H. D. Wagner

Stress-induced fragmentation of multiwall carbon nanotubes in a polymer matrix

We report the observation of single nanotube fragmentation, under tensile stresses, using nanotube-containing thin polymeric films. Similar fragmentation tests with single fibers instead of nanotubes are routinely performed to study the fiber-matrix stress transfer ability in fiber composite materials, and thus the efficiency and quality of composite interfaces. The multiwall nanotube-matrix stress transfer efficiency is estimated to be at least one order of magnitude larger than in conventional fiber-based composites.

Evaluation of Young's Modulus of Carbon Nanotubes by Micro-Raman Spectroscopy

Micro-Raman spectroscopy is used to monitor the cooling-induced compressive deformation of carbon nanotubes embedded in an epoxy matrix. Youngs modulus of single- and multiwall nanotubes may then be derived from a concentric cylinder model for thermal stresses, using the D * -band shift for each tube type. The resulting values of the elastic moduli are in very good agreement with predicted theoretical values, and with the published experimental data set of Treacy et al., Nature (London) 381 , 678 (1996).

A round-robin programme on interfacial test methods

Abstract A round-robin programme has been undertaken to assess the compatibility in the micromechanical techniques used to evaluate the interfacial shear strength of the fibre/matrix bond in composite materials. The tests selected for evaluation were the single-fibre pull-out test, the microdebond test, the fragmentation test and the micro-indentation test. Twelve laboratories were invited to participate in this programme. Each laboratory was supplied with Caurtaulds XA fibre in the untreated condition and with a standard surface treatment, and a quantity of epoxy resin, hardener and catalyst, all from the same batch. Some laboratories were supplied with composite bars made with the same materials. A common cure cycle was chosen for sample preparation. Each laboratory conducted the tests to its own procedures. The results showed that the scatter within each laboratory was acceptable but the scatter between laboratories for a particular test was high. The results are discussed and possible explanations are presented for these observations. The indications are that the fundamental procedures used in each laboratory are sound. The results also suggest that there is great potential for achieving standard procedures and reducing the inter-laboratory scatter. A further round-robin programme is proposed to generate test protocols.

On the relationship between the microstructure of bone and its mechanical stiffness

A recent study of bone structure shows that the plate-shaped carbonate apatite crystals in individual lamellae are arranged in layers across the lamellae, and that the orientation of these layers are different in alternate lamellae. Based on these findings, a new micromechanical model for the Youngs modulus of bone is proposed, which accounts for the anisotropy and geometrical characteristics of the material. The model incorporates the platelet-like geometry of the basic reinforcing unit, the presence of alternating thin and thick lamellae, and the orientations of the crystal platelets in the lamellae. The thin and thick lamellae are modeled as orthotropic composite layers made up of thin rectangular apatite platelets within a collagen matrix, and classical orthotropic elasticity theory is used to calculate the Youngs modulus of the lamellae. Bone is viewed as an assembly of such orthotropic lamellae bent into cylindrical structures, and having a constant, alternating angle between successive lamellae. The micromechanical model employs a modified rule-of-mixtures to account for the two types of lamellae. The model provides a curve similar to the published experimental data on the angular dependence of Youngs modulus, including a local maximum at an angle between 0 and 90 degrees. A rigorous testing of the model awaits additional experimental data.

Transmission electron microscopy observations of fracture of single-wall carbon nanotubes under axial tension

Well-aligned bundles of single-wall carbon nanotubes under tensile stresses were observed to fracture in real-time by transmission electron microscopy. The expansion of elliptical holes in the polymer matrix results in a tensile force in bridging nanotubes. The polymer matrix at both ends of the bundles deforms extensively under the tension force, and fracture of the nanotubes occurs in tension within the polymer hole region rather than in shear within the gripping polymer region at the ends of the bundles. This provides evidence of significant polymer-nanotube wetting and interfacial adhesion.

Hybrid effects in composites: conditions for positive or negative effects versus rule-of-mixtures behaviour

A positive or negative hybrid effect in hybrid composites is defined as a positive or negative deviation of a certain mechanical property from the rule-of-mixtures behaviour. The question of hybrid effects is first examined with special hybrids which have been chosen so that the effect of the fibre-matrix interface is minimized. The hybrids examined consisted of two types of carbon fibres with different mechanical properties but similar surface treatments. The results of all the mechanical properties examined (modulus, strength, stress intensity factor, fracture energies) under quasi-static and fast testing conditions do not show any synergism. In view of these results a second hybrid system of E-glass fibre/AS carbon fibre-reinforced epoxy has been chosen. In this system both the mechanical properties of the fibres and the interface which they form with the resin are entirely different. None of the mechanical properties, excluding the fracture energies, show any signs of a hybrid effect. The fracture energy results, however, show the existence of a negative hybrid effect. A theory which sets upper and lower bounds for the hybrid effect is proposed, and the conditions for the occurrence of either a positive or a negative effect are discussed.

Evidence of stress transfer and formation of fracture clusters in carbon nanotube-based composites

We report the first observation of the formation of damage doublets in adjacent carbon nanotubes embedded in a polymer matrix. Such damage clusters are comparable to those arising in fiber-reinforced composite materials as a result of the redistribution of stress from a failed fiber to its unfailed adjacent neighbors. This observation may help shed light on fracture nucleation and growth in nanotube-based composites.

Interfacial properties of polymer composites measured by push-out and fragmentation tests

Abstract The interfacial properties for E-glass/epoxy composites were measured using push-out tests and single fiber fragmentation tests. Theoretical models for both stress-based and energy-based criteria were used to interpret the experimental results. Fibers treated with γ-aminopropyl-triethoxysilane (γ-APS) showed higher bond strength (∼1.7 times higher) and interfacial toughness (∼1.9 times higher) than those of unsized E-glass based composites. However, the average interfacial toughness obtained from fragmentation tests was about six times higher than that obtained from push-out tests. Considering the analytical frameworks employed to interpret the values measured in the present work, the fragmentation test is a more appropriate method to obtain interfacial energy for polymeric composites, but both methods are appropriate for relative measurements of interface strength.

Orientation of carbon nanotubes in polymers and its detection by Raman spectroscopy

Abstract Small amounts of single-wall carbon nanotubes embedded in a polymer matrix were used to sense the mechanical response of the polymer using microRaman spectral data. A flow orientation method was applied to align the nanotubes in the matrix. The Raman spectra obtained for specimens cut both parallel and perpendicular to the flow direction were found to be significantly different, as a function of mechanical strain. Thus Raman spectroscopy combined with mechanical testing provides a way to probe the alignment of nanotubes in composites. The Raman shift-strain response for samples loaded perpendicular to the flow direction suggests that nanotube reorientation is achieved upon straining the polymer beyond its yield point. Our data suggest that the adhesion between the nanotubes and the polymer exceeds the shear yield strength of the matrix. We show that a stress–strain curve for the polymer may be produced directly by means of Raman spectroscopy.

Friction mechanism of individual multilayered nanoparticles

Inorganic nanoparticles of layered [two-dimensional (2D)] compounds with hollow polyhedral structure, known as fullerene-like nanoparticles (IF), were found to have excellent lubricating properties. This behavior can be explained by superposition of three main mechanisms: rolling, sliding, and exfoliation-material transfer (third body). In order to elucidate the tribological mechanism of individual nanoparticles in different regimes, in situ axial nanocompression and shearing forces were applied to individual nanoparticles using a high resolution scanning electron microscope. Gold nanoparticles deposited onto the IF nanoparticles surface served as markers, delineating the motion of individual IF nanoparticle. It can be concluded from these experiments that rolling is an important lubrication mechanism for IF-WS2 in the relatively low range of normal stress (0.96±0.38 GPa). Sliding is shown to be relevant under slightly higher normal stress, where the spacing between the two mating surfaces does not permit free rolling of the nanoparticles. Exfoliation of the IF nanoparticles becomes the dominant mechanism at the high end of normal stress; above 1.2 GPa and (slow) shear; i.e., boundary lubrication conditions. It is argued that the modus operandi of the nanoparticles depends on their degree of crystallinity (defects); sizes; shape, and their mechanical characteristics. This study suggests that the rolling mechanism, which leads to low friction and wear, could be attained by improving the sphericity of the IF nanoparticle, the dispersion (deagglomeration) of the nanoparticles, and the smoothness of the mating surfaces.