Digital image correlation (DIC) is a non-contact optical method where digital images of the surface of an object are captured and analysed to extract full field shape, deformation and/or motion measurements. A non-periodic contrasting stochastic pattern that adheres and deforms with the surface is generally applied to enable measurements to be made. A grid of points are created on the surface using the pattern and then tracked through a sequence of images, typically as the surface undergoes some form of deformation. The method compares images of the object in a non deformed and deformed state in order to obtain relative measurements. A data point is created from a neighbourhood of pixels (subset) within the digital image at the reference stage; this subset and associated data point is then tracked at sub pixel accuracy using correlation methods to match the same area of pixels based on the intensity values, in subsequent stages of deformation.
Numerous variables can introduce errors into the measurement chain of a digital image correlation (DIC) system. These can be grouped into two categories: measurement quality and the correlation principle. Although previous studies have attempted to investigate each error source in isolation, there are still no comprehensive, standardized procedures for calibrating DIC systems for full-field strain measurement.
Aim
The aim of this study was to develop an applied experimental method that would enable a DIC practitioner to perform a traceable full-field measurement calibration to evaluate the accuracy of a particular system setup in a real-world environment related to their specific application.
Approach
- Development of a traceable material measure as follows.
- Digital images of a synthetic speckle pattern generated in MATLAB and deformed in 10% increments from 0 to 50%
- Patterns for each deformation stage incorporated into a speckle pattern board (SPB) design and printed on fine art paper to create a material measure
- Material measure SPBs for every deformation stage calibrated through the measurement of the speckle pattern dimensions using a SmartScope Flash 200 multi-sensor optical measuring machine calibrated by the NIST.
- Images of each deformation stage captured using Photron high-speed cameras and processed using GOM ARAMIS DIC software
Key Findings
- A method has been developed that allows the experimental calibration of a 3D-DIC system using a novel, traceable, material measure consisting of a synthetic speckle pattern, artificially deformed and printed onto speckle pattern boards.
- Optical measurements of the printed boards indicated that the patterns were slightly longer than designed by approximately 0.1–0.2 mm which was believed to be related to the printer.
- The traceable calibration performed on the printed boards, however, enabled any deviations in the printed pattern length from the designed length to be accounted for.
- Measured strain was found to be slightly overestimated, on average, by a nominal value of approximately 0.02% strain, with a typical measurement error range of ±0.34% strain at a 95% confidence interval.
- Location within the measurement volume was not found to have a significant effect on error distributions.
- The methodology has the potential to enable DIC practitioners to be able to assess the accuracy of their system in their particular working environment.
Publications
- Blenkinsopp, R., Roberts, J., Harland, A., Sherratt, P., Smith, P., Lucas, T. (2019). A Method for Calibrating a Digital Image Correlation System for Full-Field Strain Measurements during Large Deformations. Appl. Sci., 9(14), 2828.
Researchers
Other Resources
A complete set of high resolution speckle pattern boards can be downloaded using the following link: