Development and Validation of a Single-Camera Video Extensometer
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Mechanical testing is used to estimate the properties of materials and constructs. A video extensometer is a common measurement device in mechanical testing which uses a video camera to track fiducial markers on a specimen. This work presents the development and validation of a custom video extensometer that was assembled from readily available components and can be used to track two dimensional motion for mechanical testing. The video extensometer was used to track the motion of fiducials affixed to a linear XY translation stage, relative to static fiducials, to validate the accuracy in a static case. The sensor error for tracking fiducials from targeted position to actual position was less than half a pixel. A second validation assessed the accuracy in a low-speed mechanical testing case by tracking the motion of fiducials on the machine head of a Universal Testing Machine relative to static fiducials. The sensor error of video measurements compared to Universal Testing Machine position data for tracking fiducials was less than a quarter pixel. The video extensometer was then used as the measurement device in mechanical testing of distal radius fracture plates. Fractured plastic distal radius models, repaired using either locking or non-locking screws, were tested through a simulated healing and rehabilitation period. Changes with respect to failure criteria were measured using calipers and the video extensometer. Similar trends were found between video and caliper measurements; the differences were largely due to angular changes in the constructs causing differences in the caliper measurements, as well as to uncertainties intrinsic to caliper measurements, such as repeatability of measurements and assumptions of verticality. There was no clear advantage for either fixation method found in this study. This work shows that a high accuracy video extensometer for planar motion tracking was easy to implement using commonly available materials and could have applications in a variety of fields.