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Bismaleimide (BMI) resins are used in carbon fiber reinforced composites for high-temperature applications. Conventional BMI resins require high cure and post-cure processing temperatures, which limit fabrication methods and contribute to residual stress and microcracking. Recently, BMIs with lower cure temperatures have been developed; however, little data on the properties of low-temperature cure carbon/BMI composites are available in the open literature. In this study, processing and properties of a low-temperature cure BMI resin system, referred to here as BMI-2, were evaluated. Along with evaluation of neat resin properties, multi-directional woven carbon fiber reinforced BMI-2 composite laminates were fabricated by an out-of-autoclave, vacuum-bag-only resin infusion method. New data on the mechanical, thermal, and moisture absorption properties are presented. It was found that the resin infusion method produced laminates with fiber volume fraction of approximately 55% and immeasurably low void content. BMI-2 was found to attain a degree of cure of 96% and glass transition temperature of 366°C following a 163°C cure cycle. Cured laminates did not suffer from process-induced cracking at the inter- or intra-laminar levels. Tensile strength and modulus of quasi-isotropic specimens with woven AS4 carbon fiber were 443 MPa and 47.6 GPa, respectively, and failure under quasi-static tension occurred by delamination. Compression strength was stable across a wide temperature range, with quasi-isotropic specimens retaining 60% of their room temperature compression strength at 260°C. Equilibrium moisture content of composite specimens was 1.2% by weight.

Pitch-based carbon fiber reinforced epoxy composites are used in specialized applications for their high-modulus and thermal conductivity; however, little data on their fatigue performance are available in the open literature. In this study, fatigue behaviors of ultra-high-modulus pitch-based carbon fiber and standard-modulus polyacrylonitrile (PAN)-based carbon fiber were compared in woven quasi-isotropic epoxy matrix composites subject to uniaxial tension. It was found that the pitch fiber composite possessed higher normalized tensile fatigue strength and that its stress-life ( S-N) curve is less steep. Extrapolation suggests the pitch fiber composite is more fatigue-resistant in higher cycle regimes (specifically, N > 107). Cyclic loading of the pitch fiber composite resulted in minimal matrix damage, and the eventual fractures were localized and fiber-dominated for all stress levels. Cyclic loading of the PAN fiber composites resulted in widespread matrix cracking and delamination. The difference in fatigue behavior is attributed to the different strain levels attained at similar stress levels and the consequent difference in matrix damage development.

Carbon fiber reinforced polymer (CFRP) composites hybridized with hydrogen-doped NiTi wires can be used to design structures requiring high stiffness and high damping in the low frequency range, such as helicopter blades. The current work investigates aging and hydrogen-doping for high damping without hydrogen embrittlement. We establish a hydrogenation treatment that (i) results in a response that is repeatable in the martensitic phase and after exposure to composite processing temperatures and (ii) increases the loss factor in NiTi wires by nearly 470%. By embedding H-doped wires exhibiting the highest damping into the interlayers of a [0/±45]s carbon/epoxy laminate at a volume fraction of 0.1, the hybrid NiTi-CFRP composite loss factor increases by 170%. The measured dynamic properties were found to be close to micromechanical predictions based on the properties of the NiTi and CFRP.

Traditional lag dampers used in rotorcraft applications are unable to produce adequate levels of damping for high-speed coaxial compound rotorcraft that use rigid, hingeless blades. The current pap...

Additive manufacturing (AM) has been used widely to produce three-dimensional (3D) parts from computer-aided design (CAD) software. Traditional Fused Deposition Modeling (FDM) 3D printed polymer parts lack the necessary strength to be used for functional parts in service. The potential of printing continuous fiber reinforced composites has resulted in parts with better mechanical properties and enhanced performance. Very few studies have investigated the impact energy absorption of continuous fiber reinforced 3 D printed composites. The purpose of this work is to investigate the effect of different fiber patterns (unidirectional versus concentric), different stacking patterns (consolidated versus alternating layers), and fiber orientations (0°, 90°, 45°) on the impact energy absorption of 3 D printed continuous Kevlar fiber reinforced Onyx composites. Charpy impact testing was used to determine the impact energy absorption of the specimens. It was concluded that alternating the fiber and matrix layers as opposed to consolidating all the fiber layers in the center of the specimen results in lower impact energy absorption. Additionally, the specimens with unidirectional 90° fiber orientation had the lowest impact energy absorption among the specimens with alternating stacking pattern and those with consolidated [Formula: see text]45° angle-ply fiber orientations had the highest impact energy absorption.

Fiber volume fraction is a driving factor in mechanical properties of composites. Micromechanical models are typically used to predict the effective properties of composites with different fiber volume fractions. Since the microstructure of 3D-printed composites is intrinsically different than conventional composites, such predictions need to be evaluated for 3D-printed composites. This investigation evaluates the ability of the Voigt, Reuss, and Halpin–Tsai models to capture the dependence of modulus and strength of 3D-printed composites on varying fiber volume fraction. Tensile coupons were printed with continuous carbon fiber-reinforced Onyx matrix using a Markforged Mark Two printer. Specimens were printed at five different volume fractions with unidirectional fibers oriented at either [Formula: see text] to obtain longitudinal, shear, and transverse properties, respectively. It is shown that the Voigt model provides an excellent fit for the longitudinal tensile strength and a reasonable fit for the longitudinal modulus with varied fiber content. For the transverse direction, while the Reuss model fails to capture the transverse modulus trend, the Halpin–Tsai model provides a reasonable fit as it incorporates more experimental parameters. Like conventional composites, addition of fibers degrades the transverse strength, and the transverse strength decreases with increasing fiber volume fraction. The shear modulus variation with fiber content could not be fitted reasonably with either Halpin–Tsai model or Reuss model.

Tensile quasi-static and fatigue behaviors of out-of-autoclave, quasi-isotropic, E-glass/epoxy composites with woven and stitched fiber architectures are compared. Some specimens were cyclically loaded to failure, while others were tested for residual modulus and ultimate strength. Both laminates survived 107 cycles at a maximum stress of 94.8 MPa but failed prior to 107 cycles at a maximum stress greater than or equal to 124.1 MPa. For the stress-lifetime diagram and residual strength, the woven laminate had superior properties below 104 cycles and the stitched laminate had superior properties above 105 cycles. While intralaminar crack density was higher in the stitched laminate, the woven laminate demonstrated a higher percentage of delaminated area.

Toughened composites reinforced with nanofillers show improved mechanical performance such as increased abrasion resistance, fracture toughness, and fracture energy. The degree of these improvements is influenced by the degree of dispersion of the nanofillers which can be analyzed using force microscopy (AFM), a technique that allows for mapping the local height and elastic modulus of a surface. However, current AFM apparatuses can only measure a narrow range of moduli according to the type of tip, which complicates the full-field measurement of moduli in nanocomposites with nanosilica (~72 GPa) embedded in epoxy (0.1 – 5 GPa). Moreover, height mapping can only visualize filler particles exposed at the surface. These limitations make it challenging to determine the 3D location of nanoparticles near the surface of a composite. To overcome these limitations of conventional AFM, we used a combination of data science, micromechanics, and experimental data from AFM to locate the centroidal position of nanosilica (NS) particles relative to the surrounding epoxy surface. Using finite element simulations, a theoretical dataset of modulus values as a function of particle position relative to the epoxy surface was created as a training set. Bayesian optimization determines the “best” particle position that results in minimum error between simulated and experimental modulus contours. The algorithm returns the 3D position of the fully or partially embedded NS particle relative to the epoxy surface. The algorithm has shown the ability to partially produce simulated modulus contours that resemble the experimental modulus contours.

Hybrid fiber composites offer designers a means of tailoring the stress–strain behavior of lightweight materials used in high-performance structures. While the longitudinal stress–strain behavior of unidirectional hybrid fiber composites has been thoroughly evaluated experimentally and analytically, relatively little information is available on the transverse behavior. The objective of the current investigation is to present data on the transverse modulus of elasticity of unidirectional composites with five different ratios of carbon and glass fiber and to compare the data with predictive and fitted models. The transverse modulus increases monotonically with the proportion of glass fiber in the composite. Finite element analysis was used to evaluate different ways to model voids in the matrix and allowed the unknown transverse properties of the carbon fibers to be backed out using experimental data from the all-carbon composite. The finite element results show that the transverse modulus can be accurately modeled if voids are modeled explicitly in the matrix region and if modulus is calculated based on stress applied along the minimum interfiber distance path between adjacent fibers arranged in a rectangular array. The transverse modulus was under-predicted by the iso-stress model and was well predicted by a modified iso-stress model and a modified Halpin–Tsai model.

A commercial carbon whisker reinforced polylactic acid composite made by a type of three-dimensional (3D) printing known as material extrusion additive manufacturing is investigated. The primary ob...

Atomic oxygen (AO) is one of the most abundant species present in the lower earth orbit and is responsible for the aggressive degradation of polymers used in spacecraft structures. In this investigation, we use ReaxFF reactive force field molecular dynamics simulations to evaluate the disintegration of several different thermosetting epoxy polymers subjected to hypervelocity AO impact. Our simulations indicate that epoxy with aromatic curative displays higher resistance to the AO impact because of its stable benzene functionality. Decreased cross-linking density and increased simulation temperature both lead to faster disintegration of the polymer. Our simulation results indicate that ReaxFF force field simulations can be a useful tool to evaluate the response of various thermosetting epoxies to AO impact and identify promising candidate materials for spacecraft applications.

A multilaboratory investigation into the durability of glass fiber–reinforced polymer (GFRP) bars extracted from eleven 15- to 20-year-old bridges in the US will be performed. Part 1 (Benz...

Glass fiber–reinforced polymer (GFRP) bars have been used in concrete structures as an alternative to steel bars due to their noncorrosive behavior. However, due to the lack of full unders...

Carbon nanotubes can provide significant vibration damping to carbon/epoxy composites, which is a benefit for high vibration applications such as rotorcraft. One means of maximizing the volumetric concentration of CNTs, and thus damping, in composite laminates is to concentrate the CNTs in the interlayer regions rather than disperse them throughout the matrix. Also, aligning the CNTs along the principal strain direction maximizes damping. However, there is concern that the weak CNT/epoxy bond will compromise the interlaminar shear strength (ILSS) of the laminate. The current investigation aims to evaluate the ILSS of unidirectional carbon/epoxy with an interlayer consisting of CNT yarns. The ASTM D2344 short beam strength (SBS) test is used to evaluate the ILSS of 0-deg. unidirectional carbon/epoxy with a single layer of aligned CNTs at the midplane, where failure occurs predominantly due to the applied shear stress. The main variables in the tests are the non-ionic surfactant and the orientation of the CNTs (parallel or perpendicular to the shear direction). The SBS results indicate negligible reduction in ILSS due to the addition of just surfactant to the composite, although the glass transition temperature of the composite is reduced by 10°C. Upon the addition of surfactant-treated CNTs parallel to the shear direction, the ILSS reduction relative to the baseline material is small (4%). When the surfactanttreated CNTs are oriented perpendicular to the shear direction, the reduction in ILSS relative to the baseline is also small (8%). It is concluded that adding concentrated surfactant-treated CNT interlayers to carbon/epoxy laminates has a small adverse effect on ILSS and glass transition temperature.

The mechanical behavior of the concrete–epoxy bonded interface is of high importance in concrete strengthening and repair systems. This paper presents the effects of cure time, sustained lo...

Chemical structure governs the bulk material properties of thermoset polymers at the macro-scale. In this study, three different amine curing agents — aromatic, cyclo-aliphatic and aliphatic — are reacted with bisphenol-A epoxide using a recently developed accelerated ReaxFF simulation approach for kinetically slow reactions. Accelerated ReaxFF simulations provide reactants with energy (comparable to the barrier energy) to form a stable transition state that, depending upon the proximity and the path of approach, leads to successful cross-linking. While cyclic curing agents result in simulated polymers with local heterogeneities in the molecular structure that can be resolved by annealing, strain rate dependence is manifested prominently in polymers with aliphatic curing agents and less prominent in polymer with aromatic curing agent. Based on the good correlation obtained between simulations and experiments, this work demonstrates that accelerated cross-linking and virtual testing of the polymer can capture the translation of variable chemical structure to thermo-mechanical properties of thermosets.

Small-scale plain concrete notched beams strengthened with carbon and glass fiber reinforced polymer (CFRP and GFRP) sheets were tested in flexure to assess the change in debond onset strain following sustained flexural loading in indoor and outdoor environments for as long as 13 years. The bond tests were all conducted quasi-statically in a normal laboratory atmosphere. For indoor conditioning, debond onset strain was practically unchanged in the CFRP beams over 13 years, while a decrease was seen in the GFRP beams. For outdoor conditioning, which included freeze-thaw cycles, rain, hot weather, and ultraviolet exposure, clear reductions in debond onset strain were observed in both GFRP and CFRP strengthened beams.

Carbon nanotubes (CNTs) embedded in carbon/epoxy (c/ep) composites offer a lightweight, stiff solution with high damping for structural components. To manufacture CNT/c/ep hybrid composites with high concentrations of CNTs, a CNT yarn interlayer concept is used. Through a mechanism known as stick-slip, the interface between CNTs and c/ep laminates dissipate energy during dynamic cycling. It is predicted that altering the interfacial bond strength can enhance dynamic properties: loss factor, loss modulus, and storage modulus. In this research, the effect of surface treatments on CNT yarns prior to inserting the CNTs into the laminate was explored to determine if the dynamic properties of the composite were enhanced. It was determined that 2,3-dibromo-1,4-butanediol (23D14B), Triton X-100 (TX) and a solution of sulfuric and nitric (S/N Acid) were the best treatments of 12 tested based on the increased dynamic properties. For a composite with 5 vol% CNT yarn interlayers, 23D14B, TX, and S/N Acid increased Loss Factor by 230%, 130%, and 160%, respectively. Elevated temperature, moisture-saturated/elevated temperature, and cyclic loading frequency were investigated on 5vol. % CNT interlayers to determine their effects on dynamic properties of the CNT hybrid composites.

Laminated fiber reinforced polymer composites are known for high specific strength and stiffness in the plane of lamination, yet relatively low out-of-plane impact damage tolerance due to matrix dominated interlaminar mechanical properties. A number of factors including the toughness of the matrix can influence the response of composites to impact. The objective of the current investigation is to evaluate the ballistic impact response of carbon/epoxy tubes with variable amounts of nanosilica particles added to the matrix as a toughening agent. Mass density, elastic modulus, glass transition temperature and Mode I fracture toughness of the matrix materials were measured. Tubes manufactured with these matrix materials were ballistically impacted using a round steel projectile aimed at normal incidence across the major diameter. After impact, the tubes were nondestructively inspected and subjected to mechanical tests to determine the residual shear strength in torsion. Increasing concentrations of nanosilica monotonically increased the modulus and fracture toughness of the matrix materials. Tubes with nanosilica had smaller impact damage area, higher residual shear strength, and higher energy absorbed per unit damage area versus control materials with no nanosilica. Overall, the addition of nanosilica improved the impact damage resistance and tolerance of carbon/epoxy tubes loaded in torsion, with minimal adverse effects on mass density and glass transition temperature.

Triaxially braided fabrics with equal volume fractions of fibers oriented at 60- degree intervals allow for the quick and easy hand-layup of quasi-isotropic (QI) composite laminates, in comparison to unidirectional tape and biaxial-woven fiber architectures. The material system for this study, T700S/MTM45-1, is the first material used in full-scale testing to demonstrate lightweight aircraft fuselage shielding for openrotor engine applications. The objective of the current investigation is to measure the in-plane tensile modulus, Poisson’s ratio, failure strain, and tensile strength of a triaxially braided QI T700S/MTM45-1 carbon/epoxy laminate. Straight-sided specimens were cut from panels with the axial tow of the fabric oriented at angles of 0o, 30o, 45o, 60o, and 90o relative to the loading direction. Results were compared to previously published test results for composites consisting of the same fibers and fiber architecture but with a different (PR520) epoxy resin. Test results showed a considerable variation of strength with in-plane orientation. The initial elastic modulus and ultimate strength of the T700S/MTM45-1 material were also analyzed using classical laminated-plate theory (CLPT) and ply-discount (PD) to account for matrix cracking. As a predictive model, the quasi-laminar CLPT/PD model accurately predicted modulus but over-predicted strength. Using the model to back-calculate the best fiber-direction ply-level properties in the axial and bias directions of the braid indicated that significant adjustments of the modulus and strength can be implemented to match the initial experimental modulus and strength for nearly all specimen orientations. Reductions in these properties relative to the equivalent unidirectional tape properties were greatest in the plies representing the bias tow plies, indicating a greater degree of undulation in the bias directions due to the manufacturing process.

Experimental research has shown that the incorporation of nanosilica (NS) filler in carbon/epoxy (c/ep) cylindrical tubes decreases the damaged area and increases the residual torsional strength of following normal incidence ballistic impact. The current investigation aims to determine the effect of NS on the residual torsional strength (RTS) of c/ep cylindrical tubes subjected to a static torsional load during oblique-incidence ballistic impact, where the projectile grazes the outer radius of the tube at a 45° angle in order to remove the largest amount of material (slashing impact). A total of four tubes were evaluated—two with 15% NS as a percent of matrix weight and two without NS filler. One of the impact tests resulted in two distinct holes in the tube rather than an elliptical gouged-out area, resulting in a reduced overall damage area and less degraded RTS versus the properly impacted tubes. The RTS retention of the three tubes having the intended impact trajectory was approximately 12% of the estimated strength of undamaged tubes. Likewise, the size of the damage zone surrounding the impact site, determined by cointapping, was also similar among the properly impacted tubes. Overall, the results are comparable with previously published slashing impact data for carbon fiber composite tubes made with various matrix materials and highlight the dominant influence of fiber damage versus matrix damage on the RTS of composite tubes following slashing ballistic impact.

Serviceability requirements can often control the design of glass fiber–reinforced polymer (GFRP)-reinforced concrete flexural members due to the lower stiffness of GFRP than of steel. ACI ...

Carbon nanotubes (CNTs) are known to increase damping in continuous fiber reinforced polymer composites without a loss of stiffness, which is important for designing materials for vibration- and weight-sensitive aircraft structures. The purpose of the current investigation is to explore new types of hybrid CNT - carbon/epoxy composites tailored for high damping and stiffness. Initially, different CNT surfactant treatments, orientations, and concentrations were evaluated. Based on this comparison, the material system with the best performance was identified and the dynamic behavior of this system was further investigated using a [0/±45]s laminate subjected to different cyclic strain excursions and different temperatures. It is shown that a 10% volumetric concentration of highly aligned CNT yarns treated with a non-ionic surfactant, located at the interlayers and oriented along the loading direction, provides superior damping and stiffness characteristics. Damping increased monotonically with increasing cyclic strain amplitude, but was nearly insensitive to the mean strain. For a tensile strain amplitude of ~3000 μe, the loss factor, loss modulus, and storage modulus of a hybrid [0/±45]s laminate increased by 454%, 529%, and 14%, respectively, due to the addition of CNT yarns. Sensitivity of the dynamic properties to elevated temperatures was slightly increased in the hybrid laminate due to the surfactant, although the room temperature tensile strength was nearly unchanged in comparison to the baseline laminate.

An innovative concept for a multifunctional structural battery using lithium-ion battery materials as load bearing elements in a sandwich panel construction has been demonstrated. The structural battery prototype has exhibited an initial capacity of 17.85 Ah, an energy density of 248 Wh L−1, a specific energy of 102 Wh kg−1, and a capacity retention of 85.8% after 190 charge–discharge cycles at ~ C/3 rate and eight mechanical loading cycles (upto 1060 N). The mechanical stiffness in three-point bend tests follows expectations based on sandwich beam theory, proving that the battery materials are sharing in the load-carrying function of the sandwich panel. While areas for improvement of the fabrication and performance of the prototype still exist, the results of the current investigation demonstrate the promising potential of the proposed structural battery concept for the efficient use of space and mass in an electric vehicle.

Carbon nanofiller-modified composites possess extraordinary potential for structural health monitoring because they are piezoresistive and therefore self-sensing. To date, considerable work has been done to understand how strain affects nanocomposite conductivity and to utilize electrical impedance tomography for detecting strain or damage-induced conductivity changes. Merely detecting the occurrence of mechanical effects, however, does not realize the full potential of piezoresistive nanomaterials. Rather, knowing the mechanical state that results in the observed conductivity changes would be much more valuable from a structural health monitoring perspective. Herein, we make use of an analytical piezoresistivity model to inversely determine the displacement field of a strained carbon nanofiber/polyurethane nanocomposite from conductivity changes obtained via electrical impedance tomography. From the displacements, kinematic and constitutive relations are used to calculate strains and stresses, respectively. A commercial finite element simulation is then used to validate the accuracy of these predictions. These results concretely demonstrate that it is possible to inversely determine displacements, strains, and stresses from conductivity data thereby enabling unprecedented insight into the mechanical response of piezoresistive nanofiller-modified materials and structures.

Fiber reinforced polymer composites are commonly bonded to concrete structures for strengthening reasons, even though little is known about the evolution of bond strength while the strengthening systems are subject to sustained loads and elevated temperatures. This investigation uses single-lap shear specimens to examine the effects of sustained load and elevated temperature on the time-dependent deformation of a carbon fiber reinforced polymer (CFRP) composite bonded to concrete as well as the pull-off strength of the composite after the sustained loading period. Increased temperature during the sustained loading period led to increased slip of the CFRP, whereas increased curing time of the polymer resin prior to the sustained loading period led to reduced slip. Specimens that underwent sustained loading had increased pull-off strength and interfacial fracture energy following the sustained loading period. This beneficial effect was most significant for roughly 30 days of sustained loading. The analysis of strain distributions and fracture surfaces indicated that stress relaxation of the adhesive occurred in the 30 mm closest to the loaded end of the CFRP during sustained loading, which increased the pull-off strength provided the failure locus remained mostly in the concrete. For longer sustained loading times, the bond failure locus shifted to the epoxy/concrete interface, which diminished part of the strength increase brought on by the stress relaxation of the adhesive.

In filament-wound composites, the existence of fiber undulation introduces unique challenges in the calculation of compressive modulus and strength using traditional composite theories. In the current work, a previously developed three-dimensional continuum representation of undulated fibers was incorporated into a multi-scale homogenization process to simulate the effective longitudinal stress-strain behavior of filament-wound cylinders and compressive failure in the undulation regions. Calculated properties were compared to previously obtained experimental data for carbon fiber cylinders made with various matrix materials and winding parameters. The average difference between predicted and measured properties was 14% and the predicted failure modes were consistent with the experimental observations.

Molecular dynamics simulations of polymers can help in understanding the dependence of molecular structure, cross-linking and the chemistry of the polymer chain and resulting thermo-mechanical properties of the polymer. Apart from these thermo-mechanical properties, molecular dynamics is also a powerful tool to examine the durability of polymers with potential aerospace applications under harsh environmental conditions. Ultraviolent radiation from the sun results in dissociation of molecular oxygen into atomic oxygen (AO) which is abundant in lower earth orbit. Testing composites under AO impact requires an extensive experimental setup to simulate low earth orbit (LEO) conditions and is therefore expensive. Using a newly developed accelerated cross-linking methodology in the framework of ReaxFF, bisphenol F and diethyltoluenediamine epoxy polymer chains are manufactured virtually. This simulated polymer is virtually tested for modulus, glass transition temperature, density and is impacted by atomic oxygen at 8 km/s. Thermomechanical properties show good agreement between experiments and simulations. Simulations the polymer during AO impact using ReaxFF provides useful insight to the degradation mechanism in terms of polymer chemistry and thermal profile.

The addition of functionalized nanosilica (NS) particles to epoxy resins is known to improve certain mechanical properties such as modulus of elasticity and fracture toughness. In the current investigation, epoxies with and without NS reinforcement were investigated. Four NS concentrations were evaluated: 0, 15, 25 and a maximum wt% NS dependent on which of the two curing agents was used. The tensile modulus of elasticity and quasi-static Mode I fracture toughness were measured and the Mode I fracture surfaces were examined using a field emission scanning electron microscope for general imaging and a scanning laser confocal microscope for quantitative information on surface morphology. Fracture toughness, as measured by critical strain energy release rate (GIc), and fracture surface area increased monotonically with increased NS content in the epoxy cured with diethyltoluenediamine (DETDA). However, for the material cured at a higher temperature with 4-4’ diamino diphenyl sulfone (DDS), GIc and surface area reach their respective peaks at NS concentrations less than the maximum value. The primary morphological toughing mechanisms observed were particle pullout and crack deflection. The DDS cured system had higher surface area than DETDA system for any non-zero NS content, but less GIc. Analysis of the experimental results led to the conclusion that GIc of the DETDA was mostly explainable in the context of NS particle pullout, as both fracture surface area and GIc varied in rough proportion to NS content. In the DDS system, however, such proportional behavior was not observed and it is believed that competing mechanisms influence GIc at NS concentrations above 15 wt%.

The development of next-generation rotorcraft requires advancement in certain key areas such as cruise speed, payload, range, along with cost considerations. Coaxial and tiltrotors are two promising candidates for future of high speed vertical flight. Traditional lag dampers are unable to produce adequate levels of damping for the rigid fiber composite blades of coaxial rotorcraft. A promising approach for achieving lightweight damping in composite blades is to add carbon nanotubes (CNTs) to the matrix material. The current investigation aims to predict the damping behavior of a rotating beam that represents a simplified fiber reinforced polymer/ CNT nanocomposite blade. The developed model takes in account different physical variables such as the critical shear stress ( ) for slip onset of the CNTs and CNT aspect ratio (l/d), volume fraction ( ), and orientation. The study first looks at a case where the nano-inclusions are aligned with the direction of loading for the rotor blade. The model predicts maximum damping for low in combination with relatively low l/d. The damping trend increases with increase in of the nanoinclusions. A damping ratio of 7.3% and 8% was observed with optimally configured parameters for aligned and randomly oriented CNTs respectively. Overall, the results suggest significant damping augmentation from the CNTs.

Carbon nanotubes (CNTs) are promising materials for increasing the damping of fiber reinforced composites on account of a hypothesized stick-slip dissipation mechanism. The current investigation aims to increase the damping of a carbon/epoxy composites by adding different types and amounts of CNTs to the interlayer regions. [90] , [0] and [0/±45] carbon/epoxy laminates were manufactured using various types and concentrations of CNTs and surfactant. Dynamic behavior was characterized in terms of the storage and loss moduli and loss factor under tensile cyclic loading. For [0] and [0/±45] laminates, the maximum increases in loss factor and loss modulus of roughly 400-600% were obtained with 10 volume percent CNT yarns aligned in the 0-deg. loading direction. The storage modulus of these laminates was not appreciably affected by the CNT yarns. Aligned CNT yarns provided higher damping than equivalent amounts of unaligned CNT buckypapers. In a series of tests on 0-deg. laminates with and without aligned CNT yarns, damping increased with strain amplitude but not with mean strain, while damping in the baseline laminate was nearly invariant with strain. This series of tests also detected no lower threshold of strain for the onset of slip by CNT and a 10x reduction in the rate of increase of damping with strain amplitude when the amplitude exceeded 190 μ.

Highly flexible nanocomposites have tremendous potential as smart, self-sensing materials because their conductivity is inherently linked to their mechanical state. Herein, carbon nanofiber (CNF)/polyurethane (PU) nanocomposites are studied for tactile imaging and distributed strain sensing via electrical impedance tomography (EIT) by investigating the influence of filler volume fraction on microscale morphology, piezoresistive response while bonded to mechanically loaded substrates, and sensitivity to distributed strain. Load testing of the bonded sensor reveals that viscoelasticity and filler volume fraction profoundly affect the piezoresistive response. EIT is able to accurately capture and discern between multiple points of contact in each volume fraction with lower volume fractions being more sensitive thereby demonstrating the potential of utilizing tomographic methods for tactile imaging and distributed strain sensing in PU-based nanocomposites.

Analytical models have been developed in recent years for predicting the elastic properties of fiber reinforced composite laminates with undulated laminas, such as woven and filament wound composites. In these models, a representative volume element (RVE) is defined, assumptions about how strains or stresses vary throughout the RVE are made, and then the homogenized constitutive behavior of the RVE is calculated. The constitutive behavior has been modeled in the context of either two-dimensional plate behavior or three-dimensional continuum behavior. Of the latter genre, the few existing models capable of handling undulated laminas have not captured the full three-dimensional constitutive behavior, including all the out-of-plane couplings arising from the most general undulation configurations. Building on the existing models, the current investigation utilizes micromechanics to determine the three-dimensional stiffness at discrete stations along the length of an RVE containing an undulated lamina, and determines the overall RVE constitutive behavior by averaging either the stiffness or compliance along the length of the RVE, thus providing bounds on the material behavior. The new model is evaluated by comparison with existing models as well as experiments for laminates with and without undulations. Similarities and differences with previous models are discussed.

Single-piece composite driveshafts offer the potential to reduce weight, parts inventory, and maintenance costs in rotorcraft, compared to multi-segmented drivelines with flexible connectors. This innovative application of composites is enabled by the high cyclic strain capacity of composites, which allows operation under misaligned conditions. Trade space visualization was used to demonstrate how material, ambient temperature, bending strain, and torque/speed combination (for constant power) affect the design space. Design variables included stacking sequence and number of layers and hanger bearings. Failure criteria included overheating, whirl instability, torsional buckling, and material failure. The design goals minimized mass and maximized the lowest factor of safety by adaptively generating solutions to the multi-objective problem. Rate- and temperature-dependent viscoelastic properties and temperature-dependent strengths of five composites were experimentally characterized and used as model inputs. Design space inspection highlighted shaft whirling as a common limiting factor in addition to fiber direction compressive strength for flexible composites. Mass reductions of 15.15 and 13.65 kg were realized for rigid and flexible composite driveshafts respectively (original mass of 31.3 kg) in a case study, primarily due to the elimination of mid-span flexible couplers and one hanger bearing. Larger mass reductions were demonstrated by trading operating speed and torque.

The principles of sandwich construction were used to design, manufacture, and analyze multifunctional sandwich beams that can withstand structural load and also store electrochemical energy. Three sandwich beams were manufactured with aluminum face skins adhesively bonded to cores consisting of unidirectional layers of lithium-ion battery anode and cathode materials stacked across the width of the beam so that their edges can be bonded to the face skins. This novel arrangement of the battery materials enables them to contribute significantly to the through-thickness shear rigidity of the beam. All three beams were tested structurally in three-point bending and one of the beams was also tested for electrochemical performance after being filled with electrolyte. The electrochemically tested beam demonstrated an energy density of 223 W h/L, which is comparable to typical lithium-ion batteries. Stiffness of all the beams was predictable within 10% using a first-order sandwich beam analysis, confirming the existence of composite behavior between the face skins provided by the novel core design. Electrochemical testing had only a minor effect on beam stiffness. The mechanical testing had no significant effect on the battery discharge capacity. The electrochemical test results suggested that compaction of the battery materials and the quality of the electrical connections need to be improved.