13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- The sensor is shown schematically in Figure 3. A 0.8 mm thick carbon steel sheet was machined to the size of 64 mm x 64 mm to work as the substrate. After ultrasonically degreasing the substrate in acetone, an insulating paste with BN filler and epoxy resin, was printed on the substrate with a thickness of 20 µm. A conductive paste with Ag filler and epoxy resin was then printed on top of the insulating layer with a thickness of 15 µm. This conductive paste acted as the cathode, and the exposed area of steel substrate, as the anode. 3. Experiment and Simulation 3.1. FEM model to evaluate the measurement accuracy of the metal film As showed in Figure 4, the crack length could not be estimated accurately because sometimes the resistance of the existing ion-sputtered metal film was unstable and did not increase monotonically as the crack length increased. Figure 4. The correlation between normalized crack length observed by microscope and the resistance of sputtered metal film measured by digital multimeter (DMM). Figure 5. Definition of ac and Wc in the ion-sputtered metal film, and the boundary condition on FEM.[6] In order to figure out the optimum shape of the conductive film, electrical potential analysis by FEM was performed by changing the crack length ac=0 to 1.9 mm under the fixed voltage. Because
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