KR102353635B1 - Structure health monitoring device and method using digital image correlation technique - Google Patents

Abstract

The present invention relates to an apparatus and method for monitoring structure health using a DIC technique, wherein the apparatus provides an external stimulus to a specific structure and acquires an image of the specific structure at each time point before and after the external stimulus is provided Acquisition unit, a dynamic characteristic calculator for calculating dynamic characteristics including natural frequency, damping ratio, and mode shape for the specific structure based on the image, and by detecting a change in the dynamic characteristics It includes a health monitoring unit for detecting damage to the specific structure.

Description

Device and method for monitoring structural health using DIC technique

The present invention relates to a structure health monitoring technology, and more particularly, to a structure health monitoring apparatus and method using the DIC technique for monitoring the health of the structure by photographing the vibration of the structure with a camera and observing the change in the vibration characteristics of the structure. it's about

A thermal protection system (TPS) is a system necessary for supersonic aircraft and spacecraft to withstand aerodynamic heating and acoustic loads during supersonic flight. The TPS panel acts as a shielding cover for the vehicle fuselage and can withstand impact damage from debris. Various types of TPS, including metallic TPS, multilayer TPS, integrated TPS, and bioinspired TPS, have been proposed and tested through thermal and thermodynamic performance tests.

Damage to the TPS panel can expose the vehicle's fuselage to more extreme heat, pressure and shock loads. Therefore, TPS panels must be in good condition prior to launch, as they play an important role in protecting the vehicle's structure, subsystems and even humans. Monitoring the structural response and damage status of TPS panels can be very important in the design and maintenance process.

Nevertheless, it is difficult and expensive to inspect TPS panels by traditional methods, so the development of new and simple inspection methods is required.

Korea Patent No. 10-0553124 (2006.02.10)

An embodiment of the present invention is to provide a structure health monitoring apparatus and method using the DIC technique for monitoring the structure health by photographing the vibration of the structure with a camera and observing changes in the vibration characteristics of the structure.

An embodiment of the present invention is to provide an apparatus and method for monitoring the health of a structure using the DIC technique, which can secure the safety of workers by non-destructively inspecting the structure.

Among the embodiments, the structure health monitoring apparatus using the DIC technique provides an external stimulus for a specific structure, and a structure image acquisition unit that acquires an image of the specific structure at each time point before and after the external stimulus is provided; A dynamic characteristic calculator for calculating dynamic characteristics including a natural frequency, a damping ratio, and a mode shape for the specific structure based on the basis, and damage to the specific structure by detecting a change in the dynamic characteristics It includes a health monitoring unit that detects

The specific structure is a metal TPS panel (metallic TPS panel), a front plate forming a rectangular plate-like structure, a rear plate spaced apart from the front plate at a predetermined distance and disposed in parallel, and disposed between the front and rear plates an insulation material, a plurality of bolted joints for coupling between the front and rear plates, spacers coupled to each of the plurality of bolted joints to form the gap, and the spacer It may include washers (washers) coupled to both ends of the.

The structure image acquisition unit may provide a physical contact by an impact hammer as the external stimulus to the specific structure vertically supported in a free boundary condition by at least two fine nylon cords.

The structure image acquisition unit uses at least two high-speed cameras having a resolution of 1024 * 1024 pixels and a focal length of 50 mm to respond to the external stimulus according to a shooting option set as a combination of frame rate, shutter speed, and aperture size As a result, the vibration of the specific structure can be measured.

The dynamic characteristic calculator may perform a preprocessing operation of magnifying the image using a phase-based magnification method to which magnification parameters related to frame rate, bandwidth, and coefficient are applied.

The dynamic characteristic calculator may generate a predefined deformation response of the optical object (DOF) as a result of applying a digital image correlation (DIC) technique to the image as an input.

The dynamic characteristic calculator may calculate spectral data for the strain response using an FFT algorithm and calculate the natural frequency and the attenuation ratio from the spectral data.

The health monitoring unit may detect the damage based on changes in the natural frequency and the damping ratio, or detect the damage based on a Modal Assurance Criterion (MAC) value related to the similarity between the mode shapes.

In embodiments, the method for monitoring structure health using the DIC technique includes: acquiring an image of the specific structure at each time point before and after providing the external stimulus while providing an external stimulus to the specific structure; based on the image Calculating dynamic characteristics including natural frequency, damping ratio, and mode shape for a specific structure and detecting damage to the specific structure by detecting a change in the dynamic characteristics include

The disclosed technology may have the following effects. However, this does not mean that a specific embodiment should include all of the following effects or only the following effects, so the scope of the disclosed technology should not be understood as being limited thereby.

The apparatus and method for monitoring structure health using the DIC technique according to an embodiment of the present invention may monitor the health of the structure by photographing the vibration of the structure with a camera and observing changes in the vibration characteristics of the structure.

The apparatus and method for monitoring structure health using the DIC technique according to an embodiment of the present invention can secure the safety of workers by non-destructively inspecting the structure.

1 is a view for explaining a structure health monitoring system according to the present invention.
FIG. 2 is a view for explaining a functional configuration of the structure health monitoring device of FIG. 1 .
3 is a flowchart illustrating a DIC-based structure health monitoring process according to the present invention.
4 is a view for explaining an embodiment of a specific structure according to the present invention.
5 is a view for explaining a test structure according to the present invention.
6 is a view for explaining the experimental conditions of the dynamic test according to the present invention.
7 is a view for explaining the test structure and the numbering system according to the present invention.
8 is a view for explaining the response and spectral data of the load transfer plate near the edge of the test structure according to the present invention.
9 is a view for explaining the natural frequency and mode shape of the load transfer plate according to the present invention.
10 is a view for explaining a combination of a motion magnification and a 3DPT method to obtain an enlarged displacement of a TPS panel.
11 is a view for explaining a damage detection method using the DIC and MAC methods according to the present invention.

Since the description of the present invention is merely an embodiment for structural or functional description, the scope of the present invention should not be construed as being limited by the embodiment described in the text. That is, since the embodiment may have various changes and may have various forms, it should be understood that the scope of the present invention includes equivalents capable of realizing the technical idea. In addition, since the object or effect presented in the present invention does not mean that a specific embodiment should include all of them or only such effects, it should not be understood that the scope of the present invention is limited thereby.

On the other hand, the meaning of the terms described in the present application should be understood as follows.

Terms such as “first” and “second” are for distinguishing one component from another, and the scope of rights should not be limited by these terms. For example, a first component may be termed a second component, and similarly, a second component may also be termed a first component.

When a component is referred to as being “connected to” another component, it may be directly connected to the other component, but it should be understood that other components may exist in between. On the other hand, when it is mentioned that a certain element is "directly connected" to another element, it should be understood that the other element does not exist in the middle. Meanwhile, other expressions describing the relationship between elements, that is, "between" and "between" or "neighboring to" and "directly adjacent to", etc., should be interpreted similarly.

The singular expression is to be understood as including the plural expression unless the context clearly dictates otherwise, and terms such as "comprises" or "have" refer to the embodied feature, number, step, action, component, part or these It is intended to indicate that a combination exists, and it should be understood that it does not preclude the possibility of the existence or addition of one or more other features or numbers, steps, operations, components, parts, or combinations thereof.

In each step, identification numbers (eg, a, b, c, etc.) are used for convenience of description, and identification numbers do not describe the order of each step, and each step clearly indicates a specific order in context. Unless otherwise specified, it may occur in a different order from the specified order. That is, each step may occur in the same order as specified, may be performed substantially simultaneously, or may be performed in the reverse order.

The present invention can be embodied as computer-readable codes on a computer-readable recording medium, and the computer-readable recording medium includes all types of recording devices in which data readable by a computer system is stored. . Examples of the computer-readable recording medium include ROM, RAM, CD-ROM, magnetic tape, floppy disk, optical data storage device, and the like. In addition, the computer-readable recording medium is distributed in a computer system connected to a network, so that the computer-readable code can be stored and executed in a distributed manner.

All terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs, unless otherwise defined. Terms defined in general used in the dictionary should be interpreted as having the meaning consistent with the context of the related art, and cannot be interpreted as having an ideal or excessively formal meaning unless explicitly defined in the present application.

1 is a view for explaining a structure health monitoring system according to the present invention.

Referring to FIG. 1 , the structure health monitoring system 100 may include a vibration test system 110 , a structure health monitoring device 130 , and a database 150 .

The vibration test system 110 may correspond to a system for performing a vibration test on a structure, and may include an impact hammer 111 , a specific structure 113 , and a high-speed camera 115 . The impact hammer 111 may correspond to a hammer that applies a certain impact to the specific structure 113 , the specific structure 113 may correspond to a structure for measuring soundness, and the high-speed camera 115 is a structure according to vibration It may correspond to a camera that takes an image of In addition, the vibration test system 110 may operate by being connected to the structure health monitoring device 130 by wire or wirelessly.

The impact hammer 111 may be implemented in various structures, shapes, and materials according to the vibration test and characteristics of the structure. The vibration test system 110 may be implemented by including at least two high-speed cameras 115 , and the high-speed camera 115 includes a separate trigger device and a separate trigger device to initiate an image capturing operation in accordance with the operation of the impact hammer 111 . It can be connected and operated. The trigger device may provide a trigger signal to the high-speed camera 115 according to the movement of the impact hammer 111 .

In one embodiment, the specific structure 113 is a metallic TPS panel (metallic TPS panel). An insulation material disposed between the plurality of bolted joints for coupling between the front and rear plates, and a spacer coupled to each of the plurality of bolted joints to form a predetermined interval It may be configured to include washers (Washers) coupled to both ends of the and spacers.

In Figure 4, (a) is a side view (Side view) of the metal TPS panel, Figure (b) is a top view (Top view) of the metal TPS panel, Figure (c) is a three-dimensional model of the metal TPS panel, Figure (d) is a bolted joint, and Figure (e) is a washer.

The structure health monitoring device 130 may be implemented as a server corresponding to a computer or program capable of monitoring the health of the structure by photographing the vibration of the structure with a camera and observing changes in the vibration characteristics of the structure. In addition, the structure health monitoring apparatus 130 may be connected to the user terminal through a wired network or a wireless network such as Bluetooth or WiFi, and may communicate with the user terminal through a wired or wireless network. Here, the user terminal is various computing devices operated by an individual and may be implemented as a smartphone, a notebook computer, or a computer, but is not limited thereto, and may be implemented in various devices such as a tablet PC.

In addition, the structure health monitoring apparatus 130 may store various information required in the structure health monitoring process in conjunction with the database 150 . Meanwhile, the structure health monitoring apparatus 130 may be implemented by including the database 150 therein, unlike FIG. 1 . The structure health monitoring apparatus 130 may be implemented including a processor, a memory, a user input/output unit, and a network input/output unit as a basic system configuration, and a detailed description thereof will be omitted.

The database 150 may store various pieces of information necessary for the structure health monitoring apparatus 130 in a DIC-based structure health monitoring process. For example, the database 150 may store vibration data of a structure collected from the vibration test system 110, and may store information on dynamic characteristics calculated through analysis of vibration data, but is not necessarily limited thereto, Information collected or processed in various forms during the DIC-based structural health monitoring process can be stored.

FIG. 2 is a view for explaining a functional configuration of the structure health monitoring device of FIG. 1 .

Referring to FIG. 2 , the structure health monitoring apparatus 130 may include a structure image obtaining unit 210 , a dynamic characteristic calculating unit 230 , a health monitoring unit 250 , and a control unit 270 .

The structure image acquisition unit 210 may acquire an image of the specific structure at each time point before and after the provision of the external stimulus while providing the external stimulus with respect to the specific structure 113 . To this end, the structure image acquisition unit 210 may operate in conjunction with the vibration test system 110 , and may be independently connected to the high-speed camera 115 to acquire an image regarding a specific structure 113 .

In one embodiment, the structure image acquisition unit 210 externally applies a physical contact by an impact hammer 111 to a specific structure 113 supported vertically in a free boundary condition by at least two fine nylon cords. It can be provided as a stimulus. To this end, the structure image acquisition unit 210 may operate in conjunction with the vibration test system 110 . The specific structure 113 may be implemented as a plate-like structure of a square shape, and may include holes connected to two fine nylon cords, respectively, at the corners of the square shape, and the fine nylon cords may be connected to each hole to form a specific structure ( 113) can be supported by being spaced apart from the ground by a predetermined distance. On the other hand, the impact hammer 111 may be operated by a driving device, and may provide an impact for generating vibration to the specific structure 113 .

In one embodiment, the structure image acquisition unit 210 is set to a combination of frame rate, shutter speed and aperture size using at least two high-speed cameras 115 having a resolution of 1024 * 1024 pixels and a focal length of 50 mm. Vibration of a specific structure can be measured as a response to an external stimulus depending on the imaging option being used. For example, the structure image acquisition unit 210 uses two high-speed cameras 115 according to the shooting options of a frame rate of 3000 frames per second, a shutter speed of 1/6000 second, and an aperture size of f/11 to capture the image of a specific structure. Vibration can be measured.

On the other hand, the structure image acquisition unit 210 may operate in conjunction with the vibration test system 110, provide a shooting option as parameter information to the vibration test system 110, and receive information measured through the shooting option. can

The dynamic characteristic calculator 230 may calculate dynamic characteristics including a natural frequency, a damping ratio, and a mode shape of a specific structure based on the image. Dynamic properties are inherent properties of a structure and may vary depending on the shape and support state of the structure. If the dynamic characteristics of the structure can be obtained, the shape of an object that changes as the frequency changes (Mode shape) can be known in advance through simulation, and the maximum deformation point or the point that never moves (Node) can be known. It is possible to find out the dangerous frequency band and change the design measures so that the frequency and the operating speed do not match.

In an embodiment, the dynamic characteristic calculator 230 may perform a preprocessing operation of magnifying an image using a phase-based magnification method to which magnification parameters related to frame rate, bandwidth, and coefficient are applied. For example, the dynamic characteristic calculator 230 may perform an image enlargement operation based on a frame rate of 3000 frames per second, a triple magnification factor, and an magnification bandwidth of 355 to 365 Hz.

In an embodiment, the dynamic characteristic calculator 230 may generate a predefined deformation response of the optical object DOF as a result of applying a digital image correlation (DIC) technique to an image as an input. Here, the optical object DOF may be set on the surface of the specific structure 113 , and the plurality of optical objects may form a predetermined pattern. For example, a predefined deformation response of an optical object can be generated as an output of DIC-based ARAMIS software.

In an embodiment, the dynamic characteristic calculator 230 may calculate spectral data for the strain response using an FFT algorithm, and calculate a natural frequency and an attenuation ratio from the spectral data. For example, spectral data for the strain response can be generated as the output of a MATLAB program using an FFT algorithm.

The health monitoring unit 250 may detect damage to a specific structure by detecting a change in dynamic characteristics. That is, the health monitoring unit 250 may detect whether the structure is damaged as a result of observing the change in dynamic characteristics caused by an external stimulus such as an impact applied to the structure.

In an embodiment, the health monitoring unit 250 may detect damage based on a change in natural frequency and damping ratio or may detect damage based on a Modal Assurance Criterion (MAC) value related to similarity between mode shapes.

The control unit 270 controls the overall operation of the structure health monitoring device 130 , and manages the control flow or data flow between the structure image acquisition unit 210 , the dynamic characteristic calculation unit 230 , and the soundness monitoring unit 250 . have.

3 is a flowchart illustrating a DIC-based structure health monitoring process according to the present invention.

Referring to FIG. 3 , the structure health monitoring apparatus 130 may obtain an image of a specific structure at each time point before and after the provision of the external stimulus while providing an external stimulus to the specific structure through the structure image acquisition unit 210 . There is (step S310). The structure health monitoring apparatus 130 calculates dynamic characteristics including a natural frequency, a damping ratio, and a mode shape for a specific structure based on the image through the dynamic characteristic calculation unit 230 . It can be (step S330). The structure health monitoring apparatus 130 may detect damage to a specific structure by detecting a change in dynamic characteristics through the health monitoring unit 250 (step S350).

Hereinafter, a DIC-based structure health monitoring method according to the present invention will be described in more detail with reference to FIGS. 4 to 11 .

3. Material and method

3.1 Material preparation

The metal TPS panel used in the experiment was made of Inconel 625 for the front plate, titanium Ti-6Al-4V alloy for the back structure, bolted joints made of stainless steel S304, spacers and washers, and thermal fiber insulation material. (thermal fibrous insulation material). The nominal dimensions of the TPS panel are 170*170*29.54 mm ³ . The front plate with projections on the four corners is called the front load-carrying plate. The size of this front load transfer plate is 240*240 mm ² . The protrusion can create an overlapping area between adjacent TPS panels to prevent the ingress of hot air. Table 1 below shows the size and weight of the components in the TPS panel.

Components Load-carrying plate back plate Fiber insulation layer Fully-assembled TPS panel Size (mm) 240*240 170*170 170*170 Weight (g) 1229 137 72 1547

In this study, we collect three experimental data cases including testing of a single load-carrying plate (Inconel plate), a metal TPS panel without thermal fiber insulation material, and a fully assembled metal TPS panel. First, a single load-carrying plate was determined to be tested as it corresponds to one of the most important components of a TPS panel, which dominates the weight of the TPS panel. That is, the relationship between the dynamic characteristics of the corresponding plate and the dynamic characteristics of the TPS panel is investigated. The second case is for an assembled TPS panel without a fibrous insulating material called the backbone of the TPS panel. This is to show how much damping is included without adding fibrous insulating material (non-structural mass). In the third case, in the case of a fully assembled TPS panel, it is divided into four test sub-cases to compare the dynamic characteristics of the TPS panel under healthy and damaged conditions of a specific bolted joint.

A photograph of the test TPS panel is shown in Fig. 5 (a). To create a healthy state, you can use a torque wrench with a torque value of 4Nm for the bolted joint as shown in Figure (b). The bolt type used for the TPS panel is M5 grade 6.8, and it may be desirable to tighten with a torque value of less than 4.56 Nm. Damage can be created by loosening the bolted joint as shown in Figure (c). The three damage states of bolted joints can be defined as loose bolts at corner 1, loose bolts at corners 1 and 3, and loose bolts at corners 1 and 4. Details of the experimental case may be determined as shown in Table 2 below.

Case Load-carrying plate Assembled TPS panel without fibrous insulation material Assembled TPS panel with fibrous insultation material State single plate healthy condition healthy condition Damage at specific corners(#) #One #1 and #3 #1 and #4

3.2 Experimental setup

6 shows an experimental setup of a dynamic test using a high-speed 3-dimensional point tracking (3DPT) method and a roving method using an accelerometer. Two high-speed cameras with a resolution of 1024*1024 pixels and a focal length of 50mm can capture the vibrations of the TPS panel. In this measurement, the frame rate can be chosen at 3000 frames per second, the shutter speed set at 1/6000 second, and the aperture size selected at f/11. An optical target may be attached to the front surface of the test structure in such a way that the overall shape formed by the attached target may represent the shape of the test structure. Two high-speed cameras can be installed on a tripod stand with a built-in spirit level to ensure leveling. The 3DPT method allows the camera system to be calibrated to identify the distance and angle between the two cameras in the step before starting the measurement. The calibration deviation is measured as 0.022 pixels, which may correspond to 0.006 mm for a measurand of ^{295*300*300 mm 3 .} The spatial resolution can be calculated as 0.29 mm/pixel. The sensitivity of the out-of-plane measurement is about 1/30 pixels, which in this test could correspond to about 10 µm. The noise floor can be measured by taking 100 images in a stationary state. An intermediate filter can be applied to reduce most noise in the measurement system to less than 10 μm.

The camera system may be positioned to face the front of the test structure to which the optical target is attached. The test structure can be suspended vertically using two fine nylon wires to create a free boundary condition. As shown in FIG. 6 , a photoelectric sensor may be disposed immediately behind the test structure so that an electrical signal for triggering the camera is transmitted when an impact hammer passes through the optical transmitter of the photoelectric sensor. The impact point may be selected in the protruding area of the front load bearing plate as shown in FIG. 6 . If the impact point is on the back side of the structure (titanium plate), the fibrous insulating material of the TPS structure may absorb the impact energy and the test structure may not be well excitation. At this time, the camera captures an image of the test structure. This can include several images before impact and many images during impact. The captured image can then be processed with DIC-based ARAMIS® software to acquire out-of-plane deformation. Then, the measured strain from the 3DPT method can be transferred from the time domain to the frequency domain to obtain the ODS FRF function of the test structure. Signal noise can be reduced by using the spectrum averaging technique and the Savitzky-Golay smoothing method. A reference response of 1 mm strain can be used in this study.

At the same time, an accelerometer can be used to measure the response of the test structure. The roving impact hammer method can be used to acquire the FRF at predefined points on the test structure. The accelerometer was attached to a fixed point near the corner, and an impact hammer was used to stimulate the TPS panel at the marked point to define the modal shape. For the load transfer plate and the rear structure (titanium), 36 and 25 points, respectively, are available.

7 shows a circuit diagram and numbering system of marked points on the test structure. Figure 7 (a) is a load transfer plate, Figure (b) is the 36-point number system marked on the load transfer plate, Figure (c) is the front load transfer plate and the rear structure of the metal TPS panel, and the figure ( d) is the numbering system of 61 points marked on the TPS structure. Signals from both the accelerometer and the impact force of the impact hammer can be processed ^{in the B&K Pulse TM} and ME'scopeVES ^{TM software.} The FRF between each point of influence and the fixed response point can be calculated one at a time. As a result, the FRF matrix can be completed for the price of 61.

In empirical modal analysis (EMA) theory, the modal parameters, natural frequencies, damping ratios and mode shapes of the test structure can be obtained from the Frequency Response Function (FRF), which is defined as the complex ratio of the output to the input. Due to current hardware limitations, the input signal of the impact hammer and the output response of the high-speed camera may not be sampled simultaneously. Therefore, FRF cannot be obtained and OMA can be used instead. This method can use the output response to characterize the modal parameters of the test structure. The impact force is assumed to be Gaussian noise, and the ODS FRF is calculated using the measured response of the test structure, and an Operational Deflection Shape (ODS) is constructed. This new type of measurement is defined as ODS FRF measurement or transmittance measurement. If the ODS at or near the resonant frequency is dominated by the corresponding single-mode shape at that resonant frequency, then the ODS can approximate the mode shape. Therefore, in this study, the mode shape of the roving hammer method using ODS of ODS FRF measurement and the mode shape of the numerical simulation method of verification are compared.

3.3 Finite element analysis

Using the OMA-based 3DPT method with the dynamic properties of the load-carrying plate, the measurement method for the proposed test structure can be verified, and a finite element analysis of the load-carrying plate can also be performed. In addition, the dynamic characteristic result of the accelerometer measurement can also be obtained. Comparison of natural frequency and mode shape between measurement results (3DPT and roving hammer) and finite element analysis is shown in FIGS. 8 and 9 .

Fig. 8(a) shows the deformation response of the load transfer plate in the time domain measured by the 3DPT method. A fast Fourier transform (FFT) may be performed on the measured response of the load transfer plate. Figure 8 (b) shows the smoothing curve (Savitzky-Golay smoothing method) of the ODS FRF result of a single FFT, the average result of 20 measurements, and the average result measured by 3DPT. There may be four peaks in the ODS FRF of the load transfer plate using the 3DPT method. Figure 8 (c) also shows the values of similar peak frequencies present on the FRF curve of the load transfer plate measured by the roving hammer method. It can be seen that the measured frequencies are consistent between the 3DPT method and the roving hammer method. The measured response of the optical target can then be used to obtain the ODS of the load-carrying plate with in-house Python code. Since the ODS is not scaled by the impact force, it can be defined as an unscaled mode shape or a working shape as shown in Fig. 9 (a). Meanwhile, Figure 9 (b) shows the mode shape corresponding to the natural frequency measured by the roving hammer method, and Figure (c) shows the modal parameters obtained by the FEM method. Contour colors defined in ARAMIS software follow a scale of -1 to 1, and in ME'scope software and finite element analysis it is an absolute scale of 0 to 1. In the overall result, the natural frequency and mode shape of the 3DPT method can be similar to that of the roving hammer method and the finite element analysis. Therefore, the current 3DPT method achieves acceptable accuracy and may be suitable for investigating the properties of TPS panels throughout this study.

3.4 Phase-based motion magnification

A disadvantage of the displacement-based measurement method such as the 3DPT method is that when the structure is excited with only a small amount of energy, the structure may experience a small deformation that cannot be recognized by the raw data of the 3DPT method. This can be explained by a small displacement at high frequencies, which may be the cause of missing the natural frequency corresponding to a specific mode. After many impact tests in this study, the optimal impact point can be determined as the point of the protruding area on the rear side of the front load transfer plate (Inconel) as shown in FIG. 6 . Since the amount of excitation energy is not sufficient to drive the entire structure, certain modes may not be significantly excitation and may not be clearly visible in the original ODS FRF curve. For complex geometries such as the metal TPS panel proposed in this measurement, it may be difficult to extract all modes from the OMA test due to local excitation points in the OMA laboratory test. Therefore, there is a need for a way to find modes that are not well excitation.

Studies on vibration measurements using 3DPT and phase-based motion amplification methods are widely used to identify the modal parameters and structural dynamics of structures. In the present invention, the signal-to-noise ratio in a series of captured images can be improved by using phase-based motion amplification to use the 3DPT method as a method for dynamic characteristic identification of a metal TPS panel under the above-mentioned conditions. The motion of the captured image with indistinct peaks in the ODS FRF curve is magnified by a certain magnification.

The present invention can use a phase-based motion amplification technique for a case study of a healthy fully assembled TPS panel. That is, the motion expansion method is applied to a specific bandwidth in the frequency domain. The algorithm can decompose the input signal into local spatial amplitude and phase signals using a complex steerable pyramid filter. After reconstruction of the image, higher amplitudes are achieved in the broadened frequency band. The requirements of the magnification parameters are: frame rate, magnification bandwidth and magnification factor. 10 shows an overview of the combination of motion magnification and 3DPT method to obtain magnified displacement of the TPS panel.

3.5 Damage detection method

Two methods can be used to evaluate the damage status of TPS panels. The first method is based on changes in natural frequency and damping ratio. Another method evaluates the mode shape based on the MAC. Among the many damage states of TPS panels in this study, bolted joint loosening could be important for vehicle safety. Loose bolted joints can significantly change the frequency, damping ratio, and mode shape of the metal TPS panel in most test cases. Here, we investigate the applicability of the damage detection method by focusing on this damage problem.

Defining the threshold of damage state can be very important and difficult because there is measurement uncertainty due to noise and environmental conditions of the acquisition device. This measurement uncertainty causes slight changes in frequency, damping ratio, and mode shape. Therefore, proper experimental setup may be required to minimize noise. We assume that the camera system is well calibrated and that the contrast of the optical object is good. Three factors can be adjusted until most of the noise floor achieves the desired accuracy: lighting, camera position, and optical target. The temperature around the environment may also be maintained at room temperature. The maximum noise floor is 10 μm and the maximum strain response of the TPS panel is about 300 μm. This represents a measurement uncertainty of 3.3%. In this study, we propose that the criterion for impaired status be greater than the 3.3% difference between healthy and impaired status.

The MAC can measure the consistency between estimates of mode types. This method can compare mode shape vectors between normal and damaged cases and provide MAC values for each mode. If the two mode shapes are the same, the MAC value becomes 1, and when they are completely different, the MAC value may be 0. The mathematical expression of MAC is expressed as Equation 1 below.

[Equation 1]

Here, ψ _ur is the intact modal form, ψ _dr is the damaged modal form, the subscript r denotes the r-th mode, and the superscript T denotes the transposition of the vector. A criterion of 3.3% difference can be applied to the damaged state of MAC.

11 shows a flowchart of a proposed damage detection method using a combination of DIC and MAC methods. The deformation response of the predefined DOF (optical object) in the case study is output from the DIC-based ARAMIS® software. You can use the FFT algorithm in a MATLAB program to compute spectral data for the strain response. The natural frequency corresponds to the peak of the ODS FRF curve, and the damping ratio can be calculated by a half-power method. The modal vector can be calculated by examining the imaginary part of the ODS FRF. The modal vector of the r-th mode may be scaled to a maximum value among predefined DOFs of the r-th mode. Then, in order to evaluate the consistency of the two case studies, the MAC matrix may be obtained from Equation 1 above. It then identifies damage detection based on changes in natural frequency, damping ratio, and mode shape.

4. Results and discussion

4.1 Dynamic characteristics of a healthy fully-assembled TPS panel

This section describes the dynamic properties of healthy fully assembled TPS panels as measured by the roving hammer method. By looking at the FRF curve of the measurement response at a specific point on the front of the fully assembled TPS panel, the peak of the FRF curve can be clearly identified. The corresponding natural frequencies are 192 Hz, 330 Hz, 361 Hz and 464 Hz, respectively.

We can use the FRF data of the 61 marked points on both the load transfer plate and the back structure (titanium) to construct the first four modal shapes of a healthy fully assembled TPS panel. ME's Scope ^TM software provides mode types corresponding to natural frequencies. The modal shape obtained from the proposed metal TPS panel under free conditions can match the shape from free vibration of the sandwich panel.

A first mode of the TPS panel is a torsional mode, and the second and third modes are a bending mode. The fourth mode is a bending-torsion mode in which bending and torsion are combined.

Next, we measure the dynamic properties of healthy fully assembled TPS panels using the roving response and 3DPT method.

A series of captured images of the vibration test can be magnified in motion using the phase-based magnification method as described above. The frame rate is 3000 frames per second, the multiplier is 3x, and the frequency band is 355Hz to 365Hz. The enlarged image can then be processed with the DIC algorithm to obtain the enlarged displacement. The signal-to-noise ratio increases significantly as a result of the new spectral data after motion amplification. The peak at 362 Hz can be clearly seen in the new spectral data. The natural frequencies and corresponding ODSs obtained from the 3DPT method and motion magnification can be consistent with the accelerometer measurement method.

Although motion magnification was used to identify blind modes in the 3DPT method, it may not be essential for quick testing and damage detection. The two main modes such as the first two modes of the proposed TPS panel can be used to detect the damage condition. For further investigation of the damage state in high-frequency modes, phase-based magnification may be needed to magnify the small displacement response used to identify eigenmodes and compute MAC values between health and damage states.

By removing the fibrous insulating material from a healthy fully assembled TPS panel, it is possible to study the effect of the fibrous insulating material on the structural mechanics of the TPS panel. When tightening bolts, use a torque value of 4Nm. Obviously, the deformation history of the skeletal TPS panel without the fibrous insulating material can last longer than that of the healthy fully assembled TPS panel with the fibrous insulating material due to the damping effect of the fibrous insulating material. This produces a clear peak in the third eigenmode (363 Hz). The natural frequency of the skeletal TPS panel can be increased correspondingly from the TPS panel with a fibrous insulating material. Since the fibrous insulating material is considered a non-structural mass, the increase in natural frequency may be due to the reduced mass of the skeletal TPS panel without the fibrous insulating material.

4.2 Damage detection

In general, the natural frequency of a damaged TPS panel can be changed in a normal prefabricated TPS panel. Specifically, the natural frequency may be reduced due to degradation of rigidity when the TPS panel is damaged. Performance degradation can increase as the number of damaged edges increases. A TPS panel that suffered damage at corner #1 due to damage at corner #1 may have a higher stiffness than the other two cases (damage at both corners). This increases the natural frequency of the TPS panel, which can cause damage in a single corner. It can be seen that the damage of the two corners #1 and #3 (two diagonal bolted parts) has higher rigidity of the TPS panel compared to the damaged of corners #1 and #4 (two vertical bolted parts). The percentage difference between the normal damaged state and the normal state exceeds the criterion of a 3.3% difference. Therefore, it is possible to detect damage to the TPS panel using the change in natural frequency.

A smoothing technique may be applied to the calculated spectral curve of the test structure. A smoothing curve can be plotted together with the calculated spectral curve. Modal damping is estimated from this smoothed spectral curve. The damping ratio can be calculated using the half-power bandwidth method. The effect of the fiber insulation material is explained by considering the damping ratio of each test case. The bandwidth can be calculated using a 3 dB cutoff frequency. As can be seen from the damping ratio, a single load-carrying plate can have the lowest damping ratio. A TPS panel with a fibrous insulating material can exhibit a higher damping ratio than a TPS without a fibrous insulating material due to the damping ability of the fibrous insulating material.

The damping ratio of a damaged case may be higher than that of a healthy fully assembled TPS panel, which is due to the increase in the thickness of the insulating layer (increasing porosity) as damage occurs (the bolt joint loosens), resulting in a higher damping capacity of the fibrous insulating material. Because. The damping ratio of a damaged TPS panel in a single corner may be lower than that of a TPS panel in two damaged corners. The damping ratio of the damaged TPS panel along the diagonal (corners #1 and #3) may be lower than the damping ratio of the damaged TPS panel along the vertical (corner #1 and #4). This is because the damage along the diagonal still ensured compression of the fiber insulation material, while the greater expansion of the insulation layer occurred in the damage along the vertical line. These results indicate that the change in the damping ratio can also be used to detect the damage state of the TPS panel.

The ODS of the test case is clearly related to the ODS of the load transfer plate. The ODS of a healthy fully assembled TPS panel with and without fibrous insulating material is nearly identical to that of the load carrying plate. In all conditions, the first ODS of the test TPS panel is the torsional mode, which may be the same as the first ODS of the load-carrying plate.

The second ODS of the test TPS panel may be different depending on the case of damage. A TPS panel with damage in a single corner (Corner #1) exhibits an asymmetrical deflection shape, which deflects heavily at the edge close to the damaged corner. There was damage to the bolted joints along the diagonal (corners #1 and #3), but the second ODS in this case still doesn't change much in health. This may be due to symmetry about the center point of the damaged location. The second ODS of the TPS panel with damage to the bolted joint along a vertical line (corners #1 and #3) has two edges (between corners #1 and #4 and between corners #1 and #3) and a middle along the vertical central axis. shows the highest bias in the region. The bending ODS is symmetrical about the vertical central axis.

The third ODS of the test TPS panel may also have a different shape for each damaged case, and the bending shape of the TPS panel with damage in a single corner may be slightly different from that of the healthy case. The edges along the diagonal (corners #1 and #3) exhibit a bend shape that is symmetrical to the diagonal because there is no constraint along the diagonal due to damage. The highest deflection is found near the edges (between corners #1 and #4) due to the unconstrained constraints along the vertical line in the TPS panels (corners #1 and #4) with damage to both corners along the vertical line.

The calculated MAC provides information about shape correlation. The calculated MAC values of the first four ODSs of the normal TPS panel are 1 in the diagonal element and less than 0.1 in the other elements. This means that the MAC calculation is correct. The damage state can be identified by a criterion of 0.96 (3.3% difference).

5. Conclusion

We can successfully investigate dynamics and damage detection using a combination of DIC and MAC methods. The ODS of a healthy fully assembled TPS panel or a damaged panel is related to the ODS of the load transfer plate due to the major weight of the load transfer plate in the metal TPS panel. The proposed ODS FRF is an effective measurement method to obtain modal parameters such as natural frequency, damping ratio and ODS of the test structure. 3DPT, a variant of the DIC method, can provide a method to quickly and accurately evaluate the dynamic properties of complex and large structures.

A change in the frequency of a particular eigenmode or a change in a particular ODS in damage detection can be an indication of a failure. Localized excitation may result in modes not found in OMA. Motion magnification should be used for high frequency mode to detect possible damage in test mode. The ODS obtained in this study can be very useful in identifying invisible damage such as loosening of bolted joints in metal TPS panels. However, obtaining all the rescue frequencies or ODSs is not necessarily necessary for fast inspection.

The reduced MAC values in the case study show the level of damage of the metal TPS panel, especially the number of damaged edges. The combination of DIC method and MAC provides an excellent approach to evaluate ODS and identify the damage status of metal TPS panels. The method according to the present invention corresponds to a viable non-contact and SHM technique that can be applied with high reliability to study the structural dynamics of metal TPS panels and to identify damage states of metal TPS panels due to loose bolted coupling. can do.

Although the above has been described with reference to the preferred embodiment of the present invention, those skilled in the art can variously modify and change the present invention within the scope without departing from the spirit and scope of the present invention as described in the claims below. You will understand that it can be done.

100: structure health monitoring system
110: vibration test system 111: impact hammer
113: specific structure 115: high-speed camera
130: structure health monitoring device
150: database
210: structure image acquisition unit 230: dynamic characteristic calculation unit
250: health monitoring unit 270: control unit

Claims (9)

a structure image acquisition unit that provides an external stimulus to the specific structure and acquires an image of the specific structure at each time point before and after the external stimulus is provided;
a dynamic characteristic calculator for calculating dynamic characteristics including a natural frequency, a damping ratio, and a mode shape for the specific structure based on the image; and
A health monitoring unit for detecting the change in the dynamic characteristics to detect damage to the bolt joint of the specific structure,
The specific structure is a metal TPS panel (metallic TPS panel), the front plate forming a rectangular plate-like structure; a rear plate spaced apart from the front plate at a predetermined distance and disposed in parallel with the front plate; an insulation material disposed between the front and rear plates; a plurality of bolted joints for coupling between the front and rear plates; spacers coupled to each of the plurality of bolt joints to form the gap; and washers coupled to both ends of the spacers,
The structure image acquisition unit makes physical contact by an impact hammer to the point of the protruding area on the rear surface of the front plate for the specific structure supported vertically in a free boundary condition by at least two fine nylon cords. provided as a stimulus,
The health monitoring unit detects damage to the bolt joint based on a change in a Modal Assurance Criterion (MAC) value related to the natural frequency, the damping ratio, and the similarity between the mode shapes,
The mode shape of the metal TPS panel includes a torsion mode, a bending mode and a bending-torsion mode, wherein the damage of the bolt joint includes damage of a single corner and damage of two corners, and the damage of the two corners includes damage of a diagonal corner And structure health monitoring device using the DIC technique, characterized in that it includes damage to the corner of the vertical line.
delete delete According to claim 1, wherein the structure image acquisition unit
of the specific structure as a response to the external stimulus according to a shooting option set as a combination of frame rate, shutter speed and aperture size using at least two high-speed cameras with a resolution of 1024 * 1024 pixels and a focal length of 50 mm Structure health monitoring device using DIC technique, characterized in that the vibration is measured.
The method of claim 1, wherein the dynamic characteristic calculating unit
A structure health monitoring apparatus using a DIC technique, characterized in that a preprocessing operation of magnifying the image is performed using a phase-based magnification method to which magnification parameters related to frame rate, bandwidth and coefficient are applied.
The method of claim 1, wherein the dynamic characteristic calculating unit
A structure health monitoring device using the DIC technique, characterized in that the deformation response of a predefined optical object (DOF) is generated as a result of applying a digital image correlation (DIC) technique by inputting the image as an input.
The method of claim 6, wherein the dynamic characteristic calculating unit
A structure health monitoring apparatus using a DIC technique, characterized in that the spectrum data for the deformation response is calculated using an FFT algorithm, and the natural frequency and the damping ratio are calculated from the spectrum data.
delete acquiring an image of the specific structure at each time point before and after providing the external stimulus while providing the external stimulus with respect to the specific structure;
calculating dynamic characteristics including a natural frequency, a damping ratio, and a mode shape for the specific structure based on the image; and
Detecting a change in the dynamic characteristics to detect damage to the bolt joint of the specific structure,
The specific structure is a metal TPS panel (metallic TPS panel), the front plate forming a rectangular plate-like structure; a rear plate spaced apart from the front plate at a predetermined distance and disposed in parallel with the front plate; an insulation material disposed between the front and rear plates; a plurality of bolted joints for coupling between the front and rear plates; spacers coupled to each of the plurality of bolt joints to form the gap; and washers coupled to both ends of the spacers,
The step of acquiring the image comprises physical contact by an impact hammer to the point of the protruding area on the back side of the front plate for the specific structure supported vertically in free boundary conditions by at least two fine nylon cords. providing as the external stimulus,
The detecting of the damage includes detecting the damage of the bolt joint based on a change in the MAC (Modal Assurance Criterion) value regarding the natural frequency, the damping ratio, and the similarity between the mode shapes,
The mode shape of the metal TPS panel includes a torsion mode, a bending mode and a bending-torsion mode, wherein the damage of the bolt joint includes damage of a single corner and damage of two corners, and the damage of the two corners includes damage of a diagonal corner And Structure health monitoring method using the DIC technique, characterized in that it includes damage to the corner of the vertical line.

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