A Novel Modal Strain Method for Damage Detection in SHM Based on FBG and iFEM

Damage detection is the key content of structural health monitoring. The current damage indices based on strain have two premises: complete measured modes, modal parameters before and after damage. It is very difficult to get the two contents in practical engineering. In this paper, using Fiber Bragg grating (FBG) strain sensor and inverse Finite Element Method (iFEM), the global strain field is reconstructed based on measured strain data of limited points. And an approximate complete measured mode is acquired based on global strain field. In this way the build of strain modal damage index has natural advantage making use of strain directly. Then a direct damage detection index based on strain mode shape is proposed, which can locate damage with only the strain modal data after injury, without the data before damage. The basic differential equations with equal and unequal spacing of damage strain modal are deduced. The calculation results show that the positive and negative changes occurred in strain modal difference curve at damage location. According to the theoretical derivation and statistical numerical simulation of damage location, a criterion of damage detection is proposed. An actual plate experiment results show the effectiveness of the proposed method.