A unified criterion for initiation of sediment motion and inception of sheet flow under water waves

A unified criterion is developed for initiation of non-cohesive sediment motion and inception of sheet flow under water waves over a horizontal bed of sediment based on presently available experimental data. The unified threshold criterion is of the single form, U-o = 2 pi C[1 + 5(T-R/T)(2)](-1/4), where U-o is the onset velocity of sediment motion or sheet flow, T is wave period, and C and T-R are the coefficients. It is found that for a given sediment, U-o initially increases sharply with wave period, then gradually approaches the maximum onset velocity U-o = 2 pi C and becomes independent of T when T is larger. The unified criterion can also be extended to define sediment initial motion and sheet flow under irregular waves provided the significant wave orbital velocity and period of irregular waves are introduced in this unified criterion.

Analysis of strip electric saturation model of crack problem in piezoelectric materials

This paper presents a fully anisotropic analysis of strip electric saturation model proposed by Gao et al. (1997) (Gao, H.J., Zhang, T.Y., Tong, P., 1997. Local and global energy release rates for an electrically yielded crack in a piezoelectric ceramic. J. Mech. Phys. Solids, 45, 491-510) for piezoelectric materials. The relationship between the size of the strip saturation zone ahead of a crack tip and the applied electric displacement field is established. It is revealed that the critical fracture stresses for a crack perpendicular to the poling axis is linearly decreased with the increase of the positive applied electric field and increases linearly with the increase of the negative applied electric field. For a crack parallel to the poring axis, the failure stress is not effected by the parallel applied electric field. In order to analyse the existed experimental results, the stress fields ahead of the tip of an elliptic notch in an infinite piezoelectric solid are calculated. The critical maximum stress criterion is adopted for determining the fracture stresses under different remote electric displacement fields. The present analysis indicates that the crack initiation and propagation from the tip of a sharp elliptic notch could be aided or impeded by an electric displacement field depending on the field direction. The fracture stress predicted by the present analysis is consistent with the experimental data given by Park and Sun (1995) (Park, S., Sun, C.T., 1995. Fracture criteria for piezoelectric materials. J. Am. Ceram. Soc 78, 1475-1480).

金属长杆撞击带薄盖板约束炸药的引爆现象研究

通过实验的方法研究了金属长杆撞击带薄盖板约束炸药的引爆现象。观察带有随机偏航角的金属长杆以不同速度撞击炸药的响应情况。找出了金属长杆在不同入射角度和炸药约束条件下，炸药的冲击爆轰阈值。讨论了判别未知炸药发生爆轰、延迟爆轰、非爆轰性爆炸、点火后淬熄和不反应的试验测量手段和方法。结果表明：冲击引爆阈值速度，对于不覆板的裸炸药，随入射角度（0、30、45）的增加而增加；随着覆板厚度的增加而增加。炸药的侧向约束情况对发生延迟爆轰有影响。射弹偏航也是影响炸药引爆的因素之一。Jacobs 判据是关于炸药冲击引爆阈值的经验判据，它可以适用于8701 炸药，文中给出了对平头钢弹、钢覆板情况下,用实验数据拟合的Jacobs公式。由于其它机理引发延迟爆轰（XDT）的现象是存在的，Jacobs 判据对这类现象不适用。

Simulating shock to detonation transition: Algorithm and results

An algorithm based on flux-corrected transport and the Lagrangian finite element method is presented for solving the problem of shock dynamics. It is verified through the model problem of one-dimensional strain elastoplastic shock wave propagation that the algorithm leads to stable, non-oscillatory results. Shock initiation and detonation wave propagation is simulated using the algorithm, and some interesting results are obtained. (C) 1999 Academic Press.

Localized Solid-State Amorphization at Grain Boundaries in a Nanocrystalline Al Solid Solution Subjected to Surface Mechanical Attrition

Using high-resolution electron microscopy, localized solid-state amorphization (SSA) was observed in a nanocrystalline (NC) Al solid solution (weight per cent 4.2 Cu, 0.3 Mn, the rest being Al) subjected to a surface mechanical attrition treatment. It was found that the deformation-induced SSA may occur at the grain boundary (GB) where either the high density dislocations or dislocation complexes are present. It is suggested that lattice instability due to elastic distortion within the dislocation core region plays a significant role in the initiation of the localized SSA at defective sites. Meanwhile, the GB of severely deformed NC grains exhibits a continuously varying atomic structure in such a way that while most of the GB is ordered but reveals corrugated configurations, localized amorphization may occur along the same GB.

炸药装药的发射起爆概率与质量限定条件初探

讨论火炮发射时炮弹炸药装药的发射起爆概率、寻找制订炮弹炸药装药质量限定条件的方法。

用环形激波聚焦实现爆轰波直接起爆的数值模拟

利用基元反应模型和有限体积法对环形激波在可燃气体中聚焦实现爆轰波直接起爆进行了数值模拟.研究结果表明,标准状态下的氢气-空气混合气体在马赫数为3.1以上的环形激波聚焦产生的高温高压区作用下会诱发可燃气体的直接起爆形成爆轰波,爆轰波与激波和接触间断相互作用产生了复杂的波系结构;爆轰波爆点位置在对称轴上并不是固定的点,而是随着初始激波马赫数的变化而发生移动;可燃气体初始温度和压力对起爆临界马赫数都有影响,但是初始温度的影响大得多.

Deformation and Failure of Polymer Bonded Explosives under Diametric Compression Test

The tensile deformation and failure of polymer bonded explosives (PBXs), a particulate composite, is studied in this paper. Two HMX-based PBXs with different binder were selected for study. A diametric compression test, in which a disc-shaped specimen is loaded diametrically, was chosen to generate tensile failure in the materials. The quasi-static tensile properties and the tensile creep properties were studied by using conventional displacement transducers to measure the lateral strain along the horizontal diameter. The whole-field in-plane creep deformation was measured by using the technique of high resolution moire´ interferometry. Real time microscopic examination was conducted to monitor the process of deformation and failure of PBXs by using a scanning electron microscope equipped with a loading stage. A manifold method (MM) was used to simulate the deformation and failure of PBX samples under the diametric compression test, including the crack initiation, crack propagation and final cleavage fracture. The mechanisms of deformation and failure of PBXs under diametric compression were analyzed. The diametric compression test and the techniques developed in this research have proven to be applicable to the study of tensile properties of PBXs.

Mesofracture mechanics: a necessary link

Classical fracture mechanics is based on the premise that small scale features could be averaged to give a larger scale property such that the assumption of material homogeneity would hold. Involvement of the material microstructure, however, necessitates different characteristic lengths for describing different geometric features. Macroscopic parameters could not be freely exchanged with those at the microscopic scale level. Such a practice could cause misinterpretation of test data. Ambiguities arising from the lack of a more precise range of limitations for the definitions of physical parameters are discussed in connection with material length scales. Physical events overlooked between the macroscopic and microscopic scale could be the link that is needed to bridge the gap. The classical models for the creation of free surface for a liquid and solid are oversimplified. They consider only the translational motion of individual atoms. Movements of groups or clusters of molecules deserve attention. Multiscale cracking behavior also requires the distinction of material damage involving at least two different scales in a single simulation. In this connection, special attention should be given to the use of asymptotic solution in contrast to the full field solution when applying fracture criteria. The former may leave out detail features that would have otherwise been included by the latter. Illustrations are provided for predicting the crack initiation sites of piezoceramics. No definite conclusions can be drawn from the atomistic simulation models such as those used in molecular dynamics until the non-equilibrium boundary conditions can be better understood. The specification of strain rates and temperatures should be synchronized as the specimen size is reduced to microns. Many of the results obtained at the atomic scale should be first identified with those at the mesoscale before they are assumed to be connected with macroscopic observations. Hopefully, "mesofracture mechanics" could serve as the link to bring macrofracture mechanics closer to microfracture mechanics.

脉冲爆轰发动机热射流起爆机理数值分析

应用频散可控耗散差分格式,求解具有化学反应项的Euler方程,探讨了热射流起爆可燃混合气缩短DDT过程的物理机制.数值研究模拟了不同条件下的起爆过程,从氢氧链式反应出发详细分析了氢氧爆轰直接起爆的SWACER（能量释放而形成激波或压缩波的相干放大）机制的建立条件,讨论了热射流起爆存在超临界、临界和亚临界三种直接起爆机制.

Numerical Investigation On Evolution Of Cylindrical Cellular Detonation

Cylindrical cellular detonation is numerically investigated by solving two-dimensional reactive Euler equations with a finite volume method on a two-dimensional self-adaptive unstructured mesh. The one-step reversible chemical reaction model is applied to simplify the control parameters of chemical reaction. Numerical results demonstrate the evolution of cellular cell splitting of cylindrical cellular detonation explored in experimentas. Split of cellular structures shows different features in the near-field and far-field from the initiation zone. Variation of the local curvature is a key factor in the behavior of cell split of cylindrical cellular detonation in propagation. Numerical results show that split of cellular structures comes from the self-organization of transverse waves corresponding to the development of small disturbances along the detonation front related to detonation instability.

Novel Mechanical Behavior Of Zno Nanorods

A novel stress-strain relation with two stages of linear elastic deformation is observed in [0 0 0 1]-oriented ZnO nanorods under uniaxial tensile loading. This phenomenon results from a phase transformation from wurtzite (WZ, P6(3)mc space group) to a body-centered tetragonal structure with four-atom rings (denoted as BCT-4) belonging to the P4(2)/mnm space group. The analysis here focuses on the effects of nanorod size and temperature on the phase transformation and the associated mechanical behavior. It is found that as size is increased from 19.5 to 45.5 angstrom, the critical stress for nucleation of the transformation decreases by 25% from 21.90 to 16.50 GPa and the elastic moduli of the WZ- and BCT-4-structured nanorods decrease by 24% (from 299.49 to 227.51 GPa) and 38% (from 269.29 to 166.86 GPa), respectively. A significant temperature effect is also observed, with the critical stress for transformation initiation decreasing 87.8% from 17.89 to 2.19 GPa as temperature increases from 300 to 1500 K. (c) 2007 Elsevier B.V. All rights reserved.

Investigation Of Mode Ii Crack Growth Following A Very High Speed Impact

A recoverable plate impact testing technology has been developed for studying fracture mechanisms of mode II crack. With this technology, a single duration stress pulse with submicrosecond duration and high loading rates, up to 10(8) MPam(1/2)s(-1), can be produced. Dynamic failure tests of Hard-C 60# steel were carried out under asymmetrical impacting conditions with short stress-pulse loading. Experimental results show that the nucleation and growth of several microcracks ahead of the crack tip, and the interactions between them, induce unsteady crack growth. Failure mode transitions during crack growth, both from mode I crack to mode II and from brittle to ductile fracture, were observed. Based on experimental observations, a discontinuous crack growth model was established. Analysis of the crack growth mechanisms using our model shows that the shear crack extension is unsteady when the extending speed is between the Rayleigh wave speed c(R) and the shear wave speed c(S). However, when the crack advancing speed is beyond c(S), the crack grows at a steady intersonic speed approaching root 2c(S). It also shows that the transient mechanisms, such as nucleation, growth, interaction and coalescence among microcracks, make the main crack speed jump from subsonic to intersonic and the steady growth of all the subcracks causes the main crack to grow at a stable intersonic speed.

Dynamic Fracture Instability Of Tough Bulk Metallic Glass

We report the observations of a clear fractographic evolution from vein pattern, dimple structure, and then to periodic corrugation structure, followed by microbranching pattern, along the crack propagation direction in the dynamic fracture of a tough Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit.1) bulk metallic glass (BMGs) under high-velocity plate impact. A model based on fracture surface energy dissipation and void growth is proposed to characterize this fracture pattern transition. We find that once the dynamic crack propagation velocity reaches a critical fraction of Rayleigh wave speed, the crack instability occurs; hence, crack microbranching goes ahead. Furthermore, the correlation between the critical velocity of amorphous materials and their intrinsic strength such as Young's modulus is uncovered. The results may shed new insight into dynamic fracture instability for BMGs. (C) 2008 American Institute of Physics.

Research on laser welding of cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft

Exploratory experiments of laser welding cast Ni-based superalloy K418 turbo disk and alloy steel 42CrMo shaft were conducted. Microstructure of the welded seam was characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectrometer (EDS). Mechanical properties of the welded seam were evaluated by microhardness and tensile strength testing. The corresponding mechanisms were discussed in detail. Results showed that the laser-welded seam had non-equilibrium solidified microstructures consisting of FeCr0.29Ni0.16C0.06 austenite solid solution dendrites as the dominant and some fine and dispersed Ni3Al gamma' phase and Laves particles as well as little amount of MC short stick or particle-like carbides distributed in the interdendritic regions. The average microhardness of the welded seam was relatively uniform and lower than that of the base metal due to partial dissolution and suppression of the strengthening phase gamma' to some extent. About 88.5% tensile strength of the base metal was achieved in the welded joint because of a non-full penetration welding and the fracture mechanism was a mixture of ductility and brittleness. The existence of some Laves particles in the welded seam also facilitated the initiation and propagation of the microcracks and microvoids and hence, the detrimental effects of the tensile strength of the welded joint. The present results stimulate further investigation on this field. (c) 2006 Elsevier B.V. All rights reserved.