Visualization and quantification of low-velocity impact damage of three-dimensional braided CFRP composites by micro-computed tomography.

In this paper, the low-velocity impact response and damage of three-dimensional five-directional (3D5d) braided carbon/epoxy composites under different energies (30 J, 65 J, 100 J) are studied. Importantly, the damage is segmented from the axial and radial directions of the yarn to visualize and quantify the damage distributions in different regions based on micro-CT. The results show that as compared with the samples under 30 J and 100 J, the samples under 65 J energy perform higher impact properties. Moreover, as the impact energy increases from 30 J to 100 J, the corresponding damage volume increases from 6.886 mm3 to 477.133 mm3. Furthermore, the damage distributions are almost symmetrical in space.

Micromechanisms and Characterization of Low-Velocity Impact Damage in 3D Woven Composites.

Low-velocity impact (LVI) damage of 3D woven composites were experimentally and numerically investigated, considering different off-axis angles and impact energies. The impact responses were examined by LVI tests, and the damage morphology inside the composites was observed by X-ray micro-computed tomography (µ-CT). Yarn-level damage evolution was revealed by developing a hybrid finite element analysis model. The results show that the impact damage has significant directionality determined by the weft/warp orientation of the composites. The damage originates at the bottom of the impacted area and then expands outwards and upwards simultaneously, accompanied by in-plane and out-of-plane stress transfers. The straight-line distributed weft/warp yarns play an important role in bearing loads at the beginning of loading, while the w-shape distributed binder warp yarns gradually absorb impact deformation and toughen the whole structure as the loading proceeds. The effect of directional impact damage on post-impact performance was explored by performing compressing-after-impact (CAI) tests. It is revealed that the CAI properties along principal directions are more sensitive to the low-velocity impact, and the damage mode is significantly affected by the loading direction.

Controllable Crimpness of Animal Hairs via Water-Stimulated Shape Fixation for Regulation of Thermal Insulation.

Animals living in extremely cold plateau areas have shown amazing ability to maintain their bodies warmth, a benefit of their hair's unique structures and crimps. Investigation of hair crimps using a water-stimulated shape fixation effect would control the hair's crimpness with a specific wetting-drying process thereafter, in order to achieve the regulation of hair thermal insulation. The mechanism of hair's temporary shape fixation was revealed through FTIR and XRD characterizations for switching on and off the hydrogen bonds between macromolecules via penetration into and removal of aqueous molecules. The thermal insulation of hairs was regulated by managing the hair temporary crimps, that is, through managing the multiple reflectance of infrared light by hair hierarchical crimps from hair root to head.

The Study on Semi-Blunt Puncture Behavior of Nanofiber Membrane/ Non-Woven Composite Material.

BACKGROUND: Nanofiber membrane/non-woven composite material is composed of electrospinning nanofiber membrane and non-woven fabric, which combines the supporting role of nonwoven material and the special nano-size effect of nanomaterials. OBJECTIVE: These composite material can be widely used in biomedical, filtration and other related fields. In the actual use process, nanofiber membrane/non-woven composite material is often subjected to external forces such as puncture or bursting. As a result, the mechanical study of nanofiber membrane/ non-woven composite materials has a high value and practical significance. METHODS: The nanofiber membrane/non-woven composite material was obtained by spraying solution (different concentrations of titanium dioxide-loaded Poly (vinyl alcohol) (PVA)) on meltblown polyester non-woven fabric. The surface morphology and fiber diameter of different concentrations nanotitanium dioxide-loaded Poly (vinyl alcohol) fiber were investigated by Field Emission Scanning Electron Microscopy (FESEM). The surface distribution of TiO2 on the electrospun fibrous membranes was characterized by Energy Disperse Spectroscopy (EDS). The semi-blunt puncture behavior of different concentrations of nano-titanium dioxide-loaded nanofiber membrane/non-woven composite material was conducted by universal material machine. RESULTS: With the increase of concentrations of nano-titanium dioxide particles, the surface smoothness of nanofibers diminishes, the unevenness of the diameter distribution of the fiber increased and the maximum semi-blunt puncture strength increased. CONCLUSION: The addition of hard particles does contribute to improving the puncture properties of the composite materials. Several patents, related to electrospinning and bubble electrospinning equipment for nanofiber fabrication, have been reported.

Origin of tensile strength of a woven sample cut in bias directions.

Textile fabrics are highly anisotropic, so that their mechanical properties including strengths are a function of direction. An extreme case is when a woven fabric sample is cut in such a way where the bias angle and hence the tension loading direction is around 45° relative to the principal directions. Then, once loaded, no yarn in the sample is held at both ends, so the yarns have to build up their internal tension entirely via yarn-yarn friction at the interlacing points. The overall fabric strength in such a sample is a result of contributions from the yarns being pulled out and those broken during the process, and thus becomes a function of the bias direction angle θ, sample width W and length L, along with other factors known to affect fabric strength tested in principal directions. Furthermore, in such a bias sample when the major parameters, e.g. the sample width W, change, not only the resultant strengths differ, but also the strength generating mechanisms (or failure types) vary. This is an interesting problem and is analysed in this study. More specifically, the issues examined in this paper include the exact mechanisms and details of how each interlacing point imparts the frictional constraint for a yarn to acquire tension to the level of its strength when both yarn ends were not actively held by the testing grips; the theoretical expression of the critical yarn length for a yarn to be able to break rather than be pulled out, as a function of the related factors; and the general relations between the tensile strength of such a bias sample and its structural properties. At the end, theoretical predictions are compared with our experimental data.