·Table of Contents
Fabrication of a piezoelectric impact hammer and its application to the measurements of in-situ wave velocities of Concrete
J. -H. Tong, T. -T. Wu, C.-K. Lee
Institute of Applied Mechanics,
National Taiwan University, Taipei, TAIWAN
In this paper, a piezoelectric impact hammer is designed and fabricated for generating stable repeated impact forces on a concrete specimen surface. The piezoelectric impact hammer is consisted of a stacked multi-layer PZT, a flying head system with an on-line piezoelectric sensor embedded and a holding fixture. To measure the longitudinal or Rayleigh waves of a concrete specimen, two horizontally polarized conical transducers are utilized to receive the elastic waves generated by the piezoelectric impact hammer. Since, the distance between the two receivers is known, the propagating time can be measured by using the wave-front arrival method (Longitudinal wave) and the cross correlation method (Rayleigh wave). Due to the stable impact generation of the piezoelectric impact hammer, the experimental results showed that the accuracy of the wave velocity measurement was enhanced significantly through the signal averaging. It is worth noting that the results of this paper offer a precise and convenient way in measuring in-situ elastic properties of concrete specimens.
In the field of nondestructive evaluation of concrete, the ultrasonic wave velocity is usually measured and utilized to predict or correlate the strength of concrete [1,2]. In the measurement, two low frequency ultrasonic transducers with relatively large aperture are usually employed. Depending on the in-situ concrete structure geometry, different transducer arrangements were suggested. In addition to the ultrasonic method, various methods based on the point-source/point-receiver technique that utilize transient elastic wave response have been proposed, and several successful applications have been reported [3-6]. In those methods that are based on the transient elastic wave responses, steel ball impact sources and conical PZT displacement sensors are usually adopted.
Recently, Wu and Fang  proposed a method for determining the elastic wave velocities of a concrete specimen using transient elastic waves. In the method, the Rayleigh wave velocity is determined based on the cross correlation of the surface responses and the longitudinal wave velocity is determined by measuring the wave-front arrival of the skimming longitudinal wave. The proposed method is based on the measurements of horizontally polarized surface responses. The longitudinal wave-front can be identified from the surface response directly. In the above-mentioned method, a hand held steel ball impactor was used to generate the elastic waves. However, due to the difficulty in controlling the impact force and the time origin, the averaging method, which is commonly adopted in ultrasonics to improve the signal to noise ratio, cannot be applied. In addition, on lacking the information of the impact time origin, the application of the point-source/point-receiver technique to the imaging of crack in concrete has some limitations.
To reduce the noise level in the stress wave measurements on concrete, Popovics et al.  used a DC-powered solenoid with a spring-loaded steel shaft as the stress wave source and thus reduced the noise level by averaging the repeated impact signals. However, a quantitative report of the noise level reduction has not been included. In addition, the time origin of the impact is still hard to find. On the other hand, the wave-guide concept has been applied to design a high speed piezoelectric driven system for application in the printer industry by Chang and Wang [9,10]. Later, the mechanism was improved by Lee and Wu , which add an on-line load cell to detect the impact force for micro-impact applications such as the study of the mechanical properties of amorphous carbon under high rate indentation. The high speed piezoelectric hammer has also been applied recently to the structural modal testing of small metallic structure to give precise impact timing and positioning by Lee et al. . To the best of the authors' knowledge, application of the wave-guide based piezoelectric hammer to the NDE of concrete structure has not been reported.
In this paper, a piezoelectric impact hammer is designed and fabricated for applications in the nondestructive evaluation of concrete structures. To obtain a large enough impact force, special design in the holding fixture of the impact hammer is utilized. With an on-line sensor, the impact time origin resolution can be controlled in the range of sub-microsecond. To demonstrate the accuracy of the newly fabricated impact device, measurements of the longitudinal and Rayleigh wave velocities of a concrete specimen were conducted. Due to the stable impact generation of the piezoelectric impact hammer, the experimental results showed that the accuracy of the wave velocity measurement is enhanced significantly through signal averaging.
2. Piezoelectric impact hammer system for concrete testing
The piezoelectric material is known to have the ability for generating large force but with very little deformation. In order to transform the blocking force of the piezoelectric material into a free fly motion, the wave-guide concept  is adopted in this paper. Fig. 1 shows the basic design of a piezoelectric impact hammer for generating elastic waves in concrete structures. The hammer is composed of three major parts, the PZT stack, the wave-guide, and the assembling of the flying head. The cross section of the PZT rectangular stack is 10mm x 10mm and the length is 20mm. The maximum generation force of the PZT stack is 3500 N, while its maximum displacement is 18.4 mm and the corresponding resonant frequency is 69 kHz. The diameter of the wave-guide is 5 mm with 37 mm in length. The flying head is consisted of a semi-sphere impactor with 3 mm in radius and an on-line piezoelectric sensor. The sensor is a tiny PZT disk with around 2 mm in diameter.
Fig 1: The assembly of the piezoelectric impact hammer.
It is worth noting that the capacitance of the PZT stack is as high as 5.4 mF and the driving voltage is 150 V, therefore, to generate the large enough power to drive the impact hammer, a high voltage pulse generator is needed. In this study, a 150 V voltage pulse generator, which is composed of a capacitor bank and an N channel MOSFET, is used in this study. In addition, a gated circuit was designed to gate the impact signal from the on-line piezoelectric sensor and to send a trigger signal synchronous with the impact time origin.
3. Calibration of the impact hammer system
The time origin of an impact induced by the piezoelectric hammer can be estimated from the triggering signal of the hammer and the fixed flying time from the wave-guide to the testing surface. For a flat sample surface and a relatively small impactor, the above estimation method in a well-posed experiment can results to a resolution in the microsecond range. However, this is not true for the case of testing in a concrete specimen. The surface of a concrete specimen is uneven as compared with that of a metal surface, in general. Therefore, the distance the flying head flies from the wave-guide to the surface of the specimen is not a constant. The deviation of the flying distance results to deviation in the flying time estimation. In addition, there is also a deviation in the speed of the flying head for different triggering, which will cause errors in the flying time estimation too. For example, the flying time variance is about 20 ms for testing on a concrete specimen using the current fabricated piezoelectric hammer. This variation is unacceptable for a precise measurement.
To overcome the above-mentioned difficulties, an on-line piezoelectric sensor was utilized to determine the precise time origin of the impact at the specimen surface. The upper trace shown in Fig. 2 is the signal detected by the on-line piezoelectric sensor for a particular impact on a concrete specimen. Three separated pulses are found in the signal. Pulse 1 is the first received signal by the on-line sensor for wave transmitting from the wave-guide and arriving at the sensor. Pulse 2 denotes the back propagation wave induced by the impact of the flying head on the testing surface. Pulse 3 is generated by the impact between the bounced back flying head and the wave-guide.
Fig 2: Signal from the on-line piezoelectric sensor (upper trace), gated region (middle trace) and the time reference (lower trace).
To reduce the error from in-situ applications, the time reference of this impact device must be deduced from the true impact signal, pulse 2, which is induced by the flying head hitting the specimen. A gated circuit was designed to catch the signal from the on-line piezoelectric sensor and send a synchronous pulse out as the time reference of the measuring system (Fig.2). With this circuit, the accuracy of time reference of the impact can be improved to the range of sub-microsecond.
The time reference mentioned above was utilized to average the repeated signals to enhance the S/N ratio. However, some application of the impact hammer, such as crack depth evaluation, needs not only an accurate time reference but also the true impact time origin. To obtain the impact time origin, calibration experiment on a steel block was conducted, as shown in Fig. 3. A 9 V battery was used as the voltage source to give the impact time origin. As the flying head hits the steel surface, a circuit loop is formed and this gives a step signal with a very short rise time (about 0.02 ms). The experimental result (Fig. 4) showed that the time delay between the time reference and the 9V-step signal is about 7.2 ms.
Fig 3: Arrangement of the calibration experiment.
Fig 4: Experimental result of the calibration experiment.
4. Principle of in-situ wave velocity measurements
4.1 Longitudinal wave velocity measurement
Analysis made by Wu and Fang  has shown that for elastic wave generation by a point source in a half space, the amplitude of the tangential component of the skimming longitudinal wave is four to five times larger than that of the normal component. In addition, it gives a much clear displacement jump at the wave-front arrival. For this reason, a pair of horizontal polarized conical PZT transducers were employed is this paper to measure the wave-front arrival of the surface skimming longitudinal wave.
4.2 Rayleigh wave velocity measurement
In the measurement of Rayleigh wave velocity using the transient elastic wave, the method proposed by Wu et al. was adopted . In the method, the relative positions of an impact source and two receivers (S1 and S2) are shown in Fig. 5. The distance between the source and the first receiver S1 is r1, and the distance between the first and second receiver S2 is d. In order to utilize the cross correlation method to determine the Rayleigh wave transit time between the first receiver and the second receiver, it is required that the wave responses received at these two receivers must be dominated by the Rayleigh waves. Therefore the distance r1 must be large enough to ensure that the Rayleigh wave carries most of the energy. Let h1(t), h2(t) be the signals recorded at S1 and S2. The cross correlation function of these two signals is defined as
Fig 5: Relative position of the impact source and receivers.
If the signals h1(t) and h2(t) are similar with a time delay Dt, the maximum of z(t) occurs at Dt. In other words, the time delay between h1(t) and h2(t) can be determined by finding the corresponding time at which z(t) is a maximum.
5. Experimental verification
In the measurements of in-situ wave velocities of concrete specimens, instead of finding the time origin of an impact source, two transducers to measure the time delay between the wave-front arrivals received at the two different positions can be used. The source and the receivers are located on the same straight line. Since the distance between the receivers is known, the wave velocity can be obtained without knowing the time origin of the impact source. However, the piezoelectric impact hammer does give us stable impact with repeatable impact force and constant time reference. Therefore, the signal to noise ratio for wave signal in concrete can be improved significantly by the averaging method. In this study, two home-made horizontally polarized conical transducers were utilized to measure the tangential component of the surface displacement signals. The received voltage signal from the conical transducer was amplified by a preamplifier and recorded by a 100 MHz digital oscilloscope (LeCroy 9314L). To minimize the electric noise, a pre-amplifier is embedded in the transducer assembly.
The dimensions of the concrete specimen used in this study are 60cm x 80cm in width and length and 30 cm in height. The water-to-cement (W/C) ratio of the concrete specimen is 0.58.
5.1 Longitudinal wave measurement
In determine the longitudinal wave-front arrival using a hand held impactor , to enhance the resolution of the delay time measurement, the recorded signal was enlarged and processed by a linear phase finite impulse response filter. Shown in Fig. 6 are the measured signals by the two horizontally polarized conical transducers. The distance of the impact source to the first receiver was kept at the value of r1=7 cm. The distance between the first receiver and the second receiver was kept at a constant distance 10 cm. The signals showed that there are a lot of noises riding on top of the elastic wave signals, and hence to determine the true arrival of the longitudinal wave-front, special care is needed to ensure a reliable measurement. Although the hand held impactor method can give a measurement of the longitudinal wave-front arrival with reasonable accuracy, it is relative time consuming and is not practical for in-situ measurement.
Fig 6: Received displacement signals of the longitudinal wave measurement using a hand-held impactor.
Fig 7: Averaged (40 times) displacement signals of the longitudinal wave measurements.
Fig. 7 is the results of the longitudinal wave-front measurements using the newly fabricated piezoelectric hammer with 40 times averaging. The results showed that the signal to noise ratio has been improved significantly, and therefore, the wave-front arrivals can be detected automatically by setting a common triggering level. Table 1 shows the results for four different measurements of the longitudinal wave velocities at the same location of the concrete specimen. The average longitudinal wave velocity is 3974 m/s and the standard deviation of the four measurements is 0.47%, which is very small for concrete applications.
|Standard deviation = 0.47%
|Table 1: Four different measurements of the longitudinal wave velocities at the same location of the concrete specimen.|
5.2 Rayleigh wave measurement
Figs. 8 and 9 shows the Rayleigh wave signals generated by a hand held impactor and the piezoelectric impact hammer (20 times averaging), respectively. Significant improvement on the signal to noise ratio has been achieved. The signals after the peaks of the Rayleigh waves have been removed before conducting the cross correlation of the two signals. Table 2 is the result for four different measurements of the Rayleigh wave velocities at the same location of the concrete specimen. The average Rayleigh wave velocity is 2178 m/s and the standard deviation of the four measurements is 0.08%.
Fig 8: Received displacement signals of the Rayleigh wave measurement using a hand-held impactor.
Fig 9: Averaged (20times) displacement signals of the Rayleigh wave measurement
|Standard deviation = 0.08%
|Table 2: Four different measurements of the Rayleigh wave velocities at the same location of the concrete specimen.|
In this paper, a piezoelectric impact hammer has been designed for applications in the nondestructive evaluation of concrete structures. Accurate impact time origin has been achieved by embedding a tiny piezoelectric sensor in the flying head. With the on-line sensor, the impact time origin resolution can be controlled in the range of sub-microsecond. Measurements of the longitudinal and Rayleigh wave velocities of a concrete specimen were conducted and the results demonstrated the accuracy of the newly fabricated impact device.
The authors thank the financial support of this research from the National Science Council of ROC through the grant NSC87-2732-E-002-002.
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