NASA Buckling Test with Real Time Module
For most of us, the crushing of aluminum cans is a familiar scene. But, what if you had to precisely model the crush load of a can? And, what if the “can” was a complex structure with a diameter of 27.5 feet? This was the challenge facing NASA engineers.
Cylindrical rocket shells are subject to loads similar to soda cans. Their strength under lifting loads is referred to as a “Shell Buckling Knockdown Factor” or SBKF. The original SBKF models were developed back in the 1960s. The available technology of the era limited the accuracy of these models. As a result, rocket designs had to be over-engineered for safety, and were unnecessarily heavy.
Today, technologies such as Finite Element Modeling permit more detailed models. But, these models must be validated through the testing of real specimens. Actual specimen displacements under load are a critical part of the validation process, and when it comes to this data, more is definitely better.
In preparation for a new generation of space flights, NASA started a program to update their SBKF models. An important milestone was reached on March 23, 2011, when NASA successfully tested a full-scale specimen to failure. This test was broadcast live on the internet and you can watch it here.
To get the quantity and quality of displacement data they would need, NASA relied on 3D digital image correlation (DIC) systems supplied by Correlated Solutions, Inc. Random black-and-white patterns were applied to the surface of the specimen (see photo to the left and note the size of the mobile lift below the specimen). Digital cameras continuously monitored the entire surface of the specimen, and VIC-3D software with the Real-Time module allowed NASA engineers to monitor detailed full-field three-dimensional displacement and strain data in real time! A total of seven systems were used to cover the entire 360-degree area.
The specimen was pressurized to 1psi and gradually loaded to more than 800,000 pounds. Although the outer wall was smooth, the effects of internal ribs and welds can be plainly seen in the out-of-plane displacement data w or z (see image left). As one NASA test engineer in the video feed put it: “this is the type of real-time data we get to observe during the test so we know exactly what’s going on.”
The real-time VIC-3D data was used to monitor and control the testing process. High-speed cameras captured images at 3,000 frames per second at the moment of failure. And, all of the synchronized video images were saved. As a result, NASA engineers have access to detailed, full-field, 3D deformation measurements of the specimen, throughout the entire test cycle.
Digital Image Correlation has played a key role in the successful NASA SBKF test program. As the video footage shows, the VIC-3D system provides information that would be unimaginable with any other technology. Used in conjunction with FEA modeling, it will allow NASA to decrease weight and increase the payload of future rockets.
