Optical Strain Measurement by Digital Image Analysis

A routine for contactless measurements of uniaxial strain by serial analysis of digital images.

Traditionally, precise strain measurements during uniaxial tensile testing can be performed by clamping a strain gauge to the sample. However, if the sample is very small, stripe-shaped or very compliant, this may alter the testing geometry and hence influence the measurement significantly. Contactless strain measurement is possible with laser extensometers or by using cameras combined with digital image correlation software. The drawback of such systems is that they are often quite expensive, usually predefined and thus have a limited flexibility. Strain determination by digital image correlation may be challenging if the sample surface is reflective. Additionally, it is nearly impossible if the surface structure changes significantly during the course of experiment, for example due to thin film fragmentation or buckling under externally applied strain. Motivated by these difficulties, we have developed this program as a basis for simple, yet effective and universally applicable uniaxial strain measurement.

The basic principle of the program is to monitor the positions of two parallel markers by analysis of a digital images series. These images can be taken either during microscope analysis or separately with a digital camera. Depending on the objective used, the working distance may vary widely. The marker displacement is determined by analyzing the intensity profile of the images along the tensile direction, and hence the strain of the sample can be calculated.

Basic concepts of the program have been motivated by a MATLAB code for digital image correlation written by Christoph Eberl et al. [Eberl C., Thompson R., Gianola D.: “Digital Image Correlation and Tracking”]. Our program has been optimized for strain determination by monitoring the displacement of two parallel markers. Due to the graphical user interface, the use of the program is straightforward and hence, the user does not need any MATLAB skills. The strain determination procedure is effective and accurate, which makes this method a true alternative to commercial products.

See the documentation file for a detailed description of the program.