1999 Vol. 16, No. 4
1999, 16(4): 1-7.
Abstract:
The cost effective potassium titanate whiskers are artificial fibers with high performance. In this paper, their characteristics and properties, state of the art and method of preparation were reviewed, and their applications in composite materials were introduced in details. Also pointed out were some problems to be solved and the researching trend to follow. Studies of reducing the cost of whiskers, treating surface of whiskers and composing technology between whiskers and matrix are the most important topics in the near future.
The cost effective potassium titanate whiskers are artificial fibers with high performance. In this paper, their characteristics and properties, state of the art and method of preparation were reviewed, and their applications in composite materials were introduced in details. Also pointed out were some problems to be solved and the researching trend to follow. Studies of reducing the cost of whiskers, treating surface of whiskers and composing technology between whiskers and matrix are the most important topics in the near future.
1999, 16(4): 8-12.
Abstract:
The high temperature ductile deformation behavior of TiC sintered body with various content of Cr3C2 addition was investigated. The results show that the descending trend of the effective conductivity of TiC/Ni3Al FGMs with the numbers of shock cycles relates closely to the ductile deformation behavior of TiC, and the vertical cracks appearing on the TiC surface is caused by large radial compressive stresses.
The high temperature ductile deformation behavior of TiC sintered body with various content of Cr3C2 addition was investigated. The results show that the descending trend of the effective conductivity of TiC/Ni3Al FGMs with the numbers of shock cycles relates closely to the ductile deformation behavior of TiC, and the vertical cracks appearing on the TiC surface is caused by large radial compressive stresses.
1999, 16(4): 13-16.
Abstract:
A cost effective SiC/Si3N4 nano composite was obtained by in situ synthesis of precursor, using polycarbosilane as precursor. The results showed that the ambitious flexural strength and fracture toughness of this material had improved. The formation of this composites microstructure and the mechanism of fracture were discussed at the same time.It was important to control the formation of SiC nano crystals and the growth of rod like β Si3N4 grain.The formation of the intragranular structure and the rod like β Si3N4 grains(aspect ratio more than 7.5) was the reason for the improvement properties.
A cost effective SiC/Si3N4 nano composite was obtained by in situ synthesis of precursor, using polycarbosilane as precursor. The results showed that the ambitious flexural strength and fracture toughness of this material had improved. The formation of this composites microstructure and the mechanism of fracture were discussed at the same time.It was important to control the formation of SiC nano crystals and the growth of rod like β Si3N4 grain.The formation of the intragranular structure and the rod like β Si3N4 grains(aspect ratio more than 7.5) was the reason for the improvement properties.
1999, 16(4): 17-23.
Abstract:
In this paper, TEM technology was used to analyze the microstructure and interface structure of Al 4.7Cu 1.73Mg/SiCP composites produced by spray co deposition; meanwhile, the mechanical properties and wear resistance ability of the materials were given.
In this paper, TEM technology was used to analyze the microstructure and interface structure of Al 4.7Cu 1.73Mg/SiCP composites produced by spray co deposition; meanwhile, the mechanical properties and wear resistance ability of the materials were given.
1999, 16(4): 24-29.
Abstract:
Densified TiC-Ni cermets were produced by SHS/PHIP. Theoretical analysis and experiments show that T ad and T c decrease with Ni increasing, combustion temperature influences the morphology of the products, and TiC particles become smaller and tend to be regular sphere when Ni content is increased. The products consist of TiC phase and Ni binder. Relative density gets higher with the increase of Ni content. Two factors determine the value of products hardness, and a maximum value of HRA appears at 20 wt% Ni.
Densified TiC-Ni cermets were produced by SHS/PHIP. Theoretical analysis and experiments show that T ad and T c decrease with Ni increasing, combustion temperature influences the morphology of the products, and TiC particles become smaller and tend to be regular sphere when Ni content is increased. The products consist of TiC phase and Ni binder. Relative density gets higher with the increase of Ni content. Two factors determine the value of products hardness, and a maximum value of HRA appears at 20 wt% Ni.
1999, 16(4): 30-34.
Abstract:
In situ Al/Mg2Si/Si composites were fabricated by using simple weight cast technique. Effects of extra Si contents on microstructure of Al/Mg2Si composite were investigated by optical microscopy and SEM with an EDXA accessory. Al Mg2Si Si ternary diagram was also introduced to analyze the microstructure. Experimental results indicated that extra Si in Al/Mg2Si system produced sphere like α Al grains and ternary eutectics with low aspect ratio. These microstructure improvements resulted in a significant increase in the materials' tensile properties.
In situ Al/Mg2Si/Si composites were fabricated by using simple weight cast technique. Effects of extra Si contents on microstructure of Al/Mg2Si composite were investigated by optical microscopy and SEM with an EDXA accessory. Al Mg2Si Si ternary diagram was also introduced to analyze the microstructure. Experimental results indicated that extra Si in Al/Mg2Si system produced sphere like α Al grains and ternary eutectics with low aspect ratio. These microstructure improvements resulted in a significant increase in the materials' tensile properties.
1999, 16(4): 35-39.
Abstract:
Nickel phthalocyanine (NiPc) Fe3O4 nanoparticles complexes were prepared and their chemical stability and magnetic property were measured. The results show that NiPc forms complex layers on the surface of Fe3O4 nanoparticles and there is somewhat chemical bonding between them. NiPc can protect Fe3O4 nanoparticles effectively from oxidizing and improve their antioxidation ability greatly. In addition, H c of NiPc Fe3O4 nanoparticles complex is much lower than that of Fe3O4 nanoparticles.
Nickel phthalocyanine (NiPc) Fe3O4 nanoparticles complexes were prepared and their chemical stability and magnetic property were measured. The results show that NiPc forms complex layers on the surface of Fe3O4 nanoparticles and there is somewhat chemical bonding between them. NiPc can protect Fe3O4 nanoparticles effectively from oxidizing and improve their antioxidation ability greatly. In addition, H c of NiPc Fe3O4 nanoparticles complex is much lower than that of Fe3O4 nanoparticles.
1999, 16(4): 40-43.
Abstract:
The processes for preparing Fe Al intermetallic compounds by solid state reaction were presented. Under the conditions of 20 h high energy ball milling and pre heated at 750℃, the Fe Al intermetallic compounds powder was prepared.
The processes for preparing Fe Al intermetallic compounds by solid state reaction were presented. Under the conditions of 20 h high energy ball milling and pre heated at 750℃, the Fe Al intermetallic compounds powder was prepared.
1999, 16(4): 44-49.
Abstract:
In the present work, the electric resistance of copper filled composite paint is reduced ten times by addition of coupling agent. And the electric conductivity of the composite paint keeps stable during 500 hours of exposure to air. The mechanism of resisting oxidation is investigated by Flourier Transfer Infrared Spectrometry. It is suggested that the hydrogen bond and the covalence bond between coupling agent and the copper powders are formed. This leads to formation of an oxidation resistant film on the copper powders.
In the present work, the electric resistance of copper filled composite paint is reduced ten times by addition of coupling agent. And the electric conductivity of the composite paint keeps stable during 500 hours of exposure to air. The mechanism of resisting oxidation is investigated by Flourier Transfer Infrared Spectrometry. It is suggested that the hydrogen bond and the covalence bond between coupling agent and the copper powders are formed. This leads to formation of an oxidation resistant film on the copper powders.
1999, 16(4): 50-53.
Abstract:
A new fabrication process on Fe3Al/Al2O3 composite has been researched. This is important not only to produce Fe3Al/Al2O3 composite by sintering process that does not employ pressure to composite, but also to open up and use this composite. This new fabrication process can be called melting & casting sintering process (MCSP). MCSP has proved to be a good desirable way by experimental study.
A new fabrication process on Fe3Al/Al2O3 composite has been researched. This is important not only to produce Fe3Al/Al2O3 composite by sintering process that does not employ pressure to composite, but also to open up and use this composite. This new fabrication process can be called melting & casting sintering process (MCSP). MCSP has proved to be a good desirable way by experimental study.
1999, 16(4): 54-57.
Abstract:
The curing mechanism and kinetics of epoxy resin with cured boron amine complexes containing oxaborinane were studied by means of DSC and IR. The results indicated that the first complexes containing hydrogen proton came into being only after the reaction of boron amine complexes with hydrogen groups of the system, and the polyether cross network was formed by initiation epoxy groups. The study of the curing kinetics reveals that the whole curing process follows the first order kinetics.
The curing mechanism and kinetics of epoxy resin with cured boron amine complexes containing oxaborinane were studied by means of DSC and IR. The results indicated that the first complexes containing hydrogen proton came into being only after the reaction of boron amine complexes with hydrogen groups of the system, and the polyether cross network was formed by initiation epoxy groups. The study of the curing kinetics reveals that the whole curing process follows the first order kinetics.
1999, 16(4): 58-63.
Abstract:
The friction and wear properties of polytetrafluoroethylene (PTFE) composites filled with MoS2, CuS, PbS, and graphite (addition quantity is 30 vol% in volume fraction) sliding against GCr15 bearing steel under dry friction conditions were studied by using an MHK 500 ring block wear tester (Timken wear tester), then the wear debris and the frictional surfaces of these PTFE composites were examined by using a scanning electron microscope (SEM) and an optical microscope. Experimental results show that graphite reduces the friction coefficient of the PTFE, but MoS2, CuS and PbS increase the friction coefficient of the PTFE. Meanwhile, filling PTFE with MoS2, CuS, PbS, or graphite can reduce the wear of PTFE by 2 orders of magnitude, but the wear reducing action of PbS is the most effective, while that of MoS2 is the worst. SEM and optical microscope investigations show that the MoS2, CuS, PbS, and graphite not only prevent the banded structure of PTFE from destruction, but also enhance the adhesion of the transfer films to the counterfaces, so they greatly reduce the wear of the PTFE.
The friction and wear properties of polytetrafluoroethylene (PTFE) composites filled with MoS2, CuS, PbS, and graphite (addition quantity is 30 vol% in volume fraction) sliding against GCr15 bearing steel under dry friction conditions were studied by using an MHK 500 ring block wear tester (Timken wear tester), then the wear debris and the frictional surfaces of these PTFE composites were examined by using a scanning electron microscope (SEM) and an optical microscope. Experimental results show that graphite reduces the friction coefficient of the PTFE, but MoS2, CuS and PbS increase the friction coefficient of the PTFE. Meanwhile, filling PTFE with MoS2, CuS, PbS, or graphite can reduce the wear of PTFE by 2 orders of magnitude, but the wear reducing action of PbS is the most effective, while that of MoS2 is the worst. SEM and optical microscope investigations show that the MoS2, CuS, PbS, and graphite not only prevent the banded structure of PTFE from destruction, but also enhance the adhesion of the transfer films to the counterfaces, so they greatly reduce the wear of the PTFE.
1999, 16(4): 64-67.
Abstract:
In this paper,the reinforcing effect, the stretching and compressing destructive mechanism in the polyurethane foam plastics reinforced by short cutting glass fibres have been studied. The results show that the reinforcing effect is best when the fibre length is 12 mm and the fibre content is 6 wt%. The stretching intensity, the compressing intensity and the compressing modulus are increased by 75%, 25% and 30%, respectively. When the foam reinforced by fibres is stretched, the cracks are produced and the cracks spread. They may stop spreading (as the stress is low) or may present deflection (as the stress is high) when the cracks meet fibres. Different destructive shapes (the fibres are pulled out, broken and so on) may occur when the foam is destroyed. The prop of the foam structure may be crooked, twisted and out of shape when the foam reinforced by fibres is compressed destructively. It leads to rupture of foam wall, losing stability of prop and destruction of material.
In this paper,the reinforcing effect, the stretching and compressing destructive mechanism in the polyurethane foam plastics reinforced by short cutting glass fibres have been studied. The results show that the reinforcing effect is best when the fibre length is 12 mm and the fibre content is 6 wt%. The stretching intensity, the compressing intensity and the compressing modulus are increased by 75%, 25% and 30%, respectively. When the foam reinforced by fibres is stretched, the cracks are produced and the cracks spread. They may stop spreading (as the stress is low) or may present deflection (as the stress is high) when the cracks meet fibres. Different destructive shapes (the fibres are pulled out, broken and so on) may occur when the foam is destroyed. The prop of the foam structure may be crooked, twisted and out of shape when the foam reinforced by fibres is compressed destructively. It leads to rupture of foam wall, losing stability of prop and destruction of material.
1999, 16(4): 68-71.
Abstract:
Unidirectional glass/polypropylene composites was fabricated in a hot press by using air jet commingled yarn of 50∶50 wt% glass fibers/polypropylene filaments. According to ASTM D792, the void content sample of composites was measured, and bending test of composites was performed on DCS500 tester by using three point loading system. Experimental results showed that the relation between void content and bending properties of air jet commingled yarn composites was a negative exponential function; meanwhile, the functions were various for different bending properties such as strength and modulus.
Unidirectional glass/polypropylene composites was fabricated in a hot press by using air jet commingled yarn of 50∶50 wt% glass fibers/polypropylene filaments. According to ASTM D792, the void content sample of composites was measured, and bending test of composites was performed on DCS500 tester by using three point loading system. Experimental results showed that the relation between void content and bending properties of air jet commingled yarn composites was a negative exponential function; meanwhile, the functions were various for different bending properties such as strength and modulus.
1999, 16(4): 72-78.
Abstract:
Department of Modern Mechanics, Based on thermo electro elastic coupled differential equations, composite laminates with anisotropic piezo thermo elastic layers under cylindrical bending are analyzed via Fourier series method. Because no simplification is adopted, the method can be used to analyze thin and thick plates exactly. By using complex exponents, the deduction and programming are simplified greatly. In the paper, all possible models for the heat, electric and mechanical boundaries are taken into account. Furthermore, the interactions between the coupled components and their individual characteristics are analyzed, and a marked difference between the structural response under electric load and thermo load is found.
Department of Modern Mechanics, Based on thermo electro elastic coupled differential equations, composite laminates with anisotropic piezo thermo elastic layers under cylindrical bending are analyzed via Fourier series method. Because no simplification is adopted, the method can be used to analyze thin and thick plates exactly. By using complex exponents, the deduction and programming are simplified greatly. In the paper, all possible models for the heat, electric and mechanical boundaries are taken into account. Furthermore, the interactions between the coupled components and their individual characteristics are analyzed, and a marked difference between the structural response under electric load and thermo load is found.
1999, 16(4): 79-84.
Abstract:
Experiments showed that progressive failure was the main feature of glass epoxy cylindrical shells subjected to axial impact. Based on the one dimensional wave propagation theory, a simple dynamic model was presented to analyze the failure process and energy absorption of the shells under impact loading. It was found that this theoretical analysis was in agreement with the progressive failure.
Experiments showed that progressive failure was the main feature of glass epoxy cylindrical shells subjected to axial impact. Based on the one dimensional wave propagation theory, a simple dynamic model was presented to analyze the failure process and energy absorption of the shells under impact loading. It was found that this theoretical analysis was in agreement with the progressive failure.
1999, 16(4): 85-89.
Abstract:
From the observation of biological structure, it was found that the fiber changes its density and direction continuously when passing around a hole. Some experiments have demonstrated that the mechanical properties of this kind of structure were much better than those of a structure containing drilled or punched holes. An iteration method was proposed for the design of pre formed hole structure of a single ply. Then the mechanical property of the laminates consisting of this kind of ply was analyzed. Calculation showed that the obtained laminates have higher load carrying capability than the conventional ones.
From the observation of biological structure, it was found that the fiber changes its density and direction continuously when passing around a hole. Some experiments have demonstrated that the mechanical properties of this kind of structure were much better than those of a structure containing drilled or punched holes. An iteration method was proposed for the design of pre formed hole structure of a single ply. Then the mechanical property of the laminates consisting of this kind of ply was analyzed. Calculation showed that the obtained laminates have higher load carrying capability than the conventional ones.
1999, 16(4): 90-93.
Abstract:
The pretightening force of FRP construction bolt joint has been researched in the theory of bolt washer stress. The computed pretightening force of bolt and washer size reach the prefigurative result in the construction test, but the pretightening force of bolt was decreased obviously by the time increase for matrix relaxation effect, and then tending towards a stable value.
The pretightening force of FRP construction bolt joint has been researched in the theory of bolt washer stress. The computed pretightening force of bolt and washer size reach the prefigurative result in the construction test, but the pretightening force of bolt was decreased obviously by the time increase for matrix relaxation effect, and then tending towards a stable value.
1999, 16(4): 94-100.
Abstract:
The expressions for the stress field distributions in the matrix and the fiber were derived using the theory of elasticity and the shear lag theory. The effects of fiber volume fraction, fiber aspect ratio and matrix yield strength on the stress distributions and the stress transfer were investigated. It is shown that the stress distributions in the matrix and the fiber and the plastic behavior in the matrix are of significant non uniformity; there are significant stress transfer from the matrix to the fiber and the partition between the matrix and the fiber.
The expressions for the stress field distributions in the matrix and the fiber were derived using the theory of elasticity and the shear lag theory. The effects of fiber volume fraction, fiber aspect ratio and matrix yield strength on the stress distributions and the stress transfer were investigated. It is shown that the stress distributions in the matrix and the fiber and the plastic behavior in the matrix are of significant non uniformity; there are significant stress transfer from the matrix to the fiber and the partition between the matrix and the fiber.
1999, 16(4): 101-106.
Abstract:
The expressions for the elastic modulus and the yield strength of the composite were derived based on the deformation model and the results of the stress analysis. The predictions of the expressions exhibit good agreement with the experiments and also with those from the Eshelby model and finite element numerical calculations. It was shown that the increased elastic modulus and yield strength relative to the elastic modulus and the yield strength of the matrix depend mainly on the contribution from the stress transfer.
The expressions for the elastic modulus and the yield strength of the composite were derived based on the deformation model and the results of the stress analysis. The predictions of the expressions exhibit good agreement with the experiments and also with those from the Eshelby model and finite element numerical calculations. It was shown that the increased elastic modulus and yield strength relative to the elastic modulus and the yield strength of the matrix depend mainly on the contribution from the stress transfer.
1999, 16(4): 107-111.
Abstract:
type piezoelectric composites using thickness shear mode (PCMTS) were selected as the cores of the sandwich structure, which can generate the thickness shear deformation and actuate the sandwich structure to give rise to the desired transverse deflection. The actuating principle of the PCMTS cores was analyzed, and the experimental investigations were carried out to evaluate the sandwich beam characteristics featuring the actuation of the PCMTS cores. Meanwhile, the active vibration control of the sandwich beam with the PCMTS cores was studied. The results indicate that, using the PCMTS cores as the actuators, an ideal deflection of the sandwich beam is generated and the vibration of the sandwich beam is suppressed effectively.
type piezoelectric composites using thickness shear mode (PCMTS) were selected as the cores of the sandwich structure, which can generate the thickness shear deformation and actuate the sandwich structure to give rise to the desired transverse deflection. The actuating principle of the PCMTS cores was analyzed, and the experimental investigations were carried out to evaluate the sandwich beam characteristics featuring the actuation of the PCMTS cores. Meanwhile, the active vibration control of the sandwich beam with the PCMTS cores was studied. The results indicate that, using the PCMTS cores as the actuators, an ideal deflection of the sandwich beam is generated and the vibration of the sandwich beam is suppressed effectively.
1999, 16(4): 112-120.
Abstract:
The effect of interface properties on failure modes in particle filled composite is studied by using a numerical code, MFPA2D , newly developed by CRISR at Northeastern University, Shenyang, P.R.China. Two kinds of interface, ideal and non ideal, are considered. The single particle is studied firstly, then multi particles are modeled. The results from both the single particle model and the multi particle model show that the failure mechanisms are strongly dependent on the interface properties. The simulations reproduce the crack initiation, propagation and coalescence in brittle and heterogeneous matrix. Interactions between the particles are also modeled. The numerical analysis elucidates the effect of interface properties on the anticipated failure modes. In the ideal interface condition, the failure mainly occurred in the location near to the interface. However, in the non ideal interface condition, the failure mainly occurred in the interface, which results in the interfacial debonding.
The effect of interface properties on failure modes in particle filled composite is studied by using a numerical code, MFPA2D , newly developed by CRISR at Northeastern University, Shenyang, P.R.China. Two kinds of interface, ideal and non ideal, are considered. The single particle is studied firstly, then multi particles are modeled. The results from both the single particle model and the multi particle model show that the failure mechanisms are strongly dependent on the interface properties. The simulations reproduce the crack initiation, propagation and coalescence in brittle and heterogeneous matrix. Interactions between the particles are also modeled. The numerical analysis elucidates the effect of interface properties on the anticipated failure modes. In the ideal interface condition, the failure mainly occurred in the location near to the interface. However, in the non ideal interface condition, the failure mainly occurred in the interface, which results in the interfacial debonding.
1999, 16(4): 121-129.
Abstract:
The generalized Donnell type equations governing large deflection of antisymmetrically laminated cross ply cylindrical shells counting for transverse shear deformation are presented. An asymptotic series solution is constructed by the perturbation technique of deflection type for buckling and postbuckling behaviors of the cylindrical shells under external pressure. Boundary layer effects at both ends of shells are considered and, for consistancy of the boundary value problem, the boundary layer solutions are also designed to match the out of plane boundary conditions by the singular perturbation approach. Meanwhile, effects of transverse shear deformation, Batdorf parameter, modulus ratio and initial geometrical imperfection on the buckling and postbuckling of the shells are examined. Numerical results computed from the present solution present the buckling loads and postbuckling equilibrium paths of the specific cylindrical shells under external pressure. It has been shown by the present solution that the decrease in buckling loads due to transverse shears is significant.
The generalized Donnell type equations governing large deflection of antisymmetrically laminated cross ply cylindrical shells counting for transverse shear deformation are presented. An asymptotic series solution is constructed by the perturbation technique of deflection type for buckling and postbuckling behaviors of the cylindrical shells under external pressure. Boundary layer effects at both ends of shells are considered and, for consistancy of the boundary value problem, the boundary layer solutions are also designed to match the out of plane boundary conditions by the singular perturbation approach. Meanwhile, effects of transverse shear deformation, Batdorf parameter, modulus ratio and initial geometrical imperfection on the buckling and postbuckling of the shells are examined. Numerical results computed from the present solution present the buckling loads and postbuckling equilibrium paths of the specific cylindrical shells under external pressure. It has been shown by the present solution that the decrease in buckling loads due to transverse shears is significant.
1999, 16(4): 130-135.
Abstract:
The progressive interfacial debonding process and its influence on the elasto plastic stress strain relation of the particle reinforced composites under triaxial tension were studied. Weibulls theory of statistics is used here to express the damage process. Secant moduli are applied when the system is beyond the elastic limit. Numerical analysis shows that the damage procedures depend greatly upon the loading mode, strength of the interfaces and the shape of the particles.
The progressive interfacial debonding process and its influence on the elasto plastic stress strain relation of the particle reinforced composites under triaxial tension were studied. Weibulls theory of statistics is used here to express the damage process. Secant moduli are applied when the system is beyond the elastic limit. Numerical analysis shows that the damage procedures depend greatly upon the loading mode, strength of the interfaces and the shape of the particles.
1999, 16(4): 136-141.
Abstract:
The experimental method on the mechanical properties of four directional braided carbon/epoxy composites is discussed. The corresponding elastic constants and parameters depend on braided angles and nonlinear behavior of materials. Finally, the effect of braided parameters on the failure mode of materials is analyzed and discussed.
The experimental method on the mechanical properties of four directional braided carbon/epoxy composites is discussed. The corresponding elastic constants and parameters depend on braided angles and nonlinear behavior of materials. Finally, the effect of braided parameters on the failure mode of materials is analyzed and discussed.
1999, 16(4): 142-146.
Abstract:
A lower cost method for predicting the composite safe life is given in this paper. It can combine the previous data with the current data to calculate the safe life for composite structures. In the present method, the current test data information is used independently while predicting the safe life. The information available is much more in this paper's method than in the currently available method. As a result, it has not only saved much more specimens, but also improved the predicting precision. Therefore, it solved the problems such as more specimens, longer test period and greater cost because of the large scatter of composite life. Furthermore, some fatigue test data for many kinds of advanced fiber reinforced composite materials are analyzed by this lower cost method. It shows that fewer specimens (only about half of the specimens in the current method) are required in this method and the result is more accurate for the same number of specimens by these contrast examples.
A lower cost method for predicting the composite safe life is given in this paper. It can combine the previous data with the current data to calculate the safe life for composite structures. In the present method, the current test data information is used independently while predicting the safe life. The information available is much more in this paper's method than in the currently available method. As a result, it has not only saved much more specimens, but also improved the predicting precision. Therefore, it solved the problems such as more specimens, longer test period and greater cost because of the large scatter of composite life. Furthermore, some fatigue test data for many kinds of advanced fiber reinforced composite materials are analyzed by this lower cost method. It shows that fewer specimens (only about half of the specimens in the current method) are required in this method and the result is more accurate for the same number of specimens by these contrast examples.
