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What’s one major thing that airplanes, bridges, and weigh stations have in common? If you answered strain gauges, you were right. These simple devices measure the strain, or pull, on an object. The resistance of an object will change as you apply deformation to (stretch) the strain gauge, which will then tell you how much deformation the object, like an airplane wing, bridge, or car, is currently undergoing.
We’ve seen 3D printed strain gauges and sensors before, but Rahul Panat, an associate professor of mechanical engineering at Carnegie Mellon University (CMU), is leading a collaborative team of researchers that have developed a new method for 3D printing strain gauges that significantly improves their sensitivity, along with making them capable of being used in high-temperature applications.
Panat, who is also affiliated with the university’s NextManufacturing Center, said, “Anywhere there is deflection of a mechanical system, you will see strain gauges; which is a lot of places!”
Poisson’s ratio of 0.5: green cube is unstrained, orange is expanded in x direction due to tension and contracted in y and z directions. [Image: Wikipedia]
“More contraction means more sensitivity, so we get a much more sensitive strain gauge by adopting this new manufacturing method, where we print nanoparticles of a material and create this porosity by controlled sintering,” explained Panat.
Strain gauges made from traditional manufacturing methods take on the form of a solid film. But the team used aerosol jet 3D printing to make the strain gauge, which uses heat to control the sintering of nanoparticles that partially coalesce, thus forming a porous film. When this film, which contains many tiny holes due to the method of 3D printing, is stretched, it can contract more than a solid film can.
Schematic of the porous film under linear strain showing enhanced lateral contraction; Poisson ratio greater than 0.5. [Image: CMU College of Engineering]
Panat said, “Because of the porosity of the film, we are seeing an effective Poisson Ratio of approximately 0.7'”which means we have about a 40% increase in the lateral contraction for a given deformation of the film. That makes the strain gauge much more sensitive to measurement.”
The team recently published a paper on their new method, titled “3D Printed High Performance Strain Sensors for High Temperature Applications,” in the Journal of Applied Physics; co-authors include Md Taibur Rahman, Russell Moser, Hussein M. Zbib, C.V. Ramana, and Panat.
According to the abstract, “Realization of high temperature physical measurement sensors, which are needed in many of the current and emerging technologies, is challenging due to the degradation of their electrical stability by drift currents, material oxidation, thermal strain, and creep. In this paper, for the first time, we demonstrate that 3D printed sensors show a metamaterial-like behavior, resulting in superior performance such as high sensitivity, low thermal strain, and enhanced thermal stability. The sensors were fabricated using silver (Ag) nanoparticles (NPs), using an advanced Aerosol Jet based additive printing method followed by thermal sintering.”
(a) Schematic view of the experimental set up, (b) CAD model of the high temperature dynamic strain sensing set up, and (c) actual image of the strain sensing set up with the use of bent cantilever to measure strain.
The researchers 3D printed the sensors on an Optomec Aerosol Jet 300 Series system, and then tested them under cyclic strain, at temperatures up to 500'‰ °C. In addition to the strain gauges having an increased sensitivity, it was discovered that the gauge factor was nearly 60% higher than commercially available gauges.
“The reason why a material will show thermal strain is because material naturally expands when it is heated. In our case, the overall expansion of the porous film because of heat alone is much smaller than if it were a solid film,” Panat said. “The films created with this new