CN111257236A - Double-pulse laser ultrasonic detection device and detection method thereof - Google Patents
Double-pulse laser ultrasonic detection device and detection method thereof Download PDFInfo
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Abstract
The invention discloses a double-pulse laser ultrasonic detection device and a detection method thereof, relating to the technical field of ultrasonic detection, wherein the double-pulse laser ultrasonic detection device comprises a pulse laser emitting device and a continuous laser emitting device which are respectively arranged at the front side and the rear side of a sample, a first beam splitter and a reflector which are used for dividing a pulse laser beam into two beams vertical to the front surface of the sample are arranged right in front of the emission of the pulse laser emitting device, and a lens is arranged between the first beam splitter and the surface of the sample; and one side of the rear surface of the sample is also provided with a photorefractive dual-wave hybrid interferometer and a photoelectric detector, a second beam splitter is arranged in front of the continuous laser emission device and divides a continuous laser beam into two beams, one beam is obliquely emitted to the rear surface of the sample and then reflected to enter the photorefractive dual-wave hybrid interferometer, and the other beam is directly emitted to the photorefractive dual-wave hybrid interferometer. The invention has the advantages of reducing damage and improving measurement precision.
Description
Technical Field
The invention relates to the technical field of ultrasonic detection, in particular to a double-pulse laser ultrasonic detection device and a detection method thereof.
Background
Ultrasonic testing has a dominant role in nondestructive testing, and ultrasonic nondestructive testing (NDT) is a common technique for monitoring the health of structures. The ultrasonic nondestructive technology is used for detecting the thickness and the internal quality of a part to be detected on the premise of not damaging a workpiece or a raw material. The method is widely applied to the fields of wafer detection, airplane composite material part detection, seamless steel pipe wall thickness measurement and the like. The ultrasonic signal is initially generated by a pulse, penetrates deep into the metallic material (specimen) by means of ultrasonic energy, and enters from one section into another, and when the echo is reflected from the rear surface of the specimen to the ultrasonic probe, the thickness of the specimen can be calculated. When the thickness is measured, even if slight change occurs, the measurement accuracy is high. The same inspection scheme can also be used to identify the presence of defects or cracks within the sample if the ultrasonic propagation velocity is calibrated and calibrated.
The photoacoustic (pa) effect refers to the target tissue absorbing a short pulse of laser light and emitting ultrasound. When the target object is irradiated by laser, electrons in the target object can absorb the energy of the laser, so that the temperature of the target object rises, and the energy of the laser is transferred into crystal lattices, so that the target tissue structure is slightly changed, namely, the phenomenon of expansion caused by heat and contraction caused by cold of surface atoms occurs. Depending on the peak power density of the laser pulse, the ultrasound can generate three states: thermoelastic, ablative, and constrained states. In the thermoelastic state, the peak power density is lower than 106W/cm2Ultrasonic waves are generated; in the ablation state, the peak power density is higher than 106W/cm2And the surface of the sample is melted and evaporated, and ablation materials and plasma are generated. However, during ablation, the recoil effect of ablated material expulsion and the momentum created by the downward plasma pressure cause a higher ultrasonic signal. Ultrasonic waves can be generated in all three states, but are different for surface damage. To reduce surface damage, the signal of a Photoacoustic (PA) system is enhanced by using a laser in the thermoelastic state or a laser with lower peak power in the ablative state.
Non-contact detection methods, in particular the above-mentioned photoacoustic detection methods, have considerable advantages over other methods (contact methods, liquid immersion methods); 1. the contact method is mostly manual detection, the operation is convenient, the equipment is simple, the cost is low, but the detection result is greatly influenced by human factors. But the contact method can directly couple, has small incident sound energy loss and can provide larger thickness penetration capacity. In addition, the contact method requires a coupling medium, coupling is not stable, and the surface to be inspected is required to be flat and have small roughness. The piezoelectric type electric sensor made of the lead zirconate titanate composite material is a typical contact type nondestructive detection sensor; 2. the liquid immersion method has the advantages that the probe is not contacted with the workpiece to be detected, the service life of the probe is long, the transmission and the receiving of ultrasonic waves are stable, and the influence of surface roughness is small; the automatic detection is convenient to realize, and the human factors influencing the detection reliability are reduced. The liquid immersion method has the disadvantages that the reflection of ultrasonic waves on the surfaces of liquid and metal loses a large amount of energy and needs to adopt higher gain. When detecting high attenuation materials or large thickness materials, there may not be enough energy. At higher gains, noise interference may also occur. 3. The advantages of the photoacoustic method are similar to those of the liquid immersion method, the probe is not in contact with the workpiece to be detected, the transmission and the reception of ultrasonic waves are stable, the influence of surface roughness is small, automatic detection is convenient to realize, human factors influencing the detection reliability are reduced, but the energy utilization rate is high, but if the peak power density of laser pulse is not well controlled, the surface to be detected is ablated, and the surface structure is changed.
Disclosure of Invention
In order to overcome the defects of the background technology, the invention provides a double-pulse laser ultrasonic detection device and a detection method thereof, wherein the double-pulse laser ultrasonic detection device can reduce damage and improve measurement precision.
The technical scheme adopted by the invention is as follows: the double-pulse laser ultrasonic detection device comprises a pulse laser emitting device and a continuous laser emitting device which are respectively arranged on the front side and the rear side of a sample, wherein a first beam splitter and a reflector which are used for splitting a pulse laser beam into two beams perpendicular to the front surface of the sample are arranged right in front of the pulse laser emitting device, and a lens is arranged between the first beam splitter and the surface of the sample;
a photorefractive double-wave hybrid interferometer and a photoelectric detector are further arranged on one side of the rear surface of the sample, a second beam splitter is arranged in front of the continuous laser emitting device and divides a continuous laser beam into two beams, one beam is obliquely emitted to the rear surface of the sample and then reflected to enter the photorefractive double-wave hybrid interferometer, the other beam is directly emitted to the photorefractive double-wave hybrid interferometer, and the photoelectric detector receives photoelectric signals from the photorefractive double-wave hybrid interferometer.
And a filter plate is arranged between the pulse laser emitting device and the first polarization beam splitter.
The pulse laser emitting device generates a beam of Nd YAG pulse laser with the wavelength of 1064nm and the energy of 40mJ, the laser pulse duration is 8ns, and the repetition frequency is 20 Hz.
The continuous laser emitting device emits a beam of continuous laser with the wavelength of 532 nm.
The lens concentrates the focus of the two converged pulse lasers on the front surface of the sample, and the reflection point of the continuous laser and the focus of the pulse laser are positioned on the same straight line.
The invention adopts another technical scheme that: the double-pulse laser ultrasonic detection method comprises the following steps:
A. acquiring a photoacoustic signal through a common photoacoustic system;
B. adding a first beam splitter and a lens on the front surface of the sample, and controlling the front and back positions of the lens through a control module to acquire photoacoustic signals at different positions;
C. by contrast, the optimum lens position is obtained.
The step A comprises the following steps:
a1, emitting a pulse laser beam parallel to the front surface of the sample by a pulse laser emitting device, wherein the pulse laser beam is vertically emitted to the front surface of the sample through a filter and a reflector;
a2, emitting a connection laser beam inclined to the back surface of the sample by a continuous laser emitting device, dividing the continuous laser beam into two continuous laser beams by a second beam splitter, wherein one continuous laser beam is emitted to the back surface of the sample and then reflected into a photorefractive dual-wave hybrid interferometer, the other continuous laser beam is directly emitted to the photorefractive dual-wave hybrid interferometer, and the photorefractive dual-wave hybrid interferometer interferes the incident photoacoustic signal and then emits the signal to a photoelectric detector for phase demodulation.
The step B comprises the following steps:
b1, converging the focus of the parallel double-pulse laser beam on the surface of the sample after passing through the lens to obtain a photoacoustic signal;
b2, adjusting the position of the lens through the control module, moving the lens 0.5cm towards the sample direction, enabling the focus of the double-pulse laser beam to fall in the middle of the material, and acquiring a photoacoustic signal;
b3, adjusting the position of the lens through the control module, moving the lens to the sample direction by 0.5cm, enabling the focus of the double-pulse laser beam to fall on the back surface of the material, and acquiring the photoacoustic signal.
The invention has the beneficial effects that: compared with single pulse irradiation, the double pulse laser irradiation with low energy density can enhance photoacoustic signals on the surface of a sample, and the signal enhancement by using a double pulse method is beneficial to structural health detection and improves the detection sensitivity.
Drawings
Fig. 1 is a schematic structural diagram of a double-pulse laser ultrasonic detection apparatus according to an embodiment of the present invention.
Fig. 2 is a flow chart of the double-pulse laser ultrasonic detection device.
Fig. 3 is a schematic structural diagram of a common laser ultrasonic detection device.
Fig. 4 is a comparison graph of the superposition of a double-pulse photoacoustic signal and a single-pulse photoacoustic signal.
Fig. 5 is a schematic diagram of two forms of ultrasonic waves generated on the surface of a sample by a pulsed laser.
Fig. 6 is a graph of the ultrasonic coupling signal generated inside the sample and irradiated by the double pulse laser.
Fig. 7 is a display diagram of the ultrasonic flaw detector.
Detailed Description
The embodiments of the invention will be further described with reference to the accompanying drawings in which:
as shown in the figure, the double-pulse laser ultrasonic detection device comprises a pulse laser emitting device 1 and a continuous laser emitting device 2 which are respectively arranged on the front side and the rear side of a sample, a first beam splitter 3 and a reflector 4 which are used for dividing a pulse laser beam into two beams perpendicular to the front surface of the sample are arranged right in front of the pulse laser emitting device 1, and a lens 5 is arranged between the first beam splitter 3 and the surface of the sample;
a photorefractive dual-wave hybrid interferometer 6 and a photoelectric detector 7 are further arranged on one side of the rear surface of the sample, a second beam splitter 8 is arranged in front of the continuous laser emitting device 2, the second beam splitter 8 divides a continuous laser beam into two beams, one beam is obliquely emitted to the rear surface of the sample and then reflected to enter the photorefractive dual-wave hybrid interferometer 6, the other beam is directly emitted to the photorefractive dual-wave hybrid interferometer 6, the photoelectric detector 7 receives a photoelectric signal from the photorefractive dual-wave hybrid interferometer 6, ultrasonic waves generated by laser can be used for measuring the thickness of the material or detecting defects inside the material, compared with single-pulse irradiation, the low-energy-density dual-pulse laser irradiation can enhance the photoacoustic signal on the surface of the sample, the dual-pulse method is used for enhancing the signal, the structural health detection is facilitated, and the detection sensitivity is.
A filter 9 is arranged between the pulse laser emitting device 1 and the first polarization beam splitter and used for adjusting the intensity of the pulse laser, so that the intensity of the two beams of pulse laser is equal, and the propagation of the two beams of light is not delayed.
The pulse laser emitting device 1 generates a beam of Nd YAG pulse laser with the wavelength of 1064nm and the energy of 40mJ, the laser pulse duration is 8ns, and the repetition frequency is 20 Hz.
The continuous laser emitting device 2 emits a continuous laser beam with a wavelength of 532 nm.
The lens 5 concentrates the focus of the two converged pulse lasers on the front surface of the sample, and the reflection point of the continuous laser and the focus of the pulse laser are positioned on the same straight line.
The general nondestructive detection has low precision and is greatly influenced by human factors, and some methods also lose a large amount of energy during detection and need high cost. The photoacoustic effect has many advantages in nondestructive testing applications, and has small influence on human factors and high energy utilization rate and measurement accuracy. It is known that ultrasound can be generated by the action of laser light on a material, and that non-destructive testing can be performed by receiving ultrasound. However, signals are not all received, so in photoacoustic nondestructive testing, high-energy pulse laser is required to generate strong ultrasonic signals to achieve higher detection accuracy. However, the single-pulse laser with excessively high peak power density can ablate the surface of the material, and in order to solve the problem, the double-pulse photoacoustic detection method is obtained through a design scheme and continuous experiments. The method can weaken the influence of the single-pulse laser with high peak power density on the material, and besides, under the condition of using the same energy, the double-pulse photoacoustic detection method can obtain stronger photoacoustic signals than the common photoacoustic detection method.
To obtain a high amplitude photoacoustic signal, we select appropriate pulsed laser parameters to optimize the absorption of the optical energy. By increasing the incident intensity of the laser beam, a higher amplitude signal can be obtained, where appropriate in the ablation zone. Whether two laser pulse spots overlap is selected by irradiating different areas in the vicinity of the target. When the laser spots overlap, interference between the two lasers causes an increase in signal intensity. Without overlap of laser spots, the generated ultrasound is focused inside the sample, enhancing the Photoacoustic (PA) signal. Therefore, the method of double pulse is used to enhance the photoacoustic signal.
In the embodiment, a YAG laser with the model number of Quanta Q2-1064 is used to generate a Nd YAG pulse laser with the wavelength of 1064nm and the energy of 40mJ, the Nd YAG pulse laser is incident into a polarization beam splitter through a filter, the filter is used to adjust the intensity of light, so that the intensities of the two beams of light are equal, then the laser is split into two beams in a cross beam mode, the two beams of light are parallel and are incident into a lens through a reflector and the polarization beam splitter, the two beams of light are converged onto the surface of a sample material to be detected through the convergence characteristic of the lens, the propagation of the two beams of light is not delayed, the duration of the laser pulse is 8ns, the repetition frequency is 20Hz, the laser is propagated onto the sample, then an ultrasonic signal is generated according to the thermal effect, and the signal can be emitted from the rear surface by using the performance of the ultrasonic energy input into.
A laser generator is placed at the position of a rear surface, a laser beam with the wavelength of 532nm is emitted, the laser beam is divided into two beams through a beam splitter, one beam of laser beam emits to the rear surface of a sample, ultrasonic signals penetrating through the front surface are guided to reflect and enter a sensitive photorefractive dual-wave hybrid interferometer, the other beam of laser (reference light signals) directly enters the dual-wave hybrid interferometer, the sensitive photorefractive dual-wave hybrid interferometer is used for interfering the entering photoacoustic signals, and then phase demodulation is carried out by a high-response Avalanche Photodetector (APD), the photoelectric detector can quickly and sensitively acquire the internal defect size or thickness value of a material, and the minimum plate thickness can be detected to be 0.26mm at present.
The general photoacoustic system can detect the structural health in real time, such as detecting the thickness of an unknown sample or detecting the tiny defects of the sample under the condition of known thickness. In this experiment, we hope to realize the double-pulse technology by adding some optical elements, and because the general photoacoustic system has the defects of weaker photoacoustic signal, partial limitation and no material defect beyond thermal diffusion is detected, we need to effectively obtain the maximum photoacoustic signal intensity by some means; the direction of the other beam of light after the light which is emitted to the reflector and the polarization beam splitting is controlled by a rotary actuator in the polarization beam splitter, and the position and the placing angle of the reflector and the positions of the polarization beam splitter and the lens are calculated and adjusted by using a control module. As shown in fig. 4, adjusting the lens position results in different concentrations of the two beams at the sample surface.
The double-pulse laser ultrasonic detection method comprises the following steps:
A. acquiring a photoacoustic signal through a common photoacoustic system;
B. a first beam splitter 3 and a lens 5 are added on the front surface of the sample, the front and back positions of the lens 5 are controlled through a control module, and photoacoustic signals at different positions are obtained;
C. by contrast, the optimum lens position is obtained.
The step A comprises the following steps:
a1, emitting a pulse laser beam parallel to the front surface of the sample by a pulse laser emitting device 1, wherein the pulse laser beam vertically emits to the front surface of the sample through a filter 9 and a reflector 4;
a2, emitting a connection laser beam inclined to the back surface of the sample by the continuous laser emitting device 2, dividing the continuous laser beam into two continuous laser beams by the second beam splitter 8, wherein one beam is emitted to the back surface of the sample and then reflected into the photorefractive dual-wave hybrid interferometer 6, the other beam is directly emitted to the photorefractive dual-wave hybrid interferometer 6, the photorefractive dual-wave hybrid interferometer 6 interferes the incident photoacoustic signal, and then the reflected photoacoustic signal is emitted to the photodetector 7 for phase demodulation.
The step B comprises the following steps:
b1, converging the focus of the parallel double-pulse laser beam on the surface of the sample after passing through the lens 5 to obtain a photoacoustic signal;
b2, adjusting the position of the lens 5 through the control module, moving the lens 5 to the sample direction by 0.5cm, enabling the focus of the double-pulse laser beam to fall in the middle of the material, and acquiring a photoacoustic signal;
b3, adjusting the position of the lens 5 through the control module, moving the lens 5 to the sample direction by 0.5cm, enabling the focus of the double-pulse laser beam to fall on the back surface of the material, and acquiring the photoacoustic signal.
If we want to increase the signal intensity by increasing the peak power parameter of the laser, the damage to the surface by the single pulse laser is very large, and if we use the double pulse method, the damage can be effectively reduced. Through comparison between the first step and the second step, the two laser beams are better in focusing coupling than a single pulse laser beam, because the ultrasonic waves generated by the two laser beams can effectively enhance the strength of the photoacoustic signal.
When the focus of the double-pulse laser beam is located on the surface of the material in the second step, the energy of the two beams is gathered at one point on the surface to generate a large thermal effect. We adjust the focal position by adjusting the focal length so that the focal point does not fall on the surface, as shown in fig. 5. By separating the two beams of laser on the surface of the material, the laser pulse energy density on the surface can be reduced, and the damage to the surface in the laser irradiation process can be reduced.
We can see step three versus step four from fig. 6. The position and angle of the component are adjusted by the control module, the focal points of the two beams of light are respectively on the surface (as shown in fig. 6a (i)), the inside (as shown in fig. 6b (i)) and the rear surface (as shown in fig. 6c (i)) of the material, and the acquisition signals corresponding to different focal positions are shown in the right diagram. Because the directivity of ultrasonic propagation is consistent with the incident direction of laser, the maximum constructive interference point of detection depends on the incident angle between the two beams of light; therefore, the alignment between the light beams (adjustment of the focal point position) can effectively enhance the photoacoustic signal; according to fig. 6b (ii) and 6c (ii), comparing the waveform shows that the photoacoustic signal increases as the imaginary focal point moves backward.
In the case of nondestructive inspection, the defect wave part is a key part of the inspection information, fig. 6a (ii) is a signal diagram obtained when the focal point is on the front surface (as a basis for comparison), it is obvious that the defect wave part of fig. 6B (ii) is much stronger than the defect wave part of fig. a (ii), and fig. 6c (ii) is much weaker than fig. 6a (ii) (see the peak and valley and the rugged form), so that the coupling signal obtained by the step B2 is stronger than those of B1 and B3 by comparing three sets of data.
To summarize: the laser generated ultrasonic waves can be used to measure material thickness or to detect defects within the material. Compared with single-pulse irradiation, the low-energy-density double-pulse laser irradiation can enhance the photoacoustic signal on the surface of the sample; the signal is enhanced by using a double-pulse method, so that the structural health detection is facilitated, and the detection sensitivity is improved.
In the description of the present invention, it should be noted that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate orientations or positional relationships based on those shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art. In addition, in the description of the present invention, "a plurality" means two or more unless otherwise specified.
The skilled person should understand that: although the invention has been described in terms of the above specific embodiments, the inventive concept is not limited thereto and any modification applying the inventive concept is intended to be included within the scope of the patent claims.
Claims (8)
1. The double-pulse laser ultrasonic detection device is characterized by comprising a pulse laser emitting device (1) and a continuous laser emitting device (2) which are respectively arranged on the front side and the rear side of a sample, wherein a first beam splitter (3) and a reflector (4) which are used for dividing a pulse laser beam into two beams perpendicular to the front surface of the sample are arranged right in front of the pulse laser emitting device (1) in a transmitting manner, and a lens (5) is arranged between the first beam splitter (3) and the surface of the sample;
the device is characterized in that a photorefractive double-wave hybrid interferometer (6) and a photoelectric detector (7) are further arranged on one side of the rear surface of the sample, a second beam splitter (8) is arranged in front of the continuous laser emitting device (2), a continuous laser beam is divided into two beams by the second beam splitter (8), one beam is obliquely emitted to the rear surface of the sample and then reflected to enter the photorefractive double-wave hybrid interferometer (6), the other beam is directly emitted to the photorefractive double-wave hybrid interferometer (6), and the photoelectric detector (7) receives photoelectric signals from the photorefractive double-wave hybrid interferometer (6).
2. The double-pulse laser ultrasonic inspection apparatus according to claim 1, wherein: and a filter (9) is arranged between the pulse laser emitting device (1) and the first polarization beam splitter.
3. The double-pulse laser ultrasonic inspection apparatus according to claim 1, wherein: the pulse laser emitting device (1) generates a beam of Nd YAG pulse laser with the wavelength of 1064nm and the energy of 40mJ, the laser pulse duration is 8ns, and the repetition frequency is 20 Hz.
4. The double-pulse laser ultrasonic inspection apparatus according to claim 3, wherein: the continuous laser emitting device (2) emits a continuous laser beam with the wavelength of 532 nm.
5. The double-pulse laser ultrasonic inspection apparatus according to claim 1, wherein: the reflection point of the continuous laser and the focus of the double-pulse laser are positioned on the same straight line.
6. The double-pulse laser ultrasonic inspection method is characterized in that the double-pulse laser ultrasonic inspection apparatus according to claim 1 includes the steps of:
A. acquiring a photoacoustic signal through a common photoacoustic system;
B. a first beam splitter (3) and a lens (5) are added on the front surface of the sample, the front and back positions of the lens (5) are controlled through a control module, and photoacoustic signals at different positions are acquired;
C. by contrast, the optimum lens position is obtained.
7. The double-pulse laser ultrasonic testing method according to claim 6, wherein the step A comprises:
a1, emitting a pulse laser beam parallel to the front surface of the sample by a pulse laser emitting device (1), wherein the pulse laser beam vertically irradiates to the front surface of the sample through a filter plate (9) and a reflector (4);
a2, emitting a connection laser beam inclined to the rear surface of the sample through a continuous laser emitting device (2), dividing the continuous laser beam into two continuous laser beams through a second beam splitter (8), wherein one continuous laser beam is emitted to the rear surface of the sample and then reflected to enter a photorefractive dual-wave hybrid interferometer (6), the other continuous laser beam is directly emitted to the photorefractive dual-wave hybrid interferometer (6), the photorefractive dual-wave hybrid interferometer (6) interferes the entering photoacoustic signal, and then the phase demodulation is carried out on the photoacoustic signal emitted to a photoelectric detector (7).
8. The double-pulse laser ultrasonic testing method according to claim 6, wherein the step B comprises:
b1, converging the focus of the parallel double-pulse laser beam on the surface of the sample after passing through the lens (5) to obtain a photoacoustic signal;
b2, adjusting the position of the lens (5) through the control module, moving the lens (5) to the sample direction by 0.5cm, enabling the focus of the double-pulse laser beam to fall in the middle of the material, and acquiring a photoacoustic signal;
b3, adjusting the position of the lens (5) through the control module, moving the lens (5) to the sample direction by 0.5cm, enabling the focus of the double-pulse laser beam to fall on the back surface of the material, and acquiring the photoacoustic signal.
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