Laminated composite plates have low delamination resistance. Since delamination crack creation is a difficult problem for visual inspection, delamination causes low reliability for primary structure of laminated composites. To improve this low reliability, identifications of delamination cracks in-service are required. The present study employs an electric-resistance change method in an attempt to identify internal delaminations experimentally. In our previous paper, a two-prove method was adopted for the electric resistance change measurements because of the simplicity. Instead, the present paper adopts multiple-prove method for the measurements of electric resistance changes. Electric current is charged from the different electrodes to measure the voltage changes. The measurements of electric voltage change at multiple points are robust against electric resistance change at the electrodes, and the method is similar to the four-probe method for high precision measurements of electric resistance change. In the present study, high precise measurement system of electric voltage change is developed, and the electric voltage measurement method is adopted for identifications of embedded delamination location and size. As a result, the improved electric resistance change method is shown to be effective for the identifications of embedded delamination cracks of graphite/epoxy laminated composites.

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