초음파 서모그래피를 적용한 내연기관 피스톤의 비파괴 신뢰성평가

Abstract

Passenger cars, trucks, buses, and motorcycles are moved by converting the heat energy generated by burning fuels such as gasoline, diesel, and LPG inside the engine to mechanical energy. Such engine is called “combustion engine,” which is generally used in motor vehicles. The number of automobile engine parts manufacturers in South Korea was 1,290 in 2008, representing a 200.10% increase from 642 in 2003 and a 962% increase from 134 in 1991. The annual average growth rate of these manufacturers was 13.95% from 1991 to 2008 and 24.23% from 2001 to 2003. Thus, the growth rate during the recent three years increased compared to the growth rate from 1991 to 2003. The domestic automobile engine parts manufacturing industry continuously expanded from 1991 at the annual average growth rates of 5-30%, and the production has also steadily increased. This growth trend is expected to continue for more than five years in the future. While the domestic automobile engine parts manufacturing industry is continuously growing, the nondestructive testing(NDT) technology for engine parts is still weak. In South Korea, NDT is used in the three areas of quality control, quality evaluation, and maintenance. The domestic NDT service market is mostly for quality control and quality evaluation related to the product manufacturing processes. The NDT market for maintenance is limited to nuclear-power-related facilities. It is true that the technical level of the South Korean NDT industry is weaker than those of Japan and the U.S., but South Korea has competitiveness in RT, UT, MT, and PT. The percentage of RT and UT, which accounts for over 50% of the NPT market, is 70% in South Korea compared to 55.7% in Japan. This situation will be gradually improved by the increasing share of UT. Among the existing NDT techniques, some are not easy to automate (e.g.,MT) and some have been designated as objects of environmental regulation due to pollution. Thus, the percentages of the use of NDT technologies will gradually change in the future. It is expected that UT, RT, IR, optical, and photonic tests will be used most frequently. Regarding the status of infrared and thermal testing (IR), the demand for IR for NDT is increasing, thanks to the dramatic development of thermography technology in the last few years and the rapid digitalization from the use of microprocessor and computer technologies. At present, cutting or radiographic testing is generally used to test automobile parts. The cutting test can accurately determine the internal quality and can find the defects, but it can damage some parts and can be used only for cutting surfaces. On the other hand, the radiographic test, although nondestructive, has difficulty identifying the exact shape and location of defects. To solve this problem, H Company is researching on the use of computed tomography (CT). CT with several hundreds of kV can effectively analyze the exterior and interior of aluminum parts, but its use is limited for steel, plastic, or polymer parts due to its permeability and resolution problems. Furthermore, the device is voluminous, and it takes a long time for it to produce one image of aluminum parts; it thus takes about five hours for it to scan large parts such as engine blocks and one to two hours to scan small parts. Therefore, CT is appropriate for use in sampling or internal-quality tests on small products, but it is difficult to apply CT to mass production lines that require instant internal-quality tests. Ultrasound thermography detects defects by radiating 20-30 kHz ultrasound waves to the samples and capturing the heat generated from the defects with the use of an infrared thermographic camera. This technology is being spotlighted as a next-generation NDT technique for the automobile and aerospace industries because it can test large areas and can detect defects such as cracks and exfoliations in real time. The heating mechanism of the ultrasound vibration has not been accurately determined, but the thermomechanical coupling effect and the surface or internal friction are estimated to be the main causes. When this heat is captured by an infrared thermographic camera, the defects inside or on the surface of objects can be quickly detected. Although this technology can construct a testing device relatively simply and can detect defects within a short time, there are no reliable data about the factors related to its detection ability. The ultrasound waves with a frequency of 20 kHz have elements with wavelengths of several tens of centimeters. Furthermore, they have sufficient amplitude energy even when they progress for a much longer distance than the wavelength. If there is no loss in the material, ultrasound waves can spread for a distance of several wavelengths without attenuation. The typical speed of sound waves in solids is several kilometers per second. When an ultrasound excitation pulse is applied, the sound field completely penetrates the entire test region of a structure with the size of 1 ㎡ or less. If a high-speed infrared thermographic camera with the image-capturing time of several kHz is used, defects with the size of several tens of ㎲ can be detected. Thus, to effectively cause heat at the defects, the excitation frequencies of 15~40 kHz at the boundary of audible frequencies are used. In general, 20 kHz frequencies are widely used. In this study, the ultrasound thermography technique was used to manufacture gasoline and diesel engine piston specimens, and nondestructive reliability tests to verify the applicability and validity of the ultrasound thermography technique.