Application of multiple digital image correlation sensors in earthquake engineering

As structural testing has become more expensive, researchers are pushing to capture more and better data with each structural test conducted. Digital image correlation (DIC) is a tool that is gaining popularity as one way to capture more detailed information in testing programs. In three-dimensional DIC (3D-DIC), the measured object is photographed with a pair of digital cameras before and after a load event and a stochastic pattern marked on the object is tracked through the images such that a near full field of displacements is derived. Setup of the system involves mounting the two cameras securely to a rigid bar, and capturing photographs of a NIST certified calibration object. These calibration images are used to determine the orientation of the cameras with respect to one another, and then during testing, 3D locations of pattern sub-regions (called facets) are determined via photogrammetric triangulation principles. Historically, the cost of the system, knowledge of how to correctly specify appropriate testing protocol, and how to correctly interpret the results have limited the application of this promising technology to structural testing. The current paper focuses on three main aspects of DIC technology. First, a treatment of the basic theory behind the method is provided. Included are recommendations and guidelines for accuracy and other lessons learned during deployment over a broad range of projects. The final portion of the paper is a case study of the deployment of DIC on the large scale lateral load testing of a novel reinforced concrete coupled wall system. The bottom three stories of an eight story building were constructed in the laboratory - the top five stories were simulated using hydraulic actuators at the top of the test specimen. Fourteen DIC sensors were deployed simultaneously during the test, believed to be the largest number of simultaneous deployment for structural testing.