阅读说明：本技术 一种检测黏弹性固体的超声叠片换能器 (Ultrasonic laminated transducer for detecting viscoelastic solid ) 是由 杨顺民 宋文爱 陈以方 董晓丽 张世雄 于 2021-09-06 设计创作，主要内容包括：本发明公开一种检测黏弹性固体的超声叠片换能器,包括壳体、绝缘材料、背衬、压电晶片、匹配层、导电铜片、地线、信号线和电缆接头,所述壳体设有内腔,内腔的底部设置匹配层,匹配层上方设有晶片组,晶片组由四片4MHz的压电晶片叠堆构成,相邻两压电晶片之间夹合一所述导电铜片,相邻两压电晶片极性相同的表面相对,压电晶片为圆形的实心结构；晶片组上方设置所述背衬,所述背衬上方的内腔填充有绝缘材料,所述壳体外部设置所述电缆接头,所述电缆接头分别通过所述信号线和地线内接所述晶片组的电极两端。本案弥补超声波在黏弹性固体中传播时的高衰减特性,有效提高超声波检测黏弹性固体的分辨率。(The invention discloses an ultrasonic laminated transducer for detecting viscoelastic solids, which comprises a shell, an insulating material, a back lining, piezoelectric wafers, a matching layer, a conductive copper sheet, a ground wire, a signal wire and a cable joint, wherein the shell is provided with an inner cavity, the bottom of the inner cavity is provided with the matching layer, a wafer group is arranged above the matching layer, the wafer group is formed by stacking four piezoelectric wafers of 4MHz, one conductive copper sheet is clamped between every two adjacent piezoelectric wafers, the surfaces of the two adjacent piezoelectric wafers with the same polarity are opposite, and the piezoelectric wafers are of a round solid structure; the back lining is arranged above the wafer group, an inner cavity above the back lining is filled with insulating materials, the cable joints are arranged outside the shell, and the cable joints are respectively connected with two ends of the electrode of the wafer group in an inscribed mode through the signal wire and the ground wire. The present case remedies the high attenuation characteristic of ultrasonic wave when spreading in the viscoelastic solid, effectively improves the resolution ratio of ultrasonic detection viscoelastic solid.)

1. The ultrasonic laminated transducer for detecting the viscoelastic solid is characterized by comprising a shell, an insulating material, a back lining, piezoelectric wafers, a matching layer, a conductive copper sheet, a ground wire, a signal wire and a cable joint, wherein the shell is provided with an inner cavity, the matching layer is arranged at the bottom of the inner cavity, a wafer group is arranged above the matching layer, the wafer group is formed by stacking four piezoelectric wafers of 4MHz, the conductive copper sheet is clamped between every two adjacent piezoelectric wafers, the surfaces of the two adjacent piezoelectric wafers with the same polarity are opposite, and the piezoelectric wafers are of a round solid structure; the back lining is arranged above the wafer group, an inner cavity above the back lining is filled with insulating materials, the cable joints are arranged outside the shell, and the cable joints are respectively connected with two ends of the electrode of the wafer group in an inscribed mode through the signal wire and the ground wire.

2. An ultrasonic laminated transducer for detecting viscoelastic solids as claimed in claim 1 wherein: and the conductive copper sheet and the piezoelectric wafer are bonded through epoxy resin.

3. An ultrasonic laminated transducer for detecting viscoelastic solids as claimed in claim 1 wherein: the piezoelectric wafer is made of PT material.

Technical Field

The invention belongs to the technical field of viscoelastic solid detection equipment, and particularly relates to an ultrasonic lamination transducer for detecting viscoelastic solids.

Background

Transducer means a device for interconversion between electric energy and acoustic energy. Common ultrasonic transducers convert electrical energy into ultrasonic waves for underwater communication. According to the related documents at home and abroad, the piezoelectric wafer stacking technology is mainly applied to the underwater acoustic transducer, and the underwater acoustic transducer and the industrial component flaw detection transducer have larger structural difference, so the underwater acoustic transducer is not directly used for flaw detection of the industrial component. No relevant applications were retrieved in the ultrasonic inspection transducer. When the underwater transducer is used for detecting viscoelastic solids, the performance index of the underwater transducer is greatly reduced due to the large attenuation of acoustic energy, so that the underwater transducer cannot meet normal requirements.

For plane simple harmonics in an infinite viscoelastic medium, the solution can be obtained according to the corresponding principle of the simple harmonic problem. That is, the solution of the wave equation of the viscoelastic solid can be obtained by replacing the elastic material constant with the complex function of the viscoelastic material in the elastic solution, wherein the longitudinal wave solution is:

the transverse wave solution is:

or

From the above equations, it can be seen that the viscoelastic solution and the elastic solution, although formally identical, are simply exchanged for λ, μ^*(i ω) and μ^*(i ω), but the characteristics of wave propagation are significantly affected, mainly expressed as:

1) let the viscoelastic longitudinal and transverse wave velocities be denoted as c_Lv、c_TvThen, then

Because of lambda^*(i ω) and μ^*(i ω) are all functions of ω, so c_Lv、c_TvAre a function of angular frequency ω, and therefore dispersion (chromatic dispersion, dispersion) occurs as the wave propagates through the viscoelastic medium.

2) In an ideal elastic medium, plane waves are not attenuated, whereas in a viscoelastic medium, plane waves attenuate with increasing propagation distance. Let the attenuation coefficients of longitudinal and transverse waves be respectively expressed as alpha_L，α_TThen there is

Where Re represents the real part of the complex number and Im represents the imaginary part of the complex number.

From the above research results, it can be seen that the ultrasonic wave causes greater energy attenuation in the viscoelastic solid due to dispersion and viscoelastic properties when propagating in the viscoelastic solid, compared to the elastic solid, which brings great challenge to the ultrasonic method for detecting the viscoelastic solid. To obtain the same detection resolution as that of the elastic solid, a high-energy ultrasonic transducer needs to be developed.

Disclosure of Invention

The invention aims to provide an ultrasonic laminated transducer for detecting a viscoelastic solid, which has a good detection effect and is suitable for flaw detection and material performance characterization of the viscoelastic solid.

In order to achieve the above purpose, the solution of the invention is: an ultrasonic laminated transducer for detecting viscoelastic solids comprises a shell, an insulating material, a back lining, piezoelectric wafers, a matching layer, a conductive copper sheet, a ground wire, a signal wire and a cable joint, wherein the shell is provided with an inner cavity, the bottom of the inner cavity is provided with the matching layer, a wafer group is arranged above the matching layer and consists of four piezoelectric wafers of 4MHz in a stacked mode, the conductive copper sheet is clamped between every two adjacent piezoelectric wafers, the surfaces of the two adjacent piezoelectric wafers with the same polarity are opposite, and the piezoelectric wafers are of a round solid structure; the back lining is arranged above the wafer group, an inner cavity above the back lining is filled with insulating materials, the cable joints are arranged outside the shell, and the cable joints are respectively connected with two ends of the electrode of the wafer group in an inscribed mode through the signal wire and the ground wire.

Preferably, the piezoelectric wafer is made of PT material.

Preferably, the conductive copper sheet and the piezoelectric wafer are bonded through epoxy resin.

After the scheme is adopted, the invention has the beneficial effects that: the invention fills the defect of the field of ultrasonic transducers in detecting the viscoelastic solid, and aiming at the viscoelastic solid material with larger attenuation amplitude, the invention not only overcomes the characteristic, but also has better detection effect, high gain can better make up the high attenuation characteristic of ultrasonic waves when the ultrasonic waves are transmitted in the viscoelastic solid, and can effectively improve the resolution of the ultrasonic waves for detecting the viscoelastic solid, so that the transducer of the invention can be completely competent in industrial flaw detection.

Drawings

FIG. 1 is a schematic diagram of the construction of a transducer of the present invention;

FIG. 2 is an amplitude versus frequency relationship for a viscoelastic material of the present invention (where realizations represent viscoelastic material and dashed lines represent elastic material);

FIG. 3 is a schematic diagram of a piezoelectric wafer assembly consisting of four identical piezoelectric wafers according to the present invention, which are electrically connected in parallel;

FIG. 4 is an electromechanical equivalent circuit diagram of a single piezoelectric wafer of the present invention;

FIG. 5 is an electromechanical equivalent circuit diagram of a stacked piezoelectric wafer of the present invention;

FIG. 6 is a schematic illustration of the resonant frequency of the present invention;

FIG. 7 is a schematic view of a simulation model of a transducer of the present invention;

FIG. 8 is a diagram of the sound field energy of a conventional transducer;

FIG. 9 is a diagram of the sound field energy of a transducer of the present invention;

FIG. 10 is a time domain waveform diagram of a conventional transducer;

fig. 11 is a time domain waveform diagram of a transducer of the present invention.

Description of reference numerals:

the piezoelectric ceramic package comprises a shell 1, an insulating material 2, a back lining 3, a piezoelectric wafer 4, a matching layer 5, a conductive copper sheet 6, a ground wire 7, a signal wire 8, a cable joint 9 and a viscoelastic solid 10.

Detailed Description

The invention is described in detail below with reference to the accompanying drawings and specific embodiments.

The invention provides an ultrasonic laminated transducer for detecting viscoelastic solids, which comprises a shell 1, an insulating material 2, a backing 3, a piezoelectric wafer 4, a matching layer 5, a conductive copper sheet 6, a ground wire 7, a signal wire 8 and a cable joint 9, wherein the shell 1 is used for protecting internal components of the transducer, the shell 1 is provided with an inner cavity, the bottom of the inner cavity is provided with the matching layer 5, a wafer group is arranged above the matching layer 5, the wafer group is formed by stacking four piezoelectric wafers 4 with 4MHz, the piezoelectric wafer 4 mainly realizes the conversion between electric energy and sound energy, and the matching layer 5 is used for relieving the difference of acoustic impedance between the piezoelectric wafer 4 and a detected object so that the sound energy generated by the piezoelectric wafer 4 is incident into the detected object as much as possible. The matching layer is also called as a sound-transmitting film, and the thickness of the matching layer is generally d equal to 1/4 lambda, so that the phase of reflected waves of the front interface and the rear interface of the sound-transmitting film are inversely counteracted by utilizing half-wave loss and path difference in the reflection process, and the transmitted waves are strengthened by the accumulation superposition effect of residual waves in the sound-transmitting film; in addition, the requirement for the acoustic impedance z of the sound-transmitting film is that the total transmittance t is 2 × z/(z1+ z) × 2 × z2/(z + z2) after the sound-transmitting film is added between the two media, and z2 is satisfied to maximize the total transmittance, that is, z1 × z 2. The raw material formula of the matching layer needs to be developed according to the acoustic impedance value of the detected object, so that the acoustic energy can be incident into the detected object to the maximum extent.

The conductive copper sheet 6 is clamped between two adjacent piezoelectric wafers 4, the surfaces of the two adjacent piezoelectric wafers 4 with the same polarity are opposite, and the conductive copper sheet 6 is used for realizing the parallel connection of the electrodes between the piezoelectric wafers 4. The piezoelectric wafer 4 is a circular solid structure, and is different from the piezoelectric wafer of the underwater acoustic transducer in that the piezoelectric wafer is annular or bent; the back lining 3 is arranged above the wafer group, and the back lining 3 is used for controlling the vibration degree of the piezoelectric wafer 4 on one hand and absorbing sound energy on the back surface of the piezoelectric wafer 4 on the other hand. In order to reduce the specific gravity of the reflected wave, it is required that the acoustic impedance of the backing material is close to that of the stacked piezoelectric wafer, and the acoustic impedances of the obtained backing materials are greatly different because the backing material contains different proportions of tungsten powder, epoxy resin, curing agent, adhesive and the like. Epoxy resin and tungsten powder are mixed according to the mass ratio of 1: and 6, preparing epoxy tungsten powder according to the proportion, and heating by a hot air blower after the epoxy tungsten powder is prepared to remove bubbles.

The cavity above the backing 3 is filled with insulating material 2, the insulating material 2 plays an insulating role between a component below the backing and the shell 1, the cable joint 9 is arranged outside the shell 1, the cable joint 9 is respectively connected with two ends of the electrode of the wafer group through the signal wire 8 and the ground wire 7, the signal wire 8 and the cable joint 9 realize the transmission of ultrasonic signals, in the working process of the ultrasonic transducer, on one hand, a transmitting/receiving circuit generates excitation pulses, the excitation pulses are transmitted to the piezoelectric wafer group through the signal wire 8 and the ground wire 7 through the cable joint 9, and the piezoelectric wafer group is driven to vibrate to generate ultrasonic waves, so that the conversion of electric energy to sound energy is realized; on the other hand, when the ultrasonic wave is transmitted in the detected object, the ultrasonic transducer receives the reflected sound wave and converts the reflected sound wave into a pulse signal, and then the pulse signal is transmitted to the transmitting/receiving circuit through the signal line 8 and the ground wire 7 through the cable joint 9 to form an ultrasonic wave echo signal, and whether the material characteristics and the internal structure of the detected object have damaged parts or not is judged according to the echo signal.

According to the invention, the ultrasonic laminated transducer, also called industrial flaw detection laminated transducer, ultrasonic laminated piezoelectric transducer and the like, is designed according to the structure, and for a viscoelastic solid material, a linear relation shown in figure 2 exists between the echo amplitude of ultrasonic waves and the frequency of the transducer. It can be seen from fig. 2 that the amplitude of the ultrasonic echo signal decreases linearly with a proportion of the transducer frequency for the elastic material (dashed line in the figure). And the amplitude of the ultrasonic echo signal is attenuated rapidly along with the increase of the frequency of the transducer by the viscoelastic material (a solid line in the figure), and the amplitude of the ultrasonic echo signal is almost attenuated to 0 when the frequency is 3 MHz. Therefore, for the detection of viscoelastic materials, the frequency of the ultrasonic transducer should be as low as possible to ensure the amplitude of the ultrasonic echo signal. Therefore, the scheme selects 1MHz as the resonant frequency of the laminated transducer.

The natural resonant frequency of a single piezoelectric wafer depends primarily on the thickness of the piezoelectric wafer 4 and the speed of propagation of the ultrasonic waves in the wafer material. In order to obtain high acoustoelectric conversion efficiency, the piezoelectric wafer 4 must operate in a resonance state. According to the standing wave theory, the wafer thickness is one half of the wavelength at this time, namely:

wherein t is the thickness of the wafer in mm; lambda [ alpha ]_L-longitudinal wavelength in the wafer, in mm; c. C_L-the wave velocity of the longitudinal wave in the wafer, in m/s; f. of₀The natural frequency of the wafer, in Hz.

Because the PT material has small radial vibration energy and large thickness vibration energy, the method is favorable for industrial flaw detection. Therefore, the flaw detection laminated transducer adopts the piezoelectric wafer 4 made of PT material, and the longitudinal wave speed of the piezoelectric wafer is 4350 m/s. When the structure of the laminated transducer is designed, four 4MHz wafers are adopted for parallel stacking, and a copper sheet with the thickness of 0.2mm is placed between the stacked wafers, so that the good conductivity is ensured, meanwhile, the welding of an electrode lead is facilitated, when the natural frequency is 4MHz, the thickness of the wafer is 0.54375mm according to the formula (1). The laminated transducer is made to have a frequency of 1MHz while ensuring high transmission energy. The piezoelectric wafer 4 has a stacked structure as shown in fig. 3, in the stacking process, the surfaces of the piezoelectric wafers 4 with the same polarity are opposite, the conductive copper sheet 6 is placed in the middle and is bonded by using an adhesive, and in order to obtain a better sound transmission effect, the adhesive uses epoxy resin.

Whether the actual resonance frequency of the stacked four piezoelectric wafers 4 reaches the ideal resonance frequency of 1MHz or not is verified by calculation. The thickness vibration of the thin wafer (i.e. the piezoelectric wafer 4) is convenient to adopt a cylindrical coordinate, the electrodes are plated on two circular surfaces vertical to a z axis, the polarization direction is the z axis, and the corresponding relation between the basic coordinate of the piezoelectric wafer 4 and the cylindrical coordinate is

Since the piezoelectric wafer 4 is a thin wafer (i.e., the thickness of the wafer is much smaller than the wavelength in the operating frequency range), and the upper and lower electrode surfaces are free, the shear stress T is_zr、T_zθ、T_zzAnd axial displacement component xi_zIs zero, i.e.:

the wafer is axially symmetric in vibration, the geometric shape, constraint condition and external force of the elements are all symmetric to the z-axis, and all the stress component, strain component and displacement component are also respectively symmetric to the z-axis. In this case, each component is a function of r and z, independent of θ, and then:

equation of motion is simplified to

The relationship between strain and displacement is simplified to

Because the electric field is applied to the z-axis, the boundary effect of the electric field is ignored, and only E is₃Not equal to 0, then the piezoelectric equation is simplified to

To give out T_rAnd T_θTo obtain

In the formulaIs the Young's modulus under the action of a constant electric field,the Poisson (Poisson) coefficient under the action of a constant electric field.

The mechanical vibration equation is

Wherein F is the circumferential stress, rho is the density,for the propagation velocity of the wave, S2 pi at, k ω/v wavenumber, ω 2 pi f angular frequency, f resonant frequency, J₀(ka) is a zero-order Bessel-like function, J₁(ka) is a first order class of Bessel function,the vibration velocity of the circumference is the vibration velocity of the circumference,v is the applied voltage of the wafer, which is the electromechanical conversion coefficient.

The state equation of the circuit is

In the formulaIs a two-dimensional cutoff capacitance.

An electromechanical equivalent diagram of the single piezoelectric wafer 4 can be obtained from the mechanical vibration equation and the circuit state equation, as shown in fig. 4.

If the piezoelectric wafer 4 is freely vibrated, i.e., if F is 0, the resonance frequency equation is obtained as

kaJ₀(ka)＝(1-σ)J₁(ka)

From this, the admittance equation of the single piezoelectric chip 4 can be derived as

The piezoelectric wafer group consisting of p identical piezoelectric wafers 4 are electrically connected in parallel and mechanically connected in series. Theoretically, p identical four-terminal networks are cascaded. According to the cascade theory in the circuit, an electromechanical equivalent circuit diagram of the stacked piezoelectric wafers 4 can be obtained, as shown in fig. 5.

The laminated transducer is formed by stacking piezoelectric wafers 4 to form a cylindrical radiation surface, and the piezoelectric wafers 4 are connected in parallel. The admittance of a stacked transducer is thus a superposition of the admittances of the piezoelectric wafers 4. And because the material characteristics and the sizes of the piezoelectric circular wafers are the same, the admittance equation after the four piezoelectric wafers 4 are stacked can be deduced according to the admittance equation of the single piezoelectric wafer 4, wherein the admittance equation is as follows:

according to the definition of the resonance frequency, when Y → ∞ the transducer enters a resonance state, and the vibration frequency at this time is the resonance frequency of the transducer. As can be seen from the admittance equation of the stacked transducer, the transcendental equation kaJ is only required to satisfy the condition of Y → ∞₀(ka)＝(1-σ)J₁(ka) is the value of the resonant frequency of the four piezoelectric wafer 4 stack transducer. Radius a of piezoelectric wafer 4 is 12mm, and compliance constant of piezoelectric wafer 4 Density rho 7500kg/m³To obtainThe resonant frequency of the stacked transducer can be found graphically as shown in fig. 6.

From the graph, it can be seen that the resonance frequency of the transducer in which four piezoelectric wafers 4 of 4MHz are stacked is 1MHz, which is in agreement with the actually measured value of 1.03MHz, and the relative error is 0.3%.

The gain effect of the invention is that:

the circular laminated transducer is of an axisymmetric structure, and a simplified structural model can be adopted, namely, one fourth of the structure of the transducer is taken, so that the precision requirement is ensured, and the time of simulation calculation is reduced. The simulation model is shown in fig. 7 and mainly comprises a viscoelastic solid 10, a piezoelectric wafer 4(PT) and a conductive copper sheet 6, wherein the radius of a sector area formed by the viscoelastic solid 10 is 36 mm; the radius of the piezoelectric wafer 4 is 12mm, the thickness is 0.5mm, and the frequency is 4 MHz; the radius of the conductive copper sheet 6 is 12mm, and the thickness is 0.2 mm. The parameters of the materials used in the simulation are shown in the following table.

Material	Density (kg/m)3)	Longitudinal wave velocity (m/s)	Poisson ratio
				Viscoelastic material	1500	2000	0.5
PT material	7750	4350	0.34
				Copper sheet	8600	4700	0.37

In the simulation process, under the condition of comprehensively considering the accuracy and the calculation speed, 10 units are divided in one wavelength. A symmetric boundary condition is applied in the x direction, the polarization directions of adjacent piezoelectric wafers 4 are opposite, the load Volt on the upper surface of the piezoelectric wafer 4 is 1, and the lower surface is grounded. The simulation results for the conventional single-chip transducer are shown in fig. 8, and the simulation results for the four-chip stacked transducer of the present invention are shown in fig. 9. As can be seen from the figure, the sound field energy distribution of the laminated transducer is more concentrated than that of the conventional transducer, and the gain is 25dB higher. Therefore, the high gain of the laminated transducer can better compensate the high attenuation characteristic of the ultrasonic wave when the ultrasonic wave is transmitted in the viscoelastic solid, and the resolution of the ultrasonic detection viscoelastic solid can be effectively improved to a certain extent.

The time domain waveforms for a conventional single-chip transducer are shown in fig. 10, and the time domain waveforms for a four-chip stacked transducer of the present invention are shown in fig. 11. In the experimental process, the sample to be detected is industrial rubber with the thickness of 35mm, and a penetration method is adopted so as to obtain better waveform amplitude. From the experimental results, it can be seen that the gain of the conventional transducer is 35.6dB and the gain of the laminated transducer is 20.4dB for the same thickness of industrial rubber. I.e., the laminated transducer gains 15.2dB higher than the single wafer transducer and less than 25dB of the simulation results, mainly because the simulation was performed under conditions where some edge effects and losses were ignored in order to reduce the computational effort. Therefore, the 1MHz four-lamination transducer provided by the invention can ensure the defect resolution of the viscoelastic solid ultrasonic detection method.

Compared with an underwater acoustic transducer, the transducer has the following four different characteristics:

(1) an upper portion of the piezoelectric wafer stack. The industrial flaw detection transducer is a backing material and is used for controlling the vibration of the wafer on one hand and absorbing the sound energy of the back surface of the wafer on the other hand; the underwater acoustic transducer is a rubber sheet and a metal cover plate and is used for protecting the piezoelectric wafer and preventing the back propagation of acoustic energy.

(2) A piezoelectric wafer stack. The wafer shape of the laminated transducer for industrial inspection is circular and is determined by the thickness vibration mode. While the wafer shape of the other transducers is circular, determined by the radial vibration modes.

(3) A lower portion of the piezoelectric wafer stack. The industrial flaw detection transducer is a matching layer, the acoustic impedance of the industrial flaw detection transducer is generally the product of the acoustic impedance of the piezoelectric wafer stack and the acoustic impedance of the detected object, and the acoustic energy is made to enter the detected object as much as possible; the underwater acoustic transducer is a rubber sheet and a metal cover plate and is used for protecting the piezoelectric wafer and adjusting the radiation angle of a sound field.

(4) An application scenario. The 1MHz laminated ultrasonic transducer provided by the invention is mainly used for flaw detection and material performance characterization of viscoelastic solids. Whereas underwater transducers are mainly used for underwater communication in the ocean.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the design of the present invention, and all equivalent changes made in the design key point of the present invention fall within the protection scope of the present invention.