The Calibration Process and Setting of Image Brightness to Achieve Optimum Strain Measurement Accuracy Using Stereo-Camera Digital Image Correlation

Abstract

Combining the drilling method with the digital image correlation (DIC) method is becoming more common to speed up the measurement and evaluate the strains relieved at several locations. However, to obtain the most accurate results, it is necessary to be aware of the influence of possible aspects that could adversely affect the results of the strain/stress analysis carried out using DIC. The paper describes several analyses to assess the influence of the 3D DIC system’s calibration procedure for strain/stress analysis of the specimen with a hole loaded with four different levels of tensile force. In addition, the paper also deals with the analysis of the influence of the image brightness, which was modified by changing the exposure time of the cameras. Based on the results of strain/stress analyses performed on small areas (approx. 25 × 25 mm) of a specimen with a hole by a stereo-camera DIC system, it can be concluded that both analysed factors can negatively influence the results. The most accurate results are ensured using the calibration target of very high manufacturing precision sized similarly to the field of view observed in correlation mode. The optimal image brightness is adjusted when the mean grey value of the image is from the range of 56 up to 171 with as evenly distributed image point intensities as possible.

Keywords

• digital image correlation
• calibration parameters
• image brightness
• strain/stress analysis
• stress concentrator
• residual stress