CN111551503B - Laser ultrasonic system and method for detecting elastic modulus of material in non-contact manner - Google Patents
Laser ultrasonic system and method for detecting elastic modulus of material in non-contact manner Download PDFInfo
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- CN111551503B CN111551503B CN202010358275.8A CN202010358275A CN111551503B CN 111551503 B CN111551503 B CN 111551503B CN 202010358275 A CN202010358275 A CN 202010358275A CN 111551503 B CN111551503 B CN 111551503B
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Abstract
The invention discloses a laser ultrasonic system and a laser ultrasonic method for non-contact detection of elastic modulus of a material. The invention controls the phase mask plate adding and removing system by adjusting the phase mask plate electric adjuster, and combines the adjustment of the parallel flat plate, so that the system can be quickly converted in a system for exciting and measuring surface acoustic wave signals and a system for exciting and detecting longitudinal wave and transverse wave signals. By measuring the wave speeds of acoustic surface waves, longitudinal waves and transverse waves measured in the two systems, the elastic modulus of the material is obtained when the density and the Poisson ratio of the material are unknown.
Description
Technical Field
The invention belongs to the field of laser ultrasonic non-contact detection, and particularly relates to a laser ultrasonic system and a laser ultrasonic method for detecting the elastic modulus of a material in a non-contact manner.
Background
With the development of material science, various novel materials emerge continuously, and many materials show extraordinary mechanical properties, so that the relevant measurement of mechanical parameters of the materials is particularly important. Among them, the modulus of elasticity is one of the very important parameters characterizing the mechanical properties of a material. The conventional method for measuring the elastic modulus of the material is a tensile test method, and the method has the defects of complicated process, long measuring time and certain damage to the material. The laser ultrasonic method is to excite the sound wave in the material by using the exciting light and measure the wave velocity of the sound wave so as to deduce the mechanical parameters of the sample. The laser ultrasonic method is non-contact direct excitation and measurement, and does not damage a sample, so the laser ultrasonic method becomes a research hotspot for measuring the elastic modulus of the material.
According to a calculation formula of the wave velocity of the sound wave in the sample, the mechanical property of the material can be deduced by detecting the sound wave in the object. For a fluid medium (air, water, etc.), when ultrasonic waves propagate in the medium, only volume deformation and no shear deformation occur in the medium, so that only ultrasonic longitudinal waves exist. In elastic solids, however, the propagation of sound waves is much more complex. Therefore, the measurement of relevant parameters deriving from the solid material by detecting the propagation characteristics of the acoustic wave in the solid becomes relatively complex. The method for solving the elastic modulus of the material by measuring the wave velocity by using the common laser ultrasonic method is a time difference method, namely, when the surface acoustic wave is measured, the excitation line source is continuously moved, the distance between the excitation line source and a detection point is changed, the time of the acoustic wave reaching the detection point at different excitation points is measured, and the wave velocity of the surface acoustic wave is obtained by averaging by using a velocity formula. According to the method, the position of the excitation line source needs to be moved for multiple times of measurement, and the moving accuracy cannot be guaranteed. When measuring the transverse wave, due to the limitation of the signal-to-noise ratio, the excitation point and the receiving point need to be separated by a certain distance, and the transverse wave signal with high signal-to-noise ratio is obtained. But the final result is affected due to measurement errors in the distance between the excitation line source and the detection point.
In summary, although the "time difference method" in the laser ultrasonic method can avoid damage to the sample compared with the tensile experiment method, the final result is not very accurate due to the moving distance and the error of the measured distance in the measuring process.
Disclosure of Invention
The invention aims to provide a laser ultrasonic system for detecting the elastic modulus of a material in a non-contact mode.
The technical scheme for realizing the purpose of the invention is as follows: a laser ultrasonic system for non-contact detection of elastic modulus of materials comprises an excitation light emission module, a detection light emission module, a parallel flat plate, a dichroic mirror, a phase mask plate electric regulator, a light chopper, a first achromatic lens, a phase regulator, a second achromatic lens, a mobile sample stage and a signal receiving module, wherein the parallel flat plate, the dichroic mirror and the light chopper are sequentially arranged on an optical axis of the excitation light emission module;
the object carrying surface of the movable sample stage is a back focus of the second achromatic lens;
the optical axis of the signal receiving module is symmetrical to the optical axes of the first achromatic lens and the second achromatic lens with respect to the normal of the moving sample table;
the phase mask plate and the phase mask plate electric regulator are arranged between the dichroic mirror and the light shield, and the phase mask plate is added into or removed out of the light path by controlling the phase mask plate electric regulator.
Preferably, the excitation light emitting module includes a pulse laser, an attenuation sheet, and a cylindrical mirror, the pulse laser, the attenuation sheet, and the cylindrical mirror are located on the same optical axis, and the cylindrical mirror focuses the excitation light onto the phase mask.
Preferably, the detection light emitting module comprises a continuous laser (5) and a spherical lens, and the continuous laser (5) and the spherical lens are located on the same optical axis.
Preferably, the signal receiving module comprises a focusing sphere lens and an optoelectronic signal receiver:
when the phase mask plate electric regulator is controlled to enable the phase mask plate to be in the light path, the focusing spherical lens is used for coupling the detection light carrying the sample information excited by the exciting light and the reference light into the photoelectric signal receiver to obtain a heterodyne signal of the surface acoustic wave of the sample; when the phase mask plate is not in the light path, the focusing spherical lens is used for coupling single detection light carrying sample information excited by the exciting light into the photoelectric signal receiver to obtain longitudinal wave and transverse wave signals.
Preferably, the detection light and the excitation light form a fixed angle, ensuring that the phase adjuster does not block the excitation light. .
The invention also provides a method for detecting the elastic modulus of the material in a non-contact manner, which comprises the following steps:
constructing a laser ultrasonic system, and enabling a phase mask plate to be in a light path to obtain heterodyne signals of the excited surface acoustic wave of the sample;
resolving to obtain the wave velocity of the surface acoustic wave according to the heterodyne signal of the surface acoustic wave;
enabling the phase mask plate not to be in the light path, and acquiring excited longitudinal wave and transverse wave signals of the sample;
resolving to obtain the wave velocity of longitudinal waves and transverse waves according to the signals of the longitudinal waves and the transverse waves;
and obtaining the elastic modulus of the material according to the wave speeds of the acoustic surface wave signal, the longitudinal wave signal and the transverse wave signal.
Preferably, when the phase mask electric regulator is controlled to enable the phase mask to be located in the optical path and enable the system to be a system for exciting and measuring the surface acoustic wave signal, the heterodyne signal u of the surface acoustic wave is obtained r (t)。
The specific method for obtaining the wave velocity of the surface acoustic wave by resolving according to the heterodyne signal of the surface acoustic wave comprises the following steps:
for surface acoustic wave heterodyne signal u r (t) obtaining the center frequency f of the narrow-band surface acoustic wave signal after Fourier change, wherein the surface acoustic wave heterodyne signal u r The (t) is specifically:
u r (t)＝U 0 sin(qx)
wherein, U 0 Is the amplitude of the ultrasonic displacement and,
d is the distance between reticle lines of the phase mask plate;
determining the speed of the surface acoustic wave according to the center frequency of the signal and the wavelength of the excited surface acoustic wave as follows:
c r ＝f·Λ
in the formula, f is the center frequency of the narrow-band surface acoustic wave signal, and Λ is the excited surface acoustic wave wavelength.
Preferably, the phase mask electric adjuster is controlled so that the angle of the phase mask is symmetrical to that of the dichroic mirror, the excitation light and the detection light are overlapped at the same position of the front focus of the first achromat lens and the sample, and the longitudinal wave signal u is detected l (t) echo time t of longitudinal wave reflected from the bottom of sample with thickness h l ；
The specific formula for obtaining the longitudinal wave velocity by resolving according to the signals of the longitudinal waves is as follows:
preferably, when the phase mask electric adjuster is controlled so that the phase mask is not in the optical path, and the angle of the parallel flat plate is adjusted so that the angle is asymmetric to the angle of the dichroic mirror, the excitation light and the detection light are separated at the front focal point of the first achromatic lens and the focusing position of the sample, and the transverse wave signal u is detected t (t) echo time t of transverse wave reflected from the bottom surface of the sample with thickness h t The separation distance between the exciting light and the detecting light is L;
the specific formula for obtaining the longitudinal wave velocity by resolving according to the transverse wave signal is as follows:
preferably, the specific method for obtaining the elastic modulus of the sample by calculation according to the wave velocity of the surface acoustic wave, the wave velocity of the longitudinal wave and the wave velocity of the transverse wave comprises the following steps:
according to the wave velocity c of surface waves r Velocity of longitudinal wave c l The formula:
and the wave velocity c of the surface wave obtained by resolving r With the velocity c of the longitudinal wave l Obtaining a poisson ratio v;
according to the formula of the wave velocity of the shear wave:
and the calculated transverse wave velocity c t The density ρ and the elastic modulus E of the material are obtained.
Compared with the prior art, the invention has the following remarkable advantages: based on the principles of photoacoustic excitation and detection, the method is non-contact direct excitation and measurement, does not damage a sample and does not damage the sample, and combines a photoacoustic nondestructive detection technology with a heterodyne detection technology to enable a detected surface acoustic wave signal to be stronger; an electric regulator is added under a phase mask plate, so that the phase mask plate is added into or removed from an optical path, and the system can be quickly and conveniently converted into a system for measuring longitudinal wave and transverse wave signals from a system for measuring surface acoustic wave signals in the same system; when the wave velocity of the surface acoustic wave is measured, repeated measurement is not needed, and the position of an excitation point is not needed to be moved, so that the measurement accuracy is ensured; when the longitudinal wave velocity is measured, the angle symmetry of the parallel flat plate and the dichroic mirror and a 4f system consisting of an achromatic lens are utilized to ensure that an excitation point and a detection point coincide on a sample and ensure the accuracy of measurement; and when measuring the wave velocity of the transverse wave, the accurate rotation angle of the parallel flat plate is utilized, so that the separation distance can be accurately calculated, and more accurate wave velocity of the transverse wave is obtained. The measured related information is more accurate, so that the elastic modulus of the analyzed material is more accurate.
Drawings
FIG. 1 is a schematic structural diagram of a laser ultrasonic system for non-contact detection of elastic modulus of a material.
Fig. 2 is a schematic top view of a part of the structure of the present invention with a phase mask added. Wherein the rotation angle of the parallel flat plate is finely adjusted to make the excitation point coincide with the receiving point.
FIG. 3 is a diagram illustrating a simulation of interference fringes formed by focusing the excitation light at the sample.
Fig. 4 is a schematic top view of a part of the structure of the present invention after removing the phase mask, wherein the rotation angle of the parallel plate is finely adjusted to separate the excitation point from the receiving point.
Fig. 5 is a simulation diagram of the present invention in which the phase mask is removed and the rotation angle of the parallel plate is finely adjusted to separate the excitation point from the reception point.
Detailed Description
As shown in fig. 1, a laser ultrasonic system for non-contact detection of elastic modulus of a material includes an excitation light emission module, a detection light emission module, a parallel plate 4, a dichroic mirror 7, a phase mask 8, a phase mask electric actuator 9, a shutter 10, a first achromatic lens 11, a phase actuator 12, a second achromatic lens 13, a mobile sample stage, and a signal receiving module, and controls the phase mask electric actuator 9 to add or remove the phase mask 8 into or from an optical path, and the system can be quickly and conveniently converted from a system for measuring surface acoustic wave signals to a system for measuring longitudinal wave and transverse wave signals by adjusting the parallel plate.
As shown in fig. 2, when the phase mask electric actuator 9 is controlled so that the phase mask 8 is positioned in the optical path, the system at this time is a system for measuring a surface acoustic wave signal. At the moment, excitation light emitted by the excitation light emitting module is transmitted through the parallel flat plate 4 and the dichroic mirror 7 and then is focused on the phase mask plate 8, the phase mask plate 8 divides the excitation light into two beams, and the two beams of excitation light are sequentially focused on the surface of a sample arranged on the movable sample stage through the light chopper 10, the first achromatic lens 11 and the second achromatic lens 13;
the detection light emitted by the detection light emitting module is reflected by the dichroic mirror 5 and then focused on the phase mask 8, the angle of the parallel flat plate 4 is finely adjusted, so that the angle of the parallel flat plate 4 is 'symmetrical' to the angle of the dichroic mirror 5, the detection light reflected by the dichroic mirror 5 and the excitation light transmitted by the dichroic mirror 5 are converged at the same point of the phase mask 8, the phase mask 8 divides the detection light into two beams, one beam is used as detection light and sequentially focused on the surface of a sample arranged on a movable sample stage through the light chopper 10, the first achromatic lens 11, the phase adjuster 12 and the second achromatic lens 13, and the other beam is used as reference light and sequentially focused on the surface of the sample arranged on the movable sample stage through the light chopper 10, the first achromatic lens 11 and the second achromatic lens 13;
the two excitation lights are overlapped with the detection light and the reference light at the same position of the sample;
and the signal receiving module is used for receiving the surface acoustic wave heterodyne signals after the reflection and diffraction of the sample.
Specifically, the phase mask 8 is located on the focal plane of the cylindrical mirror, and when the angles of the parallel flat plate 4 and the dichroic mirror 7 are "symmetrical", the parallel flat plate effect generated after the excitation light passes through the dichroic mirror is cancelled, so that the parallel flat plate 4 and the dichroic mirror can be regarded as air when a light path is designed, and only the height and the position of the excitation light need to be considered. When the optical path element is controlled to cause the excitation light and the detection light to coincide on the phase mask 8, since the phase mask 8 is located on the front focal plane of the first achromatic lens 11, and the first achromatic lens and the second achromatic lens constitute a "4f" system, the excitation light and the detection light coincide on the sample. When the excitation light is focused on the phase mask 8, the phase mask 8 diffracts the excitation light into various levels of diffraction light with different energies, the diffraction light has different grating periods, and a sample can be detected by using different grating periods for multiple times, so that heterodyne signals under different grating periods can be obtained.
The shutter 10 is used to block diffracted light of each level which is not used to excite acoustic waves after the excitation light is diffracted by the phase mask 8.
Specifically, the first achromatic lens 11 and the second achromatic lens 13 are configured to focus excitation light diffracted by the phase mask 8 and used for exciting a surface acoustic wave of the sample on the surface of the sample, so that two excitation lights spatially and temporally overlap on the surface of the sample to form interference fringes and generate surface ultrasound, as shown in fig. 3.
In a further embodiment, the light emitting module includes a pulse laser 1, an attenuation sheet 2, and a cylindrical mirror 3, where the pulse laser 1, the attenuation sheet 2, and the cylindrical mirror 3 are located on the same optical axis, and the cylindrical mirror 3 focuses the excitation light onto a phase mask 8.
Specifically, the 532nm pulse laser 1 is used to generate excitation light.
The attenuation sheet 2 is used for adjusting the energy of laser emitted by the 532nm pulse laser to excite an acoustic signal, so that the material is prevented from being damaged by strong laser.
The cylindrical mirror 3 is used for focusing the excitation light on the phase mask 8, and the height of the excitation light in the vertical direction is the same as that of the laser light outlet hole.
In a further embodiment, the detection light emitting module includes a continuous laser 5 and a spherical lens 6, and the continuous laser 5 and the spherical lens 6 are located on the same optical axis.
Specifically, the continuous laser 5 is used to generate detection light.
The spherical mirror 6 is used to focus the detection light onto the phase mask 8.
Specifically, when the detection light reflected by the dichroic mirror 7 is focused on the phase mask 8, the phase mask 8 diffracts the detection light into different levels of diffraction light with different energies, the diffraction light has different grating periods, and a sample can be detected by using different grating periods for multiple times, so that heterodyne signals under different grating periods can be obtained.
Specifically, the phase adjuster 12 is an electro-optical phase modulator based on an electro-optical effect, applies a voltage to an input terminal electrode to generate a phase difference controlled by the voltage, but does not affect a polarization direction, adjusts a laser beam to a desired state by adjusting the voltage, can accurately and dynamically adjust a phase condition of incident light, optimizes a heterodyne phase after phase adjustment by allowing probe light and reference light to reach a phase matching condition, and finally can detect an ultrasonic micro-vibration condition.
In a further embodiment, the signal receiving module includes a focusing sphere lens 15 and an optoelectronic signal receiver 16:
the focusing sphere lens 15 is used for coupling the detection light carrying the sample information excited by the excitation light and the reference light into the photoelectric signal receiver 16 to obtain a heterodyne signal.
In particular, the moving sample stage 14 is a three-dimensional motorized translation stage for moving the sample to a focal spot of the excitation light.
In a further embodiment, the detection light reflected by the dichroic mirror 7 and the excitation light transmitted by the dichroic mirror 7 converge on the same point of the phase mask 8, and the detection light and the excitation light form a fixed angle, so that the phase adjuster 12 only allows the detection light to pass through without blocking the excitation light.
Specifically, the angle of the detection light to the excitation light is 7 °.
As shown in fig. 4, when the phase mask electric actuator 9 is controlled so that the phase mask 8 is moved out of the optical path, the system at this time is a system for measuring signals of longitudinal waves and transverse waves.
Specifically, the angle of the parallel flat plate 4 is still ensured to be 'symmetrical' to the angle of the dichroic mirror 7, other elements are not changed, and at the moment, the excitation light emitted by the excitation light emitting module is transmitted through the parallel flat plate 4 and the dichroic mirror 7 and then focused on the surface of a sample arranged on the movable sample stage through the light chopper 10, the first achromat lens 11 and the second achromat lens 13 in sequence; the detection light emitted by the detection light emitting module is reflected by the dichroic mirror 7 and then focused on the front focus of the first achromatic lens 11, and the detection light is focused on the surface of a sample arranged on the movable sample stage through the light chopper 10, the first achromatic lens 11 and the second achromatic lens 13 in sequence. Because the angle of the parallel flat plate 4 is still ensured to be symmetrical to the angle of the dichroic mirror 7, and other elements are not changed, the exciting light and the detection light are still superposed on the sample, and at the moment, the signal receiving module obtains a longitudinal wave signal with a strong signal-to-noise ratio.
Specifically, the angle of the parallel plate 4 is finely adjusted to be "asymmetric" with respect to the angle of the dichroic mirror 7, other elements are not changed, the excitation light and the detection light are separated at the front focal point of the first achromat lens 11 and the focusing position of the sample, as shown in fig. 5, and the separation distance L between the excitation light and the detection light can be calculated by the precisely adjusted angle, at which the transverse wave signal with a strong signal-to-noise ratio is detected.
And (3) obtaining the wave velocity of each modal wave by using the acoustic surface wave, longitudinal wave and transverse wave signals measured by the system, and further obtaining the elastic modulus of the material. The measured related information is more accurate, so that the elastic modulus of the analyzed material is more accurate.
A method for detecting the elastic modulus of a material in a non-contact mode specifically comprises the following steps:
constructing a laser ultrasonic system;
in some embodiments, the specific steps of constructing the laser ultrasound system are:
The height of the center of each element in the direction perpendicular to the fixed base plate is the same, and is 68.5mm.
The reason why the cylindrical mirror is selected instead of the spherical mirror is that the cylindrical mirror collects energy with a larger density than the spherical mirror and the amplitude of the excited sound wave is larger under the condition of the same focal spot width.
And 2, adjusting an electric adjuster of the phase mask plate to enable the phase mask plate to be positioned in the light path, enabling the system to become a system for exciting and measuring the surface acoustic wave signals, and setting the period of the phase mask plate.
Firstly, the phase mask plate electric adjuster is adjusted to enable the phase mask plate to be positioned in the light path, and then the phase mask plate is placed on the minimum grating period. Since the diffraction angle is the largest when the period is the smallest, the optical element size is guaranteed to meet the maximum requirements. For example, if the minimum period is 4 μm, the wavelength of continuous excitation light is 830nm, and the probe light and the reference light are selected to be 0-order and-2-order diffracted lights of 830nm laser light, respectively, the maximum diffraction angle is calculated to be 11.97 °. Whereas the diffraction angle of the probe/reference light changes due to the changing grating period. The period becomes larger, and the two beams of the probe light/the reference light are converged toward the center of the system.
And 3, adjusting the angle between the parallel flat plate and the dichroic mirror.
The material and thickness of the flat plate and the dichroic mirror are the same. The dichroic mirror is fixed at 48.5 degrees in the horizontal direction, the angle between the exciting light and the detection light is 7 degrees, and the spatial separation of the exciting light and the detection light/reference light is ensured.
The dichroic mirror is fixed at 11.97 degrees in the vertical direction, the angle is obtained according to the condition that the period of a phase mask plate is 4 mu m, and the selected probe light and the reference light are 0-order and-2-order diffraction light of 830nm laser respectively.
In order to eliminate the parallel plate effect generated by exciting light passing through the dichroic mirror and further ensure that the exciting light is coincided with a phase mask plate under detection light and the surface of a sample, the dichroic mirror is fixed at-48.5 degrees in the horizontal direction and at 11.97 degrees in the vertical direction.
And 4, shielding unnecessary exciting light by using a light shield, and reserving useful exciting light.
Light of grade 1 of 532nm laser is selected as exciting light. The +/-1-level exciting light passing through the light shielding plate is focused on a sample through the achromatic lens, two beams of light are overlapped in space and time to form interference fringes, slight pulse heating is generated in a space geometrical structure of an optical interference pattern, thermal expansion is generated, and therefore surface acoustic waves which are propagated in opposite directions are emitted, and finally narrow-band surface acoustic standing waves are formed.
And 5, sequentially mounting the 830nm continuous laser and the spherical lens on the bottom plate. Wherein, the 830nm continuous laser light-emitting hole and the spherical lens are positioned on the same horizontal light path. The phase mask passes through the achromatic lens focus.
The height and angle of the light outlet hole of the 830nm laser and the height and position of the spherical lens are adjusted to ensure that the 830nm laser and the 532nm laser are overlapped on the grating.
The phase adjuster is fixed in the optical path of the detection light, and the phase condition of the incident light can be accurately and dynamically adjusted. The selected detection light and the reference light are 0-order and-2-order diffracted lights of 830nm laser respectively, and the phase adjuster is used for adjusting the phase of the 830nm 0-order diffracted light. The 0-order diffraction light and the-2-order diffraction light after phase adjustment reach a phase matching condition, heterodyne phase optimization can be achieved, and finally the ultrasonic micro-vibration condition of nanometer magnitude can be detected. Taking the grating period of 4 μm, the selected detection light is 0-order diffraction light of 830nm laser, the focal length of the achromatic lens is 85mm as an example, and the vertical height of the center of the phase adjuster is adjusted to 48mm.
Due to the size limitation of the phase adjuster, if the excitation light and the detection light are in the same plane, the phase adjuster will block the excitation light. Therefore, the horizontal light path where the 532nm pulse laser light outlet hole, the attenuation sheet, the cylindrical mirror and the phase mask plate are located, the light chopper, the achromatic lens and the phase adjuster are located, and 7 degrees is formed in the horizontal light path, the separation of exciting light and detecting light/reference light on the space is guaranteed at the angle, and therefore the phase adjuster is guaranteed not to shield light.
And 5, installing a three-dimensional electric translation table. The three-position electric translation stage is horizontally arranged at an angle of 45 degrees with the optical path of the achromatic lens.
And 6, sequentially installing a focusing spherical lens and a photoelectric signal receiver in the signal receiving system. The focusing sphere is in the same horizontal optical path as the photoelectric signal receiver and is 90 degrees to the optical path of the achromatic lens.
Step 7, a 532nm pulse light laser, an 830nm continuous laser, a three-dimensional electric translation table, a photoelectric signal detector and an electric regulator for controlling a phase mask plate are started, so that the phase mask plate is positioned in a light path, the system becomes a system for exciting and detecting a surface acoustic wave signal, two excitation light beams are superposed on the surface of a sample to form interference fringes, the interference fringes excite a narrow-band surface acoustic wave due to a thermoelastic effect, and the surface acoustic wave signal detected by a heterodyne system is u r (t)。
The specific method for obtaining the wave velocity of the surface acoustic wave by resolving according to the heterodyne signal of the surface acoustic wave comprises the following steps:
according to the velocity formula of the surface acoustic wave:
c r ＝f·Λ
in the formula, f is the center frequency of the narrow-band surface acoustic wave signal, and Λ is the excited surface acoustic wave wavelength.
The heterodyne signal of the surface acoustic wave measured by the system is as follows:
u r (t)＝U 0 sin(qx)
wherein, U 0 Is the amplitude of the ultrasonic displacement and,
according to the obtained surface acoustic wave heterodyne signal u r (t), the center frequency f of the signal is obtained by fourier transform.
When the distance d between the scribed lines of the phase mask (8) is known, the excited surface acoustic wave wavelength can be obtained:
according to the formula, the wave velocity of the surface acoustic wave can be obtained through calculation.
And 8, adjusting the electric adjuster of the phase mask plate to move the phase mask plate out of the light path, ensuring that the angle of the dichroic mirror is symmetrical to that of the dichroic mirror, and ensuring that other elements are not changed, wherein the system becomes a system for exciting and measuring longitudinal wave signals. The exciting light and the detecting light are overlapped at the same position of the front focus of the first achromat and the sample, and then a longitudinal wave signal u with stronger signal-to-noise ratio is detected l (t), echo time t of longitudinal wave reflected from the bottom surface of the sample with thickness h l 。
The specific method for obtaining the longitudinal wave velocity by resolving according to the longitudinal wave signal comprises the following steps:
according to a longitudinal wave velocity formula:
the longitudinal wave velocity can be obtained by calculation.
And 9, adjusting the phase mask electric adjuster to move the phase mask out of the optical path. The angle of the parallel flat plate is finely adjusted, so that the angle of the parallel flat plate is asymmetric to the angle of the dichroic mirror, other elements are not changed, and the system becomes a system for exciting and measuring transverse wave signals. The exciting light and the detecting light are separated at the front focus of the first achromat and the focusing position of the sample, and the transverse wave signal u with strong signal-to-noise ratio is detected at the moment t (t) echo time t of transverse wave reflected from the bottom surface of the sample with thickness h t And the separation distance L between the exciting light and the detection light can be calculated through the accurately adjusted angle.
The specific method for obtaining the longitudinal wave velocity by resolving according to the transverse wave signal comprises the following steps:
according to the formula of the wave velocity of the shear wave:
and resolving to obtain the transverse wave velocity.
The specific method for obtaining the elastic modulus of the sample by resolving according to the wave velocity of the surface acoustic wave, the wave velocity of the longitudinal wave and the wave velocity of the transverse wave comprises the following steps:
according to the wave velocity c of surface waves r Velocity of longitudinal wave c l The formula:
and the wave velocity c of the surface wave obtained by resolving r With the velocity c of the longitudinal wave l Obtaining the poisson ratio v;
and then according to a transverse wave velocity formula:
and the calculated transverse wave velocity c t The density ρ and the elastic modulus E of the material can be obtained.
The invention is based on the principles of photoacoustic excitation and detection, is non-contact direct excitation and measurement, does not damage the sample and does not damage the sample, and combines the photoacoustic nondestructive detection technology and the heterodyne detection technology to ensure that the detected surface acoustic wave signal is stronger. The electric regulator is added under the phase mask plate, so that the phase mask plate is added into or removed from the light path, and the system can be quickly and conveniently converted into a system for exciting and measuring longitudinal wave and transverse wave signals from a system for exciting and measuring surface acoustic wave signals in the same system. When the wave velocity of the surface acoustic wave is measured, repeated measurement is not needed, and the position of an excitation point is not needed to be moved, so that the measurement accuracy is ensured; when the longitudinal wave velocity is measured, the angle symmetry of the parallel flat plate and the dichroic mirror and a 4f system formed by an achromatic lens are utilized to ensure that an excitation point and a detection point coincide on a sample and the measurement accuracy is ensured; when the wave velocity of the transverse wave is measured, the separation distance can be accurately calculated by utilizing the accurate rotation angle of the parallel flat plate, and the more accurate wave velocity of the transverse wave is obtained. The measured related information is more accurate, so that the elastic modulus of the analyzed material is more accurate.
Claims (10)
1. The laser ultrasonic system for detecting the elastic modulus of a material in a non-contact manner is characterized by comprising an excitation light emitting module, a detection light emitting module, a parallel flat plate (4), a dichroic mirror (7), a phase mask (8), a phase mask electric regulator (9), a light chopper (10), a first achromat lens (11), a phase regulator (12), a second achromat lens (13), a mobile sample stage and a signal receiving module, wherein the parallel flat plate (4), the dichroic mirror (7) and the light chopper (10) are sequentially arranged on an optical axis of the excitation light emitting module, the first achromat lens (11), the phase regulator (12) and the second achromat lens (13) are sequentially arranged behind the light chopper (10), and light emitted by the excitation light emitting module and the detection light emitting module passes through the first achromat lens (11) and the second achromat lens (13), and only light emitted by the detection light emitting module passes through the phase regulator (12);
the object carrying surface of the movable sample stage is the back focus of a second achromatic lens (13);
the optical axis of the signal receiving module is symmetrical to the optical axes of the first achromatic lens (11) and the second achromatic lens (13) with respect to the normal of the moving sample table;
the phase mask plate (8) and the phase mask plate electric regulator (9) are arranged between the dichroic mirror (7) and the light shield (10), and the phase mask plate (8) is added into or removed out of the light path by controlling the phase mask plate electric regulator (9).
2. The laser ultrasonic system for detecting the elastic modulus of a material in a non-contact manner according to claim 1, wherein the excitation light emitting module comprises a pulse laser (1), an attenuation sheet (2) and a cylindrical mirror (3), the pulse laser (1), the attenuation sheet (2) and the cylindrical mirror (3) are located on the same optical axis, and the cylindrical mirror (3) focuses excitation light onto a phase mask (8).
3. The laser ultrasonic system for non-contact detection of the elastic modulus of a material according to claim 1, wherein the detection light emitting module comprises a continuous laser (5) and a spherical lens (6), and the continuous laser (5) and the spherical lens (6) are located on the same optical axis.
4. The laser ultrasonic system for non-contact detection of the elastic modulus of a material according to claim 1, wherein the signal receiving module comprises a focusing sphere lens (15) and an optoelectronic signal receiver (16):
when the phase mask electric regulator (9) is controlled to enable the phase mask (8) to be in the light path, the focusing spherical lens (15) is used for coupling the detection light carrying the sample information excited by the excitation light and the reference light into the photoelectric signal receiver (16) to obtain a heterodyne signal of a sample surface acoustic wave; when the phase mask plate (8) is not in the light path, the focusing spherical lens (15) is used for coupling single detection light carrying sample information excited by the exciting light into the photoelectric signal receiver (16) to obtain longitudinal wave and transverse wave signals.
5. The laser ultrasonic system for non-contact detection of the elastic modulus of a material according to claim 1, wherein the detection light and the excitation light form a fixed angle, and the phase adjuster (12) is ensured not to block the excitation light.
6. A method for detecting the elastic modulus of a material in a non-contact manner is characterized by comprising the following steps:
constructing the laser ultrasonic system as claimed in any one of claims 1 to 5, and enabling a phase mask plate (8) to be in a light path to obtain heterodyne signals of excited sample surface acoustic waves;
resolving to obtain the wave velocity of the surface acoustic wave according to the heterodyne signal of the surface acoustic wave;
enabling the phase mask plate (8) not to be in the light path, and acquiring excited longitudinal wave and transverse wave signals of the sample;
resolving to obtain the wave velocity of longitudinal waves and transverse waves according to the signals of the longitudinal waves and the transverse waves;
and obtaining the elastic modulus of the material according to the wave speeds of the acoustic surface wave, the longitudinal wave and the transverse wave signals.
7. The method for detecting the elastic modulus of the material in a non-contact manner according to claim 6, wherein the specific method for obtaining the wave velocity of the surface acoustic wave by resolving according to the surface acoustic wave heterodyne signal comprises the following steps:
for surface acoustic wave heterodyne signal u r (t) Fourier transform is carried out to obtain the central frequency f of the narrow-band surface acoustic wave signal, wherein the surface acoustic wave heterodyne signal u r The (t) is specifically:
u r (t)＝U 0 sin(qx)
wherein, U 0 Is the amplitude of the ultrasonic displacement and,
d is the distance between reticle lines of the phase mask plate (8);
determining the speed of the surface acoustic wave according to the center frequency of the signal and the wavelength of the excited surface acoustic wave as follows:
c r ＝f·Λ
in the formula, f is the center frequency of the narrow-band surface acoustic wave signal, and Λ is the excited surface acoustic wave wavelength.
8. The method for non-contact measurement of the elastic modulus of a material according to claim 6, wherein the phase mask electric actuator (9) is controlled so that the phase mask (8) is not in the optical path, the angle of the parallel plate (4) is adjusted to be symmetrical to the angle of the dichroic mirror (7), the excitation light overlaps the detection light at the front focal point of the first achromat and at the same position of the sample, and the longitudinal signal u is detected l (t), echo time t of longitudinal wave reflected from the bottom surface of the sample with thickness h l ；
The specific formula for obtaining the longitudinal wave velocity by resolving according to the signals of the longitudinal waves is as follows:
9. the method for non-contact measurement of the elastic modulus of a material according to claim 6, wherein when the phase mask electric actuator (9) is controlled so that the phase mask (8) is not in the optical path and the angle of the parallel plate (4) is adjusted so as to be asymmetric to the angle of the dichroic mirror (7), the excitation light and the detection light are separated at the front focal point of the first achromatic lens and the focused position of the sample, and the transverse wave signal u is detected t (t) echo time t of transverse wave reflected from the bottom surface of the sample with thickness h t The separation distance between the exciting light and the detecting light is L;
the specific formula for obtaining the longitudinal wave velocity by resolving according to the transverse wave signal is as follows:
10. the method for detecting the elastic modulus of the material in a non-contact manner according to claim 6, wherein the specific method for obtaining the elastic modulus of the sample by calculation according to the wave velocity of the surface acoustic wave, the wave velocity of the longitudinal wave and the wave velocity of the transverse wave comprises the following steps:
according to the wave velocity c of the surface wave r Velocity of longitudinal wave c l The formula is as follows:
and the wave velocity c of the surface wave obtained by resolving r With the velocity c of the longitudinal wave l Obtaining Poisson ratio v;
according to the formula of the wave velocity of the shear wave:
and the calculated transverse wave velocity c t And obtaining the density rho and the elastic modulus E of the material.
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