Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2000-05-01
2001-03-20
Moller, Richard A. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S630000, C073S631000, C073S579000
Reexamination Certificate
active
06202490
ABSTRACT:
TECHNICAL FIELD
This invention relates to a nondestructive testing apparatus for investigating, managing and evaluating construction and degradation of a structure or a material such as concrete in a building and an engineering work.
BACKGROUND ART
In order to enforce nondestructive testing of a concrete structure by using an elastic wave, it is necessary to precisely detect a reflection wave or a transmission wave propagating through the interior of the concrete. Namely, in view of a magnitude of a bridge, a road, dam, a building or the like to be inspected, a wave transmitter is required to have an ability to effectively injecting, into the measurement object, the elastic wave in the level of a frequency of vibration detectable even if the wave transmits the interior of the concrete for several tens cm to several tens m. Unlike a uniform material such as metal, in case of the concrete, if an acoustic wave in the band of about several MHz to be used for a metal material is introduced into the concrete, the attenuation is severe and the reach distance is short. Thus, it is impossible to use this acoustic wave for the concrete. For this reason, the band from several tens Hz to several tens kHz has to be utilized.
Conventionally, in the nondestructive testing of such a engineering building structure made of concrete, the measurement of the acoustic velocity or a thickness of the measurement object and an inference of an interior structure or a position of an abnormal portion are performed by a impact elastic wave method using an impact hammer, and by supersonic tests such as a pulse reflection method in which a probe using a piezoelectric vibrator is used as a wave transmitter for a supersonic wave, a transmission method and a resonance method.
The piezoelectric material constituting the piezoelectric vibrator that has been extensively used as a wave transmitter for a supersonic wave generates a strain in accordance with a magnitude of an electric field. The piezoelectric vibrator has a structure in which the piezoelectric material is sandwiched by electrodes and outputs a large vibration at a frequency at which its thickness corresponds to a half-wavelength of a longitudinal wave of the piezoelectric material. In general, a mechanical Q value of the piezoelectric material is high and the output efficiency of other than this mechanical resonance point is markedly degraded. Accordingly, in order to obtain a vibration in the band that is required for diagnosis of the concrete structure, the thickness of the piezoelectric vibrator should be several tens cm or more. However, it is very difficult to structure such a large size piezoelectric vibrator.
Namely, the probe using the piezoelectric vibrator is suitable for outputting a supersonic wave having a uniform frequency that is equal to or more than several tens MHz due to its characteristics. For this reason, there is a limit to the measurement in the nondestructive testing of the concrete structure in which the high frequency wave is remarkably attenuated. In particular, since it is difficult to obtain the energy of a low frequency wave that is needed for detecting the supersonic wave propagating through several tens m or longer by the piezoelectric vibrator, an impact hammer, a drop of a metal weight or the like is utilized.
The impact hammer is extensively used in a wide field for the reasons such as its simpleness and the magnitude of impact energy. The vibration band is several tens kHz. It is also applied to the nondestructive testing of a long and large concrete structure. In such a concrete structure, in order to obtain the reflection wave, a wave having a certain wavelength or more is required. However, the reflection wave might be buried in a signal of the impact. It is necessary to suitably adjust the intensity of the impact to meet the purpose of measurement. Namely, this largely depends upon the experiences or a sense of the tester. On the other hand, it is difficult to always keep the vibration force constant. The waveform observed varies for every impact. This leads to the fluctuation in evaluation. The vibration band is about 1 kHz but it is impossible to control the frequency as desired. In particular, it is difficult to detect the reflection wave in a short distance due to the affect of its reverberation wave.
The operation of a supersonic wave testing method using the piezoelectric vibrator will now be described.
A pulse reflection method and a transmission method are known as one of the measurement methods using the supersonic wave.
The pulse reflection method and transmission method are a method in which the pulsational supersonic wave is introduced from the surface of the structure, a time period until the reflection wave thereof comes back or a time period until the transmission wave propagates is measured, the acoustic propagation speed or the thickness or the distance to the reflection surface of the measurement object is obtained from the time period to thereby infer the interior structure or the absence/presence of the damage of the measurement object. On the other hand, the resonance method is a method in which a wavelength of the supersonic wave to be introduced into the measurement object is continuously changed by sweeping the frequency of a piezoelectric type oscillator to measure the resonance frequency and the plate thickness is measured from the frequency.
Furthermore, as a method of measuring an acoustic propagation speed or a thickness of the measurement object, there is a sing around method in which the introduced supersonic wave pulse is detected by a wave receiver at the end face and the detected pulse is used as a trigger to repeat the oscillation of the supersonic wave pulse. According to this method, a pulse row is generated at a constant cycle. This cycle is identical with the delay time for the pulse to propagate through the measurement object. It is therefore possible to obtain the acoustic propagation speed or the thickness of the measurement object.
The operation of the impact elastic wave method using the impact hammer will now be described.
The impact elastic wave method is a method in which the hammer impact is given to the measurement object so that a proper vibration owned by the object per se is stimulated and utilized in measurement. This method may be widely applied to a concrete, building stone, a brick material, a timber structure, a laminated material, an underground buried object or the like and is widely used as a nondestructive inspection method owing to its easiness to carry out the test.
FIG. 20
is a view showing the constitution of the impact elastic wave method using the hammer. In the drawing, reference numeral
311
denotes a hammer, reference numeral
312
denotes an impact receiving sensor, reference numeral
313
denotes an elastic wave receiver, reference numeral
314
denotes a storage oscilloscope and reference numeral
315
denotes a measurement object.
The operation of the reflection wave measurement method by hammering will now be described. The impact receiving sensor
312
is applied to the measurement objective surface of the measurement object
315
and the hammer
311
impacts the surface. In order to enhance the precision of measurement at this time, it is necessary to be attentive and adjust the intensity of the impact depending upon the measurement purpose or the material of the measurement object
315
and make sure that the impact is given only once. The elastic wave introduced into the measurement object
315
by the impact advances through the interior of the object
315
to be measured while reflected at an abnormal portion such as the confronting surface of the measurement objective surface, an internal structure or a damage or a gap, so that a part thereof reaches the impact receiving sensor
312
. The output of the impact receiving sensor
312
passes through a filter and the waveform of the frequency corresponding to the purpose of measurement is extracted. When the pulse wave by the impact is given as a trigger signal of the storage osc
Hattori Shinichi
Matsuhashi Kanji
Sakamoto Takahiro
Shimada Takashi
Taniguchi Ryosuke
Mitsubishi Denki & Kabushiki Kaisha
Moller Richard A.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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