Grid-based photogrammetry system for large scale sheet metal strain measurement

This paper presents a new approach to fast strain measurement with high accuracy for large scale sheet metal based on the surface circular grid and digital close range photogrammetry. A multi-block measuring method of discretization is implemented to archive large scale measurement. The sheet metal is separated into several blocks for respective calculating and joined together by common reference points. A surface circular searching method is presented for fast and robust 3D grid generation. A flexible bundle adjustment method is proposed for large amount 3D grid nodes reconstruction, which employs the conception of sampling points and is proved to be efficient. Furthermore, a multi-stage grid registration method is introduced to improve the accuracy of strain field by correcting the true deformation gradient tensor. A novel system is developed and performances well in actual large scale sheet metal strain measurement. Two accuracy tests confirm that the system strain measurement error is less than 0.2%.