Fluid transport through microvascular networks-a hallmark for homeostasis in living systems-has transcended to engineered materials, primarily made possible because of modern manufacturing advancements. Vascular-enabled multifunctionality, including thermal regulation and self-healing, holds great potential for extending the lifetime of structural materials and expanding the operational envelope. Prior studies on vascular-based active cooling use a "combined" heat transfer coefficient (HTC): a single parameter lumps convection and radiation effects. Although the resulting mathematical models are linear-an attractive feature for computational modeling, the combined coefficient approach may not be accurate or even applicable if the operating temperature is unknown, which is the case with many thermal regulation applications (e.g., space probes). In this paper, we illustrate the remarked limitations of the lumped approach and advocate the need to use a decoupled HTC by splitting convective and radiative heat transfer modes. We show the broad applicability of the proposed method by applying it to three material systems: glass and carbon fiber-reinforced polymer composites and an additive manufactured metal. We show, using numerical simulations, the differences in the predictions from the decoupled approach with that of the combined HTC; these differences are prominent at higher heat fluxes. Also, the decoupling has enabled us to establish a scaling law that allows transferring of solutions fields across material systems, strengthening further the validity and utility of our approach. This work's significance is two-fold. First, the research is fundamental, providing accurate measurement protocols for critical model parameters. Second, this work facilitates the development of mathematical models for vascular-based thermal regulation that are predictive even for hostile environments (which are often difficult to realize in laboratories), such as outer space.

Nearly all-natural and synthetic composites derive their characteristic attributes from a hierarchical makeup. Engineered metamaterials exhibit properties not existing in natural composites by precise patterning, often periodically on size scales smaller than the wavelength of the phenomenon they influence. Lightweight fiber-reinforced polymer composites, comprising stiff/strong fibers embedded within a continuous matrix, offer a superior structural platform for micro-architectured metamaterials. The emergence of microvascular fiber-composites, originally conceived for bioinspired self-healing via microchannels filled with functional fluids, provides a unique pathway for dynamic reconfigurable behavior. Demonstrated here is the new ability to modulate both electromagnetic and thermal responses within a single structural composite by fluid substitution within a serpentine vasculature. Liquid metal infiltration of varying density micro-channels alters polarized radio-frequency wave reflection, while water circulation through the same vasculature enables active-cooling. This latest approach to control bulk property plurality by widespread vascularization exhibits minimal impact on structural performance. Detailed experimental/computational studies, presented in this paper, unravel the effects of micro-vascular topology on macro-mechanical behavior. The results, spanning multiple physics, provide a new benchmark for future design optimization and real-world application of multifunctional and adaptive microvascular composite metamaterials.

Nearly all-natural and synthetic composites derive their characteristic attributes from a hierarchical makeup. Engineered metamaterials exhibit properties not existing in natural composites by precise patterning, often periodically on size scales smaller than the wavelength of the phenomenon they influence. Lightweight fiber-reinforced polymer composites, comprising stiff/strong fibers embedded within a continuous matrix, offer a superior structural platform for micro-architectured metamaterials. The emergence of microvascular fiber-composites, originally conceived for bioinspired self-healing via microchannels filled with functional fluids, provides a unique pathway for dynamic reconfigurable behavior. Demonstrated here is the new ability to modulate both electromagnetic and thermal responses within a single structural composite by fluid substitution within a serpentine vasculature. Liquid metal infiltration of varying density micro-channels alters polarized radio-frequency wave reflection, while water circulation through the same vasculature enables active-cooling. This latest approach to control bulk property plurality by widespread vascularization exhibits minimal impact on structural performance. Detailed experimental/computational studies, presented in this paper, unravel the effects of micro-vascular topology on macro-mechanical behavior. The results, spanning multiple physics, provide a new benchmark for future design optimization and real-world application of multifunctional and adaptive microvascular composite metamaterials.

Fiber-reinforced polymer (FRP) composites, consisting of stiff/strong fibers embedded within a continuous matrix, are a lightweight structural platform supporting an array of modern applications. Bioinspired vascularization of fiber-composites can augment existing performance with dynamic functionalities via liquid infiltration of the internal micro-fluidic network. Some vascular-enabled capabilities include self-healing to repair delamination damage and active-cooling to prevent thermal degradation. While such attributes have been demonstrated in separate platforms, research investigations that combine functionalities within a single composite have been limited. Here we provide a recent study that highlights a promising pathway for achieving both multifunctional, and reconfigurable behavior in microvascular FRP composites. Specifically, we detail the ability to regulate temperature and modulate electromagnetic signature via fluid substitution within the same serpentine vasculature. Varying microchannel density alters both active-cooling efficiency by water circulation and polarized radio-frequency wave reflection by liquid metal infiltration. We control these bulk property pluralities by widespread vascularization, while minimizing impact on structural performance, and decode the effects of micro-vascular topology on macromechanical behavior. Our in-depth experimental and computational investigation provides a new benchmark for future design optimization and real-world translation of multifunctional and adaptive microvascular composites.

Printed Circuit Heat Exchangers (PCHEs) are well-suited for Very High Temperature Reactors (VHTRs) due to high compactness and efficiency for heat transfer. The design of PCHE must be robust enough to withstand possible failure caused by cyclic loading during high temperature operation. The current rules in ASME Code Section III Division 5 to evaluate strain limits and creep-fatigue damage based on elastic analysis method have been deemed infeasible at temperatures above 650°C. Hence, these rules are inapplicable for temperatures ranging from 760–950°C for VHTRs. A full inelastic analysis method with complex constitutive material description as an alternative, on the other hand, is time consuming; hence impracticable. Therefore, the simplified Elastic-Perfectly Plastic (EPP) analysis methodology is used as a solution in ASME Code Section III Division 5. The current literature, however, lacks any study on the performance evaluation of PCHE through EPP analysis. To address these issues, this study initiates the pathway of EPP evaluation of an actual size PCHE starting with elastic orthotropic analysis in the global scale. Subsequently, preliminary planning for analyzing intermediate and local submodels are provided to determine channel level responses to evaluate PCHE performance against strain limits and creep-fatigue damage using Code Case-N861 and N862 respectively.

Construction of bioinspired vasculature in synthetic materials enables multi-functional performance via mass transport through internal fluidic networks. However, exact reproduction of intricate, natural microvascular architectures is nearly impossible and thus there is a need to create practical, manufacturable designs guided by multi-physics principles. Here we present a Hybrid Topology/Shape (HyTopS) optimization scheme for microvascular materials using the Interface-enriched Generalized Finite Element Method (IGFEM). This new approach, which can simultaneously perform topological changes as well as shape optimization of microvascular materials, is demonstrated in the context of thermal regulation. In the current study, we present a new feature that enables the optimizer to augment network topology by creating/removing microchannels during the shape optimization process. This task has been accomplished by introducing a new set of design parameters, which act analogous to the penalization factor in the Solid Isotropic Material with Penalization (SIMP) method. The analytical sensitivity for the HyTopS optimization scheme has been derived and the sensitivity accuracy is verified against the finite difference method. We impose a set of geometrical constraints to account for manufacturing limitations and produce a design which is suitable for large-scale production without the need to perform post-processing on the obtained optimum. The method is validated by active-cooling experiments on vascularized carbon-fiber composites. Finally, we compare various application examples to demonstrate the advantages of the newly introduced HyTopS optimization scheme over solely shape optimization for microvascular materials. (C) 2019 Elsevier Ltd. All rights reserved.

To meet the increased energy demand efficiently and economically, significant research and development of the Next Generation Nuclear Plant’s (NGNP’s) high temperature components, such as compact heat exchanger (CHX), are underway. The CHXs have intricate geometry which provides high compactness and large surface area for heat transfer. These properties make CHX an attractive and economical option for NGNPs. The complex geometry of the CHX is fabricated through two step processes, first, high temperature alloy plates are photochemically etched to get the desired shape, size and dimensions of hot and cold fluid channels, followed by the plates to be stacked up and diffusion bonded. The diffusion bonding process changes the base material properties. A question that will be addressed is that if the diffusion bonded materials can be assumed as a homogeneous material for design and analysis of CHX. To answer this question, a set of tension and creep tests were conducted on diffusion bonded Alloy 800H specimens at room temperature, 650℃ and 760℃. In addition, a unified constitutive model of the Chaboche type is calibrated to simulate the material responses. These experimental and simulated responses will be presented. In addition, the creep, fatigue and creep-fatigue tests to be performed in this study will be presented and discussed.

Printed Circuit Heat Exchangers (PCHEs) have high compactness and efficiency for heat transfer, which makes them an attractive option for the Very High Temperature Reactors (VHTRs). Design methodology of PCHE for non-nuclear service is well established in the ASME Code, Section VIII; however, ASME Code rules for PCHE nuclear services are yet to be developed. Towards developing the ASME Section III code rules for PCHE, the study started with the design of PCHE core specimens for testing following the ASME section VIII methodology. The failure responses of these PCHE specimens are investigated by using Finite Elements Analyses (FEA). Two dimensional isothermal plane strain analyses are performed using an uncoupled constitutive material model. Parametric studies by varying shape and size of semicircular channels, PCHE core size, and loading cases are performed to quantify the critical parameters which influence the PCHE failure responses under pressure creep and pressure burst loadings. Results indicate that the maximum creep strain and its location are dependent on the PCHE core size. Significant reduction in creep strains are observed at the channel sharp corners by considering a realistic semielliptical channel shape instead of a semicircular channel in the analysis.

This paper presents a numerical modeling and finite element analysis on the behavior of steel hollow section (SHS) columns strengthening with Carbon Fibre Reinforced Polymer (CFRP) wrappings, engaging FEA software ABAQUS 6.14-4. A three dimensional finite element model of steel SHS column was developed using both shell and solid element considering both material and geometric nonlinearities whereas CFRP wrappings with different orientations were incorporated in the model with both conventional (S4R) and continuum shell (SC8R) element to capture actual behavior of CFRP retrofitted SHS column. The proposed nuemrical model was then incorporated into the ABAQUS to simulate some of the experimental studies found in relevent literatures. It has been found that good agreement exists between numerical analysis and past experimental results, which has established the acceptability and validity of the proposed finite element model to carry out further investigation.

This paper presents a numerical finite element investigation to study the behavior of steel hollow structural section (HSS) columns strengthened with CFRP (Carbon Fiber Reinforced Polymer) materials. A three dimensional finite element model of steel HSS column was developed using shell element considering both material and geometric nonlinearities whereas CFRP strengthening was incorporated in the model with additional layers of shell elements. The developed finite element model was then used to simulate experimental studies done by past researchers. It has been found that good agreement exists between numerical analysis and past experimental results, which has established the acceptability and validity of the proposed finite element model to carry out further investigation. © 2014 Institution of Engineers, Bangladesh. All rights reserved.

This paper presents a numerical finite element investigation on the behavior of steel square hollow structural section (HSS) columns strengthened with CFRP. Three dimensional finite element (FE) models of square HSS sections were developed using shell elements considering both material and geometric nonlinearities whereas CFRP strengthening was incorporated with additional layers of shell elements. The developed FE models were used to simulate experimental studies done by past researchers. Good agreement has been found between numerical analysis and past experimental results, which has validated the acceptability of the FE model to carry out further investigation. Study is then focused on some selected non-compact AISC square HSS columns and the effects of number of CFRP layers, slenderness ratio and cross-sectional geometry on the strength gain of those columns has been observed. It is observed that CFRP strengthening is comparatively effective for higher slenderness ratios. For smaller sections strengthening tends to be effective at smaller slenderness ratios as well. For relatively large AISC square HSS columns, with increasing number of CFRP layers (from 1 to 5 layers) the axial strength gain is only approximately by about 1 to 20%. For medium and small square HSS sections, effectiveness of CFRP strengthening increases approximately by about 10 to 90%. The findings of the present study provide us a better understanding of the behavior of HSS sections strengthened with CFRP and shall be useful to engineers in applying CFRP retrofitting techniques to strengthen steel columns.