CN105372330A - Non-linear Lamb wave frequency mixing method for detecting microcrack in plate - Google Patents

Abstract

A non-linear Lamb wave frequency mixing method for detecting a microcrack in a plate comprises firstly prefabricating microcracks with different length on a to-be-detected object by a high-frequency fatigue experiment machine, exciting two different frequencies under combine mode based on a non-linear ultrasonic detection system, and exciting the interaction between Lamb wave single S0 modal and the microcracks in the steel plate by using a probe with a fixed oblique angle; extracting the amplitude of fundamental frequency and sum frequency component in a signal by employing an FFT analysis method for direct wave passing through the microcracks, and performing fundamental-frequency normalization; observing the sum frequency variation rule, and obtaining the result, wherein the result shows that the sum frequency amplitude is positively correlated with the microcrack length; and finally evaluating different microcrack length in the steel plate according to a nonlinear coefficient, and confirming the Lamb wave modal of the sum frequency component through a time-frequency analysis method, and obtaining the result, wherein the result shows that the fundamental frequency and the sum frequency component in the signal are of S0 modal. The method fully verifies the detection feasibility and correctness of non-linear Lamb wave frequency mixing on cracks in the plate.

Description

The non-linear Lamb wave frequency mixing method of microcrack zone in plate

Technical field

The present invention relates to a kind of non-linear Lamb wave detection method, particularly a kind of non-linear Lamb wave mixing detection method based on mixing effect, for microcrack zone in plate and evaluation, belongs to field of non destructive testing.

Background technology

Tabular or class tabular (as housing) metal construction widely use in all kinds of equipment, as fuselage, the wing and hatch door etc. of large-scale storage tank, aircraft.Under arms in process, these plate metalloid components are under internal factor (as the alternate load that moving component transmits) and environmental factor (as burn into temperature, extraneous load etc.) effect, can be aging gradually, show as appearance damage in various degree.In all kinds of damage, the potential hazard of fatigue crack to structure is maximum.According to statistics, the inefficacy of component of machine of more than 80% is caused by fatigue damage.Main cause is, fatigue crack is a kind of instability defect, under structure service condition, even very little crackle, under the impact of the factor such as alternate load, temperature, also may expand rapidly, produces and ruptures suddenly, cause the generation of catastrophic failure.Therefore, take the early detection of effective means realization carrying tabular Structural Metallic Fatigue crackle, location and assessment, most important for the normal operation and people's security of the lives and property guaranteeing great infrastructure.

The ultrasound wave wave mode used in Ultrasonic Detection has multiple, as bulk wave (compressional wave and shear wave), Lamb wave, surface wave etc.Plate structure is detected, mostly uses Lamb wave in existing research, because the supersonic guide-wave propagated in plate structure is called Lamb wave (LambWave).Because sound field when Lamb wave is propagated in plate is throughout whole wall thickness, propagation distance is longer and decay is less, and therefore Lamb wave is widely used in the Non-Destructive Testing of platy structure.The linear features such as tradition Lamb wave technology method reflects when utilizing ultrasound wave to pass through damage, scattering and MODAL TRANSFORMATION OF A, can realize the greater losses such as plate split shed crackle and corrosion very well detects, but is difficult to realize fatigue statistic detection in plate.Nonlinear Lamb wave propagation detection technique makes evaluation according to ultrasound wave and the defect nonlinear effect produced that interacts to structural damage situation, and it shows very high susceptibility for the microdefect (fire damage, fatigue damage, micro-crack etc.) in plate.

Non-linear ultrasonic technology mainly pays close attention to the nonlinear acoustics response of material, realizes structure earlier damage detect according to the change of other frequency component in non-actuated bandwidth in detection signal.According to Cleaning Principle, non-linear ultrasonic detects can be divided into chatter modulation method, nonlinear resonance method, mixing method and Harmonic Method, wherein Harmonic Method uses maximum methods in research at present, but the harmonic effects of the examined instrument of testing result and the non-linear generation of probe itself is large; Chatter modulation technique detects interface contact state and close crack has advantage, but the method needs additionally on test specimen to be detected, apply low-frequency vibration and have boundary nonlinear to affect, and detection system is more complicated; And nonlinear resonance method to detection system and transducer band requirement higher.Frequency mixing technique (also known as wave beam aliasing technology), based on the ripple interaction feature in media as well of two row different frequencies, realizes structural damage detection.If medium is continuous print, when two train waves meet, meets linear superposition theorem, new frequency component can not be produced; If medium has uncontinuity, namely there is nonlinear area, when two train waves meet in this region, will interact, new frequency component can be observed in a frequency domain, and the testing result of the method is not by the nonlinear interference of instrument.

Chinese scholars has carried out a large amount of fruitful research work in the Lamb wave nonlinear effect and damage check of metal plate structure.In these research work, at " in planar solid structure the generation of Lamb wave second harmonic and Communication Research ", he gives the analytic solution that Lamb wave accumulates second harmonic sound field to the distinct professor of Logistics Engineering College of PLA Deng first in theory, and demonstrates the strong nonlinear effect of Lamb wave by experiment.The build-up effect that Deng and PruellC etc. have studied the second harmonic of non-linear Lamb wave respectively in " Analysisofsecond-harmonicgenerationofLambwavespropagatin ginlayeredplanarstructureswithimperfectinterfaces " " Evaluationoffatiguedamageusingnonlinearguidedwaves.Smart MaterialsandStructures " is analyzed the feasibility that the fatigue damage in plate detects.PruellC etc. in " EvaluationofplasticitydrivenmaterialdamageusingLambwaves AppliedPhysicsLetters " by non-linear Lamb wave to plate in fatigue damage analyze.The distinct Nonlinear Cumulative damage proposing the fatigue of concept to layered solid plate structure of stress wave damage factor, high-temerature creep and surface nature subtle change " Nonlinear Lamb wave propagation of layered structure surface nature levies method surely " of Deng is studied.XuH etc. in " Time-frequencyanalysisfornonlinearLambwavesignal " by the method for time frequency analysis to the launching efficiency by the second harmonic of non-linear Lamb wave in theoretical and experimental analysis aluminium sheet.The non-linear Lamb wave of application in " DetectionofthermalfatigueincompositesbysecondharmonicLam bwaves " such as LiW carries out test and assessment to the material of fire damage.The Kim professor of Georgia Institute of Technology of the U.S. detects in Qu Jianmin professor defect such as micro-crack, moulding strain, fatigue damage in " Ultrasonic NDT that material mechanical performance is degenerated and evaluation " is to different metal plate structure of " Feasibilityofusingnonlinearguidedwavestomeasureacousticn onlinearityofaluminum ", Northwestern Univ USA.Below be all by the Second Harmonic Method of non-linear Lamb wave to the detection of fatigue damage in plate, but the harmonic effects of the examined instrument of testing result and the non-linear generation of probe itself is large, causes net result inaccurate.Sun Junjun etc. to adopt in " research of structural hair cracking mixing non-linear ultrasonic detection method " under heteropleural incentive mode close crack in steel test block to carry out conllinear mixing and detect.The auspicious grade of Fanzhong carries out mixing detection based on non-colinear detection method to close crack in steel test block in patent [a kind of non-colinear frequency mixing ultrasonic detection method for micro-crack linear measure longimetry].Croxford etc. utilize angle probe at aluminum test specimen two ends excitation oblique incidence shearing wave in " Theuseofnon-conlinearmixingfornonlinearultrasonicdetecti onofplasticityandfatigue ", achieve the detection of plastic yield and fatigue damage in test specimen.Straight dry grade is at " SiC _pparticle enhanced aluminum-based composite material non-colinear nonlinear response experimental observation " and patent [a kind of non-colinear non-linear ultrasonic lossless detection method] in utilize non-colinear motivational techniques to SiC _pthe homogeneity of particle enhanced aluminum-based composite material has carried out mixing detection.Above method is all detect based on bulk wave mixing, because the method for bulk wave mixing cannot detect microdefect in sheet material.At present, in the research of plate structure Nonlinear Lamb wave propagation, except fire damage in utilizing mixing effect to plate structure in document " AfeasibilitystudyforLambwavemixingnonlineartechnique " has carried out feasibility checking, utilize mixing effect to plate structure earlier damage, the particularly detection of fatigue crack, also rarely seen report.So a kind of method based on non-linear Lamb wave mixing effect of invention detects micro-crack in plate.The present invention can overcome instrument and the nonlinear impact of probe itself, uses the method for non-linear Lamb wave mixing detect accurately the micro-crack in plate and assess.

Summary of the invention

A kind of method that the present invention is intended to propose non-linear Lamb wave mixing carries out test and repair to micro-crack in plate.The method adopts one to swash one and receives detecting pattern, and namely a fixed angle angle probe encourages the single S0 mode Lamb wave signal of two frequencies by the micro-crack of different length in different steel plate test specimen simultaneously, then receives with a fixed angle angle probe.The signal received is carried out fft analysis, extracts and frequency composition and two fundamental components, last foundation nonlinear factor is evaluated different micro-crack length.Finally determine that produce with mode that is frequency composition according to time frequency analysis.

For achieving the above object, the technical solution used in the present invention is a kind of non-linear Lamb wave frequency mixing ultrasonic detection method, and when there is close crack in sheet material, one-dimensional nonlinear Time Migration of Elastic Wave Equation is expressed as

$\frac{{\∂}^{2}u}{\∂t^2}-c^2\frac{{\∂}^{2}u}{\∂x^2}=c^2\mathrm{\β}\frac{\∂u}{\∂x}\frac{{\∂}^{2}u}{\∂x^2}---\left(1\right)$

In formula: c is sound wave velocity of propagation in media as well; X is the distance of Acoustic Wave Propagation; β is nonlinear acoustics characteristic coefficient; U is particle vibration displacement, and the pass between itself and strain stress (x, t) is

Theoretical according to wave disturbance, and when utilizing that progressively approximate solution solves, if the solution of formula (1) is:

u(x,t)＝u ⁽⁰⁾+βu ⁽¹⁾(2)

In formula: u ⁽¹⁾represent due to the non-linear displacement caused.If definition u ⁽¹⁾be directly proportional to direction of wave travel, then:

u ⁽¹⁾＝xf(τ)(3)

In formula: τ=t-x/c; And f (τ) is unknown number undetermined.

Now define the ultrasonic signal of input two frequencies, namely

u ⁰(x,t)＝A ₁cos(f ₁τ)+A ₂cos(f ₂τ)(4)

In formula: A ₁, A ₂for harmonic amplitude; f ₁, f ₂be respectively the centre frequency of two harmonic waves, with velocity of wave c, wave number k _ipass be f _i=k _ic.

Formula (4) and (3) are substituted into (2), substitute into formula (1) further and obtain:

$f\left(\mathrm{\τ}\right)=-\frac{A_{01}^2{k}_1^2}{8}cos\left(2f_1\mathrm{\τ}\right)-\frac{A_{02}^2{k}_2^2}{8}cos\left(2f_2\mathrm{\τ}\right)+\frac{A_1{A}_2{k}_1{k}_2}{4}\[cos(f_1-f_2)\mathrm{\τ}-cos(f_1+f_2)\mathrm{\τ}\]---\left(5\right)$

Therefore try to achieve particle vibration displacement components u (x, t) and be

$\begin{array}{c}u(x,t)=u^{\left(0\right)}+{\mathrm{\βu}}^{\left(1\right)}\\ =A_1\cos (f_{1}t-k_{1}x)+A_2\cos (f_{2}t-k_{2}x)\\ +x\mathrm{\β}\{-\frac{A_1^2{k}_1^2}{8}\cos (2f_1-2k_{1}x)-\frac{A_2^2{k}_2^2}{8}\cos (2f_{2}t-2k_{2}x)\\ +\frac{A_1{A}_2{k}_1{k}_2}{4}\{\cos \[(f_1-f_2)t-(k_1-k_2)x\]-\cos \[(f_1+f_2)t-(k_1+k_2)x\]\}\}\end{array}---\left(6\right)$

From (6), except signal frequency f ₁and f ₂outward, also high-frequency harmonic 2f is produced ₁and 2f ₂and mixed frequency signal frequency content, i.e. f ₁-f ₂and f ₁+ f ₂.Therefore, the material nonlinearity that occurs under being interpreted as the superposition situation of two or more displacement field of mixing phenomenon is ultrasonic.Fundamental frequency amplitude A (f is obtained by formula (6) ₁)=A ₁, A (f ₂)=A ₂, and frequency amplitude $A(f_1+f_2)=\frac{{\mathrm{\βxA}}_1{A}_2{k}_1{k}_2}{4}$

If can extract the amplitude of each order frequency component from measuring-signal, nonlinear factor is calculated as follows

$\mathrm{\β}=\frac{4}{{\mathrm{xk}}_1{k}_2}\frac{A(f_1+f_2)}{A\left(f_1\right)A\left(f_2\right)}---\left(7\right)$

Under the condition that propagation distance is identical with wave number, be reduced to nonlinear factor β as follows,

$\mathrm{\β}=\frac{A(f_1+f_2)}{A\left(f_1\right)A\left(f_2\right)}---\left(8\right)$

For realizing the analysis of technique scheme, the test unit that this method adopts is see Fig. 2, and this system comprises computing machine (1), nonlinear acoustics measuring system (2), signal selector (3), digital oscilloscope (4), 50 Ω loaded impedance coupling (5), adjustable attenuators (6), ultrasonic exciting probe (7), test specimen to be measured (8), ultrasound wave receiving transducer (9); Be connected by signal wire between computing machine (1) with nonlinear acoustics measuring system (2), adopt the excitation of software set various state modulator nonlinear acoustics measuring system hardware and received ultrasonic signal and signal is processed; The high energy pulse signal output part of nonlinear acoustics measuring system (2) is by 50 Ω loads (5), adjustable attenuator (6), and then pop one's head in ultrasonic exciting (7) are connected; Ultrasound wave receiving transducer (9) receives the ultrasonic signal through test specimen, one road signal directly sends into nonlinear acoustics measuring system passage 1, be connected with signal selector (3) by the radiofrequency signal monitoring client of nonlinear acoustics measuring system (2), Received signal strength sent into digital oscilloscope (4).

In this paperly based on one non-linear Lamb wave frequency mixing technique, testing methods for tiny crack in plate to be realized by following steps:

1) according to test unit Fig. 2 joint test equipment, by the both sides that ultrasonic exciting is popped one's head in (7), ultrasound wave receiving transducer (9) is arranged in micro-crack.

2) frequency response characteristic of excitation and receiving transducer is measured according to SNAP system, according to excitation and the frequency response characteristic of receiving transducer in the frequency range that amplitude attenuation is less than-3dB and according to the phase velocity of steel plate and group velocity dispersion curve, and consider receiving transducer to produce with frequency composition receiving ability and two fundamental components and and frequently composition must meet phase velocity and Group-velocity Matching principle.The Lamb wave mode determined two excitation frequencies based on above reason and encourage.Last two the frequency single S0 mode Lamb wave signals encouraged under acoustics nonlinear system SNAP system combine pattern.

3) according to the S0 mode Lamb wave speed under different frequency, calculate the delay inequality of pumping signal, guarantee that the signal that two probes send arrives cracks simultaneously.

4) from oscillograph, intercept direct-path signal, carried out Fast Fourier Transform (FFT) and extract and frequency composition and two fundamental components.Calculate nonlinear system numerical expression (8).

5) above-mentioned steps 1 is repeated) ~ step 3), the test specimen of different micro-crack length is detected, carries out fundamental frequency normalization by after Fast Fourier Transform (FFT), observe its Changing Pattern.Calculate respective nonlinear factor, and then utilize nonlinear factor to evaluate different micro-crack length.

6) finally the confirmation of mode is carried out to what produce with frequency composition according to time frequency analysis.From experimental result, verify that fundamental component meets phase velocity matched and Group-velocity Matching principle with frequency composition.

Accompanying drawing explanation

Fig. 1 processes the scale diagrams of test specimen;

Fig. 2 system chart;

The frequency response curve figure of Fig. 3 .1 incentive probe;

The frequency response curve figure of Fig. 3 .2 receiving transducer;

The phase velocities dispersion curve figure of Fig. 4 .11.7mm steel plate;

The group velocity dispersion curve map of Fig. 4 .21.7mm steel plate;

The incident angle dispersion curve figure of Fig. 4 .31.7mm steel plate;

Time-domain diagram before Fig. 5 .1 encourages 0.45MHz receiving transducer to move;

Time-domain diagram after Fig. 5 .2 encourages 0.45MHz receiving transducer to move 5cm;

Time-domain diagram before Fig. 5 .3 encourages 0.6MHz receiving transducer to move;

Time-domain diagram after Fig. 5 .4 encourages 0.6MHz receiving transducer to move 5cm;

The flawless time-domain diagram of Fig. 6 .1;

Fig. 6 .2 has the time-domain diagram of crackle;

The nonlinear factor β of the different micro-crack length of Fig. 7 schemes;

Fig. 8 is the non-linear Lamb wave frequency mixing method implementing procedure figure of microcrack zone in plate.

In figure: 1, computing machine, 2, nonlinear acoustics measuring system, 3, signal selector, 4, digital oscilloscope, 5,50 Ω loaded impedances couplings, 6, adjustable attenuator, 7, ultrasonic exciting probe, 8, test specimen to be measured, 9, ultrasound wave receiving transducer.

Embodiment

Below in conjunction with specific embodiment, the invention will be further described:

The present embodiment implementation process comprises the following steps, as shown in Figure 8:

1) this experiment institute detected object is Q235 steel plate (as shown in Figure 1), and wherein thickness is 1.7mm, and central authorities have the Linear cut breach of long 2mm with prefabricating fatigue crack.The different fatigue crack length of processing is respectively 0cm, 1cm, 1.5cm, 2cm.

2) incentive probe is placed on apart from fatigue crack right-hand member 10cm, receives incentive probe and be placed on apart from fatigue crack right-hand member 5cm place (as shown in Figure 2).

3) frequency response characteristic (as shown in Fig. 3 .1 and Fig. 3 .2) of excitation and receiving transducer is measured.In conjunction with 1.7mm steel plate phase velocities dispersion curve if Fig. 3 .1, group velocity dispersion curve are as Fig. 3 .2, according to phase velocity matched and Group-velocity Matching principle, excitation frequency under SNAP system combine pattern is 0.45MHz and 0.6MHz, the wedge angle of excitation and receiving transducer is 30 °, the single S0 mode of excitation Lamb wave.

4) the single S0 mode of calculating excitation Lamb wave passes through the group velocity in 1.7mm steel plate respectively under 0.45MHz and 0.6MHz frequency, the group velocity of the single S0 mode of 0.45MHz calculated according to correlation algorithm (as shown in Fig. 5 .1 and Fig. 5 .2) is 5403.1m/s, and the group velocity of the single S0 mode of 0.6MHz in like manner calculated as figure (as shown in Fig. 5 .3 and Fig. 5 .4) is 5359.1m/s.For making two column signals arrive fatigue crack place simultaneously, by calculating excitation 0.45MHz signal demand time delay 1 μ s.

5) from oscillograph, intercept direct-path signal, carried out Fast Fourier Transform (FFT) and extract and frequency composition and two fundamental components.Calculate nonlinear factor such as formula (8).

Repeat above-mentioned steps 2) ~ step 5), the test specimen of different micro-crack length is detected, carries out fundamental frequency normalization by after Fast Fourier Transform (FFT), observe its Changing Pattern, as Fig. 6 .1, Fig. 6 .2.Along with the increase of micro-crack length, and frequency composition increases gradually.Extract fundamental frequency (f ₁and f ₂) amplitude A (f ₁), A (f ₂) and and frequency (f ₁+ f ₂) amplitude A (f ₁+ f ₂), calculate nonlinear factor.The result repeating to obtain for five times is evaluated different fatigue crack length.As shown in Figure 7, along with the increase of micro-crack length, nonlinear factor β increases gradually, so well can be detected the micro-crack in plate by the method for non-linear Lamb wave mixing.

6) determine that excitation and the mode produced are carried out analysis and judged according to Time-Frequency Analysis Method, can determine to encourage mode and generation and mode to be frequently S0 mode.Fully demonstrate fundamental component and meet phase velocity matched and Group-velocity Matching principle with frequency composition.

Be more than a typical apply of the present invention, application of the present invention is not limited to microcrack zone in steel plate, also can be used for other sheet material and carries out detection as aluminium sheet, poly (methyl methacrylate) plate etc.

Claims (2)

1. the non-linear Lamb wave mixing system of microcrack zone in plate, is characterized in that: this system comprises computing machine (1), nonlinear acoustics measuring system (2), signal selector (3), digital oscilloscope (4), 50 Ω loaded impedance coupling (5), adjustable attenuators (6), ultrasonic exciting probe (7), test specimen to be measured (8), ultrasound wave receiving transducer (9); Be connected by signal wire between computing machine (1) with nonlinear acoustics measuring system (2), adopt the excitation of software set various state modulator nonlinear acoustics measuring system hardware and received ultrasonic signal and signal is processed; The high energy pulse signal output part of nonlinear acoustics measuring system (2) is by 50 Ω loads (5), adjustable attenuator (6), and then pop one's head in ultrasonic exciting (7) are connected; Ultrasound wave receiving transducer (9) receives the ultrasonic signal through test specimen, one road signal directly sends into nonlinear acoustics measuring system passage 1, be connected with signal selector (3) by the radiofrequency signal monitoring client of nonlinear acoustics measuring system (2), Received signal strength sent into digital oscilloscope (4).

2., according to the non-linear Lamb wave mixing system of microcrack zone in plate according to claim 1, the non-linear Lamb wave frequency mixing method of microcrack zone in plate, is characterized in that: this method is realized by following steps:

1) according to test unit joint test equipment, by the both sides that ultrasonic exciting is popped one's head in (7), ultrasound wave receiving transducer (9) is arranged in micro-crack;

2) frequency response characteristic of excitation and receiving transducer is measured according to SNAP system, according to excitation and the frequency response characteristic of receiving transducer in the frequency range that amplitude attenuation is less than-3dB and according to the phase velocity of steel plate and group velocity dispersion curve, and consider receiving transducer to produce with frequency composition receiving ability and two fundamental components and and frequently composition must meet phase velocity and Group-velocity Matching principle; The Lamb wave mode determined two excitation frequencies based on above reason and encourage; Last two the frequency single S0 mode Lamb wave signals encouraged under acoustics nonlinear system SNAP system combine pattern;

3) according to the S0 mode Lamb wave speed under different frequency, calculate the delay inequality of pumping signal, guarantee that the signal that two probes send arrives cracks simultaneously;

4) from oscillograph, intercept direct-path signal, carried out Fast Fourier Transform (FFT) and extract and frequency composition and two fundamental components;

5) above-mentioned steps 1 is repeated) ~ step 3), the test specimen of different micro-crack length is detected, carries out fundamental frequency normalization by after Fast Fourier Transform (FFT), observe its Changing Pattern; Calculate respective nonlinear factor, and then utilize nonlinear factor to evaluate different micro-crack length;

6) finally the confirmation of mode is carried out to what produce with frequency composition according to time frequency analysis; From experimental result, verify that fundamental component meets phase velocity matched and Group-velocity Matching principle with frequency composition.