Structural Damage Detection Using Digital Video Imaging and Wavelet Transformation Upendra P Poudel 1 , Gongkang Fu 2 , Jian Ye 1 1 Graduate Research Assistant, 2 Professor and Director Center for Advanced Bridge Engineering, Civil and Environmental Engineering Department Wayne State University 5050 Anthony Wayne Drive, Rm 2157 Detroit, MI, 48202, USA Upendra@wayne.edu ABSTRACT Non-contact measurement offers convenient and less expensive setup. Sometimes, they may also be the only resort because the structure is too small to be instrumented with contact sensors and/or the additional mass of the sensor is too large to use. In other cases, the structure is inaccessible due to obstruction for the case of tall/large structures such as bridges or towers. The digital imaging techniques offer an alternative to contact measurement. In this paper, use of digital video imaging is proposed for detecting damage in structures. The theory of measuring structural vibration using high-resolution images based on sub-pixel edge detection technique is demonstrated with extraction of displacement time series from video images. From displacement time series, characteristic dynamic properties, i. e., natural frequency, damping and mode shapes, are established. Mode shape difference function is introduced and derived for damage detection purpose. These mode shape difference functions are subjected to continuous wavelet transformation for determining their singularity (discontinuity) at locations of damage. A laboratory test program was carried out to implement the concepts using a high-speed digital video camera. Results show that the proposed approach of data acquisition using digital video camera is effective to provide spatially intensive time-series data for non-contact structural testing. Furthermore, it is shown that the wavelet transformation of mode shape difference functions to isolate locations of singularity is able to identify the damage and their locations. INTRODUCTION Many structures in the civil infrastructure system need constant monitoring for deciding repair, maintenance, and rehabilitation. Research efforts have been reported in the past for monitoring these structures using their dynamic characteristics (Doebling et al [24]), such as the natural frequencies and mode shapes. Previously proposed approaches overwhelmingly used accelerometers for measurement, attached to the structure at a limited number of points. The effectiveness of these approaches still needs to be demonstrated because the resolution of the acquired data is limited. Such approaches also require access to the structure, which may be costly or impractical. To circumvent these difficulties, dynamic digital imaging is proposed here for structural health monitoring. Aspects related to this approach are briefly reviewed next, before it is fully presented later. Damage and alteration to structures change their behaviors. If these changes are accurately measured, they can be used to identify and locate the structural damage and alteration. This process of identification is referred to as damage detection. Such detection should cover at least two key aspects: 1) detecting presence of damage, and 2) identifying the damage locations or neighborhoods. Doebling et al [24] presented a review for damage detection of civil structures using vibration measurement, and Dimarogonas [1] for vibration based methods for detecting cracks in particular. A number of previously proposed methods require a comprehensive dynamic analysis of the structure including a finite element analysis. This analysis is meant to establish a reference to be compared with measured results for damage identification. There are two main issues associated with this approach. 1) It is not always cost-effective to conduct dynamic analysis of a structure. 2) It is always very costly, if not impossible, to obtain valid models of finite element analysis for as-built civil structures. Without physical testing, the validity of these models cannot be confirmed. During the last two decades, structural dynamic system parameters such as natural frequencies, damping, and mode shapes have been investigated for their possible use in structural damage identification and localization. While changes in natural frequencies may be used to detect the existence of damage, the mode shapes are more important indices for damage location