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Micro-mechanics of parachute fabrics under tensile loads is studied using in situ X-ray micro-tomography. Results are presented for two nylon textiles commonly used in parachute systems, MIL-C-7020H Type III and MIL-C-44378(GL) Type II. Textiles are subjected to incremental tension using a custom apparatus that loads the fabric radially, and the microstructure is imaged sequentially at steady load conditions. Micro-tomography images are processed using learning-aided segmentation and a custom processing pipeline that tracks the locations and morphological properties of individual tows on 3-D datasets. Results are used to reconstruct tow micro-scale properties and meso-scale strains. Our findings reveal a direct relation between the fabric architecture and the meso-scale mechanics. Warp tow pretensioning during manufacturing is found to affect decrimping and the anisotropy of the textile strains. Areal porosity increase with tension is quantified and a geometric model for pore opening under incremental load is validated., Comment: 21 pages, 15 figures, to be published in AIAA Journal

The influence of nitrogen on the oxidation behavior of hot-pressed zirconium carbide was investigated using a flow-tube furnace at temperatures ranging from 1000 to 1600 {\deg}C. Mass gain, oxide formation characteristics, and oxide transitions were evaluated at various experimental conditions. Differences in oxidation behavior across the range of temperatures investigated show both kinetic and microstructural dependence with implications pointing to this materials efficacy in ultra-high temperature applications. Results suggest that at temperatures above 1400 {\deg}C, although oxidation mechanisms remain dominant, nitridation and reduction mechanisms may be appreciable enough to require consideration. Supporting discussions regarding polymorphism and microstructural influences are outlined.

One of the ways we perceive shape is through seeing motion. Visual motion may be actively generated (for example, in locomotion), or passively observed. In the study of how we perceive 3D structure from motion (SfM), the non-moving, passive observer in an environment of moving rigid objects has been used as a substitute for an active observer moving in an environment of stationary objects; the 'rigidity hypothesis' has played a central role in computational and experimental studies of SfM. Here we demonstrate that this substitution is not fully adequate, because active observers perceive 3D structure differently from passive observers, despite experiencing the same visual stimulus: active observers' perception of 3D structure depends on extra-visual self-motion information. Moreover, the visual system, making use of the self-motion information treats objects that are stationary (in an allocentric, earth-fixed reference frame) differently from objects that are merely rigid. These results show that action plays a central role in depth perception, and argue for a revision of the rigidity hypothesis to incorporate the special case of stationary objects.

Inductively coupled plasma wind tunnels are crucial for replicating hypersonic flight conditions in ground testing. Achieving the desired conditions (e.g., stagnation-point heat fluxes and enthalpies during atmospheric reentry) requires a careful selection of operating inputs, such as mass flow, gas composition, nozzle geometry, torch power, chamber pressure, and probing location along the plasma jet. The study presented herein focuses on the influence of the torch power and chamber pressure on the plasma jet dynamics within the 350 kW Plasmatron X ICP facility at the University of Illinois at Urbana-Champaign. A multi-domain analysis of the jet behavior under selected power-pressure conditions is presented in terms of emitted light measurements collected using high-speed imaging. We then use Gaussian Process Regression to develop a data-informed learning framework for predicting Plasmatron X jet profiles at unseen pressure and power test conditions. Understanding the physics behind the dynamics of high-enthalpy flows, particularly plasma jets, is the key to properly design material testing, perform diagnostics, and develop accurate simulation models, Comment: 22 pages (including figures, appendix, and references); 13 figures

The B' formulation can be understood as a mass and energy conservation formalism at a reacting singular surface. In hypersonics applications, it is typically used to compute the chemical equilibrium properties of gaseous mixtures at ablating surfaces, and to estimate the recession velocity of the interface. In the first half of the paper, we derive the B' formulation to emphasize first principles. In particular, while we eventually specialize to the commonly considered case of chemical equilibrium boundary layers that satisfy the heat and mass transfer analogy, we first derive a general interface jump condition that lets us highlight all the underlying assumptions of the well-known B' equations. This procedure helps elucidate the nature of the B' formalism and it also allows us to straightforwardly extend the original formulation. Specifically, when applied at the interface between a porous material and a boundary layer (as in thermal protection systems applications), the original formulation assumes unidirectional advective transport of gaseous species from the porous material to the boundary layer (i.e., blowing). However, under conditions that may appear in hypersonic flight or in ground-based wind tunnels, boundary layer gases can enter the porous material due to a favorable pressure gradient. We show that this scenario can be easily handled via a straightforward modification to the B' formalism, and we demonstrate via examples that accounting for gas entering the material can impact the predicted recession velocity of ablating surfaces. In order to facilitate the implementation of the extended B' formulation in existing material response codes, we present a short algorithm in section 5 and we also refer readers to a GitHub repository where the scripts used to generate the modified B' tables are publicly available.

Mesoscale simulations of woven composites using parameterized analytical geometries offer a way to connect constituent material properties and their geometric arrangement to effective composite properties and performance. However, the reality of as-manufactured materials often differs from the ideal, both in terms of tow geometry and manufacturing heterogeneity. As such, resultant composite properties may differ from analytical predictions and exhibit significant local variations within a material. We employ mesoscale finite element method simulations to compare idealized analytical and as-manufactured woven composite materials and study the sensitivity of their effective properties to the mesoscale geometry. Three-dimensional geometries are reconstructed from X-ray computed tomography, image segmentation is performed using deep learning methods, and local fiber orientation is obtained using the structure tensor calculated from image scans. Suitable approximations to composite properties, using analytical unit cell calculations and effective media theory, are assessed. Our findings show that an analytical geometry and sub-unit cell geometry provide reasonable predictions for the effective thermal properties of a multi-layer production composite.

Quantitative microstructural analysis of Room Temperature Vulcanized (RTV) silicone pyrolysis at high temperatures is presented. RTV is used as a bonding agent in multiple industries, particularly filling gaps in ablative tiles for hypersonic (re-)entry vehicles and fire prevention. Decomposition of RTV is resolved in real time using in situ high-temperature X-ray computed micro-tomography. Full tomographies are acquired every 90~seconds for four different linear heating rates ranging from 7 to 54 C/min. The microstructure is resolved below 5 micro-meters/pixel, allowing for a full quantitative analysis of the micro-structural evolution and porous network development. Results are highly heating rate dependent, and are evaluated for bulk sample volume change, porosity, pore network size, and observed densification from X-ray attenuation. The outcome of this work is critical to develop multi-physics models for thermal response., Comment: 8 pages, 5 figures

A series of experiments was performed to obtain permeability data on FiberForm(R), a commercial carbon preform used for manufacturing thermal protection systems. A porous sample was placed in a quartz flow-tube heated by an isothermal furnace. The setup was instrumented to measure mass flow through and pressure drop across the sample. The intrinsic permeability and the Klinkenberg correction, which accounts for rarefied effects, were computed from the experimental data. The role of the gas temperature and pressure on the effective permeability is shown, and it is demonstrated that with proper data reduction, the intrinsic permeability is strictly a function of the micro-structure of the material. A function for the effective permeability of FiberForm, dependent on temperature, pressure, pore geometry, and type of gas is proposed. The intrinsic permeability was evaluated at $K_0 = 5.57 \times 10^{-11}$ m$^2$, with a Klinkenberg parameter of $8c/d_p = 2.51 \times 10^5$ m$^{-1}$ and a reference porosity of $\phi^\dagger = 0.87$.

Imagine entering Earth’s atmosphere after returning from the outer solar system. A heat shield less than 2 inches thick protects you from temperatures up to 2,900° Celsius (5,252° Fahrenheit). Such conditions were experienced by NASA’s Stardust capsule during reentry in 2006. The only materials capable of providing the necessary protection are composites with complex microstructures. Evaluating these materials is difficult, requiring precise knowledge of their properties. To this end, NASA scientists are developing research codes to compute material properties and simulate ablation at the microscale using agency supercomputers. Utilizing these tools, along with experiments, researchers are working to push the limits of spaceflight, allowing for greater flexibility in future space missions.