D. Satpathi

Assessment of electronic shearography for structural inspection

Electronic shearography (ES), a laser interferometry technique, has the potential for large-scale structural inspection and for identifying cracks and strain anomalies. A system based on this technology could possibly be used for noninvasive inspection of structures with high insensitivity and robustness. One major problem is the existence of largerigid 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 that is obtained by illuminating the object surface. This nullifies any possibility of interference between speckled images obtained at different stages of loading, as is necessary for interferometry applications. A systematic study was conducted to characterize the speckle pattern obtained from typical civil structures as a function of the illumination and imaging system. Experiments were carried out to quantify speckle decorrelation as a function of object motion and instrumentation characteristics. A fracture mechanics based finite element (FEM) analysis was carried out on an existing fractured bridge to determine the strains and displacements. The results of these studies were subsequently used to define the parameters of a structural inspection system based on ES for field applications.

Electronic shearography for bridge inspection

Electronic shearography (ES) is a laser based non-destructive testing method that has the potential to be developed into a commercial bridge monitoring technique. The primary advantage of ES over other similar techniques like electronic speckle pattern interferometry (ESPI) is its decreased sensitivity to in-plane rigid body movement and vibrations. Bridge inspection with ES has proven to be a daunting task so far. The main problem has been the inability of the method to handle the large deflections and vibrations that might be expected in a typical bridge subjected to normal service loads. Earlier research has shown that the extent of in-plane movement that can be tolerated by the system is dependent on the speckle size. The speckle size also affects the fringe quality by imposing resolution requirements on the imaging device. This article shall undertake the study of speckle size as a function of the focal length of the imaging lens, object distance and illumination wavelength using high resolution holographic film and a high magnification optical microscope.

Electronic shearography for inspecting steel bridges: a feasibility study

Although a large number of nondestructive techniques based on laser interferometry have been developed over the last few decades, most of them are incapable of handling large structures. Shearography has shown potential as a full field nondestructive technique, capable of being used on large structures like bridges. The method however has been beset with the problem of poor fringe quality, and as any other interferometric technique, suffers from the problem of speckle de-correlation due to rigid body motions. This paper investigates the effect of rigid body motion on fringe visibility with an emphasis on applications in nondestructive testing of bridges. To this effect, a finite element and fracture mechanics analysis is carried out on a real bridge to identify the constraints on shearography in full scale structural testing.