The effect of biaxiality of loading in the orientation of caustics

The influence of biaxiality of loading on the orientation of caustics formed from the tips of a slant crack in an infinite plate was studied. Some intricate results on the subject were, also, emphasized. The correlation between characteristic values of stress-intensity factors and several combinations of the biaxiality ratio and the angle of inclination was established.     

Effects of particle size and region width on the mixing and dispersion of pebbles in two-region pebble bed

Granular systems and their mixing processes are widely used in various industries. The effects of size ratio and region width on the mixing and dispersion of pebbles in the two-region pebble bed are studied by discrete element method (DEM). The particle-scale mixing index (PSMI) and mixing entropy are used to quantify the particle mixing process. A similarity of mixing under different discharge rates is found after a stable two-region distribution structure is established. From qualitative observation and quantitative analysis, the mixing region is complicated in shape for the cases with greater particle size ratios. There is an increasing tendency of PSMI for two-region pebble bed when the region ratio or size ratio rises. The size ratio has greater effects on the mixing and dispersion of pebble than the region ratio, since the occupied volumes and entropies in the mixing region in the relatively steady state are more strongly affected by the size ratio rather than by the region ratio. As particles in the side regions can be easily driven by large ones, the stagnant region will decrease greatly when large size particles flow in the central region.     

Influence of loading path on fatigue crack growth under multiaxial loading condition

In this study, the specimens made of carbon steel S45 with an initial surface straight edge notch were subjected to combined cyclic axial‐torsion loading at room temperature. The fatigue life, surface crack extension direction and crack length were experimentally investigated. The effects of loading path, stress amplitude ratio and phase angle on the crack growth behaviour were also discussed. The results showed that, under the combination of cyclic axial and torsion loading, the tension stress amplitude had more effect on the initial crack growth path than the latter. The shear stress amplitude contributed mainly to the latter crack extension. The crack extension path was mainly determined by the stress amplitudes and the ratio of the normal stress to the shear stress, and almost independent of the mean stresses. The increase of the tension stress amplitude and shear stress amplitude would both accelerate the crack growth rate.     

The loading history and crystal orientation effects on the size-dependency of single crystal diamond properties

Much research has been conducted to study the size-dependency of material properties, as can be found from the open literature. Recent results on combined size, rate and thermal effects further demonstrate the dominant influence of specimen size on material strength, as compared with the loading rate and thermal effects. However, little has been done to understand the loading history and crystal orientation effects on the size-dependency of material properties. To evaluate the safety and integrity of MEMS devices under general loading conditions, a series of molecular dynamics simulations are performed to investigate the size-dependency of single crystal diamond properties with various crystal orientations under shear/tension and tension/shear loading conditions. It appears from the preliminary findings that the loading history and crystal orientation do have certain influence on the size-dependency of material properties. Specifically, the failure pattern is insensitive to the loading history, which provides useful information for formulating a multi-scale material model under general loading conditions.     

The effect of cristallographic orientation on damage in MgO due to spherical particle impact

A single stage gas gun has been used to fire spherical chrome steel and WC particles at variously oriented and prepared MgO single crystals at velocities of up to 350 m sec^–1. The resultant damage has been studied by optical and scanning electron microscopy, dislocation etching and surface profilometry, and mass losses have been determined gravimetrically. The measured crater dimensions and the mass loss data have been compared with the predictions of a simple analytical model of the impact event.     

The influence of biaxiality of loading on the form of caustics in cracked plates

Summary The optical method of reflected caustics was applied up-to-now to problems of cracked plates under uniaxial loading. Only the problem of the biaxial tension of the plate has been considered for the particular case where the crack is transverse to the longitudinal axis of the plate which coincided with the loading axis. In this paper the influence of a biaxial loading of the plate on the form and orientation of the caustic was studied in connection with the orientation of the crack. New modified relations were given for the evaluation of the complex stress intensity factorK=K _I –iK _II in terms of the angle of the angular displacement of the caustic axis. For the accurate evaluation ofK _I andK _II nomograms of correction factors _y ^max , _x ^max and _x ^max were given in terms of the angle of inclination of the crack =(90–) and the biaxiality factork. Experimental evidence with PMMA internally cracked plates corroborated the results of theory.List of Symbols (z), (z) complex-stress function of Muskhelishvili - _xx , _yy , _xy crack tip stress referred to Cartesian coordinate system - r, polar coordinate system centered at crack tip - K _I ,K _II stress intensity factors for ModeI andII loading, respectively - angle of inclination of the crack - 90°- - k ratio of stresses at infinity - ₁, ₂ principal stresses at crack tip - a crack length - stress applied at infinity along the transverse boundaries of the plate - X _r,f ,Y _r,f parametric equations of the reflected caustics referred to the Cartesian systemOXY on the reference screen: (r) reflected caustics from rear face of the specimen and (f) reflected caustics from the front face of the specimen - r ₀ radius of the generatrix curve on the specimen around the crack tip (initial curve) - c _r,f optical constants of the material for reflections from the rear and front faces of the specimen respectively - _m magnification ratio of the optical set-up - z ₀ distance between the reference-screen and the middle plane of the specimen - z _i distance between the focus of the light beam and the middle plane of the specimen - d thickness of specimen - 2 for the reflected caustics from the rear face of the specimen and 1 for the reflected caustics from the front face of the specimen - C _r,f z ₀ dc _r,f / _m (2)^1/2 - v Poisson's ratio - E elastic modulus of the material - A (1+k)+(1–k) cos 2 - B (1–k) sin 2 - C 1+k ²+(1–k ²) cos 2 2 tan^–1 (B/A)=2 tan^–1 (K _II/K_I) - D _y ^max ,D _x ^max ,D _x ^min the maximum and the ninimum diameter of caustics along the axisOy andOx of the crack respectively - _y ^max , _x ^max , _x ^min the correction factors forD _y ^max ,D _x ^max andD _x ^min respectively - D _t ^max ,D _l ^max the maximum transverse and longitudinal diameters of the caustics respectively - _t ^max , _l ^max the correction factors forD _t ^max ,D _l ^max respectively With 13 Figures     

活化条件对提纯海泡石脱色性的影响

海泡石因有着独特的结构,具有开阔的孔道和较大的比表面积,它拥有良好的吸附脱色性及离子交换性等物理化学性能。以川北朝天海泡石为原料,研究了不同活化条件(酸浓度、温度、时间)对活性白土脱色性能的影响。通过研究取得的主要成果:活化酸浓度为0.6mol/L,时间为60min,温度为100℃时活化后的海泡石脱色力最好。最后得出结论,活化酸浓度、时间、温度对海泡石脱色性有影响,并得到活化的最佳酸浓度、时间及其温度。     

The influence of crystallographic orientation on the wetting of silicon on quartz single crystals

Dynamic hexagonal spreading patterns of small silicon droplets on the basal plane (001) of quartz were observed by video microscopy. A detailed analysis of the hexagonal triple line demonstrates that the patterns show slight chiral distortions that can be attributed to the screw axis of the substrate crystal. This article reveals the detailed influence of crystal symmetry on the anisotropy of reactive wetting. In this context, a first discussion about the interplay of wetting and etching of a crystal is provided.     

溅射时间对Mo薄膜结晶取向的影响

采用直流磁控溅射法在SLG衬底上沉积Mo薄膜,并用XRD、SEM、四探针等对薄膜进行表征,研究了沉积时间对薄膜晶体结构、表面形貌以及电学性能的影响。研究发现,沉积时间能够调节Mo薄膜的择优取向。溅射时间较短(5～10min)时,沉积的Mo薄膜呈(110)择优取向。溅射时间超过15min后,薄膜呈现(211)取向,且(211)晶面择优程度随沉积时间的增加而提高。随着择优取向的改变,薄膜的表面形貌由三角形颗粒变为长条形颗粒,电阻率也发生相应变化,由3.92×10-5Ω·cm增加到4.27×10-5Ω·cm再降低,对应薄膜生长的晶带模型由晶带T型组织变为晶带2组织。     

Effects of non-proportional loading paths on the orientation of fatigue crack path

Fatigue crack path prediction and crack arrest are very important for structural safety. In real engineering structures, there are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path, etc. In this paper, both experimental and numerical methods are applied to study the effects of loading path on crack orientations. Experiments were conducted on a biaxial testing machine, using specimens made of two steels: 42CrMo4 and CK45 (equivalent to AISI 1045), with six different biaxial loading paths. Fractographical analyses of the plane of the stage I crack propagation were carried out and the crack orientations were measured using optical microscopy. The multiaxial fatigue models, such as the critical plane models and also the energy‐based critical plane models, were applied for predicting the orientation of the critical plane. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the shear‐based multiaxial fatigue models provide good predictions for stage I crack growth for the ductile materials studied in this paper.     

The effect of particle orientation and/or position on two-dimensional shape measurements

Particle shapes were measured based on the random positioned (R) method, the stable positioned (S) method and the stable positioned/constant orientated (S /C) method. The mean values ϕ and ζ of two shape indices (circularity ϕ and smoothness ζ), previously defined, were calculated. It was found that smoothness ζ was little affected by shape measuring methods, while circularity ϕ differed according to methods. By analyzing the change in circularity using model ellipsoidal particles, it was confirmed that the R method could more effectively evaluate particle shapes than can the S method, based on the results of the S/C method.     

The influence of heterogeneous meninges on the brain mechanics under primary blast loading

In the modeling of brain mechanics subjected to primary blast waves, there is currently no consensus on how many biological components to be used in the brain–meninges–skull complex, and what type of constitutive models to be adopted. The objective of this study is to determine the role of layered meninges in damping the dynamic response of the brain under primary blast loadings. A composite structures composed of eight solid relevant layers (including the pia, cerebrospinal fluid (CSF), dura maters) with different mechanical properties are constructed to mimic the heterogeneous human head. A hyper-viscoelastic material model is developed to better represent the mechanical response of the brain tissue over a large strain/high frequency range applicable for blast scenarios. The effect of meninges on the brain response is examined. Results show that heterogeneous composite structures of the head have a major influence on the intracranial pressure, maximum shear stress, and maximum principal strain in the brain, which is associated with traumatic brain injuries. The meninges serving as protective layers are revealed by mitigating the dynamic response of the brain. In addition, appreciable changes of the pressure and maximum shear stress are observed on the material interfaces between layers of tissues. This may be attributed to the alternation of shock wave speed caused by the impedance mismatch.     

Biolevitation of pebbles on desert surfaces

Certain desert surfaces called desert pavements are characterized by a nearly stone-free layer of fine granular material, mostly silt (diameter of hundredths of mm) and sand (tenths of mm) capped by a monolayer of pebble-sized (few cm) stones. The fine granular material is deposited as dust from the air, so the stones must be levitated to accommodate emplacement of this material. In so-called “normal grading” of stratigraphic deposits of geologic material, as in many stream deposits, finer grained material tends to settle on top of coarser grains which have faster settling velocities. Anecdotal observations in the field suggested that the observed “reverse grading” of pavement surfaces is a consequence of the activity of small animals that provide through their activities sufficient input of energy to the surface to cause large particles to “float” on the accumulating fine sediment. To test this idea a laboratory experiment with a test organism, the large desert beetle Eleodes, was carried out to observe possible sorting process. Essentially random foraging motions of the beetles in an environment in which fine sediment was periodically added from above led to sorting of stones that were moveable by the organisms, burial of immovable stones, as well as burial and diffusion of marked grains that were comparable in size to the fine sediment. These results help explain the existence of a geologic deposit with an unusual layering of grain sizes.     

The influence of heterogeneous meninges on the brain mechanics under primary blast loading

In the modeling of brain mechanics subjected to primary blast waves, there is currently no consensus on how many biological components to be used in the brain–meninges–skull complex, and what type of constitutive models to be adopted. The objective of this study is to determine the role of layered meninges in damping the dynamic response of the brain under primary blast loadings. A composite structures composed of eight solid relevant layers (including the pia, cerebrospinal fluid (CSF), dura maters) with different mechanical properties are constructed to mimic the heterogeneous human head. A hyper-viscoelastic material model is developed to better represent the mechanical response of the brain tissue over a large strain/high frequency range applicable for blast scenarios. The effect of meninges on the brain response is examined. Results show that heterogeneous composite structures of the head have a major influence on the intracranial pressure, maximum shear stress, and maximum principal strain in the brain, which is associated with traumatic brain injuries. The meninges serving as protective layers are revealed by mitigating the dynamic response of the brain. In addition, appreciable changes of the pressure and maximum shear stress are observed on the material interfaces between layers of tissues. This may be attributed to the alternation of shock wave speed caused by the impedance mismatch.