Effect of out-of-plane specimen movement on strain measurement using digital-image-correlation-based video measurement in 2D and 3D

This study determined the effect of specimen out-of-plane movement relative to the sensor, on the accuracy of strains measured made applying 2D and 3D measurement approaches employing the state-of-the-art digital-image-correlation (DIC)-based tool iMETRUM. DIC provides a convenient and inexpensive non-contact approach to monitor structural health by measuring strains in structural systems and linking them to structural damage.

This investigation was motivated by initially undetected damage at low strains in connections of a real-world bridge, whose detection would have prevented its spread, resulting in lower repair costs. This study builds upon an initial investigation that concluded that out-of-plane specimen movement reduces the accuracy of DIC-based strain measurements.

Consequently, the effect of specimen out-of-plane displacement on the accuracy of strain measurements using the 2D and 3D measurement techniques was determined over a range of strain values and specimen out-of-plane displacements. It was concluded that the 2D system could measure strains as camera focus was being lost due to specimen out-of-plane movement, the effect of which became noticeable at about 0.025% strain and 2.5 mm displacement.

The corresponding value for the 3D system was 0.06% strain at 0.5 mm out-of-plane displacement. Furthermore, it was concluded that the 2D system can measure strains in a real bridge, but it would be challenging to use the 3D system for this task. Furthermore, the 2D iMETRUM system is easier and less costly to implement in monitoring localized strains in steel bridges.