High-selectivity imaging of closed cracks using elastic waves with thermal stress induced by global preheating and local cooling

Abstract

The authors developed a practical method based on the thermal stress dependence of cracks for the accurate imaging of closed cracks with high selectivity. A tightly closed fatigue crack was visualized in linear phased array (PA) images by opening the crack with the tensile thermal stress induced by global preheating and local cooling (GPLC), although the conventional method of using only local cooling could not fully open the crack. Furthermore, by the subtraction of the PA images before and after GPLC, only the closed crack was extracted with high selectivity while canceling the other linear scatterers.

High-Selectivity Ultrasonic Imaging of Closed Cracks Using Global Preheating and Local Cooling

“…5(e)5(g) between t = 4 and t = 6 s. Importantly, the crack depth measured is the same as the actual value of 11.3 mm. 20) This shows that GPLC is significantly effective in opening a tightly closed crack. Then, as the cooling time increased at t ² 7 s, the crack appeared at shallower part, as shown in Figs.…”

“…It is an option of the load difference phased array (LDPA). 19) After confirming the crack opening capability of the global preheating and local cooling (GPLC), 20) we applied STIS to the PA images. As a result, only the crack responses were successfully extracted in the subtracted image, with eliminating the responses of linear scatterers, whereas the conventional subtraction method of subtracting a PA image obtained before a start of LC from that after the start of LC could not eliminate the responses of linear scatterers.…”

“…On the other hand, the average wave velocity of the specimen is higher at t > 0 than that at t B because the average temperature of the specimen is lower at t > 0 than that at t B due to LC. If the change in wave velocity is not negligible, r C ðtÞ, r W ðtÞ, and r B ðtÞ slightly change depending on t. Therefore, in applying the conventional subtraction method 20) that subtracts the PA image at t B from that at t > 0, not only the crack but also the ghosts remains in the subtracted image, as shown in Fig. 2(d).…”

“…As another approach for applying a tensile stress to cracks, we have developed a practical crack opening method (GPLC), that combines global preheating (GP) and local cooling (LC), of applying tensile thermal stress to cracks. 20) GPLC can readily increase the tensile stress by varying GP temperature so that it enables us to open tightly closed cracks which cannot be opened only by LC. 21,22) It was demonstrated that the combination of GPLC and LDPA is useful in enhancing the selectivity of closed cracks, where a linear phased array (PA) image before LC was subtracted from that after the onset of LC for LDPA.…”

“…4. 20) To image the crack, a PA was used. The PZT array (manufactured by Imasonic) used has 32 elements with a center frequency of 5 MHz, where the element size and pitch are 10 and 0.5 mm, respectively.…”