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International audienceA methodology has been proposed for statistical characterization of transport behavior of a typical random fibrous medium, i.e. the Chopped Strand Mat (CSM). For any given digital images of fabric sample, statistical description of the random microstructure is employed to evaluate the permeability field, in the framework of the statistical continuum approach. By choosing suitable sampling strategy, the evaluated permeability field can be used to predict the realistic fluctuation in the flow pattern in the RTM mold-filling process with a high accuracy, as validated by the radial injection experiments. The method can be generalized to other random fibrous media. Using a database of CSM samples, statistical characterization of fiber volume fraction and permeability fields is performed respectively. Important statistical properties, e.g. the RVE size, marginal PDF and correlation length have been provided for the transport properties of the CSM, as the basis for the input data for stochastic simulation of composite processing

A cryo-quenched 70 wt % Fe-15 wt% Cr-15 wt% Ni single-crystal alloy with fcc (face centered cubic), bcc (body centered cubic), and hcp (hexagonal close packed) phases was implanted with 200 keV He+ ions up to 2 × 1017 ions·cm−2 at 773 K. Surface-relief features were observed subsequent to the He+ ion implantation, and transmission electron microscopy was used to characterize both the surface relief properties and the details of associated “swelling effects” arising cumulatively from the austenitic-to-martensitic phase transformation and helium ion-induced bubble evolution in the single-crystal ternary alloy. The bubble size in the bcc phase was found to be larger than that in the fcc phase, while the bubble density in the bcc phase was correspondingly lower. The phase boundaries with misfit dislocations formed during the martensitic transformation and reversion processes served as helium traps that dispersed the helium bubble distribution. Swelling caused by the phase transformation in the alloy was dominant compared to that caused by helium bubble formation due to the limited depth of the helium ion implantation. The detailed morphology of helium bubbles formed in the bcc, hcp, and fcc phases were compared and correlated with the characters of each phase. The helium diffusion coefficient under irradiation at 773 K in the bcc phase was much higher (i.e., by several orders of magnitude) than that in the fcc phase and led to faster bubble growth. Moreover, the misfit phase boundaries were shown to be effective sites for the diffusion of helium atoms. This feature may be considered to be a desirable property for improving the radiation tolerance of the subject, ternary alloy.

Selective laser melting (SLM) is a potential additive manufacturing (AM) technology. However, the application of SLM was confined due to low efficiency. To improve efficiency, SLM fabrication with a high layer thickness and fine powder was systematically researched, and the void areas and hollow powders can be reduced by using fine powder. Single-track experiments were used to narrow down process parameter windows. Multi-layer fabrication relative density can be reached 99.99% at the exposure time-point distance-hatch space of 120 µs-40 µm-240 µm. Also, the building rate can be up to 12 mm 3 /s, which is about 3-10 times higher than the previous studies. Three typical defects were found by studying deeply, including the un-melted defect between the molten pools, the micro-pore defect within the molten pool, and the irregular distribution of the splashing phenomenon. Moreover, the microstructure is mostly equiaxed crystals and a small amount of columnar crystals. The averages of ultimate tensile strength, yield strength, and elongation are 625 MPa, 525 MPa, and 39.9%, respectively. As exposure time increased from 80 µs to 200 µs, the grain size is gradually grown up from 0.98 µm to 2.23 µm, the grain aspect ratio is close to 1, and the tensile properties are shown as a downward trend. The tensile properties of high layer thickness fabricated are not significantly different than those with a coarse-powder layer thickness of low in previous research.

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We report an efficient and novel method to functionalize graphene oxide nanosheets with hyperbranched polysiloxane and successfully compound them with dicyclopentadiene bisphenol dicyanate ester resin to prepare nanocomposites. Fourier transform infrared spectra were used to examine the surface functionalization of graphene oxide. The effects of functionalized graphene oxide on the mechanical, thermal, dielectric and water-resistant properties of dicyclopentadiene bisphenol dicyanate ester resin were investigated systematically. Results of differential scanning calorimetry (DSC) show that the addition of modified graphene oxide can facilitate the curing reaction of dicyclopentadiene bisphenol dicyanate ester. Compared with pure dicyclopentadiene bisphenol dicyanate ester resin, the impact and flexural strengths of the nanocomposite materials are improved markedly with up to 60% and 47% increasing magnitude, respectively. Meanwhile, the modified graphene oxide/dicyclopentadiene bisphenol dicyanate ester systems show lower and more stable dielectric constant and loss than pure dicyclopentadiene bisphenol dicyanate ester resin over the testing frequency from 10 to 60 MHz. In addition, the thermal stability and moisture resistance of modified graphene oxide/dicyclopentadiene bisphenol dicyanate ester nanocomposties are also superior to that of pure dicyclopentadiene bisphenol dicyanate ester resin.