Detection and Characterization of Long-Pulse Low-Velocity Impact Damage in Plastic Bonded Explosives

Damage not only degrades the mechanical properties of explosives, but also influences the shock sensitivity, combustion and even detonation behavior of explosives. The study of impact damage is crucial in the vulnerability evaluation of explosives. A long-pulse low-velocity gas gun with a gas buffer was developed and used to induce impact damage in a hot pressed plastic bonded explosive. Various methods were used to detect and characterize the impact damage of the explosive. The microstructure was examined by use of polarized light microscopy. Fractal analysis of the micrographs was conducted by use of box counting method. The correlation between the fractal dimensions and microstructures was analyzed. Ultrasonic testing was conducted using a pulse through-transmission method to obtain the ultrasonic velocity and ultrasonic attenuation. Spectra analyses were carried out for recorded ultrasonic signals using fast Fourier transform. The correlations between the impact damage and ultrasonic parameters including ultrasonic velocities and attenuation coefficients were also analyzed. To quantitatively assess the impact induced explosive crystal fractures, particle size distribution analyses of explosive crystals were conducted by using a thorough etching technique, in which the explosives samples were soaked in a solution for enough time that the binder was totally removed. Impact induces a large extent of explosive crystal fractures and a large number of microcracks. The ultrasonic velocity decreases and attenuation coefficients increase with the presence of impact damage. Both ultrasonic parameters and fractal dimension can be used to quantitatively assess the impact damage of plastic bonded explosives.

Reversible jump sampler for autoregressive time series

We use reversible jump Markov chain Monte Carlo (MCMC) methods to address the problem of model order uncertainty in autoregressive (AR) time series within a Bayesian framework. Efficient model jumping is achieved by proposing model space moves from the full conditional density for the AR parameters, which is obtained analytically. This is compared with an alternative method, for which the moves are cheaper to compute, in which proposals are made only for new parameters in each move. Results are presented for both synthetic and audio time series.

Dynamic constitutive equation of GFRP obtained by Lagrange experiment

The note presents a method of constructing dynamic constitutive equations of material by means of Lagrange experiment and analysis. Tests were carried out by a light gas gun and the stress history profiles were recorded on multiple Lagrange positions. The dynamic constitutive equations were deduced from the regression of a series of data which was obtained by Lagrange analysis based upon recorded multiple stress histories. Here constitutive equations of glass fibre reinforced phenolic resin composite(GFRP) in uniaxil strain state under dynamic loading are given. The proposed equations of the material agree well with experimental results.

钢基身管内镀铬层下的激光淬火基体界面腐蚀与铬层剥落

钢基身管镀铬层在高温腐蚀性气体中服役,采用YAG激光器对身管内表面作螺旋线状离散淬火预处理后再电镀铬的复合工艺,形成了基体界面呈激光淬火区/激光未处理区周期性变化的铬层/基体结构.对该身管进行高温腐蚀介质为火药气体的靶场实验.对实验后的基体界面腐蚀形貌与铬层剥落的关系研究表明:次界面裂纹的形成是基体界面高温气体腐蚀的根本原因,基体界面腐蚀的结果是形成基体烧蚀坑,烧蚀坑上的铬层以断裂形式剥落.激光淬火预处理钢基体通过抑制次界面裂纹的形成避免了基体界面高温气体腐蚀损伤,提高了铬层的抗剥落能力.

The investigation on dynamic fracture behavour of materials under compressive-shear combined stress waves

In this paper, an improved plate impact experimental technique is presented for studying dynamic fracture mechanism of materials, under the conditions that the impacting loading is provided by a single pulse and the loading time is in the sub-microsecond range. The impacting tests are carried out on the pressure-shear gas gun. The loading rate achieved is dK/dt similar to 10(8) MPa m(1/2) s(-1). With the elimination of influence of the specimen boundary, the plane strain state of a semi-infinite crack in an infinite elastic plate is used to simulate the deformation fields of crack tip. The single pulses are obtained by using the "momentum trap" technique. Therefore, the one-time actions of the single pulse are achieved by eradicating the stress waves reflected from the specimen boundary or diffracted from the crack surfaces. In the current study, some important phenomena have been observed. The special loading of the single pulse can bring about material damage around crack tip, and affect the material behavior, such as kinking and branching of the crack propagation. Failure mode transitions from mode I to mode II crack are observed under asymmetrical impact conditions. The mechanisms of the dynamic crack propagation are consistent with the damage failure model.

Arc Root Motions In An Argon-Hydrogen Direct-Current Plasma Torch At Reduced Pressure

Arc root motions in generating dc argon-hydrogen plasma at reduced pressure are optically observed using a high-speed video camera. The time resolved angular position of the arc root attachment point is measured and analysed. The arc root movement is characterized as a chaotic and jumping motion along the circular direction on the anode surface.

Multi-scale analysis of AFM tip and surface interactions

Thoroughly understanding AFM tip-surface interactions is crucial for many experimental studies and applications. It is important to realize that despite its simple appearance, the system of tip and sample surface involves multiscale interactions. In fact, the system is governed by a combination of molecular force (like the van der Waals force), its macroscopic representations (such as surface force) and gravitational force (a macroscopic force). Hence, in the system, various length scales are operative, from sub-nanoscale (at the molecular level) to the macroscopic scale. By integrating molecular forces into continuum equations, we performed a multiscale analysis and revealed the nonlocality effect between a tip and a rough solid surface and the mechanism governing liquid surface deformation and jumping. The results have several significant implications for practical applications. For instance, nonlocality may affect the measurement accuracy of surface morphology. At the critical state of liquid surface jump, the ratio of the gap between a tip and a liquid dome (delta) over the dome height (y(o)) is approximately (n-4) (for a large tip), which depends on the power law exponent n of the molecular interaction energy. These findings demonstrate that the multiscale analysis is not only useful but also necessary in the understanding of practical phenomena involving molecular forces. (c) 2007 Elsevier Ltd. All rights reserved.

Embryo-damage induced nucleation of microcracks in an aluminum-alloy under impact loading

The nucleation of microdamage under dynamic loading was investigated through planar impact experiments accomplished with a light gas gun. The microscopic observation of recovered and sectioned specimens showed that microcracks were nucleated only by cracking of brittle particles inside material. However, for comparison the in situ static tensile tests on the same material conducted with a scanning electron microscope showed that the microcracks were nucleated by many forms those were fracture of ductile matrix, debonding particles from matrix and cracking of brittle particles. The quantitative metallographic observations of the specimens subjected to impact loading showed that most of the cracked particles were situated on grain boundaries of the aluminium matrix. These facts suggested the concept of critical size and incubation time of submicroscopic cavities in the dynamic case and the mechanism of embryo-damage induced nucleation by fracture of brittle particles in the aluminium alloy under impact loading was proposed.

Spall damage in aluminum-alloy

A void growth relations for ductile porous materials under intense dynamic general loading condition is presented. The mathematical model includes the influence of inertial effects, material rate sensitivity, as well as the contribution of void surface energy and material work-hardening. Numerical analysis shows that inertia appears to resist the growth of voids. The inertial effects increase quickly with the loading rates. The theoretical analysis suggests that the inertial effects cannot be neglected at high loading rates. Plate-impact tests of aluminum alloy are performed with light gas gun. The processes of dynamic damage in aluminum alloy are successfully simulated with a finite-difference dynamic code in which the theoretical model presented in this paper is incorporated.

Ballistic impact of ceramics

The impact behaviour of a range of glass and ceramic materials has been studied using high-speed photography. A gas gun was used to project hardened spheres at plate specimens in the velocity range 30 to 1000m s-1. The target materials included soda-lime glass, boron carbide and various glass ceramics and aluminas. The performance of a particular ceramic was found to depend on a combination of parameters but of key importance was the relative hardness of the projectile and target materials. The fracture toughness, K(IC), had only a secondary effect.

Observar el movimiento : construcción del formato de campo “el salto en psicomotricidad durante el tercer año de vida”

[Es]Este artículo repasa los hitos más significativos de la elaboración del instrumento de observación del formato de campo “el salto en psicomotricidad durante el tercer año de vida”. La aplicación de la Metodología Observacional a la Práctica Psicomotriz Aucouturier (PPA)es posible ya que existe una doble complementariedad entre ambas: la metodología observacional es la única que puede evaluar y explicar, y no sólo describir, el complejo flujo de actividad psicomotriz infantil o expresividad psicomotriz y la práctica psicomotriz aucouturier, a su vez, cumple el perfil óptimo de la metodología observacional. El proceso metodológico observacional impone entre sus condiciones la sistematización del instrumento de observación; lo que comienza siendo un sistema de categorías en base a la propuesta observacional de la propia PPA, termina siendo un formato de campo al servicio del objetivo inicial de la investigación, que es la evaluación del comportamiento psicomotor infantil.

The Evaluation of Fisheries Management: A Dynamic Stochastic Approach

In this article, we analyze how to evaluate fishery resource management under “ecological uncertainty”. In this context, an efficient policy consists of applying a different exploitation rule depending on the state of the resource and we could say that the stock is always in transition, jumping from one steady state to another. First, we propose a method for calibrating the growth path of the resource such that observed dynamics of resource and captures are matched. Second, we apply the calibration procedure proposed in two different fishing grounds: the European Anchovy (Division VIII) and the Southern Stock of Hake. Our results show that the role played by uncertainty is essential for the conclusions. For European Anchovy fishery (Division VIII) we find, in contrast with Del Valle et al. (2001), that this is not an overexploited fishing ground. However, we show that the Southern Stock of Hake is in a dangerous situation. In both cases our results are in accordance with ICES advice.

Mortality estimates of the four major Cichlid fishes of Umuoseriche Lake, Imo State, Nigeria

The mortality of the four major cichlid fishes of Urnuoseriche Lake is the subject of this paper. Mortality I as estimated by five techniques, vary amongst the cichlid fishes, viz, Tilapia carbrae, Tilapia mariac, Tilapia zilli cend (hrornoditilapfa guntheri. The highest mortality rate was recorded for T mariac where the total mortality (Z) was 2.06; and natural mortality (M) was 1.8949. This species was also the most highly exploited species of fish with an exploitation ratio of0.566 (56.6%) and exploitation rate of 0.494. The least exploited cichlid fish is (. gun/hen where an exploitation ratio of 0.43209%) and exploitation rate of 0.2225 was recorded. In C'. guntheni, total mortality was 0.726 and natural mortality was 0.413 1. In T zilli, total mortality was 1.0547 wile exploitation ratio was 0.3674 (3 6.74%) and an exploitation rate was 0.2394. In T cahrae. total mortality was 1.8662: exploitation ratio was 0.4786 with an exploitation rate of 0.4045. (7 page document)

身管内膛复合镀铬层的组织退化特征

研究了某型号身管后膛射击后复合镀铬层(由顶层的高收缩性铬和底层的低收缩性铬组成)的组织及显微硬度退化特征.结果显示:经3000发射击后,复合镀铬层沿着涂层厚度方向呈现梯度性再结晶,其组织由射击前的柱状晶粒团簇逐渐转变成等轴晶粒团簇,这导致镀铬层的硬度梯度分布;高收缩性铬层的表层晶粒成长最大,其相应的硬度由初始的960HV左右急剧下降到330HV左右.另外,高收缩性铬层许多短小的镶嵌微裂纹消失,而且一些穿透镀铬层的主裂纹形成并扩展到基体.根据服役条件下镀铬层的组织及性能退化特征,调整优化镀铬层的组织或许能有效地延长身管内膛镀铬层的寿命.

El salto a los tres años en psicomotricidad : observación del comportamiento psicomotor infantil

[ES] El objetivo de esta investigación es evaluar la actividad de saltar de un grupo escolar natural de 15 criaturas entre los dos y los cuatro años en el contexto de la Práctica Psicomotriz Aucouturier. La perspectiva teórica adoptada ha sido la teoría psicogenética y dialéctica de Henri Wallon. Según ésta, saltar es un automatismo natural regulado por el aparato funcional del equilibrio que se aprende en la primera infancia. La metodología utilizada ha sido la observacional. El diseño es nomotético, de seguimiento y multidimensional, y el instrumento de observación es el formato de campo "el salto en psicomotricidad durante el tercer año de vida". Los resultados, obtenidos mediante la aplicación prospectiva del análisis secuencial de retardos, informan sobre la adquisición del automatismo o proceso de aprendizaje, sobre el contenido del mismo o manejo del vértigo y sobre su resultante conjunta o tipos básicos de salto en psicomotricidad, sus características y su evolución.