Engineers and researchers typically reach for sensors when they want to evaluate the stress-strain characteristics of materials or look at how applied forces affect objects, but now there’s a better way.

Rather than directly instrumenting the object under study, you can use high-speed cameras and specialized software to optically measure deformation, displacement, and strain. The technique is called Digital Image Correlation (DIC), and it’s rapidly growing in popularity in the aerospace, defense, and automotive industries.

When performing a DIC analysis, first apply a speckle pattern to the surface of the object you want to study. The cameras and DIC software then record and analyze the pattern’s movement in response to an applied force, revealing areas of strain and displacement. DIC can be performed with single or multiple cameras that have been synchronized to record at various angles. A one-camera setup will produce a 2D image to analyze elongation, tensile strength or single-plane distortion. Multiple camera systems, which are more common, produce 3D images that offer insight into multi-plane vibration or distortion analysis.

Whichever type of DIC setup you choose, the technique has compelling advantages over traditional sensing methods. DIC is noncontact, a key feature because sensors’ mass can alter an object’s displacement or deformation, and provides more data than the discrete data points collected by sensors. DIC is also fast, inexpensive, and easily integrated.

Here’s a look at three projects that have applied DIC to vibration analysis, impact testing, and large-scale structural analysis.

Diagnosing Vibration in Rotating Machines

Measuring the vibration data of rotating structures is challenging because traditional sensors must pass signals through electrically noisy slip rings. There is also a limit to how many sensors can be attached to rotating structures without changing how the structure moves. University of Texas at Austin researchers showed that DIC makes it possible to collect vibration data from a rotating blade and conduct modal analysis (Rizo-Patron, 2015).

To capture images, the investigators used two high-speed digital cameras, which can record 3,200 images per second at full 1280 x 800 resolution. They tested their approach on a two-meter helicopter rotor excited by a jet of compressed air, capturing images at 1,000 Hz with the blade rotating at 900 RPM. Results suggest that combining DIC with high-speed cameras is effective for determining the modal parameters of rotating systems.

Measuring Blunt Impact Forces on Football Helmets

Researchers at the University of Southern Mississippi used 3D DIC to evaluate the performance of commercially available football helmets under blunt impacts. A helmet was placed on a linear impact table, where it was strapped to a simulation dummy head and then struck by a pressurized punching device at high velocities. The researchers used cameras which captured impact at 3,333 fps, and cameras to capture impact at higher frame rates. Using DIC algorithms, the software produced corresponding videos of the principle strain, displacement, and tangential strain caused by the impact, providing quantitative data that will influence the development of safer helmets.

Scaling Analysis to Large Structures

Conducting large-scale analysis with traditional sensors can be onerous, so University of Grenoble in France researchers used DIC for full-field analysis to study how timber-framed structures respond to seismic activity (Sieffert, 2016). The analysis involved capturing the displacement of a full-scale house undergoing simulated earthquakes on a shake table. The researchers had to balance image resolution to clearly see damage, while gathering full-scale measurements. They used software and a camera at a maximum resolution of 2560 x 1600. For each simulated earthquake, they acquired 7,599 images, tracking almost 4,000 pixels.

Researchers also added contact measurement devices to the structure to measure shake table displacement, which DIC analysis provided in the x and y directions. DIC also exposed an opening in infill material and provided information on the timber’s flexural behavior. The researchers concluded that DIC field displacement measurements provided direct proof of the seismic-resistant behavior of a filled timber-framed structure.

These three applications are just a glimpse of what DIC can do. With the right cameras and software, DIC’s ability to provide full-field, quantitative information about stress, strain, and vibration continues to open up new applications every day.