13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- Figure 10. Schematic of estimation method of fatigue damage using single material. Figure 11. Estimation map of (a) fatigue cycles and (b) stress amplitude of steel structure as a function of normalized crack length in two sensors made of same material. 5. Conclusions In the present study, near-threshold fatigue crack growth behavior in thin pure copper sheet with pre-crack was evaluated under strain-controlled fatigue testing in order to obtain the sensor characteristics of smart stress-memory patch and the following conclusions were obtained. (1) Using a function describing the initiation as well as the stable growth of fatigue cracks, relationship between stress intensity factor range and crack growth rate of ED-Cu was successfully fitted to one equation regardless of strain amplitude, strain ration and specimen geometry. (2) It was shown that the stress amplitude and the cyclic number can be estimated from the crack lengths of two sensors made of ED-Ni and ED-Cu. Furthermore, using S-N curve of structural material, the cumulative fatigue damage could be evaluated as a function of crack length of smart patch. (3) Based on these experimental results, a new equation for estimating fatigue cycles and stress amplitude from fatigue crack length of two specimens was derived. This equation overcame the limitation that the patch requires two or more materials with different fatigue characteristics.
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