This critical review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphology, crystal structure, and properties. Also described are the self-assembly and rheological properties of cellulose nanoparticle suspensions. The methodology of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Additionally, advances in predictive modeling from molecular dynamic simulations of crystalline cellulose to the continuum modeling of composites made with such particles are reviewed (392 references).

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Cellulose nanomaterials review: structure, properties and nanocomposites

This work presents the analytic and experimental development of piezoelectric actuators as elements of intelligent structures, i.e., structures with highly distributed actuators, sensors, and processing networks. Static and dynamic analytic models are derived for segmented piezoelectric actuators that are either bonded to an elastic substructure or embedded in a laminated composite. These models lead to the ability to predict, a priori, the response of the structural member to a command voltage applied to the piezoelectric and give guidance as to the optimal location for actuator placement. A scaling analysis is performed to demonstrate that the effectiveness of piezoelectric actuators is independent of the size of the structure and to evaluate various piezoelectric materials based on their effectiveness in transmitting strain to the substructure. Three test specimens of cantilevered beams were constructed: an aluminum beam with surface-bonded actuators, a glass/epoxy beam with embedded actuators, and a graphite/epoxy beam with embedded actuators. The actuators were used to excite steady-state resonant vibrations in the cantilevered beams. The response of the specimens compared well with those predicted by the analytic models. Static tensile tests performed on glass/epoxy laminates indicated that the embedded actuator reduced the ultimate strength of the laminate by 20%, while not significantly affecting the global elastic modulus of the specimen.

Use of piezoelectric actuators as elements of intelligent structures

This paper provides an overview of recent progress made in the area of cellulose nanofibre-based nanocomposites. An introduction into the methods used to isolate cellulose nanofibres (nanowhiskers, nanofibrils) is given, with details of their structure. Following this, the article is split into sections dealing with processing and characterisation of cellulose nanocomposites and new developments in the area, with particular emphasis on applications. The types of cellulose nanofibres covered are those extracted from plants by acid hydrolysis (nanowhiskers), mechanical treatment and those that occur naturally (tunicate nanowhiskers) or under culturing conditions (bacterial cellulose nanofibrils). Research highlighted in the article are the use of cellulose nanowhiskers for shape memory nanocomposites, analysis of the interfacial properties of cellulose nanowhisker and nanofibril-based composites using Raman spectroscopy, switchable interfaces that mimic sea cucumbers, polymerisation from the surface of cellulose nanowhiskers by atom transfer radical polymerisation and ring opening polymerisation, and methods to analyse the dispersion of nanowhiskers. The applications and new advances covered in this review are the use of cellulose nanofibres to reinforce adhesives, to make optically transparent paper for electronic displays, to create DNA-hybrid materials, to generate hierarchical composites and for use in foams, aerogels and starch nanocomposites and the use of all-cellulose nanocomposites for enhanced coupling between matrix and fibre. A comprehensive coverage of the literature is given and some suggestions on where the field is likely to advance in the future are discussed.

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Review: current international research into cellulose nanofibres and nanocomposites

Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

The numerical implementation of the model of brittle fracture developed in Francfort and Marigo (1998. J. Mech. Phys. Solids 46 (8), 1319–1342) is presented. Various computational methods based on variational approximations of the original functional are proposed. They are tested on several antiplanar and planar examples that are beyond the reach of the classical computational tools of fracture mechanics.

Numerical experiments in revisited brittle fracture

A simple and accurate finite cylindrical element method was formulated by the authors to determine stresses and deformations in laminated thick-walled cylindrical composite shells. This method reduces the overall problem to a one-dimensional one. In the previous study it was found that significant tensile radial strains occur near the inner shell surface, when the shell is subjected to external pressures. These radial tensile strains are of the same magnitude as the failure strains for the composite shells for shell dimensions and pressures of practical use. In the present study various stacking sequences are investigated to determine the one resulting in the lowest radial tensile strains. Using this optimum stacking sequence, results are obtained for shells composed of E-glass/epoxy, T300/SP-286 graphite/epoxy, and Kevlar 49/epoxy. In addition, two hybrid composite systems consisting of graphite/epoxy and glass/epoxy are studied.

Analysis of various thick-walled cross-ply composite cylindrical shells subjected to lateral pressures

A cylindrical composite shell with a non-circular, symmetric cross-section of flat sides and circular arc corners is analyzed using the abaqus finite element program. A trade study on the effects of various corner radii and shell lengths is performed. The response of the shell to constant internal pressurization is studied, with particular attention given to the bending boundary layer (BBL) near the ends. It is found that the extent of the BBL in the non-circular case is 2.5–4 times longer than that predicted by the classical equation, however, the ‘intensity’ of the bending boundary layer is reduced. An unusual ‘compounding’ effect in boundary layer response for short non-circular shells is described.

Unique bending boundary layer behaviors in a composite non-circular cylindrical shell

Experimental and analytical study on the behavior of foam core sandwich composite plates subjected to in-plane shear stress

: Results of the analyses of stresses, strains and displacements in both the adhesive bond and the adherends of bonded composite material structures and bonded metallic structures are presented. Joint configurations include the single lap, the double lap, the single doubler, the double doubler, and the stepped lap joint. The methods will be useful in designing and analyzing such joints under static, dynamic and thermal loads, as well as accounting for stresses incurred during fabrication as well as hygrothermal histories. Also shown are the effects of important geometric and material parameters on the stress, strain and displacement distributions. (Author)

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA106720&Location=U2&doc=GetTRDoc.pdf

Analysis of Bonded Joints in Composite Materials Structures Involving Hygrothermal Effects

Experimental results are presented for the mechanical properties of Kevlar 29 / Polyethylene thermoplastic matrix composite in compression at high strain rates. Results of quasi static tests are also presented. Fiber volume fraction (Vf) in the composite is 85%. The composite is quasi - isotropic. All the tests were conducted at room temperature. High strain rate tests were conducted using a Split Hopkinson Pressure Bar (SHPB) and the quasi static tests were conducted in an Instron machine. The yield stress and yield strain are presented for strain rates varying from 400/s to 1535/s in one of the in-plane (i.e. 1 and 2 directions) directions and from 1010/s to 2165/s in the 3 direction. The data from the 29 dynamic tests and 6 quasi static tests were statistically analyzed and discussed in some detail. The dynamic test results are represented by equations wherever possible and compared to the static test results. Significant increase in compressive yield strength and decrease in yield strain are observed at high strain rates compared to those of the material at quasi static tests.

Jack R. Vinson

Papers

Analysis of various thick-walled cross-ply composite cylindrical shells subjected to lateral pressures

Unique bending boundary layer behaviors in a composite non-circular cylindrical shell

Experimental and analytical study on the behavior of foam core sandwich composite plates subjected to in-plane shear stress

Analysis of Bonded Joints in Composite Materials Structures Involving Hygrothermal Effects

High strain rate compression testing of kevlar 29 / polyethylene composite with very high fiber volume fraction