Thermal regulation of structural components is critical to modern applications, particularly those subjected to erratic operating conditions and environments. For instance, space probe solar panels and electric vehicle battery packaging avail vascular-based active cooling via fluid circulation to safeguard these critical onboard assemblies from thermal overload during charging/discharging. To conduct systematic heat transfer studies- involving modeling and/or experimentation-and develop suitable designs, it is essential to estimate the convective heat transfer coefficient (HTC). While the convective HTC, herein denoted by hconv, is a useful parameter, it is neither a material property nor a fundamental thermodynamic quantity, and is thus challenging to accurately estimate. Precise measurement of hconv from thermal experiments requires either achieving adiabatic conditions (a theoretical ideal often not realized in practice) or quantifying boundary loss (a random and nonlinear phenomenon). Additionally, thermal measurements, such as those obtained with infrared (IR) imaging, are prone to noise, further complicating the calculation. To overcome these challenges, we propose a new approach that combines experimental transient thermal measurements (together with noise filtering techniques) and a parametric estimation method based on a reduced order model (ROM). This approach allows one to determine both the convective HTC and boundary loss simultaneously and systematically. Moreover, incorporating boundary loss into the calculation of hconv enhances the accuracy, which in turn leads to higher fidelity numerical models that can be used to better understand and design more efficient thermal regulation systems.

Nature's structural composites, such as bone and wood, achieve mechanical performance through hierarchical multimaterial design. Though, their real vantage lies in the exceptional ability to repeatedly heal after damage. Synthetic fiber-reinforced polymer (FRP) composites also leverage material hierarchy via fibrous reinforcement encapsulated within a polymer matrix, maximizing stiffness and strength. However, the layered architecture of laminated FRP composites makes them vulnerable to interlaminar delamination-debonding of fibers from the matrix-which significantly compromises structural integrity. Recently, we introduced a self-healing strategy via in situ heating, where soft yet tough thermoplastic inclusions achieve interlaminar fracture recovery via polymer chain re-entanglement, i.e., thermal remending. Here, in our latest embodiment, by automating in situ thermo-mechanical experiments, we achieve an order-of-magnitude enhancement in self-healing repeatability-reaching an unprecedented 1,000 cycles. Healing begins at 175% and slowly declines to 60% of the mode-I fracture resistance of a plain (nonhealing) composite, revealing unique chemo-physical mechanisms that govern this behavior. Both fiber-debris accumulation in the molten poly(ethylene-co-methacrylic acid) (EMAA) healing agent, and waning interfacial chemical reactions between the EMAA and epoxy matrix, contribute. A Weibull distribution capturing this complex fracture recovery predicts an asymptotic healing limit above 40%, suggesting sustained repair is possible. Translating these newfound thermal remending results into real-world context, a modest quarterly self-healing schedule could maintain interlaminar fracture repair of FRP composites for over 125 y-well beyond the typical design life of many modern structures including aircraft and wind turbines. Thus, this latest self-healing paradigm effectively eliminates delamination as a failure mode.

The hierarchical microstructure of fiber-reinforced composites (FRC) enables lightweight materials with exceptional mechanical properties. However, their layered architecture is prone to interfacial damage, notably delamination. An effective strategy to mitigate delamination is by integrating thermoplastic interlayers, which not only enhance FRC resistance to interfacial fracture, but also enable self-repair of cracks through thermal mending. In this study, we demonstrate for the first time, repeated in situ self-healing of FRC laminates under both mode-I fracture (via the double cantilever beam) and 3-point flexure (employing short-beam shear). Remarkably, we achieve nearly complete restoration over ten consecutive healing cycles from thermal remending of 3D-printed poly(ethylene-co-methacrylic acid) (EMAA) interlayer inclusions. To understand the mechanical effects of such soft inclusions, we conduct a comprehensive experimental and numerical investigation. Our research findings reveal: (i) Markedly different strain states in short-beam shear with soft inclusions compared to FRC without. (ii) The necessity of incorporating contact algorithms for accurate finite element (FE) simulation of local stress/strain fields and global structural responses. (iii) Adjustments in the density and layer placement of printed EMAA domains can tailor both interlaminar shear strength (ILSS) and mode-I fracture resistance (GIC). This research offers newfound insights into realizing self-healing in actual structures, reliable and efficient simulation strategies for modelers, and advancements towards more modern design motifs and suitable materials testing protocols.

Interlaminar delamination is a prevalent and insidious damage mode limiting the mechanical integrity and lifetime of fiber-reinforced composites. Conventional resolution involves over-design, laborious inspection, and repair/replacement at cost to the economy and environment. Self-healing via in situ thermal remending of thermoplastic interlayers offers a promising solution. However, better understanding of the healing agent and related mechanisms is necessary to tailor healing performance. Here, we compare non-neutralized (copolymer) and metallic-ion neutralized (ionomer) poly(ethylene-co-methacrylic acid) (EMAA) thermoplastics for healing interlaminar fracture. We reveal (i) how EMAA chemistry affects the interfacial reactions driving healing and (ii) the influence of molten viscosity on repair efficiency. At fixed viscosity, higher methacrylic acid content, chain mobility, and lower neutralization positively influence healing, where lower melt viscosity at fixed temperature improves delamination recovery. Thus, this study deepens scientific understanding of key variables for healing interlaminar fracture with EMAA, providing new insight for the design of multifunctional composites.

Microvascular materials containing internal microchannels are able to achieve multi-functionality by flowing different fluids through vasculature. Active cooling is one application to protect structural components and devices from thermal overload, which is critical to modern technology including electric vehicle battery packaging and solar panels on space probes. Creating thermally efficient vascular network designs requires state-of-the-art computational tools. Prior optimization schemes have only considered steady-state cooling, rendering a knowledge gap for time-varying heat transfer behavior. In this study, a transient topology optimization framework is presented to maximize the active-cooling performance and mitigate computational cost. Here, we optimize the channel layout so that coolant flowing within the vascular network can remove heat quickly and also provide a lower steady-state temperature. An objective function for this new transient formulation is proposed that minimizes the area beneath the average temperature versus time curve to simultaneously reduce the temperature and cooling time. The thermal response of the system is obtained through a transient Geometric Reduced Order Finite Element Model (GRO-FEM). The model is verified via a conjugate heat transfer simulation in commercial software and validated by an active-cooling experiment conducted on a 3D-printed microvascular metal. A transient sensitivity analysis is derived to provide the optimizer with analytical gradients of the objective function for further computational efficiency. Example problems are solved demonstrating the method’s ability to enhance cooling performance along with a comparison of transient versus steady-state optimization results. In this comparison, both the steady-state and transient frameworks delivered different designs with similar performance characteristics for the problems considered in this study. This latest computational framework provides a new thermal regulation toolbox for microvascular material designers.

Critical to finite element (FE) analysis of fiber-reinforced composites is accurately reproducing microstructural features via high-quality meshes such that the material heterogeneity and anisotropy are properly captured. Here we present an integrated computational framework for generating realistic FE models of woven composites with high fiber volume fractions (>50%). This framework relies on a virtual microstructure reconstruction algorithm that first generates a geometric model of loosely-woven yarns (i.e., bundles of fibers), followed by performing an FE compaction simulation to create the final textile composite microstructure. A non-iterative meshing algorithm, i.e., conforming to interface structured adaptive mesh refinement (CISAMR), has been adapted to automatically transform synthesized microstructures into conforming FE meshes. CISAMR can handle challenging geometrical features such as thin resin interstices and yarn interpenetrations (an artifact of the FE compaction) without the need to reprocess the digital geometry before mesh generation. We show that the homogenized elastic moduli obtained from FE simulations of an 8-harness satin woven composite laminate agree well with experimental measurements (within 3%), thereby validating the accuracy of the framework. We also present several other mechanical analysis examples, including a nonlinear damage simulation, that further demonstrate the ability of this framework to construct high-fidelity FE models of intricate woven composites with varying 2D and 3D woven architectures.

Modulating temperature fields is indispensable for advancing modern technologies: space probes, electronic packing, and implantable medical devices, to name a few. Bio-inspired thermal regulation achieved via fluid flow within a network of embedded vesicles is notably desirable for slender synthetic material systems. This far-reaching study-availing theory, numerics, and experiments-reveals a counter-intuitive yet fundamental property of vascular-based fluid-flow-engendered thermal regulation. For such thin systems, the mean surface temperature and the outlet temperature-consequently, the heat extracted by the flowing fluid (coolant)-are invariant under flow reversal (i.e. swapping the inlet and outlet). Despite markedly different temperature fields under flow reversal, our newfound invariance-a discovery-holds for anisotropic thermal conductivity, any inlet and ambient temperatures, transient and steady-state responses, irregular domains, and arbitrary internal vascular topologies, including those with branching. The reported configuration-independent result benefits thermal regulation designers. For instance, the flexibility in the coolant's inlet location eases coordination challenges between electronics and various delivery systems in microfluidic devices without compromising performance (e.g. soft implantable coolers for pain management). Last but not least, the invariance offers an innovative way to verify computer codes, especially when analytical solutions are unavailable for intricate domain and vascular configurations.

Structural health monitoring (SHM) of safety-critical composite components is essential to ensure mechanical stasis and detect local damage before it can produce global failure. SHM technologies must also adapt to ever-evolving materials and geometries, but traditional piezoresistive strain sensors lack the ability for end-user customization and modifications throughout service-life. Additive manufacturing via fused filament fabrication (FFF) provides a practical pathway to overcome such sensor shortcomings. However, the electrical conductivity of conventional polymer feedstock is not sufficient for accurate strain measurements without compromise to melt viscosity and thus printability. Here we report the development of a new 3D-printable and electrically responsive nanocomposite by melt-mixing poly(ethylene-co-methacrylic acid) (EMAA) with multi-walled carbon nanotubes (MWCNT). Bulk electrical conductivity of 43.9 S m-1 is achieved at only 5 wt% loading - higher than comparable materials - where the nano-dispersion heterogeneity of MWCNT in EMAA is linked to the favorable conductivity while retaining molten flowability. FFF is employed to print thin (150 mu m) serpentine strain sensors onto the surface of a glass fiber-reinforced polymer composite, which exhibit strong adhesion and accurate piezoresistive sensing under cyclic flexural loading. Twenty consecutive cycles with converged sensor readings (i.e., < 1% variation in measured resistance) demonstrates reliable performance across a relevant service strain range (0.4 - 0.8 %) for such fiber-composites. This rapid fabrication and transferable sensing strategy, suitable for new and existing structures, thus provides a crosscutting SHM solution.

Microwave absorbing materials for high-temperature harsh environments are highly desirable for aerodynamically heated parts and engine combustion induced hot spots of aircrafts. This study reports ceramic composites with excellent and stable high-temperature microwave absorption in air, which are made of polymer-derived SiOC reinforced with core–shell nanophase structure of ZrB2/ZrO2. The fabricated ceramic composites have a crystallized t-ZrO2 interface between ZrB2 and SiOC domains. The ceramic composites exhibit stable dielectric properties, which are relatively insensitive to temperature change from room temperature to 900 °C. The return loss exceeds − 10 dB, especially between 28 and 40 GHz, at the elevated temperatures. The stable high-temperature electromagnetic (EM) absorption properties are attributed to the stable dielectric and electrical properties induced by the core–shell nanophase structure of ZrB2/ZrO2. Crystallized t-ZrO2 serve as nanoscale dielectric interfaces between ZrB2 and SiOC, which are favorable for EM wave introduction for enhancing polarization loss and absorption. Existence of t-ZrO2 interface also changes the temperature-dependent DC conductivity of ZrB2/SiOC ceramic composites when compared to that of ZrB2 and SiOC alone. Experimental results from thermomechanical, jet flow, thermal shock, and water vapor tests demonstrate that the developed ceramic composites have high stability in harsh environments, and can be used as high-temperature wide-band microwave absorbing structural materials.

Interlaminar delamination in fiber-reinforced composites limits structural capacity and service life. Delaminations, which occur subsurface and can lead to catastrophic failures, are hard to detect and repair. Contrasting traditional mitigation strategies (e.g., inspection, over-design), proactive toughening and responsive self-healing of damaged interfaces offer practical, cost-effective solutions. Our recently developed strategy to address interlaminar damage—3D-printed thermoplastic interlayers and structurally integrated heaters—has been shown to achieve composite toughening and in situ self-healing via thermal remending, abetting repeated repair and improved delamination resistance. Here, we leverage this latest thermal remending strategy to investigate the effects of 3D-printed pattern topology on damage resistance and self-healing response. The chief attributes are: (i) realizing up to 450% increase in mode-I fracture resistance, (ii) restoring up to 100% of the increased fracture resistance for ten consecutive healing cycles, and (iii) achieving in situ self-healing below the thermoset-matrix glass transition temperature, thereby preserving structural integrity during repair. The proposed damage mitigation strategy fosters structural reliability, reduces failure risk, and increases service lifetime—three essential attributes in meeting the multifaceted demands of modern composite infrastructure.

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.

Natural processes continuously degrade a material's performance throughout its life cycle. An emerging class of synthetic self-healing polymers and composites possess property-retaining functions with the promise of longer lifetimes. But sustained in-service repair of structural fiber-reinforced composites remains unfulfilled due to material heterogeneity and thermodynamic barriers in commonly cross-linked polymer-matrix constituents. Overcoming these inherent challenges for mechanical self-recovery is vital to extend in-service operation and attain widespread adoption of such bioinspired structural materials. Here we transcend existing obstacles and report a fiber-composite capable of minute-scale and prolonged in situ healing - 100 cycles: an order of magnitude higher than prior studies. By 3D printing a mendable thermoplastic onto woven glass/carbon fiber reinforcement and co-laminating with electrically resistive heater interlayers, we achieve in situ thermal remending of internal delamination via dynamic bond re-association. Full fracture recovery occurs below the glass-transition temperature of the thermoset epoxy-matrix composite, thus preserving stiffness during and after repair. A discovery of chemically driven improvement in thermal remending of glass- over carbon-fiber composites is also revealed. The marked lifetime extension offered by this self-healing strategy mitigates costly maintenance, facilitates repair of difficult-to-access structures (e.g., wind-turbine blades), and reduces part replacement, thereby benefiting economy and environment.

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.

Fiber-reinforced polymer composites are attractive structural materials due to their high specific strength/stiffness and excellent corrosion resistance. However, the lack of through-thickness reinforcement in laminated composites creates inherent susceptibility to fiber-matrix debonding, i.e., interlaminar delamination. This internal damage mode has proven difficult to detect and nearly impossible to repair via conventional methods, and therefore, remains a significant factor limiting the reliability of composite laminates in lightweight structures. Thus, novel approaches for mitigation (e.g., self-healing) of this incessant damage mode are of tremendous interest. Self-healing strategies involving sequestration of reactive liquids, i.e. microcapsule and microvascular systems, show promise for the extending service- life of laminated composites. However, limited heal cycles, long reaction times (hours/days), and variable stability of chemical agents under changing environmental conditions remain formidable research challenges. Intrinsic self- healing approaches that utilize reversible bonds in the host material circumvent many of these limitations and offer the potential for unlimited heal cycles. Here we detail the development of an intrinsic self-healing woven composite laminate based on thermally-induced dynamic bond re-association of 3D-printed polymer interlayers. In contrast to prior work, self-repair of the laminate occurs in situ and below the glass-transition temperature of the epoxy matrix, and maintains >85% of the elastic modulus during healing. This new platform has been deployed in both glass and carbon-fiber composites, demonstrating application versatility. Remarkably, up to 20 rapid (minute-scale) self-healing cycles have been achieved with healing efficiencies hovering 100% of the interlayer toughened (4-5x) composite laminate. This latest self-healing advancement exhibits unprecedented potential for perpetual in-service repair along with material multi-functionality (e.g., deicing ability) to meet modern application demands.

Liquid metals adhere to most surfaces despite their high surface tension due to the presence of a native gallium oxide layer. The ability to change the shape of functional fluids within a three-dimensional (3D) printed part with respect to time is a type of four-dimensional printing, yet surface adhesion limits the ability to pump liquid metals in and out of cavities and channels without leaving residue. Rough surfaces prevent adhesion, but most methods to roughen surfaces are difficult or impossible to apply on the interior of parts. Here, we show that silica particles suspended in an appropriate solvent can be injected inside cavities to coat the walls. This technique creates a transparent, nanoscopically rough (10-100 nm scale) coating that prevents adhesion of liquid metals on various 3D printed plastics and commercial polymers. Liquid metals roll and even bounce off treated surfaces (the latter occurs even when dropped from heights as high as 70 cm). Moreover, the coating can be removed locally by laser ablation to create selective wetting regions for metal patterning on the exterior of plastics. To demonstrate the utility of the coating, liquid metals were dynamically actuated inside a 3D printed channel or chamber without pinning the oxide, thereby demonstrating electrical circuits that can be reconfigured repeatably.

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.

Internal delamination in fiber-reinforced composites is difficult to detect and nearly impossible to repair by conventional methods. Bioinspired, microvascular self-healing strategies based on sequestering reactive liquids within hollow conduits have shown promise for repairing internal damage and extending lifetimes of laminated composites. However, difficulties in achieving in situ mixing of two-part healing agents, polymerization times on the order of hours/days at room temperature, and propensity for flow blockages from cross-contamination are research challenges that have limited the adoption of this technology. This paper describes an interdisciplinary research effort to develop self-healing microvascular composites using a one-part, photo-reactive chemistry. The new platform employs microvascular networks for functional fluid transport combined with optical waveguides for rapid, light-activated healing. An epoxy-based, cationic photochemistry was developed to polymerize upon sub-hour exposure to visible light. The one-part cationic chemistry is able to achieve higher recovery ( 75%) in mode-I fracture toughness compared to a commercial free-radical photochemistry ( 55%) and two-part epoxy/amine system ( 65%). In addition to providing targeted light delivery for photo-conversion, the optical waveguides are also intended to serve dual purpose as a self-sensing conduit to relate internal polymerization with mechanical self-recovery. Towards this in situ sensing target, Raman spectroscopy is conducted to show chemical conversion along with fracture recovery increases with increasing light exposure. This novel approach to integrated self-sensing/healing in structural composites outlines a new paradigm in multifunctional material design.