The effect of Microstructure on the mechanical properties and adiabatic shear band formation in a medium carbon steel

Des essais de déformation à haute vitesse utilisant une barre d'Hopkinson en torsion ont été faits sur un acier a teneur de carbone moyen soumis a des traitements thermiques différents. Les essais avaient un taux de déformation de 1000 s-1. Les traitments thermiques étaient les suivants : tel que recu, recuit et refroidi dans l'air, et soumis à un traitement de trempe et revenu a 315, 480 at 650°C pendant une heure. Les microstructures observées étaient soit une pearlite épaisse, une pearlite non-dissoute dans une matrice de ferrite et trois variétés de martensite revenues. On a observé que la ferrite se déforme préférentiellement par rapport aux autres composants dans la microstructure. Aussi, quand la ferrite est uniforme, les bandes de cisaillement adiabatique sont aussi uniformes. Mais, quand la ferrite est ségrégée, les bandes de cissaillement observées sont plutôt granulaires.Using a torsional split-Hopkinson bar, high strain rate tests in the order of 1000 s-1 were conducted on 5 different heat treatments of a medium carbon steel. The heat treatments were ; as received, fully annealed and air cooled and quench tempered at 315, 480 and 650 °C, each for 1 hour. The microstructures obtained were coarse pearlite, unresolved pearlite in a ferrite matrix and three different tempers of martensite. It was found that ferrite in the microstructures would always deform preferentially over any other phase present in the material. In addition, if the ferrite was uniformly distributed, the ASB would be uniformly deformed. However, when it was segregated, granular type shear band surfaces would be observed

Monitoring the GAP catalyzed H-Ras GTPase reaction at atomic resolution in real time

The molecular reaction mechanism of the GTPase-activating protein (GAP)-catalyzed GTP hydrolysis by Ras was investigated by time resolved Fourier transform infrared (FTIR) difference spectroscopy using caged GTP (P(3)-1-(2-nitro)phenylethyl guanosine 5′-O-triphosphate) as photolabile trigger. This approach provides the complete GTPase reaction pathway with time resolution of milliseconds at the atomic level. Up to now, one structural model of the GAP⋅Ras⋅GDP⋅AlF(x) transition state analog is known, which represents a “snap shot” along the reaction-pathway. As now revealed, binding of GAP to Ras⋅GTP shifts negative charge from the γ to β phosphate. Such a shift was already identified by FTIR in GTP because of Ras binding and is now shown to be enhanced by GAP binding. Because the charge distribution of the GAP⋅Ras⋅GTP complex thus resembles a more dissociative-like transition state and is more like that in GDP, the activation free energy is reduced. An intermediate is observed on the reaction pathway that appears when the bond between β and γ phosphate is cleaved. In the intermediate, the released P(i) is strongly bound to the protein and surprisingly shows bands typical of those seen for phosphorylated enzyme intermediates. All these results provide a mechanistic picture that is different from the intrinsic GTPase reaction of Ras. FTIR analysis reveals the release of P(i) from the protein complex as the rate-limiting step for the GAP-catalyzed reaction. The approach presented allows the study not only of single proteins but of protein–protein interactions without intrinsic chromophores, in the non-crystalline state, in real time at the atomic level

Effects of artificial weathering in NR/SBR elastomer blends

Degradation of polymer blends occurs by the constituent phases undergoing distinct chemical changes that depend on their unique chemical structures. This makes predicting and establishing a structure-property relationship for each phase necessary as well as challenging. In this work, the molecular and physical changes occurring in sulfur-cross-linked natural rubber (NR), styrene-butadiene rubber (SBR), and their 50/50 blend subjected to accelerated weathering are analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, atomic-force microscopy (AFM), and dynamic mechanical thermal analysis (DMTA). NMR transverse relaxation time (T2) studies suggest the formation of rigid components due to weathering. FTIR and AFM reveal that this is related to the formation of a stiff surface due to chemical modifications, which shows up as an additional thermal transition in the DMTA curves. Low-field double-quantum (DQ) NMR studies of the cross-link density, by the residual dipolar coupling constant (Dres), of SBR show a continuous increase in its cross-link density over the weathering duration (988 h). In contrast, NR exhibits dominant chain scission reactions resulting in defects, with both materials demonstrating the formation of different chain lengths. During the first 168 h, NR also undergoes modification of sulfur bond lengths, which is also observed in the blend. The blend largely follows an intermediate trend of cross-link densities compared to the two polymers but shows signs of lesser chain modifications than a weighted average of the two polymers. This is confirmed by phase-resolved DQ magic-angle spinning (MAS) NMR experiments whereby the peak-specific Dres of the blend was measured to be lesser than that of the individual vulcanizates, thus proving that the blend is more resistant to weathering than its constituent elastomers