阅读说明：本技术 一种铁磁性金属构件疲劳损伤的磁致声发射检测方法 (Magneto acoustic emission detection method for fatigue damage of ferromagnetic metal component ) 是由 沈功田 沈永娜 张文君 于 2020-05-28 设计创作，主要内容包括：一种铁磁性金属构件疲劳损伤的磁致声发射检测方法,首先,采用正弦波电压信号产生激励磁场,逐步提高加载电压,直至获得具有明显尾峰的双峰包络的磁声发射信号,此时所对应的电压峰峰值作为参考电压；随后,以等于或高于参考电压的方波电压信号产生激励磁场,获取具有“T”型包络的磁声发射信号；再次,计算多个周期内磁声发射信号的峰峰值平均值作为特征参数,其随疲劳裂纹萌生、扩展的发展出现拐点变化,从而据此识别出疲劳微裂纹的萌生和扩展,对构件失效作出及时预警。本发明采用方波电压激励获得“T”形包络信号,其波形清晰易辨,在参考电压确保了足够磁场强度的基础上,大幅提升了MAE的抗噪声能力,实现了MAE疲劳监测工程化应用,检测结果稳定可靠。(A ferromagnetic metal component fatigue damage magnetic acoustic emission detection method, firstly, adopt the sine wave voltage signal to produce the excitation magnetic field, raise the loading voltage step by step, until obtaining the magnetic acoustic emission signal with double peak envelope of the obvious tail peak, the peak value of voltage peak corresponded to at this moment is regarded as the reference voltage; then, generating an excitation magnetic field by a square wave voltage signal equal to or higher than a reference voltage, and acquiring a magnetoacoustic emission signal with a T-shaped envelope; and thirdly, calculating the average value of the peak value and the peak value of the magnetic acoustic emission signals in a plurality of periods as characteristic parameters, wherein the average value of the peak value and the peak value of the magnetic acoustic emission signals in the plurality of periods is changed along with the inflection point of the fatigue crack initiation and expansion development, thereby identifying the initiation and expansion of the fatigue crack and giving timely early warning on the component failure. The square wave voltage is adopted for excitation to obtain the T-shaped envelope signal, the waveform is clear and easy to distinguish, the anti-noise capability of the MAE is greatly improved on the basis that the reference voltage ensures enough magnetic field intensity, the MAE fatigue monitoring engineering application is realized, and the detection result is stable and reliable.)

1. A magnetic acoustic emission detection method for ferromagnetic metal member fatigue damage specifically comprises the following steps:

firstly, a U-shaped electromagnetic yoke and an acoustic emission sensor are arranged in a detected area of a detected ferromagnetic metal component which is not subjected to fatigue load in a relatively fixed position, the yoke and the component form a magnetic loop, and the acoustic emission sensor is used for collecting a magneto acoustic emission signal;

the method is characterized in that the subsequent detection step is that square wave excitation is adopted on the basis of obtaining sine wave reference voltage, so that a stable and effective excitation source is obtained, the magnetization intensity inside the material of a detected component is kept saturated in the fatigue process, and the finally obtained peak-to-peak voltage of the MAE signal with the T-shaped envelope effectively indicates the starting time of rapid crack expansion in the II stage along with the inflection point change of a fatigue cycle;

the detection steps are as follows:

(1) exciting a magnetic field by a sine wave to obtain a reference voltage Vpp 1;

loading a sine wave voltage signal to a coil of a U-shaped electromagnetic yoke, and adjusting the peak-to-peak value of the loading signal from small to large until the time domain waveform of the acquired magneto acoustic emission signal has a double-peak envelope with an obvious tail peak, wherein the voltage peak-to-peak value of the loading signal is recorded as a reference voltage Vpp 1;

(2) a square wave excitation of Vpp1 based on a reference voltage;

keeping the frequency of the signal unchanged, setting the voltage peak value of the signal to be n times of Vpp1, wherein n is more than or equal to 1, generating an excitation magnetic field by adopting a square wave voltage signal with the duty ratio of 50%, and at the moment, the time domain waveform of the obtained magneto acoustic emission signal has a T-shaped envelope;

(3) filtering the signal;

filtering the magnetic emission signals of M magnetization periods, extracting the peak-to-peak voltage of the magnetic emission signals excited by the square wave voltage, and averaging the peak-to-peak voltage in the M magnetization periods;

(4) monitoring fatigue and judging the occurrence of fatigue crack propagation in the second stage;

and (3) detecting the same region of the detected ferromagnetic metal component subjected to different fatigue cycles by adopting the square wave voltage signals with the frequency and the voltage peak-to-peak value in the step (2), and analyzing a variation curve of the peak-to-peak value voltage average value of the magneto acoustic emission signal along with the detection cycle to judge whether the detected component starts the quick expansion of the fatigue crack in the second stage.

2. The method for detecting the magnetoacoustic emission of fatigue damage of a ferromagnetic metal member as set forth in claim 1, wherein the step (4) takes an average of peak-to-peak voltages of the magnetoacoustic emission signals of M magnetization cycles as a characteristic parameter, and the characteristic parameter is a starting time of rapid fatigue crack propagation in phase II when an inflection point appears after continuous monotonic decrease along a variation curve of a fatigue cycle:

(4.1) with the increase of the fatigue period, the peak-to-peak voltage of the obtained magneto acoustic emission signal fluctuates firstly and then monotonically decreases, so that the fatigue crack of the component does not rapidly expand in the second stage;

(4.2) with the increase of the fatigue period, the peak-to-peak voltage of the obtained magnetoacoustic emission signal changes from monotone reduction to gradual increase, and the component enters the stage II fatigue crack to rapidly propagate.

3. A method for detecting the fatigue damage of a ferromagnetic metal member as set forth in claim 1, wherein in the step (2), n is 1 to n₁,n₁The output maximum voltage/Vpp 1 of the signal generator providing the voltage signal.

4. A method for detecting a fatigue damage of a ferromagnetic metal member as set forth in claim 1, wherein M in step (3) is 20 or more.

5. A method for detecting a fatigue damage of a ferromagnetic metal member by a magnetoacoustic emission as claimed in claim 1, wherein the U-shaped electromagnetic yoke is replaced with a coil, and the detected region is disposed inside the coil.

Technical Field

The invention belongs to the technical field of nondestructive detection of material damage, and particularly relates to a method for detecting the fatigue damage of a ferromagnetic metal member by using magnetic acoustic emission.

Background

The fatigue life of a metal component subjected to cyclic loads for a long period of time can be divided into three phases: (1) the first stage is a fatigue hardening or softening stage, the microstructure of the material in the first stage is changed integrally, and a stress strain concentration area is finally formed to generate fatigue damage; (2) the second stage is a crack initiation stage, in which the crack length is generally not greater than the grain size (10 μm-100 μm); (3) the third stage is a crack propagation stage, which is divided into a stage I stable crack propagation stage and a stage II fast crack propagation stage, wherein the stage I stable crack propagation refers to the propagation of the microcracks along the original direction, and the stage II fast fatigue crack propagation refers to the main crack formed by the microcracks and the main crack propagating along the direction vertical to the maximum main tensile stress. Once the fatigue crack in the II stage rapidly expands, the component is often failed quickly, so that the starting time of the fatigue crack expansion in the II stage is determined, early warning is carried out, and great significance is brought to guarantee the safe operation of the component.

The traditional nondestructive detection technology, such as magnetic powder, eddy current, infiltration and the like, can only detect macroscopic or visual fatigue cracks, namely, the fatigue cracks in the II stage can rapidly expand to certain length. Magnetic Acoustic Emission (MAE) refers to a phenomenon that a ferromagnetic metal material generates acoustic emission due to the change of a magnetic domain structure under the action of an alternating magnetic field, and an MAE signal is very sensitive to a microstructure and a stress state of the ferromagnetic metal material. At present, for an alternating magnetic field used for MAE detection, a sine wave or a triangular wave current or voltage signal with 50% of symmetry is usually excited to generate, and the maximum intensity of the alternating magnetic field is determined by the amplitude and the frequency of the current or voltage signal. Research on the application of magnetoacoustic emission in nondestructive rail performance testing [ J ] experimental mechanics, 1998 (01): 99-105), MAE (mean square root) of U74 rail steel with different fatigue cycles is tested by adopting sine wave voltage signals, and the result shows that the RMS (root mean square) voltage of the MAE signals increases and then decreases monotonically with the increase of the fatigue cycle times, reaches the maximum value at about 20% of the fatigue life, and the law is more obvious when the magnetic field strength Hmax is larger (figure 1), but the stable expansion of the fatigue crack in the I stage and the rapid expansion of the fatigue crack in the II stage cannot be distinguished. Similarly, the inventor of the present application adopts a sine wave voltage signal to excite Q235 steel to obtain the variation of the root mean square voltage of the MAE signal with the fatigue cycle, and also obtains a similar rule (fig. 2), and cannot distinguish two stages of the stable expansion of the fatigue crack in stage I and the rapid expansion of the fatigue crack in stage II.

The inventor analyzes the influence of an alternating magnetic field on the change rule of the MAE along with fatigue damage through research on the relation between the amplitude and the waveform of an excitation signal and the intensity of the alternating magnetic field, and discovers that the process is accompanied with stress release and discontinuous formation of the surface of the material by analyzing the change of the stress state and the microstructure state of the material or a member when a fatigue crack is stably expanded from a first stage to a second stage and is rapidly expanded, wherein (1) the MAE signal intensity is increased through the stress release (the MAE signal intensity is inversely related to the stress), and (2) the magnetization intensity of the material is reduced through the discontinuous surface of the material, so that the number of magnetic domain walls which are irreversibly moved is reduced, and the MAE signal intensity is reduced, namely the change of the MAE signal intensity is closely related to the intensity. The change trend of stress release is the key point of stable expansion of the microcrack to the rapid expansion of the macroscopic main crack, so the inventor provides the detection method based on the idea of improving the alternating magnetic field intensity in order to solve the problem that the stable expansion of the fatigue crack in the stage I and the rapid expansion of the fatigue crack in the stage II cannot be identified on the basis of analyzing the influence factors of the MAE signal intensity.

Disclosure of Invention

In order to realize the detection of the rapid crack propagation starting time in the fatigue II stage, the invention provides a magneto acoustic emission detection method for the fatigue damage of a ferromagnetic metal member, which specifically comprises the following steps:

firstly, a U-shaped electromagnetic yoke and an acoustic emission sensor are arranged in a detected area of a detected ferromagnetic metal component which is not subjected to fatigue load in a relatively fixed position, the yoke and the component form a magnetic loop, and the acoustic emission sensor is used for collecting a magneto acoustic emission signal;

the subsequent detection step is that square wave excitation is adopted on the basis of obtaining sine wave reference voltage, so that a stable and effective excitation source is obtained, the magnetization intensity inside the material of the detected component is kept saturated in the fatigue process, and the finally obtained peak-to-peak voltage of the MAE signal with the T-shaped envelope effectively indicates the starting time of rapid crack expansion in the II stage along with the inflection point change of the fatigue period;

the detection steps are as follows:

(1) exciting a magnetic field by a sine wave to obtain a reference voltage Vpp 1; loading a sine wave voltage signal to a coil of a U-shaped electromagnetic yoke, and adjusting the peak-to-peak value of the loading signal from small to large until the time domain waveform of the acquired magneto acoustic emission signal has a double-peak envelope with an obvious tail peak, wherein the voltage peak-to-peak value of the loading signal is recorded as a reference voltage Vpp 1;

(2) a square wave excitation of Vpp1 based on a reference voltage;

keeping the frequency of the signal unchanged, setting the voltage peak value of the signal to be n times of Vpp1, wherein n is more than or equal to 1, generating an excitation magnetic field by adopting a square wave voltage signal with the duty ratio of 50%, and at the moment, the time domain waveform of the obtained magneto acoustic emission signal has a T-shaped envelope;

(3) filtering the signal;

filtering the magnetic emission signals of M magnetization periods, extracting the peak-to-peak voltage of the magnetic emission signals excited by the square wave voltage, and averaging the peak-to-peak voltage in the M magnetization periods;

(4) monitoring fatigue and judging the occurrence of fatigue crack propagation in the II stage;

detecting the same region of the detected ferromagnetic metal component subjected to different fatigue cycles by adopting the square wave voltage signals with the frequency and the voltage peak-to-peak value in the step (2), and analyzing a variation curve of the peak-to-peak value voltage average value of the magnetoacoustic emission signal along with the detection cycle to judge whether the detected component starts the fatigue crack expansion of the II stage:

further, the peak-to-peak voltage of the magnetic acoustic emission signals of M magnetization periods is averaged to be used as a characteristic parameter, and the characteristic parameter is the starting time of fatigue crack propagation in the II stage when the change curve of the characteristic parameter along with the fatigue period continuously and monotonically decreases and then has an inflection point.

(4.1) with the increase of the fatigue period, the peak-to-peak voltage of the obtained magneto acoustic emission signal fluctuates firstly and then monotonically decreases, so that the fatigue crack expansion of the component in the II stage does not occur;

(4.2) as the fatigue period increases, the peak-to-peak voltage of the resulting magnetoacoustic emission signal changes from a monotonic decrease to a gradual increase, and the component has entered phase II fatigue crack propagation.

Further, in the step (1.2), n is 1 to n₁，n₁The output maximum voltage/Vpp 1 of the signal generator providing the voltage signal.

Further, in the step (3), M is 20 or more.

Further, the U-shaped electromagnetic yoke may be replaced with a coil, in which case the detected region is disposed inside the coil.

Compared with the prior art, the invention has the beneficial effects that:

(1) in the existing MAE detection, only an excitation signal of a sine wave or a triangular wave with 50% of symmetry is adopted in the detection process, and only the fact that the root mean square voltage of an MAE signal is increased and then monotonically decreased along with the increase of the fatigue cycle times can be found, so that the expansion change of fatigue microcracks cannot be identified, and the failure of a member cannot be warned; according to the invention, a square wave voltage signal of reference voltage adjusted based on a sine wave is used as an excitation signal, on the basis that the reference voltage ensures strong alternating magnetic field intensity, a T-shaped envelope signal is obtained by utilizing square wave excitation, after the signal is increased and then monotonically decreased along with the increase of fatigue cycle times, inflection point change occurs along with the continuous development of fatigue cracks, and the development inflection point of stable expansion and rapid expansion of fatigue microcracks can be identified, so that timely early warning is given to component failure.

(2) In the existing MAE detection, an excitation signal of a sine wave or a triangular wave with 50% of symmetry is adopted in the monitoring process, under the noise interference of fatigue monitoring of a component service site, the obtained MAE signal waveform envelope changes and fluctuates, common signals are hump envelopes or envelope overlapping, the root mean square voltage of the signals is greatly influenced by the environment, and the reliability and accuracy of judging and identifying fatigue damage based on the characteristic parameters are difficult to ensure; the square wave voltage signal is used as the excitation signal, the obtained MAE signal has T-shaped envelope, the waveform of the signal is clear and easy to identify, the anti-noise interference capability of the MAE is further greatly improved on the basis that the reference voltage ensures enough magnetic field intensity, the extraction of the peak-to-peak voltage is accurate and convenient, the engineering application of the MAE technology is realized by fatigue monitoring, and the detection result is stable and reliable.

Drawings

FIG. 1 shows a VRMS-N/Nf curve under different excitation voltages for the conventional MAE detection method;

FIG. 2 shows the variation of RMS value of MAE signal with fatigue period under different excitation voltages of sine wave;

FIG. 3 is a block diagram of the steps of the method for detecting fatigue damage of ferromagnetic metal member by magnetic acoustic emission;

FIG. 4 is a graph of a bimodal envelope MAE signal with a distinct tail peak in the time domain waveform over a magnetization period;

FIG. 5 is a graph of the MAE signal with a "T" shaped envelope of the time domain waveform over one magnetization period;

the peak-to-peak voltage of the MAE signal of fig. 6Q 235 is plotted against the fatigue cycle.

Detailed Description

The invention is further described with reference to the following figures and specific embodiments.

The step block diagram of the detection method of the invention is shown in FIG. 3, and the steps are as follows;

1) the home-made U-shaped electromagnetic yoke and acoustic emission transducer were placed in a relatively fixed position in the central region of the Q235 steel member that was not fatigue loaded.

2) And loading a periodic sine wave voltage signal to a coil of the U-shaped electromagnetic yoke, wherein the frequency of the signal is 20Hz, and the voltage signal is generated by a function generator, amplified by 10 times by a power amplifier and then input into the coil. Because the MAE signal is generated under the alternating magnetic field excited by the sine wave voltage signal, the Q235 steel component generates two identical MAE signals in one magnetization period, namely the MAE signal frequency is twice of the alternating magnetic field frequency.

The peak-to-peak value of the output voltage signal is increased from 0.5Vpp by taking 0.5Vpp as a gradient through a function generator, and when the peak-to-peak value is increased to 4Vpp, the magneto-acoustic emission signal with a double-peak envelope of a time-domain waveform with an obvious tail peak can be obtained as shown in FIG. 4. At this time, the dynamic hysteresis loop of the material tends to be saturated, indicating that the maximum alternating magnetic field strength Hmax generated by the voltage signal is close to the saturation magnetization of the material.

When the magnetic field is a constant magnetic field and the magnetic field strength is greater than or equal to the saturation magnetization of the material, the effect of the crack or surface discontinuity on the magnetization of the material is negligible. Compared with a constant magnetic field with the same strength, the alternating magnetic field excited by the sine wave voltage has relatively weaker magnetization capacity on the material, because when the magnetic field is excited by the sine wave voltage signal, the larger the signal frequency is at a certain voltage, the smaller the alternating magnetic field intensity generated by the signal frequency is. The voltage platform of the square wave voltage signal is equivalent to a sine wave voltage signal with infinite frequency, so that the square wave voltage signal can effectively improve the alternating magnetic field intensity and the magnetization capacity of the material. Therefore, on the basis of the alternating magnetic field intensity corresponding to the 4Vpp voltage, Hmax is further improved in the square wave excitation mode, and the influence of surface discontinuity on the MAE signal intensity is reduced or eliminated.

3) Changing the sine wave voltage signal in 2) into a square wave voltage signal with a duty ratio of 50%, keeping the frequency of the signal at 20Hz, and setting the peak-to-peak value at 4Vpp, wherein the acquired MAE signal has a T-shaped envelope, as shown in FIG. 5.

4) Filtering the MAE signal of 25 magnetization periods, and then extracting the characteristic parameters of the MAE signal: peak-to-peak voltages, and averages the peak voltages of the MAE signal in these periods, respectively.

The specific method is that firstly, the component with the frequency lower than 20kHz is filtered, the peak voltage of the waveform signal in a magnetization period is obtained, and then the average value of the peak-to-peak voltage of a plurality of magnetization periods is obtained.

5) And (3) collecting MAE signals of the Q235 steel component subjected to different fatigue cycles by using the voltage signals in the step 3) and analyzing the MAE signals in the step 4), and obtaining peak-to-peak voltage average values of the MAE signals of the component in different fatigue cycles in 25 magnetization cycles, as shown in figure 6.

For a ferromagnetic metal material, the irreversible magnetization process is mainly irreversible displacement of magnetic domain walls and generation and annihilation of the magnetic domain walls, and the MAE signal intensity is closely related to the number of domain walls of the irreversible displacement and the displacement size. In the first stage of fatigue, dislocations in the material proliferate rapidly, with their morphology developing from discrete dislocations into dislocation junctions, dislocation envelopes and eventually forming "persistent slip bands". The formation of dislocation knots, dislocation envelopes and the like, on one hand, the number of irreversible moving magnetic domain walls is increased as pinning points of magnetic domains, and then the MAE signal is enhanced; on the other hand, the irreversible displacement of the domain wall is reduced, thereby reducing the MAE signal strength. The two aspects act together to cause the MAE signal intensity to fluctuate; in the second stage of fatigue, the original magnetic domain structure is broken by the initiation of the microcracks, and the irreversible displacement of the magnetic domain wall is reduced, so that the MAE signal intensity is further reduced; in the third stage of fatigue, along with the rapid crack propagation in the second stage of fatigue, on one hand, stress is released to a certain extent, so that the strength of the MAE signal is increased, but on the other hand, the surface discontinuity formed by the member due to the crack propagation strain causes the reduction of the magnetization of the material, so that the number of magnetic domain walls generating irreversible displacement is reduced, and the strength of the MAE signal is reduced. Therefore, the change in MAE signal intensity during the third stage of fatigue is determined by both stress relief and material surface discontinuity. When the external magnetic field is a constant magnetic field and the magnetic field intensity is greater than or equal to the saturation magnetization of the material, the influence of the discontinuity of the material surface on the magnetization of the material can be ignored. Therefore, to overcome the effect of surface discontinuity on excitation magnetization and to reflect the MAE change caused by stress release, the inventors propose that when the surface of the component is discontinuous by increasing Hmax, the effect of the surface discontinuity on material magnetization is weakened or eliminated, so that the effect of stress release on MAE strength increase is not masked by the effect of surface discontinuity, and the signal characteristic value appears with the "inflection point" of the fatigue cycle.

The acquisition of the reference voltage firstly ensures the saturation magnetization of the material, on the basis, square wave excitation provides a clear and easily-distinguished T-shaped envelope on one hand, and on the other hand, the voltage platform of the square wave voltage signal is equivalent to a sine wave voltage signal with infinite frequency, the average voltage of the square wave voltage signal is higher than the average voltage of the sine wave, and the intensity of an excitation magnetic field is substantially further improved. The method acquires a stable and effective excitation source in a mode of combining sine waves and square waves, so that the peak-to-peak voltage of a T-shaped envelope signal can be used as an analyzed characteristic parameter to effectively indicate the starting moment of rapid crack propagation in the II stage.

The embodiment proves the effectiveness of the invention, namely, the peak-to-peak voltage average value of the magnetoacoustic emission signals of the parts to be detected in different fatigue periods is compared, so that the method can be intuitively and conveniently used for analyzing the fatigue damage degree of the region to be detected, and the reliability and the effectiveness of the detection result are ensured.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations and/or simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and shall be included in the scope of the present invention.