3D shape shearography with integrated structured light projection for strain inspection of curved objects

Abstract

Shearography (speckle pattern shearing interferometry) is a non-destructive testing technique that provides full-field surface strain characterization. Although real-life objects especially in aerospace, transport or cultural heritage are not flat (e.g. aircraft leading edges or sculptures), their inspection with shearography is of interest for both hidden defect detection and material characterization. Accurate strain measuring of a highly curved or free form surface needs to be performed by combining inline object shape measuring and processing of shearography data in 3D. Previous research has not provided a general solution. This research is devoted to the practical questions of 3D shape shearography system development for surface strain characterization of curved objects. The complete procedure of calibration and data processing of a 3D shape shearography system with integrated structured light projector is presented. This includes an estimation of the actual shear distance and a sensitivity matrix correction within the system field of view. For the experimental part a 3D shape shearography system prototype was developed. It employs three spatially-distributed shearing cameras, with Michelson interferometers acting as the shearing devices, one illumination laser source and a structured light projector. The developed system performance was evaluated with a previously reported cylinder specimen (length 400 mm, external diameter 190 mmm) loaded by internal pressure. Further steps for the 3D shape shearography prototype and the technique development are also proposed.

Development of the 3D Shape Shearography Technique for Strain Inspection of Curved Objects

“…The earlier proposed approach [6] for surface strain measuring of curved objects by the 3D shape shearography is used in this research. The approach is based on transferring shearography data from the system to each point of the point cloud, which can be done in 4 steps:…”

“…The developing 3D shape shearography technique uses a multiple viewing approach with three shearing cameras and a single illumination direction ( Fig. 1 (b)) [6,9]. Simultaneous use of three cameras is needed to isolate the in-and out-of-plane surface strain components [2,4].…”

“…The well-known Zhang's camera model [10] can be used for this purpose. Previously in [6] the cameras and the projector were calibrated in pairs as stereovision systems [11,12]. Recently global calibration of the cameras and the projector (as an inverse camera) was done using the multiple view computer vision approach [13].…”

“…The complete procedure of the 3D shape shearography system calibration and data processing were also presented [6]. Current research follows the earlier work [6,7] and is devoted to crucial practical questions of the 3D shape shearography system development. This includes ways to estimate the shear distance in 3D and the camera-projector system calibration.…”

“…The shape information is required for correction of the shear distances within the field of view [2,5] together with sensitivity vector correction [2]. A general solution to the strain inspection of curved objects was reported before [6]. The 3D shearography setup was equipped with an integrated structured light projector for inline shape measuring.…”