Yuris A. Dzenis

Simultaneously strong and tough ultrafine continuous nanofibers.

Strength of structural materials and fibers is usually increased at the expense of strain at failure and toughness. Recent experimental studies have demonstrated improvements in modulus and strength of electrospun polymer nanofibers with reduction of their diameter. Nanofiber toughness has not been analyzed; however, from the classical materials property trade-off, one can expect it to decrease. Here, on the basis of a comprehensive analysis of long (5-10 mm) individual polyacrylonitrile nanofibers, we show that nanofiber toughness also dramatically improves. Reduction of fiber diameter from 2.8 μm to ∼100 nm resulted in simultaneous increases in elastic modulus from 0.36 to 48 GPa, true strength from 15 to 1750 MPa, and toughness from 0.25 to 605 MPa with the largest increases recorded for the ultrafine nanofibers smaller than 250 nm. The observed size effects showed no sign of saturation. Structural investigations and comparisons with mechanical behavior of annealed nanofibers allowed us to attribute ultrahigh ductility (average failure strain stayed over 50%) and toughness to low nanofiber crystallinity resulting from rapid solidification of ultrafine electrospun jets. Demonstrated superior mechanical performance coupled with the unique macro-nano nature of continuous nanofibers makes them readily available for macroscopic materials and composites that can be used in safety-critical applications. The proposed mechanism of simultaneously high strength, modulus, and toughness challenges the prevailing 50 year old paradigm of high-performance polymer fiber development calling for high polymer crystallinity and may have broad implications in fiber science and technology.

Novel method for mechanical characterization of polymeric nanofibers

A novel method to perform nanoscale mechanical characterization of highly deformable nanofibers has been developed. A microelectromechanical system (MEMS) test platform with an on-chip leaf-spring load cell that was tuned with the aid of a focused ion beam was built for fiber gripping and force measurement and it was actuated with an external piezoelectric transducer. Submicron scale tensile tests were performed in ambient conditions under an optical microscope. Engineering stresses and strains were obtained directly from images of the MEMS platform, by extracting the relative rigid body displacements of the device components by digital image correlation. The accuracy in determining displacements by this optical method was shown to be better than 50 nm. In the application of this method, the mechanical behavior of electrospun polyacrylonitrite nanofibers with diameters ranging from 300 to 600 nm was investigated. The stress-strain curves demonstrated an apparent elastic-perfectly plastic behavior with elastic modulus of 7.6+/-1.5 GPa and large irreversible strains that exceeded 220%. The large fiber stretch ratios were the result of a cascade of periodic necks that formed during cold drawing of the nanofibers.

Asymptotic decay of radius of a weakly conductive viscous jet in an external electric field

Motion of a weakly conductive viscous jet accelerated by an external electric field is considered. Nonlinear rheological constitutive equation applicable for polymer fluids (Oswald–deWaele law) is applied. A differential equation for the variation of jet radius with axial coordinate is derived. Asymptotic variation of the jet radius at large distances from the jet origin is analyzed. It is found that the well-known power-law asymptote for Newtonian fluids with the exponent 1/4 holds for more general class of fluids, i.e., pseudoplastic (shear thinning) and dilatant (shear thickening) fluids with the flow index between 0 and 2. Dilatant fluids with the flow index greater than 2 exhibit power-law asymptotes with the exponents depending on the flow index. Results can be applied for the analysis of viscous polymer jets in the electrospinning process.

Elasticity of planar fiber networks

A micromechanics model is proposed for the elasticity of planar fiber networks (FNs). The FN is created by random deposition of linearly elastic straight rods within a region. The rods are bonded rigidly at contacts. Under external in-plane loading, the FN deformation consists of fiber bending, elongation, and contraction. An effective constitutive relation for fiber network is developed by averaging the strain energy dissipated by all possible fiber deformations in all directions. Numerical calculations are performed to analyze the effects of fiber aspect ratio and fiber concentration on the effective stiffness of the planar random FN. Finite element analysis (FEA) is performed and compared with the theoretical predictions of the effective FN moduli at several fiber concentrations. FEA results are in good agreement with theoretical predictions. The present model can be used for the prediction of mechanical properties, scaling analysis, and optimization of fiber assemblies.

Analysis of microdamage evolution histories in composites

Abstract Evolution of microdamage in advanced composites was experimentally studied in this paper. A new method of acoustic emission (AE) analysis of histories of different damage mechanisms was formulated based on a combination of transient AE classification and multiparameter filtering. The capabilities of the method were illustrated on examples of damage evolution in several graphite/epoxy composites. Three characteristic AE waveforms with different frequency spectra were identified based on the transient analysis. Regions occupied by these waveforms in the amplitude–rise time parametric space were identified for the [0]8 and [90]16 unidirectional composites. Multiparameter filtering was applied to extract evolution histories for the characteristic waveforms. The results were compared with actual damage in the specimens and the three characteristic AE waveforms were associated with matrix cracks, fiber breaks, and ‘macrodamage’, such as delaminations or longitudinal splitting in unidirectional plies. The multiparameter filters based on the analysis of the unidirectional composites were used to extract the damage evolution histories for the cross-ply [0/90]3S and angle-ply [±45]4S composites. The results compared favourably with the observed damage in these materials. An inverse analysis of the quality of the multiparameter filtering for the laminated composites indicated that the filters developed for unidirectional composites can be applied to the analysis of laminated composites with reasonable reliability. The new method of acoustic emission analysis of damage micromechanisms is expected to be especially advantageous for fatigue damage evolution studies in composites and structures.

Mechanical deformation and failure of electrospun polyacrylonitrile nanofibers as a function of strain rate

The mechanical deformation of 12μm long electrospun polyacrylonitrile (PAN) nanofibers with diameters of 300–600nm was investigated. The nanofibers were subjected to cold drawing in atmospheric conditions and at strain rates between 10−2 and 10−4s−1. The ultimate strain of the PAN nanofibers was 60%–130% varying monotonically with the strain rate. On the contrary, the fiber tensile strength, ranging between 30 and 130MPa, varied nonmonotonically with the slowest drawing rate resulting in the largest ductilities and fiber strengths. At the two faster rates, the large fiber ductilities originated in the formation of a cascade of ripples (necks), while at the slowest strain rate, the nanofibers deformed homogeneously allowing for the largest engineering strengths and extension ratios.

Extraordinary Improvement of the Graphitic Structure of Continuous Carbon Nanofibers Templated with Double Wall Carbon Nanotubes

Carbon nanotubes are being widely studied as a reinforcing element in high-performance composites and fibers at high volume fractions. However, problems with nanotube processing, alignment, and non-optimal stress transfer between the nanotubes and surrounding matrix have so far prevented full utilization of their superb mechanical properties in composites. Here, we present an alternative use of carbon nanotubes, at a very small concentration, as a templating agent for the formation of graphitic structure in fibers. Continuous carbon nanofibers (CNF) were manufactured by electrospinning from polyacrylonitrile (PAN) with 1.2% of double wall nanotubes (DWNT). Nanofibers were oxidized and carbonized at temperatures from 600 °C to 1850 °C. Structural analyses revealed significant improvements in graphitic structure and crystal orientation in the templated CNFs, with the largest improvements observed at lower carbonization temperatures. In situ pull-out experiments showed good interfacial bonding between the DWNT bundles and the surrounding templated carbon matrix. Molecular Dynamics (MD) simulations of templated carbonization confirmed oriented graphitic growth and provided insight into mechanisms of carbonization initiation. The obtained results indicate that global templating of the graphitic structure in fine CNFs can be achieved at very small concentrations of well-dispersed DWNTs. The outcomes reveal a simple and inexpensive route to manufacture continuous CNFs with improved structure and properties for a variety of mechanical and functional applications. The demonstrated improvement of graphitic order at low carbonization temperatures in the absence of stretch shows potential as a promising new manufacturing technology for next generation carbon fibers.

Analysis of single lap adhesive composite joints with delaminated adherends

Polymer composites have high potential for applications in repair of aerospace structures. Adhesively bonded composite patches minimize balance and clearance problems on control surfaces and can be readily formed to complex aircraft contours. Delamination in composites can be detrimental to joint performance and durability and should be accounted for in joint design. In this work, a simple engineering model is developed for cracked adhesive joints with arbitrary orthotropic laminated adherends. Joints with unidirectional and cross-ply adherends are analyzed and compared as an example. Variations of the strain energy release rates with the crack location and size are calculated. It is shown that joints with the cross-ply adherends have higher energy release rates than the joints with the unidirectional adherends. Experimental observations of the delamination growth are also performed and found to corroborate the theoretical predictions.

Cycle-based analysis of damage and failure in advanced composites under fatigue. 1. Experimental observation of damage development within loading cycles

Abstract Damage development within loading cycles and the possibility of cycle-based modeling of fatigue damage evolution and failure in advanced composites are addressed in this work. In Part 1 of the work, the fatigue damage development in an unnotched composite laminate is studied by an advanced acoustic emission (AE) technique. The overall time history of the acoustic emission is recorded and separated into the emission from loading and unloading phases of the loading cycles. The source location histories and distributions of the AE events over the fatigue stress range are extracted and analyzed. Qualitatively different time histories and stress distributions for the emission generated during loading and unloading are observed and analyzed, for the first time. It is concluded that most of the emission from the unloading phase is due to internal friction between the crack faces and most of the damage is developed during the loading phase of the cycles. The location distribution of the fatigue damage is found to be fairly random to failure, indicating that the dominating fatigue fracture process in an unnotched laminate is scattered damage nucleation and accumulation. The time history of the overall damage evolution is found to exhibit the classical initial, steady, and final failure development stages. Within the loading cycles, the damage is developed over the entire stress range, with the substantial amount developed at stresses below the maximum fatigue stress. The latter observation sheds light on the reported effects of the loading cycle shape on the fatigue behavior and life of composites. The results of this study provide an insight into fatigue damage development in composites and constitute a fundamental basis for the development of a cycle-based model in Part 2 of this work.

Passive biaxial mechanical properties and in vivo axial pre-stretch of the diseased human femoropopliteal and tibial arteries.

Surgical and interventional therapies for atherosclerotic lesions of the infrainguinal arteries are notorious for high rates of failure. Frequently, this leads to expensive reinterventions, return of disabling symptoms or limb loss. Interaction between the artery and repair material likely plays an important role in reconstruction failure, but data describing the mechanical properties and functional characteristics of human femoropopliteal and tibial arteries are currently not available. Diseased superficial femoral (SFA, n = 10), popliteal (PA, n = 8) and tibial arteries (TA, n = 3) from 10 patients with critical limb ischemia were tested to determine passive mechanical properties using planar biaxial extension. All specimens exhibited large nonlinear deformations and anisotropy. Under equibiaxial loading, all arteries were stiffer in the circumferential direction than in the longitudinal direction. Anisotropy and longitudinal compliance decreased distally, but circumferential compliance increased, possibly to maintain a homeostatic multiaxial stress state. Constitutive parameters for a four-fiber family invariant-based model were determined for all tissues to calculate in vivo axial pre-stretch that allows the artery to function in the most energy efficient manner while also preventing buckling during extremity flexion. Calculated axial pre-stretch was found to decrease with age, disease severity and more distal arterial location. Histological analysis of the femoropopliteal artery demonstrated a distinct sub-adventitial layer of longitudinal elastin fibers that appeared thicker in healthier arteries. The femoropopliteal artery characteristics and properties determined in this study may assist in devising better diagnostic and treatment modalities for patients with peripheral arterial disease.