Structural Damage Detection and Evaluation Using a Scanning Laser Vibrometer

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Inspection of aging civil structures, after-earthquake buildings and bridges, aging aircraft, and mechanical systems under fatigue loading requires a method that can perform on-site quick and accurate damage detection of large structures. Unfortunately most of conventional non-destructive inspection methods are local methods; it is slow and expensive to use them for on-site inspection of large structures. Because damage signals are always contained in dynamic responses of structures, this research project is to develop a dynamics-based global inspection method for the detection and estimation of defects in large composite and built-up structures. Toward a global inspection method a scanning laser vibrometer will be used to provide accurate and dense measurement of Operational Deflection Shapes (ODSs) of structures, and a newly developed Boundary Effect Evaluation (BEE) method will be used to extract damage-induced boundary effects from the ODSs to reveal damage locations. The BEE method is a model-independent method; it uses a sliding-window least-squares curve-fitting method to decompose an ODS into central solutions and boundary solutions. Because damage introduces new boundaries to a structure, boundary solutions exist around damage and structural boundaries and hence can reveal damage locations. Moreover, crack sizes can be estimated by using stress intensity factors from fracture mechanics. Experiments have been performed to verify the capability of this BEE method in locating surface slots, edge slots, surface holes, internal holes, fatigue cracks, and stiffened sections. Numerical and experimental results indicate that the BEE method is more sensitive and reliable than other dynamics- or deformation-based methods. The accuracy and capability of this BEE method will be improved by investigating the noise patterns of ODSs, the use of low-order transverse ODSs, a new data acquisition method using a lock-in amplifier, and a systematic method of selecting ODSs for examination. This project will result in an efficient and reliable damage detection and estimation method for performing condition-based structural safety inspection of large civil, mechanical, and aerospace structures.