A high-sensitivity stress nondestructive testing method based on equivalent magnetization surface effect

Abstract

This invention discloses a highly sensitive stress nondestructive testing method based on the equivalent magnetization surface effect, belonging to the field of testing, and includes the following steps: Step 1: An external magnetic field is generated by using a square wave with a constant voltage amplitude as the excitation power supply and applied to the surface of the material under test; Step 2: An electromagnetic signal generated by magnetic domain wall displacement is collected using a multi-turn coil with a magnetic core, and the electromagnetic signal generated by the equivalent magnetization surface displacement is separated and recorded as the secondary response signal; Step 3: The characteristic value of the electromagnetic signal generated by the equivalent magnetization surface displacement is calculated; Step 4: The characteristic value of the electromagnetic signal generated by the equivalent magnetization surface displacement is used for stress detection. This invention uses a square wave excitation method to generate an equivalent magnetization surface effect in the material under test, exciting the generation of a secondary response electromagnetic signal, and using the secondary response electromagnetic signal and its characteristic value for stress detection to achieve highly sensitive stress nondestructive testing.

Description

Technical Field

[0001] This invention specifically relates to a high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect, belonging to the field of testing. Background Technology

[0002] The domain structure in ferromagnetic materials is affected by external stress (tensile stress reduces domain wall energy, while compressive stress increases it). Furthermore, if alternating magnetic fields are applied to a ferromagnetic material, its domain walls will move and emit electromagnetic pulses under the influence of the external magnetic field. This movement (wall displacement) is closely related to the domain wall energy. Therefore, the stress state of the material can be detected by detecting the electromagnetic pulse signals emitted during the movement of the domain walls, thus achieving non-destructive stress detection.

[0003] Based on this principle, Barkhausen noise technology is currently used for stress detection, and its system structure is as follows: Figure 1 As shown. The continuously changing current signal (such as a sine wave or triangular wave) generated by the signal generator is amplified by the power amplifier and applied to both ends of the magnetizing coil, causing the magnetizing coil to generate an alternating magnetic field, which locally magnetizes the ferromagnetic material under test, thereby exciting a Barkhausen noise signal inside the test piece. The signal detection coil consists of a cylindrical multi-turn coil with a magnetic core or a Hall sensor, used to collect the effective Barkhausen noise signal. The function of the signal conditioning circuit is to amplify (more than 1000 times), filter (1k to 1MHz) the signal picked up by the detection coil, suppress interference signals, and then extract the feature values ​​of the signal. The data acquisition card is used to collect the signal from the output of the conditioning circuit and transmit it to the PC for further signal processing and feature value extraction. Based on the above system (sine excitation, such as...), Figure 2 The Barkhausen signal acquired (as shown by the curve in the middle) is as follows: Figure 2 As shown, L is the duration of the Barkhausen signal under sinusoidal excitation.

[0004] The degree of influence of stress on magnetic domain walls with different domain wall energies varies. On the one hand, it causes changes in the intensity of the electromagnetic signal generated by the wall displacement, and on the other hand, it causes a shift in the timing of the wall displacement.

[0005] The current technical solution (based on the Barkhausen effect) causes the excitation signal to change continuously, and the magnetic field it excites is also a continuously changing external magnetic field acting on the ferromagnetic material. This results in all the magnetic domain walls in the material being in a changing magnetic field at all times. The electromagnetic signals generated by the magnetic domain walls with different degrees of stress influence are superimposed together, which inevitably leads to low sensitivity to stress detection.

[0006] Furthermore, since the working principle of the signal acquisition probe is to be sensitive to the magnetic field generated by electromagnetic pulses, thereby generating an induced electromotive force, the current technology is constantly changing the excitation power supply while acquiring electromagnetic pulse signals in materials. As the magnetic field induced in space by the changing power supply (including higher harmonics) is also partially sensed by the acquisition probe to generate an induced electromotive force, the signal-to-noise ratio is reduced and it is not conducive to improving the sensitivity of stress detection.

[0007] Therefore, it can be seen from the principle of Barkhausen stress detection technology that the existing excitation method determines that its stress detection sensitivity is difficult to improve further. Summary of the Invention

[0008] The purpose of this invention is to address the shortcomings of existing technologies by proposing a highly sensitive stress nondestructive testing method based on the equivalent magnetization surface effect. This method employs a square wave excitation mode to generate an equivalent magnetization surface effect in the material under test, thereby generating a secondary response electromagnetic signal. The secondary response electromagnetic signal and its characteristic values ​​are then used for stress detection to achieve highly sensitive stress nondestructive testing.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0010] A high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect includes the following steps:

[0011] Step 1: Use a square wave with a constant voltage amplitude as the excitation power supply to generate an external magnetic field, which is applied to the surface of the material being tested;

[0012] Step 2: Use a multi-turn coil with a magnetic core to collect the electromagnetic signal generated by the magnetic domain wall displacement, and separate the electromagnetic signal generated by the displacement of the equivalent magnetization surface, which is recorded as the secondary response signal;

[0013] Step 3: Calculate the characteristic values ​​of the electromagnetic signal generated by the equivalent magnetization surface displacement;

[0014] Step 4: Use the electromagnetic signal characteristic values ​​generated by the equivalent magnetization surface displacement for stress detection.

[0015] Furthermore, the square wave excitation frequency in step 1 is ≤5Hz to obtain a complete sub-response signal.

[0016] Preferably, in step 1, the square wave excitation is generated by a signal generator and the amplitude of the square wave signal is increased by a power amplifier to drive the excitation coil to generate an external magnetic field that acts on the surface of the material being tested.

[0017] Preferably, the equivalent magnetization surface is equivalent to a single magnetization surface, which is the domain wall that is simultaneously displaced by the external magnetic field in the multi-domain structure.

[0018] Furthermore, step 2 requires preprocessing the electromagnetic signals during acquisition, specifically including the following steps:

[0019] A: The acquisition coil is placed close to the surface of the material being tested to collect electromagnetic pulse signals generated by the movement of magnetic domain walls in the material;

[0020] B: The signal from the acquisition coil is amplified by a hardware amplifier circuit;

[0021] C: A data acquisition card is used to acquire the output signal of the amplifier circuit at high speed.

[0022] Preferably, the amplification factor of the hardware amplifier circuit is greater than 1000, and the high-speed acquisition frequency of the data acquisition card is greater than or equal to 500kHz.

[0023] Furthermore, the secondary response signal is the electromagnetic pulse induced voltage generated by the domain wall motion after the excitation power supply completes the level transition and maintains it for a period of time.

[0024] Furthermore, by extracting the first two groups of signals containing power supply induced spikes from the electromagnetic signal graph, and continuing the signal until the next excitation transition edge, the equivalent magnetization surface response signal is obtained.

[0025] Furthermore, when the square wave excitation voltage frequency is 2Hz or 5Hz, on the electromagnetic signal diagram, the starting point of the secondary response signal is taken as 50ms after the excitation zero crossing point, and the ending point of the secondary response signal is taken as 56ms.

[0026] Furthermore, the eigenvalues ​​of the secondary response signal are solved by root mean square and then normalized.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] This invention proposes a high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect. It adopts a square wave excitation method to generate an equivalent magnetization surface effect in the material under test, thereby generating a secondary response electromagnetic signal. The secondary response electromagnetic signal and its characteristic values ​​are then used for stress detection to achieve high-sensitivity stress nondestructive testing. Attached Figure Description

[0029] Figure 1 Here is a block diagram of the existing Barkhausen noise detection system.

[0030] Figure 2 The Barkhausen signal diagram is obtained using existing technology;

[0031] Figure 3 This is a technical framework diagram of the present invention;

[0032] Figure 4 This is a waveform diagram of the excitation source signal of the present invention;

[0033] Figure 5 This is an enlarged view of the electromagnetic signal under square wave excitation of the present invention and its local area.

[0034] Figure 6 This invention compares the sensitivity of root mean square eigenvalues ​​to existing technologies using sine waves, triangular waves, and square waves under excitation. Detailed Implementation

[0035] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] During scientific research, this invention discovered that when an external magnetic field is applied to a ferromagnetic material, its magnetization is not instantaneous but rather involves a magnetization process (with a magnetization time on the order of milliseconds, significantly longer than the wall displacement time of a single domain wall on the order of nanoseconds). When a large number of magnetic domain structures exist in the material, domains with lower domain wall energies are more prone to movement than those with higher domain wall energies. Simultaneously, the movement of domain walls inevitably leads to changes in magnetic vectors. Under the influence of microscopic eddy currents, even if the external magnetic field is removed at this point, the low-energy domain walls have already completed wall displacement and magnetic vector changes. This disrupts the condition for minimizing the total domain wall energy within the material, thus initiating the intrinsic magnetization stage. During this stage, the domain walls of different magnetic domains interact through microscopic eddy current damping, forming an equivalent magnetization surface with a multi-domain wall structure (a concept first proposed internationally by the inventors, where multiple adjacent domains undergoing wall displacement simultaneously when an external magnetic field is applied to both ends of the material constitute an equivalent magnetization surface). All domain wall energies within the material will reach re-equilibrium (total domain wall energy at its minimum). The domain wall displacement occurring during this stage generates electromagnetic signal transmission again after the applied magnetic field ceases to change, thus achieving electromagnetic signal separation. It is particularly important to note that the electromagnetic signal generated after the applied magnetic field remains unchanged depends entirely on the stress-induced change in the material's microscopic eddy current effect and does not include spatial interference from the excitation power supply. Although its total energy is less than the electromagnetic signal generated by the aforementioned Barkhausen effect, its sensitivity to stress should be higher than that of the Barkhausen noise signal. This is precisely the objective of this invention.

[0037] like Figure 3 As shown, the present invention includes the following steps:

[0038] Step 1: Use a square wave with a constant voltage amplitude as the excitation power supply to generate an external magnetic field, which is applied to the surface of the material being tested;

[0039] Step 2: Use a multi-turn coil with a magnetic core to collect the electromagnetic signal generated by the magnetic domain wall displacement, and separate the electromagnetic signal generated by the displacement of the equivalent magnetization surface, which is recorded as the secondary response signal;

[0040] Step 3: Calculate the characteristic values ​​of the electromagnetic signal generated by the equivalent magnetization surface displacement;

[0041] Step 4: Use the electromagnetic signal characteristic values ​​generated by the equivalent magnetization surface displacement for stress detection.

[0042] Furthermore, the square wave excitation frequency in step 1 is ≤5Hz to obtain a complete sub-response signal.

[0043] In step 1, the square wave excitation is generated by a signal generator and the amplitude of the square wave signal is increased by a power amplifier to drive the excitation coil to generate an external magnetic field that acts on the surface of the material being tested.

[0044] Step 2 requires preprocessing the electromagnetic signals during acquisition, specifically including the following steps:

[0045] A: The acquisition coil is placed close to the surface of the material being tested to collect electromagnetic pulse signals generated by the movement of magnetic domain walls in the material;

[0046] B: The signal from the acquisition coil is amplified by a hardware amplifier circuit;

[0047] C: A data acquisition card is used to acquire the output signal of the amplifier circuit at high speed.

[0048] Among them, the amplification factor of the hardware amplifier circuit is >1000, and the high-speed acquisition frequency of the data acquisition card is ≥500kHz.

[0049] First, this invention requires the acquisition of electromagnetic signals generated by the movement of magnetic domain walls (it should be noted that these electromagnetic signals are not generated by alternating magnetic fields, and therefore are not Barkhausen noise). Therefore, its signal acquisition section is related to... Figure 1 The structures within the red boxes are identical, namely, the acquisition coil, signal conditioning, and data acquisition, followed by transmission to a PC for signal analysis. To obtain electromagnetic signals solely caused by microscopic eddy current effects within the material, the excitation source used in this invention is an alternating change + voltage holding (i.e., square wave excitation), such as... Figure 4 As shown, each excitation cycle consists of four stages: rising edge, falling edge, high level, and low level.

[0050] The electromagnetic signal waveforms obtained under the experimental system and excitation parameters described in this invention are as follows: Figure 5As shown, the power supply induced spikes correspond to the impulse voltages induced in the detection coil at the rising and falling edges of the excitation signal, and their values ​​are significantly higher than the induced voltages generated by domain wall motion. The main response signal corresponds to the electromagnetic pulse induced voltage generated by a large amount of domain wall motion immediately after the excitation completes the level transition. The secondary response signal corresponds to the electromagnetic pulse induced voltage generated by domain wall motion after the excitation power supply completes the level transition and maintains it for a period of time (approximately 90 milliseconds). Specifically, according to the definition of Barkhausen noise, when the excitation voltage remains constant, there should be no [noise spikes]. Figure 5 (b) shows the electromagnetic signal; furthermore, in comparison Figure 2 and Figure 5 The Barkhausen signal generated under sinusoidal excitation is continuous in the time domain, while the electromagnetic signal generated under square wave excitation is clearly divided into two parts (referred to as the main response and secondary response in this invention), with a significant discontinuity between the secondary response and the main response (ignoring instrument white noise). Based on these two differences, the electromagnetic signal (secondary response signal) analyzed in this invention is not Barkhausen noise, but an electromagnetic pulse signal generated by the intrinsic magnetization process caused by the equivalent magnetization surface effect.

[0051] Secondly, according to the theoretical analysis of this invention, the sensitivity of feature extraction for all electromagnetic signals is reduced due to the aliasing of signals from different energy level domain walls and high-order harmonic induction signals from the power supply. However, the difference in domain wall energy changes between low and high energy level domain walls caused by stress will weaken the microscopic eddy current effect (mainly manifested as eddy current damping) in the "intrinsic magnetization" process, thereby obtaining a separated sub-response signal. This part of the signal concentrates the motion characteristics of the high domain wall energy part of the magnetic domain affected by stress. Therefore, although the overall signal energy is low, the signal-to-noise ratio for stress detection should be significantly improved.

[0052] Finally, based on the above analysis, this invention uses the root mean square (RMS) parameter, which reflects the average energy of domain wall motion, to characterize stress changes. The RMS parameter selected in this invention is calculated as shown in formula (1).

[0053]

[0054] Where n is the number of sampling points and X is the amplitude of the electromagnetic signal at the sampling point.

[0055] This invention, through theoretical analysis and experimental verification, proposes using a square wave excitation voltage to separate the primary and secondary electromagnetic response signals using the Barkhausen effect and the equivalent magnetization surface effect. Only the secondary response signal is used to calculate the RMS eigenvalue to characterize stress. Compared to sinusoidal wave excitation, the root mean square parameter under square wave excitation has a better linear relationship with stress. This is because changes in stress will affect the Barcol noise generation process under square wave excitation, and this is reflected in the overall energy of the Barcol noise. Based on the above analysis, the stress value inside or on the surface of the specimen will differ from that of the control sample or other parts depending on its internal structure, state, and defects. This enables the detection of stress using the Barcol signal under square wave excitation.

[0056] Depend on Figure 6 It can be seen that, by normalizing the root mean square eigenvalues, when the applied stress changes from 0 to 150 MPa, the relative change in RMS under 2Hz sinusoidal / triangular wave excitation does not exceed 30%, and the relative change in RMS under 5Hz excitation is about 20%. However, under 2Hz / 5Hz square wave excitation, the relative change rate of the secondary response signal RMS caused by the equivalent magnetization surface effect exceeds 60%, especially under 2Hz square wave excitation, the relative change of its eigenvalues ​​exceeds 70%, and the stress detection sensitivity is 2-3 times that of Barkhausen noise under sinusoidal / triangular wave excitation.

[0057] In particular, it can be seen that the RMS of the Barkhausen noise signal decreases with increasing compressive stress, while the RMS of the electromagnetic pulse signal caused by the "equivalent magnetization surface" effect described in this invention increases with increasing compressive stress. This also confirms that the electromagnetic signal of this invention is not a traditional Barkhausen noise signal.

[0058] This invention employs a square wave excitation method to generate an "equivalent magnetization surface" effect in ferromagnetic materials. Due to this effect, when the excitation voltage remains constant, a secondary response electromagnetic signal is generated in the material. Only the eigenvalues ​​of the secondary response signal are calculated, using the root mean square (RMS) calculation method employed in this invention. The signals and eigenvalues ​​obtained from the above steps are used for stress detection. Furthermore, to ensure the acquisition of the secondary response signal, the square wave excitation frequency does not exceed 5Hz; this invention uses an excitation frequency below 5Hz, achieving good results. Theoretically, this method is also applicable to the detection of changes in the microstructure of materials caused by other factors, such as surface hardness and corrosion.

[0059] Physical principles reveal that ferromagnetic materials contain numerous magnetic domain structures, with magnetic vector changes primarily achieved through the movement of domain walls. The movement of a single domain wall is inevitably influenced by the magnetic vectors of its surrounding domains (microscopic eddy current effect). Furthermore, theoretical studies have demonstrated that domain walls possess energy, and these energies are distinct. This means that the movement of numerous domain walls under an applied magnetic field cannot occur simultaneously, resulting in a significant magnetization time. Moreover, the excitation signal inevitably contains harmonic components. If the excitation remains constantly changing during signal acquisition, higher-order harmonics can easily induce noise voltage in the sensor probe. Based on these analyses, the inventors propose an "equivalent magnetization surface" model, which equates the simultaneous displacement of domain walls in a multi-domain structure under the influence of an applied magnetic field to a single "magnetization surface." Clearly, domain walls with lower energy will displace faster along the applied magnetic field direction than those with higher energy, causing the "equivalent magnetization surface" to bend. Due to the microscopic eddy current effect between domain walls, after the applied magnetic field reaches a steady-state value for a certain period of time, a large number of magnetic domain walls within the material will redistribute their domain wall energy, ultimately achieving the stable existence of the magnetization surface. During the process of the "equivalent magnetization surface" bending and reaching a stable position, the interaction of adjacent domain walls generates a regenerative electromagnetic pulse signal (secondary response signal). (At this point, the applied magnetic field needs to remain constant, which requires the excitation to change rapidly and then maintain a stable value; therefore, square wave excitation is the only suitable excitation method.) Treating the microscopic eddy current effect as damping the movement of the "equivalent magnetization surface," and drawing on a second-order system response model, as the damping increases, the bending of the "equivalent magnetization surface" becomes more significant, resulting in a more pronounced secondary response signal. Correspondingly, the amplitude and energy of the secondary response signal increase, so the signal characteristic values ​​(RMS, peak value) all show a positive correlation. Since the secondary response is generated entirely by the bending of the "equivalent magnetization surface" inside the material to "reset", it is more correlated with the stress. Theoretically, although the signal amplitude is lower than that of the main response, the signal is more significantly affected by stress. This is the theoretical basis for the high sensitivity described in this invention.

[0060] Furthermore, as can be seen from the above principles, the excitation needs to remain constant for a period of time after the change; and if the excitation is constantly changing, it will cause the domain walls of different energy levels in the material to gradually shift, which will overlap with the domain wall displacement caused by the microscopic eddy current effect, making it difficult to separate the secondary response signal generated by the "equivalent magnetization surface".

[0061] Furthermore, since the occurrence time of the secondary response lags significantly behind the zero-crossing point of the excitation, the excitation frequency must be kept low in order to obtain a complete secondary response signal. This invention has verified through experiments that a square wave excitation below 5Hz can obtain a complete secondary response signal.

[0062] Therefore, the core innovation of this invention is to use low-frequency square wave excitation to generate an external magnetic field to separate the secondary response electromagnetic signal generated by the "equivalent magnetization surface". The feature extraction of the obtained secondary response signal and the comparison with the loading stress of the material prove that the method described in this invention does have the ability to improve the sensitivity by 100% compared with the existing Barkhausen method.

[0063] In the description provided herein, a large number of specific details are described. However, it is understood that embodiments of the present invention can be practiced without these specific details. In some instances, well-known methods, structures, and techniques are not shown in detail so as not to obscure the understanding of this description.

Claims

1. A high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect, characterized in that, Includes the following steps: Step 1: Use a square wave with a constant voltage amplitude as the excitation power supply to generate an external magnetic field, which is applied to the surface of the material being tested; Step 2: Use a multi-turn coil with a magnetic core to collect the electromagnetic signal generated by the magnetic domain wall displacement, and separate the electromagnetic signal generated by the displacement of the equivalent magnetization surface, which is recorded as the secondary response signal; Step 3: Calculate the characteristic values ​​of the electromagnetic signal generated by the equivalent magnetization surface displacement; Step 4: Use the electromagnetic signal characteristic values ​​generated by the equivalent magnetization surface displacement for stress detection; An equivalent magnetization surface is a single magnetization surface that represents the domain walls in a multi-domain structure that are simultaneously displaced by an external magnetic field. The response signal is the electromagnetic pulse induced voltage generated by the domain wall motion after the excitation power supply completes the level transition and maintains it for a period of time.

2. The high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect according to claim 1, characterized in that: The square wave excitation frequency in step 1 is ≤5Hz to obtain a complete sub-response signal.

3. The high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect according to claim 1, characterized in that: Step 1: The square wave excitation is generated by a signal generator and the amplitude of the square wave signal is increased by a power amplifier to drive the excitation coil to generate an external magnetic field that acts on the surface of the material being tested.

4. The high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect according to claim 1, characterized in that, Step 2 requires preprocessing the electromagnetic signals during acquisition, specifically including the following steps: A: The acquisition coil is placed close to the surface of the material being tested to collect electromagnetic pulse signals generated by the movement of magnetic domain walls in the material; B: The signal from the acquisition coil is amplified using a hardware amplifier circuit; C: A data acquisition card is used to acquire the output signal of the amplifier circuit at high speed.

5. The high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect according to claim 4, characterized in that: The amplification factor of the hardware amplifier circuit is >1000, and the high-speed acquisition frequency of the data acquisition card is ≥500kHz.

6. The high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect according to claim 1, characterized in that: The secondary response signal is obtained by extracting the continuous signal between the first two groups of signals containing power-induced spikes on the electromagnetic signal graph.

7. The high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect according to claim 1, characterized in that, When the square wave excitation voltage frequency is 2Hz or 5Hz, on the electromagnetic signal diagram, the starting point of the secondary response signal is taken as 50ms after the excitation zero crossing point, and the ending point of the secondary response signal is taken as 56ms.

8. The high-sensitivity stress nondestructive testing method based on the equivalent magnetization surface effect according to claim 1, characterized in that: The eigenvalues ​​of the response signal are solved by root mean square and then normalized.

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