CN113504182A - Object surface crack online detection method based on laser surface acoustic wave - Google Patents
Object surface crack online detection method based on laser surface acoustic wave Download PDFInfo
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Abstract
The invention relates to an object surface crack online detection method based on laser surface acoustic waves, which comprises the following steps: soaking an object to be detected in a water tank to enable the surface of the object to be detected to be provided with a water restraint layer; the upper computer controls the laser pulse generator to emit a high-energy laser pulse beam; the high-energy laser pulse beam is incident to the surface of an object to be measured through a light path adjusting system; measuring high-frequency laser ultrasonic waves caused by the irradiation of laser on the surface of an object by using a laser interferometer; the ultrasonic signals obtained by measurement are stored in a data analysis system through a data acquisition system; the object surface crack detection can be realized through a data analysis system. The detection method has the advantages that comparison is not needed for intact samples, the laser energy for detection can be higher than the damage threshold of the object to be detected, the detection precision is high, and the like.
Description
Technical Field
The invention relates to the technical field of surface crack detection, in particular to an object surface crack online detection method based on laser surface acoustic waves.
Background
Various metal and non-metal material components are important parts in national defense industrial development and national economic construction, are widely applied to a plurality of important fields such as weaponry, aerospace, transportation, construction and infrastructure, energy chemical industry and mechanical manufacturing, and play a great role in maintaining the national defense industrial development, promoting the national economic construction and improving the quality of life of people. Modern equipment mostly runs under the conditions of high temperature, high pressure, high speed or high load, such as aircraft skin, train rails, oil and gas storage tanks and the like, which puts higher requirements on the reliability of various components. However, in the process of processing and service, various cracks are inevitably generated on the surface of the component under the influence of various complex alternating and cyclic stresses. If the cracks cannot be detected in time and necessary measures are taken for maintenance, the cracks can gradually expand, so that the stability and reliability of a single component and even the whole equipment system are influenced, even disastrous sudden results can be caused, and huge economic loss is caused. Due to the safety and economic requirements, the method has extremely important significance for detecting the cracks on the surface of the component.
The existing surface crack detection methods at home and abroad are mainly divided into two types: contact detection and non-contact detection. The contact detection method is gradually replaced by non-contact detection due to the defects of low detection efficiency, possible damage to a detection object, environmental unfriendliness and the like. Among the non-contact detection methods, infrared detection, camera imaging detection, and ultrasonic detection are currently used. The infrared detection performs crack identification by scanning and recording temperature changes on the surface of a detected object caused by defects or different thermal properties of materials, but the method is greatly influenced by the ambient temperature and the surface temperature of the detected object, and is very easy to generate measurement errors. The camera imaging detection has higher requirements on the testing environment, such as light intensity and the like, and the detection of fine cracks in a larger detection area is difficult to realize due to imaging pixels, so the application of the camera imaging detection is limited in some occasions. The ultrasonic detection has the advantages of strong penetration capacity, accurate defect positioning, high defect detection sensitivity, low detection cost, no harm to human bodies and environment and the like, and is widely used. The laser ultrasonic nondestructive testing technology is an important branch of the ultrasonic nondestructive testing technology, and because of the advantages of wide ultrasonic frequency spectrum (up to 100MHz and above), high spatial resolution, capability of realizing non-contact excitation and receiving and the like, the laser ultrasonic nondestructive testing technology gradually replaces the traditional ultrasonic wave to detect and analyze the surface cracks in practical use. For example, the serial No. CN111855801A, a method for accurately measuring the size of a defect of a rough part based on laser ultrasonic imaging, which is a typical application case of measuring the surface defect of a component by using a laser surface acoustic wave.
The technical proposal represented by the above patent can utilize laser surface acoustic wave to carry out imaging detection on the surface cracks of the component, but still has the following defects: 1) when extracting the crack signal, it is necessary to identify the crack by comparing the "surface wave signal excited by the defect-free sample as a reference signal". In the detection process, the non-crack initiation deviation may exist between the surface acoustic wave signal measurement result of the crack-free (defect-free) contrast sample and the actual signal measurement result of the object to be detected under the influence of manual operation errors and external environment changes, so that the crack false detection is caused. Meanwhile, in the actual detection process, it is generally difficult to obtain a crack-free sample for comparison which is completely consistent with the object to be detected in the aspects of material, shape and the like; 2) in order to realize true nondestructive detection, the energy of a laser pulse beam for exciting the surface acoustic wave cannot be too large. For example, if the object to be measured is made of metal, the energy of the laser pulse beam used generally cannot be greater than 1 × 107W/cm2[ Shenzhong, Yuan Ling, Zhang hong super, etc. [ M ] laser ultrasound in solid]First edition. Beijing, people post and post press, 2015.]. The limitation of the energy threshold directly influences the energy of the surface acoustic wave excited by the laser, so that the precision and the sensitivity of crack detection are directly influenced, and the limitation is also one of important reasons that the crack detection method based on the laser surface acoustic wave is not popularized and used so far; 3) generally, a crack detection method based on laser surface acoustic wave uses the linear characteristic of the laser surface acoustic wave, and the main principle is that the crack detection is carried out based on the phenomena of reflection, diffraction, attenuation or mode conversion and the like after the surface acoustic wave and the crack act, and the crack detection method is insensitive to surface microcracks [ Mezil S, Chigarev N, Tournat V.evaluation of crack parameters by a nonlinear frequency detection method [ J].Ultrasonics,2016,69:225-235.]The surface microcracks cannot be accurately detected.
Therefore, there is a need for an improvement and optimization of existing methods for detecting cracks on the surface of an object based on laser surface acoustic waves.
Disclosure of Invention
The invention aims to overcome the defects in the prior art, and provides the object surface crack on-line detection method based on the laser surface acoustic wave, the detection method can accurately detect the micro cracks on the object surface, no crack sample is needed to be compared in the detection process, the object surface to be detected is not damaged, the energy of the laser pulse beam for detection can be higher than the damage energy threshold of the object to be detected, and the crack detection efficiency, the precision and the sensitivity can be further improved.
The technical scheme adopted by the invention for solving the problems is as follows: an object surface crack online detection method based on laser surface acoustic waves comprises the following steps:
the method comprises the following steps: soaking an object to be detected in a water tank to enable the surface of the object to be detected to be provided with a water restraint layer; the water restraint layer can limit the gasification phenomenon of the surface material of the object to be detected under the irradiation of the laser pulse beam, and improve the recoil force of the splashed substance on the surface of the object to be detected after the action of the pulse beam, so that the energy value of the laser surface acoustic wave excited by the laser pulse beam on the surface of the object to be detected is improved, and the crack detection efficiency, the accuracy and the sensitivity are improved;
step two: the upper computer controls the laser pulse generator to emit a high-energy laser pulse beam, and the high-energy laser pulse beam is divided into two beams after the action of the light path adjusting system: one beam is transmitted to an enabling port of the high-speed data acquisition card to enable the high-speed data acquisition card to enter a working state, and the other beam is transmitted to the surface of an object to be detected as a laser pulse beam for detection to excite a laser surface acoustic wave; the scanning galvanometer in the light path adjusting system can control the laser pulse beam for detection to scan and detect the surface of the object to be detected;
step three: placing a laser interferometer on the same side of the measured surface of the object to be measured, and receiving laser surface acoustic waves excited on the surface of the object to be measured by a laser pulse beam for detection; the received laser surface acoustic wave signals are stored in a data analysis system through a data acquisition system; when laser surface acoustic waves excited by a laser pulse beam for detection are transmitted on the surface of an object to be detected, the laser surface acoustic waves are influenced by cracks on the surface of the object, the nonlinear modulation phenomenon can occur on the laser surface acoustic waves, the time-frequency characteristics of the laser surface acoustic waves are obviously changed, and the crack information can be inverted by analyzing the change amount of the time-frequency characteristics through a data analysis system, so that the real-time imaging detection can be performed on the cracks on the surface of the object to be detected;
in the process of inverting the crack information, analyzing the laser surface acoustic wave time-frequency characteristic parameter variation caused by the crack by utilizing a regularized nonlinear prediction error value NE;
the extraction method of the regularization nonlinear prediction error value NE comprises the following steps:
the first step is as follows: controlling the detection laser pulse beam to perform matrix scanning on the surface of the object to be detected through an upper computer and a light path adjusting system;
the second step is that: measuring response signals of laser surface acoustic waves of each point on the surface of the object to be measured on the matrix route by using a laser interferometer;
the third step: randomly selecting a response point C in the matrix route, and reconstructing a phase space according to the collected response signals, wherein the phase space is called as a phase space I;
the fourth step: randomly selecting Q base points y (i) (i is 1,2, …, Q) from a point set motion track of a phase space I, wherein N is the number of corresponding data points measured by a high-speed data acquisition card;
the fifth step: selecting a nearest point B of the response point C in the third step, reconstructing a phase space II by using a response signal acquired by the point B, and searching P space near points x (j) (1, 2, …, P) for each base point y (i) on a point set track of the phase space II, wherein the space near points x (j) are directly selected according to Euclidean distance; p is N/1000, wherein N is the number of corresponding data points measured by the high-speed data acquisition card;
and a sixth step: advancing each base point and the proximate point by L steps of distance along the respective trajectory, at which time each proximate point centroid of point y (i) can be represented as:
the calculated prediction error value PE may be expressed as:
the normalized nonlinear prediction error value NE corresponding to the point B and point C signalsBCIt can be expressed as:
wherein sigmai 2Is the variance value of the measurement signal;
the seventh step: finally determining a regularization nonlinear prediction error value NE according to the selected position of the response point C:
in the scanning point matrix, n adjacent points (including point B) are arranged around any one response point C, in this case, regularized nonlinear prediction error values NE between the signal at the response point C and the n peripheral point signals (including point B) are respectively calculatedk(k ═ i, 2, …, n), so the expression for the regularized nonlinear prediction error value NE is:
judging the actual value of n according to the position of the actually selected response point C, wherein when the point C is in the middle position of the scanning point matrix, n is 8; when the point C is on the edge of the scanning point matrix, n is 2 or n is 3.
Preferably, in the third step, the process of imaging the crack according to the extracted regularized nonlinear prediction error value NE is as follows: arranging the NE characteristic values of the points obtained by calculation according to the spatial positions of the scanning points, representing the size of the characteristic values of the points by the depth of color, wherein the maximum is red, and the minimum is blue, and further performing tenfold bilinear interpolation on the scanning points by using a bilinear interpolation method; at the position of the crack, the initial condition of the interaction between the surface acoustic wave signal and the member is changed, meanwhile, the surface acoustic wave generates the nonlinear modulation action at the position of the crack, the surface acoustic wave time-frequency characteristics are changed, the difference between the attractor track in the phase space and the corresponding attractor track in the phase space corresponding to each point at the adjacent position is obtained through reconstruction, the calculation result of the NE characteristic value is different, and the NE characteristic value is displayed as a red point in an imaging graph; at the crack-free position, the surface acoustic wave waveform characteristics are almost unchanged, the NE characteristic value is small, and blue dots are displayed in the graph; and finally, in an imaging picture, the red area represents the position of the crack, and the shape of the red area represents the shape of the crack, namely, the imaging processing of the crack is realized.
Preferably, in the first step, the top of the water tank is open, a lifting platform is arranged in the water tank, the object to be measured is placed on the lifting platform, and the lifting of the lifting platform is controlled by an upper computer; the thickness of the water restraint layer is 0.1 mm.
Preferably, the optical path adjusting system comprises a first convex lens, a spectroscope, a second convex lens and a scanning galvanometer; the first convex lens is placed on a light path of a high-energy laser pulse beam emitted by the laser pulse generator and used for focusing the high-energy laser pulse beam; the spectroscope is arranged at the focus position of the first convex lens and divides the incident high-energy pulse laser beam into a first laser beam transmitted in the horizontal direction and a second laser beam transmitted in the vertical direction; the first laser beam is focused by the second convex lens and then enters the central position of the scanning galvanometer, and the scanning galvanometer controls the first laser beam to scan and enter the surface of the object to be detected within a certain range at a certain speed so as to excite the surface of the object to be detected to generate laser surface acoustic waves; the second laser beam is transmitted to an enabling port of the high-speed data acquisition card through the optical fiber, so that the high-speed data acquisition card enters a working state.
Preferably, the data acquisition system comprises a laser interferometer, a signal amplifier and a high-speed data acquisition card, wherein the laser interferometer is used for acquiring surface acoustic waves excited by laser beams on the surface of an object to be detected; the signal amplifier is used for amplifying the surface acoustic wave signal output by the laser interferometer and transmitting the surface acoustic wave signal to the inlet of the high-speed data acquisition card.
Preferably, the data analysis system is used for performing time-frequency characteristic analysis on the surface acoustic wave signal data acquired by the high-speed data acquisition card, and finally displaying the crack imaging result on the display in real time.
In the invention, the principle of constructing the phase space representing the dynamic characteristic of the object to be measured according to the signals acquired by the laser interferometer is as follows: the phase space is a multidimensional space used for representing all possible states of an actual object, the multidimensional space is composed of dynamic vectors of all dimensions of the object to be measured, for example, the space of the object to be measured can be reconstructed through the speed and acceleration vectors of the object to be measured after being excited by incident laser surface acoustic waves, the laser interferometer can acquire the speed and acceleration vectors of the object to be measured, and the signals acquired by the laser interferometer are vector signals required for reconstructing the space of the object to be measured; each state of the object to be measured has a corresponding point in the phase space, that is, each point in the phase space maps a dynamic feature of the object to be measured one by one, and the points move along a specific trajectory line in the phase space and finally gradually converge to one or more specific points in the phase space along the trajectory line, and the specific points are called singular points in the phase space.
The dynamic characteristics of the object to be measured can be visually described by the tendency of point concentration moving trajectory lines in the phase space of the object to be measured. When the object to be detected is abnormal, for example, cracks appear on the surface, the track line of the point set in the space of the object to be detected, which represents the dynamic characteristics of the object to be detected and converges to the singular point, is changed. Crack information can be inverted by defining a particular parameter value to define the change and correlating it with crack signature information.
Compared with the prior art, the invention has the following advantages and effects:
(1) in the detection process, a crack-free sample is not required to be used as a reference standard, so that the influence of manual operation errors and external environment changes on the detection result is avoided, and the detection efficiency and the detection precision are improved;
(2) by forming the water restraint layer on the surface of the object to be detected, the energy value of the laser pulse beam for detection can be obviously improved, and the detection efficiency, the detection precision and the detection sensitivity are improved;
(3) the surface of the object to be detected is not damaged;
(4) the nonlinear characteristic parameter value NE of the inversion crack characteristics is provided, and the detection precision of the surface micro cracks is high.
Drawings
In order to illustrate the embodiments of the present invention or the solutions in the prior art more clearly, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without inventive effort.
Fig. 1 is a schematic structural diagram of a detection system used in the embodiment of the present invention.
FIG. 2 is a diagram of surface cracks of an actual test piece to be tested used in the example of the present invention.
Fig. 3 is a matrix scanning route diagram of the high-energy laser pulse beam according to the embodiment of the present invention.
Fig. 4 is a phase space reconstruction diagram in an embodiment of the present invention.
FIG. 5 is a graph showing the results of surface crack imaging of an actual test piece used in the examples of the present invention.
Reference numerals: a first convex lens 1; a spectroscope 2; a second convex lens 3; an object 4 to be measured; a lifting platform 5; a water tank 6; a scanning galvanometer 7; a laser pulse generator 8; an upper computer 9; a high-speed data acquisition card 10; a laser interferometer 11.
Detailed Description
The present invention will be described in further detail below by way of examples with reference to the accompanying drawings, which are illustrative of the present invention and are not to be construed as limiting the present invention.
Examples are given.
See fig. 1-5.
The embodiment discloses an object surface crack online detection method based on laser surface acoustic waves, which is implemented by adopting a set of detection system.
Referring to fig. 1, the detection system includes a water immersion system, a light path adjustment system, a laser pulse generator 8, an upper computer 9, a data acquisition system, and a data analysis system.
The water immersion system comprises a water tank 6 and a lifting platform 5. The lifting platform 5 is arranged in the water tank 6, and the object 4 to be measured is placed on the lifting platform 5. The top of the water tank 6 is open and is mainly used for soaking the object 4 to be measured so as to form a water constraint layer on the surface of the object 4 to be measured. The lifting of the lifting platform 5 is controlled by an upper computer, and the lifting platform 5 can finely adjust the up-down displacement of the object 4 to be measured within the precision range of + -10 um.
The light path adjusting system comprises a first convex lens 1, a spectroscope 2, a second convex lens 3 and a scanning galvanometer 7. The first convex lens 1 is disposed on the optical path of the high-energy laser pulse beam emitted from the laser pulse generator 8, and is used for focusing the high-energy laser pulse beam. The spectroscope 2 is arranged at the focus position of the first convex lens 1, and the spectroscope 2 divides the incident high-energy laser pulse beam into a first laser beam transmitted in the horizontal direction and a second laser beam transmitted in the vertical direction; the first laser beam is focused by the second convex lens 3 and then enters the central position of the scanning galvanometer 7, and the scanning galvanometer 7 controls the first laser beam to carry out scanning incidence in a certain range on the surface of the object 4 to be detected at a certain speed so as to excite the surface acoustic wave of the laser on the surface of the object 4 to be detected; the second laser beam is transmitted to the enable port of the high speed data acquisition card 10 via the optical fiber, causing the high speed data acquisition card 10 to enter a working state.
The data acquisition system comprises a laser interferometer 11, a signal amplifier and a high-speed data acquisition card 10, wherein the laser interferometer 11 is used for acquiring surface acoustic waves excited by laser beams on the surface of the object 4 to be detected; the signal amplifier is used to amplify the surface acoustic wave signal outputted from the laser interferometer 11 and transmit it to the entrance of the high speed data acquisition card 10. The laser interferometer 11 and the laser pulse generator 8 are both in communication connection with the upper computer 9.
The data analysis system is used for carrying out time-frequency characteristic analysis on the surface acoustic wave signal data acquired by the high-speed data acquisition card 10, and finally displaying the crack imaging result on the display in real time.
In this embodiment, the method for performing online detection of the object surface crack based on the laser surface acoustic wave by using the detection system includes the following steps:
the method comprises the following steps: soaking an object 4 to be detected in a water tank 6, so that the surface of the object 4 to be detected is provided with a water restraint layer; the water restraint layer can limit the gasification phenomenon of the surface material of the object to be detected 4 under the irradiation of the laser pulse beam, and improve the recoil force of the splashed substance on the surface of the object to be detected 4 after the action of the pulse beam, so that the energy value of the laser surface acoustic wave excited by the laser pulse beam on the surface of the object to be detected 4 is improved, and the crack detection efficiency, the crack detection precision and the crack detection sensitivity are improved; the thickness of the water restraint layer on the surface of the object 4 to be measured is 0.1 mm;
step two: the detection instruction is sent to the upper computer 9, the laser pulse generator 8 is controlled to emit the high-energy laser pulse beam through the upper computer 9, and the high-energy laser pulse beam is divided into two beams under the action of the light path adjusting system: one beam is transmitted to an enabling port of a high-speed data acquisition card 10 to enable the high-speed data acquisition card to enter a working state, the other beam is transmitted to the center of a scanning vibrating mirror 7 as a laser pulse beam for detection, under the action of the scanning vibrating mirror 7, the laser pulse beam for detection is incident to any point on the surface of an object 4 to be detected according to a set scanning path, and the point and a water constraint layer interact to excite an explosion shock wave, so that a laser surface acoustic wave is excited on the surface of the object 4 to be detected, and the laser surface acoustic wave is diffused around the surface of the object 4 to be detected by taking an irradiation point as a center;
step three: placing a laser interferometer 11 on the same side of the measured surface of the object 4 to be measured, controlling a detection port of the laser interferometer 11 to enable a detection beam of the laser interferometer 11 to irradiate a fixed point on the upper surface of the object 4 to be measured so as to receive a laser surface acoustic wave which is excited by a laser pulse beam for detection and freely propagates on the surface of the object 4 to be measured; the received laser surface acoustic wave signals are stored in a data analysis system through a data acquisition system; when laser surface acoustic waves excited by laser pulse beams for detection are transmitted on the surface of an object 4 to be detected, the laser surface acoustic waves are influenced by cracks on the surface of the object, nonlinear modulation phenomena can occur on the laser surface acoustic waves, the time-frequency characteristics of the laser surface acoustic waves are obviously changed, the change quantity of the time-frequency characteristics is analyzed through a data analysis system, crack information can be inverted, and therefore real-time imaging detection is conducted on the cracks on the surface of the object 4 to be detected, and crack imaging detection results are displayed on a display screen in real time.
In this embodiment, in the first step, the principle that the laser surface acoustic wave energy value excited by the laser pulse beam on the surface of the object to be measured can be increased after the water confinement layer is added on the surface of the object to be measured is as follows: the laser pulse beam power density required to break down water is about 1.8 x 1011W/cm2Comparison with the damage threshold of a general metal member of 1X 107W/cm2It is clear that the laser power density that can be tolerated by water is 3 to 4 orders of magnitude higher. When the power density is more than 1 x 107W/cm2But less than 1.8X 1011W/cm2When the laser pulse beam irradiates the water restraint layer on the surface of the object to be detected, the water restraint layer can directly absorb the energy of the laser pulse beam and quickly release heat energy to generate shock waves, and the shock waves act as a vibration source after acting on the surface layer of the metal member to be detected, so that surface acoustic waves are excited in the metal member, and the energy value of the excited surface acoustic waves is far larger than that of the metal member within the damage energy threshold range (1 multiplied by 10)7W/cm2) And absorbing the energy value of the surface acoustic wave excited by the laser pulse beam.
In this embodiment, in the third step, the basic principle that the data analysis system inverts the crack information according to the acquired signal and performs imaging processing is as follows:
(1) constructing a phase space representing the dynamic characteristics of the object 4 to be measured according to signals acquired by the laser interferometer 11, wherein the phase space is a multi-dimensional space used for representing all possible states of an actual object, the multi-dimensional space is composed of dynamic vectors of all dimensions of the object 4 to be measured, for example, the phase space of the object 4 to be measured can be reconstructed through the speed and acceleration vectors of the object 4 to be measured after being excited by incident laser surface acoustic waves, the laser interferometer 11 can acquire the speed and acceleration vectors of the object 4 to be measured after responding, and the signals acquired by the laser interferometer 11 are vector signals required for reconstructing the phase space of the object 4 to be measured; each state of the object 4 to be measured has a corresponding point in the phase space, that is, each point in the phase space maps a dynamic feature of the object 4 to be measured one by one, and the points move along a specific trajectory line in the phase space and finally gradually converge to one or more specific points in the phase space along the trajectory line, and the specific points are called singular points in the phase space;
(2) the dynamic characteristics of the object 4 to be detected can be visually described by the trend of point set moving trajectory in the phase space of the object 4 to be detected, when the object 4 to be detected is abnormal, for example, when cracks appear on the surface, the trajectory of a point set representing the dynamic characteristics of the object 4 to be detected in the phase space of the object 4 to be detected converging to a singular point can be changed, the change quantity is defined by defining a specific parameter value, and the change quantity is associated with crack characteristic information, so that the crack information on the surface of the object 4 to be detected can be obtained through inversion; and finally, representing the magnitude of the change quantity of each point according to the color depth degree, so that the imaging processing of the crack can be realized.
In this embodiment, the defined characteristic parameter value describing the variation of the point set trajectory in the phase space of the object to be measured 4 is a regularized nonlinear prediction error value NE, and the method for extracting the regularized nonlinear prediction error value NE by using the collected laser surface acoustic wave signal includes the following steps:
the first step is as follows: controlling the detection laser pulse beam to perform matrix scanning on the surface of the object 4 to be detected through the upper computer 9 and the light path adjusting system;
the second step is that: measuring a response signal of the laser surface acoustic wave of each point on the surface of the object 4 to be measured on the matrix route by using the laser interferometer 11;
the third step: randomly selecting a response point C in the matrix route, and reconstructing a phase space according to the collected response signals, wherein the phase space is called as a phase space I;
the fourth step: randomly selecting Q base points y (i) (1, 2, …, Q) from a point set motion track of a phase space I, wherein the value of Q is selected so that a prediction error calculation value is not interfered by the change of the point number, in the embodiment, Q is N/100, and N is the corresponding data point number measured by a high-speed data acquisition card;
the fifth step: selecting a nearest point B of the response point C in the third step, reconstructing a phase space II by using a response signal acquired by the point B, and searching P space near points x (j) (1, 2, …, P) for each base point y (i) on a point set track of the phase space II, wherein the space near points x (j) are directly selected according to Euclidean distance, namely, points which are closest to y (i) on a track line are taken as near points; the P value is selected to ensure that local characteristics of the attractor can be completely characterized, and meanwhile, the calculated value is insensitive to background noise, in this embodiment, P is N/1000, where N is the number of corresponding data points measured by the high-speed data acquisition card;
and a sixth step: advancing each base point and the proximate point by L steps of distance along the respective trajectory, at which time each proximate point centroid of point y (i) can be represented as:
the calculated prediction error value PE may be expressed as:
the normalized nonlinear prediction error value NE corresponding to the point B and point C signalsBCIt can be expressed as:
wherein sigmai 2Is the variance value of the measurement signal;
the seventh step: finally determining a regularization nonlinear prediction error value NE according to the selected position of the response point C:
in the scanning point matrix, n adjacent points (including point B) are arranged around any one response point C, in this case, regularized nonlinear prediction error values NE between the signal at the response point C and the n peripheral point signals (including point B) are respectively calculatedk(k ═ 1,2, …, n), the regularized nonlinear prediction error value NE is expressed as:
referring to fig. 3, the actual value of n should be determined according to the position of the actually selected response point C, where n is 8 when the point C is at the middle position of the scanning point matrix, and n is 2 or 3 when the point C is on the edge of the scanning point matrix.
In this embodiment, specifically, the process of imaging the crack according to the extracted regularization nonlinear prediction error value NE is as follows: arranging the NE characteristic values of the points obtained by calculation according to the spatial positions of the scanning points, representing the size of the characteristic values of the points by the depth of color, wherein the maximum is red, and the minimum is blue, and further performing tenfold bilinear interpolation on the scanning points by using a bilinear interpolation method; at the position of the crack, the initial condition of the interaction between the surface acoustic wave signal and the member is changed, meanwhile, the surface acoustic wave generates the nonlinear modulation action at the position of the crack, the surface acoustic wave time-frequency characteristics are changed, the difference between the attractor track in the phase space and the corresponding attractor track in the phase space corresponding to each point at the adjacent position is obtained through reconstruction, the calculation result of the NE characteristic value is different, and the NE characteristic value is displayed as a red point in an imaging graph; at the crack-free position, the surface acoustic wave waveform characteristics are almost unchanged, the NE characteristic value is small, and blue dots are displayed in the graph; and finally, in an imaging picture, the red area represents the position of the crack, and the shape of the red area represents the shape of the crack, namely, the imaging processing of the crack is realized.
The above method is further described in a specific experimental case as follows: the experimental process is as follows:
the first step is as follows: selecting a test piece to be tested containing surface cracks, wherein the specific shape of the surface cracks on the test piece is shown in figure 2, and figure 2 is obtained by shooting with a kirschner VHX-100 model electron microscope;
the second step is that: placing a to-be-tested piece on a lifting platform 5, and controlling the lifting platform 5 to descend through an upper computer 9 until the upper surface of the to-be-tested piece is completely immersed in a water tank 6, wherein in the example, the thickness of a water constraint layer on the upper surface of the to-be-tested piece is 0.1 mm;
the third step: an enabling instruction is sent to the laser pulse generator 8 through the upper computer 9, and the upper computer 9 enables the laser pulse generator 8 to emit a high-energy laser pulse beam; under the action of the light path adjusting system, laser pulses emitted by the laser pulse generator 8 are divided into two beams, one beam is transmitted to an enabling port of the high-speed data acquisition card 10, the high-speed data acquisition card 10 is enabled to enter a working state, and the other beam is focused and then transmitted to the center of the scanning galvanometer 7; in this example, the energy of the emitted laser pulse beam is 1 × 1010W/cm2The energy is larger than the damage energy threshold of the tested piece;
the fourth step: under the action of the scanning galvanometer 7, a laser beam for detection can be incident to any point on the upper surface of the to-be-tested piece according to a set scanning path, and is interacted with the water constraint layer at the point to generate explosion shock waves, so that a laser surface acoustic wave is excited on the surface of the to-be-tested piece and is diffused towards the periphery of the surface of the to-be-tested piece by taking an irradiation point as a center; in this example, the scanning path diagram is shown in fig. 3, and the distance between two adjacent scanning points is 0.1 mm;
the fifth step: controlling a detection port of a laser interferometer 11 to enable a detection beam of the interferometer to irradiate a fixed point on the upper surface of the to-be-tested piece so as to receive a laser surface acoustic wave which is excited by a detection laser beam and freely propagates on the upper surface of the to-be-tested piece;
and a sixth step: surface acoustic wave signals acquired by the interferometer are transmitted to a data analysis system through the data acquisition system, NE values corresponding to all points on a scanning path are calculated through the data analysis system, NE characteristic values of all points obtained through calculation are arranged according to spatial positions of all scanning points, the size of the characteristic values of all points is represented according to the depth of color, the maximum is red, the minimum is blue, then a bilinear interpolation method is used for performing tenfold bilinear interpolation on the scanning points, and finally a result graph is obtained and is shown in figure 5; it can be seen from fig. 5 that the surface crack of the tested sample has been clearly shown, and comparing the position, shape and direction of the surface crack in fig. 5 with those in fig. 2, it can be found that the position, shape and direction of the surface crack are substantially the same, and the surface of the tested sample is not damaged when the used laser pulse energy is greater than the damage threshold of the tested sample.
Although the present invention has been described with reference to the above embodiments, it should be understood that the scope of the present invention is not limited thereto, and that various changes and modifications can be made by those skilled in the art without departing from the spirit and scope of the present invention.
Claims (6)
1. An object surface crack online detection method based on laser surface acoustic waves is characterized in that: the method comprises the following steps:
the method comprises the following steps: soaking an object to be detected in a water tank to enable the surface of the object to be detected to be provided with a water restraint layer;
step two: the upper computer controls the laser pulse generator to emit a high-energy laser pulse beam, and the high-energy laser pulse beam is divided into two beams after the action of the light path adjusting system: one beam is transmitted to an enabling port of the high-speed data acquisition card to enable the high-speed data acquisition card to enter a working state, and the other beam is transmitted to the surface of an object to be detected as a laser pulse beam for detection to excite a laser surface acoustic wave; the scanning galvanometer in the light path adjusting system can control the laser pulse beam for detection to scan and detect the surface of the object to be detected;
step three: placing a laser interferometer on the same side of the measured surface of the object to be measured, and receiving laser surface acoustic waves excited on the surface of the object to be measured by a laser pulse beam for detection; the received laser surface acoustic wave signals are stored in a data analysis system through a data acquisition system; when laser surface acoustic waves excited by a laser pulse beam for detection are transmitted on the surface of an object to be detected, the laser surface acoustic waves are influenced by cracks on the surface of the object, the nonlinear modulation phenomenon can occur on the laser surface acoustic waves, the time-frequency characteristics of the laser surface acoustic waves are obviously changed, and the crack information can be inverted by analyzing the change amount of the time-frequency characteristics through a data analysis system, so that the real-time imaging detection can be performed on the cracks on the surface of the object to be detected;
in the process of inverting the crack information, analyzing the laser surface acoustic wave time-frequency characteristic parameter variation caused by the crack by utilizing a regularized nonlinear prediction error value NE;
the extraction method of the regularization nonlinear prediction error value NE comprises the following steps:
the first step is as follows: controlling the detection laser pulse beam to perform matrix scanning on the surface of the object to be detected through an upper computer and a light path adjusting system;
the second step is that: measuring response signals of laser surface acoustic waves of each point on the surface of the object to be measured on the matrix route by using a laser interferometer;
the third step: randomly selecting a response point C in the matrix route, and reconstructing a phase space according to the collected response signals, wherein the phase space is called as a phase space I;
the fourth step: randomly selecting Q base points y (i) (i is 1,2, …, Q) from a point set motion track of a phase space I, wherein N is the number of corresponding data points measured by a high-speed data acquisition card;
the fifth step: selecting a nearest point B of the response point C in the third step, reconstructing a phase space II by using a response signal acquired by the point B, and searching P space near points x (j) (1, 2, …, P) for each base point y (i) on a point set track of the phase space II, wherein the space near points x (j) are directly selected according to Euclidean distance; p is N/1000, wherein N is the number of corresponding data points measured by the high-speed data acquisition card;
and a sixth step: advancing each base point and the proximate point by L steps of distance along the respective trajectory, at which time each proximate point centroid of point y (i) can be represented as:
the calculated prediction error value PE may be expressed as:
the normalized nonlinear prediction error value NE corresponding to the point B and point C signalsBCIt can be expressed as:
wherein sigmai 2Is the variance value of the measurement signal;
the seventh step: finally determining a regularization nonlinear prediction error value NE according to the selected position of the response point C:
in the scanning point matrix, n adjacent points (including point B) are arranged around any one response point C, in this case, regularized nonlinear prediction error values NE between the signal at the response point C and the n peripheral point signals (including point B) are respectively calculatedk(k ═ 1,2, …, n), the regularized nonlinear prediction error value NE is expressed as:
judging the actual value of n according to the position of the actually selected response point C, wherein when the point C is in the middle position of the scanning point matrix, n is 8; when the point C is on the edge of the scanning point matrix, n is 2 or n is 3.
2. The on-line detection method for the object surface crack based on the laser surface acoustic wave as claimed in claim 1, characterized in that: in the third step, the process of imaging the crack according to the extracted regularization nonlinear prediction error value NE is as follows: arranging the NE characteristic values of the points obtained by calculation according to the spatial positions of the scanning points, representing the size of the characteristic values of the points by the depth of color, wherein the maximum is red, and the minimum is blue, and further performing tenfold bilinear interpolation on the scanning points by using a bilinear interpolation method; at the position of the crack, the initial condition of the interaction between the surface acoustic wave signal and the member is changed, meanwhile, the surface acoustic wave generates the nonlinear modulation action at the position of the crack, the surface acoustic wave time-frequency characteristics are changed, the difference between the attractor track in the phase space and the corresponding attractor track in the phase space corresponding to each point at the adjacent position is obtained through reconstruction, the calculation result of the NE characteristic value is different, and the NE characteristic value is displayed as a red point in an imaging graph; at the crack-free position, the surface acoustic wave waveform characteristics are almost unchanged, the NE characteristic value is small, and blue dots are displayed in the graph; and finally, in an imaging picture, the red area represents the position of the crack, and the shape of the red area represents the shape of the crack, namely, the imaging processing of the crack is realized.
3. The on-line detection method for the object surface crack based on the laser surface acoustic wave as claimed in claim 1, characterized in that: in the first step, the top of the water tank is open, a lifting platform is arranged in the water tank, an object to be detected is placed on the lifting platform, and the lifting of the lifting platform is controlled by an upper computer; the thickness of the water restraint layer is 0.1 mm.
4. The on-line detection method for the object surface crack based on the laser surface acoustic wave as claimed in claim 1, characterized in that: the light path adjusting system comprises a first convex lens, a spectroscope, a second convex lens and a scanning galvanometer; the first convex lens is placed on a light path of a high-energy laser pulse beam emitted by the laser pulse generator and used for focusing the high-energy laser pulse beam; the spectroscope is arranged at the focus position of the first convex lens and divides the incident high-energy pulse laser beam into a first laser beam transmitted in the horizontal direction and a second laser beam transmitted in the vertical direction; the first laser beam is focused by the second convex lens and then enters the central position of the scanning galvanometer, and the scanning galvanometer controls the first laser beam to scan and enter the surface of the object to be detected within a certain range at a certain speed so as to excite the surface of the object to be detected to generate laser surface acoustic waves; the second laser beam is transmitted to an enabling port of the high-speed data acquisition card through the optical fiber, so that the high-speed data acquisition card enters a working state.
5. The on-line detection method for the object surface crack based on the laser surface acoustic wave as claimed in claim 1, characterized in that: the data acquisition system comprises a laser interferometer, a signal amplifier and a high-speed data acquisition card, wherein the laser interferometer is used for acquiring surface acoustic waves excited by laser beams on the surface of an object to be detected; the signal amplifier is used for amplifying the surface acoustic wave signal output by the laser interferometer and transmitting the surface acoustic wave signal to the inlet of the high-speed data acquisition card.
6. The on-line detection method for the object surface crack based on the laser surface acoustic wave as claimed in claim 1, characterized in that: the data analysis system is used for carrying out time-frequency characteristic analysis on the surface acoustic wave signal data acquired by the high-speed data acquisition card and finally displaying the crack imaging result on the display in real time.
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