阅读说明：本技术 检查结构的方法以及检查系统 (Method for inspecting structure and inspection system ) 是由 孙勋 刘沛沛 张振豪 于 2019-07-03 设计创作，主要内容包括：在一种检查结构的方法中,接收由第一激光束从目标结构中生成的第一超声信号。通过向所述目标结构提供从第一激发单元生成的所述第一激光束来生成所述第一超声信号。接收由不同于第一激光束的第二激光束从所述目标结构生成的第二超声信号。通过向所述目标结构提供从第二激发单元生成的所述第二激光束来生成所述第二超声信号。接收第三超声信号,所述第三超声信号通过所述第一激光束和所述第二激光束从该目标结构生成。通过将所述第一激光束和所述第二激光束同时提供至所述目标结构来生成所述第三超声信号。基于第一超声频谱、第二超声频谱、和第三超声频谱来确定所述目标结构是否被损坏,所述第一超声频谱、所述第二超声频谱、和所述第三超声频谱是通过分别转换所述第一超声信号、所述第二超声信号、所述第三超声信号所获得的。(In a method of inspecting a structure, a first ultrasonic signal generated from a target structure by a first laser beam is received. Generating the first ultrasonic signal by providing the first laser beam generated from a first excitation unit to the target structure. A second ultrasonic signal generated from the target structure by a second laser beam different from the first laser beam is received. Generating the second ultrasonic signal by providing the second laser beam generated from a second excitation unit to the target structure. Receiving a third ultrasonic signal, the third ultrasonic signal generated from the target structure by the first laser beam and the second laser beam. Generating the third ultrasonic signal by simultaneously providing the first and second laser beams to the target structure. Determining whether the target structure is damaged based on a first ultrasonic spectrum, a second ultrasonic spectrum, and a third ultrasonic spectrum, the first ultrasonic spectrum, the second ultrasonic spectrum, and the third ultrasonic spectrum being obtained by converting the first ultrasonic signal, the second ultrasonic signal, and the third ultrasonic signal, respectively.)

1. A method of inspecting a structure, the method comprising:

receiving a first ultrasonic signal generated from a target structure by a first laser beam, the first ultrasonic signal being generated by providing the first laser beam generated from a first excitation unit to the target structure;

receiving a second ultrasonic signal generated from the target structure by a second laser beam, the second ultrasonic signal generated by providing the second laser beam generated from a second excitation unit to the target structure, the second excitation unit and the second laser beam being different from the first excitation unit and the first laser beam, respectively;

receiving a third ultrasonic signal generated from the target structure by the first and second laser beams, the third ultrasonic signal generated by simultaneously providing the first and second laser beams to the target structure; and

determining whether the target structure is damaged based on a first ultrasonic spectrum, a second ultrasonic spectrum, and a third ultrasonic spectrum obtained by converting the first ultrasonic signal, the second ultrasonic signal, and the third ultrasonic signal, respectively.

2. The method of claim 1, wherein the first laser beam has a first size and the second laser beam has a second size smaller than the first size.

3. The method of claim 2, wherein a narrowband input is generated by the first laser beam and a wideband input is generated by the second laser beam.

4. The method of claim 2, wherein at least one of the first size and the second size is changeable.

5. The method of claim 1, wherein determining whether the target structure is damaged comprises:

selecting a frequency range based on the first ultrasonic frequency spectrum and the second ultrasonic frequency spectrum;

calculating a first sideband peak count value in the selected frequency range, the first sideband peak count value representing the number of first threshold peak points that are greater than or equal to a threshold among all first peak points included in the second ultrasonic spectrum range;

calculating a second sideband peak count value in the selected frequency range, the second sideband peak count value representing a number of second threshold peak points that are greater than or equal to the threshold among all second peak points included in the third ultrasonic spectral range; and

determining whether a fatigue crack is included in the target structure by comparing the first sideband peak count value with the second sideband peak count value.

6. The method of claim 5, wherein it is determined that a fatigue crack is included in the target structure when a sideband peak count difference obtained by subtracting the first sideband peak count value from the second sideband peak count value is a positive value.

7. The method of claim 6, wherein a degree of damage to the target structure increases as the sideband peak count difference increases.

8. The method of claim 5, wherein the threshold is changeable.

9. An inspection system, comprising:

a first excitation unit configured to generate a first laser beam;

a second excitation unit configured to generate a second laser beam, the second excitation unit and the second laser beam being different from the first excitation unit and the first laser beam, respectively;

a sensing unit configured to receive a first ultrasonic signal generated from a target structure by the first laser beam, receive a second ultrasonic signal generated from the target structure by the second laser beam, and receive a third ultrasonic signal generated from the target structure by the first laser beam and the second laser beam, the first ultrasonic signal being generated by providing the first laser beam to the target structure, the second ultrasonic signal being generated by providing the second laser beam to the target structure, the third ultrasonic signal being generated by simultaneously providing the first laser beam and the second laser beam to the target structure; and

a control unit configured to determine whether the target structure is damaged based on a first ultrasonic spectrum, a second ultrasonic spectrum, and a third ultrasonic spectrum obtained by converting the first ultrasonic signal, the second ultrasonic signal, and the third ultrasonic signal, respectively.

10. The inspection system of claim 9, wherein the first laser beam has a first size and the second laser beam has a second size smaller than the first size.

11. The inspection system of claim 10 wherein a narrowband input is generated by the first laser beam and a broadband input is generated by the second laser beam.

12. The inspection system of claim 10, wherein at least one of the first size and the second size is changeable by the control unit.

13. The inspection system of claim 9, wherein the control unit is configured to select a frequency range based on the first ultrasonic spectrum and the second ultrasonic spectrum, calculate a first sideband peak count value within the selected frequency range, calculate a second sideband peak count value within the selected frequency range, and determine whether a fatigue crack is included in the target structure by comparing the first sideband peak count value to the second sideband peak count value;

wherein the first side band peak value represents the number of first threshold peak points that are greater than or equal to a threshold value among all first peak points included in the second ultrasonic spectrum range, and

wherein the second sideband peak value represents the number of second threshold peak points that are greater than or equal to the threshold among all second peak points included in the third ultrasonic spectral range.

14. The inspection system of claim 13, wherein the control unit is configured to determine that a fatigue crack is included in the target structure when a sideband peak count difference obtained by subtracting the first sideband peak count value from the second sideband peak count value is a positive value.

15. The inspection system of claim 14, wherein a degree of damage to the target structure increases as the sideband peak count difference increases.

16. The inspection system of claim 13, wherein the threshold is changeable by the control unit.

Technical Field

Exemplary embodiments relate generally to inspection techniques for structures, and more particularly, to a method of inspecting a structure and an inspection system performing the method.

Background

An inspection of the structure is performed to check or determine the security of the structure. If the structure is damaged, the safety of the structure may be degraded or degraded. Therefore, it is critical to rapidly detect the damage of the structure and take appropriate action for the damaged structure, and researchers are conducting various research projects on the technology of detecting the damage of the structure.

Korean patent No.10-0784582 relates to an apparatus and method for measuring structural damage using a piezoelectric device. Specifically, in korean patent No.10-0784582, one or more piezoelectric devices are attached to an appropriate side of a structure, a longitudinal elastic wave is generated by applying a stress through a stress applying unit, impedance in a specific frequency range is analyzed through an impedance analyzer, and thus damage of the structure is easily measured using the piezoelectric devices.

Disclosure of Invention

Some exemplary embodiments provide a method of inspecting a structure that effectively detects fatigue cracks in a target structure using nonlinear ultrasonic modulation with a dual laser system.

Some exemplary embodiments provide an inspection system that is effective in detecting fatigue cracks in a target structure using nonlinear ultrasonic modulation with a dual laser system.

According to an exemplary embodiment, in a method of detecting a structure, a first ultrasonic signal generated from a target structure by a first laser beam is received. Generating the first ultrasonic signal by providing the first laser beam generated from a first excitation unit to the target structure. A second ultrasonic signal generated from the target structure by a second laser beam is received. Generating the second ultrasonic signal by providing the second laser beam generated from a second excitation unit to the target structure. The second excitation unit and the second laser beam are different from the first excitation unit and the second laser beam, respectively. Receiving a third ultrasonic signal, the third ultrasonic signal generated from the target structure by the first laser beam and the second laser beam. Generating the third ultrasonic signal by simultaneously providing the first and second laser beams to the target structure. Determining whether the target structure is damaged based on a first ultrasonic spectrum, a second ultrasonic spectrum, and a third ultrasonic spectrum, the first ultrasonic spectrum, the second ultrasonic spectrum, and the third ultrasonic spectrum being obtained by converting the first ultrasonic signal, the second ultrasonic signal, and the third ultrasonic signal, respectively.

In some exemplary embodiments, the first laser beam may have a first size and the second laser beam may have a second size smaller than the first size.

In some exemplary embodiments, a narrow band input may be generated by the first laser beam and a wide band input may be generated by the second laser beam.

In some exemplary embodiments, at least one of the first size and the second size is changeable.

In some exemplary embodiments, when determining whether the target structure is damaged, a frequency range may be selected based on the first and second ultrasonic spectra. A first sideband peak count value within the selected frequency range may be calculated. The first sideband peak count value may represent the number of first threshold peak points greater than or equal to a threshold among all first peak points included in the second ultrasonic spectrum range. A second sideband peak count value within the selected frequency range may be calculated. The second sideband peak count value may represent the number of second threshold peak points greater than or equal to the threshold among all the second peak points included in the third ultrasonic frequency spectrum range. Whether a fatigue crack is included in the target structure may be determined by comparing the first sideband peak count value with the second sideband peak count value.

In some exemplary embodiments, when a sideband peak count difference obtained by subtracting the first sideband peak count value from the second sideband peak count value is a positive value, it may be determined that a fatigue crack is included in the target structure.

In some exemplary embodiments, the degree of damage to the target structure may increase as the sideband peak count difference increases.

In some exemplary embodiments, the threshold is changeable.

According to various exemplary embodiments, an inspection system includes a first excitation unit, a second excitation unit, a sensing unit, and a control unit. The first excitation unit generates a first laser beam. The second excitation unit generates a second laser beam. The second excitation unit and the second laser beam are different from the first excitation unit and the first laser beam, respectively. The sensing unit receives a first ultrasonic signal generated from a target structure by the first laser beam, receives a second ultrasonic signal generated from the target structure by the second laser beam, and receives a third ultrasonic signal generated from the target structure by the first and second laser beams. Generating the first ultrasonic signal by providing the first laser beam to the target structure. Generating the second ultrasonic signal by providing the second laser beam to the target structure. Generating the third ultrasonic signal by simultaneously providing the first and second laser beams to the target structure. The control unit determines whether the target structure is damaged based on a first ultrasonic spectrum, a second ultrasonic spectrum, and a third ultrasonic spectrum obtained by converting the first ultrasonic signal, the second ultrasonic signal, and the third ultrasonic signal, respectively.

In some exemplary embodiments, the first laser beam may have a first size, and the second laser beam may have a second size smaller than the first size.

In some exemplary embodiments, a narrow band input may be generated by the first laser beam and a wide band input may be generated by the second laser beam.

In some exemplary embodiments, at least one of the first size and the second size is changeable by the control unit.

In some exemplary embodiments, the control unit may select a spectral range based on the first and second ultrasonic spectra, may calculate a first sideband peak count value within the selected frequency range, may calculate a second sideband peak count value within the frequency range, and may determine whether a fatigue crack is included within the target structure by comparing the first sideband peak count value and the second sideband peak count value. The first sideband peak count value may represent the number of the first threshold peak points that are greater than or equal to a threshold among all the first peak points included in the second ultrasonic spectrum. The second sideband peak count value may represent the number of the second threshold peak points that are greater than or equal to a threshold value among all second peak points included in the third ultrasonic spectrum.

In some exemplary embodiments, the control unit may determine that a fatigue crack is included in the target structure when a side band peak count difference obtained by subtracting the first side band peak count value from the second side band peak count value is a positive value.

In some exemplary embodiments, the degree of damage to the target structure may increase as the sideband peak count difference increases.

In some exemplary embodiments, the threshold value is changeable by the control unit.

Therefore, in the method of inspecting a structure and the inspection system according to exemplary embodiments, two different laser beams may be generated using two excitation units to obtain NB responses and WB responses, and obtain WB + NB responses, and thus data collection time may be reduced. Further, damage or damage characteristics of the target structure can be effectively checked or diagnosed without the reference structure.

Drawings

FIG. 1 is a block diagram illustrating an inspection system according to exemplary embodiments.

Fig. 2 is a diagram illustrating an example of first and second laser beams provided to a target structure in an inspection system according to various exemplary embodiments.

Fig. 3A, 3B, 3C, 3D, 3E, and 3F are graphs showing response characteristics according to the size of each laser beam.

Fig. 4 is a flowchart illustrating a method of inspecting a structure according to exemplary embodiments.

Fig. 5 is a flowchart illustrating an example of determining whether a target structure is damaged, which is included in a method of inspecting a structure according to exemplary embodiments.

Fig. 6A, 6B, 7A, and 7B are diagrams for describing an operation of determining whether the target structure is damaged in fig. 5.

Fig. 8, 9, 10A, 10B, 11A, 11B, 11C, 12, 13A, 13B, and 14 are graphs for describing experimental results of a method of inspecting a structure based on various exemplary embodiments.

DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION

Example embodiments will be described more fully with reference to the accompanying drawings, in which embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like reference numerals refer to like elements throughout this application.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the inventive concept. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

It will be immediately apparent that when an element is referred to as being "connected" or "coupled" to another element, the element can be directly connected or coupled to the other element and intervening elements may be present. Conversely, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a similar manner (e.g., "between … …" versus "directly between … …", "adjacent" versus "directly adjacent", etc.).

The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of the inventive concepts. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises", "comprising", "includes" and/or "including", when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, and/or components.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the inventive concepts pertain. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

The above and other features of the innovative concepts will become more apparent from the detailed description of exemplary embodiments thereof given with reference to the attached drawings. The same reference numerals are used for the same elements in the drawings, and redundant description thereof is omitted.

FIG. 1 is a block diagram illustrating an inspection system according to exemplary embodiments. Fig. 2 is an example illustrating first and second laser beams provided to a target structure in an inspection system according to embodiments. Fig. 3A, 3B, 3C, 3D, 3E, and 3F are graphs showing response characteristics according to the size of each laser beam.

Referring to fig. 1, 2, 3A, 3B, 3C, 3D, 3E, and 3F, the inspection system 10 includes a first excitation unit 200, a second excitation unit 300, a sensing unit 400, and a control unit 500.

The first excitation unit 200 generates a first laser beam LS 1. The first excitation unit 200 may include a first pulse laser generator 210 and a first rotating mirror 220. The first laser beam LS1 generated from the first pulsed laser generator 210 may be provided to a target structure (or a target sample) 100 via the first turning mirror 220. The first excitation unit 200 may be referred to as a first laser light generation unit.

The second excitation unit 300 generates a second laser beam LS2 different from the first laser beam LS 1. The second firing cell 300 is implemented differently from (i.e., distinct from or separate from) the first firing cell 200. The second excitation unit 300 may include a second pulse laser generator 310 and a second rotating mirror 320. The second laser beam LS2 from the second pulsed laser generator 310 may be provided to the target structure 100 via the second rotating mirror 320. The second excitation unit 300 may be referred to as a second laser generating unit.

In some exemplary embodiments, the first laser beam LS1 and the second laser beam LS2 may have different sizes. For example, the first laser beam LS1 may have a first dimension and the second laser beam LS2 may have a second dimension that is less than the first dimension. As shown in fig. 2, when the first and second laser beams LS1, LS2 are arranged to be circular in plan (i.e., in plan view), the first dimension of the first laser beam LS1 may be represented by a first diameter R1 (or a first radius), and the second dimension of the second laser beam LS2 may be represented by a second diameter R2 (or a second radius) that is shorter than the first diameter R1. Although fig. 2 illustrates laser beams LS1 and LS2 as being circular in plan, exemplary embodiments are not limited thereto.

In some exemplary embodiments, a Narrow Band (NB) input may be generated by the first laser beam LS1 and a Wide Band (WB) input may be generated by the second laser beam LS 2. For example, fig. 3A, 3B, 3C, 3D, 3E, and 3F show normalized velocity in the time domain and normalized amplitude in the frequency domain when the radius of the laser beam is 0.5mm, 1mm, 2mm, 4mm, 8mm, and 16mm, respectively. As shown in fig. 3A, 3B, 3C, 3D, 3E, and 3F, as the size of the laser beam increases, the response frequency in the frequency domain may become lower and the width associated with the frequency response may become narrower. It can thus be verified that the narrow band input is generated by the first laser beam LS1 having a relatively large size.

A first ultrasonic (or ultrasonic) signal US1, a second ultrasonic signal US2, and a third ultrasonic signal US3 are generated from the target structure 100 by the first laser beam LS1 and the second laser beam LS 2. For example, when the first laser beam LS1 is provided to the target structure 100, the target structure 100 may generate the first ultrasonic signal US1 by thermal expansion. Similarly, when the second laser beam LS2 is provided to the target structure 100, the target structure 100 may generate the second ultrasonic signal US2 by thermal expansion. When the first laser beam LS1 and the second laser beam LS2 are provided to the target structure 100 substantially simultaneously or simultaneously concurrently, the target structure 100 may generate the third ultrasonic signal US3 by thermal expansion.

The sensing unit 400 receives the first ultrasonic signal US1, the second ultrasonic signal US2, and the third ultrasonic signal US3 generated from the target structure 100. For example, the first, second, and third ultrasonic signals US1, US2, and US3 may be generated at different points in time, and the sensing unit 400 may sequentially receive the first, second, and third ultrasonic signals US1, US2, and US 3.

The control unit 500 receives first data DAT1, second data DAT2, and third data DAT3 corresponding to the first ultrasonic signal US1, the second ultrasonic signal US2, and the third ultrasonic signal US3 from the sensing unit 400, respectively. The control unit 500 determines whether the target structure 100 is damaged based on the first, second, and third ultrasonic signals US1, US2, and US3 (e.g., based on the first, second, and third data DAT1, DAT2, and DAT 3). For example, the control unit 500 may determine that the target structure 100 is an intact (or undamaged) structure or a damaged structure.

The control unit 500 may generate a first control signal CON1, a second control signal CON2, and a third control signal CON3 for controlling the first firing cell 200, the second firing cell 300, and the sensing cell 400, respectively. The first firing unit 200, the second firing unit 300, and the sensing unit 400 may be synchronized to perform the above-described operations under the control of the control unit 500.

In some exemplary embodiments, at least one of said first dimension of said first laser beam LS1 and said second dimension of said second laser beam LS2 may be varied by a control unit 500 or by said control unit 500. For example, the control unit 500 may change at least one of the first size and the second size based on first and second control signals CON1 and CON 2.

Fig. 4 illustrates a flow diagram of a method of inspecting a structure, according to various exemplary embodiments.

Referring to fig. 1 and 4, in a method of inspecting a structure according to embodiments, the first ultrasonic signal US1 generated by the first laser beam LS1 from the target structure 100 is received (step S100). The first ultrasonic signal US1 is generated by providing the first laser beam LS1 to the target structure 100. Step S100 may be performed by the first excitation unit 200 and the sensing unit 400.

The second ultrasonic signal US2 generated from the target structure 100 by the laser beam LS2 is received (step S200). The second ultrasonic signal US2 is generated by providing the second laser beam LS2, which is different from the first laser beam LS1, to the target structure 100. Step S200 may be performed by the second firing cell 300 different from the first firing cell 200 and the sensing cell 400.

A third ultrasonic signal US3 generated from the target structure 100 by the first laser beam LS1 and the second laser beam LS2 is received (step S300). The third ultrasonic signal US3 is generated by simultaneously providing the first laser beam LS1 and the second laser beam LS2 to the target structure 100. Step S300 is performed by the first firing cell 200, the second firing cell 300, and the sensing cell 400.

In the method of inspecting a structure according to exemplary embodiments, two different laser beams LS1 and LS2 provided to the target structure 100 may be generated from two different excitation cells 200 and 300, respectively. In particular, the excitation cells 200 and 300 should be implemented separately and/or distinguished from each other such that two different laser beams LS1 and LS2 can be provided to the target structure 100 simultaneously, as described in step S300. However, although not shown, one laser beam may be divided into two laser beams LS1 and LS 2.

In some exemplary embodiments, the generation times of the first, second, and third ultrasonic signals US1, US2, and US3 may be different from each other. For example, the first ultrasonic signal US1 may be generated by the first laser beam LS1 at a first point in time, the second ultrasonic signal US2 may be generated by the second laser beam LS2 at a second point in time later than the first point in time, and the third ultrasonic signal US3 may be generated by the first and second laser beams LS1 and LS2 at a third point in time of the second point in time. The order of generation of the first, second, and third ultrasonic signals US1, US2, and US3 may be changed according to various exemplary embodiments.

It is determined whether the target structure 100 is damaged based on a first ultrasonic (or ultrasonic) spectrum, a second ultrasonic spectrum, and a third ultrasonic spectrum obtained by converting the first ultrasonic signal US1, the second ultrasonic signal US2, and the third ultrasonic signal US3, respectively (step S400). Step S400 is performed by the control unit 500.

In some exemplary embodiments, the first, second, and third ultrasonic frequency spectrums may be obtained by the control unit 500. In other exemplary embodiments, the first, second, and third ultrasonic spectra may be obtained by the sensing unit 400 and provided to the control unit 500 in the form of first, second, and third data DAT1, DAT2, and DAT 3.

FIG. 5 is a flow diagram illustrating an example of determining whether a target structure is damaged included in a method of inspecting a structure according to embodiments. Fig. 6A, 6B, 7A, and 7B are diagrams describing an operation of determining whether the target structure is damaged in fig. 5.

Referring to fig. 1, 4, 5, 6A, 6B, 7A, and 7B, when determining whether the target structure is damaged (step S400), a frequency range may be selected based on a first ultrasonic spectrum obtained by converting the first ultrasonic signal US1 and a second ultrasonic spectrum obtained by converting the second ultrasonic signal US2 (step S410).

For example, by performing a fourier transform on said first ultrasonic signal US1 as a signal in the time domain, and thus the first ultrasonic spectrum shown in fig. 6A can be obtained. As described with reference to fig. 2, 3A, 3B, 3C, 3D, 3E, and 3F, since the first laser beam LS1 has a relatively large size, a first ultrasonic frequency spectrum obtained by converting the first ultrasonic signal US1 generated by the first laser beam LS1 may be formed in a relatively narrow band at a low frequency, and thus a lower limit of the frequency range (i.e., the lower limit LL of the frequency range FRNG in fig. 6B) may be set based on the first ultrasonic frequency spectrum (i.e., a narrow-band response or NB response).

Further, a fourier transform may be performed on the second ultrasonic signal US2 as a signal in the time domain, and thus the second ultrasonic spectrum shown in fig. 6B may be obtained. Since the second laser beam LS2 has a relatively small size, the second ultrasonic spectrum obtained by converting the second ultrasonic signal US2 generated by the second laser beam LS2 can be formed in a relatively wide band, and thus the upper limit of the frequency range (i.e., the upper limit UL of the spectral range FRNG in fig. 6B) can be set based on the second ultrasonic spectrum (i.e., the wide band response or WB response). The frequency range FRNG may be selected based on the lower limit LL and the upper limit UL.

Further, as will be described with reference to fig. 7A and 7B, a fourier transform may be performed on the third ultrasonic signal US3 as a signal in the time domain, and thus a third ultrasonic spectrum may be obtained.

A first Sideband Peak Count (SPC) value may be calculated within the selected frequency range (step S420). The first sideband peak count value may indicate a number of first threshold peak points greater than or equal to a threshold among all first peak points included in the second ultrasonic spectrum.

For example, a threshold T may be set, as shown in FIG. 6B. For example, the threshold T may be greater than or equal to zero and less than or equal to one. In the selected frequency range FRNG, the first sideband peak count value may be calculated, the first sideband peak count value indicating the number of the first threshold peak points that are greater than or equal to the threshold T among all the first peak points included in the second spectrum. For example, the first sideband peak count value may satisfy equation 1:

[ formula 1]

In equation 1, spcwb (T) represents the first sideband peak count value for the threshold T, NTWB represents the number of all first peak points included in the second ultrasound spectrum within the selected frequency range FRNG, and npwb (T) represents the number of the first threshold peak points included in the second ultrasound spectrum within the selected frequency range FRNG. The second ultrasonic spectrum may be used as a reference for determining whether the target structure 100 is damaged.

In some exemplary embodiments, the threshold T may be changed by the control unit 500 or changeable by the control unit 500.

A second sideband peak count value within the selected frequency range may be calculated (step S430). The second sideband peak count value may be indicative of a number of second threshold peak points greater than or equal to the threshold among all second peak points included in the third ultrasonic spectrum.

For example, using the first sideband peak count value, the second sideband peak count value may satisfy equation 2.

[ formula 2]

In equation 2, SPCWB + NB (T) represents the second sideband peak value for the threshold T, NTWB + NB represents the number of all the second peak points included in the third ultrasound spectrum within the selected frequency range FRNG (e.g., WB + NB response), and NPWB + NB (T) represents the number of all the second threshold peak points included in the third ultrasound spectrum relative to the threshold T within the selected frequency range FRNG.

Whether a fatigue crack is included within the target structure 100 may be determined by comparing the first sideband peak count value with the second sideband peak count value (step S440). For example, equation 3 may be satisfied by subtracting the obtained Sideband Peak Count Difference (SPCD) value from the second sideband peak count value.

[ formula 3]

SPCD(T)＝SPC_WB+NB(T)-SPC_WB(T)

In equation 3, spcd (T) represents the sideband peak count difference for a threshold T. Whether a fatigue crack is present in the target structure 100 may be determined based on the difference in the sideband peak count differences.

For example, fig. 7A illustrates an example of the third ultrasonic spectrum obtained when the target structure 100 does not include fatigue cracks (e.g., when the target structure 100 is an intact or undamaged structure). In this case, the third ultrasonic spectrum obtained by converting the third ultrasonic signal US3 generated by the first and second laser beams LS1 and LS2 is substantially the same as the sum of the first and second ultrasonic spectra. In the example of fig. 6B and 7A, the number of all first peak points included in the second ultrasound spectrum in the selected frequency range FRNG may be substantially equal to the number of all second peak points included in the third ultrasound spectrum in the selected frequency range FRNG (e.g., NTWB + NB), the number of the first threshold peak points included in the second ultrasound spectrum with respect to the threshold T in the selected frequency range FRNG and the number of the second threshold peak points included in the third ultrasound spectrum with respect to the threshold T in the selected frequency range FRNG may be five (e.g., NPWB + NB (T) ═ 5), and thus the sideband peak count difference may be zero.

For another example, FIG. 7B illustrates an example of the third frequency spectrum obtained when a fatigue crack is included within the target structure 100 (e.g., when the target structure 100 is a damaged structure). In this case, the third ultrasonic spectrum obtained by converting the third ultrasonic signal US3 generated by the first and second laser beams LS1 and LS2 is different from the sum of the first and second ultrasonic spectra. For example, non-linear modulation may occur between the wideband response and the narrowband response, and thus there may be more sideband peaks or increased sideband energy within the selected frequency band. In the example of fig. 6B and 7B, the number of the first threshold peak points included in the second ultrasonic spectrum with respect to the threshold T in the selected frequency range FRNG may be five (e.g., NPWB (T) ═ 5), the number of the second threshold peak points included in the third ultrasonic spectrum with respect to the threshold T in the selected frequency range FRNG may be twelve (e.g., NPWB + nb (T) ═ 12), and thus the sideband peak count difference may be a positive value.

In other words, when the sideband peak count difference obtained by subtracting the first sideband peak count value from the second sideband peak count value is a positive value, it may be determined that the target structure 100 includes a fatigue crack.

In some exemplary embodiments, as the sideband peak count difference increases, the damage procedure of the target structure 100 may increase.

In some exemplary embodiments, the Maximum Sideband Peak Count Difference (MSPCD) value may satisfy equation 4.

[ formula 4]

MSPCD＝|max(SPCD(T))|

Since the energy of the crack causing the non-linear modulation (e.g., the sideband) is much less than the energy of the linear response, the maximum sideband peak count difference can often occur at a relatively small threshold T.

In the method of inspecting a structure according to exemplary embodiments, two different laser beams LS1 and LS2 may be generated using two different excitation units 200 and 300 (e.g., a dual laser system) to obtain an NB response and a WB response and to obtain a WB + NB response. Thus, the broadband nature of the pulsed laser input may increase the boundary conditions for non-linear modulation generation and may reduce data collection time compared to scanning (sweep) of two single input frequencies of the same frequency band. Further, an inspection system (e.g., a dual laser ultrasonic inspection system) according to various exemplary embodiments may extract damage features without relying on any baseline data obtained from the original conditions of the target structure, as compared to including a single excitation unit or using a single laser beam. In other words, without the reference structure, the damage or the damage characteristic of the target structure can be effectively checked or diagnosed.

Fig. 8, 9, 10A, 10B, 10C, 11A, 11B, 11C, 12, 13A, 13B, and 14 are graphs describing experimental results based on a method of inspecting a structure according to various exemplary embodiments.

Referring to fig. 8 and 9, 6 samples of 3mm thick I-shaped aluminum sheet made of aluminum alloy were prepared to evaluate the performance of each exemplary embodiment. 4 aluminum plates with cracks in the middle were prepared and named first damaged sample AD1, second damaged sample AD2, third damaged sample AD3, and fourth damaged sample AD 4. As shown in fig. 8, a notch was introduced in the middle of each damaged sample, and a fatigue crack was initiated from this notch. All geometric information for the damaged sample is shown in fig. 8. Fig. 9 is a microscope picture showing cracks of the first to fourth damaged samples AD1, AD2, AD3, and AD 4. Although not shown in fig. 8 and 9, two aluminum plates without cracks were prepared and designated as a first intact sample AI1 and a second intact sample AI 2.

Further, positions about 60mm to the left and right from the sensing point are set as irradiation points of the first laser beam LS1 and the second laser beam LS2, respectively. The first excitation unit 200 for generating the first laser beam LS1 is used to excite an NB input with a pulse energy of about 20mJ and a beam radius of about 10 mm. The second excitation unit 300 for generating the second laser beam LS2 is used to excite an NB input with a pulse energy of about 15mJ and a beam radius of about 1 mm.

Referring to fig. 10A, 10B, and 10C, the method of inspecting a structure according to various exemplary embodiments was performed on the first sound sample AI1, and thus an NB response, a WB response, and a WB + NB response were obtained. Fig. 10A shows a first ultrasonic spectrum (e.g., the NB response) obtained by transforming the first ultrasonic signal US1 generated by the first laser beam LS 1. Fig. 10B shows a second ultrasonic spectrum (e.g., the WB response) obtained by converting the second ultrasonic signal US2 generated by the second laser beam LS 2. Fig. 10C shows a third ultrasonic spectrum (e.g., the WB + NB response) of the third ultrasonic signal US3 or generated by switching the first and second laser beams LS1 and LS 2.

Referring to fig. 11A, 11B, and 11C, a method of inspecting a structure according to various exemplary embodiments was performed on the first damaged sample AD1, and thus an NB response, a WB response, and a WB + NB response were obtained. Fig. 11A shows a first ultrasonic spectrum (e.g., the NB response) obtained by transforming the first ultrasonic signal US1 generated by the first laser beam LS 1. Fig. 11B shows a second ultrasonic spectrum (e.g., the WB response) obtained by converting the second ultrasonic signal US2 generated by the second laser beam LS 2. Fig. 11C shows a third ultrasonic spectrum (e.g., the WB + NB response) of the third ultrasonic signal US3 or generated by switching the first and second laser beams LS1 and LS 2.

Although not shown in fig. 10A, 10B, 10C, 11A, 11B, 11C, NB responses, WB responses, and WB + NB responses were obtained for each of the second intact sample AI2, and the second through fourth compromised samples AD2, AD3, and AD 4.

Referring to fig. 12, correlation coefficients between WB responses and WB + NB responses in each of intact samples AI1 and AI2 and compromised samples AD1, AD2, AD3, and AD4 are shown.

Referring to fig. 13A, the variation of the sideband peak count value and the variation of the sideband peak count difference value according to the variation of the threshold value T in the first intact sample AI1 are shown. Referring to fig. 13B, there are shown changes in the sideband peak count value and changes in the sideband peak count difference value according to changes in the threshold value T in the first damaged sample AD 1. In the SPC curves of fig. 13A and 13B, the solid line represents the sideband peak count value obtained from the WB response, and the dotted line represents the sideband peak count value obtained from the WB + NB.

As shown in fig. 13A, in the first intact sample AI1, there was little difference between the sideband peak count values obtained from the WB responses and the sideband peak count values obtained from the WB + NB responses, and the maximum sideband peak count difference was about 0.0136. However, as shown in fig. 13B, in the first damaged sample AD1, the difference between the sideband peak count value obtained from the WB response and the sideband peak count value obtained from the WB + NB response was relatively large, and the maximum sideband peak count difference was about 0.0656.

Although not shown in fig. 13A and 13B, the peak count value and the sideband peak count difference were obtained for each of the second intact sample AI2, and second through fourth damaged samples AD2, AD3, and AD 4.

Referring to fig. 14, the maximum sideband peak count difference is shown for each of intact samples AI1 and AI2, and compromised samples AD1, AD2, AD3, and AD 4. It can be shown that the maximum sideband peak count difference for each of damaged samples AD1, AD2, AD3, and AD4 is always greater than this value for each of intact samples AI1, AI2, and thus it can be verified that damage to the structure can be effectively determined by obtaining the sideband peak count difference, according to various exemplary embodiments.

The above-described embodiments may be applied to various safety inspection or diagnosis systems and/or measurement systems for maintenance that evaluate the existing state of various target structures, such as buildings, samples, and specimens, and infrastructure such as bridges, large-scale facilities, and underground facilities.

The foregoing is illustrative of various exemplary embodiments and is not to be construed as limiting thereof. Although a few exemplary embodiments have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the teachings and advantages of the novel concepts. Accordingly, all such modifications are intended to be included within the scope of the present inventive concept as defined in the claims. Therefore, it is to be understood that the foregoing is illustrative of exemplary embodiments and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed exemplary embodiments are intended to be included within the scope of the appended claims.