Aluminum alloy stress state online monitoring method based on weak magnetic detection

Abstract

The invention discloses an aluminum alloy stress state online monitoring method based on weak magnetic detection, and relates to the technical field of aluminum alloy stress state online monitoring. A stress-strain curve obtained by an aluminum alloy tensile mechanical property test is used as the basis of different stress damages of the aluminum alloy, including an elastic stage, a plastic stage and a necking fracture stage, and research is carried out in combination with the surface three-dimensional magnetic field intensity in the tensile process. The relationship between different stress damage stages and surface magnetic field intensity signals is analyzed, and the result shows that the surface magnetic field intensity can sensitively distinguish the elastic stage, the plastic stage and the necking fracture stage of the material, and the dangerous point yield strength of stress damage can be accurately judged.

Description

Technical Field

[0001] This invention relates to the field of online monitoring technology for the stress state of aluminum alloys, and particularly to a method for online monitoring of the stress state of aluminum alloys based on weak magnetic field detection. Background Technology

[0002] The methods for detecting internal stress in aluminum alloys include the following: using pure transverse ultrasonic waves with polarization parallel or perpendicular to the stress direction to test LY11 aluminum alloy; calculating the acoustoelastic birefringence coefficient by measuring the change in sound velocity under tensile and compressive uniaxial stress to assess internal stress; measuring residual stress at different depths by peak shift in X-ray diffraction patterns; corrosion methods that can corrode along the depth direction while simultaneously performing X-ray measurements to obtain more accurate residual stress data; and neutron diffraction technology, which can reach the centimeter level, is suitable for measuring residual stress in deeper layers.

[0003] Existing methods for detecting the magnetic field strength on the surface of aluminum alloys can only detect the magnetic field on the surface of the aluminum alloy. Therefore, they cannot effectively use changes in the detected magnetic field signal to provide early warning of stress damage to the aluminum alloy, and thus cannot accurately determine whether stress damage has occurred in the aluminum alloy.

[0004] To address the aforementioned issues, a method for detecting the surface magnetic field strength of aluminum alloys under monotonic tensile stress is proposed. Summary of the Invention

[0005] The purpose of this invention is to at least solve one of the technical problems existing in the prior art, and to provide a method for detecting the magnetic field strength of aluminum alloy surface under monotonic tensile stress, which can solve the problems mentioned above in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: an online monitoring method for the stress state of aluminum alloys based on weak magnetic field detection, comprising the following steps: S1. An external force is applied to both ends of an aluminum alloy sample to cause deformation. A high-sensitivity magnetic induction probe is placed above the sample surface in the tensile stress concentration area to monitor the change of magnetic field intensity signal in real time, thereby analyzing the influence of stress state on magnetic field distribution. S2. According to Gauss's law for magnetic fields, the magnetic field strength at the sample surface, σ0, is related to the change in internal magnetic induction intensity. The functional relationship is as follows: (1); When the detection lift-off value is At that time, the magnetic field strength is 𝐵( Equation 2 is given.

[0007] (2); From Equations 1 and 2, we can derive: (3); As can be seen from the JAS model, in Equation 3 Since it is a function of stress , the magnetic field strength is... Associating with stress γ, taking the partial derivative of Equation 3 with respect to stress γ yields: (4); According to the JAS model, the magnetic field strength of a nonferromagnetic material is: (5); coefficients in Equation 5 Demagnetization coefficient It depends only on the sample size and the applied magnetic field. In this experiment, the geomagnetic field can be considered to be independent of stress R. It is a function of stress , which varies with different stress ; therefore, the partial differential of Equation 5 with respect to stress is: (6); In the formula, for The constant; By combining equations 4 and 6, we can obtain the stress derivative of the magnetic field strength on the surface of a nonferromagnetic material. The relationship is: (7); As can be seen from Equation 7, the surface magnetic field strength of aluminum alloy materials has a stress derivative. It is a function of axial position; when an aluminum alloy is under tensile stress, its magnetic field strength is... It changes with stress R, therefore, the magnetic field strength It can effectively characterize the stress state of aluminum alloy materials under tensile stress; S3. Online fixed-point weak magnetic field testing of aluminum alloy under stress loading: Four magnetic induction probes were set up on the surface of the tensile specimen to detect the surface magnetic field strength online. Probes 1-3 were arranged sequentially along the axial centerline, with a spacing of 9 mm between each probe. Probe 4 was placed at the transverse centerline, 10 mm from the center point. Each probe collected the surface magnetic field strength of the tensile specimen, with a lift-off value set to 1 mm. The probes were connected and controlled by computer software, and the probe acquisition parameters were adjusted. The selected sampling frequency was 60 Hz. The ambient magnetic field before the test was detected, and no-load sampling was performed on the unloaded specimen. Subsequently, a static load test was conducted using a static tensile rate of 2 mm / min. Probes 1-4 were simultaneously subjected to tensile testing and data acquisition with the testing machine, from the start of the test until the specimen fractured. S4. Stress-strain curve analysis of aluminum alloy: Aluminum alloy is a metallic material with aluminum crystal as the matrix and doped with a variety of alloying elements. The microstructure of aluminum alloy is usually composed of α-Al matrix phase and different types of strengthening phases. Under external load, aluminum alloy material mainly goes through elastic deformation, plastic deformation and final fracture failure stages. OA stage: The elastic deformation stage of the material. This stage corresponds to the tensile process where the strain is defined to be less than 0.2%. When the applied stress is removed, the deformation of the specimen disappears. That is, the specimen is in the elastic deformation stage. Point A represents the yield strength or yield point of the material. As can be seen from the figure, 7075 aluminum alloy does not show an obvious yield state. AB stage: The stage of plastic deformation of the material (the stage of material strengthening), when the applied stress exceeds the stress corresponding to point A. 0.2 Subsequently, the linear relationship between stress and strain is disrupted, strain increases significantly, and the deformation exhibits a uniform distribution. The phenomenon that the resistance to plastic deformation increases with increasing plastic deformation is called strain hardening or work hardening. When the stress reaches the stress corresponding to point B... 𝑏 When the uniform deformation of the sample stops, the stress reaches its maximum value, which is called the tensile strength of the material. The stress corresponding to this point is the resistance of the material to the maximum uniform plastic deformation, which is the maximum stress that the material can withstand before tensile failure. BC stage: Necking fracture stage, when the applied stress exceeds the stress σ corresponding to point B. b At point C, the sample begins to exhibit uneven plastic deformation, and local cross-sections experience necking, leading to a decrease in stress. When the stress reaches point C, the material fractures, and the stress corresponding to this point represents the material's ultimate resistance to plasticity. For 7075 aluminum alloy material without obvious yield, 0.2% strain is taken as its yield limit, and the yield point is determined by the 0.2% offset method. A total of four sets of repeated tests are conducted. S5. Online fixed-point detection of magnetic field strength variation characteristics: The surface magnetic field strength variation trends detected in the four groups of tests are consistent. Taking sample No. 1 as the analysis, the magnetic field data of the four probes are extracted and plotted as three vectors of magnetic field (x, y, z). The measured magnetic field is compensated by the background magnetic field, and the magnetic field strength changes from zero. The three-dimensional surface magnetic field signal during the tensile process is combined with the stress curve during the loading process for research. The material elastic stage, plastic stage, and necking fracture stage of the stress curve are used as the analysis part of the magnetic field strength, which are regions I, II, and III, respectively. Analysis of magnetic field strength B xDuring the elastic deformation stage, the signals from probes 1-4 showed a decreasing trend, but with small signal fluctuations and poor magnetic field signal uniformity. During the plastic deformation stage, the magnetic field strength showed a decreasing trend and fluctuated to varying degrees. When the applied stress exceeded... Later, during the necking fracture stage, the magnetic field intensity signals from each probe showed a relatively uniform downward trend; and at the instant of sample fracture, the magnetic field intensities of each probe exhibited a rapid decrease, with probe 1's B... x The drop value was 196 nT for probe 2, 376 nT for probe 3, 619 nT for probe 4, and 343 nT for probe 4. Analysis of magnetic field strength B y During the elastic deformation stage, the measured magnetic field strength changes consistently, exhibiting a rapid increase in signal. It is noteworthy that when the applied stress does not reach the yield point... At a pressure range of 464 MPa, the B of probes 1-3 y The signal reaches its maximum value, with signal amplification values ​​of 212 nT, 248 nT, and 214 nT, respectively, at the yield point. When probe 4 reaches its maximum signal value, the signal amplification is 398 nT. During the plastic deformation stage, the B measured by each probe... y The signal exhibits a downward trend, with varying degrees of fluctuation during this process. When the applied stress exceeds... After entering the necking fracture stage, probes 1-3 maintained the same downward trend as in the plastic stage, while probe 4 showed a relatively gradual decrease. At the instant of sample fracture, the signals of probes 1-4 showed a rapid increase, with probe 1's B signal showing the highest increase. y The rise rate was 186 nT for probe 2, 146 nT for probe 3, 137 nT for probe 4, and 167 nT for probe 4. In the measured magnetic field strength B z During the elastic deformation stage, the magnetic field strength changes consistently across the four different measurement regions, exhibiting a rapid increase in magnetic field intensity; when the applied stress exceeds... When the pressure reaches 506 MPa, the signals from all probes show a slowdown, with probe 4 showing a decreasing trend. During the plastic deformation stage, the B measured by each probe... z The signals exhibited varying degrees of fluctuation. Signals from probes 1-3 showed an increasing trend, while the signal from probe 4 fluctuated within the range of 312 nT-535 nT. When the applied stress exceeded... Then it enters the necking fracture stage, and each probe B z The signals showed the same increasing trend, and at the instant the sample fractured, each probe B... z The signals all exhibited a rapid decline, with probe 1's B signal showing the highest level. zThe rapid descent amplitude was 184 nT; probe 2 was 129 nT, probe 3 was 148 nT, and probe 4 was 306 nT; the B values ​​of probes 1-3, located in the axial centerline region of the sample, were... z The signal performance was relatively uniform, but probe 4 at the transverse center point showed a small increase in signal during the stretching process and a large drop in signal at the moment of fracture. S6. Stress damage analysis of aluminum alloy under continuous tension: Analyze the magnetic field strength on the surface of the aluminum alloy under stress, using the probe's B... x The signal and applied stress were analyzed, and the stress and B during the test were recorded. x The signal change relationship, during the elastic deformation stage, is related to the surface magnetic field strength B of the tensile specimen. x All showed a monotonically decreasing trend, indicating that the stress in the elastic stage mainly caused reversible material deformation and had limited damage to the internal structure of the material. The change in magnetic field strength was mainly affected by the external load and no obvious material damage had yet occurred. During the plastic deformation and necking fracture stages, stress damage occurs inside the specimen, and the B measured by probes 2-4... x In addition to showing a downward trend, the signal was also accompanied by strong signal fluctuations; while the B signal detected by probe number 1... x The signal exhibits a trend of first increasing and then decreasing, accompanied by strong signal fluctuations. This phenomenon suggests that the effect of stress on magnetic field changes varies spatially at different measurement locations, which may be related to the degree of local plastic deformation. The causes of signal fluctuations can be explored from both macroscopic and microscopic levels. At the macroscopic level, the aluminum alloy sample undergoes localized necking during the plastic deformation stage, resulting in irreversible deformation. At the microscopic level, significant structural changes occur within the aluminum alloy during plastic deformation, including dislocation multiplication and dislocation entanglement. These changes, coupled with stress damage, lead to the formation of B... x The signal fluctuates violently. At the moment of sample fracture, the thermomechanical energy increases sharply, corresponding to a sudden change in magnetic field strength; S7, collect B data from all probes y The signal and applied stress are analyzed to show the stress recorded during the test and B. y The relationship of signal changes, with B y The stress at the maximum value of the signal is expressed as: ; Yield strength of 7075 aluminum alloy In comparison, the surface magnetic field strength (probes 1-3) collected at the axial centerline of the sample characterizes... All less than yield strength The surface magnetic field strength collected at the transverse centerline (probe 4) characterizes... With yield strength Similarly, this demonstrates that the detection technology can determine the stress damage state of aluminum alloys before their yield point. According to the thermomechanical effect theory, the thermomechanical energy decreases during the elastic stage of the aluminum alloy and increases when it enters the plastic stage. This is consistent with the surface magnetic field strength B of the aluminum alloy. y The elastic and plastic stages exhibit opposite changes, similar to B. x The signals are the same; the magnetic field strength change curve is smooth in the elastic stage. During plastic deformation of the sample, internal stress damage accumulates and increases with the application of stress. Therefore, when the stress exceeds the yield point, the magnetic field strength fluctuates. In summary, B y The signal can serve as an early warning for stress damage in aluminum alloys and is a feasible option for clearly determining stress damage in aluminum alloys. S8, Analysis of B under stress z Signals, showing the stress and B recorded during the test. z The signal change relationship, during the elastic deformation stage, the B of the tensile specimen z The signals all showed a monotonically increasing trend. During the plastic deformation and necking fracture stages, due to stress damage inside the sample, the B signal detected by probes 1-3... z In addition to showing a rapid upward trend, the signal also exhibited signal fluctuations; while the B signal detected by probe number 4... z The signal showed a downward trend, accompanied by strong signal fluctuations; the effect of stress on magnetic field changes varied significantly at different measurement locations, which may be related to the degree of local plastic deformation. In the study of stress-thermomechanical effects in aluminum alloys, the cooling phenomenon during the elastic tensile stage is a typical thermoelastic effect, and the lowest temperature at the elastic-plastic transition can be used to determine the onset of yield strength. Under the influence of thermomechanical effects, the energy changes of the material during deformation can be characterized by temperature measurements. When its internal energy changes, it will induce thermomagnetic induction phenomena in the material. During stress loading, both the elastic and plastic stages of the material are accompanied by changes in internal energy, thereby triggering corresponding changes in magnetic field strength. S9. During the elastic loading stage, the deformation of the material is mainly elastic. According to thermodynamic theory, the energy change of the material during elastic deformation mainly originates from the interconversion between the potential energy and kinetic energy of lattice atoms. For aluminum alloys, during adiabatic elastic tension or adiabatic compression, their thermomechanical energy decreases, accompanied by a corresponding change in the sample temperature; this phenomenon is called the thermoelastic effect. During continuous tensile deformation, the mechanical energy that produces plastic deformation is expressed as heat energy, manifested as a rise in the material temperature. At this point, macroscopic plastic deformation gradually becomes the dominant deformation mechanism. Therefore, during the tensile deformation of aluminum alloys, the tensile stress energy of the sample initially shows a decreasing trend, reaches a minimum value, and then rises rapidly. This behavior of strained materials can be used to determine the onset of plastic deformation, corresponding to the measured magnetic field strength B. y ; Thermomechanical effects refer to the coupling effect of thermal and mechanical behavior of materials during deformation. Thermoelastic and thermoplastic effects are two core aspects of this research. Thermoelastic effects have a clear theoretical definition, which can be derived from thermodynamic and elastic theories. Thermoelastic effects encompass changes in stress, strain, temperature, and entropy. Adiabatic thermoelastic theory is based on the premise that there is no heat exchange between the system and its surrounding environment during deformation; therefore, the change in energy within the system is solely caused by the mechanical energy of the sample.

[0008] In the case of uniaxial tensile loading of a metal specimen, a uniform strain energy will be generated inside the specimen, assuming density... Not following As E and T change, the free energy equation can be expressed as: (8); In the formula, This is for material strain. This is the specific heat capacity at constant volume.

[0009] For an adiabatic process, Equation 8 can be written as: (9); Therefore, the integrated formula 9 and the assumptions , and Since the effects of strain and temperature on parameters are negligible, it can be rewritten as: (10); Equation 10 shows the relationship between temperature change and applied stress change during adiabatic uniaxial tensile deformation due to the thermoelastic effect. It is clear from the equation that the temperature change... With elastic stress The changes show a linear relationship. The slope... This refers to the thermoelasticity factor. Therefore, during the thermoelastic effect, parameter changes at each stage of stress loading lead to changes in internal thermomechanical energy, which in turn causes changes in surface magnetic field strength. When a material is subjected to external load and is in a state of stress damage accumulation, a large number of dislocations will be generated in its internal grains. Although no macroscopic damage defects appear on the material surface during the accumulation stage, the stress concentration caused by dislocations still leads to changes in the material's magnetic permeability and thermomechanical energy. The changes in the properties of metallic materials caused by stress damage are macroscopically manifested as abnormal changes in the magnetic field strength signal on the material surface.

[0010] Preferably, research based on the S4 dislocation theory and crystallographic theory indicates that material deformation can be divided into two parts: elastic deformation and plastic deformation. Elastic deformation is caused by lattice distortion, while plastic deformation is caused by the movement of numerous dislocations within the grains along specific crystal planes. When stress is continuously applied up to the tensile strength, the aluminum alloy material undergoes necking fracture. In the elastic deformation stage, deformation is mainly achieved through atomic displacement within the lattice, i.e., lattice distortion. Under stress, a reversible change in lattice spacing occurs, and the material can return to its original state after the external load is removed. As the stress continues to increase to the yield strength, the aluminum alloy material will enter the plastic deformation stage. At this time, dislocation slip becomes the main deformation mechanism, especially in high stress concentration areas. The movement and proliferation of dislocations may lead to microstructural evolution such as grain boundary slip and the interaction between precipitates and the matrix.

[0011] Preferably, there is no effective non-destructive testing technology for online assessment of stress damage in aluminum alloys, as described in S6. When aluminum alloys exceed their yield strength, they rapidly enter the plastic deformation failure stage, a highly dangerous condition for engineering equipment. To avoid this, emergency measures should be taken promptly before the critical yield point is reached under load. In tensile tests, the surface magnetic field strength of aluminum alloy specimens exhibits corresponding signal changes before and after the yield stage, as well as during necking fracture. It is worth noting that B... y The signal exhibits a clear signal inflection point at the yield point, changing from a monotonically linear increase to a monotonically linear decrease. This phenomenon provides a basis for stress damage assessment of aluminum alloys.

[0012] Compared with the prior art, the beneficial effects of the present invention are: This online monitoring method for the stress state of aluminum alloys based on weak magnetic field detection employs a high-sensitivity magnetic induction probe to detect stress damage in aluminum alloy samples. Through surface-point detection in static tensile tests of aluminum alloys, the force-magnetic relationship is analyzed. The stress-strain curves obtained from the tensile mechanical property tests of aluminum alloys serve as the basis for different stress damage stages, including elastic, plastic, and necking fracture stages. The method also incorporates the three-dimensional surface magnetic field strength during the tensile process to analyze the relationship between different stress damage stages and the surface magnetic field strength signal. Experimental results show that the surface magnetic field strength can sensitively identify the elastic, plastic, and necking fracture stages of the material and accurately determine the yield strength, a critical point for stress damage. The reliability of the stress damage state assessment for aluminum alloys is further verified through thermomechanical effects and the weak magnetic damage mechanism, thus enabling early warning of aluminum alloy damage and facilitating stress detection. Attached Figure Description

[0013] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the weak magnetic field detection of aluminum alloy stress state according to the present invention; Figure 2 This is a schematic diagram of the positioning of the stress loading probe of the present invention; Figure 3 This is a schematic diagram of the stress-strain curve of the aluminum alloy of the present invention; Figure 4 This is a schematic diagram of the stress results of the aluminum alloy sample of the present invention; Figure 5 This is a graph showing the relationship between magnetic field strength and stress in this invention. Figure 6 The stress and magnetic field strength B during the tensile process of this invention x Relationship diagram; Figure 7 The stress and magnetic field strength B during the tensile process of this invention y Relationship diagram; Figure 8 The stress and magnetic field strength B during the tensile process of this invention z Relationship diagram. Detailed Implementation

[0014] This section will describe in detail specific embodiments of the present invention. Preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the drawings is to supplement the textual description with graphics, so that people can intuitively and vividly understand each technical feature and overall technical solution of the present invention, but they should not be construed as limiting the scope of protection of the present invention.

[0015] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0016] In the description of this invention, terms such as greater than, less than, and exceeding are understood to exclude the stated number, while terms such as above, below, and within are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0017] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0018] Please see Figure 1-4 The present invention provides a technical solution: a schematic diagram of an online monitoring method for the stress state of aluminum alloys based on weak magnetic field detection is shown below. Figure 1 As shown in Figure 1, an external force is applied to both ends of the aluminum alloy sample to cause deformation. A high-sensitivity magnetic induction probe is placed above the sample surface in the tensile stress concentration area to monitor the change of magnetic field intensity signal in real time, thereby analyzing the influence of stress state on magnetic field distribution.

[0019] According to Gauss's law for magnetic fields, the magnetic field strength σ0 at the sample surface and the change in internal magnetic induction intensity are related. The functional relationship is as follows: (1); When the detection lift-off value is At that time, the magnetic field strength is 𝐵( Equation 2 is given.

[0020] (2); From Equations 1 and 2, we can derive: (3); As can be seen from the JAS model, in Equation 3 Since it is a function of stress , the magnetic field strength is... Associating with stress γ, taking the partial derivative of Equation 3 with respect to stress γ yields: (4); According to the JAS model, the magnetic field strength of a nonferromagnetic material is: (5); coefficients in Equation 5 Demagnetization coefficient It depends only on the sample size and the applied magnetic field. In this experiment, the geomagnetic field can be considered to be independent of stress R. It is a function of stress , which varies with different stress ; therefore, the partial differential of Equation 5 with respect to stress is: (6); In the formula, for The constant; By combining equations 4 and 6, we can obtain the stress derivative of the magnetic field strength on the surface of a nonferromagnetic material. The relationship is: (7); As can be seen from Equation 7, the surface magnetic field strength of aluminum alloy materials has a stress derivative. It is a function of axial position; when an aluminum alloy is under tensile stress, its magnetic field strength is... It changes with stress R, therefore, the magnetic field strength It can effectively characterize the stress state of aluminum alloy materials under tensile stress.

[0021] Online fixed-point weak magnetic field detection of aluminum alloy under stress loading Four magnetic induction probes were installed on the surface of the tensile specimen to detect the surface magnetic field strength online. Probes 1-3 were arranged sequentially along the axial centerline, with a spacing of 9 mm between each probe. Probe 4 was placed along the transverse centerline, 10 mm from the center point. The arrangement of the probes was as follows: Figure 2 As shown. Surface magnetic field strength was collected from the tensile-loaded specimen, with a lift-off value of 1 mm. The probe was connected and controlled via computer software, and the probe acquisition parameters were adjusted. The selected sampling frequency was 60 Hz. The ambient magnetic field before the test was detected, and no-load sampling was performed on the unloaded specimen. Subsequently, a static load test was conducted using a static tensile rate of 2 mm / min. Probes 1-4 were used simultaneously with the testing machine for tensile testing and data acquisition, from the start of the test until the specimen fractured.

[0022] Aluminum alloys are metallic materials with aluminum crystals as the matrix and doped with various alloying elements. The microstructure of aluminum alloys is usually composed of α-Al matrix phase and different types of strengthening phases. Under external load, aluminum alloy materials mainly go through elastic deformation, plastic deformation and final fracture failure stages.

[0023] Studies based on dislocation theory and crystallography show that material deformation can be divided into elastic deformation and plastic deformation. Elastic deformation is caused by lattice distortion, while plastic deformation is caused by the movement of numerous dislocations within the grains along specific crystal planes. When stress is continuously applied up to the tensile strength, aluminum alloys undergo necking fracture. In the elastic deformation stage, deformation is mainly achieved through atomic displacement within the lattice, i.e., lattice distortion. Under stress, reversible changes in lattice spacing occur, and the material can return to its original state after the external load is removed. As the stress continues to increase to the yield strength, the aluminum alloy enters the plastic deformation stage. At this point, dislocation slip becomes the main deformation mechanism, especially in high stress concentration areas. The movement and proliferation of dislocations may lead to microstructural evolution such as grain boundary slip and the interaction between precipitates and the matrix. Figure 3 The figure shows the stress-strain curve of 7075 aluminum alloy under tensile loading, from which the following deformation stages can be clearly seen: OA stage: Elastic deformation stage of the material. This stage corresponds to the tensile process where the strain is defined as less than 0.2%. At this time, when the applied stress is removed, the deformation of the specimen disappears, that is, the specimen is in the elastic deformation stage. Point A represents the yield strength or yield point of the material. As can be seen from the figure, 7075 aluminum alloy does not show a clear yield state.

[0024] AB stage: The stage of plastic deformation of the material (the stage of material strengthening). When the applied stress exceeds the stress corresponding to point A... 0.2 Subsequently, the linear relationship between stress and strain is disrupted, strain increases significantly, and the deformation exhibits a uniform distribution. The phenomenon that the resistance to plastic deformation increases with increasing plastic deformation is called strain hardening or work hardening. When the stress reaches the stress corresponding to point B... 𝑏 When the uniform deformation of the sample stops, the stress reaches its maximum value, which is called the tensile strength of the material. The stress corresponding to this point is the resistance of the material to the maximum uniform plastic deformation, which is the maximum stress that the material can withstand before tensile failure.

[0025] BC stage: Necking fracture stage. When the applied stress exceeds the stress σ corresponding to point B. b When the stress reaches point C, the sample begins to exhibit uneven plastic deformation, and local cross-sections experience necking, leading to a decrease in stress. When the stress reaches point C, the material fractures, and the stress corresponding to this point represents the material's ultimate resistance to plasticity.

[0026] For 7075 aluminum alloy materials without a clear yield strength, a strain of 0.2% is used as its yield limit, and the yield point is determined by the 0.2% offset method. Four sets of repeated tests were conducted, and the yield strength and tensile strength of the four sets are as follows: Figure 4 As shown.

[0027] Online fixed-point detection of magnetic field strength variation characteristics The surface magnetic field intensity changes in the four sets of tests showed consistent trends. This section uses sample No. 1 as the case study, extracting magnetic field data from the four probes and plotting the magnetic field as three vectors (x, y, z). All measured magnetic fields were compensated for by the background magnetic field, and the magnetic field intensity changed from zero. The three-dimensional surface magnetic field signal during the tensile process was combined with the stress curve during the loading process for further analysis, as shown below. Figure 5 As shown in a, b, and c. From Figure 5 It can be seen that the material elastic stage, plastic stage, and necking fracture stage of the stress curve are taken as the analysis parts of the magnetic field strength, which are regions I, II, and III, respectively.

[0028] like Figure 5 As shown in figure a, analyze the magnetic field strength B. x During the elastic deformation stage, the signals from probes 1-4 showed a decreasing trend, but with small signal fluctuations and poor magnetic field signal uniformity. During the plastic deformation stage, the magnetic field strength showed a decreasing trend and fluctuated to varying degrees. When the applied stress exceeded... Later, during the necking fracture stage, the magnetic field intensity signals from each probe showed a relatively uniform downward trend; and at the instant of sample fracture, the magnetic field intensities of each probe exhibited a rapid decrease, with probe 1's B... x The drop value was 196 nT for probe 2, 376 nT for probe 3, 619 nT for probe 4, and 343 nT for probe 4.

[0029] like Figure 5 As shown in b, analyze the magnetic field strength B. y During the elastic deformation stage, the measured magnetic field strength changes consistently, exhibiting a rapid increase in signal. It is noteworthy that when the applied stress does not reach the yield point... At a pressure range of 464 MPa, the B of probes 1-3 y The signal reaches its maximum value, with signal amplification values ​​of 212 nT, 248 nT, and 214 nT, respectively, at the yield point. At time 4, the signal reaches its maximum value, with a signal amplification of 398 nT. During the plastic deformation stage, the B measured by each probe... y The signal exhibits a downward trend, with varying degrees of fluctuation during this process. When the applied stress exceeds... After entering the necking fracture stage, probes 1-3 maintained the same downward trend as in the plastic stage, while probe 4 showed a relatively gradual decrease. At the instant of sample fracture, the signals of probes 1-4 showed a rapid increase, with probe 1's B signal showing the highest increase. y The rise rate was 186 nT for probe 2, 146 nT for probe 3, 137 nT for probe 4, and 167 nT for probe 4.

[0030] like Figure 5 As shown in c, the measured magnetic field strength B z During the elastic deformation stage, the magnetic field strength changes consistently across the four different measurement regions, exhibiting a rapid increase in magnetic field intensity; when the applied stress exceeds... When the pressure reaches 506 MPa, the signals from all probes show a slowdown, with probe 4 showing a decreasing trend. During the plastic deformation stage, the B measured by each probe... z The signals exhibited varying degrees of fluctuation, with probes 1-3 showing an increasing trend, while probe 4's signal fluctuated within the range of 312 nT-535 nT. When the applied stress exceeded... Then it enters the necking fracture stage, and each probe B z The signals showed the same increasing trend, and at the instant the sample fractured, each probe B... z The signals all exhibited a rapid decline, with probe 1's B signal showing the highest level. z The rapid descent amplitude was 184 nT; probe 2 was 129 nT, probe 3 was 148 nT, and probe 4 was 306 nT; the B values ​​of probes 1-3, located in the axial centerline region of the sample, were... z The signal performance was relatively uniform, but probe 4 at the transverse center point showed a small increase in signal during the stretching process and a large decrease in signal at the moment of fracture.

[0031] Stress Damage Analysis of Aluminum Alloys under Continuous Tension Currently, there is no effective non-destructive testing (NDT) technique for online assessment of stress damage in aluminum alloys. Once aluminum alloys exceed their yield strength, they rapidly enter the plastic deformation failure stage, a highly dangerous condition for engineering equipment. To prevent this, emergency measures should be taken promptly before the critical yield point is reached under load. Figure 5 It can be seen that the surface magnetic field strength of aluminum alloy specimens in tensile tests exhibits corresponding signal changes before and after the yielding stage and at the necking fracture. Of particular note is B. y The signal exhibits a clear signal inflection point at the yield point, changing from a monotonically linear increase to a monotonically linear decrease. This phenomenon provides a basis for stress damage assessment of aluminum alloys.

[0032] Analyze the magnetic field strength on the surface of the aluminum alloy under stress, and use the probe's B... x Analyze the signal and the applied stress, such as Figure 6 The stress and B during the experiment were recorded. x Signal variation relationship, where B of probes 1-4 x The signals are respectively Figure 6 ad. During the elastic deformation stage, the surface magnetic field strength B of the tensile specimen. xAll showed a monotonically decreasing trend, indicating that the stress in the elastic stage mainly caused reversible material deformation, with limited damage to the internal structure of the material. The change in magnetic field strength was mainly affected by the external load, and no obvious material damage had yet occurred. In the plastic deformation and necking fracture stages, stress damage occurred inside the sample, and the B measured by probes 2-4... x In addition to showing a downward trend, the signal was also accompanied by strong signal fluctuations; while the B signal detected by probe number 1... x The signal exhibits a trend of initial increase followed by a decrease, accompanied by strong signal fluctuations. This phenomenon indicates that the influence of stress on magnetic field changes varies spatially at different measurement locations, possibly related to the degree of local plastic deformation. The causes of signal fluctuations can be explored from both macroscopic and microscopic levels. At the macroscopic level, the aluminum alloy sample undergoes local necking during the plastic deformation stage, resulting in irreversible deformation of the material. At the microscopic level, significant structural changes occur within the aluminum alloy during plastic deformation, including dislocation multiplication and dislocation entanglement. These changes, coupled with stress damage, lead to the formation of B... x The signal fluctuates violently. At the moment of sample fracture, the thermomechanical energy increases sharply, corresponding to a sudden change in magnetic field strength; B collected by all probes y Analyze the signal and the applied stress. Figure 7 The image shows the stress recorded during the test and B. y The relationship between signal changes, including B of probes 1-4. y The signals are respectively Figure 7 ad, with B y The stress at the maximum value of the signal is expressed as: ; Yield strength of 7075 aluminum alloy In comparison, the surface magnetic field strength (probes 1-3) collected at the axial centerline of the sample characterizes... All less than yield strength The surface magnetic field strength collected at the transverse centerline (probe 4) characterizes... With yield strength Similarly, this demonstrates that the detection technology can determine the stress damage state of aluminum alloys before their yield point. According to the thermomechanical effect theory, the thermomechanical energy decreases during the elastic stage of the aluminum alloy and increases when it enters the plastic stage. This is consistent with the surface magnetic field strength B of the aluminum alloy. y The elastic and plastic stages exhibit opposite changes, similar to B. xThe signals are the same; the magnetic field strength change curve is smooth in the elastic stage. During plastic deformation of the sample, internal stress damage accumulates and increases continuously with the application of stress. Therefore, when the stress exceeds the yield point, the magnetic field strength fluctuates. In summary, B y Signals can serve as an early warning system for stress damage in aluminum alloys, making them a viable option for accurately determining stress damage in aluminum alloys.

[0033] During the elastic loading stage, the deformation of the material is mainly elastic. According to thermodynamic theory, the energy change of the material during elastic deformation mainly originates from the mutual conversion between the potential energy and kinetic energy of lattice atoms. For aluminum alloys, during adiabatic elastic tension or adiabatic compression, their thermomechanical energy decreases, accompanied by a corresponding change in the sample temperature. This phenomenon is called the thermoelastic effect. During continuous tensile deformation, the mechanical energy of plastic deformation is expressed as heat energy, manifested as a rise in the material temperature. At this point, macroscopic plastic deformation gradually becomes the dominant deformation mechanism. Therefore, during the tensile deformation of aluminum alloys, the tensile stress energy of the sample initially shows a decreasing trend, reaches a minimum value, and then rises rapidly. This behavior of strained materials can be used to determine the onset of plastic deformation. Correspondingly, the detected magnetic field strength B... y .

[0034] Thermomechanical effects refer to the coupling effect of thermal and mechanical behavior of materials during deformation. Thermoelastic and thermoplastic effects are two core aspects of this research. Thermoelastic effects have a clear theoretical definition, which can be derived from thermodynamic and elastic theories. Thermoelastic effects encompass changes in stress, strain, temperature, and entropy. Adiabatic thermoelastic theory is based on the premise that there is no heat exchange between the system and its surrounding environment during deformation; therefore, the change in energy within the system is solely caused by the mechanical energy of the sample.

[0035] In the case of uniaxial tensile loading of a metal specimen, a uniform strain energy will be generated inside the specimen, assuming density... Not following As E and T change, the free energy equation can be expressed as: (8); In the formula, This is for material strain. This is the specific heat capacity at constant volume.

[0036] For an adiabatic process, Equation 8 can be written as: (9); Therefore, the integrated formula 9 and the assumptions , and Since the effects of strain and temperature on parameters are negligible, it can be rewritten as: (10); Equation 10 shows the relationship between temperature change and applied stress change during adiabatic uniaxial tensile deformation due to the thermoelastic effect. It is clear from the equation that the temperature change... With elastic stress The changes show a linear relationship. The slope... As a thermoelastic factor, the parameter changes at each stage of stress loading during the thermoelastic effect lead to changes in internal thermomechanical energy, which in turn causes changes in the surface magnetic field strength. When a material is subjected to external load and is in a state of stress damage accumulation, a large number of dislocations will be generated in its internal grains. Although no macroscopic damage defects appear on the material surface during the accumulation stage, the stress concentration caused by dislocations still leads to changes in the material's magnetic permeability and thermomechanical energy. The changes in the properties of metallic materials caused by stress damage are macroscopically manifested as abnormal changes in the magnetic field strength signal on the material surface.

[0037] A high-sensitivity magnetic induction probe was used to detect stress damage in aluminum alloy samples. Surface-point detection was performed using static tensile tests on aluminum alloys to analyze the force-magnetic relationship. The stress-strain curves obtained from the tensile mechanical property tests of aluminum alloys were used as the basis for different stress damage stages, including the elastic, plastic, and necking fracture stages. The three-dimensional surface magnetic field strength during the tensile process was also investigated to analyze the relationship between different stress damage stages and the surface magnetic field strength signal. Experimental results show that the surface magnetic field strength can sensitively identify the elastic, plastic, and necking fracture stages of the material, and accurately determine the yield strength—the critical point of stress damage. The reliability of the stress damage state assessment for aluminum alloy materials was further verified through thermomechanical effects and the damage-weakening magnetic mechanism.

[0038] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, wherein... Figure 5 In the middle, abc represents (a) B x With stress; (b) B y With stress; (c) B z With stress, Figure 6 , Figure 7 and Figure 8 In the diagram, a, b, c, and d represent: (a) probe 1; (b) probe 2; (c) probe 3; and (d) probe 4. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. An online monitoring method for the stress state of aluminum alloys based on weak magnetic field detection, characterized in that: Includes the following steps: S1. An external force is applied to both ends of an aluminum alloy sample to cause deformation. A high-sensitivity magnetic induction probe is placed above the sample surface in the tensile stress concentration area to monitor the change of magnetic field intensity signal in real time, thereby analyzing the influence of stress state on magnetic field distribution. S2. According to Gauss's law for magnetic fields, the magnetic field strength at the sample surface, σ0, is related to the change in internal magnetic induction intensity. The functional relationship is as follows: （1）； When the detection lift-off value is At that time, the magnetic field strength is 𝐵( Equation 2 （2）； From Equations 1 and 2, we can derive: （3）； As can be seen from the JAS model, in Equation 3 Since it is a function of stress , the magnetic field strength is... Associating with stress γ, taking the partial derivative of Equation 3 with respect to stress γ yields: （4）； According to the JAS model, the magnetic field strength of a nonferromagnetic material is: (5)； coefficients in Equation 5 Demagnetization coefficient It depends only on the sample size and the applied magnetic field. In this experiment, the geomagnetic field can be considered to be independent of stress R. It is a function of stress , which varies with different stress ; therefore, the partial differential of Equation 5 with respect to stress is: (6)； In the formula, for The constant; By combining equations 4 and 6, we can obtain the stress derivative of the magnetic field strength on the surface of a nonferromagnetic material. The relationship is: (7)； As can be seen from Equation 7, the surface magnetic field strength of aluminum alloy materials has a stress derivative. It is a function of axial position; when an aluminum alloy is under tensile stress, its magnetic field strength is... It changes with stress R, therefore, the magnetic field strength It can effectively characterize the stress state of aluminum alloy materials under tensile stress; S3. Online fixed-point weak magnetic field testing of aluminum alloy under stress loading: Four magnetic induction probes are set up on the surface of the tensile specimen to detect the surface magnetic field strength online. Each probe collects the surface magnetic field strength of the tensile specimen. The probes are connected and controlled by computer software, and the probe acquisition parameters are adjusted and the sampling frequency is selected. The ambient magnetic field before the test is detected, and the specimen without load is sampled. Then, the static load test is carried out. Each probe is stretched and collected simultaneously with the testing machine, from the beginning of the test until the specimen breaks. S4. Stress-strain curve analysis of aluminum alloys: The microstructure of aluminum alloys is usually composed of α-Al matrix phase and different types of strengthening phases. Under external load, the aluminum alloy material mainly goes through elastic deformation, plastic deformation and final fracture failure stages. OA stage: The elastic deformation stage of the material. This stage corresponds to the tensile process where the strain is less than a certain proportion. When the applied stress is removed, the deformation of the specimen disappears. That is, the specimen is in the elastic deformation stage. Point A represents the yield strength or yield point of the material. As can be seen from the figure, 7075 aluminum alloy does not show an obvious yield state. AB stage: The stage of plastic deformation of the material (the stage of material strengthening), when the applied stress exceeds the stress corresponding to point A. 0.2 Subsequently, the linear relationship between stress and strain is disrupted, strain increases significantly, and the deformation exhibits a uniform distribution. This phenomenon, where the resistance to plastic deformation increases with increasing plastic deformation, is called strain hardening or work hardening. When the stress reaches the stress corresponding to point B... 𝑏 When the uniform deformation of the sample stops, the stress reaches its maximum value, which is called the tensile strength of the material. The stress corresponding to this point is the resistance of the material to the maximum uniform plastic deformation, which is the maximum stress that the material can bear before tensile failure. BC stage: Necking fracture stage, when the applied stress exceeds the stress σ corresponding to point B. b At that time, the sample began to exhibit uneven plastic deformation, and local cross-sections experienced necking, leading to a decrease in stress. When the stress reaches point C, the material fractures. The stress at this point represents the material's ultimate resistance to plasticity. For 7075 aluminum alloy materials without obvious yield, a certain strain is used as its yield limit, and the yield point is determined by a certain offset method. The test is repeated in multiple groups. S5. Online fixed-point detection of magnetic field strength variation characteristics: The surface magnetic field strength variation trends detected in the four groups of tests are consistent. Taking sample No. 1 as the analysis, the magnetic field data of the four probes are extracted and plotted as three vectors of magnetic field (x, y, z). The measured magnetic field is compensated by the background magnetic field, and the magnetic field strength changes from zero. The three-dimensional surface magnetic field signal during the tensile process is combined with the stress curve during the loading process for research. The material elastic stage, plastic stage, and necking fracture stage of the stress curve are used as the analysis part of the magnetic field strength, which are regions I, II, and III, respectively. Analysis of magnetic field strength B x During the elastic deformation stage, the signals from probes 1-4 showed a decreasing trend, but with small signal fluctuations and poor magnetic field signal uniformity. During the plastic deformation stage, the magnetic field strength showed a decreasing trend and fluctuated to varying degrees. When the applied stress exceeded... When the sample entered the necking fracture stage, the magnetic field strength signals of each probe were relatively uniform and showed a downward trend; and at the moment of sample fracture, the magnetic field strength of each probe showed a rapid decrease. Analysis of magnetic field strength B y The changes in magnetic field strength measured during the elastic deformation stage are consistent, which is manifested as a rapid increase in signal. S6. Stress damage analysis of aluminum alloy under continuous tension: Analyze the magnetic field strength on the surface of the aluminum alloy under stress, using the probe's B... x The signal and applied stress were analyzed, and the stress and B during the test were recorded. x The signal change relationship, during the elastic deformation stage, is related to the surface magnetic field strength B of the tensile specimen. x All showed a monotonically decreasing trend, indicating that the stress in the elastic stage mainly caused reversible material deformation and had limited damage to the internal structure of the material. The change in magnetic field strength was mainly affected by the external load and no obvious material damage had yet occurred. During the plastic deformation stage and the necking fracture stage, stress damage occurs inside the specimen. The effect of stress on the magnetic field changes varies spatially at different measurement locations, which may be related to the degree of local plastic deformation. The causes of signal fluctuations can be explored from both macroscopic and microscopic levels. At the macroscopic level, the aluminum alloy sample undergoes local necking during the plastic deformation stage, resulting in irreversible deformation of the material. At the microscopic level, significant structural changes occur inside the aluminum alloy during plastic deformation; S7, collect B data from all probes y The signal and applied stress are analyzed to show the stress recorded during the test and B. y The relationship of signal changes, with B y The stress at the maximum value of the signal is expressed as: ; Yield strength of 7075 aluminum alloy In comparison, the surface magnetic field strength (probes 1-3) collected at the axial centerline of the sample characterizes... All less than yield strength The surface magnetic field strength collected at the transverse centerline (probe 4) characterizes... With yield strength Similarly, this demonstrates that the detection technology can determine the stress damage state of aluminum alloys before their yield point. According to the thermomechanical effect theory, the thermomechanical energy decreases during the elastic stage of the aluminum alloy and increases when it enters the plastic stage. This is consistent with the surface magnetic field strength B of the aluminum alloy. y The elastic and plastic stages exhibit opposite changes, similar to B. x The signals are the same; the magnetic field strength change curve is smooth in the elastic stage. During plastic deformation of the sample, internal stress damage accumulates and increases continuously with the application of stress. Therefore, when the stress exceeds the yield point, the magnetic field strength fluctuates. In summary, B y The signal can serve as an early warning for stress damage in aluminum alloys and is a feasible option for clearly determining stress damage in aluminum alloys. S8, Analysis of B under stress z Signals, showing the stress and B recorded during the test. z The signal change relationship, during the elastic deformation stage, the B of the tensile specimen z The signals all showed a monotonically increasing trend. During the plastic deformation and necking fracture stages, due to stress damage inside the sample, the B signal detected by probes 1-3... z In addition to showing a rapid upward trend, the signal also exhibited signal fluctuations; while the B signal detected by probe number 4... z The signal showed a downward trend, accompanied by strong signal fluctuations; the effect of stress on magnetic field changes varied significantly at different measurement locations, which may be related to the degree of local plastic deformation. In the study of stress-thermomechanical effects in aluminum alloys, the cooling phenomenon of the material during the elastic tensile stage is a typical thermoelastic effect. The lowest temperature at the elastic-plastic transition can be used to determine the start of yield strength. Under the action of thermomechanical effects, the energy change of the material during deformation can be characterized by temperature measurement. When its internal energy changes, it will cause thermomagnetic induction phenomenon in the material. During stress loading, the elastic and plastic stages of the material are accompanied by changes in internal energy, which in turn cause corresponding changes in magnetic field strength. S9. During the elastic loading stage, the deformation of the material is mainly elastic. According to thermodynamic theory, the energy change of the material during elastic deformation mainly originates from the mutual conversion between the potential energy and kinetic energy of the lattice atoms. For aluminum alloys, during adiabatic elastic tension or adiabatic compression, their thermomechanical energy decreases, accompanied by a corresponding change in the sample temperature. This phenomenon is called the thermoelastic effect. During continuous tensile deformation, the mechanical energy of plastic deformation is expressed as heat energy, manifested as a rise in the material temperature. At this time, macroscopic plastic deformation gradually becomes the dominant deformation mechanism. Therefore, during the tensile deformation of aluminum alloys, the stress tensile energy of the sample initially shows a decreasing trend, reaches a minimum value, and then rises rapidly. This behavior of strained materials can be used to determine the onset of plastic deformation. Correspondingly, the detected magnetic field strength B... y ; Thermomechanical effects refer to the coupling effect of thermal and mechanical behavior of materials during deformation. Among them, thermoelastic and thermoplastic effects are the two core aspects of research. Thermoelastic effects have a clear theoretical definition and can be derived from thermodynamic and elastic theories. Thermoelastic effects include changes in stress, strain, temperature and entropy. The adiabatic thermoelastic theory is based on the fact that there is no heat exchange between the system and the surrounding environment during deformation. Therefore, the change in energy within the system is only caused by the mechanical energy of the sample. In the case of uniaxial tensile loading of a metal specimen, a uniform strain energy will be generated inside the specimen, assuming density... Not following As E and T change, the free energy equation can be expressed as: (8)； In the formula, This is for material strain. Specific heat capacity at constant volume For an adiabatic process, Equation 8 can be written as: (9)； Therefore, the integrated formula 9 and the assumptions , and Since the effects of strain and temperature on parameters are negligible, it can be rewritten as: (10)； Formula 10 shows the relationship between temperature change and applied stress change during adiabatic uniaxial tensile deformation through the thermoelastic effect. It is clear from the equation that the temperature change... With elastic stress The changes are linearly related, where the slope As a thermoelastic factor, the parameter changes at each stage of stress loading during the thermoelastic effect will lead to changes in internal thermomechanical energy, thereby causing changes in surface magnetic field strength. When a material is subjected to external load and is in a state of stress damage accumulation, a large number of dislocations will be generated in its internal grains. Although no macroscopic damage defects appear on the material surface during the accumulation stage, the stress concentration caused by dislocations will still cause changes in the material's magnetic permeability and thermomechanical energy. The changes in the properties of metallic materials caused by stress damage are macroscopically manifested as abnormal changes in the magnetic field strength signal on the material surface.

2. The online monitoring method for the stress state of aluminum alloy based on weak magnetic field detection according to claim 1, characterized in that: The research on S4 dislocation theory and crystallography shows that material deformation can be divided into two parts: elastic deformation and plastic deformation. Elastic deformation is caused by lattice distortion, while plastic deformation is caused by the movement of a large number of dislocations within the grains along specific crystal planes. When the stress is continuously applied to the tensile strength, the aluminum alloy material undergoes necking fracture. In the elastic deformation stage, the deformation is mainly achieved through atomic displacement within the lattice, i.e., lattice distortion. Under stress, a reversible change in lattice spacing occurs. When the external load is removed, the material can return to its original state. As the stress continues to increase to the yield strength, the aluminum alloy material will enter the plastic deformation stage. At this time, dislocation slip becomes the main deformation mechanism. Especially in high stress concentration areas, the movement and proliferation of dislocations may lead to microstructural evolution such as grain boundary slip and the interaction between precipitates and the matrix.

3. The online monitoring method for the stress state of aluminum alloy based on weak magnetic field detection according to claim 2, characterized in that: The aforementioned S6 lacks an effective non-destructive testing (NDT) technique for online stress damage assessment of aluminum alloys. When aluminum alloys exceed their yield strength, they rapidly enter the plastic deformation failure stage, a highly dangerous condition for engineering equipment. To prevent this, emergency measures should be taken promptly before the critical yield point is reached under load. In tensile tests, the surface magnetic field strength of aluminum alloy specimens exhibits corresponding signal changes before and after the yield stage, as well as during necking fracture. It is noteworthy that B... y The signal exhibits a clear signal inflection point at the yield point, changing from a monotonically linear increase to a monotonically linear decrease. This phenomenon provides a basis for stress damage assessment of aluminum alloys.

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