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As a vehicle travels through space at hypersonic speeds, the gases surrounding it generate heat at temperatures that are dangerous to the driver and instrumentation inside. Designing a vehicle that can wick away heat requires an understanding of the thermal properties of the materials used to build it. A recent two-part study at the University of Illinois Urbana-Champaign developed a method for creating 3D models of fibers within composite materials, and then used that information to predict the material’s thermal conductivity.
“We used X-ray microtomography to create 3D images that show the orientation of the fibers,” said Francesco Panerai, faculty member of the UIUC Department of Aerospace Engineering. “In most engineering applications we use composite materials made with carbon fibers, but the method we have developed can be applied to any type of fiber and any type of composite.”
Panerai said microtomography is similar to a hospital CT scan, but with high-energy X-rays that can detect fine details in microfibers, which are a fraction of the diameter of a single human hair.
“The images that show how the fibers are organized are more than just beautiful images: they are a description of the material in a three-dimensional grid. Now we can use the data from the 3D grid to do simulations to calculate the properties of the material that you would otherwise have to do complicated experiments. “said Panerai.
In the first part of the study, Panerai and his colleagues tested three different methods for visualizing fibers. “We found that because different materials are made up of different architectures, some methods worked better with some fibers and textures than others.”
For example, the study concluded that the ubiquitous structure tensor approach showed very good performance on straight and random fibers, but failed to accurately estimate the orientation of a two-way tight weave.
Another artificial flow-based method demonstrated relatively good performance on two-way woven samples, but failed on straight random fibers.
The new ray casting method has shown promise of becoming a powerful approach for estimating the orientation of fibrous materials with little curvature. But its main drawback is the high computational cost.
“Now that we can follow the direction of the fibers in space and determine the space between them, we can calculate the property of the material, in this case its thermal conductivity, in three dimensions and have very precise values.
“And, to experimentally measure conductivity, you’d need to do three experiments, one for each direction. Using this new method, we can calculate the tensor and predict properties in the three directions much more quickly and cost-effectively.”
Panerai said this new method of visualizing fibers and the proven ability to determine material properties can help redesign materials.
“We can use a very specific fiber architecture to achieve a certain property such as resistance or conductivity,” he said. “Thermal conductivity is something that everyone who works with high temperature materials tries to estimate. It seems a very simple property, but it is very difficult to measure, especially for materials that are three dimensional. This is what is remarkable about the power of this method. “
Frederico Semeraro, lead author of the study at NASA Ames Research Center, said: “Calculation of thermal conductivity is critical to reliably predict a heat shield response. Furthermore, the methodology and numerical methods that have been developed are quite flexible. to allow for the calculation of many material properties. A complete understanding of the behavior of a heat shield will ultimately enable optimization of its design. “
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