KR100784582B1 - Apparatus and method for measuring the damage of a structure using piezoelectric devices - Google Patents

Abstract

The present invention relates to an apparatus for measuring a damage position of a structure using a piezoelectric element, comprising: a computer device (60) for inputting a frequency step having a measurement frequency range and storing and displaying a measurement result; At least one piezoelectric element 30 mounted on opposite sides of the measurement object 20; A stress adding means (70) for adding a load stress of the same size to a plurality of positions by starting at a position near the middle of the measurement object (20) and spaced apart by a predetermined interval; And a predetermined torque is initially applied by the stress adding means 70 to generate a longitudinal acoustic wave to the measurement object and to supply an AC signal corresponding to a frequency step having a measurement frequency range input from the computer device 60 to the piezoelectric element. An impedance analyzer 50 applied to 30 to calculate impedance through a potential change of the piezoelectric element 30; And wires 51, 52, and 53 electrically connecting each of the piezoelectric elements 30 on the opposite side and between the impedance analyzer 50 and between the impedance analyzer 50 and the computer device 60. Characterized in that made.

Impedance, damage measurement, piezoelectric element, acoustic wave,

Description

Apparatus and method for measuring the damage of a structure using piezoelectric devices}

1 is a view showing a degree of freedom model of the structure to which the piezoelectric element is attached.

2 is a view showing an impedance measurement test piece with a piezoelectric element patch attached to an aluminum beam according to the present invention.

3 is a schematic configuration diagram of a damage measuring apparatus of a structure using a piezoelectric element according to the present invention.

4 is an impedance waveform diagram of the aluminum beam of FIG. 2.

5 is a configuration diagram illustrating an impedance measuring method according to a load position of a damage measuring apparatus of a structure using a piezoelectric element according to the present invention.

6 is a graph illustrating impedance response characteristics according to the measuring method of FIG. 5.

7 is a configuration diagram illustrating an impedance measuring method according to the load size of the damage measuring device of a structure using a piezoelectric element according to the present invention.

8 is a graph illustrating impedance response characteristics according to the measuring method of FIG. 7.

FIG. 9 is a graph showing peak frequency shift amounts in an impedance response waveform in a healthy state and a damaged state.

FIG. 10 is a graph showing the rate of change of the peak amplitude ratio in an impedance response waveform in a healthy state and a damaged state.

11 is a graph showing the rate of change of the characteristic coefficient Q value ratio.

12 is a schematic flowchart of a measuring method using a damage measuring apparatus of a structure using a piezoelectric element according to the present invention.

Explanation of symbols on the main parts of the drawings

20: structure

30: piezoelectric element

50: Impedance Analyzer

60: computer

70: stress adding means

The present invention relates to a damage measuring apparatus and method of a structure using a piezoelectric element, in particular one or more piezoelectric elements attached to the opposite side of the structure and adds stress through the stress adding means to generate a longitudinal seismic wave and through the impedance analyzer The present invention relates to a damage measuring apparatus and method using a piezoelectric element to analyze the impedance in a frequency range so that the damaged state of the structure can be known.

Large structures such as bridges and steel towers are usually installed near human life. Since these collapses cause many casualties, it is necessary to detect damages early and take countermeasures to prevent them. Such damage measurement techniques include the collapse of Seongsu Bridge in 1994, the overpass collapse of Jongam-dong, Seoul, the collapse of Sampoong Department Store in 1995, the collapse of a transmission tower in Japan in 1998, and the collapse of Oakland Bay in 1984 in San Francisco. The necessity speaks well.

Types of structural damage include cracking and corrosion of steel, loosening or dropping of bolts, and the like. Steel cracks and bolts are not exposed to the outside by painting, which makes it difficult to detect them early. Moreover, steel cracks often cause metal fatigue, and the corrosion of the material lowers the strength, which leads to the risk of destruction. Will accompany. In this case, the necessity of normally inspecting the internal stress, corrosion and damage of the steel, and tightening of bolts is raised.

An object of the present invention is to solve the problems of the prior art as described above, by attaching one or more piezoelectric elements on the opposite side of the structure and by applying a stress through the stress adding means to generate a longitudinal elastic wave and impedance analyzer The present invention provides an apparatus and method for measuring damage to a structure using piezoelectric elements, by analyzing impedance in a specific frequency range.

According to a preferred embodiment of the present invention for achieving the above object, a computer device for inputting a frequency step having a measurement frequency range and storing and displaying the measurement results; At least one piezoelectric element attached to opposite sides of the measurement object; Stress adding means for adding a load stress of the same size to a plurality of locations by starting at a position near the middle of the measurement object and being spaced apart at regular intervals; And a constant torque is initially applied by the stress adding means to generate a longitudinal seismic wave to the measurement object, and apply an AC signal corresponding to a frequency step having a measurement frequency range input from the computer device to the piezoelectric element. Impedance analyzer for calculating the impedance through the potential change of; And wires electrically connecting the piezoelectric elements on each of the opposing side surfaces and between the impedance analyzer and between the impedance analyzer and the computer device.

According to another aspect of the present invention, there is provided a computer apparatus for inputting a frequency step having a measurement frequency range and storing and displaying a measurement result; At least one piezoelectric element attached to opposite sides of the measurement object; Stress adding means for adding a load stress to the same position by varying the magnitude of the stress at the initial stress portion of the measurement object; And a constant torque is initially applied by the stress adding means to generate a longitudinal acoustic wave to the measurement object, and apply an AC signal corresponding to the frequency step having a measurement frequency range input from the computer device to the piezoelectric element, An impedance analyzer for calculating impedance through a potential change of the device; And wires electrically connecting the piezoelectric elements on each of the opposing side surfaces and between the impedance analyzer and between the impedance analyzer and the computer device.

In still another aspect of the present invention, there is provided a device comprising: a first step of obtaining a first and a second impedance waveform; And a second step of measuring a peak frequency shift amount between the first and second impedance waveforms, wherein the peak frequencies of the first and second impedance waveforms are peak frequencies in a healthy state and peak frequencies after damage, respectively. A damage measurement method of a structure using a piezoelectric element is provided.

In still another aspect of the present invention, there is provided a device comprising: a first step of obtaining a first and a second impedance waveform; And a second step of measuring an amount of change in peak amplitude ratio between the first and second impedance waveforms, wherein the amplitude ratios of the first and second impedance waveforms are amplitude ratios in the health state and amplitude ratios after damage, respectively. A damage measuring method of a structure using a piezoelectric element is provided.

In still another aspect of the present invention, there is provided a device comprising: a first step of obtaining a first and a second impedance waveform; And a second step of measuring an amount of change in the ratio of the characteristic coefficient (Q) value between the first and second impedance waveforms, wherein the characteristic coefficient (Q) value is a frequency and a second impedance of the first impedance waveform. The damage measurement method of a structure using a piezoelectric element is provided by subtracting a small value from a large value of a waveform frequency and dividing it by a maximum value.

One of the materials suitable for damage evaluation is piezoelectric element, which has the function of both sensor and actuator, making it very easy to apply to impedance measurement method.

In the present invention, to detect the early damage of the structure early, and to provide an apparatus and method for quantitative measurement and evaluation.

Hereinafter, a method and apparatus for measuring damage to a structure using a piezoelectric element according to the present invention will be described in detail with reference to the accompanying drawings.

1 is a view showing a degree of freedom model of the structure to which the piezoelectric element is attached, Figure 2 is a view showing an impedance measurement test piece attached a piezoelectric element patch to the aluminum beam according to the present invention, Figure 3 is a piezoelectric element according to the present invention Figure 4 is a schematic configuration diagram of a damage measuring device of the structure used, Figure 4 is an impedance waveform diagram of the aluminum beam of Figure 2, Figure 5 is the impedance measurement according to the load position of the damage measuring device of the structure using a piezoelectric element according to the present invention 6 is a graph illustrating impedance response characteristics according to the measuring method of FIG. 5, and FIG. 7 is an impedance measurement according to a load size of a damage measuring apparatus for a structure using a piezoelectric element according to the present invention. 8 is a graph illustrating impedance response characteristics according to the measuring method of FIG. 7, and FIG. 9 is a healthy state. hy) and a graph showing the peak frequency shift amount in the impedance response waveform in the damaged state, and FIG. 10 shows the rate of change in the peak amplitude ratio in the impedance response waveform in the healthy and damaged state. 11 is a graph showing a rate of change of the characteristic coefficient Q value ratio, and FIG. 12 is a schematic flowchart of a measuring method using a damage measuring apparatus for a structure using a piezoelectric element according to the present invention.

Referring to FIG. 1, when damage is caused by any factor in a structure, the structure's stiffness, attenuation rate, or the like is changed as compared to health, and as a result, the mechanical impedance of the structure is changed. From the relationship between the occurrence of such damage and the change in mechanical impedance, it is a concept of the impedance measuring method used in the present invention to evaluate the health state (damage) of the structure by measuring the mechanical impedance. In general, however, it is difficult to measure small changes in mechanical impedance with high accuracy. Therefore, considering the model of the structure 20 to which the piezoelectric element (PZT: lead-zirconium-titanate) 30 shown in FIG. 2 is attached, the admittance (inverse of the electrical impedance) of the piezoelectric element is determined by the structure and the piezoelectric element. The mechanical impedance of can be expressed by the following equation.

Where Y: admittance

Za: Mechanical Impedance of Piezoelectric Elements

Zs: Mechanical Impedance of Structure

Yxx: Elongation modulus when the electric field of the piezoelectric element is 0

d _3x : equivalent piezoelectric constant for any direction

ε ₃₃ : dielectric constant

d: dielectric loss rate of piezoelectric element

wa, la, ha: width, length, thickness of piezoelectric element.

If no damage occurs, the mechanical impedance of the piezoelectric element is constant, so the change in the mechanical impedance of the structure becomes dominant. Therefore, the value of the mechanical impedance of the structure can be obtained by measuring the electrical impedance of the piezoelectric element 30 attached to the opposite side of the structure 20.

In the present invention, the damage state of the structure is evaluated by measuring the electrical impedance of the piezoelectric element in three relations of "damage of structure", "mechanical impedance", and "electrical impedance of piezoelectric element."

Referring to Figure 2, there is shown a measurement test piece used for the damage measurement of the structure using a piezoelectric element according to the present invention, a beam that is a basic element of the structure was used. In order to reduce the influence of boundary conditions or external excitation as much as possible, the beams were supported by foamed synthetic resin, and both ends were free ends. Two piezoelectric elements 30 were attached to the beam at the symmetrical position of the beam so that only longitudinal seismic waves were generated. The element of was driven in phase. Physical properties of the aluminum beam and the piezoelectric element 30 are omitted.

Referring to FIG. 3, a schematic diagram of a damage measurement system can be seen, and impedance measurement (HP4192A) 50 was used for impedance measurement of the piezoelectric element 30. In addition, in order to establish a quantitative evaluation method, a stress is applied to the structure 20 by using a simple suppressing device (hereinafter referred to as a stress adding means 70) in which a bolt nut device is attached to a U-shaped horseshoe as virtual damage. Torque can be adjusted quantitatively to suppress the screw.

4 is a response of an impedance real part when the voltage applied to the piezoelectric element is applied to the beam of FIG. 3 to 1 to 100 mV. Here, since both ends of the beam are free ends, the fundamental frequency of the longitudinal vibration can be obtained by Equation 2.

여기서, n=1,2,3...이다.

Where n = 1,2,3 ...

Here, c is a phase velocity propagating inside the beam 20, and the frequency of the material is determined by the elastic modulus E and the density. If the boundary condition is free at both ends, the natural frequency becomes fn = 3.01 (kHz) according to the above expression (2).

The impedance signal measured by the impedance analyzer 50 has a real part and an imaginary part, but since the signal of the impedance real part is highly sensitive to the change in the mechanical impedance of the structure, the waveform of the impedance real part is used.

In FIG. 3, it is a structural diagram of the impedance measuring apparatus which measured the response of the structure 20 by attaching the stress adding apparatus 70 to the position which assumed the damage, and applying a constant torque (T = 1.0 Nm). Through this device, the waveform of the longitudinal elastic wave as shown in Fig. 4 was obtained. From this, it is possible to measure the change in impedance and to measure the change in waveform due to the small stress change to predict the presence of minute damage.

In general, when impedance measurement is used, the change in impedance caused by damage to the structure depends on the measurement frequency band. Therefore, which frequency band to use greatly affects measurement accuracy and evaluation reliability.

Considering the vibration mode according to the length of the piezoelectric element, as shown in FIG. 4, when the piezoelectric element 30 is attached to both sides of the beam, the natural frequency of the beam can be examined. It is assumed that the piezoelectric element 30 and the aluminum beam are uniform in shape and material of the cross-sectional area, and the piezoelectric element 30 is completely fixed to the aluminum beam 20, so that the amount of vibration in the same cross section is the same. The longitudinal direction of the aluminum beam is x-axis, the x-direction displacement is u (x, t), the cross-sectional area of the beam is As, the longitudinal modulus of elasticity is Es, the mass ρs per unit volume, and the cross-sectional area of the piezoelectric element 30 is Aa. If the longitudinal elastic modulus is Ea and the mass per unit volume is ρa, the equation of motion can be written as

Therefore, the phase velocity c of the vibration mode transmitted due to the piezoelectric elements attached to both surfaces of the aluminum beam can be written as Equation 4.

However, if the standing wave exists in the part where the piezoelectric element is attached to both sides of the beam, the natural frequency depends on the boundary condition of both ends, so that the boundary condition where the natural frequency is the highest is the opposite condition of free or both ends fixed. Once the natural frequency is calculated for the free case, it can be written as Equations 5 and 6, respectively.

n=1,2,3

n = 1,2,3

n=1,2,3

n = 1,2,3

The natural frequencies of the first mode calculated by substituting the property values (not shown) of the piezoelectric elements 30 and the beams 20 into Equations 5 and 6 were fn = 49.75kHz and fn = 99.50kHz, respectively. That is, considering the actual structure, it can be expected that the output of the piezoelectric element 30 is the largest in the frequency region 49-100 kHz, which is also consistent with the impedance response result of FIG. Therefore, in the present invention, the quantitative evaluation of the damage was performed by using the object frequency band before and after the waveform having the largest impedance value in the frequency range of 49 to 100 kHz.

Referring to Figure 5, it is a diagram of a device measuring the impedance change by changing the load position, the magnitude of the load stress was fixed to the tightening torque 1.0Nm and the impedance was measured by changing the position where the stress was added. The positions of the stress load were L = 425, 445, 465, 485, and 505 mm, respectively.

Referring to FIG. 6, when the positions of the healthy structures and the loads are different, the deformation distribution of the mode is an example. It can be seen that the peak point moves to the right according to the position of the load (ΔF), and the magnitude of the waveform (A / Ao) decreases linearly.

Referring to FIG. 7, the device measuring the impedance change while changing the load size can be seen. The impedance is changed by changing the magnitude of the load stress from 0.5 Nm to 2 Nm by 0.5 Nm. Measured.

8 and 9, examples of deformation distribution of modes according to healthy structures and magnitudes of loads. Here, it can be seen that as the load stress increases, the peak point moves to the right (ΔF), and the magnitude of the waveform (A / Ao) decreases linearly.

It can be seen that the waveform change of the mode is significantly changed linearly according to the position and magnitude of the load. Here, the shift amount of the peak frequency can be used as one of the damage evaluation indexes for evaluating the impedance waveform change.

In general, when damage occurs to a structure, the elastic wave propagates inside by the stress, so the frequency response is changed accordingly, and the frequency at which the peak appears is shifted compared to the state of health. The peak frequency in a healthy state is referred to as f _pH , and the peak frequency after damage is referred to as F _pD , and the difference is defined as the peak frequency shift amount ΔF. As the torque decreases, the amount of peak frequency shift increases monotonically linearly. From this result, it is possible to quantitatively evaluate the torque change by using the peak frequency shift amount. ΔF is obtained through the following equation (7).

Impedance peak amplitudes vary significantly with damage, such as peak frequency shifts. Therefore, the change in amplitude of the impedance peak is selected as the damage evaluation index. The amplitude ratio change rate δ is defined by Equation 8 below with the amplitude Ah in a healthy state and the peak after damage as Ad.

Referring to FIG. 10, it can be seen that as the torque decreases, the peak amplitude ratio change rate increases linearly at all peaks. This result shows that the peak amplitude ratio change rate is effective for quantitative evaluation of damage.

Referring to FIG. 11, the rate of change of the Q value using the characteristic coefficient Q value as the final evaluation index is considered. After subtracting the small value from the large value of the frequency and dividing by the maximum value, the value shown in Equation (9) is obtained. Since this value is approximated by the damping ratio, it can be said to be a value determined by the damping ratio. As the damping ratio increases, the value decreases, and when the damping ratio decreases, the trend tends to increase. The increase or decrease of the damping ratio means the increase or decrease of the energy possessed by the peak and thus can be used for the evaluation of damage. In addition, this value has a slight gap depending on the peak value, but shows a tendency to increase overall, which can be useful for damage assessment.

Referring to FIG. 12, in the damage measuring method of a structure using a piezoelectric element, first, a stress is applied to the piezoelectric element 30 attached to the structure 20 through a stress adding means 70 to generate longitudinal seismic waves (ST−). 2).

Thereafter, a frequency step with a measurement frequency range is input from the computer 60 to the impedance analyzer 50 (ST-4).

By the impedance analyzer 50, a constant (1 Vrms) AC signal corresponding to the input signal is added to the piezoelectric element 30, whereby standing waves generated in the structure 20 are changed in potential in the piezoelectric element 30. Bring it.

This potential change is calculated as impedance in the impedance analyzer 50 (ST-6), and recorded as transmission data in the computer 60 (ST-8).

The stress-adding means 70, starting from a position near the middle of the measurement object, that is, the beam 20, spaced apart at regular intervals to add a load stress of the same size to a plurality of locations, and the measurement object 20 The method of adding a load stress at the same position by varying the magnitude of the stress at the position such as 0,0.5,1,1.5,2 Nm may be used.

In impedance measurement, the resolution of the excitation frequency affects the detection accuracy of structural damage, and it is necessary to set it appropriately according to the measurement object.

The impedance waveform change under stress load depends on the deformation and magnitude of the position where the stress is applied. In the case where stress is applied at a large deformation position, the measured impedance waveform has a smaller change in peak, frequency, and magnitude than the steady state waveform.

As the damage evaluation index, "peak frequency shift amount", "peak amplitude ratio change rate", and "value ratio change rate" were proposed.

As described above, according to the bolt fastening state measurement method and apparatus using the piezoelectric element according to the present invention, the impedance measurement method using the longitudinal elastic wave principle for the structure with the piezoelectric element is easy to measure the impedance resonant frequency in a wide frequency range There is a possible effect.

In addition, the present invention has an effect capable of evaluating the microscopic damage of the structure by using the impedance measurement method by the piezoelectric element for the early detection of early damage in the structure, such as steel bridges, bridges, transmission line pylons.

Although the present invention has been described based on the above-described preferred embodiment, the present invention is not limited thereto, and those skilled in the art will be able to perform various modifications based on the appended claims.

Claims (8)

delete delete delete delete delete delete A first step of obtaining a first and a second impedance waveform; And And a second step of measuring an amount of change in the ratio of the characteristic coefficient (Q) values between the first and second impedance waveforms, The characteristic coefficient (Q) value is, The damage measurement method of a structure using a piezoelectric element, characterized in that the value of the frequency of the first impedance waveform and the frequency of the second impedance waveform is divided by the maximum value after subtracting a small value. delete

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