Experimental accuracy of two dimensional strain measurements using Digital Image Correlation

The measurement of strain has always been central to developments in structural engineering. As structures around the world reach the end of their intended service life, there is a need for more robust field monitoring techniques to determine, in conjunction with numerical models, if these structures are still fit for purpose. Current strain gage technologies are only able to offer a limited number of discrete measurements, which often is not enough data to assess complex structures. This paper presents an alternative technique for measuring strain using digital images called Digital Image Correlation (DIC) that allows for the measurement of 2-D strain fields. The accuracy that can be achieved in physical experiments where sources of error such as lighting and camera quality are present is investigated. A series of tension tests on steel plates are conducted allowing measurements from the DIC technique to be compared to measurements from conventional foil strain gages. Out-of-plane motion is identified as one of the most significant potential sources of strain measurement error when using DIC. Five solutions for significantly reducing the effects of out-of-plane motion are presented and three of these solutions are applied to the steel plate experiment. Using these solutions it is possible to achieve mean strain errors of less than 5 με when comparing DIC to foil strain gage measurements, which suggests that the DIC technique has the potential to replace conventional strain gages. Areas of future research are also introduced.

► A method for measuring strains using digital images is verified experimentally.
► The technique permits non-contact 2-D strain measurements to be taken.
► Movement of the specimen relative to the camera has a significant impact on error.
► Five approaches to minimizing this movement error are proposed.
► Strain measurement errors of less than 5 με are possible after correction.