JP6688619B2 - Impact device used for impact elastic wave method - Google Patents

Description

  The present invention relates to a hitting device used in a shock elastic wave method for nondestructively inspecting defects and the like existing inside concrete and the like.

  2. Description of the Related Art Conventionally, it has been a problem that a building or civil engineering concrete structure or the like undergoes surface peeling or surface deterioration due to aging, and the concrete portion peels off to cause a serious accident.

  In order to prevent such an accident, it is important to check the presence or absence of internal defects without destroying the structure in regular maintenance. At present, the impact elastic wave method is known as one of the methods for nondestructively inspecting internal defects of concrete. The Ministry of Land, Infrastructure, Transport and Tourism has also announced "Guideline for measuring the strength of concrete structures by slightly destructive / non-destructive tests" (March 2012), and is promoting the non-destructive investigation of concrete structures by the shock elastic wave method. .

  Up to now, several methods for investigating internal defects of concrete by the impact elastic wave method have been proposed.

  For example, in Patent Document 1 below, the soundness diagnostic device is a hammer that gives a shock to the surface of a concrete structure, an acceleration sensor that is provided in the hammering part of the hammer, and the acceleration sensor is connected to enable signal transmission. And the measured time history acceleration a (t) is taken into the analysis processing device, the surface of the concrete-based structure is tapped with the impact hammer, and the time is measured in the analysis processing device. The initial acceleration velocity V0 of the impact hammer is calculated by time integration of the hysteresis acceleration a (t), and the time history impact force F (t) (= M (hammer mass) × a (t)) generated on the structure surface. ) Is divided by the impact initial velocity V0 of the impact hammer to obtain a contact impedance Z (t), and the contact impedance Z (t) is used as the concrete system. Soundness diagnosis method of Concrete Structures for evaluating the health of the creature surface is disclosed.

  Further, in Patent Document 2 below, a Fourier transform is performed on the relationship between the signal strength and the elapsed time of the signal, so that the relationship between the signal strength and the frequency is converted into a spectrum, and the threshold value is determined by sampling data detected by the sensor. Is disclosed and a non-destructive diagnosis method for a concrete structure is disclosed in which the quality of the concrete structure is determined by comparing the threshold value with the measured data.

  Further, in Patent Document 3 below, when a peeling portion is present inside the concrete of a concrete structure, flexural vibration due to flexure occurs, and when the interior of the concrete is sound, longitudinal elastic waves and Rayleigh waves are generated, and flexural vibration is , A method for exploring delamination inside concrete that uses a characteristic with a larger amplitude value than longitudinal elastic waves or Rayleigh waves, in which impact force is applied to the concrete surface and the vibration waveform generated by this impact is measured and recorded. Then, standardize this measured waveform by standardizing the maximum amplitude value, calculate the absolute value of the standardized measured waveform's amplitude value, and add the above absolute values within a fixed time to calculate the amplitude added value. However, there is disclosed a peeling exploration method inside the concrete which compares the amplitude added value with a reference value and evaluates that there is peeling when the amplitude added value is larger than the reference value.

JP, 2004-144586, A JP, 2008-20425, A JP, 2014-212333, A

  In all of Patent Documents 1 to 3 described above, in the impact elastic wave method, the concrete surface to be inspected is directly hit to generate an elastic wave in the concrete. As the striking device for generating this elastic wave, in any of the above-mentioned Patent Documents 1 to 3, a hammer of a type that is gripped with a hand and having a striking portion made of a steel ball is used.

  However, in the case of this type of grip type steel ball hammer, it is difficult to stably input a signal of a constant frequency due to various factors such as the strength of the concrete surface, the degree of deterioration, and the distribution of aggregate. there were. Therefore, in the case of measuring and inspecting by sweeping the surface of concrete (a method of exploring under the same conditions at predetermined intervals), the input frequency of the elastic wave signal generated at each measurement point is slightly different, and the analysis result However, there was a problem that the accuracy of was decreased.

  Therefore, a main object of the present invention is to stably apply an elastic wave having a constant frequency to an object to be inspected, without being affected by the state of the object to be inspected (mainly concrete), with respect to a hitting device used in the impact elastic wave method. The present invention is to provide a hitting device.

ここに、Ｃp:鋼棒の縦弾性波速度(m/s)
前記鋼球は、前記プランジャーの共振周波数に対して±１５％範囲内の周波数を打撃によって与え得る直径の鋼球を選定したことを特徴とする衝撃弾性波法に用いる打撃装置が提供される。 In order to solve the above-mentioned problems, the present invention according to claim 1 is a steel rod having a predetermined length, which is manufactured with a constant cross-sectional area over the entire length, and a tip end surface which collides with an inspection object has a spherical crown shape. and, Ri Do from the plunger upper end surface that forms a flat, a steel ball having a predetermined diameter to impinge on the upper end surface of the plunger,
The length L (m) of the plunger is expressed by the following equation (1) between the frequency f (Hz) of the elastic wave given to the inspection object by the plunger and the length L (m) of the plunger. Based on

Where Cp: longitudinal elastic wave velocity of steel bar (m / s)
A striking device for use in an impact elastic wave method is provided , wherein the steel ball has a diameter capable of giving a frequency within a range of ± 15% with respect to the resonance frequency of the plunger by striking. .

  In the invention according to claim 1, instead of directly hitting the inspection object with the steel ball, the steel ball with a predetermined diameter is applied to the inspection object via the plunger (one-dimensional rod) of the above structural condition. On the other hand, elastic waves are applied. With such an indirect impact, the impact force is generated when the steel ball collides with the upper end surface of the plunger, and is transmitted to the inspection object through the tip surface of the plunger that is in contact with the inspection object. It Since these are materials (steel) having a stable elastic coefficient, the contact time between the steel ball and the plunger is always constant, and it is possible to stably give a wave having a constant frequency.

  In addition, it is known that a plunger with a specified length generates a specific frequency that resonates with the length of the steel rod (the theory of multiple reflection of elastic waves). Are matched within a certain range, an elastic wave having a resonance frequency corresponding to the plunger length is generated in the plunger, which is transmitted to the inspection object.

  Therefore, when the plunger of the same material is hit with steel balls of the same diameter and the same mass while moving the inspection point in sequence, it is possible to stably give an elastic wave of a certain frequency to the inspection object. It will be possible.

The length L (m) of the plunger is based on the relational expression (1) between the frequency f (Hz) of the elastic wave given to the inspection object by the collision of the plunger and the length L (m) of the plunger. that determine Te. Further, as the steel ball, a steel ball having a diameter capable of giving a frequency within a range of ± 15% with respect to the resonance frequency of the plunger by hitting is selected.

  When a steel ball hits the upper end surface of the plunger, its waves undergo multiple reflections within the plunger, which can be considered as a one-dimensional rod. According to the multiple reflection theory of elastic waves, a standing wave with a period twice the length of the plunger is generated. Therefore, it is possible to generate a single frequency that can be regarded as a sinusoidal wave although it is damped by matching the elastic wave of the frequency generated by hitting a steel ball with the resonance frequency of the plunger within a certain range. Become. At this time, since the frequency f (Hz) of the elastic wave given to the inspection object by the plunger and the length L (m) of the plunger have the relationship shown by the above equation (1), this equation (1) It is possible to set the frequency (Hz) of the elastic wave given to the inspection object based on the relationship with the length L (m) of the plunger.

As the present invention according to claim 2, wherein the radius of the spherical crown distal end surface of the plunger, percussion device for use in a radius and an impact blow method according to claim 1 Symbol placement is made to coincide with the steel balls are provided.

In the invention described in claim 2, the radius of the spherical crown-shaped tip end surface of the plunger and the radius of the steel ball are made to coincide with each other. Since the tip of the plunger that contacts the object to be inspected, such as concrete, needs to be elastically deformed to some extent, the tip of the plunger has a spherical crown shape. It is desirable to match the radius of the sphere.

As a third aspect of the present invention, the plunger is held in an insertion hole of a cylindrical holder via an elastic body that is easily deformable, and the tip end surface of the holder is more than the tip end surface of the plunger. A striking device for use in the impact elastic wave method according to claim 1 , wherein the impacting device is provided with a protrusion of 1 to 2 mm, and the holder is integrally provided with an elastic wave sensor for measuring a response elastic wave. .

The invention according to claim 3 shows a first mode in which an elastic wave sensor for measuring a response elastic wave is integrally provided with a striking device. First, the plunger has a structure in which it is held in an insertion hole of a cylindrical holder via an easily deformable elastic body, and the tip end surface of the holder is 1 to 2 mm from the tip end surface of the plunger. The holder is integrally provided with an elastic wave sensor for measuring a response elastic wave.

  Therefore, when the tip of the plunger is pressed against the object to be inspected, the tip surface of the holder can be brought into strong contact with the object to be inspected, so that the response elastic wave is efficiently transmitted to the holder, and the response elastic wave is transmitted. It becomes possible to measure the wave with high accuracy by the elastic wave sensor.

  By integrally providing the impact device with an elastic wave sensor that detects a response elastic wave, the handling property is improved, and the measurement work can be efficiently performed.

As a fourth aspect of the present invention, there is provided a striking device used in the shock elastic wave method according to any one of claims 1 and 2 , wherein a strain gauge for measuring a response elastic wave is attached to an outer surface of the plunger. It

The invention according to claim 4 shows a second mode in which an elastic wave sensor for measuring a response elastic wave is provided integrally with the striking device. Specifically, a strain gauge for measuring a response elastic wave is attached to the outer surface of the plunger. Since the strain gauge that measures the response elastic wave is provided in the plunger itself that gives impact, the response elastic wave is efficiently transmitted to the plunger, and this response elastic wave is accurately measured by the strain gauge (elastic wave sensor). It becomes possible to do.

  As described above in detail, according to the present invention, with respect to the impact device used for the impact elastic wave method, it is possible to stably apply an elastic wave having a constant frequency to the inspection object without being affected by the state of the concrete. .

It is a side view showing the example of the 1st form of hitting device 1 of the present invention used for a shock elastic wave method. It is a schematic diagram showing the 1st example of the collision method of steel ball 11. It is a schematic diagram showing the 2nd example of the collision method of steel ball 11. It is a side view showing the example of the 2nd form of hitting device 1 of the present invention. It is a V section enlarged view of FIG. It is a side view showing the example of the 3rd form of hitting device 1 of the present invention. It is a schematic diagram at the time of applying the hit | damage apparatus 1 (2nd form example) which concerns on this invention to the grout filling degree inspection in the sheath pipe of PC steel material.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First embodiment]
As shown in FIG. 1, an impact elastic wave measuring device according to the present invention includes a striking device 1, an elastic wave sensor 2 for measuring a response elastic wave from an inspection object, a waveform recording device 3, and The evaluation device 4 is connected to the waveform recording device 3.

  The striking device 1 is a steel rod of a predetermined length, which is manufactured with a constant cross-sectional area over the entire length, and has a tip end surface having a spherical crown shape and an upper end surface having a flat surface shape for colliding with an inspection object. It has a plunger 10 and a steel ball 11 having a predetermined diameter that collides with the upper end surface of the plunger 10.

  Hereinafter, it will be described more specifically.

  The plunger 10 is a steel rod made of steel and having a constant cross-sectional area over the entire length. The sectional shape of the steel rod may be circular, rectangular or polygonal. Normally, it is desirable that the steel ball 11 has a circular shape in accordance with the planar shape.

  It is preferable that the tip end surface 10a of the plunger 10 has a spherical crown shape because it is necessary to elastically deform the surface of the inspection object (mainly concrete) to some extent. Further, it is desirable that the upper end surface 10b of the plunger 10 be flat so that the steel ball 11 can collide with it. It is desirable that the radius r1 of the spherical crown at the tip of the plunger and the radius r2 of the steel ball 11 match.

  It is preferable that the steel ball 11 is made of a steel material and has a diameter corresponding to the length of the plunger 10. The vibration frequency generated when the steel ball 11 collides with the upper end surface of the plunger 10 depends on the contact time between the steel ball 11 and the plunger 10, and the contact time is 1/3 power of the mass of the steel ball 11. Proportional. That is, the mass of the steel ball 11 is, in other words, proportional to the diameter of the steel ball.

  If the vibration frequency transmitted to the plunger 10 when the steel ball 11 is collided with is adjusted to the resonance frequency of the plunger 10, the vibration given by the steel ball 11 will be rapidly transferred to the resonance frequency. Since the vibration of the resonance frequency is transmitted to the inspection object, the diameter (mass) of the steel ball 11 is adjusted so that the vibration frequency applied to the plunger 10 at the time of collision becomes close to the resonance frequency of the plunger 10. It is desirable to set.

  When the striking force is applied by the steel ball 11, the contact time becomes shorter as the collision speed becomes faster. Therefore, the collision speed between the steel ball 11 and the tip of the plunger needs to be controlled within a certain range. Therefore, as a method of colliding the steel ball 11 with the upper end surface 10b of the plunger 10, for example, as shown in FIG. The force is made to collide with the upper end surface 10b of the plunger 10. The steel ball launching device 5 includes a guide tube 20 having a spring 21 therein, and a launching plate 22 capable of attracting and holding the steel ball 11 by a magnet. As shown in FIG. The spring 21 is contracted by moving 22 to the back side, and the elastic force is stored. As shown in FIG. 2 (B), the firing plate 22 is rapidly pushed out by the elastic force of the spring 21. When the launch pad 22 is at the stop position, the steel ball 11 is launched by the inertial force toward the upper end surface 10b of the plunger 10, or a device such as a guide tube as shown in FIG. The steel ball 11 is suspended by a weak spring 25 in 24, and a solenoid 23 provided at the upper end of the plunger 10 is energized to magnetize the steel ball 11, thereby rapidly pulling the steel ball 11 and pulling the plunger 10. Etc. can be adopted instrument was mechanism impinging on the upper end face 10b.

  The elastic wave sensor 2 is brought into contact with the surface of the inspection object, and measures the response elastic wave of the wave applied to the inspection object by the striking device 1. As the elastic wave sensor 2, for example, an acceleration sensor, an AE sensor, a vibration sensor or the like can be used.

  The vibration measured by the elastic wave sensor 2 is analog-digital converted by an A / D converter (not shown) and then input to the waveform recording device 3.

  The waveform recording device 3 is a device for recording an electric signal of a wave measured by the elastic wave sensor 2. The waveform recording device 3 is composed of a server for storing electric signals, a storage such as a hard disk, a recording medium such as a CD and a DVD, a memory and the like. The waveform recording device 3 can receive signals from the elastic wave sensor 2 and store them in time series.

  The evaluation device 4 is composed of an electronic device such as a PC (personal computer), a smartphone, a tablet terminal, a wearable terminal, or the like. The evaluation device 4 analyzes the waveform stored in the waveform recording device 3 to evaluate the presence or absence of an internal defect in an inspection object such as concrete. The evaluation device 4 converts the waveform data on the time axis into spectrum data on the frequency axis by, for example, performing FFT (Fast Fourier Transform) on the recorded waveform. Alternatively, the wavelet transform is performed to convert the time-frequency axis spectrum data. As a result, the evaluation device 4 determines the presence or absence of an internal defect of the inspection object such as concrete based on the presence or absence of the spectrum in the desired frequency region.

  The evaluation device 4 can display each data via a display unit such as a display (not shown). Further, the evaluation device 4 can record each of these data in the storage, display the data on the display unit based on a command from the user, or write the data in the portable memory. The user can take out the portable memory from the evaluation device 4 and carry it around freely. Furthermore, the evaluation device 4 can also transfer these pieces of data to other electronic devices via the public communication network.

[Principle of striking device 1]
The hitting device 1 does not hit the inspection object (concrete or the like) directly with the steel ball 11, but uses the steel ball 11 having a predetermined diameter through the plunger 10 (one-dimensional rod) of the above structural condition, An elastic wave is applied to the inspection object. By such indirect impact, impact force is generated by collision between the steel ball 11 and the tip of the plunger, and these are materials (steel) having a stable elastic coefficient. The contact time with is always constant and a stable frequency is generated.

  According to the multiple reflection theory of elastic waves, it is known that the plunger 10 having a specified length generates a specific frequency that resonates with the length of the steel rod. The wave particularly generated when the steel ball 11 collides with the upper end surface of the plunger 10 has a wavelength whose contact time is one cycle. When this wave is multiple-reflected in the plunger 10 that can be regarded as a one-dimensional rod, a standing wave having a period twice the length L of the plunger 10 is generated. Therefore, a stable vibration frequency is generated in the plunger 10 by adjusting the frequency of the steel ball hit and the resonance frequency of the plunger 10. When used as a striking device for a concrete surface, it is necessary to bring the tip of the plunger 10 into contact with the concrete surface, and although the frequency may slightly decrease during transmission, a single frequency that can be regarded as a substantially sine wave is applied. Can be generated.

ここに、Ｃp:鋼棒の縦弾性波速度(m/s)[5120(m/s)]
プランジャー１０内に生成される波動の周波数（共振周波数）と、プランジャー１０の長さＬと、プランジャー１０の共振周波数に近い周波数を打撃によって与え得る鋼球１１の直径(mm)との組み合わせ例を示せば、下表１のとおりである。

なお、直径の異なる鋼球によって鋼材を打撃した際に発生する波動周波数は実際の計測によって測定することが可能であるが、鋼球をプランジャーに衝突させた際に発生する波動周波数は、鋼球とプランジャーの接触時間に依存し、接触時間は鋼球の質量の1/3乗に比例することが分かっている。前記鋼球の質量は、換言すれば鋼球の直径に比例するということができる。従って、鋼球の直径によって発生する波動周波数は概ね予想することが可能である。 The frequency f (Hz) of the elastic wave given to the inspection object by the plunger 10 (the frequency of the wave generated in the plunger 10) and the length L (m) of the plunger 10 are represented by the following equation (1) ).

Where Cp: longitudinal elastic wave velocity (m / s) of steel bar [5120 (m / s)]
Of the frequency (resonance frequency) of the wave generated in the plunger 10, the length L of the plunger 10, and the diameter (mm) of the steel ball 11 that can give a frequency close to the resonance frequency of the plunger 10 by striking. Table 1 below shows examples of combinations.

The wave frequency generated when a steel material is hit by steel balls with different diameters can be measured by actual measurement, but the wave frequency generated when a steel ball collides with a plunger is It is known that the contact time depends on the contact time between the ball and the plunger, and the contact time is proportional to the 1/3 power of the mass of the steel ball. It can be said that the mass of the steel ball is in other words proportional to the diameter of the steel ball. Therefore, the wave frequency generated by the diameter of the steel ball can be roughly predicted.

  Further, in order to quickly shift the wave frequency generated by the impact of the steel ball 11 to the resonance frequency by the multiple reflection in the plunger, the steel ball 11 that can give a frequency close to the resonance frequency of the plunger 10 by the impact. It is important to choose the diameter of. However, since it is difficult to completely match the wave frequency generated when the steel ball 11 is collided with the resonance frequency of the plunger 10, it is generally within ± 15% range, preferably within ± 10% range. Is desirable.

  As described above, by specifying the length of the plunger 10 and the diameter of the steel ball 11, the frequency of the wave motion given to the inspection object via the plunger 10 can be made constant, and the accuracy of the analysis result can be improved. Will be able to improve.

  Further, as shown in Table 1, in the present striking device 1, by specifying the length of the plunger 10, it is possible to arbitrarily set the frequency applied to the inspection object. In general, the higher the frequency in relative comparison, the higher the detection accuracy tends to be, but when the depth of internal defects is deep, the vibration is greatly attenuated in the concrete at high frequencies and the inspection accuracy decreases. Tend to do. Therefore, when the internal defect is located at a deep position, it is possible to suppress the attenuation and improve the detection accuracy by selecting a relatively low frequency.

[Second embodiment example]
Next, a second example of the impact device 1 will be described in detail with reference to FIGS. 4 and 5. The present striking device 1 shows a first mode in which an elastic wave sensor for measuring a response elastic wave is integrally provided with the striking device 1.

  The striking device 1 is a steel rod of a predetermined length, which is manufactured with a constant cross-sectional area over the entire length, and has a tip end surface having a spherical crown shape and an upper end surface having a flat surface shape for colliding with an inspection object. The plunger 10, a steel ball 11 having a predetermined diameter for colliding with the upper end surface of the plunger 10, a cylindrical holder 12 for holding the plunger 10, and an elastic member integrally provided in the holder 12. It is composed of the wave sensor 2.

  Since the plunger 10 and the steel ball 11 have already been described in the first embodiment, the description thereof will be omitted, and the holder 12 and the elastic wave sensor 2 will be described.

  The plunger 10 is held in the insertion hole of the cylindrical holder 12 via an easily deformable elastic body 13 such as rubber or soft resin, and as shown in FIG. The tip end surface is made to project from the tip end surface 10a of the plunger 10 by 1 to 2 mm (symbol A size). Further, the holder 12 has a flange-shaped horizontal plate portion 12a at the upper end thereof, and the elastic wave sensor 2 is integrally provided on the upper surface thereof.

  Therefore, when the plunger 10 is pressed against the inspection object side, the tip of the plunger 10 comes into contact with the inspection object due to the deformation of the elastic body 13, and the tip end surface of the holder 12 comes into strong contact with the inspection object. As a result, the response elastic wave is efficiently transmitted to the holder 12, and the response elastic wave can be accurately measured by the elastic wave sensor 2.

[Third embodiment example]
A third form example of the hitting device 1 will be described in detail with reference to FIG. The present striking device 1 shows a second mode in which an elastic wave sensor for measuring a response elastic wave is provided integrally with the striking device 1.

  In the present embodiment, the strain gauges 6 for measuring the response elastic waves are attached to the outer surface of the plunger 10 at a position near the tip and at two positions in the 180 ° direction or four positions in the 90 ° direction. It is a thing. Since the strain gauge 6 for measuring the response elastic wave is provided on the plunger 10 itself which gives the impact, the response elastic wave is efficiently transmitted to the plunger 10, and the response elastic wave is transmitted to the strain gauge 6 (the elastic wave sensor). ) Enables accurate measurement.

[Application Example of Impact Device 1]
The striking device 1 is mainly used for detecting internal defects in concrete by the impact elastic wave method, but the structure thereof has an elongated shape as described above, and this structure advantage is utilized. Then, it can be suitably applied to the evaluation method of the grout filling degree of the PC steel material in the sheath tube in the PC structure as described later.

  Grout is filled in the sheath pipe of PC steel in PC structures such as bridges, viaducts, and buildings. When the grout filling failure occurs in such a sheath pipe of the PC steel material, there is a possibility that the PC steel material may be corroded or broken due to the defective filling. Therefore, the grout filling degree in the sheath tube of PC steel is currently investigated by the impact elastic wave method.

  Recently, as a method of investigating the grout filling degree, a continuous hole is drilled from the surface of a PC structure to the outer surface of the sheath tube, and a wave is directly applied to the surface of the sheath tube through the drilled hole. A method has been proposed in which the propagation behavior of a wave applied to the sheath tube is detected, and the filling degree of grout into the sheath tube is evaluated based on the detected wave propagation behavior. The present striking device 1 can be suitably applied as a striking device that is inserted into the inside to impart a wave to the sheath tube.

  As shown in FIG. 7, when forming a continuous hole 30a from the surface of the PC structure 30 to the outer surface of the sheath tube 31, considering the degree of body damage, the hole may have a diameter as small as possible. desirable. A hammer-type steel ball conventionally used as a striking device cannot strike the sheath tube 31 in the hole.

  Therefore, as shown in the same figure, the striking device 1 having an elongated shape (the second embodiment is shown) is inserted into the hole 30a formed from the concrete surface to the sheath tube 31 and is directly attached to the sheath tube 31. It is possible to hit the target.

  1 ... Hitting device, 2 ... Elastic wave sensor, 3 ... Waveform recording device, 4 ... Evaluation device, 5 ... Steel ball launching device, 6 ... Strain gauge, 10 ... Plunger, 10a ... Tip surface, 10b ... Top surface, 11 … Steel balls, 12… Holders, 13… Easily deformable elastic body

Claims (4)

A steel rod of a predetermined length manufactured with a constant cross-sectional area over the entire length, the tip surface for colliding with the inspection object has a spherical crown shape, and the upper end surface has a planar plunger, Ri Do and a steel ball having a predetermined diameter to impinge on the upper end surface of the plunger,
The length L (m) of the plunger is expressed by the following equation (1) between the frequency f (Hz) of the elastic wave given to the inspection object by the plunger and the length L (m) of the plunger. Based on

Where Cp: longitudinal elastic wave velocity of steel bar (m / s)
The hitting device used for the impact elastic wave method is characterized in that the steel ball is selected to have a diameter capable of giving a frequency within a range of ± 15% with respect to the resonance frequency of the plunger by hitting. It said plunger tip of the radius of the sphere crown percussion device for use in an impact blow method according to claim 1 Symbol mounting and is matched with the radius of the steel balls. The plunger is held in an insertion hole of a cylindrical holder via an easily deformable elastic body, and the tip end surface of the holder is projected by 1 to 2 mm from the tip end surface of the plunger. The impact device used for the impact elastic wave method according to claim 1 , wherein the holder is integrally provided with an elastic wave sensor for measuring a response elastic wave. The impact device used for the impact elastic wave method according to claim 1 , wherein a strain gauge for measuring a response elastic wave is attached to an outer surface of the plunger.

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