Strain Measurement in an Elastic Material under Large Deformation Using Optical Reconstruction Methods

The aim of this study was to explore photogrammetry- optical imaging method used to describe the local variation of strain in an elastic material undergoing large deformation. The material under investigation was marked with reference points (to identify the relative deformation) and imaged with two stereoscopically positioned cameras during a tensile test (strain rate 10 mm/ 60 s). The cameras were calibrated using the Bouguet calibration toolbox in Matlab to characterize their optics and establish their positions. Two dimensional (2D) images from both cameras were investigated to obtain the three dimensional (3D) position of the markers. The deformation of the elastic material was determined by 3D reconstruction of the deformed geometry. Strain results derived from Matlab were in good agreement (99 % ± 0.3 %) with a manual measurement using a caliper. Camera calibration and reconstruction accuracy results indicated an uncertainty of dot separation of the order 30 μm, with inter-point distance of approximately 3 mm.