• Journal of the Korean Society for Nondestructive Testing
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• v.24 no.5
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• pp.465-475
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• 2004

This paper is the first part of the study on the development of a smart active layer (SAL) sensor, which consists of two parts. In this first part, the theory and concept of the SAL sensor is investigated, which is designed for the detection of elastic waves caused by internal cracks and damages in structures. For the development SAL sensor, (i) the basic theory of elastic waves was studied, (ii) the feasible study of the SAL as an elastic waves detection sensor using the finite element analysis (FEA) with respect to a piezoceramic disc was performed. (iii) the comparison of performances between some piezoceramic sensors and a commercial acoustic emission (AE) sensor was accomplished to ensure the applicability by the experimental means, such as a pencil lead break test. Also, the conceptional study for the SAL sensor, which can be utilized for the effective detection and locating of defects by the arrangement of regularly distributed sensors, was discussed.

• Composites Research
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• v.14 no.1
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• pp.1-7
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• 2001

In this study, damage evaluation of glass fiber reinforced thermoplastic composites was investigated with acoustic emission method. Specimens of 1.7mm thickness laminate were made from PET and 7 layers o171ass fabrics. Notch and impact loading were added to the specimen and normal tensile test and tensile test with the dead load were carried out. AE signal was measured as the functions of notch ratio to the width0 and impact energy in order to find out the correlation between fracture mode and AE parameters. The result has shown that low amplitude of AE signal was due to the microcrack of matrix and its growth, whereas the amplitude in the mid range was the response to the delamination and interfacial separation. In the range of high amplitude above 90dB. the fracture of glass fabric was found. Tensile strength decreased with increasing notch ratio to the width and impact energy because of tile effect or delamination, the cracking of matrix and stress concentration. In proportion to the size of damaged area. AE signal showed its wider range of frequency and energy as well as increased number of hits.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.6
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• pp.1388-1398
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• 1994

The fracture toughness and micro-fracture mechanisms of the porous glass and stainless fiber reinforced glass composite were evaluated by using the acoustice mission(AE) technique, fracture toughness $test(K_{IC})$ and the macroscopic observation of the specimen surface which was being under the loading. At initial portion of the loading, the AE signals with low energy, of which origins were considered as the micro-cracks formated at the crack tip, were emitted. With increasing the applied load, AE signals having higher energies were generated due to the coalesence of micro-cracks and fast fracture. Based on the such relationship between AE emission and loading condition, fracture toughness $K_{IAE}$ could be defined successfully be using the $K_I$ value corresponding to an abrupt change of the accumulated AE signal energies emitted during the fracture toughness test. In spite of its brittleness of glass material, nonlinear deformation behavior before maximum load was observed due to the formation of micro-cracks. Further, the stainless fiber may have attributed to the improvement of fracture toughness and the resistance to crack propagation comparing to noncomposited materials Finally, models of the micro-fracture process combined with the AE sources for the porous glass material and its composite were proposed paying attention to the micro-crack nucleation and its coalescence at the crack tip. Fiber fracture and its Pullout, deformation of fiber itself were also delinated from the model.

• The Journal of Engineering Geology
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• v.21 no.4
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• pp.295-304
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• 2011

The first step in improving our understanding of uncertainties suclt as rock mass strength parameters and deformation modulus in rock masses around high-level radioactive waste disposal repositories, for improved safety, is to study the process of crack development in intact rock. Therefore, in this study, the fracture process and crack development were examined in samples of KURT granite taken from the KAERI Underground Research Tunnel (KURT), based on acoustic emission (AE) and moment tensor analysis. The results show that crack initiation, coalescence, and unstable crack occurred at rock uniaxial compressive strengths of 0.45, 0.73, and 0.84, respectively. In addition, moment tensor analysis indicated that during the early stage of loading, tensile cracks were predominant. With increasing applied stress, the number of shear cracks gradually increased. When the applied stress exceeded the stress level required for crack damage, unstable shear cracks which directly result in failure of the rock were generated along the failure plane.

• Journal of Korean Tunnelling and Underground Space Association
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• v.11 no.1
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• pp.47-56
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• 2009

Rock bolts and shotcrete play a crucial role as a main support system in the underground space. Thus, the safety of the underground space may be affected by the defect of rock bolts. In order to evaluate the rock bolt integrity by using non-destructive technique, the transmission method of the guided ultrasonic waves, which are generated by using the piezo disk elements has been successfully performed. The energy generated by the piezo disk elements, however, is not enough for the rock bolts in the field. In addition, the piezo disk elements should be installed at the end of the steel bar during construction of the rock bolts. The purpose of this study is the devolvement of the reflection method, which may generate enough energy, and the application in the field rock bolts. Both laboratory and field tests are carried out. The guided ultrasonic waves with high energy are generated by the hammer impact with the center punch, and the AE sensor is used to measure the reflected guided waves. The received guided waves are analyzed by the wavelet transform. The peak value of the wavelet transform produces the energy velocity, which is used for the evaluation of the rock bolt integrity. The energy velocity increases with an increase in the defect ratio in both laboratory and field rock bolts. This study demonstrates that the hammer-impact reflection method may be a suitable method for the evaluation of the rock bolt in the field.

• Journal of the Korean Society for Nondestructive Testing
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• v.31 no.1
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• pp.62-67
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• 2011

Nanoparticles-coated and impact-damaged carbon-fiber reinforced plastics(CFRP) laminates were tested under compression-after-impact(CAI) mode and the propagation of damage due to compressive loading has been monitored by acoustic emission(AE). The impact damage was induced not by mechanical loading but by a simulated lightning strike. CFRP laminates were made of carbon prepregs prepared by coating of conductive nano-particles directly on the fibers and the coupons were subjected to simulated lightning strikes with a high voltage/current impulse of 10~40 kA within a few microseconds. The effects of nano-particles coating and the degree of damage induced by the simulated lightning strikes on the AE activities were examined, and the relationship between the compressive residual strength and AE behavior has been evaluated in terms of AE event counts and the onset of AE activity with the compressive loading. The degree of impact damage was also measured in terms of damage area by using ultrasonic C-scan images. From the results assessed during the CAI tests of damaged CFRP showed that AE monitoring appeared to be very useful to differentiate the degree of damage hence the mechanical integrity of composite structures damaged by lightning strikes.

• Journal of the Korean Society for Nondestructive Testing
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• v.15 no.1
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• pp.277-290
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• 1995

The study was carried out to analyze the fracture behavior and the acoustic emission(AE) characteristics and to find the relationship among tensile strength, fracture toughness and cure pressure in owe process of the carbon fiber reinforced composites of two types, $[0^{\circ}/90^{\circ}]_{2S}$ and $[0^{\circ}\;_2/90^{\circ}\;_2]_S$. AE signals were detected during the curing process, tensile tests and fracture toughness tests by acoustic emission(AE) measurements, respectively. Tensile strengths showed that the less cure pressurizing steps and the side of $[0^{\circ}/90^{\circ}]_{2S}$ specimens had the higher strengths than those of the others. Fracture toughness by the change of test temperature showed nearly same values in the same temperature region, but the higher test temperature had the lower fracture toughness values. In order to examine the relationship between fracture behavior of CFRP in tensile and fracture toughness tests and AE signals, the post processing for AE parameters of AE data and the observations of microscope and SEM have been carried out respectively.

• Journal of the Korean Society for Nondestructive Testing
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• v.36 no.3
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• pp.181-187
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• 2016

A thermal power plant has a heat exchanger tube to collect and convert the heat generated from the high temperature and pressure steam to energy, but the tubes are arranged in a complex manner. In the event that a leakage occurs in any of these tubes, the high-pressure steam leaks out and may cause the neighboring tubes to rupture. This leakage can finally stop power generation, and hence there is a dire need to establish a suitable technology capable of detecting tube leaks at an early stage even before it occurs. As shown in this paper, by applying acoustic emission (AE) technology in existing boiler tube leak detection equipment (BTLD), we developed a system that detects these leakages early enough and generates an alarm at an early stage to necessitate action; the developed system works better that the existing system used to detect fine leakages. We verified the usability of the system in a 560MW-class thermal power plant boiler by conducting leak tests by simulating leakages from a variety of hole sizes (ⵁ2, ⵁ5, ⵁ10 mm). Results show that while the existing fine leakage detection system does not detect fine leakages of ⵁ2 mm and ⵁ5 mm, the newly developed system could detect leakages early enough and generate an alarm at an early stage, and it is possible to increase the signal to more than 18 dB.

• Journal of the Korean Society for Nondestructive Testing
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• v.18 no.1
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• pp.1-9
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• 1998

The attenuation of acoustic emission (AE) waves was evaluated for injection-molded short-fiber-reinforced thermoplastic composites employing simulated AE waves. Values of attenuation coefficient (${\alpha}$) decreased more with increasing fiber volume fraction ($V_f$) than that expected from a simple linear relation between ${\alpha}$ and $V_f$. The effect of wave attenuation was taken into account in a quantitative analysis of the AE peak amplitude distribution which was obtained from each zone partitioned in a specimen gage portion. The amplitude distribution compensated for the measured attenuation loss was exhibited almost similar in every zone of the specimen. Consequently, it was, shown that the AE amplitudes obtained from fiber/plastic composites were considerably affected by the attenuation.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.16 no.2
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• pp.14-22
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• 2007

In this paper, the acoustic emission(AE) behaviors were investigated with single-and 2-spot resistance spot welded SPCC specimens. Test specimens were welded horizontally and/or vertically according to the rolling direction of base netal in 2-spot welding. In the case of 2-spot welding, when tensile-shear test has below amplitudes: crack initiation $50{\sim}60dB;$ tear fracture $40{\sim}50dB$. And when cross tensile test has below amplitudes: early stage $75{\sim}85dB;$ yielding point $65{\sim}75dB;$ post yielding $40{\sim}60dB;$ plug fracture $70{\sim}80dB\;or\;90{\sim}100dB$. Also, from the b-value that is slope of AE amplitude, we knew that there are lots of low amplitudes if b-value is big(i.e., tensile-shear $specimen{\rightarrow}tear$ fracture or shear fracture), and there are lots of high amplitudes if b-value is small(i.e.. cross tensile $specimen{\rightarrow}plug$ fracture). As the results of fiacture mechanism analyses through AE amplitude distributions, change of the b-value represented fracture patterns of materials. Correspondingly, low amplitude signals appeared in crack initiation, and high amplitude signals appeared in base metal fracture. We confirmed that these amplitude distributions represented the change or degradation of materials.