CN107747922B - Method for measuring subsurface defect buried depth based on laser ultrasound - Google Patents
Method for measuring subsurface defect buried depth based on laser ultrasound Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B17/00—Measuring arrangements characterised by the use of infrasonic, sonic or ultrasonic vibrations
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/04—Analysing solids
- G01N29/07—Analysing solids by measuring propagation velocity or propagation time of acoustic waves
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
- G01N2021/1706—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in solids
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/01—Indexing codes associated with the measuring variable
- G01N2291/011—Velocity or travel time
Abstract
The invention discloses a method for measuring subsurface defect buried depth based on laser ultrasound. The method comprises the following steps: 1) mounting a laser probe on a displacement platform; 2) the pulse laser probe and the ultrasonic probe are respectively arranged on two sides of the sub-surface defect of the workpiece; 3) the pulse laser emits pulse laser to excite ultrasonic waves on a workpiece, and the ultrasonic probe respectively measures a direct surface wave signal R and a reflected signal PR after interaction with the defects; 4) controlling the displacement platform to move, enabling pulse laser to scan n (n is more than or equal to 2) points at one side of the subsurface defect at a certain distance delta d, and receiving direct waves and reflected echoes by an ultrasonic probe when each point is scanned; 5) and calculating the buried depth of the subsurface defect by the arrival time of the direct wave signal and the defect echo signal of each scanning point received by the ultrasonic probe. The method can be used for ultra-precision machining in-situ detection and detection of the subsurface defect buried depth in other special environments such as high temperature and high pressure.
Description
Technical Field
The invention relates to the field of nondestructive testing, in particular to a laser ultrasound-based nondestructive measurement method for subsurface defect burial depth.
Background
Subsurface defects, which are cracks and indentations between a few microns to a few hundred microns below the surface and between a few microns to tens of microns in size, are micro-defects generated during ultra-precision machining such as lapping, polishing, and the like. In the use process of the part, the subsurface defect can reduce the strength and the service life of the part, and has great threat to the safe operation of equipment, even can cause immeasurable results, such as equipment failure in case of light, safety accident and serious economic loss in case of heavy, and the like. Therefore, subsurface defects must be removed during subsequent processing. However, since the subsurface defect has the characteristics of invisible surface, shallow depth, small size and the like, the conventional method is not easy to detect, and the depth is not particularly limited to quantitative detection. To this end, numerous scholars have devoted themselves to the study of methods for detecting subsurface defects.
In the existing research, Balogun et al developed a set of picosecond laser ultrasound-based all-optical scanning ultrasonic microscope system, which excited the ultrasonic longitudinal wave with the frequency up to GHz by picosecond laser and had the capability of detecting and quantitatively detecting the micro superficial defect. However, since the longitudinal wave is detected only at a single point, a scanning system is required. At the same time, exciting and receiving ultrasound at the upper GHz also makes the system complex and expensive. Kromine et al propose a line source laser scanning technology-based method for detecting subsurface defects, in which line source laser excites surface acoustic waves, the surface wave echoes can change greatly during line source laser scanning, and subsurface defects are detected through the change of the signals. The method can quickly detect and locate the subsurface defect, but has no effect on measuring the buried depth of the subsurface defect. Cho detects the bonding quality by using surface waves excited by point source laser, and simultaneously locates the subsurface transverse defects by scanning through a mechanical scanning device, and the method can not quantitatively measure the subsurface defect burying depth. Other non-destructive inspection methods, such as thermal wave imaging and X-ray detection, are also used in the detection of subsurface defects. However, thermal wave imaging techniques are not sensitive to microscopic defects and cannot be quantitatively detected. Although the X-ray method is mature, the equipment cost is high, the ray is harmful to the human body, and the X-ray method cannot be well applied to in-situ measurement.
In the field of non-destructive inspection, it is also important to detect defects and to quantitatively detect the size of defects, particularly subsurface defects produced in precision and ultra-precision machining. The buried depth of subsurface defects is an important parameter for subsequent processing to remove the defect layer. In existing non-destructive inspection methods, there is little possibility of quantitative measurement of the buried depth of subsurface defects. The method can quickly and accurately detect the subsurface defect and quantitatively measure the buried depth of the subsurface defect. If a laser interferometer is adopted to detect the ultrasound, the method can also be used for in-situ measurement or defect detection in extreme environments such as high temperature and high pressure.
Disclosure of Invention
The invention provides a method for measuring the buried depth of a subsurface defect based on laser ultrasonic surface waves, which aims to detect the buried depth of the subsurface defect generated in the process of machining a precise and ultra-precise machining material so as to guide the follow-up machining to remove the defect. The specific scheme is as follows:
a sub-surface defect burying depth measuring method based on laser ultrasound comprises the following steps:
1) mounting a pulse laser probe on a displacement motion platform, wherein the motion direction of the displacement motion platform is vertical to the length direction of the subsurface defect;
2) the pulse laser probe and the ultrasonic probe are respectively arranged at two sides of the subsurface defect, and a connecting line of a laser spot irradiated on the workpiece by the pulse laser probe and the ultrasonic probe is vertical to the subsurface defect;
3) exciting ultrasonic waves on the surface and inside of the workpiece by laser spots irradiated on the workpiece by a pulse laser probe, and respectively measuring surface wave signals R by using the ultrasonic probe1And an ultrasonic signal PR reflected back from the defect1Then obtaining a surface wave signal R1Time t of arrival at the ultrasound probeR1And the ultrasonic signal PR reflected back from the defect1Time t of arrival at the ultrasound probePR1；
4) Controlling the displacement motion platform to move towards the defect by a displacement delta d, and repeating the step 3) to obtain a surface wave signal R2Time t of arrival at the ultrasound probeR2And the ultrasonic signal PR reflected back from the defect2Time t of arrival at the ultrasound probePR2Controlling the displacement motion platform to move continuously to the defectΔ d distance, and then surface wave signal R is obtained3Time t of arrival at the ultrasound probeR3And the ultrasonic signal PR reflected back from the defect3Time t of arrival at the ultrasound probePR3And repeating the steps in the same way, scanning n (n is more than or equal to 2) points in total;
5) and (4) calculating the buried depth h of the subsurface defect through the arrival time of the direct wave signal and the defect echo signal obtained by the ultrasonic probe in the steps 3) and 4).
Preferably, the excitation method of the ultrasonic source is point source excitation, that is: the pulse laser probe emits pulse laser which is focused into point source laser through the biconvex lens, and the point source laser irradiates the surface of the workpiece and excites ultrasonic waves.
Preferably, the excitation method of the ultrasonic source can be line source excitation, namely: the pulse laser probe emits pulse laser which is focused into line source laser through the cylindrical lens, irradiates the surface of the workpiece and excites ultrasonic waves; point source excitation is also possible, i.e.: the pulse laser probe emits pulse laser, the pulse laser is focused into point source laser through the focusing lens, and the point source laser irradiates the surface of a workpiece and excites ultrasonic waves.
Furthermore, the subsurface defect is a cylindrical defect, and the line source laser is parallel to the length direction of the subsurface defect.
Preferably, the formula for calculating the buried depth h of the subsurface defect in the step 5) is as follows:
wherein d is the distance between the laser spot and the subsurface defect in step 3), vRIs the propagation velocity, v, of the surface acoustic wave in the workpiecePIs the propagation velocity of a longitudinal wave in the workpiece, vSIs the propagation velocity of the transverse wave in the workpiece.
Further, the calculation formula of the angle θ in the above formula is:
θ＝arcsin(vS/vP)
compared with the prior art, the invention has the beneficial effects that: first, the present invention can perform in-situ measurement in the case of measuring ultrasonic vibration using an interferometer. The method can measure the subsurface defect of the material after the precise ultraprecise machining in situ without secondary clamping, so that the defect can be removed in the subsequent machining process. Secondly, the sub-surface of the precision ultra-precision machining has small defects and shallow buried depth, and if a longitudinal wave C scanning detection method is used, high-frequency ultrasound is needed, so that the equipment is complex and the detection speed is low. The method is simple, low in cost, high in measurement speed and high in precision.
Drawings
FIG. 1 is a schematic diagram of an inspection state of a laser ultrasound-based subsurface defect burial depth measurement method;
FIG. 2 is a schematic diagram of a point source laser and a detection point of a method for measuring the buried depth of a subsurface defect based on laser ultrasound;
in the figure, a workpiece 1, a two-dimensional motion platform 2, a pulse laser probe 3, an ultrasonic probe 4, an oscilloscope 5, a subsurface defect 6 and a laser spot 7.
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings and examples.
The embodiment of the invention relates to a method for detecting the buried depth of a subsurface defect based on laser ultrasound.
The invention discloses a laser ultrasound-based sub-surface defect width detection method, which has the basic principle partially consistent with the content of the invention and comprises the following specific steps:
1) placing a probe 3 with a pulse laser on a two-dimensional motion platform 2, and enabling one motion direction of the two-dimensional motion platform 2 to be parallel to the long edge of a workpiece and the other motion direction to be vertical to the upper surface of the workpiece; then respectively placing a pulse laser probe 3 and an ultrasonic probe 4 at two sides of the subsurface defect 6 of the workpiece (as shown in figure 1), wherein the connecting line of the ultrasonic probe 4 and a laser spot 7 is vertical to the length direction of the subsurface defect 6 (as shown in figure 2);
2) the pulse laser probe 3 emits pulse laser, the pulse laser is focused into a point source laser spot 7 through the focusing lens, ultrasonic waves are excited on the surface of the workpiece 1, and the ultrasonic probe 4 measures a direct wave signal R1And a wave signal PR scattered back from the defect1And displayed in an oscilloscope 5 to obtain a direct wave signal R1Time t of arrival at the ultrasonic probe 4R1And a wave signal PR scattered back from the defect1Time t of arrival at the ultrasonic probe 4PR1；
3) Controlling the displacement platform 2 to move towards the defect by a displacement delta d which is 1mm, and repeating the step 3) to obtain a surface wave signal R2Time t of arrival at the ultrasonic probe 4R2And the ultrasonic signal PR scattered back from the defect2Time t of arrival at the ultrasonic probe 4PR2Controlling the displacement motion platform to continuously move delta d distance to the defect so as to obtain a surface wave signal R3Time t of arrival at the ultrasonic probe 4R3And the ultrasonic signal PR reflected back from the defect3Time t of arrival at the ultrasonic probe 4PR3And the rest is repeated, and 8 points are scanned in total;
4) calculating the buried depth h of the subsurface defect by the arrival time of the direct wave signal and the defect echo signal obtained by the measurement of the ultrasonic probe 4 in the steps 2) and 4), wherein the calculation formula is as follows:
the angle θ is determined by the following equation:
θ＝arcsin(vS/vP)
wherein d is the initial distance between the laser spot 7 and the subsurface defect 6 in step 2), vRIs the propagation velocity, v, of the surface acoustic wave in the workpiece 1PIs the propagation velocity, v, of a longitudinal wave in the workpiece 1SIs the propagation velocity of the transverse wave in the workpiece 1.
The buried depth of the subsurface defect of a medium carbon steel is detected by the method, wherein the length of the steel block is 100mm, the width of the steel block is 50mm, the thickness of the steel block is 5mm, and the buried depth of the subsurface defect is measured by using KEYENCE VHX-600 as a reference. The method comprises the steps of placing a steel block on a sample platform, exciting and receiving ultrasonic waves of a meter on two sides of a subsurface defect on the steel block by using a pulse laser probe and an ultrasonic probe respectively, receiving the ultrasonic waves R directly arriving from an excitation source and the ultrasonic waves PR scattered back from the defect by the ultrasonic probe in sequence, transmitting detected signals to an oscilloscope by the ultrasonic probe, storing data and reading the data on a computer for subsequent calculation. And controlling a two-dimensional motion platform provided with a pulse laser probe to scan 8 points to the subsurface defect, and also recording an ultrasonic signal received by the ultrasonic probe to obtain ultrasonic time for calculating the subsurface defect burying depth.
The results of the measurements of the final examples and their relative errors are shown in the following table:
as can be seen from the table, the detection method has high precision for the detection result of the subsurface defect buried depth of the material, and the contact PZT probe is used in the detection method, so that the condition and equipment cost for using the method are reduced. The method is simple, quick and effective, and does not need to take the sample to be measured down to the area to be measured from processing unlike the measurement of a microscope. Meanwhile, the invention can also use the interferometer to carry out ultrasonic detection so as to realize in-situ detection and improve the detection efficiency.
The above-described embodiments are merely preferred embodiments of the present invention, which should not be construed as limiting the invention. Various changes and modifications may be made by one of ordinary skill in the pertinent art without departing from the spirit and scope of the present invention. For example, the laser spot can be a point light source or a line light source, that is: the pulse laser probe emits pulse laser, the pulse laser is focused into line source laser through the cylindrical lens, and the line source laser irradiates the surface of a workpiece and excites ultrasonic waves. Therefore, the technical scheme obtained by adopting the mode of equivalent replacement or equivalent transformation is within the protection scope of the invention.
Claims (4)
1. A method for measuring the buried depth of a subsurface defect based on laser ultrasound, wherein the subsurface defect is a crack or an indentation, is characterized by comprising the following steps:
installing a pulse laser probe (3) on a displacement motion platform (2), wherein the motion direction of the displacement motion platform is vertical to the length direction of the subsurface defect (6);
2) the pulse laser probe (3) and the ultrasonic probe (4) are respectively arranged at two sides of the subsurface defect (6), and the connecting line of a laser spot (7) irradiated on the workpiece (1) by the pulse laser probe and the ultrasonic probe (4) is vertical to the subsurface defect;
3) the laser spot (7) irradiated on the workpiece (1) by the pulse laser probe (3) excites ultrasonic waves on the surface and the inside of the workpiece (1), and the ultrasonic probe (4) is utilized to respectively measure surface wave signals R1And an ultrasonic signal PR reflected back from the defect1Then obtaining a surface wave signal R1Time t of arrival at the ultrasonic probe (4)R1And the ultrasonic signal PR reflected back from the defect1Time t of arrival at the ultrasonic probe (4)PR1；
4) Controlling the displacement motion platform (2) to move towards the defect by a displacement delta d, and repeating the step 3) to obtain a surface wave signal R2Time t of arrival at the ultrasonic probe (4)R2And the ultrasonic signal PR reflected back from the defect2Time t of arrival at the ultrasonic probe (4)PR2Controlling the displacement motion platform to continuously move delta d distance to the defect so as to obtain a surface wave signal R3Time t of arrival at the ultrasonic probe (4)R3And the ultrasonic signal PR reflected back from the defect3Time t of arrival at the ultrasonic probe (4)PR3In the same order, scanning n points in total, wherein n is more than or equal to 2;
5) calculating the buried depth h of the subsurface defect through the arrival time of the direct wave signal and the defect echo signal measured by the ultrasonic probe (4) in the steps 3) and 4), wherein the calculation formula of the buried depth h is as follows:
wherein d is step 3)Distance between the laser spot (7) and the subsurface defect (6), vRIs the propagation velocity, v, of the surface acoustic wave in the workpiece (1)PIs the propagation velocity v of the longitudinal wave in the workpiece (1)SIs the propagation speed of the transverse wave in the workpiece (1); wherein the angle θ is calculated by the formula: θ ═ arcsin (v)S/vP)。
2. The method of claim 1, wherein: the excitation method of the ultrasonic source is point source excitation, namely: the pulse laser probe (3) emits pulse laser, the pulse laser is focused into point source laser through the biconvex lens, and the point source laser irradiates the surface of the workpiece (1) and excites ultrasonic waves.
3. The method of claim 1, wherein: the excitation method of the ultrasonic source is line source excitation, namely: the pulse laser probe (3) emits pulse laser, the pulse laser is focused into line source laser through the cylindrical lens, and the line source laser irradiates the surface of the workpiece (1) and excites ultrasonic waves.
4. The method of claim 3, wherein: the subsurface defect (6) is a cylindrical defect, and the line source laser is parallel to the length direction of the subsurface defect (6).
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