Stereo digital image correlation (stereo DIC) is an optical technique that involves applying a high contrast speckle pattern to a surface of a deforming object, from which images are taken to calculate deformation and strain. This method has several benefits such as being non-contacting, having full-field data, and can be conducted at a variety of speeds (determined by the camera) and length scales (determined by the lenses). In stereo DIC, two or more cameras triangulate upon the specimen’s surface in order to detect out-of-plane displacement. The stereo camera configuration imposes angles between the orientations of the cameras and the direction normal to a specimen’s surface. As a result, portions of the surface may tilt beyond the depth of field of the cameras and lenses, resulting in blurry regions of the image. Depth of field can be hugely influential on the quality of DIC measurements.

 

This tilting phenomenon, known as the Scheimpflug effect, can be overcome with precision lens mounts that tilt the lenses with respect to the sensor plane of the camera, thus effectively rotating the image plane to be more in line with the object’s surface. This changes how the depth of field is arranged relative to the images. This effect has been known since the early 1900s and was used by photographers such as Ansel Adams with his images of the National Parks. Scheimpflug lenses (also known as tilt-tip lenses) have been demonstrated to improve stereo camera measurements in experimental fluid dynamics but the technique has only recently been introduced to solid mechanics.

 

It is well known that the stereo angle in DIC introduces a trade off in the quality of in-plane versus out of plane measurements. The goal of this research is to characterize the ability of Scheimpflug lenses to influence this trade off. In this present work, we first performed 2D-DIC with a single camera oriented at an angle with respect to the object’s surface, and applied a known, uniform, in-plane translation to the object. We then introduced a second camera and performed stereo-DIC and additionally measured known, uniform, out-of-plane translations. The displacements from each test are compared against the known value of the applied displacement to assess measurement uncertainty as a function of image location. Initial evidence shows that applying the Scheimpflug adjustment improves measurements at displacements that are farther away from the camera. This method could prove useful for specimens that have large curvatures or experience out of plane rotation relative to DIC cameras.


Introducing the Scheimpflug Effect to Improve Stereo-DIC at Varied Stereo Angles

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Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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