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 1960’s. 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.
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 is archived at www.ustream.tv (NASAtelevision: World’s Largest Can Crusher).
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 right 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 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 Δz (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.
A flow liner used in the propulsion system on the space shuttle was under evaluation to determine whether vibrations could result in cracking. The liner could be subject to a range of vibration frequencies, and it was not known which would produce the greatest vibratory strains.
The specimen under test was complex, which meant that the overall response of the part would be impossible to predict. It was also fairly lightweight, which meant that any contact with the specimen could be expected to alter its response. Although the areas of primary concern were not very large, they included geometric features which made it difficult to predict the location of peak strains. It was also difficult to know whether the areas of peak strain would be large enough to be identified with strain gages. This test was further complicated by the high frequencies required to simulate flight conditions. Vibration response was measured at frequencies up to 6kHz.
Correlated Solutions provided a Vic-3D system and a vibration synchronization module designed to accurately capture data from vibrating specimens without the expense of high-speed cameras. Because the measurements do not require contact with the specimen, the specimen’s movement was not affected by the measurement system. Strain measurements were obtained over the entire image area, allowing the true location of the maximum strain to be identified. Because of Vic-3D’s high spatial resolution, researchers were able to obtain accurate measurements even when peak strains were concentrated in a very small area.