Mechanical behaviors and damage constitutive model of ceramics under shock compression

One-stage light gas gun was utilized to study the dynamic mechanical properties of AD90 alumina subjected to the shock loading. Manganin gauges were adopted to obtain the stress-time histories. The velocity interferometer system for any reflector （VISAR） was used to obtain the free surface velocity profile and determine the Hugoniot elastic limit. The Hugoniot curves were fitted with the experimental data. From Hugoniot curves the compressive behaviors of AD90 alumina were found to change typically from elastic to ＂plastic＂. The dynamic mechanical behaviors for alumina under impact loadings were analyzed by using the path line principle of Lagrange analysis, including the nonlinear characteristics, the strain rate dependence, the dispersion and declination of shock wave in the material. A damage model applicable to ceramics subjected to dynamic compressive loading has been developed. The model was based on the damage micromechanics and wing crack nucleation and growth. The effects of parameters of both the micro-cracks nucleation and the initial crack size on the dynamic fracture strength were discussed. The results of the dynamic damage evolution model were compared with the experimental results and a good agreement was found.

Functional Morphology and Bending Characteristics of the Honeybee Forewing

The present work aimed to reveal the functional morphology and bending characteristics of the worker honeybee （Apis mellifera） forewing. Honeybee wings including the forewing and hindwing, which are mainly composed of veins and membranes, are a kind of typical hierarchical biomaterials. We investigated the cross-sections of membranes, veins and wing hairs through Scanning Electron Microscopy （SEM）. Based on the microscopic observation, it was found that the vein is a thick-walled cylinder, and the membrane possesses multilayered structure and so does the wing hair which shows the thread surface. At the vein-membrane conjunctive position, membranes and veins are assembled seamlessly and veins are packed smoothly and tightly by membranes into a whole, allowing honeybees to perform excellent flapping flight. In such a case, we also conducted the cantilevered bending experiment of honeybee forewing to explore their bending characteristics using a MTS Tytron 250 micro force tester. Experiment results indicate that the anti-bending capacity of the forewing along the spanwise direction is higher than that along the chordwise direction which is partly caused by the wing corrugation along the wing span detected by the micro-Computed Tomography （micro-CT）, and ventral load bearing ability is better than dorsal one along the spanwise and chordwise direction of the wing which is due to the stress-stiffening of membranes. It could be concluded that the structural configuration of the wing is closely relevant to wing biomechanical behaviors. All results above would provide a significant support for the design of bioinspired wings for Flapping Micro Aerial Vehicles （FMAV）.

A mass abrasion model with the melting and cutting mechanisms during high-speed projectile penetration into concrete slabs

In this study,ogive-nose projectile penetration into concrete slabs was tested at initial projectile impact velocities ranging from 1325.0 m/s to 1425.0 m/s.The depth of penetration and mass loss of the projectiles were measured,and the residual projectiles were recovered after the penetration tests.Scanning electron microscopy and metallographic microscopy of the microstructures were performed on various sections and outer surfaces of the projectiles taken from different locations of the residual projectiles,to analyze the intrinsic mechanisms of mass abrasion.The analysis results reveal that,during high-speed projectile penetration,projectile abrasion is caused by multiple mechanisms.Based on the cavity expansion theory,a projectile penetration model was established by considering the two main mass loss mechanisms observed in the microscopic tests.The theoretical predictions of the penetration depth,mass loss rate,and change of projectile head are consistent with the experimental results obtained both in this study and previous research.

Study on cell size variation in overdriven gaseous detonations

The cell size variation in overdriven gaseous detonations is studied in hydrogen/oxygen and acetylene/oxygen mixtures.The local self-similarity of Mach reflection of detonations on the wedge in the far field renders the presence of a steady overdriven Mach stem to be possible.The study focuses on the cell size change of overdriven Mach stem on the wedge surface other than on the sidewall.The detonation cell pattern on the wedge surface has a complicated process of three-stage pattern,i.e.,the cells decreasing from large to small size,and then increasing asymptotically to a medium size and keeping constant.The cell size ratio with increasing the degree of overdrive is also examined.It is found that the ratio decays as the degree of overdrive increases.However,as the wedge angle increases to a critical value,finer cells are not created on the smoke foils.Ng’s model used to predict the cell size is also found to be valid only for detonations with relative large instability parameters,but presents large errors for highly overdriven detonations with low instability.A modification to Ng’s model is proposed based on the experimental results.