CN103235039A - Ultrasonic TOFD (Time of Flight Diffraction) detection method for weld defects based on linear frequency modulation technology - Google Patents

Abstract

The invention discloses an ultrasonic TOFD (Time of Flight Diffraction) detection method for weld defects based on a linear frequency modulation technology, relating to an ultrasonic detection method for weld defects. The method solves the problem that in the existing ultrasonic detection for large-thickness welding structures, the signal propagation distance and the resolution ratio are mutually contradicted. The method comprises the following steps that: a digital signal generator produces a wideband (Linear Frequency Modulation) LFM pulse signal, and the wideband LFM pulse signal is amplified through a linear power amplifier so as to have corresponding energy for exciting an ultrasound transmitting probe; the ultrasound transmitting probe after being excited produces forced vibration and radiates ultrasonic waves to a test piece; when a control computer executes an acquisition command, a receiving probe receives a detection signal at a corresponding position, and the receiving of the detection signal is completed by a wideband signal acquisition card; and the computer carries out matched filtering processing on the received signal, so that a pulse compression signal is obtained, and according to the time that each wave reaches, the buried depth of a defect can be calculated according to a formula. The method disclosed by the invention is used for detecting weld defects.

Description

Weld defect ultrasonic TOFD detection method based on the linear frequency modulation technology

Technical field

The present invention relates to a kind of weld defect supersonic detection method.

Background technology

Big thickness welded structure is widely used in fields such as boiler, pressure vessel, shipbuilding, because these structures often are in extreme environments such as high temperature, high pressure or top load, therefore its welding quality is had relatively high expectations, in case have an accident.Ultrasonic detecting technology as one of method of quality control, in the big welded detection of thickness, occupy critical role, particularly new development in recent years (Time of Flight Diffraction, detection technique TOFD) has substituted ray detection to a certain extent based on diffraction time difference method.Yet along with large welded structures is harsh day by day to the requirement of quality and reliability, make that its requirement to ultrasonic detecting technology is also more and more higher, namely need accurately to judge and whether have defective in the weld seam, improve the sensitivity that detects, and for the assessment of structural safety and reliability, except accurate understanding defects property, also need to improve as much as possible the accuracy of identification of defective locations, size.In addition, how to solve ultrasound wave contradiction of sound wave penetration depth and detection sensitivity in big thickness welded structure detects is also paid close attention to by people always.

The ultrasonic TOFD detection technique is simple with its principle, and is easy to operate, and characteristics such as the easy storage of testing result more and more are subjected to people's favor.This technology utilizes a pair of probe to be oppositely disposed in the weld seam both sides and center probe detects at same straight line.Referring to Fig. 1, the same moved further of two probes, one another is receiving transducer R as transmitting probe T, the distance between two Probe index is called probe spacing (PSD).Suppose to have a crack defect in sheet material or the weld seam, if the crackle height is enough big, can produce the detection signal among Fig. 2.According to the length of ultrasound wave propagation distance, wave mode is that diffracted wave on lateral wave, the defective, defective lower end diffraction involve Bottom echo successively in the signal that receives.Defective upper end diffracted wave is opposite, identical with the Bottom echo phase place with the lateral wave phase place, and defective lower end diffracted wave is identical with the lateral wave phase place and opposite with the Bottom echo phase place, can judge by phase relation whether the flaw indication of acquisition derives from same defect body.This method is used longitudinal wave probe usually, and is faster than shear wave because of its velocity of sound, after selecting suitable characterization processes parameter, can get rid of the interference of transformed wave between the lateral sweep Bottom echo like this, is easy to the identification of flaw indication.But when flaw height hour, defective upper and lower end diffracted wave is overlapping.If when not having defective in the tested material, only comprise the lateral sweep Bottom echo in the detection signal.

Can determine the depth d of defective according to the time that receives each waveform in the signal, computing formula is

$d=\frac{1}{2}\sqrt{{\left(\mathrm{C\Δt}\right)}^2+4\mathrm{C\ΔtS}}---\left(1\right)$

Δ t in the formula---the mistiming (μ s) between defective diffracted signal or Bottom echo and the lateral wave;

C---longitudinal wave velocity (mm/ μ s);

Half (mm) of S---probe spacing distance.

When Δ t is Δ t among Fig. 2 _UThe time, can calculate the depth d of defective upper end _U, should Δ t be Δ t together _LThe time can calculate defective lower end position d _LThereby the height h that obtains defective is

h＝d _L-d _U (2)

By formula (1) as can be known, if will accurately calculate defective upper end and the lower end degree of depth separately, at first need accurately to extract the mistiming between defective upper end and lower end diffracted wave and the lateral wave.But when flaw height hour, and the signal time resolving power that receives is when relatively poor, makes t among Fig. 2 ₁With t ₂The signal overlap of position, can't obtain the position of defective upper and lower end this moment simultaneously, thereby can not obtain the height of defective.

Fig. 3 has represented the influence of signal length to the detection signal temporal resolution.Provided t among Fig. 3 ₁With t ₂Situation when the signal of position meets, t is the duration of signal, τ is that the minimum time of separable two signals is poor in the time domain, be equivalent to temporal resolution, so when each signal duration more in short-term, can distinguish that the mistiming τ of two signals is more little, namely resolution will be higher the time.Therefore, if can reduce the width of signal, namely adopt narrow pulse signal to detect the temporal resolution that will improve signal.

In order to obtain the signal of short period, we can expect utilizing the high-frequency narrow pulse signal to detect solving the problems referred to above, so higher temporal resolution can be arranged naturally.But can run into the acoustic wave energy attenuation problem inevitably in the ultrasound examination, because the energy of short pulse signal is low, and decay and the frequency of signal of ultrasonic energy in material is proportional, be that the ultrasonic signal frequency is more high, the signal energy decay is more fast, this will influence the penetration capacity of sound wave, so the use of high-frequency signal is limited.So the temporal resolution in the ultrasound examination and signal penetration capacity are conflicting, and detect based on the ultrasound wave TOFD of burst pulse energisation mode, also can't escape the constraint of this problem.

Therefore, for the welded Ultrasonic Detection of big thickness, be subjected to the influence of signal propagation distance and seam organization, usually adopt the major diameter low-frequency ultrasonic waves to pop one's head in to obtain enough energy, guarantee effective penetration depth, so make to defect location, quantitatively the precision of identification is not high, simultaneously to little defective to detect ability relatively poor, be unfavorable for welded quality assessment.

Summary of the invention

The purpose of this invention is to provide a kind of weld defect ultrasonic TOFD detection method based on the linear frequency modulation technology, to solve in the present big thickness welded structure Ultrasonic Detection problem conflicting between the signal propagation distance and resolution.

The present invention solves the problems of the technologies described above the technical scheme of taking to be: said method comprising the steps of: step 1, ultrasound wave transmitting probe and receiving transducer are symmetrically distributed in the both sides of detected position, first-selected digital signal generator produces broadband LFM pulse signal, produce synchronous triggering signal excitation broadband signal receiver simultaneously, broadband LFM pulse signal amplifies to possess corresponding energy by linear power amplifier and encourages the ultrasound wave transmitting probe;

The LFM pulse signal is produced by digital method, and the mathematic(al) representation of linear FM signal is

s(t)＝Arect(t/T)cos(2πf _ct±Kπt ²) |t|≤T/2 (3)

A is signal amplitude in the formula, f _cBe carrier frequency, K=B/T is the variation slope of signal transient frequency, i.e. chirp rate, and B is signal bandwidth, and T is pulse width, and rect (t/T) is rectangular signal, and

$\mathrm{rect}\left(\frac{t}{T}\right)=\left\{\begin{array}{cc}1,& \left|\frac{t}{T}\right|\≤\frac{1}{2}\\ 0,& \mathrm{elsewise}\end{array}\right.---\left(4\right)$

So the instantaneous frequency f of signal is

$f=\frac{d(f_{c}t\&PlusMinus;\frac{K}{2}{t}^2)}{\mathrm{dt}}=\frac{d(f_{c}t\&PlusMinus;\frac{B}{2T}{t}^2)}{\mathrm{dt}}=f_c\&PlusMinus;\frac{B}{T}t$ $-\frac{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">T</figure-callout>}}{2}\&le;t\&le;\frac{T}{2}---\left(5\right)$

Pumping signal s (t) is expressed as with plural form

$\tilde{s}\left(t\right)=\mathrm{Arect}(t/T)e^{j(2\mathrm{\&pi;}{f}_{c}t\&PlusMinus;\mathrm{K\&pi;}{t}^2)}---\left(6\right)$

The LFM pulse signal is carried out spectrum analysis, namely to signal

Carry out Fourier transform, then

$\tilde{S}\left(f\right)={\&Integral;}_{-\&infin;}^{\&infin;}\tilde{s}\left(t\right)e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;ft}}\mathrm{dt}=A{\&Integral;}_{-T/2}^{T/2}{e}^{j[2\mathrm{\&pi;}(f_c-f)t+\mathrm{K\&pi;}{t}^2]}\mathrm{dt}---\left(7\right)$

Arrangement by analysis, when wide-bandwidth product was very big at that time, the frequency spectrum function of LFM pulse signal can be expressed as

$\tilde{S}\left(f\right)=\left\{\begin{array}{cc}A\sqrt{T/B}{e}^{j[-\frac{\mathrm{\&pi;T}{(f-f_c)}^2}{B}+\frac{\mathrm{\&pi;}}{4}]}& |f-f_c|\&le;B/2\\ 0& |f-f_c|>B/2\end{array}\right.---\left(8\right)$

Produce forced vibration after step 2, the ultrasound wave transmitting probe excited target, radiate supersonic wave in the test specimen;

Step 3, when the control computing machine is carried out acquisition, receiving transducer receives detection signal in the relevant position, the reception of detection signal is to be finished by the broadband signal capture card;

Step 4, computing machine carry out matched filtering to received signal to be handled, thereby obtains pulse compression signal, according to the time of arrival of each waveform in the signal, can calculate the depth of burial of defective according to formula (1).

The matched filtering of chirp pulse signal,

The output signal of linear system

Frequency spectrum

It is input signal

Frequency spectrum

Product with the frequency response H (f) of receiving system

${\tilde{S}}_{\mathrm{out}}\left(f\right)=\tilde{S}\left(f\right)H\left(f\right)---\left(9\right)$

Output signal then

Can be expressed as

${\tilde{s}}_{\mathrm{out}}\left(t\right)={\&Integral;}_{-\&infin;}^{\&infin;}\tilde{S}\left(f\right)H\left(f\right){\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">e</figure-callout>}}^j\mathrm{df}---\left(10\right)$

When the frequency response function of receiving system satisfies

$H\left(f\right)=k\tilde{S}*\left(f\right)e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;f}{t}_0}---\left(11\right)$

Be frequency spectrum

Conjugate, k is the gain constant of wave filter, this moment, the input/output signal noise ratio reached maximal value, namely

$H\left(f\right)=k\left|\tilde{S}\left(f\right)\right|e^{-j\widetilde{\mathrm{\&phi;}}\left(f\right)}{e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{\mathrm{ft}}_0}=k\left|\tilde{S}\left(f\right)\right|e^{-j[\widetilde{\mathrm{\&phi;}}\left(f\right)+2\mathrm{\&pi;}{\mathrm{ft}}_0]}---\left(12\right)$

The amplitude of normalization H (f), the frequency response function that can get the matched filter of LFM pulse signal is

$H\left(f\right)=\left\{\begin{array}{cc}{e}^{j[\frac{\mathrm{\&pi;T}{(f-f_c)}^2}{B}\frac{\mathrm{\&pi;}}{4}-2\mathrm{\&pi;}{\mathrm{ft}}_0]}& |f-f_c|\&le;B/2\\ 0& |f-f_c|>B/2\end{array}\right.---\left(13\right)$

Can be got the output complex signal of LFM pulse signal matched filter by formula (8), (9) and (13)

Frequency spectrum

For

${\tilde{S}}_{\mathrm{out}}\left(f\right)=\tilde{S}\left(f\right)H\left(f\right)=A\sqrt{\frac{T}{B}}{e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{\mathrm{ft}}_0}$ |f-f _c|≤B/2 (14)

Right

Carry out inverse fourier transform, be the complex signal of matched filter output

Through putting in order,

${\tilde{s}}_{\mathrm{out}}\left(t\right)={\&Integral;}_{-\&infin;}^{\&infin;}{\tilde{S}}_{\mathrm{out}}\left(f\right){\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">e</figure-callout>}}^j\mathrm{df}={\&Integral;}_{f_c-B/2}^{f_c+B/2}A\sqrt{\frac{T}{\mathrm{<figure-callout\; id="2"\; label="B\; e\; j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">B</figure-callout>}}}{e}^j{2\mathrm{\&pi;f}(t-t_0)}^{}\mathrm{df}$

(15)

$=A\sqrt{\mathrm{BT}}\frac{\sin \left[\mathrm{\&pi;B}(t-t_0)\right]}{\mathrm{\&pi;B}(t-t_0)}{\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">e</figure-callout>}}^j$

Then

Real part be that the real signal of matched filter output is pulse compression signal

$s_{\mathrm{out}}\left(t\right)=A\sqrt{\mathrm{BT}}\frac{\sin \left[\mathrm{\&pi;B}(t-t_0)\right]}{\mathrm{\&pi;B}(t-t_0)}\cos \left[2\mathrm{\&pi;}{f}_c(t-t_0)\right]---\left(16\right).$

The present invention has following beneficial effect: the emission of LFM pulse signal is made up of digital signal generator and linear power amplifier among the present invention, the flatness of FM signal amplitude will influence system performance in its emission pulsewidth, and the reception of detection signal is finished by the broadband signal capture card.The LFM pulse signal is produced by digital method, at first calculates and storage discrete data s (n Δ t) according to the expression formula of s (t) in the formula (3), then these data is generated the LFM pulse signal through D/A converter.

The present invention has solved the G-TOFD detection method preferably and has been difficult to guarantee simultaneously the contradiction between the signal propagation distance and resolution in the thickness welded structure Ultrasonic Detection, can improve the accuracy of identification to the location of defective, improves the discrimination of little defective

Description of drawings

Fig. 1 is synoptic diagram (the transmitting probe T that pick-up unit is arranged in the prior art, another is receiving transducer R, lateral wave 1, defective upper end diffracted wave 2, defective lower end diffracted wave 3, bottom reflection echo 4), Fig. 2 is Fig. 1 diffracted signal synoptic diagram, and Fig. 3 is that the signal duration is to the synoptic diagram that influences of temporal resolution, Fig. 4 is the schematic diagram of detection system of the present invention, Fig. 5 is the imitation specimen synoptic diagram, and Fig. 6 is the side view of Fig. 5, and Fig. 7 is two kinds of weld defect supersonic detection method results' contrast synoptic diagram.

Embodiment

Embodiment one: present embodiment is described in conjunction with Fig. 4, the method of present embodiment may further comprise the steps: step 1, ultrasound wave transmitting probe and receiving transducer are symmetrically distributed in the both sides of detected position, first-selected digital signal generator produces broadband LFM pulse signal, produce synchronous triggering signal excitation broadband signal receiver simultaneously, broadband LFM pulse signal amplifies to possess corresponding energy by linear power amplifier and encourages the ultrasound wave transmitting probe;

The LFM pulse signal is produced by digital method, and the mathematic(al) representation of linear FM signal is

s(t)＝Arect(t/T)cos(2πf _ct±Kπt ²) |t|≤T/2 (3)

A is signal amplitude in the formula, f _cBe carrier frequency, K=B/T is the variation slope of signal transient frequency, i.e. chirp rate, and B is signal bandwidth, and T is pulse width, and rect (t/T) is rectangular signal, and

$\mathrm{rect}\left(\frac{t}{T}\right)=\left\{\begin{array}{cc}1,& \left|\frac{t}{T}\right|\&le;\frac{1}{2}\\ 0,& \mathrm{elsewise}\end{array}\right.---\left(4\right)$

So the instantaneous frequency f of signal is

$f=\frac{d(f_{c}t\&PlusMinus;\frac{K}{2}{t}^2)}{\mathrm{dt}}=\frac{d(f_{c}t\&PlusMinus;\frac{B}{2T}{t}^2)}{\mathrm{dt}}=f_c\&PlusMinus;\frac{B}{T}t$ $-\frac{\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">T</figure-callout>}}{2}\&le;t\&le;\frac{T}{2}---\left(5\right)$

Pumping signal s (t) is expressed as with plural form

$\tilde{s}\left(t\right)=\mathrm{Arect}(t/T)e^{j(2\mathrm{\&pi;}{f}_{c}t\&PlusMinus;\mathrm{K\&pi;}{t}^2)}---\left(6\right)$

The LFM pulse signal is carried out spectrum analysis, namely to signal

Carry out Fourier transform, then

$\tilde{S}\left(f\right)={\&Integral;}_{-\&infin;}^{\&infin;}\tilde{s}\left(t\right)e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;ft}}\mathrm{dt}=A{\&Integral;}_{-T/2}^{T/2}{e}^{j[2\mathrm{\&pi;}(f_c-f)t+\mathrm{K\&pi;}{t}^2]}\mathrm{dt}---\left(7\right)$

Arrangement by analysis, when wide-bandwidth product was very big at that time, the frequency spectrum function of LFM pulse signal can be expressed as

$\tilde{S}\left(f\right)=\left\{\begin{array}{cc}A\sqrt{T/B}{e}^{j[-\frac{\mathrm{\&pi;T}{(f-f_c)}^2}{B}+\frac{\mathrm{\&pi;}}{4}]}& |f-f_c|\&le;B/2\\ 0& |f-f_c|>B/2\end{array}\right.---\left(8\right);$

Produce forced vibration after step 2, the ultrasound wave transmitting probe excited target, radiate supersonic wave in the test specimen;

Step 3, when the control computing machine is carried out acquisition, receiving transducer receives detection signal in the relevant position, the reception of detection signal is to be finished by the broadband signal capture card;

Step 4, computing machine carry out matched filtering to received signal to be handled, thereby obtains pulse compression signal, according to the time of arrival of each waveform in the signal, can calculate the depth of burial of defective according to formula (1).

The matched filtering of chirp pulse signal,

The output signal of linear system

Frequency spectrum

It is input signal

Frequency spectrum

Product with the frequency response H (f) of receiving system

${\tilde{S}}_{\mathrm{out}}\left(f\right)=\tilde{S}\left(f\right)H\left(f\right)---\left(9\right)$

Output signal then

Can be expressed as

${\tilde{s}}_{\mathrm{out}}\left(t\right)={\&Integral;}_{-\&infin;}^{\&infin;}\tilde{S}\left(f\right)H\left(f\right){\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">e</figure-callout>}}^j\mathrm{df}---\left(10\right)$

When the frequency response function of receiving system satisfies

$H\left(f\right)=k\tilde{S}*\left(f\right)e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;f}{t}_0}---\left(11\right)$

Be frequency spectrum Conjugate, k is the gain constant of wave filter, this moment, the input/output signal noise ratio reached maximal value, namely

$H\left(f\right)=k\left|\tilde{S}\left(f\right)\right|e^{-j\widetilde{\mathrm{\&phi;}}\left(f\right)}{e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{\mathrm{ft}}_0}=k\left|\tilde{S}\left(f\right)\right|e^{-j[\widetilde{\mathrm{\&phi;}}\left(f\right)+2\mathrm{\&pi;}{\mathrm{ft}}_0]}---\left(12\right)$

The amplitude of normalization H (f), the frequency response function that can get the matched filter of LFM pulse signal is

$H\left(f\right)=\left\{\begin{array}{cc}{e}^{j[\frac{\mathrm{\&pi;T}{(f-f_c)}^2}{B}\frac{\mathrm{\&pi;}}{4}-2\mathrm{\&pi;}{\mathrm{ft}}_0]}& |f-f_c|\&le;B/2\\ 0& |f-f_c|>B/2\end{array}\right.---\left(13\right)$

Can be got the output complex signal of LFM pulse signal matched filter by formula (8), (9) and (13)

Frequency spectrum

For

${\tilde{S}}_{\mathrm{out}}\left(f\right)=\tilde{S}\left(f\right)H\left(f\right)=A\sqrt{\frac{T}{B}}{e}^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{\mathrm{ft}}_0}$ |f-f _c|≤B/2 (14)

Right

Carry out inverse fourier transform, be the complex signal of matched filter output

Through putting in order,

${\tilde{s}}_{\mathrm{out}}\left(t\right)={\&Integral;}_{-\&infin;}^{\&infin;}{\tilde{S}}_{\mathrm{out}}\left(f\right){\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">e</figure-callout>}}^j\mathrm{df}={\&Integral;}_{f_c-B/2}^{f_c+B/2}A\sqrt{\frac{T}{\mathrm{<figure-callout\; id="2"\; label="B\; e\; j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">B</figure-callout>}}}{e}^j{2\mathrm{\&pi;f}(t-t_0)}^{}\mathrm{df}$

(15)

$=A\sqrt{\mathrm{BT}}\frac{\sin \left[\mathrm{\&pi;B}(t-t_0)\right]}{\mathrm{\&pi;B}(t-t_0)}{\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HDA00003104276400012.png"\; state="\{\{state\}\}">e</figure-callout>}}^j$

Then Real part be that the real signal of matched filter output is pulse compression signal

$s_{\mathrm{out}}\left(t\right)=A\sqrt{\mathrm{BT}}\frac{\sin \left[\mathrm{\&pi;B}(t-t_0)\right]}{\mathrm{\&pi;B}(t-t_0)}\cos \left[2\mathrm{\&pi;}{f}_c(t-t_0)\right]---\left(16\right).$

Embodiment two: in conjunction with Fig. 1 present embodiment is described, the broadband signal receiver of present embodiment is UT-350 broadband signal receiver, can receive the signal of broad frequency band, and its highest sample frequency is 50MHz.Other implementation steps are identical with embodiment one.

Embodiment three: in conjunction with Fig. 1 present embodiment is described, the linear power amplifier of present embodiment is linear power amplifier A150, and it can carry out linearity and amplify to the signal of input, and gaining is 53dB.Other implementation steps are identical with embodiment one.

Embodiment four: in conjunction with Fig. 1 present embodiment is described, the digital signal generator of present embodiment is signal generator DG3061A, utilizes this signal generator can realize the editor of the following arbitrary signal of many 20MHz.Other implementation steps are identical with embodiment one.

The LFM-TOFD that detects application example-big thickness test specimen detects.

Referring to Fig. 5 and Fig. 6, the thickness h of big thickness carbon steel imitation specimen is 60mm, has two artificial defects, the one, and centre distance detection plane L1 is the horizontal through hole of the Φ 2 of 15mm, another defective is to be the dark bottom surface open slot of 50mm apart from detecting Surface L 2.

This test specimen G-TOFD detection and LFM-TOFD detection have been carried out respectively.Identical sensor is used in twice detection, and system-gain, probe spacing, sample frequency are identical with the sampling delay parameter, and wherein probe spacing is 125mm, and sensor frequency is 10MHz, and diameter is 6mm.Different is that the used pumping signal of G-TOFD detection is the burst pulse of time width 50ns, voltage 300V, be that duration is that 5 μ s, initial frequency are 1MHz, to stop frequency be the LFM pulse signal of 12MHz and LFM-TOFD detect to adopt, and driving voltage is 84V.

Referring to Fig. 7, the normalization result of twice experiment detection signal a) is the G-TOFD detection signal, because high-frequency signal energy attenuation after long Distance Transmission is serious, can only see the bottom reflection signal this moment in detection signal; B) be the LFM-TOFD signal, can clearly distinguish the diffracted signal of lateral wave and cross-drilled hole among the figure, and successfully detect the diffracted signal of two defectives, and each waveform in the detection signal is all narrower, is more conducive to the identification to the degree of depth of defective like this.By with b) in relative time difference band between defective 1, defective 2 and Bottom echo and the lateral wave go in the formula (1) (velocity of sound C is about 5900m/s), the degree of depth that then can calculate defective 1 is 15.48mm, the degree of depth of defective 2 is 49.5mm, the height of Bottom echo is 59.1mm, and maximum error is 0.5mm among the three.

From experimental result as can be seen, in G-TOFD detected, high-frequency probe utilized the sensor of 10MHz can't realize the detection of the thick test specimen of 60mm inside because signal attenuation makes range of application be restricted; And the LFM-TOFD detection method is under the condition of using high frequency sensors, realized the identification of flaw indication, this method has solved the G-TOFD detection method preferably and has been difficult to guarantee simultaneously signal penetration power and this contradiction of detection resolution, expanded the scope of the application of high frequency probe, improved the location accuracy of identification to defective, this more is conducive to find the little defective in the material.

Claims (4)

1. weld defect ultrasonic TOFD detection method based on the linear frequency modulation technology, it is characterized in that said method comprising the steps of: step 1, ultrasound wave transmitting probe and receiving transducer are symmetrically distributed in the both sides of detected position, first-selected digital signal generator produces broadband LFM pulse signal, produce synchronous triggering signal excitation broadband signal receiver simultaneously, broadband LFM pulse signal amplifies to possess corresponding energy by linear power amplifier and encourages the ultrasound wave transmitting probe;

The LFM pulse signal is produced by digital method, and the mathematic(al) representation of linear FM signal is

s(t)＝Arect(t/T)cos(2πf _ct±Kπt ²)|t|≤T/2 (3)

A is signal amplitude in the formula, f _cBe carrier frequency, K=B/T is the variation slope of signal transient frequency, i.e. chirp rate, and B is signal bandwidth, and T is pulse width, and rect (t/T) is rectangular signal, and

$\mathrm{rect}\left(\frac{t}{T}\right)=\left\{\begin{array}{cc}1,& \left|\frac{t}{T}\right|\&le;\frac{1}{2}\\ 0,& \mathrm{elsewise}\end{array}\right.---\left(4\right)$

So the instantaneous frequency f of signal is

$f=\frac{d(f_{c}t\&PlusMinus;\frac{K}{2}{t}^2)}{\mathrm{dt}}=\frac{d(f_{c}t\&PlusMinus;\frac{B}{2T}{t}^2)}{\mathrm{dt}}=f_c\&PlusMinus;\frac{B}{T}t$ $-\frac{T}{2}\&le;t\&le;\frac{T}{2}---\left(5\right)$

Pumping signal s (t) is expressed as with plural form

$\tilde{s}\left(t\right)=\mathrm{Arect}(t/T)e^{j(2\mathrm{\&pi;}{f}_{c}t\&PlusMinus;\mathrm{K\&pi;}{t}^2)}---\left(6\right)$

The LFM pulse signal is carried out spectrum analysis, namely to signal Carry out Fourier transform, then

$\tilde{S}\left(f\right)={\&Integral;}_{-\&infin;}^{\&infin;}\tilde{s}\left(t\right)e^{-j2\mathrm{\&pi;ft}}\mathrm{dt}=A{\&Integral;}_{-T/2}^{T/2}{e}^{j[2\mathrm{\&pi;}(f_c-f)t+\mathrm{K\&pi;}{t}^2]}\mathrm{dt}---\left(7\right)$

Arrangement by analysis, when wide-bandwidth product was very big at that time, the frequency spectrum function of LFM pulse signal can be expressed as

$\tilde{S}\left(f\right)=\left\{\begin{array}{cc}A\sqrt{T/B}{e}^{j[-\frac{\mathrm{\&pi;T}{(f-f_c)}^2}{B}+\frac{\mathrm{\&pi;}}{4}]}& |f-f_c|\&le;B/2\\ 0& |f-f_c|>B/2\end{array}\right.---\left(8\right);$

Produce forced vibration after step 2, the ultrasound wave transmitting probe excited target, radiate supersonic wave in the test specimen;

Step 3, when the control computing machine is carried out acquisition, receiving transducer receives detection signal in the relevant position, the reception of detection signal is to be finished by the broadband signal capture card;

Step 4, computing machine carry out matched filtering to received signal to be handled, thereby obtains pulse compression signal, according to the time of arrival of each waveform in the signal, can calculate the depth of burial of defective according to formula (1).

The matched filtering of chirp pulse signal,

The output signal of linear system Frequency spectrum

It is input signal

Frequency spectrum

Product with the frequency response H (f) of receiving system

${\tilde{S}}_{\mathrm{out}}\left(f\right)=\tilde{S}\left(f\right)H\left(f\right)---\left(9\right)$

Output signal then

Can be expressed as

${\tilde{s}}_{\mathrm{out}}\left(t\right)={\&Integral;}_{-\&infin;}^{\&infin;}\tilde{S}\left(f\right)H\left(f\right)e^{j2\mathrm{\&pi;ft}}\mathrm{df}---\left(10\right)$

When the frequency response function of receiving system satisfies

$H\left(f\right)=k\tilde{S}*\left(f\right)e^{-j2\mathrm{\&pi;f}{t}_0}---\left(11\right)$

Be frequency spectrum

Conjugate, k is the gain constant of wave filter, this moment, the input/output signal noise ratio reached maximal value, namely

$H\left(f\right)=k\left|\tilde{S}\left(f\right)\right|e^{-j\widetilde{\mathrm{\&phi;}}\left(f\right)}{e}^{-j2\mathrm{\&pi;}{\mathrm{ft}}_0}=k\left|\tilde{S}\left(f\right)\right|e^{-j[\widetilde{\mathrm{\&phi;}}\left(f\right)+2\mathrm{\&pi;}{\mathrm{ft}}_0]}---\left(12\right)$

The amplitude of normalization H (f), the frequency response function that can get the matched filter of LFM pulse signal is

$H\left(f\right)=\left\{\begin{array}{cc}{e}^{j[\frac{\mathrm{\&pi;T}{(f-f_c)}^2}{B}\frac{\mathrm{\&pi;}}{4}-2\mathrm{\&pi;}{\mathrm{ft}}_0]}& |f-f_c|\&le;B/2\\ 0& |f-f_c|>B/2\end{array}\right.---\left(13\right)$

Can be got the output complex signal of LFM pulse signal matched filter by formula (8), (9) and (13) Frequency spectrum

For

${\tilde{S}}_{\mathrm{out}}\left(f\right)=\tilde{S}\left(f\right)H\left(f\right)=A\sqrt{\frac{T}{B}}{e}^{-j2\mathrm{\&pi;}{\mathrm{ft}}_0}$ |f-f _c|≤B/2 (14)

Right

Carry out inverse fourier transform, be the complex signal of matched filter output

Through putting in order,

${\tilde{s}}_{\mathrm{out}}\left(t\right)={\&Integral;}_{-\&infin;}^{\&infin;}{\tilde{S}}_{\mathrm{out}}\left(f\right)e^{j2\mathrm{\&pi;ft}}\mathrm{df}={\&Integral;}_{f_c-B/2}^{f_c+B/2}A\sqrt{\frac{T}{B}}{e}^{j2\mathrm{\&pi;f}(t-t_0)}\mathrm{df}$

(15)

$=A\sqrt{\mathrm{BT}}\frac{\sin \left[\mathrm{\&pi;B}(t-t_0)\right]}{\mathrm{\&pi;B}(t-t_0)}{e}^{j2\mathrm{\&pi;}{f}_c(t-t_0)}$

Then

Real part be that the real signal of matched filter output is pulse compression signal

$s_{\mathrm{out}}\left(t\right)=A\sqrt{\mathrm{BT}}\frac{\sin \left[\mathrm{\&pi;B}(t-t_0)\right]}{\mathrm{\&pi;B}(t-t_0)}\cos \left[2\mathrm{\&pi;}{f}_c(t-t_0)\right]---\left(16\right).$

2. according to the described weld defect ultrasonic TOFD detection method based on the linear frequency modulation technology of claim 1, it is characterized in that the broadband signal receiver is UT-350 broadband signal receiver.

3. according to the described weld defect ultrasonic TOFD detection method based on the linear frequency modulation technology of claim 1, it is characterized in that linear power amplifier is linear power amplifier A150.

4. according to the described weld defect ultrasonic TOFD detection method based on the linear frequency modulation technology of claim 1, it is characterized in that digital signal generator is signal generator DG3061A.

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