ASSESSMENT OF ELECTRONIC SHEAROGRAPHY FOR STRUCTURAL INSPECTION

Electronic shearography (ES), a laser interferometry technique, has th e potential for large-scale structural inspection and for identifying cracks and strain anomalies. A system based on this technology could p ossibly be used for noninvasive inspection of structures with high ins ensitivity and robustness. One major problem is the existence of large -rigid body motions in typical engineering structures such as bridges and high-rise buildings. These rigid-body motions are large enough to cause a complete decorrelation of the characteristic speckle pattern t hat is obtained by illuminating the object surface. This nullifies any possibility of interference between speckled images obtained at diffe rent stages of loading, as is necessary for interferometry application s. A systematic study was conducted to characterize the speckle patter n obtained from typical civil structures as a function of the illumina tion and imaging system. Experiments were carried out to quantity spec kle decorrelation as a function of object motion and instrumentation c haracteristics. A fracture mechanics based finite element (FEM) analys is was carried out on an existing fractured bridge to determine the st rains and displacements. The results of these studies were subsequentl y used to define the parameters of a structural inspection system base d on ES for field applications.