Ductile–brittle transitions in the fracture of plastically-deforming,adhesively-bonded structures. Part I: Experimental studies

Rate effects for adhesively-bonded joints in steel sheets failing by mode-I fracture and plastic deformation were examined. Three types of test geometries were used to provide a range of crack velocities between 0.1 and 5000 mm/s: a DCB geometry under displacement control, a wedge geometry under displacement control, and a wedge geometry loaded under impact conditions. Two fracture modes were observed: quasi-static crack growth and dynamic crack growth. The quasi-static crack growth was associated with a toughened mode of failure; the dynamic crack growth was associated with a more brittle mode of failure. The experiments indicated that the fracture parameters for the quasi-static crack growth were rate independent, and that quasi-static crack growth could occur even at the highest crack velocities. Effects of rate appeared to be limited to the ease with which a transition to dynamic fracture could be triggered. This transition appeared to be stochastic in nature, it did not appear to be associated with the attainment of any critical value for crack velocity or loading rate. While the mode-I quasi-static fracture behavior appeared to be rate independent, an increase in the tendency for dynamic fracture to be triggered as the crack velocity increased did have the effect of decreasing the average energy dissipated during fracture at higher loading rates.     

海冰与波流耦合动力学的研究进展

随着全球气候变暖,北极活动频次的逐渐增加,海冰与波流的耦合作用成为了国内外的热点研究问题. 海冰的存在会改变波浪的传播特性和色散关系,也会改变海流运动的边界条件,使得海流在冰面下边界处可能产生漩涡脱落等现象;而海冰在波浪、海流等动力作用下,也会不断发生生消、断裂、重叠和堆积等动力学行为.海冰与波流耦合动力学中存在几大难点问题:一是海冰模型的构建,需要针对海冰类型和性质的不同,考虑不同区域的海冰分布情况建立合适的海冰模型.二是冰水之间的耦合问题,海冰边界影响着冰水之间的动量和能量交换,对于冰盖、浮冰等不同类型海冰,需分别处理边界问题;冰水耦合引起的海冰破坏,其破坏模式也是多样的.三是波流联合场的构建,目前尚无较好的方法构建波流联合场,导致海冰与波流联合场作用的相关研究仍极其匮乏.因此,本文针对海冰与波流相互作用问题,回顾和讨论国内外对海冰与波浪、海冰与海流及海冰与波流相互作用的研究现状和技术难点,展望了未来可以进一步深入研究的问题,提出了初步的思路以供参考.      

冲击波和高速破片联合作用下夹芯复合舱壁结构的毁伤特性

为探讨导弹战斗部近炸下舰船夹芯复合舱壁结构设计方法,采用TNT和预制破片近炸实验研究了典型夹芯复合舱壁结构在冲击波与高速破片联合作用下的破坏效应,分析了冲击波和破片联合毁伤载荷,指出了钢质面板和抗弹层的破坏模式,阐述了夹芯复合舱壁结构的防护机理。结果表明:预制破片装药近炸下,破片能远大于冲击波能,是防护结构的主要设计载荷;前面板主要是抵御冲击波,其变形破坏整体为挠曲大变形,局部为集团破片冲塞破口、破片穿孔和撞击凹坑;背板以挠曲大变形吸能为主;陶瓷材料碎裂严重,部分陶瓷碎片反向飞溅撞击前面板;纤维增强复合材料发生了纤维断裂、基体开裂、整体弯曲大变形及分层等破坏,抗弹层应避免产生穿透性破坏。     

INFLUENCE OF LOADING RATE ON DYNAMIC FRACTURE BEHAVIOR OF FIBER-REINFORCED COMPOSITES

The effect of loading rate on the dynamic fracture properties and the failure mechanisms of glass fiber-reinforced composite materials under mode I fracture is studied. Dynamic reflective caustic experiments are carried out for two loading rates. By measuring the characteristic dimensions of the shadow spots during the caustic experiments, the dynamic SIFs are calculated for different loading rates. The experimental results indicate that the dynamic fracture toughness Kid increases remarkably with increasing loading rate, and the crack grows faster under the high-velocity impact. Moreover, by examining the crack growth routes and the fracture surfaces, it is shown that the loading rate also greatly affects the failure mechanisms at micro-scale.     

Ti-6Al-4V弹体破坏模式对冲击反应的影响研究

Ti-6Al-4V材料是武器结构轻量化时的重要替代材料,其冲击反应将可能增加战斗部毁伤威力,但目前缺乏对其冲击反应条件及反应机理的研究。本文将采用试验与理论分析方法,研究结构破坏模式对Ti-6Al-4V材料冲击反应的影响,获得其冲击反应条件及反应机理。设计并开展了钛合金弹（头部与壳体均为钛合金）与复合弹（头部碳/碳复合材料、壳体空心钛合金圆柱）正侵彻混凝土试验,撞击速度在222~1008 m/s之间。钛合金弹激发了剧烈的氧化冲击反应,但复合弹未产生冲击反应。破坏模式宏细观分析显示,钛合金弹侵彻后宏观结构基本完整,仅表面发生摩擦磨损,以细观组织剪切变形为主要失效模式,形成尺寸在微米量级至百微米量级的颗粒碎片,碎片个数可高达3×106。复合弹的钛合金空心圆柱被撕裂成块,撕裂面沿剪切带方向发展,碎块尺寸在毫米或以上量级,个数至多百余个。碎片供氧和供热的效率均与碎片尺寸成反比,而特定供氧与供热条件下,碎片尺寸足够小是Ti-6Al-4V材料发生冲击反应的必要条件,这是钛合金弹发生冲击反应而钛合金空心圆柱无法激发冲击反应的本质原因。在具备冲击反应必要条件的前提下,碎片个数越多,冲击反应烈度越高。     

Friction Between Steel and a Confined Inert Material Representative of Explosives Under Severe Loadings

The ignition of a confined explosive submitted to an impact strongly depends on the friction conditions between the explosive and the confinement material (generally steel). A test has been developed to study the friction between steel and a material mechanically representative of an explosive. The scope of interest is that of high pressures and high relative velocities (respectively 20 MPa and 10 m/s). The friction device consists of making a cylinder, formed of the material, slide through a steel tube. Axial prestress enabling the steel-material contact stress to be generated is performed by means of a screw-nut system. This confinement state avoids any fracture of the material from occurring throughout the test. Two kinds of tests are carried out: low-velocity (around 1 mm/min) and high-velocity (around 10 m/s). The relative displacement is obtained using a testing machine during the low-velocity tests, and thanks to a Hopkinson bars system during the high-velocity tests. Examination of the measurements obtained during high-velocity tests shows that a workable steady state of equilibrium has been reached. As the interface stresses cannot be measured, the friction coefficient must be determined using indirect data: force measurements obtained from the machine or from the Hopkinson bars and strain measurements made on the exterior of the tube. The procedure to identify the steel-material friction coefficient from these measurements entails analytical modelling and finite element simulations of the mechanical behaviour of the tube-specimen assembly. The friction coefficient identified during the high-velocity tests is far higher than the coefficient identified during the low-velocity tests.     

船-冰作用中海冰典型破坏模式的离散元数值模拟

在破冰船破冰过程中,冰排主要表现为挤压与弯曲两种破坏模式.本文基于黏结离散单元法对船-冰作用中的这两种典型破坏模式进行数值模拟.海冰离散元数值试样采用随机排布方式生成,采用单轴压缩试验与三点弯曲试验相结合的方式标定模型中的细观参数.将船-冰碰撞中的挤压和弯曲作用方式简化为直立或倾斜 平板与海冰的作用模式,构建挤压与弯曲...     

Experimental mechanics at velocity extremes —Very high strain rates

The early experimental work of Clark and Wood with regard to von Kármán's theory on the effect of material flow and fracture at high strain rates has led to many controversial issues on these effects. Interest has been greatly revived in recent years because of the increased emphasis on such high-velocity forming processes as explosive and capacitor discharge. Considerable new work has been performed by Ling-Temco-Vought, Inc., for the Air Force, the results of which are presented in this paper. Data have been accumulated on tensile and compression specimens, spherical bulging and cylindrical bulging for a wide variety of materials. This high-speed information has been generated with the use of a special projectile impact machine and special presses utilizing various combinations of explosive and capacitor-discharge energy, with strain rates to 10¹/sec. The effect of velocity on ductility is discussed for total strain distribution, uniform strain, double necking and critical impact velocity. The modes of failure for various part shapes are presented and related to the forming velocity.     

含随机填充孔圆形蜂窝结构的面内冲击性能

研究多孔材料细观结构与宏观力学性能之间的关系, 建立具有固定相对密度的含随机固体填充孔的圆形蜂窝结构模型。在此模型的基础上具体讨论了不同孔洞填充比和冲击速度对圆形蜂窝结构变形模式、动态冲击平台应力以及能量吸收性能的影响。研究结果表明:填充孔在蜂窝变形过程中有局部牵制作用, 蜂窝材料变形模式仍为准静态模式、过渡模式、动态模式; 当变形模式为过渡模式或动态模式时, 结构的平台应力与速度的平方成线性关系, 存在明显的速度效应; 高速冲击下, 含固体填充孔的蜂窝结构单位质量吸收的能量高于规则蜂窝结构。研究结果可为蜂窝材料的研究和设计提供参考。     

单层5A06铝合金板高速撞击实验研究

采用非火药驱动二级轻气炮发射球形弹丸,对单层5A06铝合金板进行高速撞击实验研究,从而模拟空间碎片对航天器防护结构的高速撞击作用。实验得到了该铝合金板在不同的速度区间的损伤模式。结果表明,弹丸撞击速度一定时,弹坑深度和弹坑直径均与弹丸直径呈线性关系。当撞击速度在4km/s至5km/s时,靶板上的弹坑深度和弹坑直径随撞击速度的增大而减小,在其它速度范围内,弹坑深度和弹坑直径随撞击速度的增大而增大。通过固定弹丸直径,变化撞击速度,寻找临界撞击速度的方法获得了该铝合金板在弹丸撞击速度为1.0km/s至4.2km/s时的撞击极限曲线,并将实验弹坑深度与由Cour-Palais方程得到的预测弹坑深度进行了比较,实验弹坑深度大于预测值。     

Influence of ice conditions on the effectiveness of the resonant method of breaking ice cover by submarines

This paper describes a numerical model for analyzing the stress-strain state of ice cover which has fractures of various widths caused by hydrodynamic loads due to submarine motion. Calculations and experiments were performed using the model of unbreakable ice, and the results were used to obtain dependences of the amplitude of deflections and stress in ice cover on the width of the ice fracture. The behavior of ice cover under wave loading was studied.     

Numerical prediction of fracture in the Taylor test

A flat-nosed cylinder moving at a sufficiently high impact velocity in the classical Taylor test will always fracture. In this paper, fracture phenomena and fracture mechanisms in the Taylor test are investigated numerically based on a recently developed ductile fracture locus with the cut-off value for the negative stress triaxiality at −1/3. The impact velocity of the projectile ranges from 240 m/s to 600 m/s. The lower velocity is applied to a less ductile 2024-T351 aluminum alloy cylinder while the higher velocity is introduced for more ductile Weldox 460 E steel. Three distinct fracture modes are recreated numerically: the confined fracture inside the cylinder, the shear cracking on the lateral surface, and the petalling, all of which are consistent with experimental results presented in the open literature. It is found that a more ductile cylinder tends to fail by petalling while a less ductile one by shear cracking. Confined fracture is a common failure mode for both materials, which occurs in a wide range of the impact velocity. The ductile fracture criterion with the cut-off value predicts realistic fracture patterns for short cylinders deforming predominantly under compression.     

层状结构冰球的高速撞击特性实验

为研究冰雹在不同撞击速度下的破碎特性及致损能力,通过空气炮加载技术,开展了单一性状冰球和层状结构冰球高速撞击刚性靶实验,利用测力杆记录了两种类型冰球在不同撞击速度下的撞击力时程曲线,并结合高速摄影技术研究了两类冰球的高速撞击破碎特性。实验结果表明:两类冰球高速撞击刚性靶的宏观破碎特性相似,在碰撞初始阶段冰球已完全破碎,形成微颗粒团簇体,反向溅射角呈随撞击动能增大而增大的趋势;与单一性状冰球撞击力曲线相比,层状结构冰球撞击力曲线中出现二次上升信号,推测是由于破碎界面在层间界面发生偏折,小球未完全破碎再次撞击测力杆所导致;高速撞击下层状结构冰球的撞击力高于单一性状冰球,推测是由于层间结构的存在延缓了冰球的破碎进程,提升了其在冲击方向传递动量的能力,进而产生了更高的撞击力。研究结果有助于深化对冰雹在高速撞击下的破坏力学行为的认识,同时可为飞行器结构防护、冰雹撞击安全设计研究等提供参考。     

Mechanical behavior of hexagonal honeycombs under low-velocity impact – theory and simulations

Based on the cells’ collapse mechanisms of the hexagonal honeycombs revealed from the numerical simulations under the low-velocity impact, an analytical model is established to deduce the crushing strength of the honeycomb and the stress at the supporting end both as functions of impact velocity, cell size, cell-wall angle, and the mechanical properties of the base material. The results show that the honeycomb’s crushing strength increases with the impact velocity, while the supporting stress decreases with the increase of the impact velocity. Combining with the dynamic predictions under the high-velocity impact in our previous work (Hu and Yu, 2010), the crushing strength of the honeycombs can be analytically predicted over wide range of crushing velocities. The analytical expression of the critical velocity is also obtained, which offers the boundary for the application of the functions of the honeycomb’s crushing strength under the low-velocity and the high-velocity impacts. All of the analytical predictions are in good agreement with the numerical simulation results.     

Behaviour of uniaxially preloaded aluminium plates subjected to high-velocity impact

The purpose of this paper is to evaluate the behaviour of structural thin plates made of 7075-T6 aluminium alloy under normal impact of spherical steel projectiles, when they are subjected to uniaxial in-plane tensile preload. Also, nonloaded plates were tested under high-velocity impact. The impact and residual velocity were measured in all tests. From the relationships between the impact and residual velocities, the ballistic limit was estimated by a least-squares method. No significant differences were found between the ballistic limits under the two loaded conditions. Above the ballistic limit in the preloaded plates, unstable cracks that can generate the catastrophic failure of the plate were observed. At similar velocities, this phenomenon was not observed in nonloaded plates.     

Response and Damage Tolerance of Composite Sandwich Structures under Low Velocity Impact

The deformation and failure response of composite sandwich beams and panels under low velocity impact was reviewed and discussed. Sandwich facesheet materials discussed are unidirectional and woven carbon/epoxy, and woven glass/vinylester composite laminates; sandwich core materials investigated include four types of closed cell PVC foams of various densities, and balsa wood. Sandwich beams were tested in an instrumented drop tower system under various energy levels, where load and strain histories and failure modes were recorded for the various types of beams. Peak loads predicted by spring-mass and energy balance models were in satisfactory agreement with experimental measurements. Failure patterns depend strongly on the impact energy levels and core properties. Failure modes observed include core indentation/cracking, facesheet buckling, delamination within the facesheet, and debonding between the facesheet and core. In the case of sandwich panels, it was shown that static and impact loads of the same magnitude produce very similar far-field deformations. The induced damage is localized and is lower for impact loading than for an equivalent static loading. The load history, predicted by a model based on the sinusoidal shape of the impact load pulse, was in agreement with experimental results. A finite element model was implemented to capture the full response of the panel indentation. The investigation of post impact behavior of sandwich structures shows that, although impact damage may not be readily visible, its effects on the residual mechanical properties of the structure can be quite detrimental.     

空心玻璃微珠/环氧树脂复合材料简支冲击实验研究

制备了质量不同配比改性空心玻璃微珠(HGB)填充环氧树脂复合材料,对该材料标准试件进行了摆锤冲击实验,得出了不同配比材料的冲击韧度,并使用SEM观察了冲击断口的断面形貌。实验发现,玻璃微珠含量为2%的复合材料冲击韧度最小,此时材料抗冲击性能最差;而当含量超过2%,复合材料的冲击韧度会随玻璃微珠含量增加而增大;断面上玻璃微珠在基体中有拔出、断裂和破碎三种状态,以断裂破坏的玻璃微珠为主导破坏形式。以上实验发现对此类复合材料的研究和应用具有参考意义。     

破片群作用下复合材料层合板近场动力学损伤模拟

采用一种新兴的无网格法——近场动力学理论,模拟复合材料结构在破片群载荷作用下的损伤情况。根据复合材料结构受到载荷的特性,总结破片群冲击作用下复合材料结构损伤特性,分析其破坏过程,研究破片群增强效应,并对破片速度、破片数量、破片群间距对侵彻能力增强效应的影响进行分析。结果表明:层合板结构在高速破片群侵彻作用下损伤模式多样,与破片数量、速度、间距相关;破片数量的增加,对破片群侵彻能力增强效应明显;破片间距与破片群侵彻能力增强效应负相关,破片间距减小,破片群损伤效应提高;破片速度直接决定穿透时间,破片速度的提高使得穿透时间缩短,应力波的叠加效应不足以影响破片群的侵彻能力。     

陶瓷/液舱复合结构抗侵彻数值分析

为研究陶瓷/液舱复合结构抗侵彻机理,在前期弹道冲击实验结果基础上,运用LS-DYNA进行了数值模拟,再现了陶瓷/液舱复合结构在弹体冲击下的破坏过程和破坏模式,得到与实验一致的结果。结果表明:弹体撞击结构后,结构内产生的冲击波以撞击处为圆心、以球形向前传播,并在结构内来回反射振荡;弹体在水中运动时,水中形成空泡且不断扩展,弹体头部水域形成高压区域;弹体发生墩粗和侵蚀破坏,在低速冲击下,弹体破坏主要发生在穿透陶瓷和前面板过程中,在高速冲击下,弹体破坏主要发生在水中运动阶段,最终形成类似“饼状”的严重变形;前、后面板发生局部破坏和整体变形,在高速弹体撞击下,后面板将发生花瓣开裂。     

高速冲击载荷下梯度金属泡沫夹芯梁的动态响应与失效

采用泡沫弹冲击加载实验对梯度金属泡沫夹芯梁结构开展了不同冲击强度下的动态响应和失效研究,分析了由三种不同密度泡沫铝组成的等面密度的五种不同梯度的夹芯结构在夹支边界条件下的抗高速冲击性能,结合三点弯曲实验,研究梯度效应对夹芯结构抗冲击性能的影响。研究表明：密度梯度对结构的失效过程和失效模式有着明显的影响,且夹芯梁结构的初始失效模式对结构整体响应和主要的能量吸收机制起着主导作用;当冲击条件不足以使得均质芯材发生压缩时,均质及负梯度夹芯结构初始失效模式为整体弯曲变形,低强度芯层位于前两层的梯度结构随着冲击强度的变化出现不同程度的局部芯层压缩;当冲击强度较低时,梯度结构通过丰富的局部失效表现出明显优于均质结构的抗冲击变形能力;当冲击强度大于临界值时,均质结构具有更好的抗冲击变形能力。通过合理地设计密度梯度实现逐层压缩吸能,能够有效的提升防护结构的抗冲击性能。