Extended Summaries

Abstract

Laser ultrasound can be used for noncontact depth measurement of open surface cracks, which can be applied to online monitoring of surface cracks in high temperature components. Crack depth measurement can be performed by using (a) surface waves in the frequency domain, (b) surface waves in the time domain, or by (c) time of flight diffraction (TOFD). A comparison of these methods showed that the use of surface waves in the time domain is best suited for online monitoring.Crack depth measurement using surface waves in the time domain is shown schematically in Fig. 1. Surface waves generated using a generation laser are detected using a laser interferometer. The propagation distance of the surface wave which travels around the crack depends on the crack depth, so the crack depth can be obtained from the time delay of the surface wave.Experiments using a SUS316 specimen with a slant slit, whose depth a varied continuously from 0 mm to 5 mm, showed that the time delay of the surface wave varied linearly with respect to the slit depth for a > 2.3 mm, as shown in Fig. 2. The applicable range for depth measurement is estimated to be a > 2 mm.Since the propagation speed of the surface wave depends on the specimen temperature, the time delay changes with temperature. Therefore, in order to monitor the crack depth accurately, regardless of the specimen temperature, it is necessary to eliminate the effect due to temperature change. For example, the time delay for a constant slit depth of 2 mm changed by 0.26 µs over the temperature range 25°C to 500°C. On the other hand, the time delay for slit depths 2 mm and 3 mm at 500°C differed by 0.35 µs. Therefore, the change in the time delay for a temperature change from 25°C to 500°C is equivalent to that for a slit depth change of approximately 0.7 mm. Since a measurement resolution of about 0.3 mm is required for online monitoring of crack depth, some method for correcting for the temperature change is required.In order to eliminate the effect due to temperature change, a method using two surface waves was developed, which is shown in Fig. 3. Two generation lasers are used. The first laser generates a surface wave which travels around the crack (roundabout wave), and the second laser generates a surface wave which does not interact with the crack (reference wave). The time delay of the roundabout wave includes the effects of temperature change and crack depth, whereas the time delay of the reference wave only includes the effect of temperature change. Therefore, the time delay of the reference wave can be used to correct the time delay of the roundabout wave. Using this method, the time delay of the roundabout wave (for a constant crack depth) was corrected to a constant value from 25°C to 500°C. The standard deviation was comparable to the case of constant temperature, indicating that effects of temperature change were successfully corrected.The results showed that laser ultrasound can be applied to monitoring of crack depth, and that the effect of temperature change can be c...