Mechanical properties and indentation-induced damage of high-quality ZnO microwires

In this paper, we aim to discover the mechanics-related novel phenomena on high-quality ZnO microwires through nanoindentation method. The mechanical property parameters of ZnO microwires were determined and a distinctive creep damage phenomenon was revealed. By using the cycles load mode in the nanoindentation experiments, the indentation size effect was demonstrated to exist in the ZnO microwires: the elastic modulus ranged from 65.9 GPa to 12.3 GPa and the hardness changed from 11.1 GPa to 1.8 GPa with the increase of indentation depth. Furthermore, the indentation-induced mechanical and electrical damage caused a permanent plastic deformation and an increase of 41% in the longitudinal conductance of the ZnO microwires.     

Computational modelling of static indentation-induced damage in glass

The present paper is focused on the numerical simulation of a glass plate subjected to static indentation by a spherical indenter. For this purpose, a combined approach of continuum damage mechanics (CDM) and fracture mechanics is performed. Results provided by an axisymmetric finite element model were compared with analytical solutions. A CDM based constitutive model with an anisotropic damage tensor was selected and implemented into a finite element code to study the damage of glass. The numerical results were analysed through the framework of the stress and damage distribution. Various regions with critical damage values were therefore predicted in good agreement with the experimental observations in the literature. In these regions, the directions of crack propagation, including both cracks initiating on the surface as well as in the bulk, were predicted using the strain energy density factor. Predicted directions were found in good agreement with those experimentally obtained in the literature results.     

Observation of microstructure deformation and damage in nuclear graphite

A full field strain mapping technique is employed to obtain in situ, load resolved, strain fields from the surface of disc compression and tensile test specimens of Gilsocarbon IM1-24 nuclear graphite. Analysis of these fields is used to show the development of the distribution of localised deformation, which can be linked to the microstructure and damage processes. Cracks are observed to nucleate at porosity, and to propagate by coalescence. The stability of the cracks depends on the stress state. Such observations may be used to verify microstructure-based models for graphite failure.     

Fabrication and Corrosion Resistance of SiC-coated Multi-walled Carbon Nanotubes

Multi-walled carbon nanotubes （MWNTs） were coated with nanometer sized SiC layer by magnetron sputtering, and their corrosion resistances in oxygen and gaseous silicon were studied, respectively. X-ray diffraction （XRD） reveals that the as-coated MWNTs are comprised of the amorphous SiC and the phase could transfer from amorphous to polycrystalline SiC by annealing at 1360 °C. Scanning electron microscopy （SEM） observation indicates that SiC nanoparticles are uniformly coated on the MWNTs, forming a continuous SiC coating. Thermogravimetric analysis （TGA） confirms that the onset temperature of oxidation increases from 540 to 700 ~C with the SiC coating. The morphologies of non-coated and SiC-coated MWNTs after corrosion by oxygen and gaseous silicon were also characterized by SEM. The results revealed that SiC coating could protect MWNTs from the corrosion with qaseous silicon effectivelv.     

Vitreous joining of SiC-coated carbon/carbon composites

This paper investigated the feasibility of joining of carbon/carbon (C/C) composites by using a silicate as interlayer. A SiC coating was pre-prepared on C/C substrate by pack-cementation technique to improve the wettability of glass on C/C composites, then the joints were prepared by a one-shot and low cost way. Microstructure and morphologies of the as-received joints were characterized by XRD, SEM and EDS. The results indicated that the SiC coating not only had a strong bonding with C/C composites, but also had a good physical and chemical compatibility with silicate glass. The room-temperature shear strength of the joints gives encouraging results, which can be up to 26 MPa. The fracture mode and the fracture behavior were discussed also.     

Fatigue damage mechanisms and residual properties of graphite/epoxy laminates

Damage mechanisms and accumulation, and associated stiffness and residual strength reductions were studied in cross-ply graphite/epoxy laminates under cyclic tensile loading. Stress-life data were fitted by a two-parameter wearout model and by a second-degree polynomial on a log-log scale. The fatigue sensitivity is highest for the unidirectional laminates and it decreases for the crossply laminates with increasing number of contiguous 90° plies. Five different damage mechanisms were observed: transverse matrix cracking, dispersed longitudinal cracking, localized longitudinal cracking, delaminations along transverse cracks, and local delaminations at the intersection of longitudinal and transverse cracks. Failure patterns vary with cyclic stress level and number of cycles to failure. Under monotonie loading, failure is brittle-like and concentrated. At high stress amplitudes and short fatigue lives failure results from few localized flaws, whereas at lower stress amplitudes and longer fatigue lives failure results from more dispersed flaws. The residual modulus shows a sharp reduction initially, followed by a more gradual decrease up to failure. The residual strength showed a sharp reduction initially, followed by a plateau or even some increase in the middle part of the fatigue life, and a rapid decrease in the last part of the fatigue life. A tentative cumulative damage model is proposed based on residual strength and the concept of equal damage curves.     

The effect of indentation-induced cracking on the apparent microhardness

The phenomenon of apparent microhardness increase with increasing applied indentation test load, the reverse indentation size effect (RISE), was addressed from the viewpoint of indentation-induced cracking. The apparent microhardness when the cracking occurs was found to be related to the applied indentation test load as P ^5/3. Previously published results on single crystals of silicon, GaAs, GaP and InP, which differ by a factor of four, all fall on the same line when analysed through this concept. It is concluded that the RISE is a result of the specimen cracking during the indentation.     

几种单晶半导体材料在压痕下的变形与断裂行为比较

采用显微压痕方法研究了Si、Ge、GaAs和InP四种半导体单晶的变形与断裂行为.通过测量[100]取向单晶体面内的显微硬度,裂纹开裂的临界压痕尺寸以及断裂韧性,分析了这四种材料力学性能的面内各向异性行为.结果表明:在压痕载荷的作用下,Si和Ge的塑性变形以剪切断层为主,而GaAs和InP则通过滑移系的开动协调变形.[100]取向的Si、Ge、GaAs和InP四种单晶的面内显微硬度、弹性模量和断裂韧性表现出不同程度的各向异性.裂纹长度与压痕尺寸间的关系表明,与GaAs和InP相比,Si、Ge具有较小的临界压痕尺寸和拟合直线斜率,这一临界压痕尺寸和拟合直线斜率的变化规律分别与材料的硬度和断裂韧性的变化规律一致.     

High-temperature oxidation behavior of SiC-coated carbon fiber-reinforced carbon matrix composites

The high-temperature oxidation behavior of bare and SiC-coated carbon fiber-reinforced carbon matrix (C/C) composites was examined in the temperature range 900–3000 K. In the course of the measurements, the main focus was placed upon the effect of oxygen partial pressure on the oxidation behavior and the transition temperature between the passive and active oxidation regimes. To understand the oxidation behavior of SiC-coated C/C composites quantitatively, a morphological characterization of coating cracks was carried out, and then, in the oxygen diffusion controlling temperature range, an analytical model was developed for the prediction of weight loss due to oxidation of SiC-coated C/C composites. The oxidation rates derived from this model were in fairly good agreement with the experiment results.     

里氏硬度计测量布氏硬度与洛氏硬度误差分析

本文介绍了里氏硬度计的使用现状,并以实际使用情况设计了两组实验,以标准硬度块为标准,分析了里氏硬度计测量布氏硬度与洛氏硬度误差,并给出了使用里氏硬度计进行测量的建议。     

Characterization of damage initiation and propagation in graphite/epoxy laminates by acoustic emission

A method of characterizing two major failure processes in graphite/epoxy compositestransverse cracking and mode I delamination, has been studied. Representative laminates were tested in uniaxial tension and flexure. The failure processes were monitored and characterized by acoustic emission (AE). The effects of moisture on AE were also investigated. Each damage process was found to have a distinctive AE output and to follow a definite trend which was significantly affected by moisture conditions. It was concluded that AE can serve as a useful tool for detecting and identifying these failure modes in composite structures in laboratory and in service environments.     

The effect of indentation-induced microcracks on the elastic modulus of hydroxyapatite

The presence of microcracks in materials affects a wide range of mechanical properties including elastic modulus, Poisson’s ratio, fracture strength, and fracture toughness. The microcrack-induced reductions of the Young’s modulus, E, and Poisson’s ratio, υ, are functions of the size, geometry, and number density of microcracks. In this study, an array of Vickers indentation-induced microcracks was placed on the surfaces of two hydroxyapatite (HA) specimens with totals of 391 and 513 indentations per specimen. This study tests the validity of theoretical studies of microcrack damage-induced changes in E and υ, where the changes are expressed either by (i) the volumetric crack number density, N and (ii) the crack damage parameter, ε. All elasticity measurements were done via resonant ultrasound spectroscopy. For both the HA specimens included in the study and alumina specimens indented in an earlier study [J Mater Sci 38:1910. doi: , 1], E and υ decreased approximately linearly with increasing microcrack damage. The slopes of the E and υ versus N and ε are also computed and compared to the available theoretical models.     

Thermal damage effects on delamination toughness of a graphite/epoxy composite

The effects of overheating AS/3501-6 graphite/epoxy composite material have been examined after exposures in air up to 350°C. Degradation was assessed from measurements of the interlaminar fracture toughness in both Mode I tension and Mode II shear as well as the interlaminar shear strength and hardness. Exposures up to 30 minutes at 225°C and up to 15 minutes at 300°C did not significantly reduce the interlaminar toughness. However, longer exposure at 300°C and short exposures at 350°C produced catastrophic decreases in toughness values, shear strength and hardness. The primary origin of embrittlement was attributed to deterioration of the epoxy as opposed to any fibre or fibre-matrix interfacial weakening.     

Chemical sputtering and surface damage of graphite by low-energy atomic and molecular hydrogen and deuterium projectiles

We present experimental methane production yields for H⁺, ions incident on ATJ graphite in the energy range 10–250 eV/H. Below about 60 eV/H, the molecular H species give higher methane yields/H when compared with isovelocity H⁺, similar to our earlier measurements for incident deuterium atomic and molecular ions. For both D and H atomic and molecular projectiles, the yields/atom coalesce onto a single curve below projectile energies of 60 eV/atom, when plotted as a function of maximum energy transfer, under the assumption that, below this energy, the incident molecular species are largely undissociated when undergoing C–C bond breaking collisions during their collision cascade and thus produce more damage. Raman spectroscopy of a graphite sample exposed to high fluences of D⁺ and beams at high and low energies qualitatively confirmed the assumption that more surface damage is produced by the low-energy incident molecular species than by isovelocity atomic ions. While the two high-energy beam-exposed spots showed similar damage, the low-energy molecular-beam-exposed spot showed slightly more damage than the corresponding D⁺-beam-exposed spot.