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Trusses are open, three-dimensional structures with repeatable unit cells useful for high-specific strength, stiffness, and energy-dissipating applications. By infilling Ti-6Al-4V trusses with an incompressible elastomer (polydimethylsiloxane) or compressible foam (polyurethane), we demonstrate non-porous composite materials with controlled mechanical properties. The advantages of infilled composite trusses include corrosion resistance, density- or strength-matching, and the creation of a continuous external surface that can enable surface properties such as resistance to aerodynamic pressure and biomedical compatibility. Flexural and compressive testing of composite trusses show reproducible behavior dominated by the underlying truss in elastic and plastic regimes. Compared to unfilled trusses, those filled with an incompressible elastomer or compressible foam delay initial compressive failure to higher strains. For all investigated energy dissipation metrics, foam-infilled trusses demonstrated equivalent or improved performance compared to unfilled ones. This included plateau stress, volumetric energy dissipation (+19%), and energy absorption efficiency (+12%) and ideality (+20%). For the highest tested truss relative density (ρ∗ = 0.43), 150% improvement in volumetric energy absorption was observed. Compressive loading of the incompressible siloxane-filled trusses yielded higher peak failure stresses but at reduced energy dissipation. Composite trusses can therefore serve as structural solutions to improving energy absorption in higher stress applications with minimal increases to density.

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).

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.

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.

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.

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.

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.

The axial compressive mechanical response of substrate-supported carbon nanotube (CNT) arrays with heights from 35 to 1200 μm is evaluated using flat punch nanoindentation with indentation depths to 200 μm. The compressive behavior is consistent with that of an open-cell foam material with array height playing a role similar to that of occupation density for traditional foam. Mechanical yielding of all arrays is initiated between 0.03 and 0.12 strain and arises from localized coordinated plastic buckling. For intermediate CNT array heights between 190 and 650 μm, buckle formation is highly periodic, with characteristic wavelengths between 3 and 6 μm. Buckle formation produced substantial force oscillations in both the compressive and lateral directions. The compressive elastic modulus of the arrays is obtained as a continuous function of penetration depth and attains a value between 10 and 20 MPa for all arrays during mechanical yield. A qualitative model based upon concepts of cellular foam geometry is advanced to explain the observed CNT buckling behavior.

This study is a baseline assessment of the environmental durability of current state-of-the-art, fabric-reinforced shape-memory (SM) materials being considered for morphing applications. Tensile dog-bone-shaped specimens are cut along three different directions, namely, along (0°), perpendicular (90°), and oblique (45°) to the planar orientation of the fabric. The elastomeric response and shape memory properties before and after simulated environmental exposure to moisture, lubrication oil, and UV radiation are measured. Weight loss of the as-received and conditioned specimens is monitored and the dog-bone-shaped specimens are subjected to recovery following fixation. Parameters being investigated include modulus in the glassy and rubbery state, stored strain, shape fixity, recovery stress, and unconstrained shape recovery. There is a twofold decrease in the composite stiffness as the material is cycled between room and elevated (above the glass transition) temperature. At room temperature, the 0-degree specimen has the maximum stiffness (5.8 GPa), failure strength (94 MPa), and failure strain (5.4%), while above the Tg, the 90-degree specimen has the least stiffness (∼18 MPa) and largest strain to failure (>200%). Thus, the composite exhibits large deformation in its rubbery state. Parameters which are strongly affected by the damage developed during the first SM cycle include rubbery and glassy (or unloading) moduli values measured during the second SM cycle, while smaller changes are observed in shape fixity and recovery properties.

Sandwich composites that exhibit complete mechanical and geometric healing when subjected to three-point bending are realized using a novel, pressure-triggered granular core. The sandwich composites are constructed with fiberglass-reinforced, epoxy face sheets and a granular core consisting of noncohesive, glass microballoons. By controlling the granular core hydrostatic compression in the range of 0—0.083 MPa, the observed shear modulus of the core spanned nearly three orders of magnitude. Core compression is achieved by evacuating the interior of the sandwich structure and allowing the atmospheric pressure acting on the sandwich to compress the core. Although the compressed cores typically begin to yield near 1% shear strain, the degenerate nature of the core allows for complete recovery of the modulus even after a large strain. By exploiting the inherent stiffness of the sandwich skin and the tunable modulus of the granular core, these sandwich composites are able to undergo healing cycles and completely recover both their mechanical and geometric properties.

The present study is a baseline assessment of the durability of styrene- and epoxy-based shape memory polymer resin materials being considered for morphing applications when exposed to service environment. The approach for the experimental evaluation is a measurement of the shape memory properties and elastomeric response before and after separate environmental exposure to (i) water at 49°C for 4 days, (ii) in lube oil at room temperature and at 49°C for 24 h, and (iii) after exposure to xenon arc (63°C, 18 min water and light/102 min light only) and spectral intensity of 0.3—0.4 watts/m2 for 125 cycles (250 h exposure time). Parameters being investigated include modulus in the rubbery and glassy state, stored strain, shape fixity, stress recovery ratio, and linear shape recovery. In addition, we monitor changes in specimen color, weight, and dimensions along with onset of damage due to conditioning and subsequent thermomechanical cycling.

The interfacial shear strength of carbon nanotube coated carbon fibers in epoxy was studied using the single-fiber composite fragmentation test. The carbon fibers were coated with carbon nanotubes (CNT) on the fiber surface using thermal chemical vapor deposition (CVD). The CVD process was adjusted to produce two CNT morphologies for the study: radially aligned and randomly oriented. The purpose of the CNT coating was to potentially produce a multifunctional structural composite. Results of the single-fiber fragmentation tests indicate an improvement in interfacial shear strength with the addition of a nanotube coating. This improvement can most likely be attributed to an increase in the interphase yield strength as well as an improvement in interfacial adhesion due to the presence of the nanotubes.

Epoxy–nanocomposite resins filled with 12-nm spherical silica particles were investigated for their thermal and mechanical properties as a function of silica loading. The nanoparticles were easily dispersed with minimal aggregation for loadings up to 25 wt% as determined using transmission electron microscopy (TEM) and ultra-small-angle X-ray scattering (USAXS). A proportional decrease in cure temperatures and glass transition temperature (for loadings of 10 wt% and above) was observed with increased silica loading. The morphology determined by USAXS is consistent with a zone around the silica particles from which neighboring particles are excluded. The “exclusion zone” extends to 10× the particle diameter. For samples with loadings less than 10 wt%, increases of 25% in tensile modulus and 30% in fracture toughness were obtained. More highly loaded samples continued to increase in modulus, but decreased in strength and fracture toughness. Overall, the addition of nanosilica is shown as a promising method for property enhancement of aerospace epoxy composite resins.

Polymer/layered-silicate nanocomposites have unique and hierarchical structures that can provide improvements to the properties of polymeric materials. Controlling the dispersion of the nanomaterials through processing greatly influences the resulting morphology and the resulting properties of the nanocomposite. In this article, the dispersion behavior of organic layered silicates (OLS) as a function of the processing procedure is reported. The behavior of the OLS in all stages of processing—in the solvent, the epoxy prepolymer, and in the epoxy through cure—is discussed. On the basis of understanding of the dispersion behavior of the OLS in the epoxy resin at each stage of processing, a different processing procedure can be designed and used so that the morphology of the epoxy/layered-silicate nanocomposite can be regulated. Mild low-shear processing resulted in an intercalated nanocomposite with large-size aggregates (> 10 μm), and high-shear processing resulted in an intercalated nanocomposite with relatively small-size aggregates (0.5–3 μm), whereas the high-shear and ultrasonication processing procedures gave rise to an exfoliated nanocomposite. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

The rapid heating of carbon-fiber-reinforced polymer matrix composites leads to complex thermophysical interactions which not only are dependent on the thermal properties of the constituents and microstructure but are also dependent on the thermal transport between the fiber and resin interfaces. Using atomistic molecular dynamics simulations, the thermal conductance across the interface between a carbon-fiber near-surface region and bismaleimide monomer matrix is calculated as a function of the interface and bulk features of the carbon fiber. The surface of the carbon fiber is modeled as sheets of graphitic carbon with (a) varying degrees of surface functionality, (b) varying defect concentrations in the surface-carbon model (pure graphitic vs partially graphitic), (c) varying orientation of graphitic carbon at the interface, (d) varying interface saturation (dangling vs saturated bonds), (e) varying degrees of surface roughness, and (f) incorporating high conductive fillers (carbon nanotubes) at the interface. After combining separately equilibrated matrix system and different surface-carbon models, thermal energy exchange is investigated in terms of interface thermal conductance across the carbon fiber and the matrix. It is observed that modifications in the studied parameters (a-f) often lead to significant modulation of thermal conductance across the interface and, thus, showcases the role of interface tailoring and surface-carbon morphology toward thermal energy exchange. More importantly, the results provide key bounds and a realistic degree of variation to the interface thermal conductance values at fiber/matrix interfaces as a function of different surface-carbon features.

Shape memory composites (SMCs) based on shape memory alloys (SMAs) and shape memory polymers (SMPs) allow many design possibilities due to their controllable temperature-dependent mechanical properties. The complementary characteristics of SMAs and SMPs can be utilized in systems with shape recovery created by the SMA and shape fixity provided by the SMP. In this research, three SMC operating regimes are identified and the behavior of SMC structures is analyzed by focusing on composite shape fixity and interfacial stresses. Analytical models show that SMPs can be used to adequately fix the shape of SMA actuators and springs. COMSOL finite element simulations are in agreement with analytical expressions for shape fixity and interfacial stresses. Analytical models are developed for an end-coupled linear SMP–SMA two-way actuator and the predicted strain is shown to be in good agreement with experimental test results.

The deformation mechanism and mechanical properties of carbon nanotube (CNT) forests conformally coated with alumina using atomic layer deposition (ALD) are investigated using in situ and ex situ micro-indentation. While micro-indentation of a CNT forest coated with a thin discontinuous layer using 20 ALD cycles results in a deformation response similar to the response of uncoated CNT forests, a similar test on a CNT forest coated with a sufficiently thick and continuous layer using 100 ALD cycles causes fracture of both the alumina coatings and the core CNTs. With a 10 nm coating, 4-fold and 14-fold stiffness increases are measured using a flat punch and a Berkovich tip, respectively. Indentation testing with the Berkovich tip also reveals increased recoverability at relatively low strains. The results show that ALD coated CNT forests could be useful for applications that require higher stiffness or recoverability. Also, fracturing of the nanotubes shows that upper limits exist in the loading of conformally coated CNT forests.

Rate dependent mechanical response of the vertically aligned carbon nanotube arrays (VA-CNTs) has been examined with large-displacement indentation tests. The VA-CNTs are observed to exhibit elastic deformation at small displacement and then plastic deformation at large displacement. Under the cylindrical, flat-ended punch, the nanotube arrays collapse plastically at positions of immediately beneath the indenter face. The plastic zone remains stable at large displacement, because the stress/strain field under a flat cylindrical punch is relatively constant. From the normalized indentation stress-displacement curve, the critical indentation pressure (Pm), a measure of collapsing stress of the CNT arrays, is obtained. The speeds of the indenter have been varied, from 0.5 to 4 μm/s. The large displacement deformation is influenced by the effective strain rate of the material. The critical indentation pressure increases with the increase with the strain rates.

In the present study, a series of novel linear polyaspartimide-based silane endcapped (cross-linked) polymers are synthesized using 4-4′ bismaleimidodiphenylmethane, Jeffamine D-400 (BMI-JA-400), and (3-Aminopropyl) trimethoxysilane. To add strength to these systems, the trimethoxysilane moiety is cross-linked with the addition of water to create a thermosetting material with both improved toughness and variable cross-link densities. Thermal analysis is done to evaluate the developed shape-memory polymer (SMP) resin for composite processing feasibility. The solvent content in the resin and thermal stability is monitored using thermogravimetric analysis (TGA) while advanced rheometric expansion system (ARES) with parallel plate geometry is used to measure viscosity variation with temperature. The resin BMI-JA-400-Si-70/30 is chosen for making the composite based on its viscosity, weight change, and kinetic results. Differential scanning calorimetry (DSC) is performed to determine the cure kinetics including the temperatures at which the cure reaction initiates and completes in order to develop the cure cycle for composite fabrication. The selected SMP resin is hand-impregnated with T-300 plain-weave and T-700 uni-weave carbon fabric. Six-ply composites are successfully fabricated with < 2% void content using both fabric weaves. The thermo-mechanical properties of the SMP resin are measured using dynamic mechanical analysis (DMA). In addition, the shape memory cycle with free recovery is conducted on the SMP resin and composites.

There is great interest in making shape-changing aircraft structures that are more biomimetic. Cylindrical McKibben-like flexible actuators efficiently convert fluid pressure into mechanical energy and thus offer excellent force-to-weight ratios while behaving similar to biological muscle. McKibben-like Rubber Muscle Actuators (RMAs) were embedded into elastomer panels. The effect of actuator spacing on the performance of these shape-changing panels was investigated. The work included nonlinear finite element analysis, fabrication, and testing of panels where four RMAs were spaced side-by-side, 1/2, 1, and 1.3 RMA diameters apart. Nonlinear “Laminated Plate” and “Rod & Plate” finite element models of individual RMAs were created from existing RMA dimensions. After adjusting for an initial “activation pressure,” the models produced realistic RMA forces. The laminated plate models used less computer resources, but only produced small amounts of actuator contraction (actuator strain). The more resource-intensive Rod & Plate models better replicated fiber/braid re-orientation and produced axial strains up to 60% of test values. Three types of embedded RMA panel FEA models; a “2D Cross-Section,” a “Full 3D Panel” (with either Laminated Plate or Rod & Plate RMAs) and a “3D Unit Cell” (also with either Laminated Plate or Rod & Plate RMAs). The Full 3D Rod & Plate model gave the most accurate strains and forces, but required unsustainable levels of computing resources. The 2D cross-section model predicted optimal RMA spacing to be at 1 diameter. All other FEA models show optimal panel performance between 1/2 and 1 diameter spacing. Panels with embedded RMAs were fabricated and tested with air or water pressure. Panel force as a function of pressure and as a function of contraction (strain) was obtained. Overall, FEA and test results for panels indicate that optimal performance occurs when the RMAs are spaced between 1/2 to 1 diameter apart. Actuator force as a function of spacing is fairly flat in this region, indicating that minor design or manufacturing differences may not significantly affect performance. However, the total amount of axial contraction decreases significantly at greater than optimal spacing. Useful design, simulation, and test methodologies for embedded RMA panels have been demonstrated.

Shape memory composites (SMCs) based on shape memory alloys (SMAs) and shape memory polymers (SMPs) are interesting due to their controllable temperature-dependent mechanical properties. The complementary characteristics of SMAs and SMPs can be used to create materials or systems with shape recovery created by the SMA and shape fixity provided by the SMP. In this research, three SMC operating regimes are identified and the behavior of SMC structures is analyzed by focusing on composite fixity and interfacial stresses. Analytical models show that certain SMPs can achieve sufficient shape fixing. COMSOL Multi-Physics simulations are in agreement with analytical expressions for shape fixity and interfacial stresses. Analytical models are developed for an end-coupled linear SMP-SMA two-way actuation system.Copyright © 2012 by ASME

Glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCN) have been synthesized, characterized, and modeled to understand composition dependence on large amplitude, bidirectional bending and twisting deformation upon irradiation with linearly polarized blue-green (440-514 nm) light. These materials exhibit interesting properties for adaptive structure applications in which the shape of the photoresponsive solid state structure can be rapidly reconfigured with light. The basis for the photomechanical output observed in these materials is absorption of actinic light by azobenzene, which upon photoisomerization dictates an internal stress within the local polymer network. The photoinduced disruption of the order/orientation of the local polymer network accompanying photoisomerization is manifested in a macroscopic deformation. Accordingly, this work examines the polarization-controlled bidirectional bending of highly concentrated azo-LCN materials and compares the macroscopic bending to a nonlinear photoshell model. The resulting photomechanical output is highly dependent on the concentration of crosslinked azobenzene mesogens employed in the formulation. The model comparisons illustrate differences in internal photostrain and deformation rates as a function of composition.

Hierarchical carbon fibers show potential as a bio-inspired fluid flow sensor. The sensor is inspired from bat wings, which have thousands of micro-scale hairs that are deflected due to the flow and are believed to feedback flow information through force sensitive cells. Radially aligned carbon nanotube arrays on carbon fiber could function as the transducer in a similar device by decreasing resistance with the application of compressive strain. The bio-inspired flow sensor is first modeled to determine the compliance of the fiber and strains applied to the carbon nanotube arrays. Vertically aligned carbon nanotube arrays are then prepared on planar conductive substrates through transfer from insulating Si wafers; which simplifies the analysis of the mechanical properties of the material. The electromechanical material properties are measured by a modified dynamic mechanical analyzer. Results are presented along with recommendations for the next phase of electromechanical property evaluation.

A series of novel shape memory polymers, synthesized from 4-4-bismaleimidodiphenyl-methane, an extended chain aliphatic diamine, and a bis-isocyanate, have been created and characterized with the aim of providing a family of robust high temperature shape memory polymers with tailorable transition temperatures for use in reconfigurable aerospace structures. In the present study, three of the polymers are chosen for more detailed study of their thermomechanical properties. These materials are compared to commercial resins Veriflex® and Veriflex-E® which are styrene- and epoxy-based proprietary formulations, respectively. The thermal and mechanical properties are determined utilizing thermogravimetric analysis and dynamic mechanical analysis. The temperatures at which 2% weight loss is observed in dry air ranges from 272 to 305 °C for the synthesized polymers, and occurs at 242 and 317 °C for the commercial Veriflex® and Veriflex-E® respectively. The glass transition temperatures, as measured by the peak in the tan(δ) curve, for the synthesized polymers range from 110 to 144 °C which is a higher than the Veriflex® and Veriflex-E® achieve at 84.3 and 100 °C respectively. With operation temperatures of subsonic structural aircraft components often reaching 121 °C (250 °F), the transition temperatures of the bismaleimide-based shape memory polymers are clearly desirable to ensure that shape memory polymers used in aircraft structures will not be prematurely triggered by the existing heat loads. In addition, the shape memory performance of the bismaleimide-based shape memory polymers compares well with the Veriflex® and Veriflex-E® resins.Copyright © 2011 by ASME

Thermally-actuated shape memory polymers (SMPs) typically display two phases separated by the glass transition temperature (Tg). At temperatures well below the Tg, the polymer exhibits a relatively high elastic modulus. Well above the Tg the elastic modulus drops by several orders of magnitude. In this high temperature region, SMP materials can achieve strain levels well above 100 %. The complex behavior of SMPs (stiffnesses dropping to the order of 1 GPa and extremely high strain levels) precludes the use of traditional strain gages and low-contact force extensometers. The present study presents a detailed expansion of state-of-the-art thermomechanical testing techniques used to characterize the material behavior of SMPs. An MTS environmental chamber with an observation window allows for non-contact optical measurements during testing. A laser extensometer is used for measurement and active control of axial strain. The upper limit on the strain rate capability of the laser extensometer is established. In addition, the photographic strain measurement method known as digital image correlation (DIC) is incorporated, allowing for full field measurement of axial and transverse strains of SMPs over a range of temperatures and strain rates. The strain measurements of the DIC and laser extensometer are compared to each other as well as to clip-on extensometers and strain gages. The comparisons provide insight into the limitations of the traditional strain measurement systems. A series of tensile tests are performed on a commercial SMP from 25 °C up to temperatures of 130 °C and strain levels above 100 %. The laser extensometer provides a robust method for controlling the strain in the gage section of the samples. In addition, results show that the full field measurements of both the axial and the transverse strain are essential for characterizing the constitutive response of SMPs at room and elevated temperatures.

The primary purposes of a core in a sandwich composite are to keep the face sheets separated by a fixed distance and to transmit shear stresses. Syntactic foam cores consisting of hollow glass microspheres and resin can form strong, lightweight cores. By underfilling the interstitial space in a packed microsphere bed with a binder, a three-phase syntactic foam is created that has a percolated void network. In a sealed sandwich composite, a void network allows for the entire core of the sandwich composite to be evacuated and mechanically compressed by the exterior pressure. By combining this compression with a heating cycle, it is possible to repair core cracking and core/face sheet interface debonding when a reversible binder is used. Upon cooling, the healed sandwich restores its properties. We examine the relation between the mechanical properties of these sandwich composites and the healing methodologies.