应力波载荷作用下线弹性断裂过程的动态分析方法研究

利用Hopkinson单压杆实验装置 ,对材料的线弹性动态断裂特性进行了研究 ,建立了应力波载荷作用下动态裂纹起裂及扩展过程的动态分析方法 ,采用该方法可同时测得材料的动态裂纹起裂时间、断裂韧性和裂纹扩展速度。40Cr钢三点弯曲试样的实验结果表明 :该钢的动态裂纹扩展过程主要是减速过程 ,在2 2 5TPam /s的加载速率下 ,起裂时间为 2 8 0 0 s,最大裂纹扩展速度为 478 91m/s ,动态断裂韧性为6 3 12MPam。     

某船用钢动态弹塑性断裂韧性的试验测试

描述了利用Hopkinson压杆技术加载三点弯曲试样测试材料动态弹塑性断裂韧性的试验方法。材料的动态应力－应变行为测试在SHPB装置上进行，试样上的动态载荷历史由Hopkinson压杆直接测得，在此基础上，利用自行编制的ANSYS宏程序计算得到J积分历史；与起裂时间相对应的J积分值，即为动态弹塑性断裂韧性。采用上述方法进行了某船用钢的动态断裂试验，首次获得了该钢的动态弹塑性断裂韧性值，为舰船的抗爆能力计算，防动态断裂设计和安全评定提供了基础数据。     

加载角度对层理页岩裂纹扩展影响的实验研究

采用分离式霍普金森压杆（SHPB）系统对页岩进行冲击实验,研究层理角度对页岩动态断裂过程的影响,在裂尖设置裂纹扩展计,借助高速摄影和数字图像相关（DIC）技术对页岩中心切槽半圆盘弯曲（NSCB）试件断裂的全过程进行研究,得到了不同加载角度下页岩的动态起裂韧度、裂纹扩展速度、断裂过程中应变场和水平位移场的变化规律。实验发现:不同加载角度下,页岩的动态起裂韧度具有显著的各向异性,加载角度与动态起裂韧度呈正相关;加载角度对试样的裂纹扩展速度具有显著影响,与裂纹扩展速度呈负相关;当冲击速度较低时,切槽方向是裂纹扩展的优势方向,而当冲击速度较高时,试样会产生沿层理弱面的次生裂纹,次生裂纹对试样的断裂具有显著影响。     

40CrNiMoA钢的动态剪切断裂行为研究

采用Ｈｏｐｋｉｎｓｏｎ单压杆技术对４０ＣｒＮｉＭｏＡ钢单边平行双裂缝试样进行高速剪切加载,得到了在不同动态高加载率时的动态断裂起始韧性。实验表明,在实验的加载率范围内,沿原裂纹方向扩展的动态剪切型断裂存在两种不同的断裂模式,分别称为常规的韧性剪切断裂模式和绝热剪切型断裂模式。前者的断裂韧性随加载率的提高而增大,而后者的断裂韧性则随加载率的提高而减小。根据对这两种断裂行为及其相互转变的实验结果的分析探讨,认为存在一个临界应力强度因子率Ｋｃｄ,它表征两种断裂模式发生转变的条件。     

超强钢18NiC250在不同加载速率下的断裂韧性

为了揭示国产超强钢18NiC250的强度、断裂韧性随加载速率的变化规律,利用电子万能试验机和Hopkinson压杆,测试其在0.001～2 000 s-1的塑性流动应力应变曲线及在10-1~106 MPam1/2/s的断裂韧性,同时对断裂破坏机理进行了微观分析。结果表明:该材料的强度对加载速率不敏感,即流动应力基本保持在1.9 GPa;而断裂韧性很敏感,当加载速率由10-1 MPam1/2/s增大到106 MPam1/2/s时,断裂韧性降低了38.2%,断裂模式由韧窝断裂转变为解理断裂。     

混凝土材料的层裂特性

利用Hopkinson压杆作为实验设备,通过试件后面吸收杆上应变波形研究了混凝土材料的层裂特性。不同强度混凝土在不同加载率下的实验结果表明,混凝土层裂强度与其压缩强度以及加载速率有关,给出了层裂强度和压缩强度以及加载率之间关系的经验公式,并指出不同加载速度下混凝土裂纹扩展方式不同是层裂强度率效应的主要原因。加载压缩波损伤的影响、重复加载实验以及多次层裂的顺序都表明,损伤对混凝土的层裂过程有重要影响。     

温度影响下砂岩的细观破坏及变形场的DSCM表征

通过扫描电镜(SEM)实时在线观察研究了温度影响下砂岩的细观破坏,观察到砂岩的脆性断裂可同时发生在不同地方、不同矿物可能独立承载和裂纹分叉等现象. 温度低于100°C时,主裂纹附近有许多微裂纹和支裂纹发生;而温度超过150°C之后,表面很少出现支裂纹或二次裂纹. 随着温度的升高,砂岩的断裂韧性有先升高后降低的趋势,150°C左右是断裂韧性变化的临界温度. 随着温度的升高,砂岩的细观断裂机制有由脆性机制向延性机制转变的趋势,抵抗和协调变形的能力都得到增强. 细观尺度下砂岩的破坏机制有沿颗粒断裂、穿颗粒断裂及其混合断裂,其中沿颗粒断裂机制占主导地位,这是由于沿颗粒破坏需要消耗较少的能量,而穿颗粒断裂需消耗较多的能量. 利用数字散斑相关方法(digital speckle correlation method, DSCM)对SEM下砂岩破坏的细观变形场进行了测量,这表明利用DSCM表征岩石的局部变形场的连续式测量是可行的.      

岩石SHPB实验加载过程中应力平衡问题分析

运用一维应力波理论,分析了弹性应力波在分离式Hopkinson压杆(SHPB)实验中的传播过程,推 导出试件和压杆中应力分布相关计算公式。探讨了有关因素对试件应力平衡时间的影响规律,发现试件应力 平衡时间受试件/压杆广义波阻抗比和入射加载升时的影响显著,而不受试件/压杆截面积比和入射加载应力 幅值的影响。结合岩石SHPB实验,计算分析了不同入射加载应力幅值在不同入射加载升时情况下,试件达 到应力平衡时的应变变化特征,并提出了降低试件在应力平衡时的应变控制方法,使试件在未达到断裂应变 之前达到应力平衡,以保证实验的有效性。得出的结论对岩石类脆性材料SHPB实验方案设计具有一定的 参考意义。     

685均质钢静动态断裂韧性实验研究

为了解685均质钢的裂纹在静态加载和动态加载下的裂纹起裂和扩展情况,分别在静态试验机和基于Hopkinson杆技术改进的动态加载装置上采用三点弯曲试样对685均质钢的静、动态断裂韧性进行了研究。685均质钢中存在的少量孪晶马氏体组织对其断裂韧性造成了不利影响,可以适当降低碳含量来改善685均质钢的断裂韧性性能。当加载率KI≤1.8778×106MPa（m）^1/2/s时,685均质钢的动态断裂韧性值都随加载率的增加而下降。当加载率KI>1.8778×106MPa（m）^1/2/s时由于裂纹尖端热软化效应的影响,使得该材料的动态断裂韧性值又上升。采用高速摄影技术记录了裂纹的起裂和扩展,测得了裂纹的扩展速率。发现用裂纹嘴张开位移（Crack Mouth Opening Displacement,CMOD）法计算得到的动态断裂韧性值与用电阻应变片计算的结果相一致。     

多功能Hopkinson压杆型试验装置的研制与应用

对传统的Hopkinson压杆试验装置进行了改进和完善，除了用于材料的动态压缩以外，还能进行动态拉伸、绝热剪切、裂纹扩展速度测定、动态断裂韧性测试等．本文介绍了几种试验方法的基本原理，并给出了试验结果．     

动载下韧性撕裂在三种管线钢材中传播阻力的研究

阐述了用实验确定在常温及静载和动载条件下韧性撕裂在三种管线钢材中传播时的断裂比能值。实验中采用了销钉加载双面开槽的双悬脊梁(DCB)试件。应用能量平衡法对实验结果进行了分析。考察了加载速率和试件厚度对撕裂韧性的影响。结果表明,管线钢材对韧性撕裂传播的阻力在动载条件下增大,对较薄的试件及应变率敏感性较高的材料其增大更为明显。对于纯剪切断裂的传播来说,撕裂韧性一般随试件厚度的增大而增加。     

伴随微孔洞生长的裂纹弹塑性定常扩展

裂纹在韧性材料中扩展时,将们随着微孔洞的萌生和生长,孔洞的萌生和深化将直接影响着材料的总体断裂韧性和强度,以往的研究主要集中在将裂纹的扩展刻划为微孔洞的萌生、生长和汇合这样一个过程。从传统的断裂过程区模型出发研究微孔洞的萌生和生长对材料总体断裂韧性的影响,通过采用Ｇｕｒｓｏｎ模型,建立塑性增量本构关系,然后针对定常扩展情况直接进行分析,孔洞对材料断裂韧性的影响由本构关系刻划,而在孔洞汇合模型中,上     

Effects of static electric field on the fracture behavior of piezoelectric ceramics

The paper gives an overview on experimental observations of the failure behavior of electrically insulating and conducting cracks in piezoelectric ceramics. The experiments include the indentation fracture test, the bending test on smooth samples, and the fracture test on pre-notched (or pre-cracked) compact tension samples. For electrically insulating cracks, the experimental results show a complicated fracture behavior under electrical and mechanical loading. Fracture data are much scattered when a static electric field is applied. A statistically based fracture criterion is required. For electrically conducting cracks, the experimental results demonstrate that static electric fields can fracture poled and depoled lead zirconate titanate ceramics and that the concepts of fracture mechanics can be used to measure the electrical fracture toughness. Furthermore, the electrical fracture toughness is much higher than the mechanical fracture toughness. The highly electrical fracture toughness arises from the greater energy dissipation around the conductive crack tip under purely electric loading, which is impossible under mechanical loading in the brittle ceramics. The project supported by an RGC grant from the Research Grant Council of the Hong Kong Special Administrative Region, China     

加载速率和几何尺寸对国产A508-III钢断裂行为影响的实验研究

由加载速率和几何约束改变而引起的压力容器钢韧脆转变问题是核能安全领域亟待解决的关键问题. 为了准确分析国产A508-III钢的动态断裂行为, 借助INSTRON VHS高速材料试验机, 开展了不同加载速率和几何尺寸条件下的国产A508-III钢的断裂韧性试验, 研究了加载速率和几何尺寸等因素对国产A508-III钢动态断裂韧性的影响. 研究表明, A508-III钢具有良好的抗冲击韧性, 随着加载速率的提高, 试样的总冲击吸收能基本保持恒定, 裂纹萌生吸收能量不断上升, 而裂纹扩展吸收能量呈下降趋势. J-Δa阻力曲线和条件起裂韧性J_Q随着几何约束的增加而降低, 随加载速率的增加而升高. 当达到某一临界速率时, 条件起裂韧性J_Q基本恒定, 试样断裂方式也由韧性断裂转变为韧?脆?韧混合断裂. 由于出现混合断裂模式, 发生脆性断裂时的最大J积分值J_max更适于描述国产A508-III钢的断裂韧性演化规律. 随着试样面外几何约束的降低, J_max随Δa_m的增加而线性增大. 试样面内几何约束越高, J_max与Δa_m之间的线性关系斜率越大. 随着试样几何约束的增加, 材料的韧脆转变速率增加, J_max值下降. 改变几何约束只能在有限的加载速率范围内改变材料的断裂方式, 当加载速率超过某个临界值时, 加载速率成为影响材料断裂方式的主要因素.      

Effect of cooling rate on fracture behavior of polypropylene

The purpose of this work is to investigate the influence of morphology, induced by cooling rate during molding, on the time–temperature dependence of fracture behavior of polypropylene (PP). Fractures tests were performed over a range of loading rates from 0.2 mm/min to 2.5 m/s, using the single edge notched bending specimen. The results show that the transition temperature from brittle to ductile behavior increases with decreasing cooling rate. However, at very low loading speed (0.2 mm/min), an opposite effect is observed, the brittle–ductile transition temperature diminishes with lower cooling rate. At low test speeds, the fracture performance is reduced with a decreasing cooling rate. Conversely, under impact, the fracture toughness of PP is enhanced with a decrease in cooling rate. This is explained by the mechanism of blunting of the crack tip due to adiabatic heating under high loading rates. The blunting effect results in a more significant plastic deformation of the crystalline region that requires a higher energy. The brittle–ductile transition was characterized by an energy activation process expressed by the Arrhenius equation. Decreasing the cooling rate results in a decrease of both the pre-exponential factor and the energy barrier controlling the time–temperature dependence of fracture behavior. The reduction of the pre-exponential factor corresponds to a more ordered morphology due to a reduction in the entropy and is consistent with a higher crystallinity. The reduction of activation energy with higher crystalline level suggests that the brittle–ductile transition also involves the primary relaxation process that is known to occur mostly in an amorphous structure. A higher crystallinity would restrain the primary relaxation processes and the brittle–ductile transition becomes more dependent on the secondary movements of the chain segments. The results demonstrate that the relationship between deformation rate, temperature, and mechanical performance of PP is not only controlled by molecular relaxation processes, but also strongly dependent on its morphology.     

塑性动态断裂实验研究

本文利用自制的实验装置,对韧性材料在爆炸冲击载荷作用下的塑性动态断裂特性,进行了实验研究,其中包括高塑性应变速率下,塑性区裂纹扩展过程和扩展速度的测试;塑性动态断裂韧性CTOD,及其在不同裂纹扩展速度下变化规律的测试。同时,对不同裂纹扩展速度的试件断口进行微观分析。     

Effect of adhesive thickness,adhesive type and scarf angle on the mechanical properties of scarf adhesive joints

The effects of adhesive thickness, adhesive type and scarf angle, which are determined as the main control parameters by the dimensional analysis, on the mechanical properties of a scarf adhesive joint (SJ) subjected to uniaxial tensile loading are examined using a mixed-mode cohesive zone model (CZM) with a bilinear shape to govern the interface separation. Particularly, the adhesive-dependence of the vital cohesive parameters of CZM, which mainly include initial stiffness, total fracture energy and separation strength, is introduced emphatically. The numerical results demonstrate that the ultimate tensile loading increases as the adhesive thickness decreases. Cross the ultimate tension, the joint loses the load-bearing capacity when adopting the brittle adhesive but sustains partial load-bearing capacity while selecting the ductile adhesive. In addition, for the joint with the ductile adhesive, the maximum applied displacement until the complete failure of it is directly proportional to the adhesive thickness, which is different from the case using the brittle adhesive. Taking the combination of the ultimate loading and applied displacement into account, failure energy is employed to evaluate the joint performances. The results show that the failure energy of the joint with the brittle adhesive increases as the adhesive thickness decreases. Conversely, the situation of the joint using the ductile adhesive is vice versa. Moreover, the effect of the adhesive thickness becomes more noticeable with decreasing the scarf angle owing to the variation of the proportion of each component of the mixed-mode. Furthermore, all the characteristic parameters (the ultimate tensile loading, the maximum applied displacement and the failure energy) that adopted to describe the performances of SJ increase as the scarf angle decreases. Finally, the numerical method employed in this study is validated by comparing with existing experimental results.     

不同加载速率下砂岩弯曲破坏的细观机理

岩石细观破裂形貌是岩石破坏机制的重要反映,为研究不同加载速率对砂岩弯曲破坏的影响,通过三点弯曲实验和扫描电镜方法,对某煤矿关键层砂岩弯曲破断裂纹细观形态以及裂纹的自相似性进行了研究。选取6个不同加载速率对岩样进行三点弯曲实验,观察其宏观断裂情况,并利用扫描电镜对弯曲断裂面表面裂纹细观结构进行观察,并拍摄不同倍数下的扫描电镜图片。对图片进行图像处理后得到砂岩弯曲断裂破坏细观裂纹信息,并计算得到微裂纹的分形盒维数值。结果显示:随着加载速率的提高,砂岩穿晶断裂的比例也随之升高,裂纹分形维数亦随着加载速率的增大而增加,同时,分形维数还与弯曲断裂破坏荷载和抗弯强度成正比。可见,加载速率对断裂方式有一定的影响,且加载速率越大断裂所需的破坏能越大,裂纹分布越广,表明开采速度与岩爆等岩体动力灾变有密切关系。     

Effect of Notch-Tip Radius on Dynamic Brittle Fracture of Polycarbonate

Polycarbonate (PC), which is considered to be a ductile amorphous polymer, is prone to brittle fracture in the presence of sharp notches. In the present work, effect of notch-tip radius on brittle fracture of PC is studied under static and under dynamic loading (high loading rates) conditions. Towards this end, a hybrid experimental and numerical approach is adopted. Dynamic fracture experiments using Hopkinson bar setup are performed on single-edge notched specimen of PC having different notch-tip radii. Ultra-high speed imaging is used for real-time observation of the fracture process. Finite element simulations are simultaneously performed using a well calibrated elastic-viscoplastic constitutive model for polymers. In the presence of a notch, brittle fracture in PC starts with a defect nucleation ahead of it. For each notch-tip radii, we are experimentally able to capture the process of defect initiation and quantify the mean stress required, static as well as dynamic loading. We found that the mean stress required for defect nucleation increases with decreasing notch-tip radius due to increased triaxility at the notch-tip. Defect initiation stresses are also higher for dynamic conditions compared to static loading. Defect initiation toughness for dynamic loading is always higher than those for static loading, but reduction in defect initiation toughness with decreasing notch-tip is severe for dynamic loading.     

基于FDM-DEM耦合的冲击损伤大理岩静态断裂力学特征研究

为探究循环冲击损伤后大理岩的静态断裂力学特征,基于有限差分（finite difference method,FDM）-离散元（discrete element method,DEM）耦合的建模技术构建了三维分离式霍普金森压杆（split Hopkinson pressure bar,SHPB）数值模型,其中杆件系统和岩石试件分别采用FLAC3D和PFC3D程序建模。利用该模型对中心直切槽半圆盘（NSCB）试样进行了恒定子弹速度下的循环冲击,随后对受损试样进行静态三点弯曲断裂实验。通过编写Fish程序,提取试样断裂面数据,对断裂面进行重构并定量计算表面粗糙度。通过与相关室内实验结果的对比分析,验证了本文数值分析的合理性与可靠性。模拟结果表明,随着循环冲击次数的增加,试样内部微裂纹、破碎颗粒均增加。连接力场分布混乱,部分力链发生断裂。力链的变化是试样力学性能劣化的根本原因。在静态三点弯曲断裂实验中,冲击5次后试样的静态断裂韧度较天然试样产生一定程度的降低。试样在静载过程中产生的微裂纹和碎块的数量随循环冲击次数的增加而增加,断裂面粗糙度随循环冲击次数的增加而增加。