Two-level damage detection strategy based on modal parameters from distributed dynamic macro-strain measurements

The concept of distributed strain sensing techniques has been proposed in a recent research by the authors, which had been dedicated to utilize the strain distributions throughout the full or some partial areas of structures to detect the arbitrary and unforeseen damage. Based on the distributed long-gage fiber optic sensors, a two-level strategy is proposed in this study for damage detection in flexural structures using modal parameters extracted from dynamic macro-strain responses. Modal macro-strain vector (MMSV), similar to the mode shape from acceleration measurements, is utilized to construct damage evaluating indexes for Level I damage locating with no need for a detailed analytical model. Hereto, the damaged area with a space resolution of the gage lengths of fiber optic sensors can be detected and the unknown parameters to be identified are greatly reduced into the damaged domain. Then based on the concept of FE model updating, natural frequencies that present high accuracy and precision in practical measurements can be adopted for Level 2 damage locating with a space resolution of the sizes of finite elements and damage quantification. The experimental investigations on cantilevered beams before and after different levels of damage with distributed long-gage Fiber Bragg grating (FBG) sensors are carried out. The comparison between the identified damages and the exact set ones has verified the applicability and effectiveness of the proposed two-level strategy in damage locating and quantification.