Catalin Picu

Effect of defects on the intrinsic strength and stiffness of graphene

It is important from a fundamental standpoint and for practical applications to understand how the mechanical properties of graphene are influenced by defects. Here we report that the two-dimensional elastic modulus of graphene is maintained even at a high density of sp(3)-type defects. Moreover, the breaking strength of defective graphene is only ~14% smaller than its pristine counterpart in the sp(3)-defect regime. By contrast, we report a significant drop in the mechanical properties of graphene in the vacancy-defect regime. We also provide a mapping between the Raman spectra of defective graphene and its mechanical properties. This provides a simple, yet non-destructive methodology to identify graphene samples that are still mechanically functional. By establishing a relationship between the type and density of defects and the mechanical properties of graphene, this work provides important basic information for the rational design of composites and other systems utilizing the high modulus and strength of graphene.

Heterogeneity in Epoxy Nanocomposites Initiates Crazing: Significant Improvements in Fatigue Resistance and Toughening

Crazing is a failure mode of bulk polymers and occurs under predominant uniaxial tensile load when the bulk eventually forms denser ligaments (or fibrils) while preserving its continuity. [1‐2] The bridging of cracks by such fibrils is an importantmechanismforenergydissipationandtougheningin thermoplastic polymers. However, craze phenomena are not observed [3‐5] in thermosetting polymers such as epoxies due to the high crosslinking density of the epoxy chains, which limits molecular mobility and inhibits craze fibril formation. Such thermosetting epoxies typically display a brittle failure. [6‐7] We demonstrate here that thermosetting epoxies reinforced with amido-amine-functionalized multiwalled carbon nanotubes (A-MWNTs) exhibit crazing. We show order of magnitude reduction in fatigue crack growth rates as a result of the crazing. The fracture toughness and ductility of the brittle epoxy is also significantly enhanced by the crazing. Importantly these enhancements in fatigue resistance and toughness are achieved without any softening of the material. In fact, the Young’s modulus of the nanocomposite is � 30% greater and the average hardness of the nanocomposite is � 45% higher than the baseline (pristine) epoxy. We show that this effect is related to heterogeneous curing of the epoxy, which results in localized pockets of uncrosslinked epoxy that are trapped (or frozen) at the nanotube‐matrix interfaces. Under mechanical loading, these localized regions of high molecular mobility can evolve (or coalesce) to generate conditions that are favorable for crazing. Recently, in a very interesting study, [8] crazing has been reported for a poly(lactide

Stress reduction in tungsten films using nanostructured compliant layers

The residual stress in thin films is a major limiting factor for obtaining high quality films. We present a strategy for stress reduction in sputter deposited films by using a nanostructured compliant layer obtained by the oblique angle deposition technique, sandwiched between the film and the substrate. The technique is all in situ, does not require any lithography steps, and the nanostructured layer is made from the same material as the deposited thin film. By using this approach we were able to reduce stress values by approximately one order of magnitude in sputter deposited tungsten films. These lower stress thin films also exhibit stronger adhesion to the substrate, which retards delamination buckling. This technique allows the growth of much thicker films and has enhanced structural stability. A model is developed to explain the stress relief mechanism and the stronger adhesion associated with the presence of the nanostructured compliant layer.

Control of epoxy creep using graphene.

The creep behavior of epoxy-graphene platelet (GPL) nanocomposites with different weight fractions of filler is investigated by macroscopic testing and nanoindentation. No difference is observed at low stress and ambient temperature between neat epoxy and nanocomposites. At elevated stress and temperature the nanocomposite with the optimal weight fraction, 0.1 wt% GPLs, creeps significantly less than the unfilled polymer. This indicates that thermally activated processes controlling the creep rate are in part inhibited by the presence of GPLs. The phenomenon is qualitatively similar at the macroscale and in nanoindentation tests. The results are compared with the creep of epoxy-single-walled (SWNT) and multi-walled carbon nanotube (MWNT) composites and it is observed that creep in both these systems is similar to that in pure epoxy, that is, faster than creep in the epoxy-GPL system considered in this work.

Physical properties of nanostructures grown by oblique angle deposition

Isolated three-dimensional nanostructures were grown on templated or flat substrates by oblique angle deposition with or without substrate rotation where the physical shadowing effect dominates and controls the structures. The mechanical and electromechanical properties of Si springs and Co coated Si springs were measured by atomic force microscopy. The electrical property of β-phase W nanorods were measured by scanning tunneling microscopy. Examples of measurements of the elastic property of springs, electromechanical actuation, field emission of electrons, and field ionization of argon gas are presented. Potential applications and improvements of growth of uniform nanostructures are discussed.

A Coupled Fiber-Matrix Model Demonstrates Highly Inhomogeneous Microstructural Interactions in Soft Tissues Under Tensile Load

A soft tissues macroscopic behavior is largely determined by its microstructural components (often a collagen fiber network surrounded by a nonfibrillar matrix (NFM)). In the present study, a coupled fiber-matrix model was developed to fully quantify the internal stress field within such a tissue and to explore interactions between the collagen fiber network and nonfibrillar matrix (NFM). Voronoi tessellations (representing collagen networks) were embedded in a continuous three-dimensional NFM. Fibers were represented as one-dimensional nonlinear springs and the NFM, meshed via tetrahedra, was modeled as a compressible neo-Hookean solid. Multidimensional finite element modeling was employed in order to couple the two tissue components and uniaxial tension was applied to the composite representative volume element (RVE). In terms of the overall RVE response (average stress, fiber orientation, and Poissons ratio), the coupled fiber-matrix model yielded results consistent with those obtained using a previously developed parallel model based upon superposition. The detailed stress field in the composite RVE demonstrated the high degree of inhomogeneity in NFM mechanics, which cannot be addressed by a parallel model. Distributions of maximum/minimum principal stresses in the NFM showed a transition from fiber-dominated to matrix-dominated behavior as the matrix shear modulus increased. The matrix-dominated behavior also included a shift in the fiber kinematics toward the affine limit. We conclude that if only gross averaged parameters are of interest, parallel-type models are suitable. If, however, one is concerned with phenomena, such as individual cell-fiber interactions or tissue failure that could be altered by local variations in the stress field, then the detailed model is necessary in spite of its higher computational cost.

Peierls Stress of Dislocations in Molecular Crystal Cyclotrimethylene Trinitramine

Dislocation mediated plasticity in the α phase of the energetic molecular crystal cyclotrimethylene trinitramine (RDX) was investigated using a combination of atomistic simulations and the Peierls-Nabarro (PN) model. A detailed investigation of core structures and dislocation Peierls stress was conducted using athermal atomistic simulations at atmospheric pressure to determine the active slip systems. Generalized stacking fault energy surfaces calculated using atomistic simulations were used in the PN model to also estimate the critical shear stress for dislocation motion. The primary slip plane is found to be (010) in agreement with experimental observations, with the (010)[100] slip systems having the lowest Peierls stress. In addition, atomistic simulations predict the (021)[01[overline]2], (021)[100], (011)[100], (001)[100], and (001)[010] slip systems to have Peierls stress values small enough to allow plastic activity. However, there are less than five independent slip systems in this material in all situations. The ranking of slip systems based on the Peierls stress values is provided, and implications are discussed in relation to experimental data from nanoindentation and shock-induced plastic deformation.

Collagen Organization in Facet Capsular Ligaments Varies with Spinal Region and with Ligament Deformation

The spinal facet capsular ligament (FCL) is primarily comprised of heterogeneous arrangements of collagen fibers. This complex fibrous structure and its evolution under loading play a critical role in determining the mechanical behavior of the FCL. A lack of analytical tools to characterize the spatial anisotropy and heterogeneity of the FCLs microstructure has limited the current understanding of its structure-function relationships. Here, the collagen organization was characterized using spatial correlation analysis of the FCLs optically obtained fiber orientation field. FCLs from the cervical and lumbar spinal regions were characterized in terms of their structure, as was the reorganization of collagen in stretched cervical FCLs. Higher degrees of intra- and intersample heterogeneity were found in cervical FCLs than in lumbar specimens. In the cervical FCLs, heterogeneity was manifested in the form of curvy patterns formed by collections of collagen fibers or fiber bundles. Tensile stretch, a common injury mechanism for the cervical FCL, significantly increased the spatial correlation length in the stretch direction, indicating an elongation of the observed structural features. Finally, an affine estimation for the change of correlation length under loading was performed which gave predictions very similar to the actual values. These findings provide structural insights for multiscale mechanical analyses of the FCLs from various spinal regions and also suggest methods for quantitative characterization of complex tissue patterns.

A discrete network model to represent the deformation behavior of human amnion.

A discrete network model (DNM) to represent the mechanical behavior of the human amnion is proposed. The amnion is modeled as randomly distributed points interconnected with connector elements representing collagen crosslinks and fiber segments, respectively. This DNM is computationally efficient and allows simulations with large domains. A representative set of parameters has been selected to reproduce the uniaxial tension-stretch and kinematic responses of the amnion. Good agreement is found between the predicted and measured equibiaxial tension-stretch curves. Although the model represents the amnion phenomenologically, model parameters are physically motivated and their effect on the tension-stretch and in-plane kinematic responses is discussed. The model is used to investigate the local response in the near field of a circular hole, revealing that the kinematic response at the circular free boundaries leads to compaction and strong alignment of the network at the border of the defect.

Modeling the Mechanics of Semiflexible Biopolymer Networks: Non-affine Deformation and Presence of Long-range Correlations

An intertwined network of fibers forms the microstructure of many biological materials and defines their mechanical properties. Depending on the properties of individual fibers, from mechanics point of view, these fibrous materials can be considered to behave as semiflexible networks or flexible networks. While the behavior of flexible networks has been studied thoroughly, the mechanics of semiflexible networks is a less developed subject. In semiflexible networks, the filaments resist the external stresses by storing energy in both bending and axial modes of deformation. Their deformation field is non-affine and has long range correlations within a certain range of scales of observation. Due to the increasing interest in understanding the mechanical and rheological properties of complex systems such as the cell cytoskeleton and connective tissue, a growing interest was manifested in characterizing the mechanics of the semiflexible networks in the recent years. This chapter discusses recent advances in the field of semiflexible random fiber networks, including the quantification of their non-affine deformation and methods for solving boundary value problems on fibrous domains with intrinsic long range correlations.