Search

Impacts to the back of the head due to rearward falls, also referred to as “backfall” events, represent a common source of TBI for athletes and soldiers. A new experimental apparatus is described for replicating the linear and rotational kinematics of the head during backfall events. An anthropomorphic test device (ATD) with a head-borne sensor suite was configured to fall backwards from a standing height, inducing contact between the rear of the head and a ground surface simulant. A pivoting swing arm and release strap were used to generate consistent and realistic head kinematics. Backfall experiments were performed with the ATD fitted with an American football helmet and the resulting linear and rotational head kinematics, as well as calculated injury metrics, compared favorably with those of football players undergoing similar impacts during games or play reconstructions. This test method complements existing blunt impact helmet performance experiments, such as drop tower and pneumatic ram test methods, which may not be able to fully reproduce head–neck–torso kinematics during a backfall event.

This is the first comparative study of a chemically homologous series of linear and two-dimensional polyaramids at high temperatures. Detailed comparisons are made in the bulk and at free surface e...

Polymeric structures fabricated using fused filament fabrication (FFF) have limited use in engineering applications as a result of their poor inter-laminar bonding. In this study, we utilize a dual-material print head to encase a low glass transition temperature (Tg) polymer (acrylonitrile butadiene styrene) within a high-Tg shell (polycarbonate). The resulting structure, if annealed at a temperature between the core and shell polymer Tg values, creates a tough interior with high inter-laminar strength while retaining the as-printed three-dimensional geometry of the part. Fracture toughness of annealed, shelled parts was evaluated using single edge notch bend (SENB) fracture specimens and reached values more than 1800% higher than unannealed specimens. Importantly, the annealed specimens exhibited consistent ductile failure and plastic deformation, unlike the as-printed parts which exhibited brittle inter-laminar fracture. Parts with complex geometries are presented to demonstrate geometric stability during annealing and a practical load bearing application.

Polymers that extend covalently in two dimensions have attracted recent attention

The design of an adaptive helmet suspension system that provides optimized head protection under variable impact conditions is reported. The adaptive response is achieved through the use of rate activated tethers (RATs), a flexible strap-like material that uses shear thickening fluids to generate speed-sensitive extensional resistance. The RATs are integrated into a helmet by replacing the webbing system of a traditional construction hardhat with a network of RATs, and performing impact attenuation testing over a range of velocities. Impacts to the crown region of the helmet demonstrate a 50% reduction in peak acceleration experienced by the headform for impact velocities between 1.5 and 3.5 m/s compared to the conventional webbing system, and comparable response to the conventional system at 4.5 m/s. Complementary RAT extensional testing and low velocity helmet compression tests confirm that the rate-sensitive response of the RATs contributes significantly to improved system performance. Additionally, calculations for suspension model systems show that the steady yield force exhibited by RATs over long strokes is a critical feature for minimizing head acceleration, and that the RAT suspension systems are achieving responses remarkably close to the ideal suspension response. Keywords: Impact, Brain injury, Helmet, Head injury criterion, Shear thickening fluid, Rate-activated tether

Single high-performance fibers (Kevlar® K129, Twaron® 2040, Technora® T200, Dyneema® SK76, Zylon® AS, Vectran® HT, and S-glass) are subject to lateral cutting action by an industrial blade. Due to the inherent variability of such testing, 40–60 replicate experiments were performed for each fiber type, at each of three different loading orientations, and the data were fit with a generalized extreme value distribution to summarize likelihood of failure under each condition. The results show that when organic fibers are cut at an angle that introduces shear loads, cut resistance drops by 60–90% compared to normal-incidence cut loading. This trend is driven by the highly oriented and fibrillar nature of organic fibers, in contrast to the isotropic S-glass fibers which show comparatively little change in cut resistance with blade orientation. Overall, the Vectran fibers provide the best resistance to normal loading, while the S-glass fibers are most resistant to non-normal loading. Micrographs show that, for organic fibers, shear loads lead to more localized failure compared to normally loaded fibers.

Polymeric structures fabricated using Fused Filament Fabrication (FFF) suffer from poor inter-laminar fracture toughness. As a result, these materials exhibit only a fraction of the mechanical performance of those manufactured through more traditional means. Here we show that thermal annealing of confined structures manufactured using the FFF technique dramatically increases their inter-laminar toughness. Single Edge Notch Bend (SENB) fracture specimens made from acrylonitrile-butadiene-styrene (ABS) feedstock were isothermally heated in a supporting fixture, post-manufacture, across a range of times and temperatures. Fracture testing was then used to quantify the changes in inter-laminar toughness offered by annealing through measurements of the Mode I critical elastic-plastic strain energy release rate, JIc. Under the most aggressive annealing conditions, the inter-laminar toughness increased by more than 2700% over the non-annealed baseline material. Void migration and aggregation during the annealing process was analyzed using X-ray tomography and provides insight into the toughening mechanisms. Time-scales of reptation and polymer mobility at the interface during annealing are also modeled and agree with fracture experiments.

Post-impact mechanical response of 2D and 3D woven glass/epoxy composite plates and beams of equivalent areal density are evaluated using both Compression-After-Impact (CAI) and Flexure-After-Impact (FAI) testing protocols. Residual strength and stiffness for CAI and FAI are compared after normalization of impact energy with respect to specimen volume. Post-impact flexural strength and modulus from FAI testing exhibit larger reductions with respect to impact energy in comparison to CAI results. At the largest impact energies tested, FAI testing yields 70% reduction in flexural strength compared to only 20% reduction (in compressive strength). Architecturally, 3D woven composites retain greater post-impact mechanical performance as a result of the through-thickness Z-tow which suppresses delamination growth and opening during impact.

Recurrent self-healing of fracture damage in fiber-reinforced composites was accomplished by incorporating internal vascular networks for repeated delivery of reactive liquid components to an internal delamination. Double cantilever beam specimens containing embedded microvascular channels were repeatedly fractured and healed by pumping individually sequestered epoxy and amine based healing agents to the fracture plane. The effect of various pumping parameters and component delivery ratios on in situ mixing of the healing agents and the resulting healing efficiency is reported. Confocal Raman spectroscopy was used to quantify the extent of mixing of healing agents within the fracture plane. Using an optimized healing agent delivery scheme, ten cycles of fracture and healing were achieved with, on average, 55% and as high as 95%, recovery of the virgin critical strain energy release rate.

Low velocity impact damage of 2D and 3D woven glass/epoxy composites with the same areal density and material constituents were examined. Characterization of damage for both plate and beam sample geometries was investigated through the collection of high-resolution cross-sectional images after impact. Load and displacement data collected during impact testing reveals that the threshold load to introduce delamination damage is independent of the fabric architecture and is constant across a range of impact energies. Delamination length and opening of 3D woven composites was less than 2D composites impacted at the same energy as a result of suppression of delamination propagation and opening offered by the Z-tow reinforcement of the 3D fabric architecture. The formation of transverse shear cracks was independent of the fabric architecture.

Ground impact injuries are a significant mode of sports-related injuries and a particular concern for concussions caused by head-to-ground impacts. To study these injuries and develop improved technologies to reduce their likelihood and severity, a test surface must be available that replicates the dynamic mechanical response of typical playing field surfaces. In this study, a series of playing surface simulants created from stacked layers of foams and rubbers of various thicknesses and hardness values were tested under impact loading. Data are generated via ASTM F355/F1936 with a Type A cylindrical missile, implemented using a modified rail-guided test system. The results show that multi-layer stacks graded to transition from a soft impact face to a harder base layer, when subject to uniaxial impulse, produce acceleration pulse shapes, peak values, and durations comparable to a wide range of real playing surfaces. The low cost, repeatability, and facile assembly and maintenance of the playing surface simulants make them well-suited for laboratory study of ground impacts.

A process is presented for the rapid production of microstructured monofilaments via thermal drawing of additively manufactured polymer preforms. Preforms are produced wholly, or in part, via fused filament fabrication of acrylonitrile-butadiene-styrene (ABS) and polycarbonate materials. The preforms are heated and drawn under tension to convert the preforms into lengths of monofilament that closely reproduce the geometric structure of the parent preform. Example monofilaments include “microprinted” monofilaments that contain an arbitrary image embedded in the monofilament cross section; microfluidic monofilaments in which flow channels are formed by combining optically transparent and opaque materials; dual-material monofilaments that combine ABS and polycarbonate into a regular spoked geometry with five-fold symmetry; and a microfluidic preform co-fed with glass optical fiber, allowing both fluid and light transmission through the monofilament. The primary advantages of this monofilament fabrication technique include short lead times; minimal investment in materials and equipment; a means of directly combining multiple materials into a single monofilament, even if the material components have different thermorheological properties; and the ability to create arbitrary and complex geometries.

The effect of layer orientation on the fracture properties of poly(acrylonitrile-butadienestyrene) (ABS) materials fabricated through the fused filament fabrication (FFF) process was explored. Critical elastic-plastic strain energy release rates of single edge notch bend (SENB) specimens with variable crack-tip/laminae orientations were compared. Results show that the inter-laminar fracture toughness (fracture between layers) is approximately one order of magnitude lower than the cross-laminar toughness (fracture through layers) of similarly manufactured parts. Contrasting brittle and ductile fracture behavior is observed for inter-laminar and cross-laminar crack propagation, respectively, demonstrating that the elastic-plastic response of AM ABS parts is governed by the direction of crack propagation within the laminated structure. Fracture surfaces of failed specimens are examined using scanning electron microscopy to show micro- and macro-scale toughening/embrittling mechanisms. Techniques for designing tougher additively manufactured materials based on biological analogies are discussed.

During impact, protective eyewear can be dislodged and leave the eyes and face vulnerable to injury from secondary impacts. In this study, a rate-activated eyewear retention system is studied and compared to a conventional elastic retention system. The rate-activated behavior of the retention system is due to an enclosed shear-thickening fluid, a high solids-loading colloid that undergoes a rapid increase in viscosity above a critical shearing rate. Tensile testing shows that the force required to elongate this retention system is over 10 times higher when stretched at 100 mm/s, compared to 1 mm/s. A women’s lacrosse goggle mounted to an anthropomorphic test dummy head is used as a model system to compare retention system response. Goggles are impacted by an NOCSAE-certified lacrosse ball at 27 m/s, with a trajectory oriented 45° relative to the anterior side of the midsagittal plane of the head, per ASTM F3077. High-speed video analysis shows that the rate-activated system provides improved retention relative to the conventional goggle strap, with an average of 12 mm of dynamic goggle deflection compared to 65 mm for the conventional design. These results suggest that a rate-activated retention system could reduce injuries by ensuring that personal protective equipment remains in place during impact events.

Knitted fabrics are constructed from interlocking loops of yarn. Curvature in the yarn provides stretchability, making them well suited for garments that will cover areas of the body that require large relative motion. Although most current soft armors are composed of woven textiles, knitted fabrics made of aramid fibers such as Kevlar may have application for use in soft armor to provide a larger range of motion in addition to protection. However, their ballistic performance has not been well characterized. The goal of this work is to develop a computational framework to simulate a single jersey knit under ballistic loading. The path and shape of the yarn in the knit fabric is defined with parametric equations calibrated to CT images of a sample fabric. Results of finite element simulations demonstrate the unique mechanical response of knit soft armor, and show qualitative agreement with ballistic experiments.

Right circular cylinders (RCC) are a common fragment-simulating projectile used to simulate debris resulting from an improvised explosive device (IED) blast. The specifications for ballistic performance of soft body armor specify the geometry and mass of RCC projectiles used for testing. The edges on the impacting side of an RCC projectile have a fillet radius, and recent investigations indicate that the fillet radius (or “sharpness”) of the projectile may affect the ballistic limit velocity, V50. It is hypothesized that this happens because the sharpness of the projectile changes the fiber failure mechanism, so to test this, 4 gr RCC projectiles with varying fillet radii (0.64 mm, 1.27 mm and 1.70 mm) are precisely manufactured. Previously collected, unpublished data for 4 gr RCC projectiles with fillet radii of 0.10 mm, 0.18 mm and 0.25 mm are also included to assess a range of projectile sharpness values. Ballistic impact tests on single layer woven Kevlar K706 fabric are conducted using a laboratory gas gun to measure the limit velocity, and it is found that the limit velocity decreases with increasing fillet radii. Back surface images and impacted targets are examined post-mortem to identify failure mechanisms.

Polymeric components manufactured via freeform fabrication (FFF) typically have poor inter-laminar toughness resulting from incomplete bonding across layers during production. Here we study the effect of printing and post-processing on the inter-laminar toughness of additively manufactured semi-crystalline (poly-lactide (PLA)) structures. Specimens were subject to post-print thermal annealing to promote inter-laminar bonding, while post-annealing quenching rates were chosen to vary the induced degree of crystallinity in the final structure, as characterized via dynamic scanning calorimetry (DSC). Critical elastic-plastic strain energy release rates (JIc) of annealed samples were evaluated using the single edge notched bend (SENB) geometry and post-testing fractography. The results show that as-printed PLA adopts an amorphous character with good inter-laminar toughness and ductility. Post-print annealing can double the toughness via increased interfacial wetting, but only if the material is quenched rapidly to preserve the amorphous character. In contrast, post-print annealing followed by slow cooling results in a semi-crystalline state (≈25% crystallinity) with low fracture toughness and brittle fracture behavior.

The ballistic perforation response of knits formed from continuous filament aramid is reported and compared to conventional armor textiles and commodity fabrics. The ballistic experiments consist of 6.0-mm-diameter glass spheres impacted into gelatin-backed targets with areal densities from 200 to 1000 g/m 2 . These ballistic experiments are complemented with quasistatic reverse-perforation experiments to gain insights into deformation and failure for these materials. In-plane stretch experiments are also performed to quantify modulus and strain-to-failure. The results show that, while the ballistic performance of traditional woven textiles is generally superior to knitted aramids, knits formed from continuous filament aramid are significantly better than knits formed from staple yarn. Knitted structures are limited by two main factors: failure of a single yarn tends to lead to catastrophic deconstruction and perforation, and the low in-plane modulus of knits leads to poor lateral stress transfer and energy distribution during higher speed impact. Importantly, however, knits provide significantly more reversible elongation with less elastic resistance compared to other structures, such as woven textiles, making them well-suited for wearable protection in which comfort is critical. The results also show that continuous filament knits can be produced with commercial manufacturing equipment, and that barriers composed of few layers of high-denier yarn knits likely provide more efficient ballistic resistance than equivalent weight barriers composed of many layers of low-denier yarn knits.

This work introduces an innovative technique to characterize the internal morphology of high-performance fibers by using a focused ion beam (FIB) sample preparation method and subsequent atomic force microscopy (AFM). A FIB is used to mill opposing notches that facilitate direct failure along a longitudinal shear plane, and expose the internal surface of the fiber. By exposing this surface via longitudinal shear, distortion of the cleaved surface is minimal, which is an advantage over surface preparation methods such as microtoming. After cleaving the notched fibers, an AFM technique is used to generate modulus maps of the fiber fracture surfaces. These modulus maps provide qualitative and quantitative microstructural information. Initial results obtained from Kevlar KM2 and Dyneema SK76 fibers are presented and a brief analysis of the observed internal features is provided. Extending the technique to image internal features in other materials is also discussed.

Stacking up the filler material In composite materials, a strong or stiff filler is added to a softer matrix to create a combined material with better mechanical or electrical properties. To minimize the filler content, it needs to be uniformly distributed in the composite, which is particularly challenging for nanoscale materials. Liu et al. alternately stacked sheets of graphene and polycarbonate to make a base composite. By further cutting and stacking, up to 320 aligned layers were made with a very uniform filler distribution. Alternatively, the initial stack could be rolled into a rod. In both cases, the properties exceeded what might be expected from a simple combination of the two materials. Science , this issue p. 364

Fused filament fabrication (FFF), sometimes called material extrusion (ME) offers an alternative option to traditional polymer manufacturing techniques to allow the fabrication of objects without the need of a mold or template. However, these parts are limited in the degree to which the welding interface is eliminated post deposition, resulting in a decrease in the interlaminar fracture toughness relative to the bulk material. Here reptation theory under nonisothermal conditions is utilized to predict the development of healing over time, from the rheological and thermal properties of Acrylonitrile-Butadiene-Styrene (ABS). ABS is rheologically complex and acts as a gel and as such considerations had to be made for the relaxation time of the matrix which is important in predicting the degree of interfacial healing. The nonsiothermal healing model developed is then successfully compared to experimental interlaminar fracture experiments at variable printing temperatures, allowing future optimization of the process to make stronger parts.

Knit textiles constructed from continuous filament high-modulus fibers provide a unique combination of in-plane stretchability with resistance to ballistic penetration. A finite element model of ballistic impact for these materials is presented that uses beam elements overlaid on top of truss elements to capture both the tensile and bending response of the comprising knitted yarns. Accurate bending behavior, in particular, is of critical importance for knits because in-plane stretching in these materials is accommodated via bending of the multifiber yarns. Model comparisons of single layer knits impacted experimentally with spherical ballistic projectiles demonstrate improved dynamic deflection predictions compared to solid element or truss element yarn representations. Additionally, slip at the clamped fabric boundary, a common feature in ballistic experiments, is shown to be significant and is effectively represented using an elastic-plastic model at the perimeter of the simulation domain.

2D polymers have emerged as an infinitely-tailorable material with remarkable, tunable response and density-normalized mechanical properties far exceeding structural materials such as steel, high-performance fibers or reinforced composites. It is critical that the vast material design space of 2D polymers be mapped in order to achieve optimal mechanical performance, since hundreds of permutations of one class of 2D polymers known as covalent organic frameworks have already been synthesized in the decade since the introduction of these materials. To this end, this work establishes a general structure-property relationship for elastic modulus and strength for a common 2D polymer motif consisting of nodes linked by linear bridge polymer chains to form a two-dimensional network. The length of the bridge chains are parametrically varied to study the impact of chain compliance on stiffness and strength. The density-normalized isotropic strength of the graphene/polyethylene hybrid material known as graphylene begins at 0.015 GPa/kg·m3 (50% higher than that of perfect crystalline Kevlar®) and the density-normalized isotropic stiffness is 0.143 GPa/kg·m3 (31% higher than Kevlar®) and decreases non-monotonically with increasing bridge chain length. The mechanical response is mapped and correlated to the inherent molecular structure of these general 2D polymer as a framework for designing 2D polymer molecules for mechanical applications from the ground up.

The linear polymer poly(p-phenylene terephthalamide), better known by its tradename Kevlar, is an icon of modern materials science due to its remarkable strength, stiffness, and environmental resistance. Here, we propose a new two-dimensional (2D) polymer, “graphamid”, that closely resembles Kevlar in chemical structure, but is mechanically advantaged by virtue of its 2D structure. Using atomistic calculations, we show that graphamid comprises covalently-bonded sheets bridged by a high population of strong intermolecular hydrogen bonds. Molecular and micromechanical calculations predict that these strong intermolecular interactions allow stiff, high strength (6–8 GPa), and tough films from ensembles of finite graphamid molecules. In contrast, traditional 2D materials like graphene have weak intermolecular interactions, leading to ensembles of low strength (0.1–0.5 GPa) and brittle fracture behavior. These results suggest that hydrogen-bonded 2D polymers like graphamid would be transformative in enabling scalable, lightweight, high performance polymer films of unprecedented mechanical performance.

The remarkable properties of graphene, including unusually high mechanical strength and stiffness, have been well-documented. In this paper, we combine an analytical solution for ballistic impact into a thin isotropic membrane, with ab initio density functional theory calculations for graphene under uniaxial tension, to predict the penetration resistance of multi-layer graphene membranes. The calculations show that continuous graphene membranes could enable ballistic barriers of extraordinary performance, enabling resistance to penetration at masses up to 100× lighter than existing state-of-the-art barrier materials. The very high elastic wave speed and strain energy to failure are the major drivers of this increase in performance. However, the in-plane mechanical isotropy of graphene, as compared to conventional orthotropic woven textiles, also contributes significantly to the efficiency of graphene as a barrier material. This result suggests that, for barrier applications, isotropic membranes composed of covalently bonded two-dimensional molecular networks could provide distinct advantages over fiber-based textiles derived from linear polymers.

Because surface treatments of high-performance fibers have previously resulted in increased friction and improved impact performance, it was of interest to evaluate the influence of multi-walled carbon nanotubes (MWNTs) on impact performance and contributing constituent properties of Kevlar. Kevlar K129 yarns and fabrics were modified via sonication in a solution of N-methylpyrrolidone (NMP) and MWNTs. This method has the potential to both improve the intrinsic properties of the fibers themselves as well as increase the friction, with very low mass addition. Tensile, static friction, and pull-out tests were performed to compare the properties of MWNT-treated materials to neat. As a result of MWNT augmentation, yarn modulus increased up to 15 %, and static and kinetic friction coefficients increased up to 30 %. Yarn pull-out tests revealed up to a 230 % increase in the forces required to pull-out yarns. To study the effects of MWNT augmentation on dynamic performance, low-velocity impact tests of steel spheres on a single ply of fabric were performed. These experiments demonstrated approximately 50 % increase in ballistic limit for MWNT-treated Kevlar with negligible (0.4–1.4 %) increase in mass. Entanglement among MWNTs along with increased surface roughness and surface area increased the resistance to motion, improving impact performance by increasing the energy required to pull-out yarns from the textile, while inhibiting textile windowing and driving a larger number of yarn failures. The observed changes in fabric response suggest that MWNT treatments have the potential to improve the ballistic limit of fabrics through increased interfilament and interyarn friction without compromising fiber strength or adding significant mass.

Structural electrochemical composites, which are capable of carrying mechanical loads while simultaneously storing or releasing electrical energy, combine the components and behaviors of conventional polymer composite structures and electrochemical devices such as batteries and supercapacitors into a single multifunctional material. In order to analyze these systems more rigorously, this paper derives relationships and metrics for the mass savings of a multifunctional design relative to a design consisting of conventional structures and electrochemical devices. These metrics are then evaluated using structural supercapacitors composed of carbon fiber electrodes and conductive solid polymer electrolytes, as well as multifunctional supercapacitors from literature. The analysis reveals that state-of-the-art multifunctional supercapacitors are still far from reaching the levels of performance needed to supplant conventional structures and save system mass. The metrics provide further insight regarding multifunctional value of the material components as well as influence of various functionalities on system performance.

Structural capacitors are manufactured from glass fabric/epoxy prepreg dielectrics and metalized polymer film electrodes. The electrical breakdown strengths of these multifunctional materials are investigated across a wide range of electrode constructions and processing parameters. The results show that electrode selection and materials processing have a significant impact on the energy that the device can store. Also, this careful consideration of processing parameters and electrode construction has led to the development of a structural capacitor with an energy density exceeding 0.90 J/cm3, the highest yet reported.

The probabilistic impact responses of single layer greige and scoured plain-weave Kevlar KM2 fabrics are experimentally studied. Single-layer, 101 cm × 101 cm fabric targets are mounted in a novel equilateral octagon (EO) fixture that leaves the principal yarns unclamped. A probabilistic velocity response (PVR) curve, which describes the probability of fabric penetration as a function of projectile impact velocity, is generated through a series of thirty impact tests using a spherical steel projectile impacted at velocities between 69 and 113 m/s. Additional experiments are conducted by impacting targets repeatedly at identical velocities, and comparing the resulting residual velocities of the penetrating projectiles. Fabric penetration in all cases is entirely accommodated by yarn pull-out and windowing, without any principal yarn failure at the impact site. The results indicate that frictional yarn sliding and pull-out are the primary energy dissipating mechanisms during these impact conditions. Controlled yarn pull-out experiments are conducted on the same greige and scoured fabrics to statistically characterize the yarn pull-out loads. Variability in pull-out forces in the greige fabrics are measurably higher than the variability in pull-out forces for the scoured fabrics, which correlates well with variability trends in the PVR and residual velocity ballistic experiments. Additional factors, such as yarn-projectile friction and differences in filament packing efficiency, are hypothesized to also contribute to the observed differences in the greige and scoured fabric impact responses.

Additively Manufactured (AM) parts exhibit orthotropic behavior when loaded as a result of the layer-by-layer assembly commonly utilized. While previous authors have studied the effect of layer orientation on the tensile, flexural, and impact response of AM parts, the effect of layer orientation on the fracture response is not well established. Here we explore the effect of layer orientation on the fracture properties of Acrylonitrile-Butadiene-Styrene (ABS) materials fabricated through the Fused Filament Fabrication (FFF) process. Critical fracture toughness values of Single Edge Notch Bend (SENB) specimens with a pre-crack oriented either parallel or perpendicular to the direction of layer-by-layer assembly were compared. Results show that the inter-laminar fracture toughness (fracture between layers) is approximately one order of magnitude lower than the cross-laminar toughness (fracture through layers) of similarly manufactured parts. Contrasting brittle and ductile fracture behavior is observed for inter-laminar and cross-laminar crack propagation, respectively, demonstrating that the elastic-plastic response of AM ABS parts is governed by the orientation of the layers with respect to the direction of crack propagation.

The effect of friction on the interaction between a protective fabric sleeve and a manikin arm undergoing elbow flexion has been investigated both numerically and experimentally. The experimental studies used a cylindrical Kevlar sleeve on the right arm of a Hybrid III crash-test dummy. A load frame was used to produce elbow flexion and to measure the force required to produce that motion. Friction was varied by performing the elbow flexion task with and without a tight-fitting white knit polyester undergarment, and friction coefficients were determined experimentally. Corresponding numerical simulations using LS-DYNA were also performed, with the textile sleeve represented by orthotropic shell elements with geometric non-linearity based on experimental literature data. The results show that the presence of the Kevlar sleeve significantly increases the force required to execute elbow flexion, but that the addition of the friction-reducing undergarment reduces the flexion force generated by the Kevlar slee...

Conformal atomic layer deposition (ALD) of Al2O3 and TiO2 thin films on Kevlar®, poly(p-phenylene terephthalamide) (PPTA) fibers at 50 and 100 °C affects the fiber cut resistance. Systematic studies of ALD coatings between 10 to 400 A thick formed at 50 and 100 °C revealed excellent conformality, and trends in cutting performance depended on materials and process details. A 50 A/50 A TiO2/Al2O3 bilayer formed at 50 °C increased cut resistance of PPTA by 30% compared to untreated fiber materials. In situ infrared analysis shows that trimethylaluminum (TMA) Al2O3 precursor reacts sub-surface with PPTA and tends to degraded mechanical performance. The TiCl4 TiO2 precursor reacts to form a barrier that limits TMA/PPTA interactions, allowing a harder Al2O3 layer to form on top of TiO2. The thin ALD coatings do not substantially affect durability, flexibility, or weight of the PPTA, making ALD a potentially viable means to enhance the protective properties of Kevlar and other polymer fiber systems.

The cut resistance of organic and inorganic high-performance single fibers has been studied. Several fiber types were examined, including Kevlar, Twaron, Vectran, Technora, Zylon, Dyneema, carbon fiber, and S-glass. Experiments were conducted using a custom-designed fixture that forces an industrial cutting blade laterally into a single fiber at varying blade and fiber angles. The effects of cutting angle and fiber type were explored, and the detailed progression of failure was inferred from post-failure imaging. All organic fibers demonstrated similar levels of cut resistance, with both organic and inorganic fibers showing less cut resistance as cutting angle is increased. Failure in the organic fibers was dominated by the anisotropic structure of the fibers. In contrast, isotropic, inorganic glass fibers demonstrated less cutting angle dependence and failed according to simple, localized brittle fracture. The inorganic fibers demonstrated higher average cut resistance than organic fibers, most likely due to their relative hardness and expected higher transverse mechanical properties.

Composites of a polymer–matrix reinforced by polymer ribbon monofilaments are investigated as mechanically robust, transparent composite materials. Transparent nylon monofilaments are mechanically worked to form flattened nylon ribbons, which are then combined with index-matched epoxy resin to create transparent composites. A range of optical and mechanical experiments are performed on composites and surrogate systems in order to quantify properties and guide system design. The results show that these polymer–polymer composites provide good transparency over a wide temperature range, and superior ballistic penetration resistance compared to monolithic transparent polymers.

Healing of impact damage in vascular fiber-reinforced composite beam specimens was explored using a flexure after impact testing protocol. Two-part epoxy and amine based healing agents were delivered to impact-damaged beam specimens using a novel air assisted reagent delivery scheme. The incorporation of microchannels into composite specimens did not alter the flexural stiffness or strength of the composites, however, the post impact flexural strength was reduced, on average, by 25.7%. Flexure testing of healed specimens demonstrated 47% recovery of strength and 83% recovery of moduli when compared to control samples in which no agents were delivered. Optical cross-sectional micrographs reveal that not all damage regions are infiltrated during healing as a result of incomplete damage connectivity. A damage filling efficiency during healing agent infiltration was calculated and was found to positively correlate with recovery of post-impact strength.

A system for sputter deposition of thin (10–1000 nm) metal and ceramic coatings onto vibro-fluidized beds of millions of microparticles has been demonstrated. Coatings are sputtered onto microspheres of various size distributions. Experiments show that the coating deposition rate onto particles decreases with decreasing particle size, for the case of a fixed total bed mass. Factors affecting particle coating thickness also include sputter power and fluidized bed container size. Coating surface morphology is evaluated using scanning electron microscopy. Lower sputter deposition power creates smoother, less porous coatings. Fluidizing parameters, such as vibration amplitude and frequency, are also shown to affect coating morphology. Dual-layer metal-ceramic coatings are also demonstrated using the sputter deposition technique. A dual-layer aluminum/tin-oxide coating is produced on glass microspheres, which are then shown to have distinct reflectance characteristics and colors. A model is developed to estimate coating thickness based on size, shape and number of particles.

In this paper the effects of projectile size and shape on the impact response of a single-layer fully-clamped flexible woven fabric are numerically studied using a finite element analysis. Six projectiles of spherical, cylindrical, and conical shapes with varying sizes but of the same mass are considered. A yarn-level fabric model with a deterministic implementation of experimental mean yarn tensile strength and inter-yarn friction is considered. A single impact velocity is chosen for the six different projectiles to compare the fabric impact responses in terms of residual projectile velocities, fabric energy dissipations, and number of failed yarns. Two impact locations are considered, at a yarn cross-over and at the gap in-between the yarns. The sensitivity of fabric impact response to projectile geometrical characteristics and impact location is demonstrated.

A graphene/polyethylene hybrid 2D polymer, “graphylene”, exhibits a higher theoretical fracture toughness than graphene, while remaining 2× stiffer and 9× stronger than Kevlar®, per mass. Within the base structure of graphylene, the sp3-bonded polyethylene linkages provide compliance for ductile fracture, while the benzene rings contribute to high stiffness and strength. Combining stiff and compliant units to achieve enhanced mechanical performance demonstrates the potential of designing 2D materials at the molecular level.

This article details the design, fabrication, and application of a mechatronic arm exoskeleton for firearm aim stabilization (MAXFAS), which senses and damps involuntary tremors in the arm. Human subject experiments were carried out using the device in a simulated shooting and aiming task. Results indicate that MAXFAS reduced arm tremors and improved shooting performance while wearing the device. Residual performance improvement after removing the device and possible training function of MAXFAS will also be discussed.

As the simplest two-dimensional (2D) polymer, graphene has immensely high intrinsic strength and elastic stiffness but has limited toughness due to brittle fracture. We use atomistic simulations to explore a new class of graphene/polyethylene hybrid 2D polymer, “graphylene”, that exhibits ductile fracture mechanisms and has a higher fracture toughness and flaw tolerance than graphene. A specific configuration of this 2D polymer hybrid, denoted “GrE-2” for the two-carbon-long ethylene chains connecting benzene rings in the inherent framework, is prioritized for study. MD simulations of crack propagation show that the energy release rate to propagate a crack in GrE-2 is twice that of graphene. We also demonstrate that GrE-2 exhibits delocalized failure and other energy-dissipating fracture mechanisms such as crack branching and bridging. These results demonstrate that 2D polymers can be uniquely tailored to achieve a balance of fracture toughness with mechanical stiffness and strength.

The combination of extraordinary strength and stiffness in conjunction with exceptional electronic and thermal properties in lightweight two-dimensional materials has propelled graphene research toward a wide array of applications including flexible electronics and functional structural components. Tailoring graphene's properties toward a selected application requires precise control of the atomic layer growth process, transfer, and postprocessing procedures. To date, the mechanical properties of graphene are largely controlled through postprocess defect engineering techniques. In this work, we demonstrate the role of varied catalytic surface morphologies on the tailorability of subsequent graphene film quality and breaking strength, providing a mechanism to tailor the physical, electrical, and mechanical properties at the growth stage. A new surface planarization methodology that results in over a 99% reduction in Cu surface roughness allows for smoothness parameters beyond that reported to date in literature and clearly demonstrates the role of Cu smoothness toward a decrease in the formation of bilayer graphene defects, altered domain sizes, monolayer graphene sheet resistance values down to 120 Ω/□ and a 78% improvement in breaking strength. The combined electrical and mechanical enhancements achieved through this methodology allows for the direct growth of application quality flexible transparent conductive films with monolayer graphene.

The effects of projectile characteristics on the probabilistic impact response of single-layer fully-clamped flexible woven fabrics is numerically studied using a yarn-level fabric model with a statistical implementation of yarn strengths. Six small and large sized spherical, cylindrical, and conical projectiles of the same mass are considered. Probabilistic velocity response curves which describe the probability of fabric penetration as a function of projectile impact velocity are generated for each projectile type through a series of forty impact simulations at varying impact velocities. The probabilistic fabric impact response is observed to be strongly dependent on the shape of the projectile’s impact face and the manner of projectile–yarn interactions at the impact site.

The probabilistic impact response of flexible woven fabrics can be described through the V0–V100 or probabilistic velocity response (PVR) curve which describes the probability of fabric penetration as a function of projectile impact velocity. One source of variability that affects the probabilistic nature of fabric impact performance is the statistical distribution of yarn tensile strengths. In this paper the effects of the statistical yarn strength distribution characteristics on the probabilistic fabric impact response are computationally studied using five different strength distributions with differing mean strengths and distribution widths. Corresponding fabric PVR curves are generated for each strength distribution using a probabilistic computational framework that involves randomly mapping yarn strengths onto the individual woven yarns of a fabric finite element model and then running a series of impact simulations for the case of a four-sided clamped fabric impacted at the center by a spherical projectile.

The impact response of flexible woven fabrics is probabilistic in nature and described through a probabilistic velocity response curve or V 0 – V 100 curve. Computational impact analyses based on deterministic methods are incapable of predicting the experimentally observed probabilistic fabric impact response. To overcome this limitation we have developed a probabilistic computational framework within a finite element analysis to predict the V 0 –V 100 response. The finite element model is a yarn-based representation of the fabric architecture, with a principal stress based failure criterion implemented uniformly within each yarn, but varying for each yarn within the fabric. For each impact simulation, individual yarn strengths are mapped from experimentally obtained yarn strength distributions, resulting in fabric models with spatially non-uniform failure conditions. Impact simulations are run for the case of a spherical projectile of diameter 5.556 mm impacting a single layer of 50.8 × 50.8 mm, edge-clamped, unbacked, aramid fabric. Three different yarn strength models are implemented, representing spool yarns, and yarns extracted from greige and scoured woven fabrics. Decreases in yarn strength are found to correlate to decreases in the V 1 , V 50 , and V 99 velocities predicted by the simulations. The relationships between yarn strength distribution and probabilistic fabric impact response are discussed.

Dielectric capacitors with mechanical load-bearing capability have been constructed by laminating glass-epoxy prepregs with metalized film electrodes. Mechanical characterization and high-voltage testing are used to quantify the elastic modulus, mechanical strength, and dielectric energy density of these structural devices. An approach for predicting mass savings in systems utilizing multifunctional material structures is also presented. The experimental results show that, in spite of increases in void content with fiber volume fraction, overall structural capacitor performance is greatest at maximum fiber volume fraction. At these high-fiber volume fractions, the overall multifunctional performance of the structural capacitors is predicted to provide mass and volume savings over conventional designs.

In this study autonomic self-healing of impact damage in composite materials is shown using a microencapsulated healing agent. The components for self-healing, urea‐formaldehyde microcapsules containing dicyclopentadiene (DCPD) liquid healing agent and paraffin wax microspheres containing 10 wt% Grubbs’ catalyst, have been successfully incorporated in a woven S2-glass-reinforced epoxy composite. Lowvelocity impact tests reveal that the self-healing composite panels are able to autonomically repair impact damage. Fluorescent labeling of damage combined with image processing shows that total crack length per imaged cross-section is reduced by 51% after self-healing. A testing protocol based on compression after impact reveals significant recovery of residual compressive strength (RCS) in self-healing panels. Self-healing panels show a higher threshold impact energy before RCS reduction, and as impact energy increases, RCS recovery decreases. Qualitative inspection shows that crack separation increases with increasing impact energy, indicating that self-healing performance depends on the ability to adequately fill damage volume.

The cut and puncture resistance of thermoplastic (TP) impregnated, woven aramid fabric is characterized under quasi-static and dynamic stab testing conditions. Polyethylene, Surlyn®, and co-extruded polyethylene–Surlyn films of various thicknesses are laminated into fabrics and compared with neat fabrics at equal weights and layer counts. The results show that TP-laminated fabrics improve the stab and puncture resistance of the fabrics, through a combination of increased cut resistance and reduced windowing. All films are shown to be effective, although thin Surlyn films appear to provide the best overall performance. The results also show that the TP films need to be integrally bonded to the fabrics in order to achieve synergistic property enhancement.

Polymer electrolytes were developed to improve simultaneous demonstration of mechanical and electrochemical properties. Solvent-free random copolymers were synthesized using one monomer with poly(ethylene glycol) sidechains that promote lithium ion conduction and one crosslinking monomer that promotes high modulus. Sixty unique systems of monomer pairs were developed in this manner. The properties of the resulting copolymers were influenced by the monomer ratio and chemistry. The copolymers consistently exhibited improved electrochemical–mechanical multifunctionality with respect to the analogous homopolymers. The most promising systems included highly conductive components paired with highly structural components, suggesting that improved multifunctionality may be achieved through interpenetrating multicomponent systems in which each component demonstrates high efficiency in a single property. Electrochemical, mechanical, and viscoelastic properties are discussed with respect to composition and the glass transition temperature. Modeling of conductivity and modulus was employed to enable prediction of copolymer properties based on the ratio and properties of the constituents.