U. Ramamurty

Nano-indentation studies on polymer matrix composites reinforced by few-layer graphene

The mechanical properties of polyvinyl alcohol (PVA) and poly(methyl methacrylate) (PMMA)-matrix composites reinforced by functionalized few-layer graphene (FG) have been evaluated using the nano-indentation technique. A significant increase in both the elastic modulus and hardness is observed with the addition of 0.6 wt% of graphene. The crystallinity of PVA also increases with the addition of FG. This and the good mechanical interaction between the polymer and the FG, which provides better load transfer between the matrix and the fiber, are suggested to be responsible for the observed improvement in mechanical properties of the polymers.

Strain rate sensitivity of a closed-cell aluminum foam

Abstract An experimental investigation into the strain rate sensitivity of a closed-cell aluminum foam at room temperature and under compression loading is conducted. The nominal strain rates are varied by four orders of magnitude, from 3.33×10−5 to 1.6×10−1 s−1. Within this range, experimental results show that the plastic strength and the energy absorbed increase (by 31 and 52.5%, respectively) with increasing strain rate. However, the plastic strength was found to increase bilinearly with the logarithm of strain rate, whereas dense metals tend to show only a linear response. As is the case with dense metals, the strain rate sensitivity of the foam was not a constant value, but found to be dependent on the strain and incremental change in strain rate. These results are explained with the aid of suitable micromechanical models such as microinertial effects against the bucking of cell walls at high strain rates that are unique to foams.

Extraordinary synergy in the mechanical properties of polymer matrix composites reinforced with 2 nanocarbons

One of the applications of nanomaterials is as reinforcements in composites, wherein small additions of nanomaterials lead to large enhancements in mechanical properties. There have been extensive studies in the literature on composites where a polymer matrix is reinforced by a single nanomaterial such as carbon nanotubes. In this article, we examine the significant synergistic effects observed when 2 different types of nanocarbons are incorporated in a polymer matrix. Thus, binary combinations of nanodiamond, few-layer graphene, and single-walled nanotubes have been used to reinforce polyvinyl alcohol. The mechanical properties of the resulting composites, evaluated by the nanoindentation technique, show extraordinary synergy, improving the stiffness and hardness by as much as 400% compared to those obtained with single nanocarbon reinforcements. These results suggest a way of designing advanced materials with extraordinary mechanical properties by incorporating small amounts of 2 nanomaterials such as graphene plus nanodiamond or nanodiamond plus carbon nanotube.

EFFECT OF CRYSTALLINITY ON THE IMPACT TOUGHNESS OF A La-BASED BULK METALLIC GLASS

The influence of crystallinity on the ductile–brittle transition in a bulk lanthanum-based metallic glass was investigated. The degree of crystallization in the metallic glass, which was processed through the arc-melting and water-quenching route, was systematically altered by varying the annealing time above the glass transition temperature. The resulting amorphous/crystalline microstructures were characterized by XRD, DSC, and TEM techniques. Instrumented impact test results show a significant decrease, by ∼90%, in impact toughness with the introduction of a small percentage of crystallinity. This decrease in toughness was also associated with a distinct change in fracture morphology, from a ductile vein-type fracture in the bulk glass to intergranular cleavage fracture in the crystalline material. The observed fracture transition was rationalized by recourse to the mechanism of stress relaxation due to viscous flow. For this purpose, variations in elastic modulus and dynamic viscosity with respect to the crystallinity were measured using a dynamic mechanical analyzer. The characteristic relaxation times were computed from the viscosity data and were used to explain the ductile–brittle transition. Microscopic mechanisms responsible for the fracture transition are also discussed.

Embrittlement of a bulk metallic glass due to low-temperature annealing

Embrittlement of a bulk La-based metallic glass due to isothermal and isochronal annealing below the T-g was investigated. Results show that the impact toughness decreases with increasing annealing time or temperature, accompanied by a change in fracture morphology. Reasons for this are discussed in terms of structural relaxation

Nanoindentation in Crystal Engineering: Quantifying Mechanical Properties of Molecular Crystals

Nanoindentation is a technique for measuring the elastic modulus and hardness of small amounts of materials. This method, which has been used extensively for characterizing metallic and inorganic solids, is now being applied to organic and metal-organic crystals, and has also become relevant to the subject of crystal engineering, which is concerned with the design of molecular solids with desired properties and functions. Through nanoindentation it is possible to correlate molecular-level properties such as crystal packing, interaction characteristics, and the inherent anisotropy with micro/macroscopic events such as desolvation, domain coexistence, layer migration, polymorphism, and solid-state reactivity. Recent developments and exciting opportunities in this area are highlighted in this Minireview.

Impact energy absorption in an Al foam at low velocities

Energy absorption characteristics of Al foam were investigated using flat-end and spherical-end indenters with impact velocities ranging between 3 and 30 m/s. The absorbed energy increases marginally within the quasi-static regime, but increases significantly at velocities greater than \sim 10 m/s, a result of shock wave propagation effects becoming activated.

Microstructure and mechanical properties of a partially crystallized La-based bulk metallic glass

The evolution of microstructure upon partial crystallization and its influence on the mechanical properties such as hardness, elastic modulus and viscosity in a La 55 Al 25 Cu 10 Ni 5 Co 5 bulk metallic glass alloy are studied. Specimens with various volume fractions of crystalline phases were obtained by annealing the as-cast amorphous alloy above its glass transition temperature and were characterized by transmission electron microscopy. Microscopic examination of the heat-treated samples shows short-range-ordered domains prior to nanocrystallization within the amorphous matrix, followed by the growth and impingement of the crystallites. Whereas the hardness of the annealed samples increases linearly with increasing crystallinity, the elastic modulus and the viscosity both increase abruptly when the crystalline volume fraction is about 40 vol.%, with a only minor variation on either side of this range. The sudden rises in the modulus and viscosity are similar to those in the literature data on the fracture strength of partially crystallized bulk amorphous alloys that shows a steep drop in strength at 30-50 vol.% crystallinity. On the basis of the microscopic observations, it is suggested that the interaction and formation of rigid networks of crystalline phases upon the attainment of a critical second-phase volume fraction may be the possible reason for the sudden change in mechanical properties. Percolation theory is utilized in further substantiating this hypothesis.

Kinematic and mechanical profile of the self-actuation of thermosalient crystal twins of 1,2,4,5-tetrabromobenzene: a molecular crystalline analogue of a bimetallic strip.

A paradigm shift from hard to flexible, organic-based optoelectronics requires fast and reversible mechanical response from actuating materials that are used for conversion of heat or light into mechanical motion. As the limits in the response times of polymer-based actuating materials are reached, which are inherent to the less-than-optimal coupling between the light/heat and mechanical energy in them, a conceptually new approach to mechanical actuation is required to leapfrog the performance of organic actuators. Herein, we explore single crystals of 1,2,4,5-tetrabromobenzene (TBB) as actuating elements and establish relations between their kinematic profile and mechanical properties. Centimeter-size acicular crystals of TBB are the only naturally twinned crystals out of about a dozen known materials that exhibit the thermosalient effect-an extremely rare and visually impressive crystal locomotion. When taken over a phase transition, crystals of this material store mechanical strain and are rapidly self-actuated to sudden jumps to release the internal strain, leaping up to several centimeters. To establish the structural basis for this colossal crystal motility, we investigated the mechanical profile of the crystals from macroscale, in response to externally induced deformation under microscope, to nanoscale, by using nanoindentation. Kinematic analysis based on high-speed recordings of over 200 twinned TBB crystals exposed to directional or nondirectional heating unraveled that the crystal locomotion is a kinematically complex phenomenon that includes at least six kinematic effects. The nanoscale tests confirm the highly elastic nature, with an elastic deformation recovery (60%) that is far superior to those of molecular crystals reported earlier. This property appears to be critical for accumulation of stress required for crystal jumping. Twinned crystals of TBB exposed to moderate directional heating behave as all-organic analogue of a bimetallic strip, where the lattice misfit between the two crystal components drives reversible deformation of the crystal.

Mechanical properties of inorganic nanowire reinforced polymer–matrix composites

Th em echanical properties of poly(vinyl alcohol) matrix composites incorporating SiC and Al2O3 nanowires (NWs) have been investigated. A marked increase in the elastic modulus (up to 90%) has been observed even with the addition of a small quantity (0.8 vol%) of nanowires. This observation cannot be explained by iso-stress analysis, which is appropriate for describing the variation of properties with the reinforcement volume fraction in discontinuously reinforced composites. Crystallization of the polymer induced by the NWs, the high aspect ratio and the surface-to-volume ratio of the NWs as well as the possible in-plane alignment of the NWs during processing are considered to be responsible for the increase in the stiffness. A significant increase in the strength of the composite with the addition of NWs is also observed. This is due to the significant pull-out of the NWs and the corresponding stretching of the matrix due to the complete wetting of the NW surface by the polymer. The increase in tensile strength is found to saturate at higher vol% of NW addition due to the reduced propensity for shear-band induced plastic deformation. (Some figures in this article are in colour only in the electronic version)