The influence of carbonaceous nanofillers with different dimensionalities-C-60 (0-D), SWCNTs (1-D), and graphene (2-D)-on the crystallization and thermomechanical behavior of an amorphous polyetherimide (ODPA-P3) was investigated. Nanocomposite films were prepared via in-situ polymerization in N-methyl-2-pyrrolidone (NMP), followed by casting, drying, and thermal imidization. In C-60 and SWCNT, ODPA-P3 crystallization was governed by nanofiller dispersion and localized solvent retention during intermediate stages of thermal imidization. Well-dispersed C-60 induced spherulitic crystallization (chi(c) approximate to 40%). SWCNT fibrillar bundles interspersed with coaxial, crystalline ODPA-P3 form a percolated network above 0.6 vol% and resulted in chi c approximate to 43% at 2.4 vol% SWCNT with a T-g increase from 196 degrees C to 207 degrees C. Graphene dispersion and hence it nucleation ability showed a stronger dependence on flake dimensions and dispersity. Smaller, well-dispersed flakes (L approximate to 0.46 mu m, similar to 14 layers) promoted localized crystallization, whereas larger flakes (L approximate to 0.93 mu m, similar to 16 layers) aggregated and remained largely amorphous. Improved dispersion using thinner (<= 5 layers) graphene at low loadings (<= 0.052 vol%) yielded the highest crystallinity (chi(c) approximate to 43.5%). Thermomechanical analysis revealed substantial reinforcement in SWCNT nanocomposites, with storage modulus (E ') increasing from 4.0 to 6.3 GPa at 30 degrees C and from 10(-4) to 1.6 GPa at 245 degrees C (1.2 vol%). Graphene nanocomposites showed modest improvements (E ' = 5.1 GPa at 30 degrees C and 0.1 GPa at 250 degrees C at 2.4 vol%). These results demonstrate that nanofiller dimensionality, size, and dispersion collectively govern crystallization and reinforcement in ODPA-P3 nanocomposites providing design strategies for preparing reinforced polyetherimide nanocomposites.

Poly(urethane-urea) (PUU) is a biphasic, structural polymer that allows for tuning of mechanical properties through tailoring of stoichiometries and interactions of hard and soft segments. In this work, we demonstrate that PUU self-assembles into nanophase networks when polymerized in situ in the presence of high-density aligned carbon nanotube (A-CNT) arrays, where spacing between CNT fibers is 10s of nanometers, revealing strong process-structure-property relations. PUU with two stoichiometric variations were investigated: a 2:1:1 (PUU211) and a 5:4:1 (PUU541) ratio of isocyanate:diamine:polyol at three A-CNT volume percents (0, 1, and 10 vol % A-CNTs). Along the A-CNT long axis, both PUU stoichiometries form elongated hard-segment structures, attributed to the preferential bonding of the urea moieties to the surfaces of the CNTs. The CNTs influence the nanophase self-assembly prior to polymerization, which leads to oriented nanophases in the cured matrix. The more stiff PUU541, with a greater extent of urea moieties, more readily patterns hard segments off the A-CNTs. Longitudinal stiffness of PUU541 in the CNT axial direction increased from 1.35 GPa in the neat polymer to 3.20 GPa in the polymer nanocomposite (PNC) with 10 vol % A-CNTs, significantly higher than 1.30 GPa for PUU211 PNC with 10 vol % A-CNTs. This represents a stiffness increase of 2800% and 137% in the PUU211 and PUU541, respectively, as a result of 10 vol % A-CNTs, a mechanical enhancement 3x higher than expected via rule of mixtures, and attributed to PUU biphasic network morphology change. Additionally, the mechanical anisotropy observed from nanoindentation correlates well with the PUU network morphology of the nanocomposites revealed by atomic force microscopy (AFM) and the changes in phase size found from wide- and small-angle X-ray spectroscopy (WAXS and SAXS) measurements. These strong property-structure relations between biphasic PUU and densely packed A-CNTs provide additional routes toward tuning of properties for PUU nanocomposites for advanced structural composites.

We report on the morphology and mechanical properties of nanocomposite films derived from aqueous, hybrid liquid crystalline mixtures of rodlike aggregates of a sulfonated, all-aromatic polyamide, poly(2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT), and graphene oxide (GO) platelets. An isothermal step at 200 °C facilitates in situ partial thermal reduction of GO to reduced GO (rGO) in nanocomposite films. X-ray scattering studies reveal that PBDT-rGO nanocomposites exhibit both higher in-plane alignment of PBDT (the order parameter increases from 0.79 to 0.9 at 1.8 vol % rGO) and alignment along the casting direction (from 0.1 to 0.6 at 1.8 vol % rGO). From dynamic mechanical thermal analysis, the interaction between PBDT and rGO causes the β-relaxation activation energy for PBDT to increase with rGO concentration. Modulus mapping of nanocomposites using atomic force microscopy demonstrates enhanced local stiffness, indicating reinforcement. From stress-strain analysis, the average Young's modulus increases from 16 to 37 GPa at 1.8 vol % rGO and the average tensile strength increases from 210 to 640 MPa. Despite polymer alignment along the casting direction, an average transverse tensile strength of 230 MPa is obtained.

Structural polymers such as epoxy are commonly reinforced with nano-sized reinforcement particles to increase the toughness and thermo-mechanical properties. Although qualitative measurements can be made to examine particle distribution on a scratched or abraded nanocomposite surface, it is difficult to quantify the three-dimensional locations of the particles relative to the visible surface. In this investigation, we developed a method that combines experimental data obtained from atomic force microscopy (AFM), data science, and continuum micromechanics to discover the 3D position of similar to 142 nm diameter nanosilica (NS) particles relative to an abraded surface. Finite element analysis was used to develop a training set of modulus values as a function of NS particle position relative to the surface of the polymer. Bayesian optimization was then used to determine the particle position(s) by minimizing the error between simulated and experimental modulus (AFM) contours. The algorithm can consistently predict particle positions within 3 nm of the actual known positions in synthetically created AFM data. We implemented the algorithm on experimental AFM data to create simulated modulus contours that partially reproduce key features present in an experimental modulus contour. This method provides a powerful tool to map spherical particle distribution in a 3D space, allowing better processing-structureproperty understanding that can arise from resin chemistry variations, surface functionality, processing conditions, and nanofiller particle properties.

This work explores the multiscale mechanical behavior of 17-4 PH stainless steel structures processed through the atomic diffusion additive manufacturing technique (ADAM). 17-4 PH stainless steel parts were fabricated with a Markforged Metal X 3D printer and characterized with respect to variable printing orientations for samples loaded in tension, shear, and bending. Sections of material were taken from each face of a bending test sample and prepared for microscopy to quantify porosity, grain size, and local stiffness and hardness. Microscale evaluation showed a porosity content of 3.3% on average across all faces. The yz face specifically showed the same sort of packing limitations often seen in other extrusion-based methods leading to greater porosity. An electron backscatter diffraction investigation showed a mean grain size of 6.5 μm with some grain alignment in the z- direction in the xz plane. Bulk material response in tension was dependent upon the print orientation of the sample. Cases where material was extruded entirely in the direction of loading saw a stiffness, strength, and strain to failure improvement of greater that 10% compared with other infill schemes. Shear testing revealed similar increases in strain to failure for samples with material extruded in only one direction compared with cross hatching at alternating orthogonal angles. Bend test results were similar in tension and compression regardless of orientation. For a sample printed with primary loading in the print plane ( xy ), the tensile modulus was 130–140 GPa, the tensile yield and ultimate strength were 600 MPa and 800 MPa, and the shear strength was 40.6 MPa on average.

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.

Material extrusion (MatEx) is finding increasing applications in additive manufacturing of thermoplastics due to the ease of use and the ability to process disparate polymers. Since part strength is anisotropic and frequently deviates negatively with respect to parts produced by injection molding, an urgent challenge is to predict final properties of parts made through this method. A nascent effort is underway to develop theoretical and computational models of MatEx part properties, but these efforts require comprehensive experimental data for guidance and validation. As part of the AM-Bench framework, we provide here a thorough set of measurements on a model system: polycarbonate printed in a simple rectangular shape. For the precursor material (as-received filament), we perform rheology, gel permeation chromatography, and dynamical mechanical analysis, to ascertain critical material parameters such as molar mass distribution, glass transition, and shear thinning. Following processing, we conduct X-ray computed tomography, scanning electron microscopy, depth sensing indentation, and atomic force microscopy modulus mapping. These measurements provide information related to pores, method of failure, and local modulus variations. Finally, we conduct tensile testing to assess strength and degree of anisotropy of mechanical properties. We find several effects that lead to degradation of tensile properties including the presence of pore networks, poor interfacial bonding, variations in interfacial mechanical behavior between rasters, and variable interaction of the neighboring builds within the melt state. The results provide insight into the processing–structure–property relationships and should serve as benchmarks for the development of mechanical models.

This work investigates the role of a carbon nanophase on the local mechanical behavior of nano-carbon metal composites (NCMCs) produced through an electrocharge-assisted process. Nanoindentation experiments on single crystal Al, Al 1350 parent alloys, and Al 1350 NCMCs revealed variable mechanical properties, caused by an interplay between microstructure and graphitic reinforcements. TEM and AFM studies also reveal nanoscale structural changes based on the incorporation of a carbon nanophase. In order to decouple the effects of the aforementioned mechanical behaviors, molecular dynamics nanoindentation simulations were performed on the (111) surface of Al and Al NCMC samples containing semi-infinite graphene nanoribbons to examine the evolution of plasticity over time. Findings indicate that the arrangement of a finite graphene nanophase within a host matrix can alter plasticity mechanisms and therefore yield strength in near-surface mechanical behaviors with little effect on elastic properties. This understanding should enable further study into tunable bulk properties of Al-based NCMCs while isolating microstructural effects and reinforcement effects of the carbon phase. Such an understanding could lead to application-specific material geometries ranging from high-performing vehicle structures to next-generation electrical devices.

The processing conditions used in the production of advanced polymer fibers facilitate the formation of an oriented fibrillar network that consists of structures spanning multiple length scales. The irregular nature of fiber tensile fracture surfaces suggests that their structural integrity is defined by the degree of lateral (interfacial) interactions that exist within the fiber microstructure. To date, experimental studies have quantified interfacial adhesion between nanoscale fibrils measuring 10-50 nm in width, and the global fracture energy through applying peel loads to fiber halves. However, a more in-depth evaluation of tensile fracture indicates that fiber failure typically occurs at an intermediate length scale, involving fibrillation along interfaces between fibril bundles of a few 100s of nanometers in width. Interaction mechanisms at this length scale have not yet been studied, due in part to a lack of established experimental techniques. Here, a new focused ion beam-based sample preparation protocol is combined with nanoindentation to probe interfaces at the intermediate length scale in two high-performance fibers, a rigid-rod poly(p-phenylene terephthalamide) and a flexible chain ultrahigh molecular weight polyethylene fiber. Higher interfacial separation energy recorded in the rigid-rod fiber correlated with less intensive fibrillation during failure and is discussed in the context of fiber chemistry and processing. Power law scaling of the total absorbed interfacial separation energy at three different scales in the polyethylene fiber is observed and analyzed, and distinct energy absorption mechanisms, featuring a degree of self-similarity, are identified. The contribution of these mechanisms to the overall integrity of the fiber is discussed, and the importance of the intermediate scale is elucidated. Results from this study provide new insights into the mechanical implications of hierarchical lateral interactions and will aid in the development of novel fibers with further improved mechanical performance.

Purpose This study aims to discuss the effect of carbon nanotubes (CNTs) on the mechanical properties of acrylonitrile–butadiene–styrene (ABS) composites fabricated by additive manufacturing (AM). Insight into the energy-dissipation mechanisms introduced and/or enhanced by the addition of CNTs is presented in this study. Design/methodology/approach ABS/CNT filaments were fabricated with different concentrations of CNTs. Using a fused deposition modeling approach, unidirectional specimens were printed using a MakerBot Replicator 2X (MakerBot Industries, Brooklyn, NY, USA). Specimens were tested under static and dynamic conditions, with the loading coinciding with the printing direction, to determine elastic modulus, strength and viscoelastic properties. Findings A CNT reinforcing effect is evident in a 37 per cent increase in elastic modulus. Likewise, the strength of the composite increases by up to 30 per cent with an increase in weight fraction of CNTs. At low dynamic strain amplitudes (0.05 per cent), a correlation between dissipated strain energy of the butadiene phase and strength of the composite is found such that less dissipation, from constraint of the butadiene particles by the CNTs, leads to higher strength of the composite. At higher dynamic strains, the presence of a high concentration of CNT leads to increased energy dissipation, with a maximum measured value of 24 per cent higher loss factor compared to baseline specimens. Because the trend of the composite behavior is similar (with a higher absolute value) to that of neat ABS, this study’s results indicate that well-established polymer/CNT dissipation mechanisms (such as stick-slip) are not significant, but that the CNTs amplify the dissipation of the ABS matrix by formation of crazes through stress concentrations. Originality/value This study provides knowledge of the dissipation behavior in additively manufactured ABS/CNT composites and provides insight into the expansion to new printable materials for dynamics applications.

The seamless integration of functional biomolecules with advanced nanomaterials not only allows for a hybrid material yielding their combined properties but also can enable novel joint functionality. In this work, the protein bovine serum albumin is used to simultaneously reduce and stabilize graphene oxide, demonstrating complete reduction of the graphene oxide to its reduced graphene oxide form. Where previous work has utilized a protein-reduced graphene oxide hybrid material as an attachment point for subsequent metal nanoparticle binding, this effort establishes an in situ methodology to directly synthesize tailored metal nanoclusters within the stabilizing protein complex. The successful synthesis of gold metal nanoclusters within the protein component of the hybrid system is verified, allowing for the creation of high-density protein-nanocluster ensembles on the graphene substrate. Utilizing the sensitive nature of the high-density protein-nanocluster materials, a protease sensor platform is demonstrated to detect the presence of trypsin, which is a biomarker for acute pancreatitis, at concentrations below 100 ng/mL.

Structural health monitoring and nondestructive inspection techniques typically assess the lifecycle and reliability of high-value aerospace, mechanical, and civil systems. Maintenance and inspection intervals are typically time-based and dependent on the structural health monitoring/nondestructive inspection technique to detect macroscale damage resulting from fatigue or environmental damage. The current work proposes an integrated materials-structures-dynamics approach for providing state awareness of structural health. The proposed approach shifts the conventional structural health monitoring/nondestructive inspection focus of searching for cracks to a health state awareness based on tracking changes in the energetics of the materials-structures-dynamics states. Energy variations are tracked in a cantilevered structure exposed to nonlinear harmonic oscillation, where the strain energy of the beam was derived and used to determine a health state index. Nanoindentation was used to probe the near-surface mechanical properties of the beam to characterize local material variations as a function of fatigue cycles. A nonlinear ultrasonic approach was considered in order to connect the local material behavior changes to the variations in the dynamic performance of the beam. The intent of the investigation was to connect the traditionally detached materials, structural, and dynamics approaches to structural health monitoring/nondestructive inspection, while providing a framework for enabling damage precursor detection.

In this work, a damage precursor indicator for aluminum 7075-T6 is proposed based on the nonlinear dynamics of a cantilevered beam system. A shouldered beam specimen is used to move the region of highest stress away from the clamped end. The beam is subject to harmonic base excitation in a uniaxial shaker. Fatigue damage accumulates in the beam with damage forming most quickly in the region of highest stress. By monitoring the tip deflection, changes in the natural frequency and response to a given excitation are correlated to fatigue life. Despite the absence of large-scale cracks, detectable changes in the nonlinear dynamics are discovered. The nonlinear dynamic parameters are estimated capturing the change in the forward and backward nonlinear sine sweeps. These changes could lend themselves to being a trackable damage precursor. The presence of microstructural precursors are confirmed using nanoindentation.

The structural dynamic behavior of 3D-printed and conventional aluminum alloys are studied experimentally. Heat treated Scalmalloy and Al 7075 T-6 beam-like structures are fabricated using additive and extrusion manufacturing, respectively. The beams are exposed to vibration fatigue. Linear and nonlinear dynamic characterizations are performed to extract the stiffness and damping properties for pristine and fatigued structures. Additionally, the study includes optical imaging, Electron Backscatter Diffraction (EBSD), and Atomic Force Microscopy (AFM) to obtain the alloys’ materials properties. While the preliminary results show that Scalmalloy fatigue life is approximately 3-4 times lower than Al 7075 T-6, failure in Scalmalloy appears to be more graceful and predictable than that for aluminum.

The focus of this study was to apply a robust inspection technique for monitoring damage nucleation and propagation in 7075 aluminum alloy specimens exposed to cyclic loading. A previously developed specimen, linearly tapered in width along the length, was subjected to a sinusoidal tension-tension load while conductivity and strain were measured in-situ. Ex-situ measurements of modulus, hardness, surface potential, digital image correlation strain field, and neutron diffraction were made as a function of fatigue cycles. It is hypothesized that varying levels of induced stress along the length due to equal-force but varying area along the length will create a record of damage which can be probed to intuit a temporal history for the specimen. Baseline, intermediate, and failure sensor measurements for several specimens were compared and analyzed as a function of applied stress (varied linearly along the length) and fatigue cycles (constant). Mechanisms of damage nucleation and propagation due to fatigue cycling are discussed with an emphasis on which inspection methods are most promising for improving structural durability and state monitoring.

Poly(urethane-urea) (PUU) has been infused into ultrahigh volume fraction carbon nanotube (CNT) forests using a heat-curable polymer formula. Polymer nanocomposites with carbon nanotube volume-fractions of 1%, 5%, 10%, 20%, and 30% were fabricated by overcoming densification and infusion obstacles. These polymer nanocomposites were nanoindented quasi-statically and dynamically to discern process-structure-(mechanical) property relations of polymerizing PUU in such densely-packed CNT forests. A 100× increase in indentation modulus has been observed, which is attributed not only to CNT reinforcement of the matrix, but also to molecular interactions in the matrix itself. Quasi-static elastic moduli ranging from 10 MPa–4.5 GPa have been recorded. Storage modulus for all materials is found to track well at loadings of 200 Hz, with little effect observed from increasing CNT volume fraction.

The exceptional static and dynamic physical properties of polyurethane-urea) (PUU) elastomers make them prime candidates for impulsive loading structural applications, such as blast protection coatings. Since the theoretical physical properties of carbon nanotubes (CNTs) are among the best for any currently known material, a number of previous studies explored the use of CNTs as nanoscale fillers to enhance the properties of PUU nanocomposites. However, due to the challenges inherent in dispersing CNTs in a PUU matrix and the resulting random orientation of the CNTs, these previous works observed marginal improvements in physical properties, and were unable to establish clear structure-property relations. Here, we report the synthesis of aligned-CNT (A-CNT) reinforced PUU polymer nanocomposites (A-PNCs) by infusing A-CNT forests with a stoveable PUU, and establish process-structure-property relations that quantify the contribution of CNT confinement on the PUU mechanical response. This stoveable process was achieved using blocked isocyanate which prevented polymerization until the blocks were removed with heat. PUUs of two distinct compositions were explored: one with 40 wt% hard-segment content (PUU211) and the other with 66 wt% hard-segment content (PUU541). Thermogravimetric analysis indicates that A-CNTs enhance the thermal stability of the hard-segment phase in PUU A-PNCs at 340 degrees C by up to 45% over the baseline PUUs. Atomic force microscopy reveals that the elongated nanophase hard segment formations along the CNT axis observed only in the nanocomposites were of similar characteristic size to the average inter-A-CNT spacing (similar to 70 nm), indicating a strong influence of A-CNTs on the size and orientation of hard-segment nanophases, as corroborated via small angle X-ray scattering. Nanoindentation testing reveals that PUU A-PNCs possess significant elastic anisotropy, and exhibit enhanced longitudinal effective indentation moduli of similar to 460 MPa (>3 x that of the PUU211 baseline) and similar to 1350 MPa (similar to 1.5 x that of the PUU541 baseline) for PUU211 and PUU541 nanocomposites, respectively. This difference in magnitude of CNT reinforcement efficacy indicates that CNT confinement leads to significant hard-segment re-organization in the PUU211 A-PNCs, whereas the interconnected network of hard-segments in the PUU541 is affected by CNT templating to a lesser extent. Dynamic nanoindentation testing results are consistent with these interpretations, where longitudinally-loaded PUU211 A-PNCs are found to exhibit a >3 x enhancement in storage modulus at 1 Hz of similar to 730 MPa, whereas the longitudinally-loaded PUU541 A-PNCs exhibit a slightly enhanced storage modulus enhancement at 1 Hz of 2190 MPa (similar to 1.5 x that of the PUU541 baseline). Reinforcement of PUUs with A-CNTs is a promising way to tune the physical properties of the PNCs; higher A-CNT packing densities, where the inter-CNT spacing could approach the nanophase characteristic diameter, could further enhance the PUU performance in ballistic protection applications. (C) 2018 Elsevier Ltd. All rights reserved.