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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.

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.

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 describes an experimental validation set for assessing the real-time fatigue behavior of metallic additive manufacturing (AM) maraging steel structures. Maraging steel AM beams were fabricated with laser powder bed fusion (LPBF) and characterized with ex situ studies of porosity through X-ray computed tomography (CT), nano-indentation, and atomic force microscopy, as well as quasi-static testing to evaluate the as-printed state. Microscale evaluation showed void content of 0.34–0.36% with hardness and stiffness variation through the build direction on the order of 5.1–5.6 GPa and 139–154 GPa, respectively. The microscale inhomogeneities created an as-printed state where the compression and tension plasticity behavior at the macroscale was unequal in quasi-static loading, leading to greater yielding in tension. Specimens were subjected to cyclic loads, while the structural behavior was characterized through in situ magnetic permeability, digital image correlation (DIC) strain, and structural compliance measurements. In the range of 3 × 103 to 3 × 104 cycles to failure, magnetic permeability measurements were able to capture the mechanical state as early as 60% of life depending on failure location. Results are discussed with an emphasis on material-property-structure relationships in terms of the multi-scale material state and fatigue validation data for improving the durability of AM parts.

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.

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.

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 current work investigates early material changes that manifest in aluminum alloy 7075 subjected to fatigue loading. Tension-tension specimens linearly tapered in width were exposed to cyclic loading with compliance and eddy current measurements made in-situ, while nano-indentation, surface potential, x-ray diffraction, and ultrasonic response were made ex-situ. Early stage damage progression was evaluated at various size scales and used to estimate a remaining life in real time. Baseline, in-situ, and post failure measurements were compared and analyzed as a function of fatigue cycles. Eddy current and compliance measurements showed a strong correlation between signal strength and fatigue life with eddy current providing a more precise signal per specimen. Microscale electrical measurements showed a link between surface potential and regions of the specimen that experienced higher tensile stress. Results are discussed with an emphasis on multiscale damage detection and identification due to fatigue cycling, while highlighting the most promising techniques for improving structural durability and real-time state monitoring.

The incorporation of carbon nanostructures into Al alloys has the potential to further improve the mechanical and electrical properties of these alloys. We report a novel one-step fabrication method of incorporating carbon in aluminum alloys by the application of a high electric current to molten Al metal containing particles of activated carbon at high temperature and under Ar atmosphere. We propose that the mechanism for carbon incorporation is similar to electromigration where the current facilitates ionization of the carbon atoms followed by polymerization of the carbon structures and the formation of graphitic chains and ribbons along preferred directions of the Al lattice. Furthermore, the current induces transformation from amorphous carbon to crystalline graphitic structures that propagate in the metal matrix. The carbon is identified by the C-K edge in electron energy loss spectra, X-ray photoelectron spectroscopy, Raman and Kelvin probe force microscopy. The stiffness is increased in these samples; hardness, on the other hand, decreases for some samples compared to reference Al alloys with no carbon. The electrical conductivity is superior to that of Al-carbon nanotube composites previously reported. The enhanced properties of Al covetics show potential for naval and aerospace applications and power transmission lines.

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.

The standard method for estimating instantaneous peak water use in premise plumbing systems (PPSs) is Hunter’s (1940) design curve, showing water demand versus fixture units. In recent year...

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(

The Additive Manufacturing Benchmark (AM-Bench) test series was established to provide rigorous measurement test data for validating additive manufacturing (AM) simulations for a broad range of AM technologies and material systems. AM-Bench includes extensive in situ and ex situ measurements, simulation challenges for the AM modeling community, and a corresponding conference series. In 2018, the first round of AM-Bench measurements and the first AM-Bench conference were completed, focusing primarily upon laser powder bed fusion (LPBF) processing of metals, and both LPBF and material extrusion processing of polymers. In all, 46 blind modeling simulations were submitted by the international AM community in comparison with the in situ and ex situ measurements. Analysis of these submissions provides valuable insight into existing AM modeling capabilities. The AM-Bench data are permanently archived and freely accessible online.

The exceptional static and dynamic physical properties of poly(urethane-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 °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 (∼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 ∼460 MPa (>3 × that of the PUU211 baseline) and ∼1350 MPa (∼1.5 × 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 × enhancement in storage modulus at 1 Hz of ∼730 MPa, whereas the longitudinally-loaded PUU541 A-PNCs exhibit a slightly enhanced storage modulus enhancement at 1 Hz of 2190 MPa (∼1.5 × 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.

Copper-nanocarbon, called covetic, films made using pulsed laser deposition (PLD) from a target containing nominally 4 wt% carbon in the copper matrix show uniform integration of up to 4.1 ± 0.2 wt.% C. We observe evidence of s p 2 carbon in PLD Cu covetic films in XPS and a peak in the C K-edge in electron energy loss spectroscopy indicative of transitions from the 1s to π * anti-bonding unoccupied state, suggesting that the C incorporated in the film is graphitic in nature. We measure sheet resistance of 1.7 Ohm/sq and transmittance of 25% at 550 nm in a ≈ 27 nm thick PLD Cu covetic film after deposition. These films also show much reduced oxidation by scanning probe techniques and very stable resistance for over 120 days - significantly longer than e-beam films of the same thickness. Cu covetic films made by PLD show good promise as transparent electrodes.

A micro-/nano-scale technique is presented for characterizing early stage damage formation on un-sized individual carbon fibers as well as embedded carbon fibers exposed to cyclic loads. Nanoindentation and atomic force microscopy were used to study the local mechanical properties and morphology of composite constituents before and after cyclic loading. Samples were of the type: 1) un-sized individual carbon fiber and 2) carbon fibers embedded in an epoxy matrix. Contributions from the structural compliance were isolated from the indentations in order to track variations in interphase properties due to global fatigue. When tested in the radial direction, the fiber shell was found to be approximately 50% stiffer than the fiber core. Indentations on fibers within a composite cycled to 100% of fatigue life revealed a compliance effect of roughly 5 GPa with respect to fibers within a control composite. The experimental results are compared to a finite element analysis model in order to help understand the mechanical influence of the interphase in the presence of the elastically-mismatched fibers and matrix. The work is a first step toward understanding damage precursor formation in individual microfibers and composite interphase regions; the work is expected to enable multiscale composite modeling efforts as well as the development of future self-sensing materials.

The local mechanical behavior of fatigued steel specimens was probed using nanoindentation. High-carbon steel cantilevers were exposed to nonlinear harmonic oscillation. The indentation modulus on the beam surface and plastic work during indentation decreased as a function of cycles, which was attributed to grain fragmentation and reorientation as well as the continuous reduction in inherent energy dissipation capacity of the material. X-ray diffraction, electron backscatter diffraction, and atomic force microscopy were used to characterize this microstructural evolution during early stages of the beam fatigue life, which altered 1) the local mechanical properties and 2) the global structural dynamic response. The results provide insight into fatigue damage precursors and provides a framework for connecting materials evolution with nonlinear structural dynamics.

Electrocharging assisted process is a method for the incorporation of carbon in molten metals under high electric current which results in the formation of networks of carbon nanostructures inside the metal matrix, and gives the new material improved mechanical, electrical and thermal properties. Alloys produced with this method are called covetics. In our previous works, different characterization techniques such X-ray photoelectron spectroscopy (XPS), Raman, X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) confirmed the presence of sp2 crystalline graphene nanoribbons and sheets in Ag and Al-covetic samples. Here, we report on detailed Raman mapping and characterization of Al-6061, Al-7075 and Ag covetics to further investigate the fraction of sp2/sp3 bonding, strain, defects, degree of oxidation, and crystalline sizes of the graphene nanoribbons. Gradual changes of strain are observed in regions with sp2 bonding and some degree of amorphous carbon is revealed by Raman scattering. Different degrees of oxidation of the carbon nanostructures with mostly sp2 bonding are evident by EELS spectrum imaging. Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) also reveal the presence of graphene ribbons and sheets with different lengths and conductivity.

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.

Electrocharging assisted process has been used to incorporate carbon in Aluminum 6061 and 7075 alloys ensuing significant improvements of the ultimate tensile strength, hardness, and electrical conductivity. This work investigates the presence of carbon, its structure, carbon-metal bonding, surface characterization and dispersion of carbon incorporated in Al alloys by electrocharging assisted process. Networks of Graphene nanoribbons with 3D epitaxy and preferred orientation along the 〈110〉 and 〈112〉 directions of Al are evident by transmission electron microscopy and spectrum imaging of the C K edge electron energy loss spectra. X-ray photoelectron spectroscopy and Raman scattering corroborate sp 2 carbon in Al-6061, and hybrid sp 2 -sp 3 in Al-7075 with added carbon. Kelvin probe force microscopy substantiates the presence of carbon in the Al matrix. Phonon density of states derived from first-principles calculations predicts C Al Raman active modes whilst density functional theory indicates covalent bonding between carbon and Al. This method of incorporation of graphene nanostructures in metals with strong carbon-metal bonding can open up new avenues for incorporation of sp 2 carbon structures in other materials.

To date, the directed in situ synthesis of fluorescent gold nanoclusters (AuNCs) has only been demonstrated in cancerous cells, with the theorized synthesis mechanism prohibiting AuNC formation in nontumorigenic cell lines. This limitation hinders potential biostabilized AuNC-based technology in healthy cells involving both chemical and mechanical analysis, such as the direct sensing of protein function and the elucidation of local mechanical environments. Thus, new synthesis strategies are required to expand the application space of AuNCs beyond cancer-focused cellular studies. In this contribution, we have developed the methodology and demonstrated the direct in situ synthesis of AuNCs in the nontumorigenic neuronal microglial line, C8B4. The as-synthesized AuNCs form in situ and are stabilized by cellular proteins. The clusters exhibit bright green fluorescence and demonstrate low (10%) toxicity. Interestingly, elevated ROS levels were not required for the in situ formation of AuNCs, although intracellular reductants such as glutamate were required for the synthesis of AuNCs in C8B4 cells. To our knowledge, this is the first-ever demonstration of AuNC synthesis in nontumorigenic cells and, as such, it considerably expands the application space of biostabilized fluorescent AuNCs.

The aim of the present study is to utilize fractographic methods employing scanning electron microscope (SEM) images to investigate the effects of build direction and orientation on the mechanical response and failure mechanism for Acrylonitrile–Butadiene–Styrene (ABS) specimens fabricated by fused deposited modeling (FDM). The material characterized here is ABS-M30 manufactured by Stratasys, Inc. Measurements of tensile strength, elongation-at-break and tensile modulus measurements along with the failure surfaces were characterized on a range of specimens at different build direction and raster orientation: ±45°, 0°, 0/90°, and 90°. The analysis of mechanical testing of the tensile specimens until failure will contribute to advances in creating stronger and more robust structure for various applications. Parameters, such as build direction and raster orientation, can be interdependent and exhibit varying effects on the properties of the ABS specimens. The ABS-M30 specimens were found to exhibit anisotropy in the mechanical response when exposed to axial tensile loading. The stress-strain data was characterized by a monotonic increase with an abrupt failure signifying brittle fracture. In certain combinations of build direction and raster orientation tensile failure was preceded by slight softening. The tensile strength and modulus, and elongation-at-break were found to be highly dependent upon the raster orientation and build direction. The relationship between the mechanical properties and failure was established by fractographic analysis. The fractographic analysis offers insight and provides valuable experimental data for the purpose of building structures in orientations tailored to their exemplified strength. For instance, specimens loaded such that bonds between adjacent rasters are the primary load bearing mechanism offer the least significant failure resistance. Other examples are shown where artifacts of the FDM fabrication process act to enhance tensile strength when configured properly with respect to the load. The results highlighted in this study are fundamental to the development of optimal design of complex ultra-light structure weight with increased structural efficiency. The study also presents a systematic scheme employing analogs to traditional fiber-reinforced polymer composites for the designation of build orientation and raster orientation parameters.

This paper presents a nonlinear dynamic methodology for monitoring precursors of fatigue damage in metallic structures under variable rotational base excitation. The methodology accounts for important nonlinearities due to the complex loading generated by variable rotation and structural degradation. The sources of the nonlinearities include: structural stiffening due to gyroscopic motion and high-response amplitude at the fundamental mode, softening due to inertial forces and gyroscopic loads, and localized microscopic material damage and micro-plasticity. The loading intensity and number of vibration cycles increase the influence of these effects. The change in the dynamic response due to fatigue damage accumulation is experimentally investigated by exciting a cantilever beam at variable rotational base motions. The observed fatigue evolution in the material microstructure at regions of large stresses (and the resulting progressive structural softening) is tracked by quantifying the growth in the tip response, the change in the fundamental natural frequency of the beam and the skewedness of the stepped-sine response curve. Previous understanding of the structural dynamic behavior is necessary to ascertain the damage precursor location and evolution. Nanoindentation studies near the beam clamped boundary are conducted to confirm the gradual progression in the local mechanical properties as a function of loading cycles, and microstructural studies are conducted to obtain qualitative preliminary insights into the microstructure evolution. This study demonstrates that careful monitoring of the nonlinearities in the structural dynamic response can be a sensitive parameter for detection of damage precursors.

Summary A nonlinear dynamic methodology is developed for detecting and estimating fatigue damage precursor in isotropic metal structures, prior to fatigue crack initiation, based on measurement of changes in the structure nonlinear response to harmonic base excitation. As an example, a damage precursor feature was extracted for cantilever beams by quantifying the reduction in the nonlinear stiffness because of localized microscopic material softening. The nonlinear dynamic analytical model parametrically tracks changes in the nonlinear structural stiffening as a function of the structural response and the loading cycles. Experimental results are obtained by exciting the base of cantilever beams at various amplitude levels. At high response amplitudes, the beams experience three competing simultaneous nonlinear dynamic mechanisms: (i) stiffening because of high response amplitude at the fundamental mode; (ii) softening because of inertial forces; and (iii) softening because of localized microscopic material damage caused by cyclic micro-plasticity. The third mechanism—a potential precursor to fatigue crack initiation in the structure—resulted in a cumulative softening because of early accumulation of cyclic fatigue damage and is the focus of this study. The loading intensity and the number of cycles influenced the relative contribution of these dynamic mechanisms. Nanoindentation studies near the beam clamped boundary were conducted to confirm the fatigue degradation in the local mechanical properties as a function of loading cycles. The proposed detection method is simple to implement and detects the presence of damage precursors but lacks the resolution to discern spatial distributions of the damage intensity. Based on prior knowledge of the structural dynamic characteristics of the beam and using the proposed methodology, damage level and stiffness reduction can be detected and estimated based on changes in the nonlinear structural response. The nonlinear stiffness term in the equation of motion is found to be very sensitive to the proposed fatigue damage precursor, making it an effective method for monitoring structural degradation prior to crack developments. The proposed methodology provides new insights and constitutes a significant step toward the development of new inspection techniques. Copyright © 2016 John Wiley & Sons, Ltd.

Low fiber-direction compressive strength of high-modulus (HM) carbon fiber-reinforced polymers (CFRPs) has been their major weakness prohibiting implementation of such materials in aircraft primary structures despite improving mechanical stiffness at a lower weight. A new HM CFRP achieving fiber-direction compressive strength of intermediate-modulus (IM) CFRPs but with more than 30% higher axial modulus has been recently developed. This work looks into fiber-matrix interface shear strength as a potential mechanism driving the compression strength improvement of the new material system. In-situ scanning electron microscopy (SEM) based single fiber push-out experiments addressing standing challenges associated with manufacturing high-quality samples as well as distinguishing same-diameter HM and IM carbon fibers in the hybrid composite system are used to measure fiber-matrix interface shear strength. The experiments show a 29% lower average value of the fiber-matrix interface shear strength for the HM carbon fibers compared to the IM carbon fibers in the new material system. Such a significant reduction corresponds to a 22% lower fiber-direction compressive strength of the HM CFRP without the integrated IM fibers. The results support the idea of integrating off-the-shelf IM carbon fibers with a stronger fiber-matrix interface and a higher shear modulus into HM CFRPs to improve their compressive strength.

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.

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.

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. Keywords: carbon nanotubes; polymer nanocomposites; polyurethane urea; self-assembly, United States. Army Research Office (Contract W911NF-13-D-0001)

New methods are needed to reprocess the excess of plastics in the waste stream. In this work, bottle-grade polyethylene terephthalate (PET), Styrofoam, and polycarbonate from compact discs (CDs) were spun into nanofibers as fine as ca. 100 nm in diameter using the electrospinning technique. The mechanical properties of the fibers were evaluated using microtensile testing. The elastic moduli ranged from 15 to 60 MPa, and displayed stiffnesses comparable or greater than fibers made from commercial polymers of equivalent molecular weight. Nanofibers were also prepared from blends of Styrofoam and recycled polycarbonate. Recycled PET fibers were tested for application in water filtration and had greater than 99% filtration efficiency of 1 μm particles. Nanofibers from both pure and mixed waste streams are expected to have applications in myriad areas such as ultra/microfiltration, composites, and tissue engineering.

The direct synthesis of monolayer and multilayer ReS2 by chemical vapor deposition at a low temperature of 450 °C is reported. Detailed characterization of this material is performed using various spectroscopy and microscopy methods. Furthermore initial field-effect transistor characteristics are evaluated, which highlight the potential in being used as an n-type semiconductor.

Protein-encapsulated gold nanocluster (P-AuNC) synthesis was first demonstrated in 2009 [1]. Initially, these P-AuNCs were used as cellular imaging agents as the protein shell surrounding the AuNC made them highly biocompatible. However, recent studies have begun to show that these stabilizing proteins may also retain native biological function, thus giving a dual functionality to these hybrid molecules. Here, we present the synthesis of DNase 1 stabilized AuNCs (DNase 1:AuNCs) with core sizes consisting of either eight or 25 atoms. The DNase 1:Au8NCs exhibit blue fluorescence, whereas the DNase 1:Au25NCs are red emitting. Moreover, in addition to the intense fluorescence emission, the synthesized DNase 1:AuNC hybrids retain the native functionality of the protein, allowing simultaneous detection and digestion of DNA with a detection limit of 2 mg/mL (Figure 1). The DNase 1:AuNCs could be conveniently employed as efficient and fast sensors to augment the current inefficient and time-consuming DNA contamination analysis techniques.

Carbon fiber composites display considerable heterogeneity in strength and stiffness depending on the respective constituent properties, reinforcement alignments, and interfacial relationships. Typically the inherent material anisotropy and complexity lead to relatively diverse material fatigue failures depending on the loading conditions and carbon fiber reinforcement orientations. The current work seeks to better understand the mechanisms by which fatigue damage precipitates and the smallest scales (sub-micron) at which material phenomena may be observed and tracked before becoming observable on the millimeter scale or larger. Unidirectional IM7-8552 composite samples were fabricated for transverse (90°) and longitudinal (0°) fiber loading. Samples were tested in bending using a three-point set-up and in single axis tension developing the cyclic life response at several loading stresses. Baseline and failed samples were partitioned and polished for instrumented indentation and atomic force microscopy (AFM) which probed the local mechanical and morphological behavior of the composite interphase. Attempts are made to characterize the local material behavior around the fiber-matrix interphase to understand material response of this region with respect to fatigue cycling. The fiber-matrix interphase results are compared to numerical modelling in order to better understand the interphase behavior and fatigue degradation of the composite materials.

Carbon nanotube (CNT) reinforced acrylonitrile-butadiene-styrene (ABS) composites fabricated using a fused deposition modeling approach were characterized for mechanical strain energy storage and dissipation capabilities. Dynamic mechanical analysis (DMA) was performed to quantify loss factor as a function of applied dynamic strain. In addition, DMA was performed at varying temperatures to give insight into the molecular interactions present in these composites. Insight into the microstructure was provided by atomic force microscopy (AFM). The results are compared to neat ABS and ABS/CNT systems processed through an injection molding technique. Results show large energy dissipation, accompanied by permanent damage, in both injection molded and additively manufactured neat ABS samples. In contrast, the additively manufactured ABS/CNT nanocomposites exhibited strain energy dissipation but reduced the effect of cavitation and crazing by possible reinforcement. This result suggests CNT fillers have the potential to alter the dissipation mechanisms present in additively manufactured structures to control structural damping. This research provides insight into the design and additive manufacturing of materials where energy dissipation is essential to maintain structural stability and functionality under dynamic loading.

Traditionally, most mechanical testing is conducted on specimens of uniform cross-section and stress magnitude, or only a single position along the specimen length is of interest. Investigations of various stress levels therefore requires separate tests with a unique specimen at each stress level of interest. The impetus of the current work was to develop a method for the design and monitoring of a specimen that simultaneously experiences a continuum of stress magnitudes across various positions. A linearly-tapered specimen was developed and subjected to sinusoidal tension-tension fatigue until specimen failure, with the expectation that a record of damage exists along the length of the specimen due to the varying level of induced stress. Baseline and post-failure scans of x-ray diffraction, electrical resistance via four point probe, nano-indentation, eddy current, and geometric changes were compared. Attempts were made to characterize the pre- and post-test property behaviors as a function of the applied stress, which varied linearly along the specimen, and as a function of fatigue cycles, which were the same along the length of the specimen. The mechanisms of specimen damage due to fatigue cycling were investigated and analyzed to improve durability and damage tolerance understanding.