Structural Health Monitoring Based on Operational Modal Analysis from Long Gauge Dynamic Strain Measurements

Vibration-Based Monitoring (VBM) is a non-destructive method for structural damage identification and condition assessment. The main challenge for VBM is to identify structural characteristics that are as sensitive as possible to damage and at the same time as insensitive as possible to measurement noise, loading, and environmental factors such as temperature. Damage alters the stiffness, mass or energy dissipation of a structure, which are properties that are directly related to its modal characteristics. VBM aims to identify changes in modal characteristics of a structure that are directly related to damage. Natural frequencies and displacement mode shapes are probably the most commonly used modal characteristics for damage identification. However, they can exhibit a low sensitivity to certain types of damage while the sensitivity of natural frequencies to environmental influences may be sufficiently high to completely mask the effect of even severe damage. Modal strains are a promising alternative for damage identification as they are more sensitive to local damage. The use of modal strains for VBM applications is explored in this thesis. Two methods for accurate dynamic sub-microstrain measurements are  developed, to deal with the difficulties that current measurement techniques encounter in capturing the low strain levels that occur on civil structures during ambient or operational excitation. Fiber-optic Bragg gratings (FBG) are used to measure the dynamic strains, which subsequently are used in modal analysis to obtain the modal strains of the tested structures. Both methods exhibited very high accuracy in modal strain identification of laboratory and full-size structures. The damage detection and localization capabilities of modal strains and of related quantities that are directly obtained from them, such as neutral axis positions, are investigated through operational modal analyses on prestressed concrete beams that are subjected to progressive damage tests. It is demonstrated that the cracks that are induced in the beams by the progressive damage tests do have a clear and local influence on the strain mode shapes and neutral axis positions, even at low damage levels. Thorough experimental investigations are conducted in laboratory and on two railway bridges to identify the influence of temperature on modal strains. It is found that the temperature changes do not have a measurable influence on the modal strains and neutral axis positions.