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This paper investigates the mechanical properties and microstructure of additively manufactured, continuous fiber reinforced composites made by the Continuous Fiber 3D Printing (CF3D®) process. We specifically investigate composites of GF-2, a high temperature thermosetting acrylic polymer, and T-1100, a high strength carbon fiber, with a 41.5% fiber volume fraction. The resulting composites have a longitudinal tensile modulus of 122 GPa and a strength of 1599 MPa, which is 89% and 55% of the theoretical value by rule of mixtures, respectively. This appears to be the highest strength additively manufactured composites to date, largely due to the high mechanical properties of the T-1100 fiber, low defects, and substantial fiber volume fraction. Comparable results are obtained for a cationic cure resin, CATPRO14. We optically measured the fiber–fiber contact surface area fraction of GF-2 and employing the unidirectional strength model of Karam to predict a strength efficiency of 0.6, which is in good agreement with mechanical estimate of 0.55. Comparing results with the efficiency factors of high fiber volume prepreg laminates, we determine the printing process yields composites with 88%–95% of the anticipated longitudinal modulus and 65%–70% of the anticipated longitudinal tensile strength of those processed by traditional methods. To extend the printing to multifunctional microvascular composites, we consider the mechanical implications of including sacrificial filaments of 100–300 μm within each tow. A modest decrease in longitudinal strength in proportion to the microchannel volume fraction is predicted, where the decrease is mainly attributed to the increased fiber-to-fiber interactions in more tightly packed tows.

Polyimide hybrid nanocomposites with the polyimide confined at molecular length scales exhibit enhanced fracture resistance with excellent thermal-oxidative stability at low density. Previously, polyimide nanocomposites were fabricated by infiltration of a polyimide precursor into a nanoporous matrix followed by sequential thermally induced imidization and cross-linking of the polyimide under nanometer-scale confinement. However, byproducts formed during imidization became volatile at the cross-linking temperature, limiting the polymer fill level and degrading the nanocomposite fracture resistance. This is solved in the present work with an easier approach where the nanoporous matrix is filled with shorter preimidized polyimide chains that are cross-linked while in the pores to eliminate the need for confined imidization reactions, which produces better results compared to the previous study. In addition, we selected a preimidized polyimide that has a higher chain mobility and a stronger interaction with the matrix pore surface. Consequently, the toughness achieved with un-cross-linked preimidized polyimide chains in this work is equivalent to that achieved with the cross-linking of the previously used polyimide chains and is doubled when preimidized polyimide chains are cross-linked. The increased chain mobility enables more efficient polymer filling and higher polymer fill levels. The higher polymer-pore surface interaction increases the energy dissipation during polyimide molecular bridging, increasing the nanocomposite fracture resistance. The combination of the higher polymer fill and the stronger polymer-surface interaction is shown to provide significant improvements to the nanocomposite fracture resistance and is validated with a molecular bridging model.

A design algorithm is presented for finding optimal topologies and alignment profiles of additively manufactured, fiber-reinforced composite structures. A SIMP-like scheme is used to determine where a constrained amount of material is to be located in the design space, while an additional local design variable determines print speed. The local print speed affects fiber alignment based on an experimentally determined relationship, allowing for graded control over effective orthotropy. Material properties are selected from experimental results for additive printed, chopped fiber composites. Two different load cases and three different global print directions are selected for a total of six minimum compliance optimization cases. Results show the design is sensitive to both local fiber control and global print angle selection. A post-processing and manufacturing technique is also introduced for fabricating the optimized structures.

The field of multi-stable structures has been steadily growing due to a wide range of potential applications including energy harvesting, MEMS, and mechanical logic. This work focuses on utilizing elastic energy trapping and snap-through phenomena of bistable unit cells to design a latticed, hierarchical multi-stable cylinder that can articulate up to 30 degrees from its center axis. The employment of bistable elements is hypothesized to reduce the total strain energy required to articulate the cylinder, and yield faster responses with the snap-through. While multi-stable cylinders exist in previous studies, there have been no previous attempts at studying different modes of deformation beyond compressive loading. Thus, the current work presents a new problem regarding the effects of bistable elements in a latticed cylinder that is carrying tensile, compressive, and shear loadings and exhibiting large displacements as the cylinder is articulated.. The total strain energy density of the articulating cylinder is investigated as a function of the heights of the unit cells, which aids in determining an ideal height for the design that minimizes the strain energy density. Results show that the strain energy of an articulating cylinder can be minimized with the use of multi-stability, and that a multi-stable cylinder can require up to three times less loads to maintain desired articulation compared to a mono-stable structure. These results will lead to future works on further optimizing the articulating cylinder by varying additional parameters like the individual heights of rows, the thicknesses of unit cell beams, the strain energy density, and the initial loading threshold for articulation. In addition, the work in this study can yield methodologies for designing arbitrarily morphing skins beyond just cylindrical geometries.

To date, molecular modeling of cross-linking polymers has focused on those involving single-reaction cure mechanisms, such as epoxies and the epoxide–amine reaction. In this work, we have developed a novel cross-linking framework that is capable of undertaking complex cure mechanisms involving several simultaneous reaction pathways with minimal user input. As a case study, a bismaleimide (BMI) resin is considered herein which possesses multiple cure reactions and reaction pathways. Using an adaptable molecular dynamics simulation method, we highlight our framework by implementing five distinct cure reactions of Matrimid-5292 (a BMI resin) and predicting the corresponding thermomechanical properties. The method is used to establish the influence of different cure reactions and extent of curing on mass density, glass transition temperature, coefficient of thermal expansion, elastic moduli, and thermal conductivity. The developed method is further validated by comparison of these properties to experimentally...

Interlayer bonds pose regions of weakness in structures produced via melt extrusion based polymer additive manufacturing. Bond strength was assessed both between layers and within layers as a function of print parameters by performing tensile tests on ABS coupons printed in two orientations. Print parameters considered were extruder temperature, print speed, and layer height. An IR camera was used to track thermal history of interlayer bond lines during the printing process. Contact length between roads was measured from mesostructure optical micrographs. Print speed was found to have a large impact on tensile strength with high speeds generally yielding lower strength. A plateau in tensile strength of 22 MPa was observed for a normalized contact length greater than 0.6 independent of print orientation.

In this work, we exploit a confinement-induced molecular synthesis and a resulting bridging mechanism to create confined polyimide thermoset nanocomposites that couple molecular confinement-enhanced toughening with an unprecedented combination of high-temperature properties at low density. We describe a synthesis strategy that involves the infiltration of individual polymer chains through a nanoscale porous network while simultaneous imidization reactions increase the molecular backbone stiffness. In the extreme limit where the confinement length scale is much smaller than the polymer's molecular size, confinement-induced molecular mechanisms give rise to exceptional mechanical properties. We find that polyimide oligomers can undergo cross-linking reactions even in such molecular-scale confinement, increasing the molecular weight of the organic phase and toughening the nanocomposite through a confinement-induced energy dissipation mechanism. This work demonstrates that the confinement-induced molecular bridging mechanism can be extended to thermoset polymers with multifunctional properties, such as excellent thermo-oxidative stability and high service temperatures (350 °C).

Irregular interdigitated morphology is prevalent in biological sutures in nature. Suture complexity index has long been recognized as the most important morphological parameter to govern the mechanical properties of biological sutures. However, the suture complexity index alone does not reflect all aspects of suture morphology. The goal of this investigation was to determine that besides suture complexity index, whether the degree of morphological irregularity of biological sutures has influences on the mechanical properties, and if there is any, how to quantify these influences. To explore these issues, theoretical and finite element (FE) suture models with the same suture complexity index but different levels of morphological irregularity were developed. The quasi-static stiffness, strength for damage initiation and post-failure process of irregular sutures were studied. It was shown that for the same suture complexity index, when the level of morphological irregularity increases, the overall strain to failure will increase while tensile stiffness is retained; also, the total energy to fracture increases with a sacrifice in strength to damage initiation. These results reveal that morphological irregularity is another important independent parameter to govern and balance the mechanical properties of biological sutures. Therefore, from the mechanics point of view, the prevalence of irregular suture morphology in nature is a merit, not a defect.

In contrast to morphing wings, which have been intensively studied and demonstrated, relatively little research has focused on morphing fuselages, which are capable of improving agility and range by using the articulating section as an aerodynamic control surface. Herein, we investigate several spring-reinforced cylindrical geometries representational of an articulating fuselage that operate in large-scale bending without buckling. Finite element models analyze design performance under loading and in bending and are compared to experimental prototypes. Axial compression tests are conducted to compute bending work as a function of articulation angle. Strong agreement between model predictions and experimental measurements is observed. Using mechanics-based graphical materials selection, carbon fiber-reinforced polymer composites or lightweight metal alloys are determined to be the most promising materials for successful designs. For the spring-reinforced design space, machined spring-reinforced designs are superior at lower values of articulation work (25 N⋅m) for 25° of articulation. Keywords: Morphing structures, Digital image correlation, Graphical materials selection, Kinematic design, Additive manufacturing

Shape-changing aerospace structures explore the trade-offs between aerodynamic performance and actuation efficiency. Herein, a morphing cylinder is optimized to minimize both its work to actuate in bending as well as radial deviations. Based on derived linear elastic mechanics of a cylindrical shell in bending, it was determined that favorable designs have high circumferential and low axial stiffnesses. We investigate circumferentially reinforced composites that limit radial displacement and allow for out-of-plane bending. Designs of flexible skins with embedded or underlying reinforcement were modeled, fabricated, and compared to unreinforced skins. Finite element analyses predicted the bending kinematics and structural responses. Digital image correlation used during the bending of physical prototypes generated strain contour plots of cylinder surfaces. Strong agreement between model predictions and experimental measurements was observed. The dimensions of each archetype were systematically varied via finite element modeling to obtain a set of nondominated designs capable of 25° of bending without buckling or large radial displacements. For the examined designs, it was determined that flexible skins reinforced with embedded rings were optimal at low (8–15 N·m) and high (>250 N·m) values for the work to articulate while flexible skins reinforced with underlying helical springs were optimal at moderate values (15–250 N·m). Keywords: Morphing structures, Digital image correlation, Finite element analysis, Kinematic design

Extrusion-based additive manufacturing methods, such as direct-write of carbon fiber-reinforced epoxy inks, have become an attractive route toward development of structural composites in recent years, because of emerging techniques such as big area additive manufacturing. The development of improved materials for these methods has been a major focus area; however, an understanding of the effects of the printing process on the structural and dynamic recovery in printed materials remains largely unexplored. The goal of this work is to capture multiscale and temporal morphology and dynamics within thermosetting composite inks to determine the parameters during the printing process that influence the recovery of the printed material. Herein, we use X-ray photon correlation spectroscopy in small-angle scattering geometry to reveal both morphology and recovery dynamics of a nanoparticle (layered-silicate Cloisite 30B) in a thermoset epoxy resin (EPON 826) during the printing process in real time. Our results show that the dynamics of the layered silicate particles during recovery are anisotropic and slow down to behavior which is characteristic of aging in colloidal clay suspensions around t

Additive manufacturing (AM) of fiber reinforced thermosetting resins has not been well explored due to material and processing restrictions including high viscosity, poor reinforcement dispersion, dimensional tolerance, and porosity. Yet, such a process would enable tool-less and agile manufacturing of small and complex composite parts with temperature and environmental stability superior to traditional thermoplastics. Printed thermoset composites could also be combined with traditional epoxy prepregs to make complex co-cured structures. In this work, a direct write AM process for epoxy / chopped carbon fiber composites is used which has been shown to provide cured composites tensile modulus and strength values 90% and 66% of the fully aligned composite, respectively. The fibers are largely oriented in the print direction, but have sufficient localized material flow to achieve acceptable transverse properties and low porosity. Sandwich structures created with additively printed cores and co-cured prepreg face sheets were fabricated and evaluated. Rather than creating a sandwich structure by adhesively bonding standard sizes of honeycomb core to cured composite face sheets, this work examined co-curing customized additively printed core structures with prepreg face sheets. This process can be used to design and optimize properties for complex geometries while reducing the number of manufacturing steps.

This work addresses the multi-fidelity analysis-driven design of a thermal transport system based on the flow of liquid metal through a structural laminate as induced by a solid-state magneto-hydro-dynamic (MHD) pump. A full three-dimensional model of the thermal transport system is both simplified to a reduced-order algebraic model, which correctly captures trends in the global system response, and alternatively implemented in an finite element framework, which captures essential global and local aspects of the system response not attainable via reduced-order modeling. The predictions of each model are validated against previously published experimental data. It is shown in detail for the first time in the context of MHD systems that a multi-fidelity approach to the multi-objective design optimization problem can leverage both the speed of the algebraic model and the accuracy of the finite element model, leading to effective predictions of optimal system designs in a reasonable amount of time. A relatively new algorithm for multi-objective and parameterized Pareto optimization is employed, and a clear path of continued development is identified.

The development of multifunctional composite structures enabled by embedded microvascular networks offers the potential for significant increases in functionality. The addition of active thermal management capability, self-healing, or electromagnetic configurability to a structural composite is attractive. However, the tradeoffs between mechanical performance and extended functionalities remain relatively ill-quantified. Previous studies of the effects of embedded microchannels on structural capability have considered a small number of loading conditions and layups not typically used in aerospace structures. This computational work provides comprehensive structural design guidance by quantifying the negative mechanical effects of parallel and periodic microchannels in aerospace composites, where the results are synthesized from a comprehensive set of analyses performed via rigorous implementation of widely-accepted modeling methods. The impacts of channel orientation, spacing, and channel cross-sectional aspect ratio are all quantified for laminate sequences typical of those used in aerospace structures.

Additive manufacturing offers the ability to fabricate complex structures without the need for expensive tooling which is very attractive for manufacturing of structural composites. One limitation of additive manufacturing technologies hindering their adoption in high-performance structural composites is the lack of compatible high operating temperature thermosetting resins with robust thermo-oxidative stability. To address this limitation, a reactive melt extruded thermosetting polyimide filament was printed via fused filament fabrication (FFF). A crosslink chemistry has been designed with slow reaction kinetics at the printing temperature which allows printing in a similar manner to engineering thermoplastic filaments. A cure cycle for the as-printed uncrosslinked articles was developed to achieve full cure without loss of shape. The glass transition temperature of the fully cured resin is 250 °C. Comparing longitudinal and transverse tensile strengths of printed and cured coupons showed highly anisotropic behavior. Transverse and through-thickness properties for both uncured and cured printed coupons were nearly identical. The average measured tensile strength of printed and fully cured material was 86 MPa; 73% of fully cured compression molded strength. These results demonstrate the ability to 3D print high performance thermosets without changing commercially available FFF machines.

Cyclic loading conditions are commonly encountered in the applications of shape memory polymers (SMPs), where the cyclic characteristics of the materials determine their performance during the service life, such as deformation resistance, shape recovery speed and shape recovery ratio. Recent studies indicate that in addition to the physical damage or some other irreversible softening effects, the viscoelastic nature could also be another possible reason for the degraded cyclic behavior of SMPs. In this paper, we explore in detail the influence of the viscoelastic properties on the cyclic tension and shape memory (SM) behavior of an epoxy based amorphous thermosetting polymer. Cyclic experiments were conducted first, which show that although the epoxy material does not have any visible damage or irreversible softening effect during deformation, it still exhibits obvious degradation in the cyclic tension and SM behaviors. A linear multi-branched model is utilized to assist in the prediction and understanding of the mechanical responses of amorphous SMPs. Parametric studies based on the applied model suggest that the shape memory performance can be improved by adjusting programming and recovery conditions, such as lowering the loading rate, increasing the programming temperature, and reducing the holding time.

We report growth of a hierarchical nanostructure consisting of multi-layer graphene-petals decorating individual carbon nanotubes in a vertically aligned CNT array. The structure obtained by post-growth treatment of the CNT array in a microwave plasma chemical vapor deposition process leads to a remarkable increase in array stability, stiffness, elastic recovery and energy absorption capacity and a decrease in stress softening. Further, the change in properties can be tuned by controlling the petal growth time. For a 50 μm tall array, growth of graphene petals increases the buckling load and energy absorbed in load–unload cycles by a factor of about 60. The petal-coated CNT arrays also retain their morphology during solvent immersion and evaporation cycles, while arrays without petals coalesce into highly dense regions. The results open a new pathway towards tuning mechanical behavior of any arbitrary CNT array and also demonstrate the importance of structural hierarchy in tailoring the behavior of nanostructures.

Transverse and out-of-plane mechanical properties of objects fabricated by fused filament fabrication, an additive manufacturing technique, are typically 20-50% lower than bulk properties. A major contributor to this knockdown is poor raster-toraster and interlayer bond strength driven by incomplete polymer coalescence. The two major steps in the coalescence process responsible for bond strength development are neck growth and interfacial diffusion. The goal of this work is to explore the effects of temperature, time, and particle size on coalescence of ABS polymer in a controlled environment to identify appropriate thermal history to target during the printing process for enhanced raster-to-raster and interlayer bond strength. Particle size was found to have a significant impact on neck growth where doubling the size reduced the normalized neck growth by 58%. Diffusion was qualitatively assessed by tracking absorbance of red pigment from a red colored ABS particle into the neighboring white ABS particle at 550 nm. Smaller particle size held at elevated temperature for longer time had pigment diffusion across longer distances from the neck region. Results suggest that smaller diameter rasters promote faster and more complete coalescence.

Low-cost unmanned aircraft that use affordable manufacturing and have limited service life can enable mission concepts in which there is a higher tolerance for aircraft loss, or attrition. Because of their higher risk tolerance, these low-cost attritable aircraft could also integrate emerging technology which may have previously been considered too risky for integration into expensive and long life aircraft. Light-weight multifunctional structural composites have the potential to integrate additional functions and enable mission agility without significantly adding weight or reducing payload capacity. However, design and development of these material systems are often difficult because of the traditional “building block” development approach used for traditional composites, the large option space available for structural and functional properties, and the potential complexity of the multiscale and multiphysics coupling. To realize integrated functionality, new multi-scales and multi-physical experimental mechanical characterization techniques should be merged with maturing integrated materials models. We discuss this need using examples of a reconfigurable liquid metal Structurally Embedded Vascular Antenna (SEVA), a plasmonic nanoparticle based method for measuring internal temperature gradients, and embedded micro-cantilever carbon-nanotube based sensors. The latter of these is also used to discuss the potential to accelerate development of multifunctional structural concepts by provide air flow measurement and structural feedback during testing of complex structures. This could, in turn, eliminate some testing of intermediate elements in the slow and expensive traditional “building block” approach.

Within this paper, we develop an extension to traditional graphical material selection techniques to create maps of the optimal materials for designs with limiting mechanisms which vary. We apply these techniques to pressurized structures that perform multiple functions and have functional systems embedded within them. Specifically, we consider a conceptual analysis for a pressurized radome and a microvascular composite with active thermal management. The latter is the basis for a liquid metal based Structurally Embedded Vascular Antenna (SEVA) with the potential to tune over a broad range of RF frequencies.

Understanding the shear strain, viscoelastic response, and onset of damage within bonded composites is critical to their design, processing, and reliability. This presentation will discuss the multidisciplinary research conducted which led to the conception, development, and demonstration of two methods for measuring the shear within a bonded joint – dualplane digital image correlation (DIC) and a micro-cantilever shear sensor. The dual plane DIC method was developed to measure the strain field on opposing sides of a transparent single-lap joint in order to spatially quantify the joint shear strain. The sensor consists of a single glass fiber cantilever beam with a radially-grown forest of carbon nanotubes (CNTs) within a capillary pore. When the fiber is deflected, the internal radial CNT array is compressed against an electrode within the pore and the corresponding decrease in electrical resistance is correlated with the external loading. When this small, simple, and low-cost sensor was integrated within a composite bonded joint and cycled in tension, the onset of damage prior to joint failure was observed. In a second sample configuration, both the dual plane DIC and the hair sensor detected viscoplastic changes in the strain of the sample in response to continued loading.

Natural materials such as bone and tooth achieve precisely tuned mechanical and interfacial properties by varying the concentration and orientation of their nanoscale constituents. However, the realization of such control in engineered foams is limited by manufacturing-driven tradeoffs among the size, order, and dispersion uniformity of the building blocks. It is demonstrated how to manufacture nanocomposite foams with precisely controllable mechanical properties via aligned carbon nanotube (CNT) growth followed by atomic layer deposition (ALD). By starting with a low density CNT forest and varying the ALD coating thickness, we realize predictable ≈1000-fold control of Young's modulus (14 MPa to 20 GPa, where E ∼ ρ 2.8), ultimate compressive strength (0.8 MPa to 0.16 GPa), and energy absorption (0.4 to 400 J cm–3). Owing to the continuous, long CNTs within the ceramic nanocomposite, the compressive strength and toughness of the new material are 10-fold greater than commercially available aluminum foam over the same density range. Moreover, the compressive stiffness and strength equal that of compact bone at 10% lower density. Along with emerging technologies for scalable patterning and roll-to-roll manufacturing and lamination of CNT films, coated CNT foams may be especially suited to multifunctional applications such as catalysis, filtration, and thermal protection.

The microvascular strategy demonstrates unique advantages in shape-memory polymer applications as it can achieve both rapid thermal activation and deactivation during a typical shape-memory cycle. In addition, localized overheating can be avoided in comparison with other heating methods, such as the magnetic and electric heating. In this paper, a finite-element-based computational analysis was performed to study the rapid thermal response of shape-memory polymer composites with an embedded microvascular system. We show that the polymer shape-fixing speed, the shape fixity after a given cooling time, and the shape-recovery rate are significantly enhanced due to the rapid cooling and heating effect of internal microvasculature. The effect of the composite dimensions and microvascular channel arrangement on the activation and deactivation was studied. Typically, reducing the composite thickness and tube spacing increases the shape-recovery speed of the shape-memory polymer composite by effectively reducing the length of the thermal conduction pathways. Strategies for achieving an optimized tube arrangement were also discussed based on the considerations of the system mass and thermal boundary conditions. The results in this paper provide a guideline for further designs and applications of shape-memory polymer composites with embedded microvasculature.

The mechanical behavior of carbon nanotube (CNT) forests soaked in three solvents – toluene, acetonitrile, and isopropanol – is examined. Effective stiffness of the structure is evaluated in the dry and wet condition by micro-indentation using a 100 μm flat punch. With soaking of CNT forests in solvents, the stiffness decreases and deformation mechanism changes from buckling concentrated close to the bottom of the CNT forest to a distribution of local buckles along the height and global buckling of the entire length of CNTs. We use molecular dynamics simulations to relate the experimental observations to the reduced mechanical support from neighbor CNTs due to a decreased magnitude of van der Waals (vdW) interactions in the presence of solvents. Toluene, which produces the lowest average measured stiffness between the three solvents, produces the lowest vdW forces between individual CNTs. Furthermore, wet–dry cycling of CNT forests shows the reversibility and repeatability of change of stiffness by immersing in solvents. The results show that soaking CNT forests in solvents could be useful for applications such as interface materials where lower stiffness of CNT forests are needed and applications such as energy absorbing materials in which re-setting of stiffness is required.

Artificial hair sensors consisting of a piezoresistive carbon-nanotube-coated glass fiber embedded in a microcapillary are assembled and characterized. Individual sensors resemble a hair plug that may be integrated in a wide range of host materials. The sensors demonstrate an air-flow detection threshold of less than 1 m/s with a piezoresistive sensitivity of 1.3% per m/s air-flow change.

For the first time, carbon nanotube (CNT) forests are fully characterized as transversely isotropic continuum material. Each of the five independent elastic constants is experimentally obtained using a combination of nanoindenter-based uniaxial compression and shear testing, in situ SEM compression, and digital image correlation (DIC) of vertically and laterally oriented CNT microstructure columns. Material properties are highly anisotropic, with an axial modulus (165–275 MPa) that is nearly two orders of magnitude greater than the transverse modulus (2.5–2.7 MPa) and the out of plane shear modulus (0.8–1.6 MPa). The Poisson’s ratios along three mutually orthogonal axes, measured directly by simultaneous in situ DIC evaluation of axial and transverse strain, are found to be similarly anisotropic (ν12 = 0.35, ν23 = 0.1, ν21 = 0.005). A Timoshenko beam model is then developed to accurately predict the critical buckling stress of the vertically oriented columns using a subset of these anisotropic properties and considering inelastic column buckling. These results show that the critical bucking stress of CNT microstructures vary predictably with geometry and that continuum models with appropriate material constants may be applied to analyze CNT microstructures and evaluate their stability for many applications.

Elastocapillary self-assembly is emerging as a versatile technique to manufacture three-dimensional (3D) microstructures and complex surface textures from arrangements of micro- and nanoscale filaments. Understanding the mechanics of capillary self-assembly is essential to engineering of properties such as shape-directed actuation, anisotropic wetting and adhesion, and mechanical energy transfer and dissipation. We study elastocapillary self-assembly (herein called "capillary forming") of carbon nanotube (CNT) microstructures, combining in situ optical imaging, micromechanical testing, and finite element modeling. By imaging, we identify sequential stages of liquid infiltration, evaporation, and solid shrinkage, whose kinetics relate to the size and shape of the CNT microstructure. We couple these observations with measurements of the orthotropic elastic moduli of CNT forests to understand how the dynamic of shrinkage of the vapor-liquid interface is coupled to the compression of the forest. We compare the kinetics of shrinkage to the rate of evporation from liquid droplets having the same size and geometry. Moreover, we show that the amount of shrinkage during evaporation is governed by the ability of the CNTs to slip against one another, which can be manipulated by the deposition of thin conformal coatings on the CNTs by atomic layer deposition (ALD). This insight is confirmed by finite element modeling of pairs of CNTs as corrugated beams in contact and highlights the coupled role of elasticity and friction in shrinkage and stability of nanoporous solids. Overall, this study shows that nanoscale porosity can be tailored via the filament density and adhesion at contact points, which is important to the development of lightweight multifunctional materials.

Plasma-enhanced chemical vapor deposition (PECVD) is a well-known method for the synthesis of carbon nanotube (CNT) forests with the electric field in the plasma sheath being responsible for the vertical orientation of CNTs. Here, we investigate the deformation mechanism and mechanical properties of pristine and conformally coated PECVD CNT forests under compressive loading. Our in situ indentation experiments reveal that local buckles form along the height of pristine CNTs progressing downward from the starting point at the tips. For CNT forests coated from their roots to top with alumina using atomic layer deposition (ALD), the deformation mechanism depends strongly on the coating thickness. The buckling behavior does not change significantly when the coating is 5-nm thick. However, with a 10-nm-thick coating, the nanotubes fracture—that is, at both the CNT core and alumina coating. Ex situ indentation experiments with a flat punch reveal 8- and 22-fold increase in stiffness with the 5- and 10-nm coating, respectively. Comparing the behavior of the PECVD forests with CNTs grown with thermal chemical vapor deposition (CVD) shows that the mechanical behavior of PECVD CNTs depends on their characteristic morphology caused by the growth parameters including plasma. Our findings could serve as guidelines for tailoring the properties of CNT structures for various applications in which CNT compliance or deformation plays a critical role.

The majority of aerospace structural composites use thermoset resins for their processing flexibility, temperature capability, and environmental durability. In this study, the recovery behavior of Veriflex-E, an epoxy-based, thermosetting, thermally-triggered shape memory polymer (SMP) resin, is investigated in glassy (25 °C) and rubbery (130 °C) states, as a function of shape memory cycles, and as a means to evaluate its potential as a resin for a shape memory composite. At 25 °C, Veriflex-E exhibits a promising high elastic modulus and a positive, nonlinear strain rate sensitivity in monotonic loading. At 130 °C, the strain rate sensitivity in monotonic loading decreases. Stress relaxation after storage in the deformed temporary state and subsequent shape recovery is of particular interest, a challenge to measure, and has not been widely reported for SMPs. The current experimental program measures the influence of strain rate changes in the 10−4–10−2 s−1 range on the stress relaxation response of the material, as well as on the strain recovery behavior at both 25 °C and 130 °C. As expected, the post-relaxation strain is larger with faster loading. Unexpectedly, the total strain recovered after shape memory cycling is more similar to the low temperature deformation. Overall, the results suggest that, while being influenced by both the loading rate and the test temperature, Veriflex-E is a promising candidate for a shape memory composite which could enable adaptive structures.

With incorporation of gold nanoparticles, i.e., nanorods (AuNR) and nanospheres (AuNS), into a polyurethane-based shape-memory polymer (SMP) EG-72D matrix, SMP nanocomposite films capable of being remotely triggered by low-power laser are fabricated and characterized using UV-vis-NIR spectroscopy, X-ray scattering, and dynamic mechanical analysis (DMA). It is demonstrated that, with incorporation of very low concentration of gold nanorods (≈0.1 wt%), the mechanically programmed EG-72D/AuNR nanocomposite presents rapid response to low power laser irradiation (785 nm, ≈10 mW). Comparative studies on the laser irradiation response of EG-72D/AuNS and EG-72D/AuNR nanocomposite films suggest that AuNRs have significantly higher photothermal conversion efficiency than AuNS and on-resonance laser irradiation, matching the wavelength of the incident laser with the longitudinal plasmon resonance of AuNR, is necessary to induce the fast response of gold nanoparticle enabled SMP nanocomposites.

The integration of material systems to include thermal activation and deactivation of shape memory polymers represents a key challenge for adaptive systems. Microvascular fluid flow with hot or cold fluid is used as an energy transport mechanism to activate and deactivate shape memory polymers, where the maximum temperature is limited by the hot fluid temperature to prevent overheating. A thin panel was constructed from the Veriflex® shape memory polymer and included an array of 10 parallel microvascular tubes. The panel is strained orthogonally to the tube direction in the activated state. An analytical steady-state surface temperature model is applied to predict the surface temperature range during activation. Thermography is used to measure the steady-state surface temperature for heating and the dynamic surface temperature for both heating and cooling. The decay constants and surface temperature range for heating and cooling are examined as a function of applied strain (0%–20%) and fluid flow rate (0–...

In Fused Deposition Modeling (FDM), filaments of heated thermoplastic are extruded from a tip that moves in a raster-like motion. Depending on filament material properties and the processing parameters, the extruded beads and the fusion-bond between them could result in anisotropic properties (particularly with regard to strength). In this work, the focus is on characterizing the interlaminar fracture toughness of coupons produced by FDM with varying road-to-road offsets producing positive, zero and negative air gaps between neighboring roads resulting in coupons of varying density (void content). The resulting bond line formation is assessed through measuring and analyzing changes in the mesostructure and the contact attained at the interfaces between the adjoining roads. The bond quality is assessed by evaluating plane-strain fracture toughness using compact tension specimens. Results demonstrate an approximate five-fold increase in fracture toughness with varying air gaps.

The design and excitation of a liquid metal alloy antenna embedded into structural composite panel is studied. The alloy resides in micro-channels embedded into the composite panel that allows it to be pumped in and out of the panel to form the electrical paths necessary for the antenna. The antenna geometry is a derivative of a zig-zag log periodic dipole wire antenna. The excitation of the antenna from a structurally-embedded printed microstrip transmission line is also examined. The operational bandwidth and frequency response are controlled by altering the amount of alloy in the meandering structure of the channels, and preliminary results are provided.

Thermally activated shape memory polymers (SMPs) have attracted great interest in recent years for application in adaptive shape-changing (morphing) aero structures. However, these components require materials with transition temperatures well above the glass transition temperatures of most widely available SMPs while also maintaining processability and property tailorability. In the present study, a series of novel polyaspartimide-urea based polymers are synthesized and characterized. The glass transition temperature and shape memory properties are varied using a diisocyanate resin creating a urea crosslinking moiety between the polyaspartimide chains. Overall, a family of high temperature SMPs was synthesized and characterized showing high thermal stability (>300 °C), toughness, strong shape memory effects, and tailorable properties.

Spatial mapping of strain fields within compressed carbon nanotube (CNT) array columns is achieved using digital image correlation (DIC) analysis of in situ scanning electron microscopy (SEM) image sequences. Full-field displacement and strain maps are generated based upon the motion of the constituent CNTs, which serve as a traceable high-contrast speckle pattern for DIC analysis. The deformation modes and CNT array buckling characteristics vary systematically as a function of column aspect ratio, including bending, crushing, and bottom-up buckle accumulation behaviors. In spite of disparate appearing deformation modes, strain maps indicate that CNT array buckling consistently initiates at 5% local principal strain (ϵ2) for all columns. The ability to quantitatively assess the deformation modes and buckling behavior of CNT arrays at the nanoscale will enable their improved design for high-strain electrical contacts, compliant thermal interfaces, force sensors, energy-absorbing foams, or other applications.

Mechanical properties and deformation of the vertically aligned carbon nanotube arrays (VA-CNTs) has been examined with large-displacement indentation tests inside a scanning electronic microscope (SEM). The in-situ indentation allows for establishment of the load-depth responses and real-time observations of the deformation processes. Under a cylindrical, flat indenter, the VA-CNTs exhibit elastic deformation at small displacement and then plastic deformation at large displacement. The critical indentation pressure (Pm), a measure of collapsing stress of the CNT arrays, is obtained. The magnitude of Pm is approximately equal to the collapsing stress of the carbon nanotube arrays obtained under uniaxial compression.

Aerospace-grade, quasi-isotropic composite laminate panels of IM7/977-2 carbon fiber/epoxy prepreg are cured in an autoclave with microvascular channels. The channels are created both with a stainless steel tube insert and a removable mandrel to create lined and unlined passages, respectively. The interlaminar fracture toughness is examined as a function of the channel diameter and orientation to the adjacent, unidirectional fiber plies in a double cantilever beam test. The channels allow for a heat exchange fluid to flow through the composites and facilitate thermal transport. Thermography is used to examine the steady state surface temperature profiles of the horizontally oriented panels operating with internal fluid flow and external natural convection. A two-dimensional, analytic model was developed for the surface temperature profiles to understand the thermal transport within the specimens. The framework for the design and operation of multi-channel systems is established and a case study for surface temperature control is examined.

In this study, the inelastic deformation behavior of an epoxy-based, thermally triggered shape memory polymer resin, known as Veriflex-E, was investigated. The experimental program was designed to explore the influence of strain rate on monotonic loading at various temperatures which is needed to establish the design space of SMPs in load bearing applications. Thermally actuated shape memory polymers can be thought of as having two phases separated by the glass transition temperature (Tg). At temperatures below the Tg, Veriflex-E exhibits a high elastic modulus and positive, non-linear strain rate sensitivity in monotonic loading. The Poisson’s ratio at room temperature is independent of the strain rate, but dependent upon the strain magnitude. As the temperature is increased, the strain rate sensitivity in monotonic loading decreases. Well above the Tg, the elastic modulus drops by several orders of magnitude. In this high temperature region, the material achieves strain levels well above 100% and Poisson’s ratio is constant at 0.5 regardless of strain rate or strain magnitude.

Shape memory polymers (SMPs) are an emerging class of active polymers that have dual-shape capability and are, therefore, candidate materials for reconfigurable structures. However, the durability of SMPs has not been fully investigated to ensure they function properly in relevant environments. In this study, epoxy-based SMPs were conditioned separately in simulated service environments designed to be reflective of anticipated performance requirements, namely, (1) exposure to UV radiation for 125 cycles, (2) immersion in lube oil at ambient temperature, (3) immersion in water at 49 °C. A novel high-temperature indentation method was used to evaluate the shape recovery ability and mechanical properties of the conditioned SMPs. Results show that environmentally conditioned SMPs generally exhibit decreased glass transition temperatures and higher moduli and strain rate sensitivities in comparison to an unconditioned one. Environmental conditioning affects the shape recovery abilities of the SMPs if the recovery temperatures are set low. In particular, the UV exposed and water immersed SMPs exhibit noticeable lower shape recovery ratios compared to the unconditioned material. When the recovery temperature is above T g , the material’s ability to regain shape remains relatively unchanged with conditioning.

Quantitative nanoindentation of nominally 7.5 and 600 μm tall vertically aligned carbon nanotube (VACNT) arrays is observed in situ within an SEM chamber. The 7.5 μm array consists of highly aligned and weakly interacting CNTs and deflects similarly to classically defined cylindrical columns, with deformation geometry and critical buckling force well estimated using the Euler-Bernoulli theory. The 600 μm array has a highly entangled foam-like morphology and exhibits sequential buckle formation upon loading, with a buckle first forming near the array bottom at approximately 2% strain, followed by accumulating coordinated buckling at the top surface at strains exceeding 5%.

Thermally activated shape memory polymers (SMPs) can display modulus changes of approximately three orders of magnitude in transitioning from the high modulus, “glassy” state below the glass transition temperature (Tg) to the low modulus, “rubbery” state above the Tg. In the high temperature region, SMPs can achieve strain levels well above 100%. Their complex behavior includes large modulus changes to as low as ∼1 MPa, extremely high strain levels, and path dependent properties, thus precluding the use of traditional strain gages and low-contact force extensometers. The present study presents a comparison of thermomechanical testing techniques developed to characterize the material behavior of SMPs. Specifically, the performance of strain measurements using contact methods (clip-on extensometers and adhesive strain gages) are compared to non-contact methods (laser extensometer and digital image correlation). An MTS environmental chamber with an observation window allows for non-contact optical measurements during testing. A series of tensile tests are performed on a commercial SMP (with a Tg of ∼105 °C) at 25 °C and at 130 °C. It is observed that the clip-on extensometer significantly affects the SMP behavior even in the low temperature, high modulus state. Overall, the laser extensometer provides a robust method for controlling the axial strain in the gage section of the samples at moderate strain rates. The digital image correlation allows for full field measurement of both axial and transverse strains of SMPs over a range of temperatures and strain rates.