Nondestructive Evaluation of Closed Cracks Using an Ultrasonic Transit Timing Method

In this study, we investigated a method for the non-destructive detection of cracks based on an ultrasonic transit timing method. This method is considered to be an effective means for crack monitoring over an extensive coverage area. Aluminum specimens were prepared with a range of crack sizes and subjected to a tone burst wave to investigate the relationship between crack depth and the transit time difference of the surface wave. A good correlation was obtained between these two variables. The dependence of transit delay time on frequency was also examined over the frequency range from 500kHz to 5 MHz. The results showed that greater transit time differences occurred in the lower frequency range less than 1MHz. With these frequencies, ultrasonic waves can travel over a long distance with little attenuation and are better suited for crack monitoring over an extensive area. The technique was applied to real-time monitoring of fatigue crack growth in stainless steel specimens and found to be useful for crack detection. Introduction: Cracks in machineries and other structures do not usually lead to direct destruction, but affect the safety until the cracks grow to a certain size. With the exception of unavoidable disasters such as earthquakes, many damaging and destructive accidents are caused either by overlooking large cracks, or by a failure to detect the growth of known cracks. The allowable size of a crack depends on the material and the shape of the crack. With respect to nuclear facilities, allowable crack sizes range from several mm to several scores of mms [1], [2]. Periodic inspection with nondestructive techniques such as ultrasonic inspection is a useful method for detecting the presence of cracks. However, periodic inspections are not usually carried out on a daily basis and therefore potentially serious cracks may remain undetected. Furthermore, it is often difficult to predict when and where a crack will occur, and the area of monitoring may be very large. Clearly, there is a need for monitoring systems that can regularly and reliably detect cracks in mechanical structures. Such systems could also potentially provide economic benefits by reducing maintenance costs and increasing the life of the structure. Many studies have reported the use of ultrasonic waves for crack detection. Cook proposed a method for detecting small fatigue cracks using an amplitude change of Rayleigh waves [3]. Himawan used maximum 1 ADDRESS CORRESPONDENCE TO: Structural Health Monitoring Research Group, Research Institute of Instrumentation Frontier, AIST,1-1-1, Umezono, Tsukuba Central 2, Ibaraki-ken, 305-8568, JAPAN Abstract Number: 246 Name: Junji TAKATSUBO, Phone: +81-29-861-3067, Fax: +81-29-861-3126, E-mail: [email protected] amplitude of reflection echo due to cracks to evaluate them quantitatively [4]. Salam proposed an ultrasonic shear wave method for sensitive detection and sizing of small closed cracks [5]. Kawashima have investigated a nonlinear ultrasonic method to detect closed cracks with leaky surface wave [6]. Although these methods are useful for detecting and sizing cracks using portable equipment, they are not suitable for monitoring over large areas because they require several sensors or water dipping or scanning in order to detect flaws. Two important considerations for the development of novel crack monitoring techniques are reliable detection and accurate measurement of cracks. However, crack monitoring systems capable of performing both of these functions are very expensive, and therefore it is considered more practical to divide these functions into A) primary monitoring, which seeks to detect cracks, and B) secondary monitoring, which attempts to accurately measure crack dimensions. It is desirable that primary monitoring can be carried out using remote automatic measurement system, can be applied over an extensive surface area, and is low cost and simple to operate. The major requirement for secondary monitoring is that it can be carried out quickly and with high accuracy of measurement. Secondary monitoring involves the measurement of known cracks with portable equipment. Our study was carried out with the goal of developing a primary monitoring system. An ultrasonic transit timing method was used for detecting cracks over an extensive area. For open cracks (such as slits), the surface wave is known to diffract around the crack, causing an echo at the crack tip. This echo can be measured with sufficient sensitivity that the crack can be detected based on the change in the transit time of the wave. However, for closed cracks, the ultrasonic wave passes directly through the crack due to contact at the crack interface. The resulting ultrasonic wave is therefore very weak [7]. Visualization of ultrasonic propagation around cracks [8] revealed that the transit time of the ultrasonic wave increases slightly when the wave transits the closed-crack interface. This time difference is dependant on the frequency of the incident wave and is more pronounced in the low-frequency region of less than 1 MHz. Ultrasonic waves in the wavelength range of several hundred kHz are routinely used for crack inspection of concrete and are considered suitable for monitoring extensive areas because they can travel long distances with little attenuation. In this paper, we report a method for the nondestructive evaluation of fatigue cracks that makes use of the transmission time difference. Propagation Behavior of Ultrasonic Waves around a Crack: Using a laser-based ultrasonic visualizing system [8], we observed the propagation behavior of ultrasonic waves around both slit and fatigue cracks. The propagation characteristics of the two crack types were then compared. The samples used for the ultrasonic visualization are shown in Fig. 1. Compact Tension (CT) specimens with fatigue cracks were used to investigate closed cracks. After generating fatigue cracks in the specimens by applying a load of 15 kN at a frequency of 10 Hz, we cut the specimens as shown in Fig. 1. To investigate open cracks, the specimens were treated with electric discharge machining. This resulted in a slit crack of a width 0.3 mm. The material used to produce both specimens was aluminum alloy. An angle beam transducer with a frequency of 5 MHz (90o) was attached as shown in Fig. 2. Short-pulse ultrasonic waves were generated by the transmission of spike waves from the pulse generator. A heterodyne optical system (Fig. 3) was used to measure the waveform displacement of the ultrasonic wave propagating through a 15×15 mm area of the specimen. The measurement pitch was set at 0.15 mm. The displacement waveform data was stored on a computer using a digital oscilloscope. After completion of each measurement, displacement data acquired at an arbitrary time at all measurement points were intensity-modulated and displayed synchronously on a PC screen. The propagation behavior of the ultrasonic waves could be visualized using this method. The visualized images obtained using this technique are shown in Fig. 4. The slit (fatigue crack) depth was 6.0 (5.2 mm) for these images. As Fig. 4 clearly indicates, although ultrasonic waves for slit crack propagate by diffracting the tip of the slit, the waves for fatigue crack were found to pass through the interface. Few ultrasonic waves were found to detour the tip of the fatigue crack. The technique, measuring the diffracted waves across the tip of the crack was therefore considered to be of little use for the detection of closed cracks. Further studies investigating the propagation behavior of ultrasonic waves across fatigue cracks showed that the slight delay of transit time appeared when ultrasounds across the crack interface. To explain this observation, there may be some areas where the crack interfaces are in contact, and other areas where they are 60 27 24 Aluminum Crack or slit Cutting Line CT Specimen Fatigue crack Fig.1 Preparation of cracked specimen Angle Beam Transducer(90o) Laser scanning area 15mm × 15mm crack Laser Beam ultrasound Fig.2 Scanning area of laser beam, and mounting position of an angle beam transducer for visualizing ultrasonic waves. Detection Laser Digital oscilloscope