Materials characterisation part II: tip geometry of the Vickers indenter for microindentation tests

This is the second of two papers by the authors associated with materials characterisation methods based on hardness testing. It is important to have knowledge of the tip geometry of the indenter employed in the hardness test as this affects the correctness of the value of contact area parameter used to determine the mechanical properties. In this paper, outcomes of a study concerned with the tip geometry of the Vickers microindenter are presented. Results from experiment are compared with results from published works and the most current accepted analytical models. A new non-contact methodology based on a residual imprint imaging process is developed and further compared with other methods using experimental and numerical analyses over a wide range of material properties. For confirmation, an assessment was undertaken using numerical dimensional analysis which permitted a large range of materials to be explored. It is shown that the proposed method is more accurate compared with other methods regardless of the mechanical properties of the material. The outcomes demonstrate that measuring contact area with the new method enhanced the overall relative error in the resulting mechanical properties including hardness and Young’s modulus of elasticity. It is also shown that the value of the contact area using actual indenter geometry obtained from experimental load-displacement analysis or FEM numerical analysis is more accurate than the value obtained from the assumption of perfect indenter geometry and hence can be used for materials with low strain hardening property. © 2017 Springer-Verlag Londo

Strengthening mechanisms in thermomechanically processed NbTi-microalloyed steel

The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075 °C to 825 °C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly

RAFT aqueous dispersion polymerization yields poly(ethylene glycol)-based diblock copolymer nano-objects with predictable single phase morphologies

A poly(ethylene glycol) (PEG) macromolecular chain transfer agent (macro-CTA) is prepared in high yield (>95%) with 97% dithiobenzoate chain-end functionality in a three-step synthesis starting from a monohydroxy PEG113 precursor. This PEG113-dithiobenzoate is then used for the reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA). Polymerizations conducted under optimized conditions at 50 °C led to high conversions as judged by 1H NMR spectroscopy and relatively low diblock copolymer polydispersities (Mw/Mn < 1.25) as judged by GPC. The latter technique also indicated good blocking efficiencies, since there was minimal PEG113 macro-CTA contamination. Systematic variation of the mean degree of polymerization of the core-forming PHPMA block allowed PEG113-PHPMAx diblock copolymer spheres, worms, or vesicles to be prepared at up to 17.5% w/w solids, as judged by dynamic light scattering and transmission electron microscopy studies. Small-angle X-ray scattering (SAXS) analysis revealed that more exotic oligolamellar vesicles were observed at 20% w/w solids when targeting highly asymmetric diblock compositions. Detailed analysis of SAXS curves indicated that the mean number of membranes per oligolamellar vesicle is approximately three. A PEG 113-PHPMAx phase diagram was constructed to enable the reproducible targeting of pure phases, as opposed to mixed morphologies (e.g., spheres plus worms or worms plus vesicles). This new RAFT PISA formulation is expected to be important for the rational and efficient synthesis of a wide range of biocompatible, thermo-responsive PEGylated diblock copolymer nano-objects for various biomedical applications

Thermo-responsive Diblock Copolymer Worm Gels in Non-polar Solvents

Benzyl methacrylate (BzMA) is polymerized using a poly(lauryl methacrylate) macromolecular chain transfer agent (PLMA macro-CTA) using reversible addition–fragmentation chain transfer (RAFT) polymerization at 70 °C in n-dodecane. This choice of solvent leads to an efficient dispersion polymerization, with polymerization-induced self-assembly (PISA) occurring via the growing PBzMA block to produce a range of PLMA–PBzMA diblock copolymer nano-objects, including spheres, worms, and vesicles. In the present study, particular attention is paid to the worm phase, which forms soft free-standing gels at 20 °C due to multiple inter-worm contacts. Such worm gels exhibit thermo-responsive behavior: heating above 50 °C causes degelation due to the onset of a worm-to-sphere transition. Degelation occurs because isotropic spheres interact with each other much less efficiently than the highly anisotropic worms. This worm-to-sphere thermal transition is essentially irreversible on heating a dilute solution (0.10% w/w) but is more or less reversible on heating a more concentrated dispersion (20% w/w). The relatively low volatility of n-dodecane facilitates variable-temperature rheological studies, which are consistent with eventual reconstitution of the worm phase on cooling to 20 °C. Variable-temperature 1H NMR studies conducted in d26-dodecane confirm partial solvation of the PBzMA block at elevated temperature: surface plasticization of the worm cores is invoked to account for the observed change in morphology, because this is sufficient to increase the copolymer curvature and hence induce a worm-to-sphere transition. Small-angle X-ray scattering and TEM are used to investigate the structural changes that occur during the worm-to-sphere-to-worm thermal cycle; experiments conducted at 1.0 and 5.0% w/w demonstrate the concentration-dependent (ir)reversibility of these morphological transitions

Polymerization-Induced Self-Assembly of Block Copolymer Nano-objects via RAFT Aqueous Dispersion Polymerization

In this Perspective, we discuss the recent development of polymerization-induced self-assembly mediated by reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke organic diblock copolymer nano-objects of controllable size, morphology, and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells

New method to determine the recombination rate constant between nitroxides and macroradicals

International audienc

New taxonomic records and regional trends for the Marquesan Prehistoric Marine Fishery, Eiao Island, Polynesia

Eiao Island (39.2 km(2), 577 m elevation), situated at the northern extent of the Marquesas Archipelago, features rocky and steep coastlines with few sheltered embayments that allow easy access to the sea and marine resources. We report the first evidence of prehistoric fishing practices from Eiao Island based on three inland sites (possibly dating from the 14th to 17th centuries), and explore variation in fish exploitation. All previous archaeological fishing records from the archipelago are from coastal sites, with inland Eiao Island assemblages offering comparative data on site location and taxonomic composition. The Eiao Island fish bone assemblages are dominated by piscivorous taxa, specifically grouper (Serranidae). Few tuna, mackerel and bonito (Scombridae) remains were recovered from the Eiao Island assemblages, compared to reports from Ua Pou, Tahuata and Ua Huka. New family-level taxonomic records added for the archipelago include: bonefish (Albulidae), requiem sharks (Carcharhinidae), butterflyfish (Chaetodontidae), flagtail (Kuhliidae), damselfish (Pomacentridae) and rabbitfish (Siganidae). These results further contribute to our understanding of prehistoric Marquesan fishing practices and allow elucidation of subsistence in coastal versus inland settings, variability in taxonomic composition between islands of the archipelago, and importantly inform on humanenvironment interactions in East Polynesia