Some characteristics of low-speed streaks under sheared air-water interfaces

The characteristics of low-speed fluid streaks occurring under sheared air-water interfaces were examined by means of hydrogen bubble visualization technique. A critical shear condition under which the streaky structure first appears was determined to be u(tau) approximate to 0.19 cm/s. The mean spanwise streak spacing increases with distance from the water surface owing to merging and bursting processes, and a linear relationship describing variation of non-dimensional spacing <(+)over bar> versus y(+) was found essentially independent of shear stress on the interface. Values of <(+)over bar>, however, are remarkably smaller than their counterparts in the near-wall region of turbulent boundary layers. Though low-speed streaks occur randomly in time and space, the streak spacing exhibits a lognormal probability distribution behavior. A tentative explanation concerning the formation of streaky structure is suggested, and the fact that <(+)over bar> takes rather smaller values than that in wall turbulence is briefly discussed.

Effect of Particle Size on the Formation of Adiabatic Shear Band In Particle Reinforced Metal Matrix Composites

In this paper, the effect of particle size on the formation of adiabatic shear band in 2024 All matrix composites reinforced with 15% volume fraction of 3.5, 10 and 20 mum SiC particles was investigated by making use of split Hopkinson pressure bar (SHPB). The results have demonstrated that the onset of adiabatic shear banding in the composites strongly depends on the particle size and adiabatic shear banding is more readily observed in the composite reinforced with small particles than that in the composite with large particles. This size dependency phenomenon can be characterized by the strain gradient effect. Instability analysis reveals that high strain gradient is a strong driving force for the formation of adiabatic shear banding in particle reinforced metal matrix composites (MMCp).

Structural and Mechanical Properties of the Organic Matrix Layers of Nacre

The type of nanostructure referred to in biomineralization as a mineral bridge has been directly observed and measured in the organic matrix layers of nacre by transmission electron microscopy and scanning electron microscopy. Statistical analysis provides the geometric characteristics and a distribution law of the mineral bridges in the organic matrix layers. Experiments reveal that the nanostructures significantly influences the mechanical properties of the organic matrix layers. In addition, the mechanical analysis illustrates the effects of the nanostructures on the behaviors of the organic matrix layers, and the analytical results explain the corresponding experimental phenomena fairly well. The present study shows that the mineral bridges play a key role in the mechanical performances of the organic matrix layers of nacre. The results obtained provide a guide to the interfacial design of synthetic materials.

Investigation of Dynamic Fracture Toughness under Compressive-Shear Combinede Stress Wave Loading

提出亚微秒单脉冲应力波载荷作用下II型裂纹的平板冲击实验技术。加载率为dK/dt-10~8MPa·m~{”/d}·s~{-1}。实验中由锰铜应力片和弹性波理论分别测定和计算了压应力；通过微观分析确定了动态裂纹的平均扩展长度；引进等效应力强度因子,用动态断裂理论确定了60号钢的动态断裂韧性K_{Id}和K_{IId}；建立了亚微秒冲击载荷作用下确定材料动态断裂韧性的方法。

Experimental investigation of the shear strength of a unidirectional carbon/aluminum composite under dynamic torsional loading

In this paper, the dynamic shear strength of a unidirectional C/A356.0 composite and A356.0 alloy, respectively, are measured with a split Hopkinson torsional bar (SHTB) technique. The results indicate that the carbon fibers make very little contribution to the enhancement of the shear strength of the matrix material. The microscopic inspections on the fracture surface of the composite show a multi-scale zigzag feature. This implies that there is a complicated shear failure mechanism in the unidirectional carbon/aluminum composite.

The evolution of shear bands of saturated soil

The development of the shear bands of saturated soil in coupling-rate- and pore-pressure-dependent simple shear has been discussed, using a simple model and a matching technique at the moving boundary of a shear band. Tt is shown that the development of shear bands are dominated by the coupling-rate and pore-pressure effect of the material. The strength of the soil acts as a destabilizer, whilst pore pressure diffusion makes the band expand. The theory is discussed and some computational solutions have been presented.

Influence of the Processing Conditions on the Characteristics of the Clad Layers Produced with Laminar Plasma Technology

Laminar plasma technology was used to produce ceramic hardened layers of Al2O3-40% mass Ni composite powders on stainless steel substrates. In order to investigate the influences of processing conditions on the morphologies of the surface modified layers, two different powder-feeding methods were tested, one with carrier gas called the powder injection method, and the other without carrier gas called powder transfers method. The microscopic investigations demonstrate that the cross-section of the clad layers consists of two distinct microstructural regions, in which the Al2O3 phases exhibit different growth mechanisms. When the powder transfers method is adopted, the number density and volume fraction of the Al2O3 particles increase considerably and their distributions exhibit zonal periodical characteristics. When the powder-feeding rate increases, the microstructure of the Al2O3 phases changes from a small globular to a long needle shape. Finite element simulations show that the transient thermo-physical features of the pool substances, such as solidification rate and cooling rate, influence strongly the mechanisms of the nucleation and the directional growth of the Al2O3 phases in the thermal processing.

Microstructure and characteristics in the organic matrix layers of nacre

The direct observation of a type of microstructure in the organic matrix layers of nacre was obtained with a transmission electron microscope. The microstructure, which is referred to as mineral bridge in the biomineralization, is nanoscale and randomly distributed in the layers. Statistical analysis gives the distribution laws and characteristics of mineral bridges in the organic matrix layers. The existence of mineral bridges in nacre was confirmed, and it was shown that the microarchitecture of nacre should be described as a "brick-bridge-mortar" arrangement rather than traditional "brick and mortar" one.

Shear Strength Measurements in LY-12 Aluminium Alloy During Shock Loading

Lateral stress of LY-12 alummium alloy under plate impact shock loading was measured. Based on the measured data, the Hugoniot relation and shear strength were obtained. The result has demonstrated that the shear strenath of the tested material increases remarkably with the increasing longitudinal stress. This means that the assumption of constant shear strength usually adopted in shock stress calculation is not suitable for the present material.

A simple approach to the evaluation of fiber/matrix interfacial shear strength and fracture toughness

On the basis of microscopical analyses of the fiber distribution and longitudinal shear deformation in unidirectional fiber composites, a simple approach is presented for characterizing the interfacial sheer strength and fracture toughness.

Thermoplastic shear localisation in titanium alloys during dynamic deformation

Cylindrical specimens (4 mm diameter and 4 mm height) of titanium alloy bar were given various heat treatments to provide a wide range of microstructures and mechanical parameters. These specimens were then subjected to high plastic strain at a large strain rate (103 s-1 ) during dynamic compression by a split Hopkinson bar at ambient temperature. The microstructures of the localised shear bands were examined by optical and transmission electron microscopy. The results show that there are two types of localised shear bands: deformed and white shear bands. A detailed observation reveals that there is no difference in the nature of the deformed and white shear bands, but they occur at different stages of localised deformation. It is found that there is a burst of strain, corresponding to a critical strain rate at which the white shear band occurs and no phase transformation occurs in the shear bands.