3.1 Effect of temperature on laser-excited ultrasound
To obtain the ultrasonic displacement signals at different temperatures, shown in Fig. 4, the laser energy, laser duration, and other parameters such as Thermal conductivity、Conductivity、Thermal capacity were maintained at the same level; while certain other parameters, such as the yield strength, Young's modulus, coefficient of thermal expansion, density, thermal conductivity, and electrical conductivity, were set corresponding to the different temperatures. The distance of the detection point from the center line of the laser source remained unchanged. It is obvious from Fig. 4 that the shape of the wave packets generated by laser excitation at different temperatures basically remained identical, although the wave packets appear at different times. This is attributable to the decrease in the speed of sound as a consequence of the temperature rise, leading to a significant hysteresis of the ultrasonic wave. Most likely, the change in the speed could induce an error in determination of the precise location of the defects in the process of defect detection. This establishes the fact that changing the temperature only affects the time it takes for each mode wave to reach the detection point and does not essentially change the ultrasonic waveform generated by laser excitation.
Figure 5 shows the effect of temperature change on the displacement of the y-component of the laser-excited ultrasonic waves as well as the width of the wave packet. The amplitudes of the surface longitudinal waves (SL), the surface waves (RW), and the y-direction displacements of bottom-reflected transverse waves (BRSW) show an increasing trend with temperature, with a particularly more pronounced increase in the case of RW. The y-directional displacements of SL, RW, and BRSW are 0.9, 12.15, and 1.1 µm, respectively, at 25°C, while the corresponding values at 400°C are 1, 13.5, and 1.2 µm, respectively: a corresponding increase of 11, 11, and 9%, respectively. Based on the aforementioned results it can be deduced that the coefficient of thermal expansion, thermal conductivity, and heat capacity dominate the effect of laser excitation at increasing temperature by essentially increasing the ultrasonic y-axis displacement of SL, RW, and BRSW.
As can be seen from Fig. 5, the wave-packet widths of SL, RW, and BRSW show a gradual increase with rising temperature. The larger the wave-packet width, the lower the frequency of the laser-excited ultrasonic waves; it can be inferred that as the temperature increases, the lower the frequency of SL, RW, and BRSW, the lower the resolution.
3.2 Effect of Temperature on Laser-EMAT Detected Echoes
The ultrasonic signal received by the EMAT at different detection temperatures is shown in Fig. 6. It can be observed that the amplitude of the ultrasonic signals received by EMAT in the face decreases gradually with rise in temperature and the wave packet gets broader.
Moreover, in Fig. 7 (a) it can be observed that when the temperature is elevated from 25 to 400 ℃ the amplitudes of SL, RW, and BRSW received by the in-plane EMAT gradually decrease from 0.58, 1.37, and 0.52 mV to 0.29, 0.58, and 0.26 mV, respectively, amounting to a reduction of 50, 57, and 50%, respectively.
As can be seen from Fig. 7 (b), the width of the SL wave packet received by the in-plane EMAT tends to drop initially and then rise sharply as the temperature increases further, whereas the width of the RW and BRSW wave packets rise steadily with temperature.
The ultrasonic signals received by the out-of-plane EMAT at different temperatures are shown in Fig. 8. It can be observed that the amplitude of the ultrasonic signals received by the out-of-plane EMAT also decreases gradually with the increase of temperature, and the wave packet gets broader.
Similarly, it is obvious from Fig. 9 (a) that the amplitudes of SL, RW, and BRSW signals received by the out-of-plane EMAT decrease from 0.67, 7.67, and 0.63 mV at 25 ℃ to 0.31, 3.4, and 0.31 mV at 400 ℃, respectively, with rise in temperature: the average decrease is approximately 53.5%.
With regard to the packet widths, it is evident from Fig. 9 (b) that the SL and RW packet widths received by the out-of-plane EMAT exhibit a steady but shallow increasing trend corresponding to the temperature rise. In contrast, the BRSW packet widths exhibit a sharp drop initially followed by a sharp rise and subsequently exhibit a shallow decreasing trend.
The ultrasonic signals received by the MLC EMAT at different temperatures are shown in Fig. 10. It can be observed from Fig. 10 that the amplitude of the ultrasonic waves received by the MLC EMAT gradually decreases and the wave packet becomes wider corresponding with the temperature rise.
From Fig. 11 (a) it is obvious that the amplitudes of SL, RW, and BRSW signals received by the MLC EMAT gradually decrease from 0.08, 2.56, and 0.22 mV to 0.045, 1.22, and 0.13 mV, respectively, when the temperature is elevated from 25 to 400 ℃, which amount to decrease of 43.7, 52.1, and 40%, respectively.
It can be observed in Fig. 11 (b) that the wave packet widths of SL and RW signals received by the MLC EMAT exhibit a rising trend with increasing temperature, whereas the wave packet width of BRSW signal shows a decreasing trend.
3.3 Analysis of Temperature Influence Laws on the Performance of Three Types of Receiving EMATs
When generation of ultrasonic waves due to laser excitation occurs, the amplitudes of the RW signals received by the in-plane, out-plane, and MLC EMATs are 1.37, 7.67, and 2.56 mV, respectively, at 25 ℃ and 0.58, 3.4, and 1.22 mV, respectively, at 400 ℃, respectively, when the laser excitation generated ultrasound, and the amplitude of the RW received by the out-of-plane EMAT was 5.6 times that of the in-plane EMAT and 2.99 times that received by the MLC EMAT.
As the temperature rises from 25 to 400 ℃, the amplitude of the received RW signals for all three types of EMATs decreases gradually and the wave packet width increases. Compared with the RW signal amplitude received at 25 ℃, the amplitudes of the RW signals received at 400 ℃ for in-plane, out-of-plane, and MLC EMATs decrease by 57, 55.7, and 52.1%, respectively. It can be seen that the three types of EMATs are affected by temperature in the following order: in-plane EMAT > out-of-plane EMAT > MLC EMAT.
In summary, the out-of-plane EMAT is finally selected for the optimized design.