铝合金单层球面网壳的非线性稳定分析

以K8型单层球面网壳结构为例,采用有限元分析软件ANSYS进行建模和计算,对铝合金网壳进行了几何非线性稳定分析,讨论了矢跨比、荷载分布形式、初始缺陷和支承条件对网壳稳定性承载力的影响。通过与钢网壳比较,得到了一些关于铝合金单层球面网壳稳定性的结论,可为相关的工程设计提供参考。     

Deformation and fragmentation behaviour of exploded metal cylinders and the effects of wall materials,configuration, explosive energy and initiated locations

Tubular metal specimens are explosively expanded to fragmentation, and the effects of wall materials, thicknesses, notches in walls, explosive driver diameters and the initiated locations are investigated on the deformation and fracture behavior of the cylinders experimentally and numerically. In the standard tests, the driver is a column of low density powder of high explosive PETN, inserted coaxially into the bore of a smooth-walled cylinder and initiated by exploding a bundle of fine copper wires at the column axis using a discharge current from a high-voltage capacitor bank. Notched cylinders with single axial slit, various grooves in the walls, and smooth cylinders with varied wall thicknesses were tested. Low-carbon steels and an aluminum alloy A5052 were provided in addition to the standard smooth-walled 304 stainless steel cylinder, and they were fully or partially charged with varying explosive column diameters. The initiated locations in the explosive column are changed for comparison, placing the bundle of fine copper wires eccentrically from the central axis or replacing the fine wire bundle into a bold wire line except the middle portion at the central axis for central point initiation. Additionally an explosive-filled cylindrical vessel with welded endplate at the one end is initiated at the other end explosive surface exploding wire-rows and expanded by axially propagating explosive detonation to fracture for comparison with the uniform expansion. Deformation and crack initiation of expanding cylinders are observed with high speed camera, and most of the fragments have been recovered successfully. Recovered fragments have been measured and investigated using a fragmentation model. The effects of test parameters on the deformation and fracture behavior of metallic cylinders are discussed with use of numerical simulations, indicating applicability of the fragmentation model and suggesting future necessary studies.     

不同材料双层球缺罩侵彻特性的研究

采用数值模拟及钢靶侵彻试验两种方法研究了单层钛合金球缺罩、钛合金/铜、铝合金/铜双层球缺罩形成的3种杆式射流对45#钢锭靶板侵彻深度和开孔的问题。研究结果表明:不同材料的双层球缺罩形成的杆式射流对45#钢锭靶板的开孔孔径和侵彻深度大小均有直接影响,且钛合金/铜双层罩杆式射流破甲深度相对于铝合金/铜双层罩有一定提高,其开孔孔径明显增大。而单层钛合金药型罩杆式射流整体速度最大,开孔孔径较钛合金/铜、铝合金/铜双层罩杆式射流有明显提高,但破甲能力较低。研究结果对于武器战斗部设计具有一定的指导作用。     

Phenomenological aspects of a modified fragmentation test using small quantities of explosive

This paper describes the phenomenological aspects of a modified fragmentation test which utilizes an annulus of high explosive in contact with the inner wall of a cylinder of material under test. The shock interactions within the cylinder wall on detonation of the explosive are modelled using a one-dimensional Lagrangian computer code. Predictions of fragment velocities are made in this work. The velocities are found to be influenced by the spallation of the cylinder and this effect can be accounted for in the model by using the position of the spall surfaces which have been obtained from experimental firings of cylinders.

The fragment velocities are found to be much lower than those predicted for a conventional fragmentation test which utilizes a cylinder completely filled with high explosive and this suggests that the post-fragmentation damage of the fragments on recovery from the modified fragmentation test will be small. Experimental evidence from the steel cylinders tested in this work shows that the fragmentation mechanisms operating at the initiation of the test are similar to those reported for a conventional fragmentation test.     

Analysis of process parameters effects on friction stir welding of dissimilar aluminum alloy to advanced high strength steel

Thin sheets of aluminum alloy 6061-T6 and one type of Advanced high strength steel, transformation induced plasticity (TRIP) steel have been successfully butt joined using friction stir welding (FSW) technique. The maximum ultimate tensile strength can reach 85% of the base aluminum alloy. Intermetallic compound (IMC) layer of FeAl or Fe₃Al with thickness of less than 1 μm was formed at the Al–Fe interface in the advancing side, which can actually contribute to the joint strength. Tensile tests and scanning electron microscopy (SEM) results indicate that the weld nugget can be considered as aluminum matrix composite, which is enhanced by dispersed sheared-off steel fragments encompassed by a thin intermetallic layer or simply intermetallic particles. Effects of process parameters on the joint microstructure evolution were analyzed based on mechanical welding force and temperature that have been measured during the welding process.     

爆轰法制取氧化铝

为进一步探求纳米氧化铝的尺寸和晶型与混合炸药爆轰参数之间的有机联系,以九水硝酸铝和黑索金为原材料,按1∶8的质量比将两者均匀混合配制出密度约为0.8g/cm3的混合炸药在爆炸罐内进行爆轰实验研究,爆轰实验产物为灰色粉末。通过高分辨率透射电镜和X光衍射仪器对产物进行检测分析,分析结果表明产物为纳米氧化铝,氧化铝颗粒形状呈标准球形,大部分颗粒尺寸小于100nm,少量颗粒尺寸大于100nm,颗粒平均尺寸为30nm左右,氧化铝晶型为γ型。实验结果进一步证明了混合炸药中黑索金含量越高,爆轰合成的纳米氧化铝尺寸越大。     

Study on the fragmentation of shells

Fragmentation can be observed in nature and in everyday life on a wide range of length scales and for all kinds of technical applications. Most studies on dynamic failure focus on the behaviour of bulk systems in one, two and three dimensions under impact and explosive loading, showing universal power law behaviour of fragment size distribution. However, hardly any studies have been devoted to fragmentation of shells. We present a detailed experimental and theoretical study on the fragmentation of closed thin shells of various materials, due to an excess load inside the system and impact with a hard wall. Characteristic fragmentation mechanisms are identified by means of a high speed camera and fragment shapes and mass distributions are evaluated. Theoretical rationalisation is given by means of stochastic break-up models and large-scale discrete element simulations with spherical shell systems under different extreme loading situations. By this we explain fragment shapes and distributions and prove a power law for the fragment mass distribution. Satisfactory agreement between experimental findings and numerical predictions of the exponents of the power laws for the fragment shapes is obtained     

Explosive simulation to investigate enhanced ablator removal

We have developed a method to simulate with explosives the impact and penetration of a recentry vehicle (RV) shell by a high-density hypervelocity fragment. Using a two-dimensional Lagrangian hydrocode, we modeled various hypervelocity fragment impact conditions and innovative explosive configurations that simulate the impact effects. The method is based on matching the damage inflicted on the heatshield by the impact and penetration of the fragment. Specifically, we set a simulation objective of matching the hole size, the time history of the stress environment, and the final effective plastic strain field for both the silica phenolic heatshield and aluminum layers while keeping the momentum imparted to the target the same. The calculations showed that the explosive jet from an explosive charge placed inside a short disposable steel barrel produced a hole that matched the simulation criteria reasonably well except that the aluminum substrate stretched excessively before failing. A much improved simulation was obtained when the target was penetrated with a fragment projected by an explosive charge. All the simulation criteria listed above were matched very well, indicating that explosive simulation can be used to simulate the impact of hypervelocity fragments with a high degree of fidelity.     

Soft magnetic structural alloys for elevated-temperature applications

Metallurgical efforts to develop a soft magnetic material suitable for application in the rotor of a generator or motor in advanced aerospace electric systems are reviewed. Commercial materials which have been considered include AISI 4340 steel, H-11 steel, Nivco alloy, and 15- and 18-percent Ni maraging steels. Developments described have led to several new materials with combination of good mechanical and magnetic properties at elevated temperature. Such materials include an improved maraging steel a precipitation hardenable cobalt-base alloy, a carbide strengthened Co-W alloy, dispersion-strengthened soft magnetic alloys, and unidirectionally solidified Co-Nb-Fe eutectic alloys.     

Investigation of the fracture and fragmentation of explosively driven rings and cylinders

Cylinders and rings fabricated from AerMet^® 100 alloy and AISI 1018 steel have been explosively driven to fragmentation in order to determine the fracture strains for these materials under plane-strain and uniaxial-stress conditions. The phenomena associated with the dynamic expansion and subsequent break up of the cylinders are monitored with high-speed diagnostics. In addition, complementary experiments are performed in which fragments from the explosively driven cylinders are recovered and analyzed to determine the statistical distribution associated with the fragmentation process as well as to determine failure mechanisms. The data are used to determine relevant coefficients for the Hancock–McKenzie (Johnson–Cook) fracture model. Metallurgical analysis of the fragments provides information on damage and failure mechanisms.     

钨合金球形破片对低碳钢的穿甲极限

摘　要：研究特定质量钨合金球形破片对低碳钢侵彻与贯穿中的极值。针对钨合金球形破片对半无限和有限厚度低碳钢的高弹速穿甲,首先通过数值仿真得到了典型直径破片的极限侵彻深度,同时获得了球形破片直径与极限穿透厚度的关系;在此基础上,针对不同直径破片进行了穿甲实验,实验结果与数值仿真及分析具有较好的一致性,根据实验现象和数值仿真分析了不同撞靶速度下破片的破损形式。研究结果具有重要的应用价值。     

Severe plastic deformation (SPD) and nanostructured materials by machining

Large plastic strains between 1 and 15 can be imposed in chips formed by plane-strain (2-D) machining of metals and alloys. This approach has been used to examine microstructure changes induced by large strain deformation in model systems—copper and its alloys, precipitation-hardenable aluminum alloys, high-strength materials such as titanium, Inconel 718 and 52100 steel, and an amorphous alloy. It is shown that materials with average grain sizes in the range of 60 nm–1 μm can be created by varying the parameters of machining, which in turn affects the deformation processes. Furthermore, a switch-over from an elongated subgrain microstructure to an equi-axed nanocrystalline microstructure, with a preponderance of large-angle grain boundaries, has been demonstrated at the higher levels of strain in several of these materials. This switch-over can be readily controlled by varying the deformation conditions. Dynamic recrystallization has been demonstrated in select material systems under particular conditions of strain and temperature. This study may be seen as providing an important bridge between furthering the understanding of microstructural refinement by large strain deformation and the practical utilization of nanostructured materials in structural and mechanical applications. Conventional plane-strain machining has been shown to be a viable SPD method for examining the underlying processes of very large strain deformation.     

Research on explosive welding of aluminum alloy to steel with dovetail grooves

In order to explore a new method for the explosive welding of aluminum alloy to steel, a 5083 aluminum alloy plate and a Q345 steel plate with dovetail grooves were respectively employed as the flyer and base plates. The parameters adopted in the explosive welding experiment were close to the lower limit of weldable window of 5083 aluminum alloy to Q345 steel. The bonding properties of 5083/Q345 clad plate were studied through mechanical performance tests and microstructure observations. The results showed that the aluminum alloy and steel plates were welded under the actions of metallurgical bonding and meshing of dovetail grooves. The tensile shear strength of 5083/Q345 clad plate met the requirements of the bonding strength of Al/Fe clad plate. The interfaces between aluminum alloy and the upper and lower surfaces of dovetail grooves were mainly welded through direct bonding, and discontinuous molten zone emerged in the local region; while the interface between aluminum alloy and the inclined surface of dovetail grooves was bonded by continuous molten layer. The brittle intermetallic compounds FeAl₂ and Al₅Fe₂ were generated at the bonding interfaces of 5083/Q345 clad plate. The fracture surface of the tensile specimen exhibited ductile and quasi-cleavage fractures.     

Electron beam surface treatment. Part II: microstructure evolution of stainless steel and aluminum alloy during electron beam rapid solidification

Surface rapid solidification microstructures of AISI 321 austenitic stainless steel and 2024 aluminum alloy have been investigated by electron beam remelting process and optical microscopy observation. It is indicated that the morphologies of the melted layer of both stainless steel and aluminum alloy change dramatically compared to the original materials. Also, the microstructures were greatly refined after the electron beam irradiation.     

Investigation of fragments size resulting from dynamic fragmentation in melted state of laser shock-loaded tin

The understanding of dynamic fragmentation in shock-loaded metals and the evaluation of geometrical and kinematical properties of the resulting fragments are issues of considerable importance for both basic and applied science, for instance to predict the evolution of engineering structures submitted to high-velocity impact or explosive detonation. Among dynamic failure processes, spall fracture in solid materials has been extensively studied for many years, while scarce data can be found yet about how such phenomenon could evolve after partial or full melting on compression or on release. In this case, the dynamic fragmentation process, which may be referred to as ‘micro-spalling’, takes place in a liquid medium. It results in the formation of a cloud of fine molten droplets, ejected at high-velocity. The present work is devoted to experimental characterization, theoretical modelling and simulation of the ‘micro-spalling’ process in tin, with a specific emphasis on the size of the resulting fragments, namely the melted droplets. Laser-driven shock-loading experiments on tin have been performed. Post-test observations of the recovered fragments provide an insight into the actual fragmentation process and allow to infer the distribution of the fragments size which are found to be mostly sub-micrometric. Fragmentation modelling is based on a widely employed, energetic approach adapted to the case of liquids. This approach is implemented as a failure criterion in an one-dimensional hydrocode including a multiphase equation of state for tin. A fairly good agreement is obtained between experimental and computed sizes range. Some discrepancies are explained by both experimental uncertainties and model limitations which are carefully pointed out and discussed.     

超薄铝合金胶粘接及激光焊接工艺研究

目的 为了解决厚度为0.05 mm的铝合金在激光焊接过程中,材料发生变形导致的焊穿或者虚焊问题.方法 采用在下层铝合金材料表面均匀涂覆粘胶剂,将上下2层铝合金材料进行预固定,然后采用激光焊接,形成焊缝.通过控制涂覆粘胶剂的次数,得到胶层厚度为11.2μm,对涂覆粘胶剂的铝合金进行激光焊接实验.结果 当激光平均功率为150 W,焊接速度为100 mm/s,离焦量为2 mm时,焊缝剪切强度最大,为124 MPa,达到母材剪切强度的82％.结论 通过粘胶剂预固定后,在激光加热过程中,可以很好地克服材料的变形,让上下材料形成熔池,熔池冷却凝固后熔合在一起形成焊缝,可以解决材料发生变形导致的焊穿或者虚焊问题.     

Response of simulated propellant and explosives to projectile impact—II. Fragmentation

This paper is the second of a series concerned with the penetration and perforation phenomena in two types of propellant and explosive simulant, named Propergol, due to the impact at normal incidence of both blunt and conically-tipped steel strikers. The collision results in fragmentation, plug formation and generation of a cloud of debris that includes particles of measurable dimensions traveling with significant velocities. Both the fragment size and area as well as the ejecta mass are determined experimentally as a function of Propergol specimen thickness and impact velocity or energy. The cumulative number of fragments as a function of size for the Propergol is uniformly found to be a bi-linear semi-logarithmic relationship with the bifurcation occurring at the mean crystal radius. Individual crystals and the crater generated are examined by means of a scanning electron microscope.

A phenomenological model of the fragmentation process is constructed, based on an assumed spherical shape of the fragments and the bi-linear fragment distribution, using energy methods. This is combined with a perforation analysis that considers the process to be sequentially composed of initial indentation, fragmentation, and sliding and deflection of the Propergol disks. An evaluation of this model providing fragment volumes as a function of impact velocity is compared with experimental results and found to be in good agreement.     

钛-钢板爆炸焊接的影响因素及炸药配方

爆炸复合用炸药是影响爆炸焊接效果的最主要因素,不同的材料和工艺参数对炸药的爆速、比冲量和能量有不同的影响,其中炸药的密度直接影响复板斜碰撞基板的速度和角度。选用低合金高强度结构钢Q345B和工业纯钛TA2为实验材料,通过理论计算与实验研究,得到了一种较为适合钛-钢板爆炸焊接用的炸药配方,并对钛-钢爆炸焊接影响因素进行了探讨。     

TC4钛合金棒料亮条纹缺陷分析

某TC4钛合金棒料加工产品,在进行X射线检测时发现存在亮条纹缺陷,采用各种理化检验手段,分析了该TC4钛合金棒料的化学成分、显微组织和硬度。结果表明:该TC4钛合金棒料在冶炼时由于工艺控制不当,形成了一个严重的富钛贫铝贫钒区域,该区域在后期的热加工过程中形成了不同于基体的粗大单相α组织,其硬度明显低于基体硬度,所以X射线检测时呈现亮条纹形貌;这是一种典型的软偏析冶金缺陷。