Techniques for Nde of Closed Cracks

Usually ultrasonic techniques tend to evaluate the closed cracks as smaller than the actual size. In this paper, we describe two strategic techniques for accurately evaluating the closed cracks by means of ultrasound. First, we show the advanced technique utilizing the thermal stress induced by cooling a cracked part. It is shown that a small amount of temperature decrease is effective to open a closed crack and hence to increase the ultrasonic response of the closed crack. Another advanced technique gives the success in quantitative evaluation of both the crack depth and the crack closure stress by nondestructive way. The technique is based on the analysis of inverse problem, and capable of evaluating tightly closed cracks as well as open cracks under a no load condition. Introduction: Ultrasonic techniques have traditionally been used for nondestructive inspection of cracks [1-10]. Usually, nondestructive inspection is carried out when structural components are not in operation. This means that cracks are required to be detected and evaluated under a no load condition. Fatigue cracks are known to close under a no load condition due to plastic deformation in the wake of the crack which is surrounded by an elastic region, and stress corrosion cracking shows closure due to oxide between the mating crack surfaces. Therefore, the no load condition causes a significant problem of crack closure, especially in the ultrasonic techniques. If the closed crack undergoes nondestructive inspection without opening it, the crack will be erroneously evaluated as being smaller than the actual size [11,12]. Recently, the technique called thermosonics combining the sound pulse as the energy source with an infrared camera for monitoring the subsequent photons emitted in the vicinity of a surface or subsurface defect has received considerable attention [13]. The technique is effective for detecting and sizing surface or subsurface closed cracks. In this paper, we describe some strategic techniques for nondestructive evaluation (NDE) of closed cracks hidden in the components. One advanced technique is an application of cooling to a cracked part for NDE of cracks. Another advanced technique realizes the simultaneous evaluation of both the crack size and the closure stress under a no load condition. Difficulties of Sizing Closed Cracks: Common ultrasonic techniques for the evaluation of crack depth used the reflected and tip-diffracted waves of bulk shear and longitudinal waves and also the surface Rayleigh wave [1-10]. But sizing a tightly closed crack is difficult. The difficulties of sizing closed cracks are due to 1) Reduction in signal intensity caused by the sound transmission through the closed crack surfaces, and 2) Noises from the closed crack surfaces because of the contacting asperities. Figure 1 shows the echo patterns obtained by the tip-diffracted wave method [(a)] and Rayleigh wave method [(b) and (c)] [11]. A fatigue crack in austenitic stainless steel (AISI304) specimen having a size of 468 × 38 × 76 mm was examined. It is difficult to find the crack-tip signal in Fig. 1(a) obtained by the tip-diffracted wave method (6 MHz, 45 shear wave) under a no loading. The intensity of the tip-diffracted wave is weak, and in the case of closed cracks, it is further weakened by sound transmission through the closed crack surfaces. Because of the low amplitude of the diffracted wave from closed cracks and the high noise-background, signal diffracted from the tip of the crack may approach to the noise level and become undetectable. Hence it is extremely difficult to clearly distinguish the crack-tip signal from back-scattering noises. Also, when the Rayleigh wave method (5 MHz) was used under a no loading, the noises arising from tightly contacting asperities on the crack surfaces made it difficult to detect the crack-tip signal [Fig. 1(b)]. If we open the crack by an additional loading, the echo pattern clearly shows the crack-tip signal [Fig. 1(c)], and we can evaluate the crack depth from the time-of-flight of the crack-tip signal. Advanced NDE of Closed Cracks Utilizing Thermal Stress: In this paper, we propose a strategic technique using thermal stress for NDE of closed cracks. In the previous section, it was shown that additional loading is effective for opening the closed cracks and the ultrasonic response from the crack is enhanced by the loading. The technique proposed here uses the thermal stress induced simply by cooling a cracked part for effectively opening the closed crack. The technique is applicable to the hidden cracks in the components.