Shape memory strain testing method of shape memory alloy material

Abstract

The invention provides a shape memory strain testing method of a shape memory alloy material, which comprises the following steps of: firstly, acquiring the phase transformation end temperature of the material by a differential scanning calorimetry method, and then respectively preserving heat for a certain time at two constant temperature points which are 5-30 DEG C higher than the austenite phase transformation end temperature and 5-30 DEG C lower than the martensite phase transformation end temperature; measuring the size of the material; and finally calculating the shape memory strain based on the size difference of the two phases. According to the method, a test mode that continuous temperature change is needed in traditional dynamic thermomechanical analysis is abandoned, a two-point measurement method is adopted, the test efficiency is greatly improved, meanwhile, it is ensured that the material is in a pure phase state, and the measurement accuracy is improved; the method is not limited by material forms, is suitable for wires, pipes, plates and special-shaped components, can flexibly select whether to apply loads or not, truly reflects the strain capacity of the materials under actual working conditions, and provides an efficient and reliable test means for performance evaluation and device design of the shape memory alloy.

Description

Technical Field

[0001] This invention relates to the field of shape memory alloy material testing technology, and specifically to a method for testing the shape memory strain of shape memory alloy materials. Background Technology

[0002] Shape memory alloys possess unique shape memory effects and superelasticity, making them widely applicable in aerospace, biomedicine, and precision drive fields. Shape memory strain (or recoverable strain) is a key parameter for measuring their shape memory performance, directly affecting the electrical control parameters, structural design, and operational reliability of shape memory alloy devices.

[0003] Currently, there is a lack of unified and efficient methods for testing the shape memory strain of shape memory alloys. Relevant standards only define two-way memory recovery strain but do not specify concrete testing methods and operational details, resulting in a lack of standardized guidelines for practical testing. In existing technologies, some studies use dynamic thermomechanical analysis (DMA) to obtain shape memory strain through temperature-strain curves. However, this method typically requires extremely low heating and cooling rates (e.g., below 3K / min), leading to lengthy testing times and low efficiency, making it difficult to meet the needs of rapid evaluation for engineering applications. Furthermore, existing testing methods have poor adaptability to sample shapes and sizes, making them unsuitable for shape memory alloys with different forms, such as wires, tubes, and irregularly shaped components. Inaccurate temperature control or unclear phase transformation determination during testing can easily lead to shape memory strain measurement results deviating from the true value, thus affecting material performance evaluation and the rationality of device design.

[0004] Inaccurate shape memory strain values ​​used in device design can lead to serious problems: if the design value exceeds the material's actual strain capacity, excessive driving current is often required to achieve the target, causing microstructural damage due to instantaneous overheating and accelerating fatigue performance degradation; if the design value is far less than the material's actual potential, the material's effectiveness cannot be fully realized, resulting in device volume redundancy and reduced space utilization. Therefore, developing a method for rapidly and accurately measuring shape memory strain is of great significance for both material evaluation and device engineering design. Summary of the Invention

[0005] The purpose of this invention is to provide a method for testing the shape memory strain of shape memory alloy materials. This method is less restricted by the shape and size of the measured material, has high adaptability, strong operability and execution, and high efficiency.

[0006] To achieve the above objectives, the present invention proposes the following technical solution:

[0007] A method for testing the shape memory strain of a shape memory alloy material, wherein the shape memory alloy material has a first phase in a high-temperature state and a second phase in a low-temperature state;

[0008] The testing method includes the following steps:

[0009] (1) Obtain the first phase transformation end temperature and the second phase transformation end temperature of the shape memory alloy material;

[0010] (2) Hold the material at a first preset temperature above the first phase transition end temperature for a first preset time to allow the shape memory alloy material to completely transform into the first phase, and measure the first dimension of the shape memory alloy material when it is in the first phase;

[0011] (3) Keep the material at a second preset temperature below the end temperature of the second phase transformation for a second preset time to allow the shape memory alloy material to completely transform into the second phase, and measure the second dimension of the shape memory alloy material when it is in the second phase and at the same position as in step (2);

[0012] (4) Calculate the shape memory strain of the shape memory alloy material based on the first dimension and the second dimension.

[0013] As a preferred embodiment of the present invention, the first preset temperature is set to be 5 to 30°C higher than the first phase transition end temperature.

[0014] As a preferred embodiment of the present invention, the second preset temperature is set to be 5 to 30°C lower than the second phase transition end temperature.

[0015] As a preferred embodiment of the present invention, in step (1), obtaining the first phase transition end temperature and the second phase transition end temperature includes:

[0016] (1.1) The shape memory alloy material was tested using differential scanning calorimetry. The test process included at least one complete thermal cycle, and DSC curves were obtained.

[0017] (1.2) Determine the first phase transition end temperature and the second phase transition end temperature based on the obtained DSC curve.

[0018] As a preferred embodiment of the present invention, the method for calibrating the first phase transition end temperature and the second phase transition end temperature includes at least one of the tangent method and the extrapolation method.

[0019] As a preferred embodiment of the present invention, when using the differential scanning calorimetry method, both the heating rate and the cooling rate are 5 ≤ V. T ≤10 K / min.

[0020] As a preferred embodiment of the present invention, the testing process includes at least one complete thermal cycle, comprising:

[0021] Perform 2 to 3 complete thermal cycles, and take the temperature at which the first phase transition ends and the temperature at which the second phase transition ends during the first thermal cycle as the first phase transition end temperature and the second phase transition end temperature, respectively.

[0022] Alternatively, the average temperature at which the first phase transition ends after multiple thermal cycles and the average temperature at which the second phase transition ends can be taken as the first phase transition end temperature and the second phase transition end temperature, respectively.

[0023] As a preferred embodiment of the present invention, the first preset time is 10-30 minutes; the second preset time is 10-30 minutes. The first and second preset times are set based on the principle of holding the shape memory alloy material at the corresponding phase transformation temperature, thereby completing the corresponding phase transformation within the preset time.

[0024] As a preferred technical solution of the present invention, during the measurement of the first dimension and / or the second dimension, a preset load may be applied to the shape memory alloy material according to the application or requirements, or no load may be applied.

[0025] If no load is applied, the first preset temperature is set to be 5-10°C higher than the first phase transition end temperature, and the second preset temperature is set to be 5-10°C lower than the second phase transition end temperature.

[0026] If a load is applied, the first preset temperature is set to be 10-30°C higher than the first phase transition end temperature, and the second preset temperature is set to be 10-30°C lower than the second phase transition end temperature.

[0027] As a preferred embodiment of the present invention, the shape memory alloy material includes wire, rod, tube, plate or irregularly shaped component.

[0028] As can be seen from the above technical solutions, the technical solution of the present invention provides a method for testing the shape memory strain of shape memory alloy materials, which has the following beneficial effects:

[0029] 1. This invention is based on a two-point measurement method within a specific temperature range outside the phase transformation temperature point. It abandons the traditional DMA method, which requires measuring strain during continuous temperature changes, and only requires measurement at two constant temperature points, reducing the testing time from several hours to less than one hour. By measuring within the phase transformation completion zone, it ensures that the material is in a pure, stable, single-phase state (e.g., fully austenitic or fully martensitic), avoiding the uncertainty and error in dimensional measurement caused by incomplete phase transformation or phase component mixing within the phase transformation range. The principle of this method does not depend on the specific morphology of the material; as long as the dimensions can be measured at two temperature points, it overcomes the strict limitations of traditional methods on sample shape (e.g., it must be a standard tensile specimen) and size. It is suitable for the direct testing of wires, tubes, rods, irregularly shaped components, and even small devices (e.g., vascular stents).

[0030] 2. When measuring dimensions, a redundant temperature range of 5–30°C is set to avoid unnecessary energy consumption, increased equipment requirements, or potential additional material aging caused by excessively high or low temperatures. This makes the method more practical and operable in engineering while ensuring accuracy. For materials with a wide phase transition range, a larger redundant temperature range of 10–20°C can more reliably ensure phase purity, improving the adaptability and reliability of the method.

[0031] 3. This invention employs differential scanning calorimetry (DSC) to obtain the phase transition temperature and optimizes the heating and cooling rates during testing. By aligning the phase transition temperature used in shape memory strain testing with the material's own DSC thermal characteristic data, a direct and precise correlation is established between thermal and mechanical properties, providing a unified data foundation for material design and performance evaluation. The moderate heating and cooling rates are significantly faster than the less than 3K / min typically required by the DMA method, improving the efficiency of the DSC test itself. Furthermore, it effectively reduces the impact of thermal hysteresis on peak shape and calibration temperature, ensuring the accuracy of the phase transition temperature calibration.

[0032] 4. The present invention provides a complete methodological system, covering complex shape memory alloy systems with multi-step phase transformations (austenite-R-phase-martensite), thus expanding the applicability of the testing method. Furthermore, the present invention's testing method can independently characterize shape memory strain based on the austenite-R-phase transformation, which is of significant guiding value for the design of precision shape memory alloy devices utilizing the characteristics of R-phase transformation, such as small strain, narrow hysteresis, and long fatigue life. In addition, the present invention's testing method is not limited to the morphological structure of shape memory alloy materials, but is applicable to wires, rods, tubes, plates, and irregularly shaped components, highlighting the practical value of this method in directly serving engineering material evaluation and device prototype verification, and solving the pain points of existing methods that are "unable to measure" or "inaccurate in measuring" irregularly shaped parts.

[0033] 5. During the measurement of the first and second dimensions, a load can be applied depending on the actual application scenario, thus more realistically simulating the shape memory strain behavior of the material under service conditions. Compared with the traditional DMA testing method, this method can more accurately reflect the shape memory strain of the material under actual driving or load conditions, while avoiding the problems of low efficiency and complex equipment of dynamic testing methods such as DMA, and has higher testing flexibility and engineering applicability. It realistically measures the material's true strain under load.

[0034] It should be understood that all combinations of the foregoing concepts and the additional concepts described in more detail below can be considered part of the inventive subject matter of this disclosure, provided that such concepts do not contradict each other. Attached Figure Description

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art.

[0036] Figure 1 This is a DSC curve of the shape memory alloy material in Embodiment 1 of the present invention;

[0037] Figure 2 This is a DSC curve of the shape memory alloy material in Embodiment 3 of the present invention;

[0038] Figure 3 The image shows the DSC curve of a shape memory alloy material containing an intermediate phase, as described in an embodiment of the present invention. Detailed Implementation

[0039] The terms "first," "second," and similar words used in the specification and claims of this patent application do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, unless the context clearly indicates otherwise, the singular forms of "an," "a," or "the," etc., do not indicate a quantity limitation, but rather indicate the presence of at least one. Terms such as "comprising" or "including" mean that the element or object preceding "comprising" encompasses the features, integrals, steps, operations, elements, and / or components listed following "comprising" or "including," and do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or collections thereof.

[0040] This invention provides a method for testing the shape memory strain of shape memory alloy materials. The method combines DSC data to test the material dimensions above the austenitic phase transformation temperature and below the martensitic temperature. Based on the dimensions at the two temperature states, the shape memory strain is calculated. Compared with the existing DMA testing method, this method is more efficient and accurate.

[0041] The shape memory alloy material of this invention is made of shape memory alloy and generally has a first phase at a high temperature and a second phase at a low temperature. The first phase at a high temperature is often called the austenitic phase, and the second phase at a low temperature is often called the martensite phase. The specific testing steps of the method are as follows:

[0042] (1) Perform DSC testing on the shape memory alloy material to be tested and obtain DSC curves. At least one complete thermal cycle must be completed during the DSC test. The obtained DSC curves contain the heat flow change curves during the heating and cooling processes, which can be identified as endothermic peaks and exothermic peaks, respectively.

[0043] In this embodiment of the invention, DSC (Differential Scanning Calorimetry) is a thermal analysis technique. Its principle is to measure the change in heat flow difference (or power difference) between the sample and an inert reference material with temperature or time during programmed temperature control (including heating, cooling, and isothermal processes), thereby studying the thermal properties of the material and its transformation process. By directly detecting whether the material absorbs heat (endothermic peak) or releases heat (exothermic peak) during phase transformation, the phase transformation temperature of shape memory alloy materials can be accurately determined, such as the martensitic phase transformation initiation temperature (Ms) and end temperature (Mf), and the austenitic phase transformation initiation temperature (As) and end temperature (Af).

[0044] In some specific embodiments of the present invention, ASTM F2004-17 can be selected as the reference standard for DSC testing to ensure that the heating (cooling) rate, the quality of the shape memory alloy material being tested, the atmosphere, and other conditions are consistent and reasonable. This is a prerequisite for ensuring high accuracy of the phase transformation temperature calibration. For example, during the test, the heating (cooling) rate should not be too high, otherwise it will affect the authenticity and accuracy of the phase transformation temperature test data. In some preferred embodiments of the present invention, the temperature change rate of the phase transformation temperature of the Ni-Ti (nickel-titanium) shape memory alloy is ≤ 5V. T ≤10 K / min avoids uneven temperature distribution inside the sample due to excessively rapid temperature increase, which could affect the accuracy of the phase transition temperature. At the same time, a constant rate can make the phase transition peak shape clearer, which is convenient for temperature calibration.

[0045] During testing, it is necessary to ensure that the baseline is stable before and after the test to reduce the influence of thermal history on the peak shape. Preferably, the test follows a complete thermal cycle of heating measurement-cooling measurement or cooling measurement-heating measurement arm, preferably 2 to 3 complete thermal cycles, to confirm the repeatability and stability of the deformation. Optionally, the data from the first thermal cycle can be used as the standard, or the average phase transition temperature of all thermal cycles can be taken as the final result to eliminate random errors.

[0046] The inert reference material that can be selected in the embodiments of the present invention is high-purity alumina, or other inert materials that do not undergo phase change and have stable heat capacity within the test temperature range, such as quartz, platinum, etc.

[0047] (2) Perform data processing on the obtained DSC curves.

[0048] Based on the obtained DSC curve, the positions of the endothermic peak and / or exothermic peak are confirmed, and the phase transformation temperature is automatically determined or calibrated using the data processing software built into the testing equipment. The phase transformation temperature includes, but is not limited to, the martensitic phase transformation start temperature (Ms) and end temperature (Mf), the austenitic phase transformation start temperature (As) and end temperature (Af), and the phase transformation end temperature Af of the high-temperature phase and the phase transformation end temperature Mf of the low-temperature phase are calibrated.

[0049] In some specific embodiments of the present invention, shape memory alloys with certain specific compositions (e.g., nickel-rich nickel-titanium alloys) or those that have undergone special treatment do not directly transform into the martensitic phase upon cooling. Instead, they first undergo an intermediate third-phase transformation, for example, first transforming into the R phase, and finally transforming into the martensitic phase as the temperature decreases. Let Rf represent the phase transformation end temperature of this third phase, R phase. Therefore, in such cases, as... Figure 3As shown, there may be complex peak shapes or multi-step phase transformations, and the phase transformation end temperature of the low-temperature phase may also include the phase transformation end temperature of the R phase. However, it is worth noting that even if the shape memory alloy material has an intermediate phase, the method of the present invention still calculates the shape memory strain based on the dimensional change between the first phase and the second phase, without the need to separately measure the intermediate dimension, thereby simplifying the testing process, avoiding interference from multi-phase transformations, and ensuring the directness and engineering practicality of strain characterization.

[0050] When calibrating the phase transition temperature, the tangent method or extrapolation method can be optionally used to analyze the endothermic and / or exothermic peaks. Specifically, the tangent method involves drawing tangents on the DSC curve of the shape memory alloy material under test, at the starting baseline and the steepest part of the rising (falling) edge of the phase transition peak (including endothermic and exothermic peaks). The temperature corresponding to the intersection of these two tangents is defined as the phase transition start temperature (phase transition end temperature). The tangent method is suitable for phase transition peaks with clear peak shapes and flat baselines.

[0051] Extrapolation can be used for phase transitions with indistinct peak shapes, such as the R-phase transition. The specific procedure involves identifying the starting and ending points of the phase transition peak on the DSC curve. Tangents or fitted straight lines are drawn along the leading and trailing edges of the peaks, respectively. These two lines are then extrapolated until they intersect; the temperature corresponding to the intersection point is the start or end temperature of the phase transition. For multi-peak cases, such as those with the R-phase transition, extrapolation can be performed separately for each peak.

[0052] In some specific embodiments of the present invention, for cases with complex peak shapes (e.g., multiple peaks, broad peaks) or multiple phase transitions, manual verification and correction should be performed to ensure the accuracy of the phase transition temperature calibration results.

[0053] (3) Determine the dimensions of shape memory alloy materials within a specific temperature range.

[0054] The shape memory alloy material to be tested is placed under a first preset temperature and a second preset temperature, and left to stand for a first preset time, preferably 10 to 30 minutes, to ensure that the shape memory alloy material to be tested is fully heated or cooled under the first preset temperature or the second preset temperature, so that the internal temperature of the shape memory alloy material to be tested is within ±1℃ of the external environment, and the deformation reaches a stable state, completely in a single phase state. Next, the size and shape information of the shape memory alloy material to be tested are measured. Based on the size data of the shape memory alloy material in these two phase states, the shape memory strain / recoverable strain of the shape memory alloy material is quantitatively calculated.

[0055] In some preferred embodiments of the present invention, a first preset temperature is set to be 5–30°C higher than the austenitic phase transformation end temperature Af of the shape memory alloy material to be tested, i.e., Af + (5–30°C). Within this temperature range, the shape memory alloy material is entirely in the austenitic phase. A second preset temperature is set to be 5–30°C lower than the martensitic phase transformation end temperature Mf of the shape memory alloy material to be tested, i.e., Mf - (5–30°C). Within this temperature range, the shape memory alloy material is entirely in the martensitic phase. This temperature offset range of (5–30°C) covers the complete phase transformation region of most shape memory alloys while avoiding material degradation or increased burden on testing equipment due to excessively high / low temperatures.

[0056] In some specific embodiments of the present invention, for shape memory alloy materials with a phase transition temperature range greater than 50°C or with multiple overlapping phase transition peaks in the DSC curve, it is preferable to set the first preset temperature at Af+ (10-20°C) and / or the second preset temperature at Mf- (10-20°C) to ensure phase purity.

[0057] Optionally, during the measurement of the first dimension, a preset load may be applied to the shape memory alloy material, or no load may be applied, depending on the application or requirements. Similarly, during the measurement of the second dimension, a preset load may be applied to the shape memory alloy material, or no load may be applied, depending on the application or requirements. This design gives the testing method of the present invention high flexibility and engineering simulation capabilities. For example, if the test objective is to characterize the intrinsic shape memory strain of the shape memory alloy material in a free state, no load may be applied; if it is necessary to simulate the actual working state of the shape memory alloy material in a specific device, a preset corresponding load may be applied, such as testing the preload force, working resistance, etc., when used as an actuator, fastener, or structural element. Through this optional load application method, the method can more realistically reflect the strain behavior of the shape memory alloy under near-service conditions, thereby providing a direct and reliable data basis for the accurate design and performance prediction of shape memory alloy materials and their fabricated devices and components, overcoming the problem of the disconnect between traditional no-load test results and actual conditions.

[0058] If no load is applied, the first preset temperature is preferably set 5-10°C higher than the first phase transformation end temperature, and the second preset temperature is set 5-10°C lower than the second phase transformation end temperature. In some specific embodiments of the present invention, when the test target is the intrinsic shape memory strain of the material, not applying a load can eliminate the interference of external forces on the phase transformation path and deformation mechanism. Setting the temperature offset to 5-10°C is sufficient to ensure that the material fully transforms into a pure austenitic or martensitic phase above or below the phase transformation end temperature, while avoiding excessive heating / cooling that would lead to energy waste or unnecessary changes in the material's microstructure.

[0059] If a load is applied, the first preset temperature is preferably set to be 10–30°C higher than the first phase transformation end temperature, and the second preset temperature is set to be 10–30°C lower than the second phase transformation end temperature. In some specific embodiments of the present invention, when it is necessary to evaluate the strain performance of the material under actual working conditions (e.g., as an actuator, fastener, support, etc.), a preset load (e.g., prestress, working resistance, etc.) consistent with the actual working conditions can be applied. However, the application of the load will affect the phase transformation driving force and phase transformation kinetics, which may lead to a shift in the phase transformation temperature or incomplete phase transformation. Therefore, a larger temperature redundancy (e.g., 10–30°C) is required to ensure that: at high temperatures, even if there is a suppressive effect of the load on austenitization, the material can still completely transform into the austenitic phase; at low temperatures, the load may promote or hinder the martensitic phase transformation, and sufficient low-temperature offset can ensure that the martensitic phase transformation is fully completed. This design enables the test results to more realistically reflect the available strain of the material under stress, providing direct and reliable data support for device design.

[0060] It is important to note that when testing the dimensions of the shape memory alloy material under test, it should be carried out in a high-precision constant temperature chamber or liquid bath to ensure that the shape memory alloy material under test is uniformly and uniformly at the preset target temperature. The temperature control accuracy is preferably ±0.5℃. In addition, it is necessary to use a low thermal conductivity, unconstrained fixture for fixation, such as a ceramic or low thermal conductivity polymer fixture, to avoid affecting the free deformation of the shape memory alloy material under test due to the thermal expansion or mechanical constraint of the fixture.

[0061] The shape memory alloys used to prepare shape memory alloy materials in this embodiment of the invention can be nickel-titanium shape memory alloys (such as NiTi binary system, NiTiCu ternary system, NiTiNb ternary system, NiTiFe ternary system, NiTiHb ternary system, etc.); or copper-based shape memory alloys (such as CuZnAl ternary system, CuAlNi ternary system) or iron-based shape memory alloys (FeMnSi ternary system, FeNiCoTi ternary system).

[0062] In some specific embodiments of the present invention, the shape of the shape memory alloy material includes, but is not limited to: round wire, flat wire, irregular wire, rod (material), tube (material), shaft, plate (material). The shape memory alloy material can also be a standard or irregular device, component, instrument, part, etc. with the shape memory alloy material as the core component.

[0063] For measuring the length, diameter, height, or shape dimensions of wires, rods, tubes, and irregularly shaped components, non-contact or contact measurements can be performed using a laser micrometer, video extensometer, or a high-precision digital micrometer / caliper with a constant temperature chamber. Measurements should be repeated at least three times, with each measurement marked or taken at the same location on the shape memory alloy material. The accuracy of the measuring instrument itself should preferably be at least at the micrometer level.

[0064] For shape memory alloy materials of other shapes, appropriate tools such as coordinate measuring machines can be selected according to their geometric characteristics. The key is to keep the temperature constant in a constant temperature environment or when quickly removing the material from the measurement.

[0065] (4) Calculate the shape memory strain value by combining the phase transformation temperature determined by the DSC curve with the size data of the shape memory alloy material.

[0066] Example 1

[0067] The shape memory alloy material to be tested is a nickel-titanium binary shape memory alloy round wire, with a diameter of 0.25 mm and a length of 100 mm at room temperature.

[0068] (1) The shape memory alloy round wire to be tested was subjected to DSC testing. The temperature range was 0℃~150℃, and the heating (cooling) rate was 10K / min. The DSC curve was obtained, as shown in the figure. Figure 1 As shown.

[0069] (2) From the DSC curve obtained in step (1), it can be seen that the austenitic phase transformation end temperature Af: T1≈82℃ and the martensitic phase transformation end temperature Mf: T2≈29℃ of the shape memory alloy material.

[0070] (3) Calculate the first preset temperature 82℃+5℃=87℃ and the second preset temperature 29℃-5℃=24℃.

[0071] The two ends of the circular wire were gently clamped with PTFE clamps and placed in a constant temperature liquid bath with an accuracy of ±2℃. After holding at the first preset temperature of 87℃ for 20 minutes, the length of the middle section of the circular wire was measured using a laser micrometer, and the length at the first preset temperature was obtained as L1 = 94.2 mm. Then, the liquid bath was cooled to 24℃ and held for 20 minutes, and the length of the circular wire at the same position was measured, and the length at the second preset temperature was obtained as L2 = 100 mm. Finally, the shape memory strain of the shape memory alloy circular wire was calculated as: (L2-L1) / 100*100% = 5.8%.

[0072] Example 2

[0073] The shape memory alloy material to be tested is a nickel-titanium binary shape memory alloy tube with an outer diameter of 10 mm and an inner diameter of 6 mm at room temperature.

[0074] (1) The shape memory alloy tube to be tested was subjected to DSC test. Temperature range: -150℃~150℃, heating (cooling) rate: 10K / min.

[0075] (2) From the DSC curve obtained in step (1), we can see that the austenitic phase transformation end temperature of the shape memory alloy material is Af: T1 = 75℃, Mf: T2 = 25℃.

[0076] Set the first preset temperature to 75 + 10 = 90℃; set the second preset temperature to 25 - 10 = 15℃.

[0077] The shape memory alloy tube was placed in a constant temperature liquid bath and fixed with ceramic clamps. After holding at 90℃ for 20 minutes, the height was measured using a laser micrometer, and H1 = 28.30 mm was obtained. After cooling to 15℃ and holding at that temperature for 20 minutes, the height was measured again, and H2 = 30.00 mm was obtained.

[0078] The shape memory strain of the shape memory alloy tube is: (30.00-28.30) / 30.00*100%=5.6%.

[0079] Example 3

[0080] The shape memory alloy material to be tested is a nickel-titanium vascular stent with a diameter of 4.83 mm at room temperature.

[0081] (1) The material to be tested was subjected to DSC testing. The temperature range was -50℃ to 50℃, and the heating (cooling) rate was 10K / min. The DSC curve was obtained, as shown in the figure. Figure 2 As shown.

[0082] (2) According to the DSC curve of the shape memory alloy material to be tested, the austenite phase transformation end temperature Af: T1 = 15.27℃ and the R phase transformation end temperature Rf: T2 = -2.98℃.

[0083] (3) The vascular stent was placed on a constant temperature airflow stage and stabilized at 20°C for 10 minutes without load. The image was acquired by a microscopic vision system with a temperature control cavity and its diameter was measured by image analysis software. The diameter D1 of the vascular stent was 4.83 mm. The stent loading load was applied at -15°C (because the load was applied, the test temperature decreased more than the phase transition temperature). After stabilizing for 10 minutes, the diameter D2 of the vascular stent was measured to be 0.53 mm.

[0084] The recoverable strain is calculated as: (4.83-0.53) / 4.83*100%=89%. The value of 89% is a comprehensive result considering shape memory strain, support structure design, and support load, so this value is relatively large.

[0085] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A method for testing the shape memory strain of shape memory alloy materials, characterized in that, Shape memory alloys have a first phase at high temperatures and a second phase at low temperatures; The testing method includes the following steps: (1) Obtain the first phase transformation end temperature and the second phase transformation end temperature of the shape memory alloy material; (2) Hold the material at a first preset temperature above the first phase transition end temperature for a first preset time to allow the shape memory alloy material to completely transform into the first phase, and measure the first dimension of the shape memory alloy material when it is in the first phase; (3) Keep the material at a second preset temperature below the end temperature of the second phase transformation for a second preset time to allow the shape memory alloy material to completely transform into the second phase, and measure the second dimension of the shape memory alloy material when it is in the second phase and at the same position as in step (2); (4) Calculate the shape memory strain of the shape memory alloy material based on the first dimension and the second dimension.

2. The method for testing the shape memory strain of shape memory alloy materials according to claim 1, characterized in that, The first preset temperature is set to be 5 to 30°C higher than the first phase transition end temperature.

3. The method for testing the shape memory strain of shape memory alloy materials according to claim 1, characterized in that, The second preset temperature is set to be 5 to 30°C lower than the second phase transition end temperature.

4. The method for testing the shape memory strain of shape memory alloy materials according to claim 1, characterized in that, In step (1), obtaining the first phase transition end temperature and the second phase transition end temperature includes: (1.1) The shape memory alloy material was tested using differential scanning calorimetry. The test process included at least one complete thermal cycle, and DSC curves were obtained. (1.2) Determine the first phase transition end temperature and the second phase transition end temperature based on the obtained DSC curve.

5. The method for testing the shape memory strain of shape memory alloy materials according to claim 4, characterized in that, The method for calibrating the first phase transition end temperature and the second phase transition end temperature includes at least one of the tangent method and the extrapolation method.

6. The method for testing the shape memory strain of shape memory alloy materials according to claim 4, characterized in that, When using the differential scanning calorimetry method, both the heating rate and the cooling rate are 5 ≤ V. T ≤10 K / min.

7. The method for testing the shape memory strain of shape memory alloy materials according to claim 4, characterized in that, The testing process includes at least one complete thermal cycle, including: Perform 2 to 3 complete thermal cycles, and take the temperature at which the first phase transition ends and the temperature at which the second phase transition ends during the first thermal cycle as the first phase transition end temperature and the second phase transition end temperature, respectively. Alternatively, the average temperature at which the first phase transition ends after multiple thermal cycles and the average temperature at which the second phase transition ends can be taken as the first phase transition end temperature and the second phase transition end temperature, respectively.

8. The method for testing the shape memory strain of shape memory alloy materials according to claim 1, characterized in that, The first preset time is 10 to 30 minutes; the second preset time is 10 to 30 minutes.

9. The method for testing the shape memory strain of shape memory alloy materials according to claim 1, characterized in that, During the measurement of the first dimension and / or the second dimension, a preset load may be applied to the shape memory alloy material, or no load may be applied, depending on the application or requirements. If no load is applied, the first preset temperature is set to be 5-10°C higher than the first phase transition end temperature, and the second preset temperature is set to be 5-10°C lower than the second phase transition end temperature. If a load is applied, the first preset temperature is set to be 10-30°C higher than the first phase transition end temperature, and the second preset temperature is set to be 10-30°C lower than the second phase transition end temperature.

10. The method for testing the shape memory strain of shape memory alloy materials according to claim 1, characterized in that, The shape memory alloy material can be in the form of wire, rod, tube, plate or irregularly shaped component.

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