冲击载荷下分支交错层状仿生复合材料动态断裂行为的实验研究和数值模拟

通过三点弯动态冲击实验和数值模拟方法,研究了分支交错层状仿生复合材料的动态断裂韧性。首先设计并制备了分支交错层状仿贝壳复合材料试样,即将一种脆性刚性材料和一种橡胶类材料分别作为复合材料的硬质层和软胶层;随后采用改进的分离式Hopkinson压杆装置进行了三点弯冲击实验;接着讨论了初始冲击速度、硬质材料长宽比、软质材料层厚度对复合材料试样动态断裂行为的影响;最后采用ABAQUS有限元数值模拟,研究了不同宽度和不同冲击方向对复合材料试样动态断裂韧性和裂纹扩展的影响。结果表明:随着冲击速度和硬质材料长宽比增加、软胶层厚度减小,裂纹越倾向于沿直线扩展,反之,裂纹越倾向于绕过硬质材料沿着软胶层呈折线扩展;试样的峰值动载荷和起裂时间也随之增大。有限元模拟结果表明:随着结构总宽度的增大,试样断裂韧性增加,裂纹倾向于绕过硬质材料沿着软胶层扩展;采用实验设计的冲击方向时,试样的断裂韧性高于其他方向。     

含有缺陷3C-SiC陶瓷拉伸性能的分子动力学模拟

SiC纤维增韧SiC基复合材料(SiC_f/SiC)由于其优越的性能而成为新一代核能系统重要候选材料之一.材料中的缺陷会使材料的力学性能发生变化,本文运用分子动力学程序LAMMPS模拟计算了分别含有空位、微空洞和反位替代三种缺陷的3C-SiC结构体系沿[100]方向的拉伸变形过程,原子间相互作用采用Tersoff多体势描述.通过模拟得到不同缺陷体系的应力—应变曲线和拉伸过程中体系能量,通过分析应力-应变曲线,得到了不同缺陷体系的杨氏模量、断裂应变、拉伸强度随缺陷"浓度"的变化关系,最后分析了3C-SiC拉伸断裂机理.研究结果表明,空位和微空洞对杨氏模量、拉升强度的影响类似,都是随着缺陷"浓度"的增加而减小,反位替代缺陷使体系的杨氏模量随缺陷"浓度"的增加而增大.     

层状千枚岩的断裂特性

采用中心直切槽半圆盘层状岩样测试了层状千枚岩的断裂性能,并基于黏结单元建立了层状岩石的有限元数值计算模型,系统研究了层理倾角、层理强度、层理间距及切缝倾角等参数对层状千枚岩断裂特性的影响。结果表明:当层理倾角在0°～90°范围内时,Ⅰ型断裂韧度逐渐增大,峰值载荷和峰值位移也呈增大趋势;层理倾角为零时,发生张拉破坏。层理倾角在15°～45°时,剪切破坏占主导;层理倾角在60°～90°时,张拉破坏占主导。层理倾角为零时,破坏模式受层理强度影响较小;层理倾角分别为15°和30°时,随着层理强度增大,试样由剪切破坏向拉-剪耦合破坏演化;层理倾角在45°～90°时,试样均呈现拉-剪耦合破坏,且随着层理强度增大,试样有向拉伸破坏为主演化的趋势。层理间距较小时,裂纹呈沿层–穿层阶梯状扩展趋势明显;切缝倾角较大时,裂纹穿层扩展趋势明显。     

加载角度和线型缺口对锂离子电池隔膜拉伸性能的影响

锂离子电池隔膜作为防止正负极接触的物理屏障,其结构完整性对于电池安全至关重要。进行了4种商业隔膜单轴拉伸实验,分析加载角度和线型缺口对隔膜材料拉伸强度、弹性模量、断裂模式的影响。结果表明:无缺口试样在0°方向上的拉伸强度最大,90°方向上拉伸强度最小;当两个无缺口试样的加载角度互为补角时,它们的拉伸强度接近。对于缺口试样而言,缺口方向沿着90°的试样有最大破坏载荷;线型缺口试样有更高的弹性模量,但是塑性变形大幅减少。无缺口试样和缺口试样在拉伸过程中的断裂模式相同,即除0°试样沿横向断裂外,其他加载角度的试样均沿着纵向断裂。     

无机玻璃动态压缩破坏的离散元模拟

利用离散元软件PFC~(2D)(Particle Flow Code)建立了分离式霍普金森压杆(SHPB)系统,模拟了无机玻璃圆柱和圆盘试件在冲击压缩下的动态力学行为和失效破坏模式。结果表明:无机玻璃作为典型的脆性材料,其抗压强度具有明显的应变率效应,而杨氏模量则对应变率不敏感;无机玻璃圆柱的破坏过程受纵向压力、端面摩擦力以及横向惯性力的影响,初期微裂纹呈三角状分布,随着纵向应力水平的提高,出现明显的泊松效应,产生横向张应力,致使微裂纹沿纵向扩展,最终试件发生沿轴向的劈裂断裂;摩擦系数和泊松比对试件破坏模式及强度有一定影响。将建立的SHPB数值实验平台用于模拟无机玻璃巴西圆盘试验,揭示了圆盘发生中心开裂的拉伸特征及拉伸强度的应变率相关性。     

PBX炸药含各向异性损伤的黏弹性统计微裂纹本构模型初步研究

建立含损伤本构模型是研究炸药动态力学响应规律的基础。基于PBX炸药材料的宏观黏弹性特征和细观上微裂纹面的方向性,建立了含各向异性损伤的黏弹性统计微裂纹(Aniso-Visco SCRAM)本构模型,简化后得到单轴应力加载下的本构方程。利用数值计算程序,以PBX9501为例,分析了微裂纹扩展的各向异性、PBX炸药破坏强度及临界应变的拉压异性和应变率相关性,考察了微裂纹数密度、初始微裂纹尺寸、微裂纹面摩擦系数及断裂表面能4个主要参数的敏感性及影响规律。结果表明,它们对微裂纹的扩展演化有较大影响,进而导致材料表现出不同的力学响应。     

动载下准脆性材料的泛形裂纹研究

为建立动态拉伸载荷作用下准脆性材料裂纹扩展路径的泛形表征,提出了一种非均匀准脆性材料动态裂纹扩展的泛形模型,计算得到的泛形裂面复杂度与已有实验数据吻合较好。结果表明:动态拉伸载荷作用下的裂纹扩展路径是泛形的,其复杂度随加载应变率的增大而减小,并与材料动态拉伸承载能力的空间随机分布无关,且随Weibull分布形状参数m的增加而减小。研究结果为分析动态拉伸载荷作用下准脆性材料的裂纹扩展机理即泛形表征提供了依据。     

韧性材料的微裂纹扩展与分叉的晶体相场模拟

采用晶体相场方法研究韧性单晶材料在双轴拉伸条件下微裂纹扩展与分叉的演化过程,分析应变、温度、初始裂口形状等因素对裂纹扩展和分叉的影响.结果表明:对于简单的单向拉伸,应变需要达到一定的临界值,裂纹扩展才会启动.对于二组相互垂直的双轴拉伸作用,当应变达到临界值后,裂纹扩展过程中会发生分叉现象.温度越高,裂纹扩展越快且分叉越多.裂纹在扩展过程中,体系能量不断降低,当裂纹出现分叉时,体系能量降低得更快.在裂纹扩展过程中,有时是会在裂尖处前方出现微小的空洞,类似在裂纹尖端前方出现位错发射情况,这些微小的空洞逐渐扩大连成裂纹.本文所得结果与相关模拟结果和实验结果符合.     

压缩载荷下UHMWPE纤维复合材料层合板的力学性能与失效分析

为获得超高分子量聚乙烯(ultra-high molecular weight polyethylene,UHMWPE)纤维复合材料层合板在静、动态压缩载荷下的力学性能与失效模式,采用万能材料试验机和分离式霍普金森压杆对材料进行面外方向的压缩实验,获得了不同应变率下材料的应力-应变关系。通过扫描电子显微镜观察材料微观失效形貌,分析了材料的失效模式。结果表明,UHMWPE纤维复合材料层合板在应变率较低(6.7×10^-3～6.7×10^-2 s^-1)且相差较小时,无应变率效应;在高应变率(2.05×10³～5.27×10³ s^-1)下,材料具有明显的应变率效应。压缩强度随应变率的增加而增大,动态增强因子逐渐增大,具有明显的应变率强化效应。静态压缩载荷下,材料的主要破坏模式为纤维的拉伸、断裂;动态压缩载荷下,材料的主要破坏模式为纵向位错分层。     

冲击载荷下仿贝壳砖泥结构的动态响应

贝壳作为典型的抗冲击生物材料,具有轻质、高强、高韧等优异性能。通过构建仿贝壳砖泥结构有限元模型,并对其在落锤冲击载荷下的动态响应进行数值模拟,分析了堆叠层数、冲击速度及锤头类型对仿贝壳砖泥结构能量吸收性能的影响。结果表明:5类堆叠层数下的仿贝壳砖泥结构的比吸能呈先增加后减小的变化趋势,并且在所设计的5类堆叠层数结构中,3层仿贝壳砖泥结构具有最大的比吸能,其值较比吸能最小的单层结构提高了10.8%;随着冲击速度的提升,结构载荷峰值及能量吸收均略有增大;相同锤径下,圆柱形锤头较半球形锤头更易穿透模型。     

Effect of microstructure on fracture in age hardenable Al alloys

ABSTRACT

In the present study, the fracture behaviour of AA6016 alloy was investigated during bending deformation. Wrap-bend tests were conducted and the material was subjected to different bend angles to study crack propagation. The average grain size of the as-received material is approximately 45?μm. The aspect ratio of the grains was changed from 0.53 to 0.40 during bending. The presence of deformation bands was observed during bending in both tensile and compressive regions of the sample. No orientation correlation was observed between the deformation band and its corresponding parent grain. The Schmid factor inside the deformation bands was higher than that of the parent grain, which indicates that the deformation bands accommodate strain during bending. The crystallographic texture evolved significantly during bending deformation. The strength of cube texture component decreases with increasing bend angle and new texture components formed during bending. These new texture components favour either single slip or duplex slip. A mixture of intra-granular and inter-granular fracture occurs during bending. It is observed that inter-granular crack propagation is predominantly favoured along high-angle boundaries, and grain boundary de-cohesion occurs in regions where the misorientation angle is greater than 40°. The formation of deformation-induced coincidence lattice site (CSL) boundaries is also observed during bending and it is shown that the volume fraction of CSL boundaries of Σ3 type increases with increasing bend angle. The current study shows that the formation of deformation-induced CSL boundaries of Σ3 type in AA6016 alloy can improve its inherent resistance to crack propagation during bending.     

低能He~(2+)与He原子碰撞转移电离出射电子能谱研究

利用反应显微成像谱仪对70和100keV He2+与He原子碰撞转移电离(TI)过程中不同出射角度的电子能谱进行了测量,观测到出射电子能谱具有如下分布特征:出射电子速度分布介于0和入射离子速度vp之间;在不同出射角度电子能谱分布均有一极大值存在,随着出射角度的增大,能谱分布极大值逐渐减小;当电子出射角度等于45°时,多数电子集中在0eV附近。上述特征可由低能离子-原子碰撞"准分子"模型进行定性解释。在100keV He2+-He转移电离出射电子能谱中有靶电子被俘获至散射离子连续态(electron capture to continuum,简称ECC)电子的贡献,这可看做是动力学两步过程的作用。     

Temperature dependences of the strengths of a carbon fiber and a three-dimensional reinforced carbon-carbon composite

At a fixed tension rate, the ultimate tensile strength of a carbon fiber decreases nonlinearly with increasing temperature T. This nonlinearity is caused by a change in the statistics of atomic vibrations from quantum (at T < 2250 K) to classical (at T > 2250 K) statistics. To take into account the quantum statistics, quantum function F _q is introduced into Zhurkov’s equation instead of temperature; the value of this function is calculated from the temperature dependence of the specific heat of carbon. This equation gives the values of the fracture activation energy (≈16 eV) and parameter γ (≈0.15 nm³). The strength of the three-dimensional reinforced carbon-carbon composite decreases up to ≈1800 K and increases as the temperature grows further. The decrease in the strength is explained by an increase in the rate of fiber and matrix fracture with increasing temperature, and the increase in the strength is explained by a decrease in the strength of the fiber-matrix adhesion bonds at high temperatures. As a result of this decrease, fibers begin to move with respect to each other under load, and the stresses applied to them level off. Although the fiber strength continues to decrease with increasing temperature, this effect increases the composite strength.     

Crack propagation in amorphous polymers and their composites

The fracture energy of a polymer depends strongly on the viscoelastic responses of the material, and therefore is a function of temperature and crack velocity. The toughness of a composite is determined by the way in which the reinforcing filler modifies the energy dissipating mechanisms of the polymeric matrix.

The fracture toughness of a variety of polymeric glasses and their composites with glass beads, glass fibers, and rubber particles was measured. The velocity of rapidly moving cracks and the crack propagation rates under controlled loading conditions were also measured.

It was found that the crack propagation velocities in unfilled and glass bead filled materials were controlled by the longitudinal stress waves in the matrix and that the only effects of the glass beads were to blunt the crack tip and limit the viscous deformation. The effect on fracture toughness was relatively small and either positive or negative, depending on which of the above two factors dominated.

The presence of rubber particles as a second phase lowered terminal crack propagation velocities and greatly increased the fracture toughness, indicating a crack retarding effect of the rubber particles. This is related to the induction of crazes in the matrix by the rubber phase.

Glass fibers had a tendency to bridge the tip of a propagating crack, thereby greatly increasing the fracture toughness. In this case the work of fracture comes from a combination of the elastic strain energy stored in the fibers, the energy dissipated in debonding the fibers from the matrix, and the fracture energy of the matrix itself.     

Variation in electromagnetic radiation during plastic deformation under tension and compression of metals

This paper presents some significant variations in the intermittent electromagnetic radiation (EMR) during plastic deformation under tension and compression of some metals with selected crystal structure, viz. zinc, hexagonal closed packed (hcp), copper, face-centred cubic (fcc: stacking fault energy 0.08 J/m²), aluminium (fcc: stacking fault energy 0.2 J/m²) and 0.18 % carbon steel, body-centred cubic (bcc). The intermittent EMR signals starting near yielding are either oscillatory or exponential under both modes of deformation except a very few intermediate signals, random in nature, in zinc under compression. The number and amplitude of EMR signals exhibit marked variations under tension and compression. The smooth correlation between elastic strain energy release rate and average EMR energy release rate suggests a novel technique to determine the fracture toughness of metals. The first EMR emission amplitude and EMR energy release rate occurring near the yield increase, but maximum EMR energy burst frequency decreases almost linearly with increase in Debye temperature of the metals under tension while all EMR parameters decrease nonlinearly under compression. These results can be developed into a new technique to evaluate dislocation velocity. The EMR amplitude and energy release rate of the first EMR emission vary parabolically showing a maxima with increase in electronic heat constant of the metals under tension while they first sharply decrease and then become asymptotic during compression. However, the variation in EMR maximum energy burst frequency is apparently similar under both modes of deformation. These results strongly suggest that the mechanism of EMR emission during plastic deformation of metals involves not only the interaction of conduction electrons with the lattice periodic potential as presented in the previous theoretical models but also the interaction of conduction electrons with phonons. However, during crack propagation and fracture, charge oscillations at fractured surfaces due to breaking of atomic bonds constitute an additional factor.     

多晶石墨烯拉伸断裂行为的分子动力学模拟

采用分子动力学模拟方法研究了不同晶界对石墨烯拉伸力学特性及断裂行为的影响. 定义了表征晶界能量特性的新参量缺陷能, 并以此为基础分析了晶界结构的能量特性. 探讨了晶界对弹性模量和强度极限等的影响以及强度对晶界能量特性的依赖关系. 结果表明: 晶界能量特性可以间接反映晶界强度; 同时, 晶界中缺陷会使实际承载碳键数量小于名义承载碳键数, 从而在较大范围内影响弹性模量. 分析了不同晶界的断裂过程, 发现了裂纹扩展方向的强度依赖性: 低强度晶界主要是以碳键直接断裂为主要方式的沿晶断裂, 而高强度晶界通常是碳键直接断裂和Stone-Wales翻转过程交替进行下的穿晶断裂. 研究结果可为石墨烯器件的设计制造提供理论指导.     

Simulation investigation on channel surface plasmon guiding by terahertz wave through subwavelength metal V-groove

The waveguide propagation properties of terahertz wave through subwavelength metallic triangular groove channels have been simulationally investigated. The effects of groove depth, groove angle and dielectric filling materials on propagation property have been shown and discussed. The results show that with increasing of groove depth and angle degree the propagation constant of channel surface plasmon polariton mode decreases. The propagation constant of poor conductive metal is larger than that of good conductive metal, which may result from the fact that the former has larger skin depth. In addition, the confinement of CSPP mode could be improved by increasing the refractive index of dielectric filling materials.     

Fracture energy of gels

To clarify effects of crack speed and cross-link density on the fracture energy of acrylamide gels, we evaluated the roughness of the fracture surface and measured the fracture energy taking into account the roughness. The fracture energy increases linearly with crack speed Vin a fast crack speed region, and the increasing rate of fracture energy with V decreases with increasing cross-link density in the gels. In a slow crack speed region the fracture energy depends on crack speed more strongly than in the fast crack speed region. This indicates that a qualitative change exists in the fracture process of the gels. Received 8 March 2000 and Received in final form 7 July 2000     

Crack Tip Opening Angle as a Fracture Resistance Parameter to Describe Ductile Crack Extension and Arrest in Steel Pipes under Service Pressure

The angle between two element sides representing the crack tip is defined as the crack tip opening angle (CTOA). Its critical value is used as a criterion of fracture resistance for characterizing stable tearing in thin metallic materials. Various methods are used for determination of the CTOA. Optical microscopy is one of the most common methods as well as fitting of experimental load-displacement diagrams by the finite element method (DIC). Additionally, analytical analysis using the experimental load-displacement curve method (SSM) derived from the plastic hinge model of deflection in three-point bending of a ductile specimen is applied. This approach assumes a constant rotation centre distance. Values of CTOA for API 5L X65 pipe steel found by three methods—DIC, CNM, and SSM—are given. Values of CTOA given by these three methods are similar and close to 20°. A discussion on the different parameters used to characterize the fracture resistance of running cracks in a pipe under service pressure is presented. The energy of fracture at impact determined by Charpy or drop-weight tear test (DWTT) tests and the critical J energy parameter are considered as well as the yield locus after damage, cohesive zone energy, and CTOA is another approach. One notes that CTOA is assumed to be constant during stable crack extension and decreases linearly with crack length during the instable and primary phase. A numerical technique to describe a ductile running crack using the node release technique and using CTOA as the fracture resistance criterion is presented. This method is compared with three different two-curve methods (TCMs): the Battelle, high strength line pipe (HLP), and HLP-Sumitomo methods. The Batelle TCM, as the oldest method, based on Charpy energy, gives a strongly conservative prediction. Predictions by the CTOA method are close to those obtained by the HLP-Sumitomo one.