KR101257203B1 - Apparatus and method for evaluating micro damage of materials using nonlinear laser-generated surface wave - Google Patents

Abstract

PURPOSE: A micro-damage diagnosing device using a non-linear laser surface waves and a micro-damage diagnosing method are provided to enable to monitor on a real time basis in industrial field in which safety management levels with respect to structures and materials are high. CONSTITUTION: A micro-damage diagnosing device(100) using a non-linear laser surface waves comprises a surface wave exciting unit(110) and a signal analysis unit(120). The surface wave exciting unit makes the surface waves incident to an object through a slit mask(116). The signal analysis unit receives signals from the object, thereby analyzing the frequencies. The slit mask is formed so that a double frequency component included in the surface waves is minimized.

Description

Apparatus and Method for Evaluating Micro Damage of Materials using Nonlinear Laser-generated Surface Wave}

Disclosed is a microdamage diagnosis apparatus and method using a nonlinear laser surface wave. More specifically, the micro-damage diagnosis apparatus using the nonlinear laser surface wave capable of minimizing the double frequency components caused by the nonlinear characteristics inherently included in the laser surface waves and detecting only the double frequency components generated by the nonlinear characteristics of the material. The method is disclosed.

Nonlinear ultrasonic technology is a material deterioration, that is, the occurrence of potential defects (potential, precipitates, microstructural changes of the material, etc.) inside the material due to environmental influences such as stress, fatigue, heat, corrosion, etc. Applied to the measurement of physical property characteristics. For this purpose, we define a nonlinearity parameter β representing the nonlinearity of the ultrasonic wave, which is represented by calculating the magnitude (A ₁ ) and the magnitude (A ₂ ) of the fundamental frequency component of the measured signal. Lose.

In order to use this acoustic nonlinear characteristic, conventionally, the measurement was performed by using a transmission longitudinal wave using a contact probe. Has limited field applicability.

Therefore, research on nonlinear characteristic measurement using surface waves has been conducted. Surface waves are waves that propagate within a few wavelengths of one side of an inspected object, so only one side of the inspected object is required. It is also effective in exposing strong nonlinearity to surface damage due to energy concentration on the surface.

However, the influence of the wedge, the coupler, and the contact force for generating the surface wave using the contact transducer is limited to the measurement of the acoustic nonlinear characteristics.

In order to overcome this problem, a non-contact laser surface wave generation technology has been studied. The non-contact laser surface wave technology does not require a transducer, eliminating the need for a coherent, eliminating the influence of the transducer's contact force, and enabling rapid inspection, which is effective for automated inspection and constant monitoring.

However, the laser excitation surface wave is caused by the thermoelastic effect of the laser and the material, and the linear array laser beam using the slit mask is used to make a narrow band signal which is advantageous for the frequency analysis. exist. It is difficult to measure the acoustic nonlinear characteristics comparing the magnitudes of the fundamental and harmonic components due to the overlap between the harmonic components inherent by the existing harmonic components and the acoustic nonlinear characteristics, and the measured acoustic nonlinear characteristic results are also less reliable. There is a problem.

SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus and method for diagnosing microdamage using nonlinear laser surface waves capable of determining slit mask array spacing and predicting frequency characteristics in order to control the harmonic components inherent in narrowband surface wave generation using a laser. It is.

An apparatus for diagnosing microdamage using nonlinear laser surface waves according to an embodiment of the present invention includes a surface wave excitation portion for injecting surface waves into a test object through a slit mask, and a signal analyzer for receiving a surface wave from the test object and analyzing a frequency thereof. In addition, the slit mask may be formed to minimize the double frequency components included in the surface wave.

Preferably, the ratio of the slit opening width w and the slit spacing d of the slit mask may be formed to minimize the double frequency components included in the surface wave, and more preferably, 2 included in the surface wave. The double frequency component may be determined to be zero.

The surface wave excitation part may include a laser source that is in contact with the object under test, and includes the laser source, and the slit mask may be disposed between the laser source and the object under test.

In the micro-damage diagnostic method using the nonlinear laser surface wave according to the embodiment of the present invention, the defect may be detected by using the nonlinear laser surface wave through the slit mask, and the frequency of the nonlinear laser surface wave is the fundamental frequency component of the nonlinear laser surface wave, The frequency component due to the nonlinearity of the subject and the double frequency component included in the nonlinear laser surface wave may be set, and the double frequency component included in the nonlinear laser surface wave may be set to be minimized.

The double frequency component included in the nonlinear laser surface wave has a factor (p) proportional to the fundamental frequency component and may be formed to minimize the factor (p).

The longitudinal wave of the nonlinear laser surface wave is represented by the following equation, and the proportional factor p may be determined to be minimized by the ratio of the slit opening width w and the slit spacing d of the slit mask.

Where A1 is the magnitude of the fundamental frequency component, k is the wave number of the surface wave, ω is the angular frequency, β is a nonlinear parameter, and b is a phase difference.

Alternatively, the longitudinal wave of the nonlinear laser surface wave is represented by the following equation, and the double frequency component may be determined to be zero by the ratio of the slit opening width w and the slit spacing d of the slit mask.

According to an embodiment of the present invention, by using a non-contact nonlinear laser surface wave, it is possible to overcome the experimental error that occurs when using a contact probe, thereby having an effect that is less affected by the environment of the inspector.

In addition, according to an embodiment of the present invention, by the micro-damage diagnosis apparatus and method using a non-linear laser surface wave, real-time monitoring in the industrial field with high safety management level for structures and materials such as nuclear power plants, aerospace, various industrial plants, etc. There is an effect to make this possible.

1 is a block diagram of a micro-damage diagnosis apparatus using a nonlinear laser surface wave according to an embodiment of the present invention.
2 shows the deformation due to laser absorption and the shape of the ultrasonic waves generated thereby.
3 shows the shape of the surface wave generated according to the shape of the laser beam.
4 shows the shape of the slit mask and the surface wave generated at the time when the laser beam is incident on the surface of the test object.
5 shows the prediction result of the relative nonlinear parameter.
6 is a result of measuring relative nonlinear parameters using test pieces having different values of nonlinear parameters of actual materials.
7 is a graph comparing the magnitude of the fundamental frequency component and the magnitude of the double frequency component by analyzing the frequency of the laser excited surface wave generated by changing the slit opening width and the slit interval of the slit mask.

Hereinafter, configurations and applications according to embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following description is one of several aspects of the patentable invention and the following description forms part of the detailed description of the invention.

In the following description, well-known functions or constructions are not described in detail for the sake of clarity and conciseness.

1 is a block diagram of a micro-damage diagnosis apparatus 100 using a nonlinear laser surface wave according to an embodiment of the present invention.

The micro-damage diagnosis apparatus 100 includes a surface wave excitation unit 110 and a signal analyzer 120, and the surface wave excitation unit 110 includes a high power pulse laser source 112, a beam expander 124, and a slit mask 116. ). The signal analyzer 120 may include a surface wave receiver 122 and a frequency analyzer 124.

As the high power pulse laser source 112, an Nd: YAG pulse laser (wavelength 1064 nm, pulse width 5 ns) may be used, and the slit mask 116 may be attached to the surface of the object D to suppress the diffraction effect of the laser. have. The signal received by the signal analyzer 120 is amplified using a pulser / receiver and then A / D converted in a digital oscilloscope. Fast Fourier transform (FFT) may be used for such frequency analysis.

Reference may be made to FIGS. 2 and 3 to describe the surface wave shape by the laser beam from the high power pulsed laser source 112, which illustrates the shape of the ultrasonic wave generated by the deformation and stress caused by laser absorption. 3 shows the shape of the surface wave generated according to the shape of the laser beam.

Referring to FIG. 2, ultrasonic generation using a laser is attributable to thermal deformation generated when the laser beam L is incident on the surface of the test object D. When the laser beam L of the appropriate intensity is irradiated onto the surface of the object D, the surface is heated instantaneously and thermal stress is generated. Accordingly, deformation of the object D causes seismic waves in the object D. . That is, when the laser beam L is absorbed by the object D, the object D thermally expands and expands in all directions of three axes, thereby generating longitudinal waves P, transverse waves S, and surface waves R. do.

Referring to FIG. 3, the general laser beam L has a point shape, and thus the surface wave generated therein propagates in all directions and has a broadband frequency. This disperses the energy of the surface waves and propagates waves in all directions except for the direction to be examined, and also has difficulty in frequency analysis. Using the laser beam L in the form of a line improves the directionality but still has a broadband frequency. Therefore, when the laser beam L is used in the form of a line-array, the surface waves have a directional and narrow band frequency. The micro-damage diagnostic apparatus 100 according to an embodiment of the present invention forms the laser beam L in the form of a line array using the slit mask 116.

4 shows the shape of the slit mask and the surface wave generated at the time when the laser beam is incident on the surface of the test object. The shape and frequency of the laser-based surface wave are determined by conditions such as the shape, spacing, and intensity of the laser beam L incident on the inspected object D. The condition of the laser beam L is determined by the shape of the slit mask 116. In Fig. 4, d is an interval of each slit, w is an opening width of the slit, and λ is a wavelength of the generated surface wave.

, 에 의해 나타나는 것으로 고차탄성계수에 크게 의존하게 되므로 고체재료의 비선형성을 나타내게 된다. The nonlinear elastic properties of the solid test object (D) are represented by the interaction force between the atoms in the crystal and the mean atomic separation distance due to the repulsive force, which is caused by the nonlinear behavior of the physical stresses with respect to the separation distance between atoms. It is a characteristic that appears. That is, the interatomic energy of the solid inspected object D does not have a harmonic characteristic but exhibits a non-harmonic characteristic. In general, this unharmonized property is explained by the higher-order elastic theory using continuum approximation. When a sinusoidal ultrasonic wave with a given constant frequency propagates in a solid medium having sufficient amplitude and non-harmonic nature, the ultrasonic wave having a fundamental incident frequency is distorted due to the difference in local phase velocity depending on the nonlinear nature of the material. Occurs. Therefore, a higher order harmonic component corresponding to a multiple of the fundamental frequency is generated. These higher harmonic components are physically related to the nonlinear stress-strain relationship of solid materials,

This results in nonlinearity of the solid material because it is highly dependent on the higher order modulus.

In general, when the solid material deforms, the linear properties of the material, that is, the secondary elastic modulus, are regarded as little or no change. However, the absolute value is very low compared to the secondary elastic modulus, but the third and fourth modulus are greatly changed. Therefore, it is very useful to evaluate microscopic changes in materials using higher-order harmonics that depend on such higher-order modulus. As mentioned above, the ultrasonic nonlinearity is closely related to the crystal structure state or the regularity of crystals in which the ultrasonic wave propagates. This will affect the parameters.

In the case of surface waves propagating along the surface of a semi-infinite medium, the displacement component takes into account the component of the x-axis and the z-axis perpendicular to the direction of travel. At this time, the displacement potential in each direction can be represented by the following equations (1) and (2).

[Equation 1]

&Quot; (2) "

,

,

here,

,

,

In addition, B ₁ , C ₁ are arbitrary constants c _L and c _T are the speed of the longitudinal wave and the transverse wave, respectively, k, w and c are the wave number, angular frequency and propagation speed of the surface wave.

The surface wave is a composite wave of longitudinal wave and transverse wave, and considering that the wave propagates along the surface of the non-stressed state of solid state, the displacement component can be expressed as the following equation.

&Quot; (3) "

&Quot; (4) "

,

here,

,

In the case of the non-linear solid medium, the second harmonic component of the surface wave propagating a sufficient distance can be expressed as follows.

[Equation 5]

&Quot; (6) "

For surface waves, both longitudinal and transverse components contribute to ux and uz. By the way, when the medium is isotropic and has a second weak acoustic nonlinearity, the acoustic nonlinearity of the transverse wave can be ignored due to the symmetry of the tertiary elastic modulus. Therefore, the acoustic nonlinearity of the surface wave contributes only the longitudinal component, which is similar to the nonlinear characteristic of the general longitudinal wave. That is, the wave equation in the propagating surface wave can be represented by the wave equation of the longitudinal wave of Equation 7 as follows.

[Equation 7]

Here, A2 is represented as follows.

[Equation 8]

Where β is a nonlinear parameter and x is the propagation distance.

Therefore, the nonlinear parameter β is calculated as follows.

&Quot; (9) "

Nonlinear ultrasound technology may be calculated through the ratio of the fundamental frequency component and the double frequency component as shown in Equation (9). However, the surface wave generated by the laser essentially includes the double frequency component since the surface wave is generated by the laser, in addition to the double frequency component generated by the nonlinear characteristic of the material as shown in Equation (7).

This is expressed as the following equation.

&Quot; (10) "

This equation represents the fundamental frequency component of the laser excited surface wave, the component exhibited by the nonlinearity of the material, and the double frequency component essentially contained in the laser excited surface wave. The double frequency component generated by the laser-excited surface wave has a magnitude corresponding to p times the fundamental frequency component, and has a phase difference between the fundamental frequency component and b.

In the above equation, the magnitude of the double frequency component essentially included in the laser surface wave is represented by the overlap of the component generated by the nonlinearity of the material and the component generated essentially by the laser. It's hard to get it directly. Therefore, the relative nonlinear parameter β 'such as the following Equation 11 can be used.

&Quot; (11) "

)과 2배 주파수 성분의 측정 크기(

)의 비로 나타내어 진다.The relative nonlinear parameter β 'is the square of the magnitude of the measured signal of the fundamental frequency component (

) And the measured magnitude of the double frequency component (

It is represented by the ratio of).

Accordingly, the value of the relative nonlinear parameter β 'for the change in the ratio of the nonlinearity β of the material, the phase b, the magnitude of the essentially double frequency component, p' is shown in FIG. 5.

Fig. 5 shows the prediction result of the relative nonlinear parameter, and when the nonlinearity (β) of the material increases from 0.05 to 0.25 in 0.05 units according to the phase (b), the change characteristic of the value of the relative nonlinear parameter β 'is You can see the difference. That is, for b of 0 to 180 degrees, even if the nonlinearity of the material increases, the value of the relative nonlinear parameter decreases. In b of 180 to 360 degrees, the relative nonlinear parameter also increases as the material nonlinearity increases.

This means that when p = 0 the relative nonlinear parameter β ′ directly reflects the nonlinearity β of the material. In order to produce a laser excited surface wave of P = 0, it is possible to change the slim opening width w and the slit spacing d of the slit mask 116.

6 is a result of measuring the relative nonlinear parameter β 'using a test piece having a different value of the nonlinear parameter β of the actual material. The material used here is a tensile test piece of aluminum alloy, and strain / elongation means a test piece that breaks at 100% as a ratio of strain to material elongation. This material is a test piece with a larger value of β as the strain increases. Thus, an increase in strain / elongation means an increase in β.

Referring to FIG. 6, when p = 0, the value of the relative nonlinear parameter β ′ increases as the nonlinear β of the material increases, which corresponds to the case of p = 0 of FIG. 5. In contrast, in the case of p = 0.4 in FIG. 6, the relative nonlinear parameter β ′ decreases even if the nonlinearity β of the material increases, which is p = 0.4 in FIG. 5. This corresponds to the case of b = 0 to 180 degrees.

That is, since the value of the relative nonlinear parameter β 'should be a value representing the nonlinearity of the material, it can be seen that the ideal case is the case where the magnitude (p) of the essentially double frequency component is minimal.

Since the magnitude p of the double frequency component depends on the slit opening width w of the slit mask 116 and the slit spacing d, the laser corresponding to p = 0 by determining the arrangement interval of the slit mask 116. A surface wave can be obtained.

7 is an experimental result comparing the magnitude of the fundamental frequency component and the magnitude of the double frequency component by analyzing the frequency of the laser excitation surface wave generated by changing the slit opening width (w) and the slit interval (d) of the slit mask.

Referring to FIG. 7, it is preferable to make the arrangement interval determination of the slit mask 116 to minimize the magnitude p of the double frequency component through the slit opening width / slit interval d. Furthermore, by generating a laser surface wave in which the magnitude p of the double frequency component becomes zero, the β value representing the nonlinear component of the inspected object D can be obtained.

According to the present invention, by using a non-contact non-linear laser surface wave to overcome the error caused by the contact transducer it is possible to enable the micro-damage diagnosis is less affected by the environment of the examiner.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Therefore, such modifications or variations will have to be belong to the claims of the present invention.

100: micro damage diagnosis device
110: surface wave excitation part
112: laser source
116: slit mask
120: signal analysis unit

Claims (8)

A surface wave excitation unit for injecting surface waves into the test object through the slit mask; And
A signal analyzer which analyzes a frequency by receiving a signal from the object under test;
Lt; / RTI >
The slit mask is a micro-damage diagnostic apparatus using a non-linear surface wave is formed so as to minimize the double frequency components contained in the surface wave.
The method of claim 1,
The ratio of the slit opening width and the slit spacing of the slit mask is determined to minimize the size of the double frequency components included in the surface wave, micro-damage diagnostic apparatus using a non-linear surface wave.
The method of claim 2,
Micro-damage diagnostic device using a non-linear surface wave, the magnitude of the double frequency component contained in the surface wave is zero.
The method of claim 1,
The surface wave excitation unit comprises a laser source for generating a line array laser beam (L) in non-contact with the test object, and the slit mask is disposed between the laser source and the test object, micro-damage diagnosis apparatus using a non-linear surface wave.
In the micro damage diagnosis method using a nonlinear laser surface wave through a slit mask,
The frequency of the nonlinear laser surface wave includes a fundamental frequency component of the nonlinear laser surface wave, a frequency component caused by nonlinearity of the subject, and a double frequency component included in the nonlinear laser surface wave, and includes a double frequency component included in the nonlinear laser surface wave. Minimized damage diagnosis method using nonlinear laser surface wave.
The method of claim 5,
The double frequency component of the nonlinear laser surface wave has a factor proportional to the fundamental frequency component and minimizes the factor.
The method according to claim 6,
The longitudinal wave of the nonlinear laser surface wave is represented by the following equation, and the proportional factor is determined to be minimized by the ratio of the slit opening width and the slit spacing of the slit mask.

Where A ₁ : magnitude of the fundamental frequency component
k: wavenumber of surface wave
ω: angular frequency
β: nonlinear parameter
b: phase difference
p: proportional factor
The method according to claim 6,
The longitudinal wave of the nonlinear laser surface wave is represented by the following equation, and the proportional factor is determined to be zero by the ratio of the slit opening width and the slit spacing of the slit mask.

Where A ₁ : magnitude of the fundamental frequency component
k: wavenumber of surface wave
ω: angular frequency
β: nonlinear parameter
b: phase difference
p: proportional factor

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