Results 1 - 10 of 14
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[en] The present study investigated the damage resistance of two dimensional carbon fiber reinforced silicon carbide (C/SiCs) composites subjected to low velocity impact (LVI). Damage microstructures of specimens under different impact energies (Ei) were characterized by infrared thermography, X-ray computed tomography and scanning electron microscopy. The real damage radii of specimens were found to change slightly with Ei, whereas apparent damage radii where much larger. Overall, the fabricated 2D C/SiC composites exhibited good damage resistance to LVI with nominal post-impact tensile strengths remaining at 89.4%, 83.35%, 76.97%, and 74.84% of their pre-impacted counterpart of 158 MPa, for impact energies of 3, 4, 5, and 6 J, respectively. Compared with the as-received one, after LVI real tensile strengths of the C/SiC composite specimens increased by 5.84% for the Ei of 3 J, 9.27% for 4 J, −1.83% for 5 J, −3.16% for 6 J.
[en] The current study reports the enhancement of mechanical properties of carbon fiber-reinforced silicon carbide ceramic matrix composites (C/SiC CMCs) by the application of a carbon nanotube/silicon carbide (CNT/SiC) coating. CNTs were deposited on the surfaces of C/SiC composites using electrophoretic deposition (EPD), after which infiltration by SiC was achieved through a chemical vapor infiltration process. An EPD duration of 5 min was associated with a 40% increase in the ultimate flexural strength relative to that of composites with a pure SiC coating. The observed enhancement was rationalized by the microstructural observations of SiC infiltration into the porous CNT morphology and the subsequent formation of CNT/SiC layers on the surfaces of the composites and by the inherent toughness of the SiC whiskers. The flexural strength decreased with EPD durations greater than 5 min due to the formation of thick CNT meshes, which decreased the open porosity and thereby obstructed further SiC infiltration. This is a viable methodology for the improvement of mechanical properties of CMCs by the introduction of a ceramic coating containing CNT.
[en] The intensity distribution in Fresnel diffraction through a slit includes numerous small fluctuations referred to as ripples. These ripples make the modelling of the intensity distribution complicated. In this study, we examine the characteristics of the Fresnel diffraction intensity distribution to deduce the rule for the peak position and then propose two types of quantum-mathematical models to obtain the distance between the edge and the peak point. The analysis and simulation indicate that the error in the models is below . The models can also be used to detect the edges of a diffraction object, and we conduct several experiments to measure the slit width. The experimental results reveal that the repetition accuracy of the method can reach . (paper)
[en] Density defect in a 3D needled C/SiC composite was successfully prepared by chemical vapor infiltration. Infrared thermal wave imaging, X-ray radiographic methods were used to examine the sensibility to the density defect in the C/SiC composite. Mechanical properties of the C/SiC composite containing the density defects were investigated by the three-point bending tests. Results show that both thermography and radiography methods can detect the density defect of the C/SiC composites, and the former is more effective than the latter especially with respect to detecting the superficial gas pores and delamination. There exists a critical defect ration Rd of 30% (i.e., ration of defect volume to the whole specimen volume) about the effect of the density defect on the flexural strength of composites. With the increase of the defect length, the material strength decreased when the density defect ration Rd < 30%, and then the material strength slightly recovered when the Rd > 30%.
[en] The fracture toughness of a carbon fiber reinforced silicon carbide composite was investigated relating to classical critical stress intensity factor KIC, work of fracture, and acoustic emission energy. The KIC was obtained by the single edge notch beam method and the work of fracture was calculated using the featured area under the load–displacement curves. The KIC, work of fracture, and acoustic emission energy were compared for the composites before and after heat treatment and then analyzed associated with toughening microstructures of fiber pullout. It indicates that the work of fracture and acoustic emission energy can be more suitable to reflect the toughness rather than the traditional KIC, which has certain limitation for the fracture toughness characterization of the crack tolerant fiber ceramic composites.
[en] This article motivates and presents the scale relativistic approach to non-differentiability in mechanics and its relation to quantum mechanics. It stems from the scale relativity proposal to extend the principle of relativity to resolution-scale transformations, which leads to considering non-differentiable dynamical paths. We first define a complex scale-covariant time-differential operator and show that mechanics of non-differentiable paths is implemented in the same way as classical mechanics but with the replacement of the time derivative and velocity with the time-differential operator and associated complex velocity. With this, the generalized form of Newton’s fundamental relation of dynamics is shown to take the form of a Langevin equation in the case of stationary motion characterized by a null average classical velocity. The numerical integration of the Langevin equation in the case of a harmonic oscillator taken as an example reveals the same statistics as the stationary solutions of the Schrödinger equation for the same problem. This motivates the rest of the paper, which shows Schrödinger’s equation to be a reformulation of Newton’s fundamental relation of dynamics as generalized to non-differentiable geometries and leads to an alternative interpretation of the other axioms of standard quantum mechanics in a coherent picture. This exercise validates the scale relativistic approach and, at the same time, it allows to envision macroscopic chaotic systems observed at resolution time-scales exceeding their horizon of predictability as candidates in which to search for quantum-like dynamics and structures.
[en] Properties of a carbon fiber reinforced silicon carbide matrix composite were investigated in controlled environments including constant load, thermal cycling and wet oxygen atmosphere. Damage was assessed by residual mechanical properties and scanning electron microscopy characterization. Thermal strain was shown to change with cyclic temperatures over the same period (120 s). Strain varies approximately from the initial linear elastic strain of 0.63% to the final nonreversible damage strain of 1.6% during the short time of the test. The experimental strain difference between two selected temperatures is about 0.16% and the theoretical calculation value is 0.1566%. After 50 thermal cycles, the Young's modulus of the composites is reduced by a factor of 0.5 while the residual strength still retains 82% of the initial strength. It is observed that matrix cracks transversely and wave-shaped cracks are arranged on the coating surface at relatively regular spacing. A typical superficial oxidation can be found along the opening and propagating cracks beneath the coating
[en] Emodin (1,3,8-trihydroxy-6-methylanthraquinone), a major constituent of rhubarb, has a wide range of therapeutic applications. Previous studies have established that emodin inhibits cell proliferation and induces caspase 3-dependent apoptosis. However, its side-effects, particularly those on embryonic development, have not been well characterized as yet. In the current study, we examined the cytotoxic effects of emodin on mouse embryos at the blastocyst stage, subsequent embryonic attachment and outgrowth in vitro, and in vivo implantation by embryo transfer. Blastocysts treated with 25–75 μM emodin exhibited significantly increased apoptosis and a corresponding decrease in total cell number. Notably, the implantation success rate of blastocysts pretreated with emodin was lower than that of their control counterparts. Moreover, in vitro treatment with 25–75 μM emodin was associated with increased resorption of post-implantation embryos and decreased fetal weight. With the aid of an in vivo mouse model, we showed that consumption of drinking water containing emodin led to apoptosis and decreased cell proliferation, and inhibited early embryonic development to the blastocyst stage. Our findings support a degree of selective inhibition of retinoic acid receptors in blastocysts treated with emodin. In addition, emodin appears to induce injury in mouse blastocysts through intrinsic apoptotic signaling processes to impair sequent embryonic development. These results collectively indicate that emodin has the potential to induce embryonic cytotoxicity.
[en] Hi-Nicalon fibers were exposed in 8% O2/78% Ar/14% H2O atmosphere for 1 h at 1300, 1400, 1500, 1600 deg. C, respectively. Residual tensile strength was evaluated by tensile test, phases in the fibers were identified using an X-ray diffractometer (XRD), morphology of the fracture surfaces and microstructure was observed by scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. Results indicated that residual tensile strength increased with increasing temperature from 1300 to 1500 deg. C, then decreased after annealing in 1600 deg. C. The grain size of β-SiC and the amount of the stacking faults increased under the elevated temperature as well. After annealing, a passive film with a structure of α-cristobalite crystals dispersed in amorphous SiO2 phase formed on the fiber surface, the thickness of the film increased with the annealing temperature from 1300 to 1500 deg. C, after annealing in 1600 deg. C, fractional silica film spalled. Finally, relationship between the structural changes and the mechanical properties, the control effect of water vapor on formation and structural evolution of the passive film were discussed
[en] Highlights: ► A C/SiC was exposed in O2, AO, mode A (first O2 and then AO) and mode B (first AO and then O2) oxidation. ► The O2 produced SiO2 layer on the specimens, but the AO can preferentially etch off the Si of SiC. ► The mode B oxidation can induce more serious degradation for specimen. ►The mechanisms of the coupled oxidations were proposed in this study. - Abstract: The advanced high temperature composites have been considered as structural or functional material in complex and harsh environments, which usually comprise of various oxidizing particles such as molecular oxygen (MO), atomic oxygen (AO) and molecular water. This study aimed to investigate C/SiC behaviors in single factor or combination environments. In the MO oxidation, an inert surface is formed on SiC. At the AO oxidation, the Si of SiC is preferentially etched off leaving a certain depth of C layer on the sample. Two types of coupled oxidations defined as mode A (first MO and then AO oxidation), and mode B oxidation (first AO and then MO oxidation) were investigated systematically in this study, and found that the prior oxidative effect majorly determined the corresponding coupled oxidation behavior. From microscopic observation and fractural test, the mode B oxidation induced a more serious degradation upon the material than that of the mode A, which is illustrated by the proposed erosion mechanisms for the modes A and B.