Structural Health Monitoring of CFRP Laminates : Development of 90-degree Crack Detection and Suppression System

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The purpose of the present study is to develop a structural health monitoring system to detect and suppress transverse cracks in a90-degree layer which generate at the early stage of loading.Transverse crack detection was made with a small-diameter (40 μm in coating diameter, 52 μm in coating diameter) Fiber Bragg Grating (FBG) sensor embedded in the CFRP lamina neighboring to the 90-degree layer. Transverse cracks generated in the gage section of the FBG sensor induce non-uniform strain distribution in the same section and change in the reflection spectrum of the sensor. This phenomenon can be used for the detection of transverse cracks. The experiments for the transverse crack detection were successfully conducted for cross-ply laminates as well as quasi-isotropic laminates. The experimental results were also reasonably explained by the theoretical analysis of the non-uniform strain distribution and the fiber optic theory.Transverse crack suppression was made with a 40-μn thick pre-strained shape memory alloy (SMA) foil embedded in the interlaminar region. When the pre-strained SMA is heated, the SMA shrinks and the recovery stress is generated, which retards the strain level of crack initiation and reduces the crack multiplication rate. The experiments for the transverse crack suppression were successfully conducted for cross-ply laminates at different temperatures. These experimental results were also reasonably explained by the theoretical analysis based on both the modified Brinson model for SMA constitutive equation and the shear-lag analysis. Then the optimum condition was determined for the SMA location, pre-strain level and applied temperature.Then, both detection and suppression systems were implemented in the same CFRP laminate to demonstrate the simultaneous damage detection and suppression system. The damage suppression system was successfully operated after the damage detection system detected the transverse cracks in the 90-degree ply.