The L.S. Starrett Company
Athol, MA
www.starrettmetrology.com
A group of graduate students from Cal (California) State Fullerton was conducting research in the field of ground vehicle aerodynamics to test the structural capacity of custom profile stings during high-threshold testing in a wind tunnel. The stings were mounted to the top face of the wind tunnel and used to consolidate cabling and actuating rods for a kinematic system. The system served to propagate a test vehicle model in six degrees of freedom where the stings were subjected to forces being applied to the leading edge, perpendicular to its length. The stings were made from 3D-printed material with wall thicknesses of 0.03" (0.8 mm), 0.04" (1.0 mm), and 0.05" (1.3 mm).
The goal of testing was to be able to obtain the ideal wall thickness which could withstand the types of forces experienced in high-threshold wind tunnel testing, without suffering structural failure, while simultaneously providing ample space for the cabling and actuating rods. To achieve this, a pull test was designed to replicate those types of forces and accurately capture how failures occurred, where they occurred, and at what level of force they occurred, depending on the wall thickness being tested.
The force measurement system used was a Starrett FMS 1000 test stand with a 500N load cell and L2 software. Custom fixturing was developed, which worked with the force systems’ standard attachment points, while allowing the profile stings to be mounted securely to the fixture itself in a perpendicular position to the force being applied, simulating the force of the wind tunnel.
All samples encountered two forms of failure. First, a compressive buckling along the trailing edge of the sting. Second, a leading edge tearing of the layers of the sting directly above the fixture point at the base.
For the 0.03" (0.8 mm) thickness wall sample, the compressive buckling and leading-edge tear happened simultaneously, at an applied force about 0.27 Megapascals (MPa). Once the failure point was reached, there was an immediate drop-off followed by an attempt of the material to re-load itself as the load distributed to other layers, however the material never fully recovered.
The sample with a wall thickness of 0.04" (1.0 mm) displayed the broadest failure range of the three wall thicknesses. At approximately 0.29 MPa the material suffered an initial disengagement, followed by the compressive buckling stage, at which point it was able to re-engage prior to the initial leading-edge tear, occurring at about 0.34 MPa, and subsequent failure at about 4.1 MPa. While the material was able to re-engage at a higher level of applied force, it also never fully recovered to pre-failure levels.
The final wall thickness of 0.05" (1.3 mm) proved to be the sturdiest and most resilient to applied force. This sample showed a much broader elastic region than the previous two samples, with the initial compressive buckling point occurring at about 0.53 MPa. The material was able to re-engage before encountering the subsequent leading-edge tear at about 0.57 MPa, at which point sample failure also occurred. The sample recovered to similar levels as the sample with a 0.04" (1.0 mm) wall thickness, but much like the previous two samples, was unable to fully recover after structural failure was achieved.
The testing of the various wall thicknesses illustrated an appropriate material trend, showing that as the wall thickness increased, the ability to sustain greater forces increased, with similar buckling and tearing at all thicknesses prior to failure. While the sample with a 0.05" (1.3 mm) wall thickness was able to withstand the greatest forces without failure, it was determined that the sample with the 0.04" (1.0 mm) wall was ideal.
The 0.04" (1.0 mm) thickness provided ample rigidity to withstand anticipated forces in the wind tunnel. Also, the visible signs of fracture, which were observed in testing, can allow researchers to discern imminent failure ahead of time, preventing the loss of material or components stored within each sting.
This article was written by CW Moran, Metrology Marketing Manager, The L.S. Starrett Company (Athol, MA). For more information, visit here .
