METHOD FOR MEASURING DEVIATION ANGLE OF FATIGUE MICROCRACK BASED ON NONLINEAR ULTRASOUND

Abstract

Disclosed is a method for measuring a deviation angle of a fatigue microcrack based on nonlinear ultrasound, comprising: preliminarily positioning a fatigue microcrack to obtain a center of the microcrack; selecting a horizontal positive direction, and defining an orientation angle; drawing a positive circumference on a surface of a metal plate, and selecting a fixed interval angle; placing an excitation sensor and a receiving sensor on the drawn positive circumference according to the orientation angle; ultrasonically testing each group of ultrasonic sensing paths, and recording time domain waveform signals formed by each group of ultrasonic sensing paths; converting each group of time domain waveform signals into a corresponding frequency domain graph, extracting an ultrasonic fundamental wave signal amplitude and a second harmonic waveform amplitude, and calculating a relative nonlinear coefficient; drawing an orientation angle-relative nonlinear coefficient polar coordinate graph; and determining a deviation angle of the microcrack.

Claims

1. A method for measuring a deviation angle of a fatigue microcrack based on nonlinear ultrasound, wherein the method comprises the following steps of: S1: preliminarily positioning a fatigue microcrack for a metal plate containing a microcrack defect to obtain a center O of the microcrack; S2: taking the center O of the microcrack as an origin on a surface of the metal plate, selecting a direction as a horizontal positive direction, and taking a counterclockwise included angle of the horizontal positive direction as an orientation angle α.sub.x; S3: drawing a positive circumference on the surface of the metal plate with the center O of the microcrack as a center of circle and r as a radius, and selecting an appropriate fixed interval angle Δα, wherein the orientation angle α.sub.x satisfies that α.sub.x=x.Math.Δα, and 0°≤α.sub.x<360°; x is a number of change times of the orientation angle α.sub.x, and x=0, 1, 2, ..., n; and n is a value limit of the number of change times x of the orientation angle α.sub.x, $\text{n}=\frac{360°-\Delta \alpha }{\Delta \alpha };$ S4: placing an excitation sensor and a receiving sensor on the drawn positive circumference in the step S3 according to the orientation angle α.sub.x, wherein the excitation sensor is placed at the orientation angle α.sub.x, and the receiving sensor is placed at a place symmetrical with the orientation angle α.sub.x about the center O of the microcrack, and forming a group of ultrasonic sensing paths between corresponding excitation sensor and receiving sensor; sequentially increasing the number of change times x of the orientation angle α.sub.x, and changing positions of the excitation sensor and the receiving sensor to obtain n+1 groups of ultrasonic sensing paths; and ultrasonically testing each group of ultrasonic sensing paths, and recording time domain waveform signals formed by each group of ultrasonic sensing paths; S5: converting each group of time domain waveform signals into a corresponding frequency domain graph, respectively extracting an ultrasonic fundamental wave signal amplitude A.sub.1x and a second harmonic waveform amplitude A.sub.2x in each group of frequency domain graph data, and calculating each group of relative nonlinear coefficients (β'.sub.x, wherein the relative nonlinear coefficient is that β'.sub.x = $\frac{A_{2x}}{A_{1x}^2}$ ; and S6: drawing an orientation angle α.sub.x - relative nonlinear coefficient β'.sub.x polar coordinate graph, wherein the drawn polar coordinate graph is an approximately symmetrical “leaf” pattern, an included angle between a vertical direction of a long axis of the approximately symmetrical “leaf” pattern and the horizontal positive direction is a deviation angle of the microcrack, and a range of the deviation angle of the microcrack is [0°, 180°); and the orientation angle α.sub.x - relative nonlinear coefficient β'.sub.x polar coordinate graph takes the α.sub.x as a polar angle and the relative nonlinear coefficient β'.sub.x as a polar radius.

2. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 1, wherein the step S4 specifically comprises: S41: establishing a polar coordinate system on the surface of the metal plate with the center O of the microcrack as a polar point and the horizontal positive direction as a polar axis, wherein the orientation angle is that α.sub.x=x.Math.Δα, and 0°≤α.sub.x<360°, and the number of change times of the orientation angle α.sub.x is that x=0, 1, 2, ..., n; S42: when the orientation angle α.sub.x satisfies that 0°≤α.sub.x<180°, placing one excitation sensor at polar coordinates (r, α.sub.x) and one receiving sensor at polar coordinates (r, α.sub.x+180°); when the orientation angle α.sub.x satisfies that 180°<α.sub.x<360°, placing one excitation sensor at polar coordinates (r, α.sub.x) and one receiving sensor at polar coordinates (r, α.sub.x-180°); and forming an x.sup.th group of ultrasonic sensing paths between corresponding excitation sensor and receiving sensor, wherein r is a radius of the positive circumference drawn in the step S3; S43: ultrasonically testing the x.sup.th group of ultrasonic sensing paths, wherein ultrasound is excited by the excitation sensor and collected by the receiving sensor to obtain time domain waveform signals of the x.sup.th group of ultrasonic sensing paths, and recording the time domain waveform signals formed by the x.sup.th group of ultrasonic sensing paths by an oscilloscope; and S44: repeating the step S42 and the step S43 according to an order that the number of change times x of the orientation angle α.sub.x is increased from 0 to n in sequence to obtain n+1 groups of ultrasonic sensing paths in total, and recording n+1 groups of corresponding time domain waveform signals in total.

3. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 1, wherein the step S5 specifically comprises: S51: analyzing the x.sup.th group of time domain waveform signals, and converting the signals into a corresponding frequency domain graph, wherein x is the number of change times of the orientation angle α.sub.x, and x=0, 1, 2, ..., n; S52: extracting the ultrasonic fundamental wave signal amplitude A.sub.1x and the second harmonic waveform amplitude A.sub.2x from the x.sup.th group of frequency domain graph data, wherein a fundamental wave signal frequency is close to a center frequency of the excitation sensor, and a second harmonic frequency is twice that of the fundamental wave frequency; S53: obtaining the x.sup.th group of relative nonlinear coefficients β'.sub.x by calculating according to ${{\beta }^{\prime }}_x=\frac{A_{2x}}{A_{1x}^2};$ and S54: repeating the step S51 to the step S53, and calculating n+1 groups of relative nonlinear coefficients in total.

4. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 1, wherein the fixed interval angle Δα satisfies that 0°<Δα≤60°.

5. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 1, wherein the value limit n of the number of change times x of the orientation angle α.sub.x is not less than 5.

6. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 1, wherein a center frequency of the receiving sensor is twice that of the excitation sensor.

7. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 1, wherein algorithms for converting the time domain waveform signals into the frequency domain graph comprise fast Fourier transform and Fourier transform.

8. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 1, wherein the fixed interval angle Δα is 15°; or the fixed interval angle Δα is 20°; or the fixed interval angle Δα is 30°; or the fixed interval angle Δα is 45°.

9. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 6, wherein the center frequency of the receiving sensor is 5 MHz, and the center frequency of the excitation sensor is 2.5 MHz.

10. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 2, wherein a center frequency of the receiving sensor is twice that of the excitation sensor.

11. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 3, wherein algorithms for converting the time domain waveform signals into the frequency domain graph comprise fast Fourier transform and Fourier transform.

12. The method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound according to claim 4, wherein the fixed interval angle Δα is 15°; or the fixed interval angle Δα is 20°; or the fixed interval angle Δα is 30°; or the fixed interval angle Δα is 45°.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 is a schematic diagram of placement positions of a group of excitation sensor and receiving sensor in an embodiment of the present invention; wherein 1 is a sample to be tested, 2 is the excitation sensor, 3 is the receiving sensor, and 4 is a center O of a microcrack of the sample to be tested.

[0042] FIG. 2 is a polar coordinate diagram of orientation angle - relative nonlinear coefficient in the embodiment of the present invention.

DETAILED DESCRIPTION

Embodiment

[0043] In this embodiment, a 6005A aluminum alloy plate with a geometric size of 120 mm ×30 mm×4 mm is taken as a sample to be tested, it is known that there is an actually prefabricated fatigue microcrack in the sample to be tested, a center of the actually prefabricated fatigue microcrack is located at a center of the sample to be tested, and a deviation angle of the microcrack is an included angle of 60° with a horizontal direction selected later. The sample to be tested is now measured by a method for measuring a deviation angle of a fatigue microcrack based on nonlinear ultrasound of the present invention, comprising the following measurement steps.

[0044] In S1, the fatigue microcrack of the sample to be tested is preliminarily positioned to obtain a center O of the microcrack.

[0045] An appropriate rectangular testing area is selected on a surface of the metal plate, a plurality of horizontal and vertical measurement positions of an excitation transducer distributed at intervals are determined along two intersecting sides of a rectangle respectively, a receiving transducer is placed at a position on opposite sides of the rectangle and vertically symmetrical with the excitation transducer about a center line of the rectangle, a relative nonlinear coefficient is measured, the excitation transducer is moved in an appropriate horizontal or vertical direction after each measurement and the position of the receiving transducer is changed accordingly, and each group of relative nonlinear coefficients is compared, wherein a place with a maximum relative nonlinear coefficient is the center of the microcrack.

[0046] In this embodiment, it is tested that the center O of the microcrack of the sample to be tested is located at the center of the sample to be tested.

[0047] In S2, the center O of the microcrack is taken as an origin on the surface of the metal plate, a direction is selected as a horizontal positive direction, and a counterclockwise included angle of the horizontal positive direction is taken as an orientation angle α.sub.x, as shown in FIG. 1.

[0048] In S3, a positive circumference is drawn on the surface of the metal plate with the center O of the microcrack as a center of circle and r of 15 mm as a radius, as shown in FIG. 1, and a fixed interval angle Δa of 15° is selected, wherein the orientation angle α.sub.x satisfies that a.sub.x= xΔa, and 0°≤a.sub.x<360°; x is a number of change times of the orientation angle α.sub.x, and x=0, 1, 2, ..., n; and a value limit of the number of change times x of the orientation angle α.sub.x is that n = 360°-Δα .sub.= 360°-15° .sub.= 23 .sub.. That is to say, values of the Δa 15° orientation angle α.sub.x in this embodiment are 0°, 15°, 30°, ..., 345° in sequence.

[0049] In S4, an excitation sensor and a receiving sensor are placed on the drawn positive circumference in the step S3 according to the orientation angle α.sub.x, wherein the excitation sensor is placed at the orientation angle α.sub.x, and the receiving sensor is placed at a place symmetrical with the orientation angle α.sub.x about the center O of the microcrack, and a group of ultrasonic sensing paths is formed between corresponding excitation sensor and receiving sensor, as shown in FIG. 1; the number of change times x of the orientation angle α.sub.x is sequentially increased, and positions of the excitation sensor and the receiving sensor are changed to obtain 24 groups of ultrasonic sensing paths; and each group of ultrasonic sensing paths is ultrasonically tested, and time domain waveform signals formed by each group of ultrasonic sensing paths are recorded.

[0050] In this embodiment, a center frequency of the receiving sensor is twice that of the excitation sensor, and specifically, the center frequency of the receiving sensor is 5 MHz, and the center frequency of the excitation sensor is 2.5 MHz.

[0051] In this embodiment, the step S4 specifically comprises the following.

[0052] In S41, a polar coordinate system is established on the surface of the metal plate with the center O of the microcrack as a polar point and the horizontal positive direction as a polar axis, wherein the orientation angle is that a.sub.x=x.sup..Aa, and 0°≤a.sub.x<360°, and the number of change times of the orientation angle α.sub.x is that x=0, 1, 2, ..., 23.

[0053] In S42, when the orientation angle α.sub.x satisfies that 0°≤a.sub.x≤180°, one excitation sensor is placed at polar coordinates (r, a.sub.x) and one receiving sensor is placed at polar coordinates (r, a.sub.x+180°) (as shown in FIG. 1); when the orientation angle α.sub.x satisfies that 180°<a.sub.X<360°, one excitation sensor is placed at polar coordinates (r, a.sub.x) and one receiving sensor is placed at polar coordinates (r, a.sub.x-180°); and an X.sup.th group of ultrasonic sensing paths is formed between corresponding excitation sensor and receiving sensor, wherein r is a radius of the positive circumference drawn in the step S3.

[0054] In S43, the x.sup.th group of ultrasonic sensing paths is ultrasonically tested, wherein ultrasound is excited by the excitation sensor and collected by the receiving sensor to obtain time domain waveform signals of the x.sup.th group of ultrasonic sensing paths, and the time domain waveform signals formed by the x.sup.th group of ultrasonic sensing paths are recorded by an oscilloscope.

[0055] In S44, the step S42 and the step S43 are repeated according to an order that the number of change times x of the orientation angle α.sub.x is increased from 0 to 23 in sequence to obtain 24 groups of ultrasonic sensing paths in total, and 24 groups of corresponding time domain waveform signals are recorded in total.

[0056] In S5, each group of time domain waveform signals is converted into a corresponding frequency domain graph, an ultrasonic fundamental wave signal amplitude A.sub.1x and a second harmonic waveform amplitude A.sub.2x in each group of frequency domain graph data are respectively extracting, and each group of relative nonlinear coefficients β'.sub.x, is calculated, wherein the relative nonlinear coefficient is that

[00003]$\beta {\text{'}}_x=\frac{A_{2x}}{A_{1x}^2}.$

[0057] In this embodiment, the step S5 specifically comprises the following.

[0058] In S51, the x.sup.th group of time domain waveform signals is analyzed, and the signals are converted into a corresponding frequency domain graph, wherein x is the number of change times of the orientation angle α.sub.x, and x=0, 1, 2, ..., 23. An algorithm for converting the time domain waveform signals into the frequency domain graph is fast Fourier transform.

[0059] In S52, the ultrasonic fundamental wave signal amplitude A1.sub.x and the second harmonic waveform amplitude A.sub.2x are extracted from the x.sup.th group of frequency domain graph data, wherein a fundamental wave signal frequency is close to a center frequency of the excitation sensor, and a second harmonic frequency is twice that of the fundamental wave frequency.

[0060] In S53, the x.sup.th group of relative nonlinear coefficients β'.sub.x is obtained by calculating according to

[00004]$\beta {\text{'}}_x=\frac{A_{2x}}{A_{1x}^2}.$

[0061] In S54, the step S51 to the step S53 are repeated, and 24 groups of relative nonlinear coefficients are calculated in total.

[0062] In S6, an orientation angle α.sub.x - relative nonlinear coefficient β'.sub.x polar coordinate graph is drawn with the a.sub.x as a polar angle and the relative nonlinear coefficients β'.sub.x as a polar axis, wherein the drawn polar coordinate graph is an approximately symmetrical “leaf” pattern, as shown in FIG. 2, an included angle between a vertical direction of a long axis of the approximately symmetrical “leaf” pattern and the horizontal positive direction is a deviation angle of the microcrack, and the deviation angle of the microcrack measured in this embodiment is an included angle of 60° with the selected horizontal direction (which means that: in this embodiment, in the orientation angle α.sub.x - relative nonlinear coefficient β'.sub.x polar coordinate graph, when a straight line in the vertical direction of the long axis of the approximately symmetrical “leaf” pattern passes through a polar point, an counterclockwise included angle between the straight line and a direction of a polar axis within [0°, 180°)is 60°).

[0063] It can be seen from the above that a measurement result of the sample to be tested by the method for measuring the deviation angle of the fatigue microcrack based on the nonlinear ultrasound of the present invention is consistent with the deviation angle of the actually prefabricated fatigue microcrack. Therefore, the method of the present invention provides a feasibility for measuring the deviation angle of the fatigue microcrack.

[0064] In addition, the fixed interval angle Δa in this embodiment may also be other angles satisfying that 0°<Δa≤60°, such as 20°, 30° and 45°, wherein a measurement method is the same as that in this embodiment, and a measurement result is also consistent with an actual situation, which will not be repeated herein.

[0065] The above embodiments only express some specific implementations of the present invention, and the descriptions thereof are specific and detailed, but the embodiments cannot be understood as limiting the scope of protection of the present invention. It should be pointed out that those of ordinary skills in the art may further make several modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.