Experimental investigation on the mechanical behavior of 3D carbon/carbon composites under biaxial compression

The effects of complex state of stress on the compressive behaviour of 3D carbon/carbon composites are investigated by application of uniaxial and biaxial loadings using a specially developed Zwick cruciform testing facility. The shape of the biaxially loaded cruciform specimen is optimised to avoid premature fracture outside the gauge section. A semi-analytical method is proposed to determine the stress components in the gauge section of the biaxial specimen. The experimentally obtained failure stress relation, which traces an elliptical path in the principal stress space, can be well represented by the Tsai criterion with a stress interaction parameter of F12=-0.85. Macro-fracture morphology and SEM micrographs are examined and the results show that the failure mechanisms of the composites vary with the loading ratio. The results also suggest that the biaxial stress interaction effect is represented by a domain in the biaxial specimen, which is characterised by torsion and bending fractures in the dislocated fibres between two adjacent Z yarns

Hot deformation behavior and processing maps of diamond/Cu composites

The hot deformation behaviors of 50 vol pct uncoated and Cr-coated diamond/Cu composites were investigated using hot isothermal compression tests under the temperature and strain rate ranging from 1073 K to 1273 K (800 C to 1000 C) and from 0.001 to 5 s1, respectively. Dynamic recrystallization was determined to be the primary restoration mechanism during deformation. The Cr3C2 coating enhanced the interfacial bonding and resulted in a larger flow stress for the Cr-coated diamond/Cu composites. Moreover, the enhanced interfacial affinity led to a higher activation energy for the Cr-coated diamond/Cu composites (238 kJ/mol) than for their uncoated counterparts (205 kJ/mol). The strain-rate-dependent constitutive equations of the diamond/Cu composites were derived based on the Arrhenius model, and a high correlation (R = 0.99) was observed between the calculated flow stresses and experimental data. With the help of processing maps, hot extrusions were realized at 1123 K/0.01 s1 and 1153 K/0.01 s1 (850 C/0.01 s1 and 880 C/0.01 s1) for the uncoated and coated diamond/Cu composites, respectively. The combination of interface optimization and hot extrusion led to increases of the density and thermal conductivity, thereby providing a promising route for the fabrication of diamond/Cu composites

Size-dependent and tunable elastic properties of hierarchical honeycombs with regular square and equilateral triangular cells

Simple closed-form results for all the independent elastic constants of macro-, micro- and nanosized first-order regular honeycombs with square and equilateral triangular cells and for the self-similar hierarchical honeycombs were obtained. It is found that, if the cell wall thickness of the first-order honeycomb is at the micrometer scale, the elastic properties of a hierarchical honeycomb are size dependent, owing to the strain gradient effects. Further, if the first-order cell wall thickness is at the nanometer scale, the elastic properties of a hierarchical honeycomb are not only size dependent owing to the effects of surface elasticity and initial stresses, but are also tunable. In addition, the cell size and volume of hierarchical nanostructured cellular materials can be varied, and hierarchical nanostructured cellular materials could also possibly be controlled to collapse

Butterfly wing architecture assisted CdS/Au/TiO2 Z-scheme type photocatalytic water splitting

Inspired by natural Z-scheme photosynthesis and black butterfly wing's antireflection morphology, we used the wings of butterfly Papilio nephelus Boisduva as templates to synthesize CdS/Au/TiO2 with butterfly wing architecture. This combination of artificial Z-scheme photosystem and butterfly wing's hierarchical architecture was expected to enhance the light harvesting and water splitting efficiency. The finite-difference time-domain (FDTD) simulation was applied to demonstrate the optical function of the architecture inherited from butterfly wing theoretically, UV–vis spectra and photocatalytic H2 evolution rates were further recorded to experimentally demonstrate the coupled effect of butterfly wing architecture and CdS/Au/TiO2 Z-scheme components. The FDTD simulation shows that the architecture of the wing scale TiO2 effectively reduced the UV light reflection by about 40%. Meanwhile, the wing scale architecture model exhibited lower UV reflection and transmission in water than those in air, which can be attributed to the stronger diffuse reflection in water. UV–vis spectra and photocatalytic H2 evolution experiments confirmed that the combination of the wing scale architecture and CdS/Au/TiO2 Z-scheme components contributed to the enhancement of the light harvesting ability and improved the water-splitting efficiency by 200% compared to the plate architecture TiO2. Inspired by Nature, we present a promising way for constructing efficient photocatalysts for hydrogen evolution

The HtrA chaperone monitors sortase-assembled pilus biogenesis in Enterococcus faecalis.

Sortase-assembled pili contribute to virulence in many Gram-positive bacteria. In Enterococcus faecalis, the endocarditis and biofilm-associated pilus (Ebp) is polymerized on the membrane by sortase C (SrtC) and attached to the cell wall by sortase A (SrtA). In the absence of SrtA, polymerized pili remain anchored to the membrane (i.e. off-pathway). Here we show that the high temperature requirement A (HtrA) bifunctional chaperone/protease of E. faecalis is a quality control system that clears aberrant off-pathway pili from the cell membrane. In the absence of HtrA and SrtA, accumulation of membrane-bound pili leads to cell envelope stress and partially induces the regulon of the ceftriaxone resistance-associated CroRS two-component system, which in turn causes hyper-piliation and cell morphology alterations. Inactivation of croR in the OG1RF ΔsrtAΔhtrA background partially restores the observed defects of the ΔsrtAΔhtrA strain, supporting a role for CroRS in the response to membrane perturbations. Moreover, absence of SrtA and HtrA decreases basal resistance of E. faecalis against cephalosporins and daptomycin. The link between HtrA, pilus biogenesis and the CroRS two-component system provides new insights into the E. faecalis response to endogenous membrane perturbations