Surveying and mapping method for critical shear stress-temperature phase diagram of tubular NiTi shape memory alloy
By conducting isothermal hyperelastic shear stress-strain curve tests on NiTi thin tubes, the critical shear stresses of martensite and austenite were determined in reverse, solving the problem of insufficient mapping accuracy in existing technologies. This enabled efficient and accurate description of phase transformation temperatures and promoted the engineering application of NiTi actuators.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- FUZHOU UNIV
- Filing Date
- 2026-02-10
- Publication Date
- 2026-05-19
AI Technical Summary
Existing technologies cannot efficiently and accurately map the critical shear stress-temperature phase diagram of tubular NiTi shape memory alloys, and cannot meet the high performance and high reliability requirements of NiTi actuators in aerospace and other fields. Existing methods have problems with systematic errors and insufficient modeling accuracy.
By conducting isothermal hyperelastic shear stress-shear strain curve tests on NiTi thin tubes, the critical shear stresses of martensite and austenite were determined inversely. A phase diagram was constructed using linear fitting and the least squares method. Data acquisition and processing were performed using a dedicated mapping system to ensure temperature field uniformity and data accuracy.
This method enables precise description of the phase transition temperature of tubular NiTi shape memory alloys, simplifies the operation process, reduces system errors, improves the accuracy and efficiency of phase diagram plotting, provides reliable data support, and shortens the development cycle for actuator design and optimization.
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Figure CN122062964A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of materials testing and characterization technology, specifically relating to a method for mapping the critical shear stress-temperature phase diagram of tubular NiTi shape memory alloys. Background Technology
[0002] NiTi shape memory alloys, due to their excellent shape memory effect, superelasticity, and biocompatibility, have become the preferred material for core components of actuators in aerospace, intelligent equipment, and other fields. Among them, tubular NiTi shape memory alloys are particularly widely used in various torsional actuators due to their advantages of lightweight structure and excellent torque transmission characteristics. The critical shear stress-temperature phase diagram is the core basis for characterizing the phase transformation mechanical properties of tubular NiTi shape memory alloys. Its accuracy directly determines the design rationality, working accuracy, and service stability of NiTi actuators. Therefore, developing efficient and accurate methods for mapping critical shear stress-temperature phase diagrams is of great significance for promoting the engineering application of NiTi shape memory alloy actuators.
[0003] Currently, in the field of phase transformation characteristic testing and phase diagram construction of NiTi shape memory alloys, related technologies mostly revolve around the positive correlation between phase diagrams and stress-strain curves. That is, by theoretically deriving or pre-setting phase diagram parameters, stress-strain curves under different conditions are mapped. While this forward research approach can analyze phase transformation laws at the theoretical level, it contradicts the needs of practical engineering testing. In engineering practice, it is easier to directly obtain experimental stress-strain curves through mechanical experiments. However, existing technologies lack a convenient method for reverse derivation from experimental stress-strain curves to critical shear stress-temperature phase diagrams. This makes it difficult to effectively convert a large amount of measured mechanical data into phase diagram parameters, hindering the rapid provision of direct data support for the structural design and performance optimization of tubular NiTi actuators.
[0004] In conventional testing of the phase transformation temperature of NiTi alloys, differential scanning calorimetry (DSC) and dynamic thermomechanical methods are relatively mature techniques. These methods mainly determine the phase transformation characteristic temperature of the alloy under stress-free or low-stress conditions through thermal or thermomechanical coupling. However, they cannot simulate the torsional load state during the actual operation of tubular NiTi actuators, making it difficult to establish a quantitative correlation between the critical phase transformation stress and ambient temperature. Furthermore, these conventional testing methods are prone to systematic errors in phase transformation temperature measurement due to factors such as insufficient uniformity of the sample temperature field, instrument signal acquisition deviations, and heat loss during the testing process. The measured results can only reflect the phase transformation law under stress-free conditions and cannot meet the characterization requirements of the phase transformation properties of tubular NiTi alloys under actual loading conditions.
[0005] Regarding the research on constructing stress-temperature phase diagrams for NiTi alloys through reverse modeling, a few existing studies have reported inverse analysis methods based on constraint restoring force curves. These methods attempt to deduce the correlation between phase transformation stress and temperature through modeling and analyzing mechanical curves. However, most of them use cosine-like functions for fitting the numerical description of martensite volume fraction. In reality, the martensitic phase transformation process of NiTi alloys is affected by multiple factors such as stress state, temperature change, and material microstructure. The variation law of its volume fraction is not an ideal cosine curve characteristic. Using cosine-like functions for numerical description can easily lead to overfitting or underfitting problems in mechanical curve modeling, causing a systematic deviation between the modeled phase transformation temperature and the actual true value. The critical shear stress-temperature phase diagrams constructed based on this type of model have insufficient accuracy and cannot meet the design and performance evaluation requirements of high-precision NiTi actuators.
[0006] In summary, existing phase transformation testing and phase diagram construction techniques for NiTi shape memory alloys have significant shortcomings in terms of ease of reverse derivation, adaptability to stress conditions, and accuracy of modeling and analysis. In particular, for the torsional load characteristics of tubular NiTi shape memory alloys, there is currently no technical method that can accurately and efficiently map their critical shear stress-temperature phase diagram. This makes it difficult to meet the development requirements of high-performance and high-reliability NiTi actuators in high-end equipment fields such as aerospace. The optimization and innovation of related testing methods have become urgent technical problems to be solved in this field. Summary of the Invention
[0007] Aiming at the defects and deficiencies existing in the prior art, the present invention provides a method for mapping the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy and a mapping system for implementing the method. This method relies on the superelastic mechanical properties of the NiTi thin tube after complete austenitization. By means of closed-loop temperature control, the NiTi thin tube is heated above the austenite phase transformation end temperature and kept at a constant temperature under multiple temperature gradients, so that the specimen is in a superelastic state. Subsequently, a torsional loading-unloading test is applied to the NiTi thin tube under each constant temperature condition, and a predetermined unified maximum shear strain is loaded at all temperature gradients, and this shear strain is greater than the shear strain corresponding to the martensite phase transformation end shear stress to ensure complete phase transformation of stress-induced martensite. At the same time, the corresponding superelastic shear stress-shear strain curves at each temperature are collected. The present invention divides the collected curves into a loading section and an unloading section, and reversely determines the martensite start, martensite end, austenite start, and austenite end critical shear stresses at each temperature based on the linear characteristics and geometric characteristics of the curves. Specifically, by linearly fitting the austenite elastic deformation stage and the stress-induced martensite phase transformation stage of the loading section, and adopting a local linear sampling method for the latter to determine the martensite start critical shear stress by fitting the intersection point; based on the characteristics that the maximum residual strain of the NiTi thin tube is consistent at different temperatures and the unloading section of martensite is a linear section with a constant slope, a cross-temperature unified linear model is constructed, and the martensite end critical shear stress is determined by combining the least squares linear regression; the austenite start critical shear stress is identified by calculating the first derivative of the unloading section curve to identify the inflection point, and the inflection point coordinates at all temperatures are linearly fitted to obtain the idealized slope and trajectory of the austenite phase transformation stage; the fitting line segment of the austenite reverse phase transformation stage of the unloading section under the condition that the temperature is higher than the austenite phase transformation end temperature is selected, and the theoretical intersection point is obtained with the fitting line segment of the austenite elastic deformation stage of the loading section at the same temperature to determine the austenite end critical shear stress. Finally, the four critical shear stresses at each temperature are linearly fitted with the test temperature respectively to construct and generate the critical shear stress-temperature phase diagram of the NiTi thin tube. The mapping system supporting the present invention includes three major modules: test and measurement, temperature control, and data acquisition and processing. Among them, the fixture assembly of the test and measurement module uses a coupling with interference fit with the NiTi thin tube, and a stepped shaft is provided in the thin tube to provide radial support. The outer surface of the NiTi thin tube is treated with knurled mesh pattern, and the coupling is equipped with accessories with different inner diameter tolerance specifications, which can be replaced according to the wear condition of the mesh pattern to avoid radial sliding; the temperature control module realizes rapid and accurate temperature control and ensures uniform temperature field of the tube body; the data acquisition and processing module can collect the shear stress-shear strain curve and temperature signal, perform data analysis operations such as curve feature recognition, reverse determination of critical shear stress, and phase diagram generation, and can exclude noise-like abnormal data through algorithms during the curve processing process to achieve batch and stable recognition of inflection points and characteristic stages under different curve forms.This invention pioneers a testing method that inversely maps the isothermal hyperelastic shear stress-shear strain curve to a critical stress-temperature phase diagram. This avoids the overfitting and underfitting problems caused by existing technologies that use cosine-like functions to describe the martensite volume fraction, significantly reducing the systematic error in phase transformation temperature testing. Simultaneously, the unified maximum shear strain experimental design simplifies the operation process, improving the operability and efficiency of the experiment. Furthermore, this method has low dependence on specialized equipment, a short testing cycle, and can accurately construct the linear relationship between critical phase transformation stress and ambient temperature. The corresponding mapping system and testing method are highly compatible, and the collaborative work of each module ensures the accuracy of the test data and the precision of the phase diagram. The mapped critical shear stress-temperature phase diagram can accurately describe the phase transformation temperature of tubular NiTi shape memory alloys under different load levels, providing reliable data support for the design, optimization, and performance evaluation of tubular NiTi shape memory alloy actuators. This can effectively shorten the development cycle of NiTi actuators and promote their application in the aerospace industry.
[0008] The specific technical solution adopted by this invention to solve its technical problem is as follows:
[0009] A method for mapping the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy, comprising:
[0010] The NiTi thin tube was heated to the austenite phase transformation end temperature A. f The above achieves complete austenitization and maintains constant temperature under multiple temperature gradients, making the NiTi thin tube in a superelastic state;
[0011] Torsion tests were performed on the NiTi thin tube under various isothermal conditions. The NiTi thin tube was loaded to a predetermined uniform maximum shear strain at each temperature to obtain the corresponding hyperelastic shear stress-shear strain curves at each temperature.
[0012] Based on the geometric and differential characteristics of each shear stress-shear strain curve, the critical shear stresses for the start, end, austenite, and austenite termination at each temperature are determined in reverse.
[0013] The four critical shear stresses at each temperature were linearly fitted with the test temperature to construct a linear relationship between the critical phase transformation shear stress of NiTi thin tubes and the ambient temperature, thus generating a critical shear stress-temperature phase diagram.
[0014] Furthermore, the inverse determination of the critical shear stress based on geometric and differential characteristics specifically includes:
[0015] Based on the loading segment of the curve, the critical shear stress at the start of martensite is determined by the intersection of the linear fitting of the austenite elastic deformation stage and the stress-induced martensite phase transformation stage.
[0016] Based on the unified maximum shear strain and the linear characteristics of the martensite unloading segment, the critical shear stress at the end of martensite is determined by a unified linear model across temperatures.
[0017] Identify the inflection point by the change of the first derivative of the unloading section of the curve, and determine the critical shear stress at which the austenite begins to be subjected to the stress.
[0018] The critical shear stress at the end of austenite formation is determined based on the intersection of the linear fitting theory between the austenite reverse phase transformation stage of the unloading section and the austenite elastic deformation stage of the loading section.
[0019] Furthermore, the predetermined uniform maximum shear strain is greater than the shear strain corresponding to the martensitic transformation termination shear stress at each test temperature, so as to ensure the complete stress-induced martensitic transformation of the NiTi thin tube.
[0020] Furthermore, the unified linear model across temperatures is constructed based on the characteristics of the NiTi thin tube having consistent maximum residual strain at different temperatures and the martensite unloading segment being a linear segment.
[0021] Furthermore, when determining the critical shear stress at the start of martensite based on the intersection of the linear fitting of the austenite elastic deformation stage and the stress-induced martensitic transformation stage, the stress-induced martensitic transformation stage is fitted using a local linear sampling method.
[0022] Furthermore, after identifying the inflection point and determining the critical shear stress at the start of austenite, the coordinates of the inflection point at all temperatures are linearly fitted to obtain the idealized slope and trajectory of the austenite phase transformation stage.
[0023] Furthermore, when the critical shear stress of the austenite is determined, the fitting line segment of the austenite reverse phase transformation stage is selected, and the theoretical intersection point is obtained with the fitting line segment of the austenite elastic deformation stage of the loading stage at the same temperature.
[0024] Furthermore, the uniform maximum shear strain remains consistent across all temperature gradients to ensure that the stress-induced martensitic transformation of NiTi thin tubes is comparable at all temperatures.
[0025] Furthermore, a system for mapping the critical shear stress-temperature phase diagram of tubular NiTi shape memory alloys using the mapping method described above, characterized in that it comprises:
[0026] A clamping assembly for clamping NiTi thin tubes, the clamping assembly including couplings that are interference-fitted with both ends of the NiTi thin tubes, and a stepped shaft disposed in the inner hole of the NiTi thin tubes to provide radial support;
[0027] The testing machine, connected to the fixture assembly, is used to apply torsional loads to NiTi thin tubes and perform torsional tests.
[0028] A temperature control device, coupled to the clamp assembly, is used to heat the NiTi thin tube to above the austenitic phase transformation end temperature Af and maintain a constant temperature under multiple temperature gradients, so that the NiTi thin tube is in a superelastic state.
[0029] The data acquisition and processing unit is connected to the testing machine and the temperature control device, respectively, and is used to acquire shear stress-shear strain curves and temperature signals, and perform data analysis operations such as curve feature recognition, inverse determination of critical shear stress, and generation of phase diagrams.
[0030] Furthermore, the outer surface of the NiTi thin tube is knurled, and the coupling is equipped with accessories with different inner diameter tolerances. During the test, a coupling with a smaller tolerance can be replaced according to the wear of the knurling on the surface of the NiTi thin tube to avoid radial slippage between the NiTi thin tube and the coupling.
[0031] Compared to existing technologies, this invention and its preferred scheme pioneer a testing method that inversely maps the isothermal hyperelastic shear stress-strain curve of a NiTi thin tube to a critical stress-temperature phase diagram. This effectively solves the problem that existing technologies cannot conveniently obtain this phase diagram from experimental stress-strain curves, successfully constructing a quantitative correlation between the critical phase transformation stress and ambient temperature of tubular NiTi shape memory alloys, thus overcoming the limitation of conventional testing methods that can only obtain phase transformation temperatures under stress-free conditions. Through innovative data analysis and fitting methods, this invention avoids the overfitting and underfitting problems caused by using cosine-like functions to describe the martensite volume fraction in existing inverse modeling, significantly reducing the systematic error in phase transformation temperature testing, making the characterized phase transformation parameters more closely match the actual true values, and significantly improving the accuracy of the critical shear stress-temperature phase diagram. In terms of experimental design, this invention adopts a torsional loading method with a unified maximum shear strain under multiple temperature gradients, simplifying the cumbersome process of setting the phase transformation termination strain separately for different temperatures, improving the convenience and efficiency of experimental operation, while ensuring the comparability of the stress-induced martensitic phase transformation degree at different temperatures. This invention designs specific methods for determining four critical shear stresses. Through local linear sampling, construction of a unified linear model across temperatures, identification of inflection points using the first derivative, and fitting of theoretical intersection points, it can not only objectively and accurately determine the phase transformation characteristic points at each temperature, but also accurately calculate the austenite termination critical shear stress that does not directly appear in the measured curves. This achieves a complete description and characterization of the phase transformation behavior of tubular NiTi shape memory alloys. The mapping system and testing methods designed in conjunction with this invention are highly compatible. The optimized tooling structure design of the experimental testing module effectively avoids the problems of tube extrusion deformation and radial slippage during testing. The closed-loop temperature control module ensures the uniformity of the temperature field in the NiTi thin tube. The collaborative work of each module provides a reliable guarantee for the accuracy of the test data, significantly improving the stability and repeatability of the entire mapping process. Furthermore, the testing method of this invention has low dependence on specialized equipment, short testing cycle, and better overall implementation cost. The measured critical shear stress-temperature phase diagram can accurately reflect the phase transformation law of tubular NiTi shape memory alloy under different load levels, providing solid and reliable data support for the design, optimization, and performance evaluation of tubular NiTi shape memory alloy actuators. It can effectively shorten the development cycle of such actuators and provide strong technical support for their engineering applications in aerospace and other fields. Attached Figure Description
[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:
[0033] Figure 1 This is a schematic diagram of the experimental equipment and tooling structure for NiTi thin tubes according to an embodiment of the present invention;
[0034] Figure 2This is a schematic diagram of the temperature control system according to an embodiment of the present invention;
[0035] Figure 3 This is a schematic diagram of the Matlab fitting process in an embodiment of the present invention;
[0036] Figure 4 This is a schematic diagram of the fitting of the critical shear stress at the start of martensite in an embodiment of the present invention;
[0037] Figure 5 This is a schematic diagram of the fitting of the critical shear stress at the end of martensite formation in an embodiment of the present invention;
[0038] Figure 6 This is a schematic diagram of the fitting of the critical shear stress at the beginning of austenite in an embodiment of the present invention;
[0039] Figure 7 This is a schematic diagram of the fitting of the critical shear stress at the end of austenite formation in an embodiment of the present invention;
[0040] Figure 8 This is a schematic diagram of the critical stress-temperature phase diagram fitting in an embodiment of the present invention. Detailed Implementation
[0041] To make the features and advantages of the present invention more apparent and understandable, specific embodiments are described below in detail:
[0042] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.
[0043] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.
[0044] This invention discloses a method for measuring and plotting the critical shear stress-temperature phase diagram of NiTi shape memory alloys after complete austenitization, utilizing the hyperelastic mechanical properties of NiTi thin tubes. This method significantly reduces the systematic error in phase transformation temperature testing compared to differential scanning calorimetry (DSC), dynamic thermomechanical methods (DMA), and other inverse modeling methods based on mechanical load curves. By establishing a linear relationship between the critical phase transformation shear stress and ambient temperature to plot the phase diagram, the phase transformation temperature of tubular NiTi shape memory alloys under different load levels can be accurately described. The proposed testing method is convenient to operate, has a short testing cycle, and low dependence on specialized equipment, which can shorten the development cycle of NiTi actuators and further promote the application of NiTi shape memory alloy actuators in the aerospace industry.
[0045] The invention presents a novel testing method that maps the isothermal hyperelastic shear stress-strain curves of NiTi thin tubes at different temperatures in reverse to the critical stress-temperature phase diagram, which can conveniently obtain the critical stress-temperature phase diagram.
[0046] This invention utilizes the hyperelastic mechanical properties of NiTi thin tubes after complete austenitization to measure and plot their critical shear. This invention avoids the systematic errors caused by using numerical descriptions of martensite volume fraction with cosine-like functions by using inverse modeling of hyperelastic curves.
[0047] The implementation process of the present invention will be further illustrated and described below with reference to the accompanying drawings:
[0048] 1. Test module
[0049] like Figure 1 As shown, its core components include a torque testing machine (1), a coupling (2), a NiTi thin tube (5), a stepped shaft (6), and a nut (7). The NiTi tube and the coupling are connected by an interference fit to ensure reliable fastening and transmission of large torque, and to avoid radial relative slippage. The two ends of the coupling are milled into flat surfaces to facilitate stable clamping by the vise clamp of the testing machine. The stepped shaft (6) is placed inside the NiTi thin tube to provide radial support and prevent the tube from being squeezed and deformed under stress. The tail end of the stepped shaft is fixed with a nut (7) to prevent it from sliding down.
[0050] 2. Temperature control module
[0051] The temperature control system operates according to the following closed-loop process, such as... Figure 2As shown. The host computer (14) first sets the target temperature and sends a control command to the NI acquisition card (13); the NI acquisition card then drives the relay (15) to turn on, so that the heating film (12) uniformly covering the outer surface of the NiTi tube starts to work and heats the NiTi tube; the temperature sensor (11) integrated in the heating area monitors the temperature of the NiTi tube in real time and feeds back the resistance voltage signal it collects to the NI acquisition card. The NI acquisition card converts the signal into a temperature value and uploads it to the host computer, thus forming a complete closed-loop control circuit.
[0052] 3. Data Acquisition and Processing Module
[0053] Using the angle and torque sensors integrated into the torque tester, shear stress-shear strain curves were output. The experimental data were then fitted using Matlab software to output four critical phase transformation shear stresses and a critical shear stress-temperature phase diagram for NiTi thin tubes. The data fitting and critical shear stress determination methods are as follows:
[0054] The critical shear stress at the start of martensite formation: At different temperatures, the austenitic elastic loading segments of NiTi thin tubes largely overlap, indicating that the elastic modulus of austenite is temperature insensitive. The inflection point of the curve increases linearly with increasing temperature due to the critical torque at the start of martensitic transformation. Simultaneously, the slope of the stress-induced martensitic transformation plateau also increases with increasing temperature, mainly due to the differences in the martensite content and the deformation resistance of residual austenite at different temperatures. For fitting, only local linear sampling of the stress-induced martensite segment is needed; the ordinate value of the intersection point of this line with the fitted austenitic elastic segment is the critical shear stress at the start of martensite formation.
[0055] Martensite Termination Critical Shear Stress Fitting: In practical experiments, setting different martensite phase transformation termination strains for each temperature would lead to cumbersome procedures and poor operability. Therefore, this invention applies a uniform torsion angle sufficient to ensure complete stress-induced martensite phase transformation to samples at all temperatures. This uniform torsion angle can be determined through a preliminary experiment at the highest isothermal temperature. In the preliminary experiment, the torsion angle is increased sequentially. When the torsion angle increases to the point where the loading curve shows a clear second inflection point, i.e., the martensite termination critical point, this angle is sufficient to ensure complete stress-induced martensite phase transformation at all set temperatures. Based on the maximum residual strain of NiTi tubes at each temperature... Consistent, and the martensitic unloading segment has a slope of K. M Based on experimental phenomena, the theory of constant linear segments allows for the establishment of a coordinate linear model of the critical shear stress point at which martensite terminates at different temperatures. After determining the ordinate values of the inflection points at different temperatures, which represent the critical shear stress at which martensite terminates, the least squares method is used to uniformly fit all inflection points, thereby determining the slope and x-intercept of the linear relationship.
[0056] Fitting the critical shear stress at the start of austenite transformation: Ideally, according to the conservation principle of complete phase transformation, the austenitic transformation stage should have the same slope as the stress-induced martensitic transformation stage. However, in actual experiments, various factors, such as torque state and material microstructure, affect the slopes of the two stages, causing them to not completely overlap but exhibit slight differences. Partial phase transformation belongs to the category of small cycles, and the slope of its austenitic transformation stage is consistent with that of the large cycle of complete phase transformation. The first derivative of different shear strain test curves is calculated, and the ordinate value of the inflection point of all curves is determined as the critical shear stress at the start of austenite transformation. Linear fitting is performed on the coordinates of all inflection points to finally obtain the idealized slope and trajectory of the austenitic transformation stage. As a preferred method, the first derivative is calculated using the central difference method in numerical differentiation. The threshold for judging the inflection point is: the value of the first derivative is 0 and the value of the second derivative changes sign. At the same time, combined with the actual trend of the test curve, false inflection points caused by data noise are eliminated.
[0057] The fitting of the critical shear stress at the end of austenite formation: its coordinates are defined as the intersection of the fitting curve of the austenite phase transformation in the unloading section and the fitting curve of the elastic austenite formation in the loading section. This point is a theoretical reference value, as it does not directly appear in the measured curves due to plastic deformation or residual martensite in NiTi tubes. The austenite phase transformation torque plateau increases with increasing temperature, a trend consistent with the stress-induced martensitic phase transformation torque plateau; therefore, the ordinate value of this point, i.e., the critical shear stress at the end of austenite formation, also increases accordingly.
[0058] Critical shear stress-temperature phase diagram fitting: The critical shear stress at each temperature is linearly fitted, and the critical shear stress-temperature phase diagram of NiTi thin tube can be finally drawn.
[0059] Based on the design of the above key modules, the process of implementing the present invention in this embodiment is as follows: temperature setting and heating → sample loading and data acquisition → data processing and fitting → result output. Specifically, a heating film is uniformly coated on the outer surface of the NiTi tube, and the NiTi tube is heated to the austenite phase transformation end temperature (A) by a temperature control module. f The above describes setting several different temperature gradients to maintain a constant temperature. (As a preferred implementation in this embodiment, the temperature gradient settings need to cover the thermally induced phase transformation temperature range of the NiTi thin tube, and all temperature points are higher than the austenite phase transformation end temperature A.) fThe interval between adjacent temperature points can be reasonably set according to the accuracy requirements of phase diagram mapping, with a typical interval of 5~20℃. The assembled NiTi tube is clamped in the vise of a torsion testing machine. First, a forward load is applied to induce deformation; finally, a reverse load is applied until the stress reaches zero. Several different deformation values are set, with the maximum deformation value determined through preliminary experiments. In these experiments, the deformation value corresponding to the shear stress at the end of the martensitic phase transformation of the NiTi thin tube at the highest set temperature is measured, and the shear stress-strain curve is recorded and saved. The purpose of setting several different deformation values for preliminary experiments is to verify and determine the deformation value that ensures complete stress-induced martensitic phase transformation at the highest isothermal temperature through torsion tests with different deformation values. Ultimately, this deformation value is used as the unified maximum shear strain under all temperature gradients, simplifying and standardizing the experimental procedure.
[0060] The core and key point of this invention lies in the data fitting method based on Matlab, the specific process of which is as follows: Figure 3 As shown.
[0061] Step S1 divides all input test curves into two parts: the "loading segment" and the "unloading segment".
[0062] Please refer to step S2. Figure 4 For each curve's loading segment, the program first identifies the initial linear portion (elastic loading stage) and performs linear fitting. For each curve's loading segment, the program first identifies and extracts experimental data from the elastic loading stage (where the rate of change of shear stress with shear strain increases rapidly) and a portion of the stress-induced martensitic transformation stage (where the rate of change of shear stress with shear strain decreases significantly and remains essentially stable). Then, it performs linear fitting on the curves for both stages. The ordinate of the intersection of the two linearly fitted curves represents the critical shear stress at which martensite begins to form at that temperature.
[0063] Please refer to the S3 steps. Figure 5 After batch identifying the second inflection point of stress-induced martensitic transformation in test curves at different temperatures during maximum deformation, the program summarizes the coordinates of this second inflection point, i.e., the critical point for the end of martensite formation. Subsequently, it uses the least squares method to perform linear regression on these points, based on the consistency of maximum residual strain at different temperatures. The theory establishes a unified linear model. This model allows for the precise determination of the martensitic final critical shear stress at various temperatures.
[0064] Please refer to step S4. Figure 6 The inflection point locations under all strains are determined, and their ordinates are the critical shear stress at the start of austenite. The coordinates of all inflection points are linearly fitted, and finally the idealized slope and trajectory of the austenite phase transformation stage under different deformations are obtained.
[0065] Please refer to the S5 steps. Figure 7Take the fitted line segment of the austenite phase transformation stage in step S4 and the fitted line segment of the austenite elastic deformation stage at the same temperature obtained in step S2. The intersection of the two fitted line segments is defined as the ideal critical point for the end of the austenite phase transformation, and the ordinate of the intersection point is the ideal value of the critical shear stress for the end of the austenite phase transformation.
[0066] Please refer to step S6. Figure 8 The critical shear stress-temperature phase diagrams are obtained by linearly fitting the above-mentioned critical shear stress at the start of martensite, critical shear stress at the end of martensite, critical shear stress at the start of austenite, and critical shear stress at the end of austenite.
[0067] Compared with existing technologies, this invention mainly improves the experimental operation and fitting method for drawing the critical shear stress-temperature phase diagram of NiTi thin tubes, thereby improving the convenience and accuracy of phase diagram drawing and providing reliable data support for the design, optimization and performance evaluation of tubular NiTi shape memory alloy actuators.
[0068] This invention creatively applies the same shear strain to specimens under all temperature conditions, simplifying the test procedure and improving test efficiency and operability.
[0069] This invention objectively determines the inflection point of all curves by calculating the first derivative of different experimental curves, and uses the least squares method to uniformly fit the experimental curves to establish a unified linear model, thus avoiding the systematic error caused by using a numerical description of martensite volume fraction with a cosine-like function.
[0070] This invention, through linear fitting, can accurately determine the coordinates of the austenite termination critical torque point, even if it does not appear directly in the measured curve, thus fully describing the phase transformation behavior.
[0071] It should be noted that, in implementing the solutions provided above by this invention, the following points should also be noted:
[0072] 1) The temperature control module is required to heat the NiTi thin tube quickly and accurately and stabilize it at the target temperature, and ensure the uniformity of the temperature field of the tube body to avoid asynchronous phase transitions caused by temperature gradients, which would affect the subsequent fitting quality.
[0073] 2) After the outer surface of the NiTi thin tube is knurled, the knurling is prone to wear as the number of tests increases. It is necessary to replace it with a coupling with a smaller tolerance specification in time to avoid slippage caused by high shear stress during the test.
[0074] 3) The fitting algorithm should automatically eliminate abnormal data such as noise, especially when batch processing and automatically identifying inflection points. It should ensure that feature points can be stably and accurately located under various curve shapes. For example, the processing of noise-type abnormal data adopts a two-step method. First, the shear stress-shear strain curve is smoothed by moving average filtering to eliminate high-frequency noise. Then, the 3σ criterion is used to remove abnormal data points. That is, when the deviation of a data point from the average value of the set of data exceeds 3 times the standard deviation, it is judged as an abnormal data point and removed.
[0075] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.
[0076] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.
[0077] This invention is not limited to the preferred embodiment described above. Anyone inspired by this invention can derive various other methods for mapping the critical shear stress-temperature phase diagram of tubular NiTi shape memory alloys. All equivalent variations and modifications made within the scope of the claims of this invention should be included within the scope of this invention.
Claims
1. A method for mapping the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy, characterized in that, include: The NiTi thin tube was heated to the austenite phase transformation end temperature A. f The above achieves complete austenitization and maintains constant temperature under multiple temperature gradients, making the NiTi thin tube in a superelastic state; Torsion tests were performed on the NiTi thin tube under various isothermal conditions. The NiTi thin tube was loaded to a predetermined uniform maximum shear strain at each temperature to obtain the corresponding hyperelastic shear stress-shear strain curves at each temperature. Based on the geometric and differential characteristics of each shear stress-shear strain curve, the critical shear stresses for the start, end, austenite, and austenite termination at each temperature are determined in reverse. The four critical shear stresses at each temperature were linearly fitted with the test temperature to construct a linear relationship between the critical phase transformation shear stress of NiTi thin tubes and the ambient temperature, thus generating a critical shear stress-temperature phase diagram.
2. The method for mapping the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy according to claim 1, characterized in that: The inverse determination of critical shear stress based on geometric and differential characteristics specifically includes: Based on the loading segment of the curve, the critical shear stress at the start of martensite is determined by the intersection of the linear fitting of the austenite elastic deformation stage and the stress-induced martensite phase transformation stage. Based on the unified maximum shear strain and the linear characteristics of the martensite unloading segment, the critical shear stress at the end of martensite is determined by a unified linear model across temperatures. Identify the inflection point by the change of the first derivative of the unloading section of the curve, and determine the critical shear stress at which the austenite begins to be subjected to the stress. The critical shear stress at the end of austenite formation is determined based on the intersection of the linear fitting theory between the austenite reverse phase transformation stage of the unloading section and the austenite elastic deformation stage of the loading section.
3. The method for plotting the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy according to claim 1, characterized in that: The predetermined uniform maximum shear strain is greater than the shear strain corresponding to the martensitic phase transformation termination shear stress at each test temperature, so as to ensure the complete stress-induced martensitic phase transformation of NiTi thin tubes.
4. The method for mapping the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy according to claim 2, characterized in that: The unified linear model across temperatures is constructed based on the characteristics of NiTi thin tubes having consistent maximum residual strain at different temperatures and the martensite unloading segment being a linear segment.
5. The method for mapping the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy according to claim 2, characterized in that: When determining the critical shear stress at the start of martensite based on the intersection of the linear fit between the austenite elastic deformation stage and the stress-induced martensite transformation stage, the stress-induced martensite transformation stage is fitted using a local linear sampling method.
6. The method for mapping the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy according to claim 2, characterized in that: After identifying the inflection point and determining the critical shear stress at which austenite begins, linear fitting is performed on the coordinates of the inflection point at all temperatures to obtain the idealized slope and trajectory of the austenite phase transformation stage.
7. The method for mapping the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy according to claim 2, characterized in that: When determining the critical shear stress at the end of the austenite phase transformation, the fitting line segment of the austenite reverse phase transformation stage is selected, and the theoretical intersection point is obtained with the fitting line segment of the austenite elastic deformation stage of the loading stage at the same temperature.
8. The method for plotting the critical shear stress-temperature phase diagram of a tubular NiTi shape memory alloy according to claim 1, characterized in that: The uniform maximum shear strain remains consistent across all temperature gradients to ensure that the stress-induced martensitic transformation of NiTi thin tubes is comparable at all temperatures.
9. A system for mapping the critical shear stress-temperature phase diagram of tubular NiTi shape memory alloys, implementing the mapping method of any one of claims 1-8, characterized in that, include: A clamping assembly for clamping NiTi thin tubes, the clamping assembly including couplings that are interference-fitted with both ends of the NiTi thin tubes, and a stepped shaft disposed in the inner hole of the NiTi thin tubes to provide radial support; The testing machine, connected to the fixture assembly, is used to apply torsional loads to NiTi thin tubes and perform torsional tests. A temperature control device, coupled to the clamp assembly, is used to heat the NiTi thin tube to above the austenitic phase transformation end temperature Af and maintain a constant temperature under multiple temperature gradients, so that the NiTi thin tube is in a superelastic state. The data acquisition and processing unit is connected to the testing machine and the temperature control device, respectively, and is used to acquire shear stress-shear strain curves and temperature signals, and perform data analysis operations such as curve feature recognition, inverse determination of critical shear stress, and generation of phase diagrams.
10. The critical shear stress-temperature phase diagram mapping system for tubular NiTi shape memory alloys according to claim 9, characterized in that, The outer surface of the NiTi thin tube is knurled, and the coupling is equipped with accessories with different inner diameter tolerances. During the test, the coupling with a smaller tolerance can be replaced according to the wear of the knurling on the surface of the NiTi thin tube to avoid radial slippage between the NiTi thin tube and the coupling.