Keywords: non-destructive evaluation, crack identification, electric potential method, piezoelectric film, passive method, active method, pulse-echo method.

As a result of the increase in the number of aged manufacturing plants and their components, nondestructive evaluation and online monitoring becomes more and more important. Online monitoring has the advantage that it does not require shutdown of the plants. The present authors proposed the passive electric potential computer tomography (CT) method using the electric potential distribution on the surface of a piezoelectric film pasted onto a cracked body subjected to a service load [1,2,3]. It was found that a characteristic electric potential distribution appeared on the piezoelectric film when a crack existed in the body. Hence the crack can be identified from the electric potential distribution. An inverse analysis was conducted for crack identification based on a comparison of the electric potential distribution measured on the piezoelectric film and that obtained by the finite element method.

When an electric impulse is applied to the film an ultrasonic wave is emitted. Then the film can be used for an active evaluation of the crack using a pulse-echo method.

In the present study the combined use of the passive electric potential CT method and the active pulse-echo method using the piezoelectric film was examined. The electric potential distribution on the piezoelectric film was measured using the smart-layer for a body with a vertical through crack subject to a sinusoidal mechanical load. By applying an electric impulse to a point on the piezoelectric film, supersonic waves were emitted. The waves reflected at crack tips were received on several points on the film.

It was found that the active pulse-echo method was effective for the estimation of the crack depth but was not applicable for estimating the length of the vertical through cracks. On the other hand the passive electric potential CT method was useful for the estimation of the crack length and crack location. The effectiveness of the combined use of the active pulse method and the passive electric potential CT method using the piezoelectric film was shown.

Several combinations of the passive electric potential CT method and the active pulse-echo method were examined. The application of the active pulse-echo method for estimation the crack depth followed by the application of the passive electric potential CT method for estimating the crack length and the crack location worked well for the crack identification.

1

S. Kubo, T. Sakagami, K. Ohji, "Electric Potential CT Method Based on BEM Inverse Analyses for Measurement of Three-Dimensional Cracks", Computational Mechanics '86, Proc. of Int. Conf. on Computational Mechanics, vol 1, Springer, V-339-344, 1986.

2

T. Sakagami, S. Kubo, T. Hashimoto, H. Yamawaki, K. Ohji, "Quantitative Measurement of Two-Dimensional Inclined Cracks by the Electric-Potential CT Method with Multiple Current Applications", JSME Int. J., Ser. I, 31, 76-86, 1988.

3

S. Kubo, T. Sakagami, K. Ohji, "The Electric Potential CT Method for Measuring Two- and Three-Dimensional Cracks", Current Japanese Materials Research - Vol.8 Fracture Mechanics, 235-254, Elsevier Appl. Sci., Soc. Mat. Sci., Japan, 1991.

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