JP2001012933A - Method for measuring depth of crack in structure using surface wave - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure high accuracy nondestructive measurement regardless of presence of a filler or an iron rod by sampling an oscillating elastic wave astride a crack and measuring the depth of a crack, based on the amplitude of a signal in front and rear of the crack. SOLUTION: Surface of a structure is oscillated by hitting it with a hammer, or the like, and an oscillation signal is sampled by means of a plurality of acceleration pickup sensors positioned on the opposite sides of a crack. In this regard, a surface wave is employed along with energy (speed or displacement amplitude) as a physical quantity. The wave energy decreases during propagation process and attenuates slightly due to viscosity of the structural material. The wave energy is varied across a crack and the extent of variation increases as the depth of the crack increases. Based on the propagation characteristics, acceleration signal in the data sampled by each sensor is integrated and converted into a speed signal thus correcting the effect of geometric attenuation in the amplitude of surface wave speed, wall thickness or sharing wave component. Depth of a crack is calculated using a corrected amplitude ratio determined through corrective analysis and a surface wavelength.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for diagnosing cracks in a structure. More specifically, the present invention relates to a method for measuring the depth or presence or absence of cracks on the front surface or the back surface of structures such as dam surfaces, tunnels and building walls.

[0002]

2. Description of the Related Art Cracks existing in concrete walls and the like cause a lack of continuity of the entire wall, a decrease in strength, a shortened life, and the like. However, concrete materials always have problems such as temperature, drying, casting defects and stress concentration, and the occurrence of cracks is inevitable in some sense. Therefore, it is considered that confirming the crack state or grasping the progress is an important point for the inspection for diagnosing the soundness of the wall and the structure.

At present, several methods have been proposed for nondestructive measurement of crack depth. In this,
Many methods use elastic waves or sound waves that propagate through a material. These methods are methods for estimating the traveling time of the first wave of arrival or the depth of a crack due to a change in polarity.

[0004] These methods have the following common features. First, we focus on the initial component of the propagated wave. This is a method of estimating the depth of the crack based on the arrival time or phase angle of the initial wave and the geometric relationship of the crack. Next, use the ultrasonic oscillation and receiver. According to this, it is easy to grasp the time of oscillation and the like. Generally, longitudinal waves are used. Longitudinal wave propagation speed is the fastest and easy to sort.

[0005]

The above-mentioned method has been developed in fields such as metal flaw detection because of its simple operation and rigorous theory. However, the use of these methods in the fields of civil engineering and construction seems to present various problems as described below. (B) Difference in thickness dimension order: The thickness of a metal member is generally small, often several mm and several cm. On the other hand, in the case of a concrete structure, it has a thickness of several tens cm or more, such as a lining of a tunnel and a wall of a building. (B) Difference in material properties: The metallic material is relatively uniform, and the attenuation of waves propagating in it is small. However, since concrete is a composite material, it has a large amount of wave scattering and a large attenuation. (C) Difference in crack state. Cracks in the metal material have almost no filling such as water and dust. In the case of concrete, these materials are often included, and many concrete bars are buried in the concrete walls of buildings. In many cases, both sides of the crack in the concrete wall are in contact with each other.

Due to the difference between the metal member and the concrete member, it is natural that the existing method causes a problem with the concrete member. Since the energy of ultrasonic oscillation is very weak, the waves diffracting and propagating at the tip gradually become weaker and the S / N (signal / noise ratio) becomes remarkable for cracks in concrete members with large dimensions and large attenuation. Will drop. However, due to its characteristics, longitudinal waves pass through contact surfaces on both sides of the crack, dust, water, reinforcing steel, and the like. If the energy of the passing wave approaches or exceeds the energy of the wave diffracted at the tip of the crack, the passing wave is picked up, and the depth of the crack is judged to be shallower than it actually is. In particular, the energy of the diffracted wave becomes weaker as the crack becomes deeper, and the possibility of a judgment error increases. Further, in actual construction, such a judgment is dangerous.

Although there is a method for detecting cracks using electric current, paint, infrared rays, electromagnetic waves, etc., it is often difficult to apply the method to concrete structures. At present, it must be said that an effective non-destructive method for cracks in thick concrete walls has not yet been realized.
It should be noted that even if the core is removed, there is no guarantee that accurate data can be obtained due to the influence of the rebar or the inclination of cracks.

The present invention overcomes the above-mentioned drawbacks of the prior art, and enables non-destructive exploration to a depth of the order of meters without being substantially affected by the contact of cracked surfaces, fillers, reinforcing bars, etc., and the cracking of new structures. The purpose is to provide a diagnostic method for

[0009]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems by providing a surface wave (relief wave) oscillated by a hitting device on the surface of a structure and transmitted along the surface of the structure. This is a method of measuring the depth of cracks using waves. Unlike the conventional method, the present method has the following features. Since a shock wave is generated by hitting with a hammer or the like instead of ultrasonic oscillation, energy is significantly increased.
The wave used is not a conventional longitudinal wave but a surface wave (Reilly wave). Furthermore, the physical quantities used are the running time,
It is energy (speed or amplitude of displacement), not initial motion polarity.

On the surface of the concrete structure, the waves generated by the impact include various components (longitudinal waves, transverse waves, surface waves, plate waves depending on the location). The energy transmitted along the surface of the structure therein is transmitted mainly in the form of a surface wave. In the propagation process of surface waves,
As the wave front spreads, the energy density per unit area decreases more and more. In addition, there is some attenuation due to the viscosity of the concrete material. If these attenuations are corrected, the amplitude or energy of the wave should always remain constant. However, when there is a crack, the surface wave is cut off to some extent by the crack. Therefore, the energy of the wave is reduced in the place after the crack. Therefore, the energy of the wave before and after the crack changes due to the presence of the crack, and the degree of the change increases as the crack becomes deeper. Therefore, it is possible to estimate the presence or absence or the depth of the crack based on the degree of the change (so-called amplitude ratio) and the wavelength of the wave.

By focusing on energy, an overall situation that is not particular about a local area can be grasped. Even if there is an effect of the reinforcing bar passing through the crack, the energy of the wave transmitted along the reinforcing bar naturally decreases because the area of the reinforcing bar is only a few percent of the entire crack area.

The method of searching for cracks by the surface wave amplitude ratio is a method developed based on the characteristics of the surface wave.
Surface waves have the following properties. (B) Surface waves are the most important waves on the surface of a structure. A considerable part (theoretically, about 67%) of energy is transmitted in the form of a surface wave in a wave generated by hitting the surface of a structure. Note that, depending on the type of transmission, the geometric attenuation of surface waves is much smaller than that of body waves. Moreover, when actually exploring, the sensor can only be attached to the surface of the structure. Therefore, most of the energy of the waves picked up by the sensor is occupied by surface waves except for the sensor close to the source. (B) The effect of surface waves decreases rapidly with depth. In concrete, the amplitude of the surface wave falls to about 0.2 at a wavelength depth of 1 assuming that the surface of the elastic body is 1. Therefore, it can be seen that the surface wave mainly exists only within the range of the wavelength of one time the depth. Therefore,
The amplitude ratio of the wave before and after the crack depends on the wavelength of the incident surface wave. For the same degree of cracks, long wavelength waves will pass well. That is, the amplitude ratio is large. (C) Surface waves in an elastic body mainly depend on the shear characteristics (shear constant) of the material. Therefore, it is anticipated that water and dust in the crack are hardly affected, and it is considered that the degree of influence is small because the shear rigidity of the contact surface is small.

The relationship between the crack depth and the corrected amplitude ratio is as shown in FIG.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In order to measure the crack depth of a structure by the method of the present invention, the following procedure is performed.

It oscillates by hitting using a hammer or the like.
It is possible to change the wavelength of the oscillating surface wave depending on the weight of the hammer and the shape of the tip. That is, the heavier the hammer, the longer the wavelength of the generated surface wave. As measurement accuracy, a surface wave of a long wavelength is desirable for deep cracks, and a short wavelength is better for shallow cracks.

An oscillation signal is sampled. The configuration of the signal sampling system designed and produced here is shown in FIG. The sensor used is an acceleration pickup whose resonance frequency is 30 kHz or higher, and the frequency range is fc to 5 kHz ±
0.5 dB, fc to 12 KHz ± 3 dB. A / D
The conversion speed of the board can be input at 1 μs / ch and up to 16 channels. At present, 8 channels are used. The trigger is set by software, and the signal sampling is performed correspondingly (FIG. 5). If necessary, remove environmental noise using a filter. The collected acceleration signals are converted to digital signals and stored on the hard disk of the personal computer.

The crack depth is estimated by analysis. The depth of the crack is estimated by analyzing the collected data in the following order. (B) The acceleration signal is converted into a speed signal by integration. Since the speed signal directly correlates with the energy of the surface wave, the measured acceleration signal is integrated as follows and converted into a speed signal.

(Equation 2) However, the integrated speed also performs baseline correction. (B) Correct geometric attenuation. The geometric decay of the surface wave (Reilly wave) velocity amplitude is corrected by the following equation.

## EQU3 ## where A is the amplitude, and r is the distance from the source (hitting point). Footnote ₀ represents a reference point (trigger channel). (C) Correct the material attenuation. For material damping, similarly

## EQU4 ## Here, σ is the internal attenuation of the material, which depends on the loss coefficient h of the material and the wavelength λ of the surface wave.

Σ = (2πh / λ) (d) Correct the effect of the wall thickness. When the wall thickness is thin, the energy decay is larger than in the case of the semi-infinite, even if the crack depth is the same. If the wall thickness is less than 1.3 times the surface wave wavelength, the amplitude ratio before and after the crack x (the ratio between the amplitude of the velocity or displacement after the crack and the amplitude before the crack) is as follows: .

[6] However, x ₀ and x is the amplitude ratio in the case of a semi-infinite body and thin walls, respectively, L is the temporary and the wavelength λ of 1.3 times the surface waves, respectively D and H wall thickness and cracks Of depth. (E) Correct for the effects of shear wave components. Theoretically, in a wave oscillating on the surface of an elastic body, about 26% of energy is occupied by a shear wave. Since the shear wave is attenuated at r- ² along the surface of the elastic body, it can be seen that the component of the shear wave almost disappears when it is separated from the oscillation source to some extent. However, a signal taken at a location (sensor) close to the source contains a constant shear wave. This component will also be removed. (F) Calculate the crack depth. The corrected amplitude ratio x was obtained by the integration and correction analysis described above. Using this and the surface wave wavelength λ, the following equation

The crack depth H is calculated according to the following equation: H = −0.7429λln (x)

[0018]

EXAMPLES Example 1: An example of measurement on a concrete specimen. Concrete specimens with cracks of various depths were prepared and tested. FIG. 6 shows the comparison between the measured crack depth and the value estimated by the method of the present invention.

Example 2 Non-destructive measurement by the method of the present invention and measurement by coring were performed on cracks in two waterway tunnels. The resulting comparison is shown in FIG. Visual observation of the core taken out revealed that the cracked surface was in contact with the surface at a depth of several centimeters, or that a filler such as mud was present in the deep part. Despite these conditions, it was confirmed that accurate measurement was possible with this method.

Example 2 Measurement of cracks in a reinforced concrete wall (thickness: about 1 m) was performed by this method.
The results of an exploration at the place where the crack is expected to penetrate are shown in the following table.

[Table 1]

Therefore, it is presumed that the influence of the reinforcing bar is small, and even if there is a reinforcing bar, the depth of the crack can be measured with high accuracy by the present method.

[0022]

As described in detail above, the present invention is not limited by the presence or absence of a filler or a reinforcing bar therein, or by the contact of the cracked surface with respect to the crack of the structure. It is possible to measure well and non-destructively.

[Brief description of the drawings]

FIG. 1 is a view showing a measurement image of a structure crack depth by a method of the present invention.

FIG. 2 is a diagram showing a relationship between a corrected wave amplitude and a crack.

FIG. 3 is a diagram showing the relationship between crack depth, wavelength and amplitude ratio.

FIG. 4 is a diagram showing a configuration of a signal sampling system according to the method of the present invention.

FIG. 5 is a diagram illustrating a method of setting a trigger and a sampling order in order to collect a vibration signal according to the method of the present invention.

FIG. 6 is a diagram showing a comparison between an actual crack depth of a concrete specimen and a result measured by the method of the present invention.

FIG. 7 is a diagram showing a comparison between a measurement result obtained by coring and a measurement result obtained by the method of the present invention for cracks in a tunnel.

Continued on the front page F term (reference) 2F068 AA24 AA50 BB26 CC11 EE01 FF00 GG05 HH03 KK16 QQ05 QQ09 QQ12 QQ14 2G047 AA10 BA01 BC03 CA03 CB03 EA08 EA10 GG14 GG42

Claims (5)

[Claims] The oscillated elastic wave is sampled across the crack by a sensor (FIG. 1), and the depth of the crack is measured by the signal amplitude (acceleration or velocity or displacement) before and after the crack. Corrected (geometric damping, material damping, etc.)
The relationship between the amplitude and the crack is shown in FIG. 2. The crack depth measurement formula is as follows: H = C ₁ λln (x) + C ₂ H: Crack depth from the surface, λ: Surface wave wavelength, x: Amplitude ratio In this case, x is determined by the signal amplitude (corrected) collected by each sensor after cracking / the signal amplitude (corrected) collected by each sensor before cracking. C ₁ and C ₂ : a method for measuring the crack depth of a structure, which is obtained according to the following theoretical or test constants. 3. For a certain material (artificial or natural material such as concrete, asphalt, rock material, etc.), fix the oscillation mechanism and the position, the receiving sensor and the position, etc., and use the sample calibration test to determine the crack depth and signal amplitude ratio. (Corrected or uncorrected) regression equation is obtained. Using the determined regression equation, the crack depth of a similar material is measured. The signals used are any of acceleration, velocity, and displacement, and do not correct or do not correct each attenuation. 4. A method for measuring the presence or absence or depth of a crack from the back surface, even if it is not visible on the front surface, based on the principle of claim 1. 5. The effect of repairing cracks is described in claim 1.
Method based on the principle of measurement.