Identification of modal strains of a pre-stressed concrete beam during progressive damage testing

Vibration-based Structural Health Monitoring (SHM) of civil structures currently suffers from a low sensitivity of natural frequencies and modal displacements to certain types of damage while the sensitivity to environmental influences may be sufficiently high to completely mask the effect of severe damage. Modal strains and curvatures are more sensitive to local damage, but the direct monitoring of these quantities is challenging due to the very small strain levels occurring during ambient, or operational excitation. In the present work, a pre-stressed, concrete I-shaped beam was subjected to a four-point, static bending test. The beam was subjected to six loading cycles of increasing load, before failing at the last cycle. Dynamic measurements were conducted on the beam at the beginning of each cycle and hammer impacts were used as dynamic excitation. The response of the beam to the applied force was recorded with accelerometers and two chains of multiplexed Fiber-optic Bragg Grating (FBG) strain sensors attached through a novel external clamping system at the top and the bottom flange, such that macro-strains were measured. Subsequently, the identification of the modal characteristics (eigenfrequencies, mode shapes and modal strains) was conducted with subspace identification. In order to achieve accurate identification of the modal strains, a novel optical signal processing technique that enables to obtain sub-microstrain accuracy with FBG strain sensors was implemented. The evolution of the modal characteristics of the beam, after each loading cycle is investigated. Changes of the eigenfrequencies, of the mode shapes and modal strains were observed, indicating the presence of damage.status: publishe