2002 Vol. 19, No. 5
2002, 19(5): 1-6.
Abstract:
In recent years, sisal fiber has been widely used as reinforcement of polymer composites due to its renewability, cost effectiveness, low density, high specific strength and modulus. This paper presents a summary of the recent developments of sisal reinforced composites. Structure-property relationships and surface modification techniques of the fiber as well as influencing factors of mechanical performance of the composites are reviewed. It is believed that a right combination of fabrication techniques so far developed in accordance with different applications and target user groups should be found out.
In recent years, sisal fiber has been widely used as reinforcement of polymer composites due to its renewability, cost effectiveness, low density, high specific strength and modulus. This paper presents a summary of the recent developments of sisal reinforced composites. Structure-property relationships and surface modification techniques of the fiber as well as influencing factors of mechanical performance of the composites are reviewed. It is believed that a right combination of fabrication techniques so far developed in accordance with different applications and target user groups should be found out.
2002, 19(5): 7-13.
Abstract:
Rigid rodlike molecular chains-thermotropic liquid crystalline polymers (TLCP), which would self-organize into domains, showed anisotropy and excellent physical properties. In situ composites containing fibrils with high aspect ratios, high strength and high modulus as self-reinforcing components were prepared via incorporation of TLCP into thermoplastic polymers (TP), which led to significantly improve the processability due to the reduction in viscosity. A brief review on the rheology and microstructure of in situ composites was given, and factors influencing TLCP fibrillation and mechanical properties, such as chemical and physical properties of components including compatibility between TLCP and TP, as well as processing parameters were presented in this paper with the related work available in the literature. The state-of-the-art compatibility technology for TLCP/TP blends was elucidated in detail. A new concept of preparation of high performance in situ composites involving fibrils formed with the aid of application of an electric field during the processing was put forward for the first time.
Rigid rodlike molecular chains-thermotropic liquid crystalline polymers (TLCP), which would self-organize into domains, showed anisotropy and excellent physical properties. In situ composites containing fibrils with high aspect ratios, high strength and high modulus as self-reinforcing components were prepared via incorporation of TLCP into thermoplastic polymers (TP), which led to significantly improve the processability due to the reduction in viscosity. A brief review on the rheology and microstructure of in situ composites was given, and factors influencing TLCP fibrillation and mechanical properties, such as chemical and physical properties of components including compatibility between TLCP and TP, as well as processing parameters were presented in this paper with the related work available in the literature. The state-of-the-art compatibility technology for TLCP/TP blends was elucidated in detail. A new concept of preparation of high performance in situ composites involving fibrils formed with the aid of application of an electric field during the processing was put forward for the first time.
2002, 19(5): 14-21.
Abstract:
The effects of alkali solution and silane coupling agent on the structure and properties of flax fibers as well as the effect of pretreatment of flax weave with alkali solution and silane on the mechanical properties of flax weave/LLDPE laminate were studied. The experiment results show that the moisture and thermal stability of flax are improved, and the degree of crystallinity and crystal face distance of flax are decreased after the flax weave is treated with alkali solution or silane. The tensile strength is increased from 84.8 MPa to 96.9 MPa, in case the fabric is pretreated with alkali solution;the tensile strength, flexural strength and flexural modulus are increased from 84.8 MPa, 30.1 MPa, 1.95 GPa to 108.9 MPa, 64.6MPa, 7.88 GPa after the flax weave is pretreated with A-151.
The effects of alkali solution and silane coupling agent on the structure and properties of flax fibers as well as the effect of pretreatment of flax weave with alkali solution and silane on the mechanical properties of flax weave/LLDPE laminate were studied. The experiment results show that the moisture and thermal stability of flax are improved, and the degree of crystallinity and crystal face distance of flax are decreased after the flax weave is treated with alkali solution or silane. The tensile strength is increased from 84.8 MPa to 96.9 MPa, in case the fabric is pretreated with alkali solution;the tensile strength, flexural strength and flexural modulus are increased from 84.8 MPa, 30.1 MPa, 1.95 GPa to 108.9 MPa, 64.6MPa, 7.88 GPa after the flax weave is pretreated with A-151.
2002, 19(5): 22-27.
Abstract:
In the fabrication of SiC fibers via preceramic polymer pyrolysis, air-curing is necessary to render the green fiber infusible, but it introduced much oxygen to the fibers, which limited their high-temperature properties greatly. Instead, another promising approach to the preparation of low-oxygen-content SiC fibers has been realized by electron beam (EB) radiation curing under inert atmosphere. This paper studied the influence of EB radiation on the infusibility and thermal stability of polycarbosilane (PCS) fibers and analyzed the mechanism of radiation-curing. The results revealed that the onset-curing dose of PCS fibers was 15MGy and their pyrolysis was similar to that of the air-cured fibers. After pyrolysis up to 1200℃ for 30min under nitrogen atmosphere, inorganic SiC fibers with a lower oxygen content, 3.3wt%, were obtained. Their tensile strength was 1.65GPa, with a microcrystalline size of 3.4nm. After heat-treatment at 1600℃ for 30min under helium atmosphere, they lost 8wt% weight and retained 80% strength, with a grain growth of 12.9 nm.
In the fabrication of SiC fibers via preceramic polymer pyrolysis, air-curing is necessary to render the green fiber infusible, but it introduced much oxygen to the fibers, which limited their high-temperature properties greatly. Instead, another promising approach to the preparation of low-oxygen-content SiC fibers has been realized by electron beam (EB) radiation curing under inert atmosphere. This paper studied the influence of EB radiation on the infusibility and thermal stability of polycarbosilane (PCS) fibers and analyzed the mechanism of radiation-curing. The results revealed that the onset-curing dose of PCS fibers was 15MGy and their pyrolysis was similar to that of the air-cured fibers. After pyrolysis up to 1200℃ for 30min under nitrogen atmosphere, inorganic SiC fibers with a lower oxygen content, 3.3wt%, were obtained. Their tensile strength was 1.65GPa, with a microcrystalline size of 3.4nm. After heat-treatment at 1600℃ for 30min under helium atmosphere, they lost 8wt% weight and retained 80% strength, with a grain growth of 12.9 nm.
2002, 19(5): 28-32.
Abstract:
Surface treatment of isotropic pitch-based carbon fibers by electrochemical oxidation was carried out, and the processes of electrochemical oxidation were investigated through Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). The XPS studies indicated that the surface carbon and functional groups were progressively oxidized into carboxyl and carbon dioxide. The analyses of AFM images and surface roughness showed that the surface treatment first made carbon fiber smoother, and then caused striated surface after continual treatment. AFM illustrated the surface form on a microscopic scale effectively compared to SEM. Combining XPS with AFM can characterize the process of electrochemically oxidized pitch-based carbon fiber.
Surface treatment of isotropic pitch-based carbon fibers by electrochemical oxidation was carried out, and the processes of electrochemical oxidation were investigated through Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). The XPS studies indicated that the surface carbon and functional groups were progressively oxidized into carboxyl and carbon dioxide. The analyses of AFM images and surface roughness showed that the surface treatment first made carbon fiber smoother, and then caused striated surface after continual treatment. AFM illustrated the surface form on a microscopic scale effectively compared to SEM. Combining XPS with AFM can characterize the process of electrochemically oxidized pitch-based carbon fiber.
2002, 19(5): 33-37.
Abstract:
The rheological behavior of phenolic resin for resin transfer molding(RTM) was studied with DSC analysis and viscosity experiments. A rheological model based on the dual-Arrhenius equation was established and used to simulate the rheological behavior of the resin. The model prediction determined from the dual-Arrhenius equation was in good agreement with experimental data. The processing window of the resin system can be well determined based on the developed model. The rheological model is important for processing simulation and quality control of RTM processing for high performance composites.
The rheological behavior of phenolic resin for resin transfer molding(RTM) was studied with DSC analysis and viscosity experiments. A rheological model based on the dual-Arrhenius equation was established and used to simulate the rheological behavior of the resin. The model prediction determined from the dual-Arrhenius equation was in good agreement with experimental data. The processing window of the resin system can be well determined based on the developed model. The rheological model is important for processing simulation and quality control of RTM processing for high performance composites.
2002, 19(5): 38-42.
Abstract:
After treating by reductant and inorganic acid, the clays were coagulated with natural latex to prepare natural rubber-clay composites. The composites were studied in different decomposition temperatures by using in-situ FTIR. In lower decomposition temperature, the decomposing gaseous products of the composites mainly were ethylene, lower molecular weight olefin and carbonyl compounds. With the increase of degradation temperature, the contents of lower molecular weight olefin reduce and more higher molecular weight olefin and alkyl compounds form. Furthermore, with the increase of the contents of the metal component in the clay, such as Co, Mn, Cu and Fe, the decomposing gaseous products contain more olefin and carbonyl compounds as well as CO2. The metal components in the clay accelerate the oxidative degradation of natural rubber/clay composites.
After treating by reductant and inorganic acid, the clays were coagulated with natural latex to prepare natural rubber-clay composites. The composites were studied in different decomposition temperatures by using in-situ FTIR. In lower decomposition temperature, the decomposing gaseous products of the composites mainly were ethylene, lower molecular weight olefin and carbonyl compounds. With the increase of degradation temperature, the contents of lower molecular weight olefin reduce and more higher molecular weight olefin and alkyl compounds form. Furthermore, with the increase of the contents of the metal component in the clay, such as Co, Mn, Cu and Fe, the decomposing gaseous products contain more olefin and carbonyl compounds as well as CO2. The metal components in the clay accelerate the oxidative degradation of natural rubber/clay composites.
2002, 19(5): 43-46.
Abstract:
As a high performance thermal-resistant composite, carbon/carbon composites have the highest mechanical strength in the temperature above 1600℃, and the research of C/C gradient materials will extend the application of C/C as a kind of heat insulation in high temperature. To get some information concerning applying density gradient materials, thermal conductivities (with different densities and under various temperatures) were measured. In order to provide a theory in the application of density gradient carbon/carbon composites(C/C), the relationship between these thermal conductivity and density/temperature was summarized; the mechanical properties of 2DC/C and density gradient C/C were investigated. In conclusion, the relationship between the thermal conductivity and density approximately accords with linearity in the experimental range. Density gradient C/C has higher flexural strength and lower thermal conductivity than uniform C/C, which infers that the materials have superiority in heat insulation and structural application.
As a high performance thermal-resistant composite, carbon/carbon composites have the highest mechanical strength in the temperature above 1600℃, and the research of C/C gradient materials will extend the application of C/C as a kind of heat insulation in high temperature. To get some information concerning applying density gradient materials, thermal conductivities (with different densities and under various temperatures) were measured. In order to provide a theory in the application of density gradient carbon/carbon composites(C/C), the relationship between these thermal conductivity and density/temperature was summarized; the mechanical properties of 2DC/C and density gradient C/C were investigated. In conclusion, the relationship between the thermal conductivity and density approximately accords with linearity in the experimental range. Density gradient C/C has higher flexural strength and lower thermal conductivity than uniform C/C, which infers that the materials have superiority in heat insulation and structural application.
2002, 19(5): 47-51.
Abstract:
In situ gradient composites of Al-16wt%Si alloy, which possess the microstructures of accumulated primary silicon in the inner layer and eutectic in the rest layers or accumulated primary silicon from the outer layer to the inner layer,have been achieved by changing the rotation rate of centrifugal casting. The formation process of the composites has been analyzed, and the microstructure, hardness, wear ability and the phase structure of the composites have been also investigated.
In situ gradient composites of Al-16wt%Si alloy, which possess the microstructures of accumulated primary silicon in the inner layer and eutectic in the rest layers or accumulated primary silicon from the outer layer to the inner layer,have been achieved by changing the rotation rate of centrifugal casting. The formation process of the composites has been analyzed, and the microstructure, hardness, wear ability and the phase structure of the composites have been also investigated.
2002, 19(5): 52-56.
Abstract:
Aluminum matrix composites reinforced with potassium titanate whiskers were prepared by squeezing casting process. The thermal expansion and dimensional stability of such composites were investigated. Dilatometric measurements revealed that the coefficient of thermal expansion of ZL109 alloy tends to decrease with increasing whiskers content. Strain hysteresis was observed in the matrix alloy and its composites subjected to repeated thermal cycling between 50℃~280℃. Thermal cycling curves indicated that ZL109 alloy exhibits the largest thermal and residual plastic strain. The composite reinforced with 30vol% whiskers had little dimensional change owing to the absence of residual strain. Thermal cycling curves were categorized into three types and explained by the theory of plastic strain in the matrix. Finally, it is concluded that the thermal cycling curves can be used to predict the dimensional thermal stability and failure of the structural materials on exposure to fluctuating temperature environments.
Aluminum matrix composites reinforced with potassium titanate whiskers were prepared by squeezing casting process. The thermal expansion and dimensional stability of such composites were investigated. Dilatometric measurements revealed that the coefficient of thermal expansion of ZL109 alloy tends to decrease with increasing whiskers content. Strain hysteresis was observed in the matrix alloy and its composites subjected to repeated thermal cycling between 50℃~280℃. Thermal cycling curves indicated that ZL109 alloy exhibits the largest thermal and residual plastic strain. The composite reinforced with 30vol% whiskers had little dimensional change owing to the absence of residual strain. Thermal cycling curves were categorized into three types and explained by the theory of plastic strain in the matrix. Finally, it is concluded that the thermal cycling curves can be used to predict the dimensional thermal stability and failure of the structural materials on exposure to fluctuating temperature environments.
2002, 19(5): 57-61.
Abstract:
This paper deals with the relationship between thermal expansion behaviors and internal stresses of SiCW/Al composites quenched from 600℃ by water and annealed from 600℃ with a cooling rate of 1℃/min. The results show that the dislocation density, residual tensile stress in the matrix and yield strength of the quenched composite are larger than those of the annealed composite. There are two peaks on the curve of the coefficients of thermal expansion (CTE) vs heating temperature of the quenched composite. The first peak appears at about 80℃, and the second one appears at 245℃, and the latter is much higher than the former. But only one peak appears at 80℃ on the curve of the CTEs vs temperature of the annealed composite, and the peak is smaller than the first one of the quenched specimen. The CTE of the annealed specimen increases with temperature increasing gradually. The first peak on the curve of CTE vs temperature is the result of relaxation of the residual tensile stress in the matrix, while the second one is a signal of the relaxation rate of the compressive stress in the matrix of the composite.
This paper deals with the relationship between thermal expansion behaviors and internal stresses of SiCW/Al composites quenched from 600℃ by water and annealed from 600℃ with a cooling rate of 1℃/min. The results show that the dislocation density, residual tensile stress in the matrix and yield strength of the quenched composite are larger than those of the annealed composite. There are two peaks on the curve of the coefficients of thermal expansion (CTE) vs heating temperature of the quenched composite. The first peak appears at about 80℃, and the second one appears at 245℃, and the latter is much higher than the former. But only one peak appears at 80℃ on the curve of the CTEs vs temperature of the annealed composite, and the peak is smaller than the first one of the quenched specimen. The CTE of the annealed specimen increases with temperature increasing gradually. The first peak on the curve of CTE vs temperature is the result of relaxation of the residual tensile stress in the matrix, while the second one is a signal of the relaxation rate of the compressive stress in the matrix of the composite.
2002, 19(5): 62-65.
Abstract:
AlBOW whisker Al matrix composites were fabricated by squeeze casting technique. First, according to the basic principle of fluid dynamics in porous media and Laplace equation, the squeeze casting infiltration process of metal melt fluid in the porous whisker preform was analyzed. The paper indicated that metal melt fluids infiltrated the whisker preform by a disordered fluid status. And then, the effect of the length of whisker on the void structure of the porous preform was analyzed in detail.
AlBOW whisker Al matrix composites were fabricated by squeeze casting technique. First, according to the basic principle of fluid dynamics in porous media and Laplace equation, the squeeze casting infiltration process of metal melt fluid in the porous whisker preform was analyzed. The paper indicated that metal melt fluids infiltrated the whisker preform by a disordered fluid status. And then, the effect of the length of whisker on the void structure of the porous preform was analyzed in detail.
2002, 19(5): 66-70.
Abstract:
A TiB2/Cu-Zr composite was fabricated by an in-situ reaction synthesis method, and the microstructure was studied. The results showed that TiB2 particles exist in two patterns and its distribution is homogeneous with its size of 0.5 μm~10 μm. A reaction model of TiB2 in molten metal is established combined with the thermodynamics of TiB2 particles in solid and liquid matrix. Finally, the growth mechanism of two kinds of TiB2 particles is proposed.
A TiB2/Cu-Zr composite was fabricated by an in-situ reaction synthesis method, and the microstructure was studied. The results showed that TiB2 particles exist in two patterns and its distribution is homogeneous with its size of 0.5 μm~10 μm. A reaction model of TiB2 in molten metal is established combined with the thermodynamics of TiB2 particles in solid and liquid matrix. Finally, the growth mechanism of two kinds of TiB2 particles is proposed.
2002, 19(5): 71-74.
Abstract:
Carbon nanotube is a promising enhancemental material for its high strength and excellent toughness. After electroless plating, the carbon nanotube was coated with a relatively continuous and uniform layer of nickel(20 nm~40nm). It was shown by TEM images that nickel was deposited as nanoparticles on the activated sites at the pretreated surface of carbon nanotubes. When the reaction time was prolonged, the nickel layer would become thicker and joined together to form a continuous one. It is not the curvature of MWNTs but the dispersion of catalytic centers on pretreated MWNTs that plays a crucial part in electroless plating.
Carbon nanotube is a promising enhancemental material for its high strength and excellent toughness. After electroless plating, the carbon nanotube was coated with a relatively continuous and uniform layer of nickel(20 nm~40nm). It was shown by TEM images that nickel was deposited as nanoparticles on the activated sites at the pretreated surface of carbon nanotubes. When the reaction time was prolonged, the nickel layer would become thicker and joined together to form a continuous one. It is not the curvature of MWNTs but the dispersion of catalytic centers on pretreated MWNTs that plays a crucial part in electroless plating.
INITIATION OF CRACK FROM THE PORES IN CERAMICS UNDER THERMAL SHOCK ——Ⅰ: ANALYSIS OF THERMAL STRESSES
2002, 19(5): 75-78.
Abstract:
Cracking initiating from the surface of ceramics is a typical failure mode when the ceramic with high temperature is suddenly put into air or water with low temperature. So, the microstructure of the surface and near the surface of the ceramics is one of the dominant factors for the thermal shock behaviors of the ceramics. In other words, some experimental results have shown that porous ceramics manifested good thermal shock behaviors compared with dense ceramics, but the mechanism related to the fact has not been explained quantitatively. In the present paper, with the commercial finite element method (FEM) package, ABAQUS, stress distributions on and near the surface of the ceramics under sudden temperature change, particularly around the pores on the surface (open pores) and the pores near the surface (closed pores), were analyzed. It is found that the stress concentration caused by the closed pores was more serious than that caused by the open pores.
Cracking initiating from the surface of ceramics is a typical failure mode when the ceramic with high temperature is suddenly put into air or water with low temperature. So, the microstructure of the surface and near the surface of the ceramics is one of the dominant factors for the thermal shock behaviors of the ceramics. In other words, some experimental results have shown that porous ceramics manifested good thermal shock behaviors compared with dense ceramics, but the mechanism related to the fact has not been explained quantitatively. In the present paper, with the commercial finite element method (FEM) package, ABAQUS, stress distributions on and near the surface of the ceramics under sudden temperature change, particularly around the pores on the surface (open pores) and the pores near the surface (closed pores), were analyzed. It is found that the stress concentration caused by the closed pores was more serious than that caused by the open pores.
2002, 19(5): 79-83.
Abstract:
Thermal shock cracks can be initiated at the surface of ceramics materials when the surface was loaded by suddenly cooling. In the present paper, with the commercial finite element method (FEM) package ABAQUS, the initiation of crack for porous ceramics under suddenly cooling was investigated. Two cases, cracks initiating from the pores on the surface and the pores in the neighborhood of the surface of the porous ceramics were studied. For the case of crack initiating from the pores on the surface, it is found that the energy release rate will increase with increasing of the depth of the crack and the radius of the pores. For the case of crack initiating from the pores near the surface, the energy release rate will increase when the crack depth, the pore radius and the distance between the pore and the surface increase.
Thermal shock cracks can be initiated at the surface of ceramics materials when the surface was loaded by suddenly cooling. In the present paper, with the commercial finite element method (FEM) package ABAQUS, the initiation of crack for porous ceramics under suddenly cooling was investigated. Two cases, cracks initiating from the pores on the surface and the pores in the neighborhood of the surface of the porous ceramics were studied. For the case of crack initiating from the pores on the surface, it is found that the energy release rate will increase with increasing of the depth of the crack and the radius of the pores. For the case of crack initiating from the pores near the surface, the energy release rate will increase when the crack depth, the pore radius and the distance between the pore and the surface increase.
2002, 19(5): 84-89.
Abstract:
Using numerical simulation technique, the authors study the effect of the specimen size on stress distribution and fracture properties of the interface between the fiber and matrix for the fiber pull-out test. The effects of the embedded length of the fiber in the matrix and the wrapped thickness of the matrix around the fiber were considered. On the assumption of perfect interface of fiber and matrix, the numerical analysis for the interfacial shear stress and normal stress of the test specimen with different dimensions is performed. For the whole process of interfacial crack growth, the energy release rate was calculated. All materials are assumed to be linearly elastic solid. It is shown that the size of the test specimen affects dramatically the distribution of stresses and the fracture properties of the interface.
Using numerical simulation technique, the authors study the effect of the specimen size on stress distribution and fracture properties of the interface between the fiber and matrix for the fiber pull-out test. The effects of the embedded length of the fiber in the matrix and the wrapped thickness of the matrix around the fiber were considered. On the assumption of perfect interface of fiber and matrix, the numerical analysis for the interfacial shear stress and normal stress of the test specimen with different dimensions is performed. For the whole process of interfacial crack growth, the energy release rate was calculated. All materials are assumed to be linearly elastic solid. It is shown that the size of the test specimen affects dramatically the distribution of stresses and the fracture properties of the interface.
2002, 19(5): 90-94.
Abstract:
Delaminated inversion of a composite beam is investigated by the method of model analysis.The basic equations are built on the basis of elastic theory. The different sizes and positions of delamination are described easily because of the beam being divided into regions. The characteristic equations are established by using the boundary conditions, continuous conditions of the displacements and equilibrium conditions of the shearing force and moment in each region. Considering the first intrinsic frequency and vibrative shape of the system, one parameter is known by resolving the characteristic equations. The theoretical basis is provided for non-damage test in engineering.
Delaminated inversion of a composite beam is investigated by the method of model analysis.The basic equations are built on the basis of elastic theory. The different sizes and positions of delamination are described easily because of the beam being divided into regions. The characteristic equations are established by using the boundary conditions, continuous conditions of the displacements and equilibrium conditions of the shearing force and moment in each region. Considering the first intrinsic frequency and vibrative shape of the system, one parameter is known by resolving the characteristic equations. The theoretical basis is provided for non-damage test in engineering.
2002, 19(5): 95-101.
Abstract:
It is difficult to insure the quality of composites for the reason that there are complicated processes during the curing process of composites.Void action and residual stress act as the main actors of influencing the quality of composite products.In this paper, the principles of void and residual stress are analyzed. The process model is established. The computer code is made to calculate the formation of the void and residual stress. The results of computation can be the base for the design of the operation process of the composite curing process.
It is difficult to insure the quality of composites for the reason that there are complicated processes during the curing process of composites.Void action and residual stress act as the main actors of influencing the quality of composite products.In this paper, the principles of void and residual stress are analyzed. The process model is established. The computer code is made to calculate the formation of the void and residual stress. The results of computation can be the base for the design of the operation process of the composite curing process.
2002, 19(5): 102-107.
Abstract:
The long-term accelerated aging test in laboratory environment has been conducted on the composite insulating materials of actual rotating coils subjected to combined stresses including electrical, thermal, mechanical vibration and thermal cycling factors under simulated conditions. The dynamic mechanical experiment of composite insulating materials has been carried out in different aging periods using Dynamic Mechanical Analyzer (DMA) and the relationship between mechanical parameters and the aging time of composite insulating materials is also analyzed in this paper. Based on the Eyring absolute reaction speed theory, a method calculating the apparent activation energy of glass transition (AAEGT) is developed, which is used to calculate the AAEGT of composite insulating materials according to its mechanical dissipation curve in different aging periods.
The long-term accelerated aging test in laboratory environment has been conducted on the composite insulating materials of actual rotating coils subjected to combined stresses including electrical, thermal, mechanical vibration and thermal cycling factors under simulated conditions. The dynamic mechanical experiment of composite insulating materials has been carried out in different aging periods using Dynamic Mechanical Analyzer (DMA) and the relationship between mechanical parameters and the aging time of composite insulating materials is also analyzed in this paper. Based on the Eyring absolute reaction speed theory, a method calculating the apparent activation energy of glass transition (AAEGT) is developed, which is used to calculate the AAEGT of composite insulating materials according to its mechanical dissipation curve in different aging periods.
2002, 19(5): 108-113.
Abstract:
An experimental investigation was performed on the characteristics and damage of composite laminates with quasi-static indenting. Compressive behavior of laminates after quasi-static indentation was studied also in order to examine the validity of one CAI test method with the minimized specimen. Methods of ultrasonic C-scan, deply, etc were used to measure the damages of the laminates. The CAI strengths of laminates with the quasi-static indentation damage were compared with that of laminates after low velocity impact. The results showed that there were ultimate load and a threshold load or a threshold indentation depth below which no damage occurred while laminates were indented. The distributions of delamination areas along laminate thickness of the CAI test method with the minimized specimen were different from that of SACMA CAI test method. The compressive failure modes of these two methods were different.
An experimental investigation was performed on the characteristics and damage of composite laminates with quasi-static indenting. Compressive behavior of laminates after quasi-static indentation was studied also in order to examine the validity of one CAI test method with the minimized specimen. Methods of ultrasonic C-scan, deply, etc were used to measure the damages of the laminates. The CAI strengths of laminates with the quasi-static indentation damage were compared with that of laminates after low velocity impact. The results showed that there were ultimate load and a threshold load or a threshold indentation depth below which no damage occurred while laminates were indented. The distributions of delamination areas along laminate thickness of the CAI test method with the minimized specimen were different from that of SACMA CAI test method. The compressive failure modes of these two methods were different.
2002, 19(5): 114-117.
Abstract:
Some dynamic compressive tests about polyurethane rigid (PUR) syntactic foams have been done by SHPB experimental set. The stress-strain curves of PUR syntactic foams of 3 different densities are obtained at higher strain rates and their dynamic mechanical properties are also investigated at the same time. Based on all the stress-strain curves obtained, the characteristics of the energy absorption of PUR syntactic foams are analyzed and discussed furthermore. It is found that the envelope of optimum energy absorption of PUR syntactic foams is the same line in the energy absorption diagram. In addition, the dynamic failure characteristics are studied by means of SEM analyses for PUR syntactic foams specimens after loading.
Some dynamic compressive tests about polyurethane rigid (PUR) syntactic foams have been done by SHPB experimental set. The stress-strain curves of PUR syntactic foams of 3 different densities are obtained at higher strain rates and their dynamic mechanical properties are also investigated at the same time. Based on all the stress-strain curves obtained, the characteristics of the energy absorption of PUR syntactic foams are analyzed and discussed furthermore. It is found that the envelope of optimum energy absorption of PUR syntactic foams is the same line in the energy absorption diagram. In addition, the dynamic failure characteristics are studied by means of SEM analyses for PUR syntactic foams specimens after loading.
2002, 19(5): 118-124.
Abstract:
The static aeroelastic characteristics of a flexible forward-swept composite conceptual aircraft are calculated and analyzed by the NASTRAN software, in which the divergence speed of a wing alone is included, and the variation curves of flightloads, longitudinal stability derivatives and longitudinal control derivatives with respect to Mach number and dynamic pressure respectively under the subsonic case are involved emphatically. By comparison and analysis, the following results can be found: (1) At the same attitude, with Mach number increasing, for the longitudinal stability and control derivatives, the rigid one and the elastic one change with different trends respectively, and there are great differences between some variation curves; for the variation curves of the shear force, bending moment and torsional moment of the wing along the lines of span, the rigid proportions decrease while the elastic one increases; for the maximal vertical deflection of the wing (at the leading edge of the wing tip) and the maximal torsional deformation of the wing (at the wing tip), both increase a little. (2) At the same Mach number, with dynamic pressure increasing, for the longitudinal stability and control derivatives, the rigid one remains unchanged while the elastic one changes linearly; for the variation curves of the shear force, bending moment and torsional moment of the wing along the lines of span, the variation trends of rigid and elastic proportions are the same as result (1); for the maximal vertical deflection and the maximal torsional deformation of the wing, both are the same as result (1). (3) By the use of composite tailoring, wash-in of the wing can be reduced; therefore divergence speed can be enhanced, and the deformation of the wing and the elastic proportion of flightload can be decreased.
The static aeroelastic characteristics of a flexible forward-swept composite conceptual aircraft are calculated and analyzed by the NASTRAN software, in which the divergence speed of a wing alone is included, and the variation curves of flightloads, longitudinal stability derivatives and longitudinal control derivatives with respect to Mach number and dynamic pressure respectively under the subsonic case are involved emphatically. By comparison and analysis, the following results can be found: (1) At the same attitude, with Mach number increasing, for the longitudinal stability and control derivatives, the rigid one and the elastic one change with different trends respectively, and there are great differences between some variation curves; for the variation curves of the shear force, bending moment and torsional moment of the wing along the lines of span, the rigid proportions decrease while the elastic one increases; for the maximal vertical deflection of the wing (at the leading edge of the wing tip) and the maximal torsional deformation of the wing (at the wing tip), both increase a little. (2) At the same Mach number, with dynamic pressure increasing, for the longitudinal stability and control derivatives, the rigid one remains unchanged while the elastic one changes linearly; for the variation curves of the shear force, bending moment and torsional moment of the wing along the lines of span, the variation trends of rigid and elastic proportions are the same as result (1); for the maximal vertical deflection and the maximal torsional deformation of the wing, both are the same as result (1). (3) By the use of composite tailoring, wash-in of the wing can be reduced; therefore divergence speed can be enhanced, and the deformation of the wing and the elastic proportion of flightload can be decreased.
