Modeling oxidation damage of continuous fiber reinforced ceramic matrix composites

For fiber reinforced ceramic matrix composites（CMCs）,oxidation of the constituents is a very important damage type for high temperature applications. During the oxidizing process,the pyrolytic carbon interphase gradually recesses from the crack site in the axial direction of the fiber into the interior of the material. Carbon fiber usually presents notch-like or local neck-shrink oxidation phenomenon,causing strength degradation. But,the reason for SiC fiber degradation is the aw growth mechanism on its surface. A micromechanical model based on the above mechanisms was established to simulate the mechanical properties of CMCs after high temperature oxidation. The statistic and shearlag theory were applied and the calculation expressions for retained tensile modulus and strength were deduced,respectively. Meanwhile,the interphase recession and fiber strength degradation were considered. And then,the model was validated by application to a C/SiC composite.

Particle shape effects on ductility of SiC particle reinforced LD2 matrix composites investigated by AFM-based nanoindentation

An AFM (Atomic Force Microscope) based nanoindentation method for local measurement of mechanical properties near interfaces in both angular and blunted SiC particle reinforced LD2 composites is presented. The blunted composite exhibits an improved ductility than the angular counterpart. The nanoindentation examination shows that the micromechanical properties near interfaces distribute unevenly and vary with particle shape in the SiC p/LD2 composites. There are a higher nanohardness value and a lower plastic deformation capacity around an angular particle than around a blunted one. It is inferred that the residual stress and strain concentrations are severer around the angular particle, which causes matrix cracking at a lower external strain level and leads to a lower ductility of the angular composite.

Micromechanical characterization of microwave dielectric ceramic BaO-Sm_(2)O_(3)-5TiO_(2),by indentation and scratch methods

Mechanical characterization of dielectric ceramics,which have drawn extensive attention in wireless communication,remains challenging.The micromechanical properties with the microstructures of dielectric ceramic BaO-Sm_(2)O_(3)-5TiO_(2)(BST)were assessed by nanoindentation,microhardness,and microscratch tests under different indenters,along with the X-ray diffraction(XRD),scanning electron microscopy(SEM),and Raman spectroscopy.Accurate determination of elastic modulus(Err)(i.e.,260 GPa)and indentation hardness(Hrr)(i.e.,16.2 GPa)of brittle BST ceramic by the instrumented indentation technique requires low loads with little indentation-induced damage.The elastic modulus and indentation hardness were analyzed by different methodologies such as energy-based approach,displacement-based approach,and elastic recovery of Knoop imprint.Consistent values(about 3.1 MPa·m^(1/2))of fracture toughness(Kc)of BST ceramic were obtained by different methods such as the Vickers indenter-induced cracking method,energy-based nanoindentation approaches,and linear elastic fracture mechanics(LEFM)-based scratch approach with a spherical indenter,demonstrating successful applications of indentation and scratch methods in characterizing fracture properties of brittle solids.The deterioration of elastic modulus or indentation hardness with the increase in indentation load(F)is caused by indentation-induced damage and can be used to determine the fracture toughness of material by energy-based nanoindentation approaches,and the critical void volume fraction(f^(*))is 0.27(or 0.18)if elastic modulus(or indentation hardness)of the brittle BST ceramic is used.The fracture work at the critical load corresponding to the initial decrease in elastic modulus or indentation hardness can also be used to assess the fracture toughness of brittle solids,opening new venues of the application of nanoindentation test as a means to characterize the fracture toughness of brittle ceramics.

Micromechanical properties of poly(N-isopropylacrylamide)hydrogel microspheres determined using a simple method

Temperature-responsive poly（N-isopropylacrylamide） （PNIPAM） hydrogel microspheres have attracted extensive attention because of their promising diverse biomedical applications. A quantitative understanding of the micromechanical properties of these microspheres is essential for their practical application. Here, we report a simple method for the characterization of the elastic properties of PNIPAM hydrogel microspheres. The results show that PNIPAM hydrogel microspheres exhibit elastic deformation and the obtained force-deformation experimental data fits the Hertz theory well. The moduli of elasticity of the PNIPAM hydrogel microspheres prepared under different conditions were systematically investigated in this work for the first time. The PN1PAM hydrogel microsphere composition significantly affects their micromechanical properties and their temperature sensitivity behavior. PNIPAM hydrogel microspheres with a larger equilibrium volume change have a lower modulus of elasticity. The modulus of elasticity of the PNIPAM hydrogel microspheres at body temperature （37 ℃, above the lower critical solution temperature （LCST） of PNIPAM） is much higher than that at room temperature （25 ℃, below the LCST of PNIPAM） because ofthermo-induced volume shrinkage and an increase in stiffness. These results provide valuable guidance for the design of smart materials for practical biomedical applications. Moreover, the simple microcompression method presented here also provides a versatile way to investigate the micromechanical properties of microscopic biomedical materials.

Design and application of the dynamic detection platform for testing the micromechanical properties of agricultural products

The micromechanical properties of agricultural products can be used to formulate simulation models and optimize processing parameters for harvest,packaging,and storage.In this study,a detection platform was designed for conducting mechanical tests on different specimens at the tissue level.The system provided controllable and precise displacements;the relative error was less than±1.0%.The displacement and force measurements were calibrated.Micromechanical tests of apple parenchyma tissues were conducted to evaluate the performance of the proposed platform.The stress-strain curves of the specimens and the micrographs of the microstructures can be obtained simultaneously during the tests.The proposed platform has the capability to test mechanical properties and obtain micro behaviors of agricultural products at the tissue level,which helps in studying the failure mechanism of agricultural products under external loading.

Study of the Climbing Behavior of the Flagella of Calamus Simplicifolius Based on Micro-CT and Nanoindentation

Rattan is a typical tropical climbing plant that uses flagella to climb supports to grow.A comprehensive understanding of the anatomic structure and micromechanics of rattan flagella might inspire more research on biomimetic climbing materials.Here,the structure and micromechanical properties of flagella in calamus simplicifolius were examined by Micro-Computed Tomography(Micro-CT)and nanoindentation techniques,respectively.The results showed that the rachis of the flagella mainly comprised vascular bundles surrounded by basic tissues,which had a gradient density decreasing from outsides to insides.The prickles are derived from the epidermis or the epidermis and cortical tissue of the flagellum,which do not possess vascular tissue.The entire tip of the prickle was composed almost of fibrous cells.The indentation modulus of elasticity of the prickle was 17.03 GPa,which was 17.93%higher in comparison with the rachis.The hardness of the prickle was 539.27 MPa and was slightly higher than that of the rachis.The results indicated that the discrepancy of micromechanical strengths in different parts of flagella reflects on their unique roles in the process of climbing.