Assessment of the influence of the crack monitoring method in interlaminar fatigue tests using fiber Bragg grating sensors

One of the main experimental problems in performing delamination fatigue tests on composite materials is monitoring the crack length. Crack length is commonly monitored by means of traveling optical devices in a discrete representation and/or is derived from the specimen’s compliance previously calibrated. In this paper, we propose an accurate method, based on the use of fiber Bragg grating sensors, and we apply it to a carbon reinforced composite. Then, the obtained results are compared to the results achieved by the visual inspection method in order to analyze the influence of the crack growth monitoring on the crack growth rate characteristics determination: Paris law material constants and energy threshold. Results indicate that optical inspection methods predict well the exponent in the Paris law equation, but the energy threshold is strongly dependent on the method used to measure the crack growth.