locations and number of cracks, shear deformation, material properties, slenderness ratio and various end supports on the critical buckling loads of cracked FGM beams. Numerical examples show that the developed method can simply, exactly and effectively solve the buckling of cracked FGM beams with various conditions.
2010, 27(4): 131-138.
Abstract:
The structure of composite pressure vessels with ultra-thin metallic liner was analyzed using the finite element method. In the analysis process, the changes of ply angle and ply thickness along the radius of parallel circles were allowed for in the dome of the geometric model. The composite layer and the liner within the composite pressure vessel were analyzed with laminated plate theory and elastic-plasticity theory in the material model, respectively. Deformation compatibility and non-penetration characteristic between composite layer and liner were considered by introducing contact analysis. The numerical results show that in the work pressure, tension strain is in all the longitudinal direction and compressive strain exists partly in the hoop direction in the dome of the vessel. And plastic deformation occurrs in the liner within composite pressure vessel. After unloading, tension stress exists in composite layer and compressive stress develops in the liner of the vessel. Based on the loaddisplacement curve of maximal displacement point in the liner, buckling deformation of the liner was simulated using a simplified model of the vessel. The experimental data and observed result are in good agreement with the calculated results, which confirms the reliability of the calculated results in this paper.
The structure of composite pressure vessels with ultra-thin metallic liner was analyzed using the finite element method. In the analysis process, the changes of ply angle and ply thickness along the radius of parallel circles were allowed for in the dome of the geometric model. The composite layer and the liner within the composite pressure vessel were analyzed with laminated plate theory and elastic-plasticity theory in the material model, respectively. Deformation compatibility and non-penetration characteristic between composite layer and liner were considered by introducing contact analysis. The numerical results show that in the work pressure, tension strain is in all the longitudinal direction and compressive strain exists partly in the hoop direction in the dome of the vessel. And plastic deformation occurrs in the liner within composite pressure vessel. After unloading, tension stress exists in composite layer and compressive stress develops in the liner of the vessel. Based on the loaddisplacement curve of maximal displacement point in the liner, buckling deformation of the liner was simulated using a simplified model of the vessel. The experimental data and observed result are in good agreement with the calculated results, which confirms the reliability of the calculated results in this paper.
2010, 27(4): 137-146.
Abstract:
To study the mechanical performances of HPC beam mixed with CFRP/GFRP rebars during the whole course, a new three-dimensional nonlinear beam-shell composite element was derived. After the degraded shell element theory was introduced, the prestressed CFRP rebar was modeled by a spatial beam element and then based on the compatibility of displacements and rotations of the nodes of CFRP rebar element, the contribution in the stiffness matrix by the prestressed CFRP element to the beam-shell composite element was completed. The GFRP rebar and HPC beam were both modeled by the layered shell element. Combined with Jiang-yielding and Madrid-hardening criteria which were used to depict the material nonlinearity of concrete, a new nonlinear beam-shell composite element was put forward. The nonlinear analytical procedure was compiled. The calculations are in good agreements with experiment results, which shows the efficiency of the studied nonlinear beam-shell composite element and the reliability of the gained nonlinear procedure and the correctness of the adopted criteria of material nonlinearity of concrete. By the composite element, the configuration of CFRP rebar is precisely expressed and the performances such as tension, suppression, torsion and bending are comprehensively considered, which helps to synthetically embody the reinforcement of assembled rebars to the structure.
To study the mechanical performances of HPC beam mixed with CFRP/GFRP rebars during the whole course, a new three-dimensional nonlinear beam-shell composite element was derived. After the degraded shell element theory was introduced, the prestressed CFRP rebar was modeled by a spatial beam element and then based on the compatibility of displacements and rotations of the nodes of CFRP rebar element, the contribution in the stiffness matrix by the prestressed CFRP element to the beam-shell composite element was completed. The GFRP rebar and HPC beam were both modeled by the layered shell element. Combined with Jiang-yielding and Madrid-hardening criteria which were used to depict the material nonlinearity of concrete, a new nonlinear beam-shell composite element was put forward. The nonlinear analytical procedure was compiled. The calculations are in good agreements with experiment results, which shows the efficiency of the studied nonlinear beam-shell composite element and the reliability of the gained nonlinear procedure and the correctness of the adopted criteria of material nonlinearity of concrete. By the composite element, the configuration of CFRP rebar is precisely expressed and the performances such as tension, suppression, torsion and bending are comprehensively considered, which helps to synthetically embody the reinforcement of assembled rebars to the structure.
2010, 27(4): 147-151.
Abstract:
To determine the effective moisture diffusion properties of fiber polymer-matrix composites, a new solution method was established by using a concept of unit cell. The effects of temperature and fiber volume fraction on the effective moisture coefficients were investigated. In this study, the impermeable fibers were assumed to be distributed uniformly in matrix. Using the proposed model, effective moisture diffusivities were calculated at different temperatures and fiber volume fractions. The results show that effective diffusivities of the composites increase with the increase of temperature, and decrease with the increase of fiber volume fraction. Effective diffusivities of regular hexagonal composite are a bit larger than those of square one under the same temperature and volume fraction. The numerical results are compared with those from empirical formula, and good agreement is achieved. Therefore, it proves that the present method is valid and reliable to predict moisture diffusion properties for fiber polymer-matrix composites by the unit cell model. The present results are significant to explore moisture diffusion mechanism and overall properties of composites.
To determine the effective moisture diffusion properties of fiber polymer-matrix composites, a new solution method was established by using a concept of unit cell. The effects of temperature and fiber volume fraction on the effective moisture coefficients were investigated. In this study, the impermeable fibers were assumed to be distributed uniformly in matrix. Using the proposed model, effective moisture diffusivities were calculated at different temperatures and fiber volume fractions. The results show that effective diffusivities of the composites increase with the increase of temperature, and decrease with the increase of fiber volume fraction. Effective diffusivities of regular hexagonal composite are a bit larger than those of square one under the same temperature and volume fraction. The numerical results are compared with those from empirical formula, and good agreement is achieved. Therefore, it proves that the present method is valid and reliable to predict moisture diffusion properties for fiber polymer-matrix composites by the unit cell model. The present results are significant to explore moisture diffusion mechanism and overall properties of composites.
2010, 27(4): 152-159.
Abstract:
The actively cooled performance of sandwich panel with cellular materials considering heat transfer factors was analyzed, and the thermal performance indexes of cellular materials with two different heat transfer boundary conditions were derived respectively, then the factors which impact the actively cooled performance were obtained. The relationship between optimal relative density and thermal performance index was analyzed in particular relative thickness, and the hexagon has the best performance. The maximum thermal performance index and optimal relative density increase rapidly with the growth of relative thickness, while they get slowly and reach a peak value when the relative thickness grew over 20. The minimum mass was obtained, and the maximum thermal performance index tends towards a constant value with the increasing minimum mass. The minimum mass decreases with the increasing proportion of heat transfer coefficients in the same maximum thermal performance index. At last, the structure was optimized considering the structural load, and the hexagon has the superior performance to others.
The actively cooled performance of sandwich panel with cellular materials considering heat transfer factors was analyzed, and the thermal performance indexes of cellular materials with two different heat transfer boundary conditions were derived respectively, then the factors which impact the actively cooled performance were obtained. The relationship between optimal relative density and thermal performance index was analyzed in particular relative thickness, and the hexagon has the best performance. The maximum thermal performance index and optimal relative density increase rapidly with the growth of relative thickness, while they get slowly and reach a peak value when the relative thickness grew over 20. The minimum mass was obtained, and the maximum thermal performance index tends towards a constant value with the increasing minimum mass. The minimum mass decreases with the increasing proportion of heat transfer coefficients in the same maximum thermal performance index. At last, the structure was optimized considering the structural load, and the hexagon has the superior performance to others.
2010, 27(4): 160-167.
Abstract:
According to the feature of main body yarns and edge ones in 1×1 three-dimensional braiding with complex rectangular cross-section, a twice four-step braiding algorithm was described, which is realized through moving two groups of yarn one by one with the four-step braiding technique. A program to simulate the motion of yarn was worked out based on the algorithm by using the script language of Python and Tkinter, and the law of yarn motion was analyzed. The visualized models of the 3D braided preform were constructed by using visualization tool kit(VTK) and Beta-Splines, and man-machine interactive operation was permitted.
According to the feature of main body yarns and edge ones in 1×1 three-dimensional braiding with complex rectangular cross-section, a twice four-step braiding algorithm was described, which is realized through moving two groups of yarn one by one with the four-step braiding technique. A program to simulate the motion of yarn was worked out based on the algorithm by using the script language of Python and Tkinter, and the law of yarn motion was analyzed. The visualized models of the 3D braided preform were constructed by using visualization tool kit(VTK) and Beta-Splines, and man-machine interactive operation was permitted.
2010, 27(4): 168-173.
Abstract:
The purpose of this investigation is to establish a modeling strategy for accurately predicting the buckling and the progressive failure in the post-buckling of cylindrical composite shells. Finite element method was employed and Hashin failure criterion was used. A user material subroutine based on the failure criterion was developed and inserted into the commercial finite element software ABAQUS. Cylindrical composite shell panels subjected to a transversely concentrated load at its centre were analyzed. Two straight edges of the panel were elastic support and two curved edges were free. Finite element results were compared with the existing experimental data in the literature to investigate the effect of the elastic support conditions and the stress concentration caused by the concentrated load. A reasonable criterion for simulating the correct elastic boundary condition was proposed. It is found that if the elastic support and the stress concentration at the small area of the concentrated load point are considered in the modeling, the simulated results are fairly close to the experimental data and are much better than the existing predictions. Therefore, the rationality of the established modeling strategy is validated.
The purpose of this investigation is to establish a modeling strategy for accurately predicting the buckling and the progressive failure in the post-buckling of cylindrical composite shells. Finite element method was employed and Hashin failure criterion was used. A user material subroutine based on the failure criterion was developed and inserted into the commercial finite element software ABAQUS. Cylindrical composite shell panels subjected to a transversely concentrated load at its centre were analyzed. Two straight edges of the panel were elastic support and two curved edges were free. Finite element results were compared with the existing experimental data in the literature to investigate the effect of the elastic support conditions and the stress concentration caused by the concentrated load. A reasonable criterion for simulating the correct elastic boundary condition was proposed. It is found that if the elastic support and the stress concentration at the small area of the concentrated load point are considered in the modeling, the simulated results are fairly close to the experimental data and are much better than the existing predictions. Therefore, the rationality of the established modeling strategy is validated.
2010, 27(4): 174-179.
Abstract:
Abstract： To effectively simulate and accurately recover the three-dimensional stress/strain/deformation field of composite laminated plates, an asymptotic revise theory and the recovery relationship were established based on the variational asymptotic method (VAM). The original 3D stress field was expressed by one-dimensional generalized stress and warping function based on decomposition of rotation tensor (DRT) to consider all the deformations, and VAM was used to strictly split the three-dimensional problem into a two-dimensional non-linear analysis of deformation plate (equivalent single-layer plate model) and a one-dimensional linear analysis along the transverse normal direction. Then, the strain energy was asymptotic corrected to second order by taking advantage of the ratio of height to span and the order of two-dimensional strain, and the energy was converted to the form of Reissner formula for practical applications. Based on this theory, a variational asymptotic plate and shell analysis program (VAPAS) was developed. The cylindrical bending example of a 20-layer composite plate shows that the three-dimensional field recovered by this theory agrees better with the exact results than that by the first-order shear deformation theory (FOSDT) and classic laminated theory (CLT).
Abstract： To effectively simulate and accurately recover the three-dimensional stress/strain/deformation field of composite laminated plates, an asymptotic revise theory and the recovery relationship were established based on the variational asymptotic method (VAM). The original 3D stress field was expressed by one-dimensional generalized stress and warping function based on decomposition of rotation tensor (DRT) to consider all the deformations, and VAM was used to strictly split the three-dimensional problem into a two-dimensional non-linear analysis of deformation plate (equivalent single-layer plate model) and a one-dimensional linear analysis along the transverse normal direction. Then, the strain energy was asymptotic corrected to second order by taking advantage of the ratio of height to span and the order of two-dimensional strain, and the energy was converted to the form of Reissner formula for practical applications. Based on this theory, a variational asymptotic plate and shell analysis program (VAPAS) was developed. The cylindrical bending example of a 20-layer composite plate shows that the three-dimensional field recovered by this theory agrees better with the exact results than that by the first-order shear deformation theory (FOSDT) and classic laminated theory (CLT).
2010, 27(4): 180-188.
Abstract:
The fracture toughness experiments of Z-pin reinforced composite laminates were carried out. Pull-out test of Z-pins of different diameters（0.28, 0.52, 0.80mm）embedded into unidirectional laminate coupons was conducted to measure the curves of Z-pin bridging force versus pull-out displacement. Then, the mode Ⅰ and mode Ⅱ fracture toughness of Z-pinned laminates and unpinned laminates were tested respectively using the double-cantilever-beam (DCB) method and three point edge-notched-flexure (ENF) methods. Comparison was made for the measured mode Ⅰ and mode Ⅱ strain energy release rates between two types of laminates. Strain energy release rates GⅠC and GⅡC of Z-pin reinforced laminates are higher from 83% to 1110% and from 23% to 438% than those of unpinned laminates respectively. Moreover, the test results denote that enlarging the Z-pin distributive density is more effective to reinforce structural strain energy release rate than enlarging Z-pin diameter at the same volume fraction.
The fracture toughness experiments of Z-pin reinforced composite laminates were carried out. Pull-out test of Z-pins of different diameters（0.28, 0.52, 0.80mm）embedded into unidirectional laminate coupons was conducted to measure the curves of Z-pin bridging force versus pull-out displacement. Then, the mode Ⅰ and mode Ⅱ fracture toughness of Z-pinned laminates and unpinned laminates were tested respectively using the double-cantilever-beam (DCB) method and three point edge-notched-flexure (ENF) methods. Comparison was made for the measured mode Ⅰ and mode Ⅱ strain energy release rates between two types of laminates. Strain energy release rates GⅠC and GⅡC of Z-pin reinforced laminates are higher from 83% to 1110% and from 23% to 438% than those of unpinned laminates respectively. Moreover, the test results denote that enlarging the Z-pin distributive density is more effective to reinforce structural strain energy release rate than enlarging Z-pin diameter at the same volume fraction.
2010, 27(4): 189-194.
Abstract:
The quasi-static compressive and tensile experiments of hollow glass beads(HGB)/epoxy were studied. The effects of different HGB mass ratios on the density, modulus and strength of HGB/epoxy were discussed, and its stress relaxation was analyzed. It is found that its various data are decreased with the increasing of the HGB mass ratio, and the composite has the image of brittle failure, but there is a great deformation before failure, and a great spring back after failure. It shows that the elasticity of the composite is enhanced. The HGB/epoxy composite has obvious stress relaxation, and the relaxation velocity is increased with the increment of HGB mass ratio. It shows that this HGB/epoxy composite has obvious viscoelasticity.
The quasi-static compressive and tensile experiments of hollow glass beads(HGB)/epoxy were studied. The effects of different HGB mass ratios on the density, modulus and strength of HGB/epoxy were discussed, and its stress relaxation was analyzed. It is found that its various data are decreased with the increasing of the HGB mass ratio, and the composite has the image of brittle failure, but there is a great deformation before failure, and a great spring back after failure. It shows that the elasticity of the composite is enhanced. The HGB/epoxy composite has obvious stress relaxation, and the relaxation velocity is increased with the increment of HGB mass ratio. It shows that this HGB/epoxy composite has obvious viscoelasticity.
2010, 27(4): 195-199.
Abstract:
The mechanical behavior of natural ramie / isocyanate laminate composite with holes was investigated by the biaxial tensile method. The uniaxial and biaxial load tensile tests for four kinds of 0.5, 1.0, 2.0, 4.0mm aperture were conducted, and the effects of displacement and the pore size on strain were characterized by the digital speckle correlation method. The result shows that the displacement field around the hole is uniform distribution when the load changes linearly, however, while the load is closing to failure, non---linear distribution and high strain points appear, which are broken first quickly. As the radius of the pore increases, average strains in x and y directions are reduced within the same area around the hole under 1000~2000N biaxial load and fluctuation of strain decreases slightly.Mechanical characteristics of materials are different under different tensile ways, strength under biaxial load is lower than the one under uniaxial tension and the reduction ratio, which increases with the size of pore radius raising, is about 14%～27% of the uniaxial load.
The mechanical behavior of natural ramie / isocyanate laminate composite with holes was investigated by the biaxial tensile method. The uniaxial and biaxial load tensile tests for four kinds of 0.5, 1.0, 2.0, 4.0mm aperture were conducted, and the effects of displacement and the pore size on strain were characterized by the digital speckle correlation method. The result shows that the displacement field around the hole is uniform distribution when the load changes linearly, however, while the load is closing to failure, non---linear distribution and high strain points appear, which are broken first quickly. As the radius of the pore increases, average strains in x and y directions are reduced within the same area around the hole under 1000~2000N biaxial load and fluctuation of strain decreases slightly.Mechanical characteristics of materials are different under different tensile ways, strength under biaxial load is lower than the one under uniaxial tension and the reduction ratio, which increases with the size of pore radius raising, is about 14%～27% of the uniaxial load.
