Characterization of microscopic damage in composite laminates and real-time monitoring by embedded optical fiber sensors

First, a methodology for observation and modeling of microscopic damage evolution in quasi-isotropic composite laminates is presented. Based on the damage observation using both an optical microscope and a soft X-ray radiography, a damage mechanics analysis is conducted to formulate the stiffness change due to transverse cracking. Then, both energy and stress criteria are combined to provide a valid procedure to predict the transverse crack evolution. The theoretical prediction is found to agree well with the experimental results for the transverse crack density as a function of strain as well as stress–strain curves. Then, another methodology is introduced using two kinds of embedded optical fiber sensors to detect and monitor the transverse crack evolution in composite laminates. One is plastic optical fibers (POF), where the loss in optical power is generated by local deformation of POF due to transverse cracking. The other is fiber Bragg grating (FBG) sensors, where the local strain distribution within the FBG gage length due to transverse cracking alters the power spectrum of the light reflected from the FBG sensors. Embedded optical fiber sensors are found to be a powerful method to detect and monitor the transverse crack evolution in composite laminates.