University of Maryland: 3D Printing Honeycomb Structures with Buckling Initiators for Crash Mitigation
When it comes to 3D print infill, honeycomb structures are generally the most efficient, maximizing storage space and minimizing necessary material. Research also shows that the honeytube, a novel form of the honeycomb structure, can also provide 3D printed objects with excellent energy absorption.
Recently, Rachel Harvey, Norman M. Wereley, PhD, and Min Mao, PhD, researchers in the University of Maryland‘s Composites Research Laboratory (CORE) in the Department of Aerospace Engineering, published a paper, titled “Development of 3D Printed Honeycombs for Crash Mitigation Applications,” that takes a closer look at the structure, and why its mechanical properties make it “ideal for usage in crash mitigation, particularly for helicopters and automobiles.”
“Currently, when crushed by a dynamic load, there is an impulse in force prior to a steady absorption – which could be detrimental in such crash mitigation applications. In this study, 3D printed honeycombs are investigated for subsequent crush efficiency with quasi-static and dynamic crush tests,” the researchers wrote in the abstract. “3D printing, rather than conventional manufacturing, allows for structural modifications within the honeycomb that influence its force-displacement profile. Buckling initiators on the face and/or vertex of honeycombs should reduce the initial peak stress and increase the strain at which densification, the point at which the stress once again increases, begins.”
The goal was to design and fabricate 3D printed honeycombs with both diamond-shaped and circular buckling initiators, which they found will “decrease the initial peak stress of tested honeycombs.” They used a uPrint SE system to 3D print the honeycombs out of ABSplus filament, with 1 mm cell wall thickness, solid infill, and standing 30 mm tall.
First, in order to find the energy absorption and crush efficiency, they tested the in-plane direction for the honeycomb structures.
Then, the 3D printed honeycombs were tested on their out-of-plane properties on a 20,000 lb MTS machine, “for comparison with Gibson and Ashby equations and previous data.” They placed the honeycomb between two compression platens on the machine, “with a displacement of .002in/sec, .02in/sec, .2in/sec, and 2in/sec.” They raised the platens 1″ above the samples before the dynamic testing began, which was captured with a high-speed camera so they could properly document the crush testing of the structures.
They found that the two types of buckling initiators definitely influenced the 3D printed honeycomb structures’ stress-strain curves – meaning their energy absorption was ideal. You can see this reflected in the tables below.
“Initial peak stress has shown to decrease with the implementation of buckling initiators, along with later points of densification,” the researchers concluded.
The researchers will continue their work on energy absorption of 3D printed honeycomb structures.
“Future directions for the project include testing honeycombs of other materials with buckling initiators, and the implementation of variations of current buckling initiator designs,” they concluded.
They plan on testing honeycombs made with different materials, like foam and aluminum, with buckling initiators; testing different cell-designs, such as a flower petal; and conducting drop tests on 3D printed honeycombs using a high-speed camera. In addition, they will also keep testing the honeycombs they previously designed in order to compare the efficiency of these designs with the new ones.
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