A six-parameter mathematical model was introduced to simulate the stress-strain hysteresis and the sinner hysteresis of polycrystalline shape memory alloys (SMAs). By the comparison with experiments of Cu-Zn-Al SMA, i...A six-parameter mathematical model was introduced to simulate the stress-strain hysteresis and the sinner hysteresis of polycrystalline shape memory alloys (SMAs). By the comparison with experiments of Cu-Zn-Al SMA, it was shown that the model could be used to calculate the stress-strain relations with rather good accuracy. Moreover, it was found that the six parameters introduced in this paper represented the characteristics of the stress-strain hysteresis of polycrystalline SMAs and could be used to characterize the hysteresis quantitatively.展开更多
The textured shape memory alloys exhibits anisotropic because the property of single crystal is strongly orientation dependent. The effect of texture on phase transformation strain in CuZnAl shape memory sheets was in...The textured shape memory alloys exhibits anisotropic because the property of single crystal is strongly orientation dependent. The effect of texture on phase transformation strain in CuZnAl shape memory sheets was investigated. The texture of parent austenite was measured by X ray goniometer and analyzed by the orientation distribution function. Subsequently, using the texture parameters and single crystal properties, the phase transformation strains at the different directions of rolling plane by the statistically averaging method were calculated. It was showed that the experimental results are agreeable with the calculated ones. It is well explained that this anisotropy of phase transformation strain is mainly caused by the crystallographic texture of the rolled sheets.展开更多
In this paper, the analytical solution of stress field for a strained reinforcement layer bonded to a lip-shaped crack under a remote mode III uniform load and a concentrated load is obtained explicitly in the series ...In this paper, the analytical solution of stress field for a strained reinforcement layer bonded to a lip-shaped crack under a remote mode III uniform load and a concentrated load is obtained explicitly in the series form by using the technical of conformal mapping and the method of analytic continuation. The effects of material combinations, bond of interface and geometric configurations on interfaciai stresses generated by eigenstrain, remote load and concentrated load are studied. The results show that the stress concentration and interfaciai stresses can be reduced by rational material combinations and geometric configurations designs for different load forms.展开更多
The effect of pre-strain on phase transformation of TiNi shape memory alloy film was studied by differential scanning calorimeter measurement (DSC). Compared with un-defarmed TiNi film, the reverse transformation of p...The effect of pre-strain on phase transformation of TiNi shape memory alloy film was studied by differential scanning calorimeter measurement (DSC). Compared with un-defarmed TiNi film, the reverse transformation of pre-strained specimens was elevated to a higher temperature on the first heating, but martensite and reverse transformation on subsequent thermal cycles occurred at a lower temperature. The evolution of transformation behavior in pre-strained TiNi film was related to the change of elastic strain energy, irreversible energy and internal stress field.展开更多
A magnetic shape memory alloy with nonstoichiometric Ni50Mn27Ga23 was prepared by using melt-spinning technology. The martensitic transformation and the magnetic-field-induced strain (MFIS) of the polycrystalline melt...A magnetic shape memory alloy with nonstoichiometric Ni50Mn27Ga23 was prepared by using melt-spinning technology. The martensitic transformation and the magnetic-field-induced strain (MFIS) of the polycrystalline melt-spun ribbon were investigated. The experimental results showed that the melt-spun ribbons underwent thermal-elastic martensitic transformation and reverse transformation in cooling and heating process and exhibited typical thermo-elastic shape memory effect. However the start temperature for martensitic transformation decreased from 286 K for as-cast alloy to 254 K for as-quenched ribbon and Curie temperature remains approximately constant. A particular internal stress induced by melt-spinning resulted in the formation of a texture structure in the ribbons, which made the ribbons obtain larger martensitic transformation strain and MFIS. The internal stress was released substantially after annealing, which resulted in a decrease of MFIS of the ribbons.展开更多
The strain jump across the Austenite-Martensite (A-M) interface in single crystal Cu-14wt%Al-4.12wt%Ni Shape Memory Alloys (SMAs) under uniaxial tension was studied in this paper. A crystallographic-based mechanics an...The strain jump across the Austenite-Martensite (A-M) interface in single crystal Cu-14wt%Al-4.12wt%Ni Shape Memory Alloys (SMAs) under uniaxial tension was studied in this paper. A crystallographic-based mechanics analysis on the formation and microstructure of the interface was performed. By using the high sensitive Moiré interference technique, the full-field deformation patterns during the transformation process were successfully recorded. The orientation of the habit plane (A-M interface) and the magnitude of the shape strain were determined precisely from the Moiré fringe patterns. The theoretical predictions on the habit plane normal and the shape strain were compared with the measured results and good agreements were obtained.展开更多
The effects of microamount additions of RE (Tb, Sm) on martensitic transition, the magnetic-field-induced strain and the bending strength of highly textured polycrystalline Ni_(48)Mn_(33)Ga_(19) alloy were investigate...The effects of microamount additions of RE (Tb, Sm) on martensitic transition, the magnetic-field-induced strain and the bending strength of highly textured polycrystalline Ni_(48)Mn_(33)Ga_(19) alloy were investigated. The experimental results show that the addition of RE elements decreases the martensitic transformation temperature and the Curie temperature. But the bending strength of Ni-Mn-Ga-RE (RE=Tb, Sm) alloys increases remarkably because of the grain refinement. As a result, Ni-Mn-Ga-RE alloys will be applied practically with higher reliability and stability due to favorable plasticity and toughness. In addition, the replacement of small amounts Ga by Tb or Sm decreases the magnetic-field-induced strain of the alloys at room temperature.展开更多
The recovery force or recovery strain is an important indicator of NiTi-based shape memory alloy devices. However, the restoring force or recoverable strain is partially restrained due to an interaction between reorie...The recovery force or recovery strain is an important indicator of NiTi-based shape memory alloy devices. However, the restoring force or recoverable strain is partially restrained due to an interaction between reorientation and reorientation-induced plasticity. Therefore, a macroscopic multi-mechanism constitutive model was constructed to describe the degeneration of shape memory effect based on the phase diagram. The residual strain after cooling consists of reorientation strain and reorientation-induced plastic strain. An internal variable, i.e., the detwinned stress, and its evolution equation were introduced into the transformation kinetics equation to describe the nonlinear hardening characteristics induced by the combined reorien- ration and detwinning mechanisms during mechanical loading. Finally, the proposed model was numerically implemented to simulate the experiments of shape memory effect at different peak strains. Comparisons between the experimental and simulated results show that the proposed model can reasonably describe the degeneration of shape memory effect.展开更多
The mechanical behavior and the effect of pre-strain on recovery behavior of Ti50Ni47Fe3 (at. pct) alloy were investigated systematically by tensile and recovered tests accompanied by electrical resistance measurement...The mechanical behavior and the effect of pre-strain on recovery behavior of Ti50Ni47Fe3 (at. pct) alloy were investigated systematically by tensile and recovered tests accompanied by electrical resistance measurement. Ti50Ni47Fe3 alloy has different deformation behaviors at different temperature ranges, the deformation curves in different temperature range can be classified into four kinds. The start temperature of recovery increases with the increase of pre-strain. There exists an optimal deformation condition, at which the specimen exhibits maximum free recovery strain. With increasing pre-strain the recovery stress increases and reaches the maximum at 8% pre-strain. R-phase to parent transition offered about 0.2% recovery strain. With pre-strain increasing the recovery stress increases and reaches to the maximum at 8% pre-strain. The recovery stress is corresponding with the critical stress of stress-induced martensitic transformation.展开更多
The dependence of high temperature shape memory effect on the pre-deformation tempera- ture of the Ti_(50)Ni_3Pd_(37) alloy has been studied.This alloy,of which the reverse transformation start temperature on heating ...The dependence of high temperature shape memory effect on the pre-deformation tempera- ture of the Ti_(50)Ni_3Pd_(37) alloy has been studied.This alloy,of which the reverse transformation start temperature on heating is 620 K,has both one-way and two-way shape memory effects. The maximum shape memory strain of the ahoy may be obtained under pre-deformation in the range of 620 to 640 K.This seems to be related to the minimum recoverable strain energy emerged from the above mentioned temperature range.展开更多
Energy minimization has been widely used for constructing curve and surface in the fields such as computer-aided geometric design, computer graphics. However, our testing examples show that energy minimization does no...Energy minimization has been widely used for constructing curve and surface in the fields such as computer-aided geometric design, computer graphics. However, our testing examples show that energy minimization does not optimize the shape of the curve sometimes. This paper studies the relationship between minimizing strain energy and curve shapes, the study is carried out by constructing a cubic Hermite curve with satisfactory shape. The cubic Hermite curve interpolates the positions and tangent vectors of two given endpoints. Computer simulation technique has become one of the methods of scientific discovery, the study process is carried out by numerical computation and computer simulation technique. Our result shows that： （1） cubic Hermite curves cannot be constructed by solely minimizing the strain energy; （2） by adoption of a local minimum value of the strain energy, the shapes of cubic Hermite curves could be determined for about 60 percent of all cases, some of which have unsatisfactory shapes, however. Based on strain energy model and analysis, a new model is presented for constructing cubic Hermite curves with satisfactory shapes, which is a modification of strain energy model. The new model uses an explicit formula to compute the magnitudes of the two tangent vectors, and has the properties： （1） it is easy to compute; （2） it makes the cubic Hermite curves have satisfactory shapes while holding the good property of minimizing strain energy for some cases in curve construction. The comparison of the new model with the minimum strain energy model is included.展开更多
The effects of thermomechanical cycling on the shape memory behavior and transformation temperatures of a Ni50.2Ti49.8 alloy under a constant applied stress of 300 MPa were investigated, k is believed that thermomecha...The effects of thermomechanical cycling on the shape memory behavior and transformation temperatures of a Ni50.2Ti49.8 alloy under a constant applied stress of 300 MPa were investigated, k is believed that thermomechanical cycling induces defects such as dislocations, which evidently affect the shape memory behavior and transformation temperatures. The recovery strain decreases with increasing number of thermomechanical cycles, whereas the irreversible plastic strain increases, especially in the initial few cycles. The stored elastic strain energy has an important influence on transformation temperatures, the A5^σ decreases and the M5^σ increases with increasing number of thermomechanical cycles. The recovery strain, irreversible plastic strain, A5^σ , and M5^σ reach a saturation value after several cycles.展开更多
This paper aims to build a constitutive model intended to describe the thermomechanical behavior of shape memory alloys. This behavior presents many facets, among them we have considered the simple way of shape memory...This paper aims to build a constitutive model intended to describe the thermomechanical behavior of shape memory alloys. This behavior presents many facets, among them we have considered the simple way of shape memory, which is one of most important properties of shape memory alloys. Because of numerous stages of this effect, the subject was divided into three independent parts. For each part, we built the corresponding thermodynamic potential and we deduced the constitutive equations. To make this model workable, we have developed an algorithm. The simulation was performed using the NiTi as shape memory alloy.展开更多
Based on experimental study of the time dependence of the recoverable shape memory effect (SME)of shape memory alloys Ti50Ni50,Ti50Ni48Cu2 and Ti50Ni45Cu5 an empirical re- lationship between recoverable strain energy,...Based on experimental study of the time dependence of the recoverable shape memory effect (SME)of shape memory alloys Ti50Ni50,Ti50Ni48Cu2 and Ti50Ni45Cu5 an empirical re- lationship between recoverable strain energy,U_r,and temperature,T.was derived as: U_r=U_o[1-exp(-mT_R^n)] where m—characteristic constant related to material,and n-shape-temperature sensitivity. The Johnson-Mehl-Aerami's phenomenological description of the time dependence of phase transformation was adopted to obtain a semiquantitative relationship associated with recoverable SME.between thermoelastic martensite transformation and temperature.This semiquantitative relationship is correspondent with the empirical expression.展开更多
SMPU （shape memory polyurethane） non-ionomers and ionomers, synthesized with poly（c-caprolactone） （PCL）, 4, 4＇-diphenylmethane diisocyanate （MDI）, 1,4-butanediol （BDO）, dimethylolpropionic acid （DMPA） wer...SMPU （shape memory polyurethane） non-ionomers and ionomers, synthesized with poly（c-caprolactone） （PCL）, 4, 4＇-diphenylmethane diisocyanate （MDI）, 1,4-butanediol （BDO）, dimethylolpropionic acid （DMPA） were measured with cyclic tensile test and strain recovery test. The relations between the structure and shape memory effect of these two series were studied with respect to the ionic group content and the effect of neutralization. The resulting data indicate that, with the introduction of asymmetrical extender, the stress at 100% elongation is decreased for PU non-ionomer and ionomer series, especially lowered sharply for non-ionomer series; the fixation ratio of ionomer series is not affected obviously by the ionic group content; the total recovery ratio of ionomer series is decreased greatly. After sufficient relaxation time for samples stretched beforehand, the switching temperature is raised slightly, whereas the recovery ratio measured with strain recovery test method is lowered with increased DMPA content. The characterization with FT-IR, DSC, DMA elucidated that, the ordered hard domain of the two series is disrupted with the introduction of DMPA which causes more hard segments to dissolve in soft phase; ionic groups on hard segment enhance the cohesion between hard segments especially at high ionic group content and significantly facilitate the phase separation compared with the corresponding non-ionomer at moderate ionic group content.展开更多
Thermal cycle training of CuZnAl shape memory alloys with differenttransformation temperatures was carried out. The influence of different pre-strains, heat treatmentsand media on the shape memory effect(SME) of CuZnA...Thermal cycle training of CuZnAl shape memory alloys with differenttransformation temperatures was carried out. The influence of different pre-strains, heat treatmentsand media on the shape memory effect(SME) of CuZnAl alloys with different transformationtemperatures was studied by means of scanning electron microscopy(SEM) and X-raydiffractometry(XRD). Experimental results show that despite respective variation in heat treatment,medium and cycling number, the recover}'' rate always decreases as pre-strain increases. The declineis obvious when pre-strain is less than 2.6 percent but not so sharp when pre-strain exceeds 2.6percent. Larger pre-strain results in more than one slip system and causes intercutting of themartensite strips among martensitic variants, then leads to the decline of SME. The SME of alloyswith transformation temperatures below 347 K is larger than that of alloys beyond 361 K by 20percent - 40 percent. The recovery rate of two-step aged alloy is higher than that of step-quenchedalloy by 20 percent - 25 percent. During thermal cycling, the recovery rate in oil is better thanthat in water.展开更多
The combined effects of void size and void shape on the void growth are studied by using the classical spectrum method. An infinite solid containing an isolated prolate spheroidal void is considered to depict the void...The combined effects of void size and void shape on the void growth are studied by using the classical spectrum method. An infinite solid containing an isolated prolate spheroidal void is considered to depict the void shape effect and the Fleck-Hutchinson phenomenological strain gradient plasticity theory is employed to capture the size effects. It is found that the combined effects of void size and void shape are mainly controlled by the remote stress triaxiality. Based on this, a new size-dependent void growth model similar to the Rice-Tracey model is proposed and an important conclusion about the size-dependent void growth is drawn： the growth rate of the void with radius smaller than a critical radius rc may be ignored. It is interesting that rc. is a material constant independent of the initial void shape and the remote stress triaxiality.展开更多
Effect of chemical component on shape memory effect (SME) of Fe-Mn-Si-Ni-C-RE shape memory alloys was studied by bent measurement, thermal cycle training, SEM etc. Results of study indicate that the alloys with high M...Effect of chemical component on shape memory effect (SME) of Fe-Mn-Si-Ni-C-RE shape memory alloys was studied by bent measurement, thermal cycle training, SEM etc. Results of study indicate that the alloys with high Mn content (25%) appeare better SME, especially in lower strain. SME improves evidently when Si is higher content, especially it’s range from 3% up to 4%. But brittleness of Fe-Mn-Si-Ni-C-RE alloy increases by increasing the Si content. SME of the alloy is weakening gradually as carbon content increases under small strain (3%). But in the condition of large strain (above 6%), SME of the alloy whose carbon content ranges from 0.1 % to 0.12% shows small decreasing range, especially of alloy with the addition of compound RE.展开更多
Effect of carbon, compound RE, quenching temperature, pre-strain and recovery temperature on shape memory effect (SME) of Fe-Mn-Si-Ni-C-RE shape memory alloy was studied by bent measurement, thermal cycle training, SE...Effect of carbon, compound RE, quenching temperature, pre-strain and recovery temperature on shape memory effect (SME) of Fe-Mn-Si-Ni-C-RE shape memory alloy was studied by bent measurement, thermal cycle training, SEM etc. It was shown that the grains of alloys addition with compound RE became finer and SME increased evidently. SME of the alloy was weakening gradually as carbon content increased under small strain (3%). But in the condition of large strain (more than 6%), SME of the alloy whose carbon content range from 0.1% to 0.12% showed small decreasing range, especially of alloy with the addition of compound RE. Results were also indicated that SME was improved by increasing quenching temperature (>1000℃). The amount of thermal induced martensite increased and the relative shape recovery ratio could be increased to more than 40% after 3-4 times thermal training. The relative shape recovery ratio decreased evidently depending on rising of pre-strain. Furthermore, because speed of martensite transition was extremely great under higher tempering temperature (more than 450℃, ε → γ transition completed in 10s meanwhile the relative shape recovery ratio of the alloy increased rapidly.展开更多
文摘A six-parameter mathematical model was introduced to simulate the stress-strain hysteresis and the sinner hysteresis of polycrystalline shape memory alloys (SMAs). By the comparison with experiments of Cu-Zn-Al SMA, it was shown that the model could be used to calculate the stress-strain relations with rather good accuracy. Moreover, it was found that the six parameters introduced in this paper represented the characteristics of the stress-strain hysteresis of polycrystalline SMAs and could be used to characterize the hysteresis quantitatively.
文摘The textured shape memory alloys exhibits anisotropic because the property of single crystal is strongly orientation dependent. The effect of texture on phase transformation strain in CuZnAl shape memory sheets was investigated. The texture of parent austenite was measured by X ray goniometer and analyzed by the orientation distribution function. Subsequently, using the texture parameters and single crystal properties, the phase transformation strains at the different directions of rolling plane by the statistically averaging method were calculated. It was showed that the experimental results are agreeable with the calculated ones. It is well explained that this anisotropy of phase transformation strain is mainly caused by the crystallographic texture of the rolled sheets.
基金Project supported by the National Natural Science Foundation of China(Nos.10872065 and 50801025)the State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body(No. 60870005)the Doctor Station Fund of Institutions of Higher Learning(No.200805320023)
文摘In this paper, the analytical solution of stress field for a strained reinforcement layer bonded to a lip-shaped crack under a remote mode III uniform load and a concentrated load is obtained explicitly in the series form by using the technical of conformal mapping and the method of analytic continuation. The effects of material combinations, bond of interface and geometric configurations on interfaciai stresses generated by eigenstrain, remote load and concentrated load are studied. The results show that the stress concentration and interfaciai stresses can be reduced by rational material combinations and geometric configurations designs for different load forms.
文摘The effect of pre-strain on phase transformation of TiNi shape memory alloy film was studied by differential scanning calorimeter measurement (DSC). Compared with un-defarmed TiNi film, the reverse transformation of pre-strained specimens was elevated to a higher temperature on the first heating, but martensite and reverse transformation on subsequent thermal cycles occurred at a lower temperature. The evolution of transformation behavior in pre-strained TiNi film was related to the change of elastic strain energy, irreversible energy and internal stress field.
基金This work was supported by“863”Program under grant No.2001AA327022.
文摘A magnetic shape memory alloy with nonstoichiometric Ni50Mn27Ga23 was prepared by using melt-spinning technology. The martensitic transformation and the magnetic-field-induced strain (MFIS) of the polycrystalline melt-spun ribbon were investigated. The experimental results showed that the melt-spun ribbons underwent thermal-elastic martensitic transformation and reverse transformation in cooling and heating process and exhibited typical thermo-elastic shape memory effect. However the start temperature for martensitic transformation decreased from 286 K for as-cast alloy to 254 K for as-quenched ribbon and Curie temperature remains approximately constant. A particular internal stress induced by melt-spinning resulted in the formation of a texture structure in the ribbons, which made the ribbons obtain larger martensitic transformation strain and MFIS. The internal stress was released substantially after annealing, which resulted in a decrease of MFIS of the ribbons.
基金The project supported by the National Natural Science Foundation of China (19891180(3))Hong Kong Research Grant Committee (DAG 96/97. EG15)
文摘The strain jump across the Austenite-Martensite (A-M) interface in single crystal Cu-14wt%Al-4.12wt%Ni Shape Memory Alloys (SMAs) under uniaxial tension was studied in this paper. A crystallographic-based mechanics analysis on the formation and microstructure of the interface was performed. By using the high sensitive Moiré interference technique, the full-field deformation patterns during the transformation process were successfully recorded. The orientation of the habit plane (A-M interface) and the magnitude of the shape strain were determined precisely from the Moiré fringe patterns. The theoretical predictions on the habit plane normal and the shape strain were compared with the measured results and good agreements were obtained.
文摘The effects of microamount additions of RE (Tb, Sm) on martensitic transition, the magnetic-field-induced strain and the bending strength of highly textured polycrystalline Ni_(48)Mn_(33)Ga_(19) alloy were investigated. The experimental results show that the addition of RE elements decreases the martensitic transformation temperature and the Curie temperature. But the bending strength of Ni-Mn-Ga-RE (RE=Tb, Sm) alloys increases remarkably because of the grain refinement. As a result, Ni-Mn-Ga-RE alloys will be applied practically with higher reliability and stability due to favorable plasticity and toughness. In addition, the replacement of small amounts Ga by Tb or Sm decreases the magnetic-field-induced strain of the alloys at room temperature.
基金Financial supports by the National Natural Science Foundation of China （Nos. 11572265, 11532010）, the Excellent Youth Found of Sichuan Province （No. 2017JQ0019）
文摘The recovery force or recovery strain is an important indicator of NiTi-based shape memory alloy devices. However, the restoring force or recoverable strain is partially restrained due to an interaction between reorientation and reorientation-induced plasticity. Therefore, a macroscopic multi-mechanism constitutive model was constructed to describe the degeneration of shape memory effect based on the phase diagram. The residual strain after cooling consists of reorientation strain and reorientation-induced plastic strain. An internal variable, i.e., the detwinned stress, and its evolution equation were introduced into the transformation kinetics equation to describe the nonlinear hardening characteristics induced by the combined reorien- ration and detwinning mechanisms during mechanical loading. Finally, the proposed model was numerically implemented to simulate the experiments of shape memory effect at different peak strains. Comparisons between the experimental and simulated results show that the proposed model can reasonably describe the degeneration of shape memory effect.
文摘The mechanical behavior and the effect of pre-strain on recovery behavior of Ti50Ni47Fe3 (at. pct) alloy were investigated systematically by tensile and recovered tests accompanied by electrical resistance measurement. Ti50Ni47Fe3 alloy has different deformation behaviors at different temperature ranges, the deformation curves in different temperature range can be classified into four kinds. The start temperature of recovery increases with the increase of pre-strain. There exists an optimal deformation condition, at which the specimen exhibits maximum free recovery strain. With increasing pre-strain the recovery stress increases and reaches the maximum at 8% pre-strain. R-phase to parent transition offered about 0.2% recovery strain. With pre-strain increasing the recovery stress increases and reaches to the maximum at 8% pre-strain. The recovery stress is corresponding with the critical stress of stress-induced martensitic transformation.
文摘The dependence of high temperature shape memory effect on the pre-deformation tempera- ture of the Ti_(50)Ni_3Pd_(37) alloy has been studied.This alloy,of which the reverse transformation start temperature on heating is 620 K,has both one-way and two-way shape memory effects. The maximum shape memory strain of the ahoy may be obtained under pre-deformation in the range of 620 to 640 K.This seems to be related to the minimum recoverable strain energy emerged from the above mentioned temperature range.
基金Supported by the National Natural Science Foundation of China(61173174,61103150,61373078)the NSFC Joint Fund with Guangdong under Key Project(U1201258)the National Research Foundation for the Doctoral Program of Higher Education of China(20110131130004)
文摘Energy minimization has been widely used for constructing curve and surface in the fields such as computer-aided geometric design, computer graphics. However, our testing examples show that energy minimization does not optimize the shape of the curve sometimes. This paper studies the relationship between minimizing strain energy and curve shapes, the study is carried out by constructing a cubic Hermite curve with satisfactory shape. The cubic Hermite curve interpolates the positions and tangent vectors of two given endpoints. Computer simulation technique has become one of the methods of scientific discovery, the study process is carried out by numerical computation and computer simulation technique. Our result shows that： （1） cubic Hermite curves cannot be constructed by solely minimizing the strain energy; （2） by adoption of a local minimum value of the strain energy, the shapes of cubic Hermite curves could be determined for about 60 percent of all cases, some of which have unsatisfactory shapes, however. Based on strain energy model and analysis, a new model is presented for constructing cubic Hermite curves with satisfactory shapes, which is a modification of strain energy model. The new model uses an explicit formula to compute the magnitudes of the two tangent vectors, and has the properties： （1） it is easy to compute; （2） it makes the cubic Hermite curves have satisfactory shapes while holding the good property of minimizing strain energy for some cases in curve construction. The comparison of the new model with the minimum strain energy model is included.
文摘The effects of thermomechanical cycling on the shape memory behavior and transformation temperatures of a Ni50.2Ti49.8 alloy under a constant applied stress of 300 MPa were investigated, k is believed that thermomechanical cycling induces defects such as dislocations, which evidently affect the shape memory behavior and transformation temperatures. The recovery strain decreases with increasing number of thermomechanical cycles, whereas the irreversible plastic strain increases, especially in the initial few cycles. The stored elastic strain energy has an important influence on transformation temperatures, the A5^σ decreases and the M5^σ increases with increasing number of thermomechanical cycles. The recovery strain, irreversible plastic strain, A5^σ , and M5^σ reach a saturation value after several cycles.
文摘This paper aims to build a constitutive model intended to describe the thermomechanical behavior of shape memory alloys. This behavior presents many facets, among them we have considered the simple way of shape memory, which is one of most important properties of shape memory alloys. Because of numerous stages of this effect, the subject was divided into three independent parts. For each part, we built the corresponding thermodynamic potential and we deduced the constitutive equations. To make this model workable, we have developed an algorithm. The simulation was performed using the NiTi as shape memory alloy.
文摘Based on experimental study of the time dependence of the recoverable shape memory effect (SME)of shape memory alloys Ti50Ni50,Ti50Ni48Cu2 and Ti50Ni45Cu5 an empirical re- lationship between recoverable strain energy,U_r,and temperature,T.was derived as: U_r=U_o[1-exp(-mT_R^n)] where m—characteristic constant related to material,and n-shape-temperature sensitivity. The Johnson-Mehl-Aerami's phenomenological description of the time dependence of phase transformation was adopted to obtain a semiquantitative relationship associated with recoverable SME.between thermoelastic martensite transformation and temperature.This semiquantitative relationship is correspondent with the empirical expression.
基金This work was supported by Hong Kong ITF research project (No. ITS 098/02).
文摘SMPU （shape memory polyurethane） non-ionomers and ionomers, synthesized with poly（c-caprolactone） （PCL）, 4, 4＇-diphenylmethane diisocyanate （MDI）, 1,4-butanediol （BDO）, dimethylolpropionic acid （DMPA） were measured with cyclic tensile test and strain recovery test. The relations between the structure and shape memory effect of these two series were studied with respect to the ionic group content and the effect of neutralization. The resulting data indicate that, with the introduction of asymmetrical extender, the stress at 100% elongation is decreased for PU non-ionomer and ionomer series, especially lowered sharply for non-ionomer series; the fixation ratio of ionomer series is not affected obviously by the ionic group content; the total recovery ratio of ionomer series is decreased greatly. After sufficient relaxation time for samples stretched beforehand, the switching temperature is raised slightly, whereas the recovery ratio measured with strain recovery test method is lowered with increased DMPA content. The characterization with FT-IR, DSC, DMA elucidated that, the ordered hard domain of the two series is disrupted with the introduction of DMPA which causes more hard segments to dissolve in soft phase; ionic groups on hard segment enhance the cohesion between hard segments especially at high ionic group content and significantly facilitate the phase separation compared with the corresponding non-ionomer at moderate ionic group content.
基金Project(BE2004027) supported by the Science and Technology Foundation of Jiangsu Province,China
文摘Thermal cycle training of CuZnAl shape memory alloys with differenttransformation temperatures was carried out. The influence of different pre-strains, heat treatmentsand media on the shape memory effect(SME) of CuZnAl alloys with different transformationtemperatures was studied by means of scanning electron microscopy(SEM) and X-raydiffractometry(XRD). Experimental results show that despite respective variation in heat treatment,medium and cycling number, the recover}'' rate always decreases as pre-strain increases. The declineis obvious when pre-strain is less than 2.6 percent but not so sharp when pre-strain exceeds 2.6percent. Larger pre-strain results in more than one slip system and causes intercutting of themartensite strips among martensitic variants, then leads to the decline of SME. The SME of alloyswith transformation temperatures below 347 K is larger than that of alloys beyond 361 K by 20percent - 40 percent. The recovery rate of two-step aged alloy is higher than that of step-quenchedalloy by 20 percent - 25 percent. During thermal cycling, the recovery rate in oil is better thanthat in water.
基金The project supported by the National Natural Science Foundation of China(A10102006)the New Century Excellent Talents in Universities of China.
文摘The combined effects of void size and void shape on the void growth are studied by using the classical spectrum method. An infinite solid containing an isolated prolate spheroidal void is considered to depict the void shape effect and the Fleck-Hutchinson phenomenological strain gradient plasticity theory is employed to capture the size effects. It is found that the combined effects of void size and void shape are mainly controlled by the remote stress triaxiality. Based on this, a new size-dependent void growth model similar to the Rice-Tracey model is proposed and an important conclusion about the size-dependent void growth is drawn： the growth rate of the void with radius smaller than a critical radius rc may be ignored. It is interesting that rc. is a material constant independent of the initial void shape and the remote stress triaxiality.
文摘Effect of chemical component on shape memory effect (SME) of Fe-Mn-Si-Ni-C-RE shape memory alloys was studied by bent measurement, thermal cycle training, SEM etc. Results of study indicate that the alloys with high Mn content (25%) appeare better SME, especially in lower strain. SME improves evidently when Si is higher content, especially it’s range from 3% up to 4%. But brittleness of Fe-Mn-Si-Ni-C-RE alloy increases by increasing the Si content. SME of the alloy is weakening gradually as carbon content increases under small strain (3%). But in the condition of large strain (above 6%), SME of the alloy whose carbon content ranges from 0.1 % to 0.12% shows small decreasing range, especially of alloy with the addition of compound RE.
文摘Effect of carbon, compound RE, quenching temperature, pre-strain and recovery temperature on shape memory effect (SME) of Fe-Mn-Si-Ni-C-RE shape memory alloy was studied by bent measurement, thermal cycle training, SEM etc. It was shown that the grains of alloys addition with compound RE became finer and SME increased evidently. SME of the alloy was weakening gradually as carbon content increased under small strain (3%). But in the condition of large strain (more than 6%), SME of the alloy whose carbon content range from 0.1% to 0.12% showed small decreasing range, especially of alloy with the addition of compound RE. Results were also indicated that SME was improved by increasing quenching temperature (>1000℃). The amount of thermal induced martensite increased and the relative shape recovery ratio could be increased to more than 40% after 3-4 times thermal training. The relative shape recovery ratio decreased evidently depending on rising of pre-strain. Furthermore, because speed of martensite transition was extremely great under higher tempering temperature (more than 450℃, ε → γ transition completed in 10s meanwhile the relative shape recovery ratio of the alloy increased rapidly.
