霍普金森杆冲击压缩单裂纹圆孔板的岩石动态断裂韧度试验方法

对压缩单裂纹圆孔板(single cleavage drilled compression--SCDC)砂岩试样,利用分离式霍普金森压杆(SHPB)冲击加载,进行了岩石张开型(I型)动态断裂实验。分别采用2种方法确定砂岩的动态断裂韧度,第1种方法是实验-数值法:由SHPB弹性杆上应变片获得作用在试件上的加载力,然后输入有限元分析程序求得试样裂尖动态应力强度因子,对应于裂尖起裂时刻的动态应力强度因子即为材料动态断裂韧度值;第2种方法是准静态法:将载荷峰值代入静态应力强度因子公式确定动态断裂韧度。2种方法的结果差异较大,对无量纲裂纹长度a/R= 0.64(A组)试样,准静态方法确定的断裂韧度值要比实验-数值法确定的断裂韧度值平均要小35%~62%;对无量纲裂纹长度a/R=1.61(B组)试样,准静态方法的计算结果比实验-数值法的计算结果平均要小72%~83%。从原理上讲,实验-数值法比准静态法能更合理地测定岩石的动态断裂韧度。     

用圆孔内单边裂纹平台巴西圆盘和实验-数值-解析法确定砂岩的动态起裂和扩展韧度

在I型(张开型)动态断裂实验中,利用大直径(?100 mm)分离式霍普金森压杆径向冲击圆孔内单边裂纹平台巴西圆盘试样。考虑了材料惯性效应和裂纹扩展速度对动态应力强度因子的影响,用实验-数值-解析法确定了高加载率和高裂纹扩展速度情况下,砂岩的动态起裂韧度和动态扩展韧度。由动态实验获取试样的动荷载历程,采用裂纹扩展计(Crack Propagation Gauge,CPG)测定试样断裂时刻和裂纹扩展速度,获得裂纹扩展速度对应的普适函数值。然后将动荷载历程带入到有限元软件中进行动态数值模拟,求出静止裂纹的动态应力强度因子历程,再用普适函数值对其进行近似修正。最后根据试样的起裂时刻和穿过CPG中点的时刻,由相应的动态应力强度因子历程分别确定砂岩的动态起裂和动态扩展韧度,它们分别随动态加载率和裂纹扩展速度的提高而增加。     

用应变片法确定混凝土动态起裂时间的研究

在混凝土等准脆性材料的动态起裂韧度K_(1d)测试中,准确确定试件裂尖的起裂时间是测试工作的关键。采用分离式霍普金森压杆系统,对圆孔裂纹平台巴西圆盘混凝土试件进行动态径向冲击试验,通过在裂尖粘贴应变片的方法来确定起裂时间。讨论了应变片在裂纹尖端的粘贴位置、粘贴方向等因素对起裂时间测试值的影响,结果表明裂尖应变片的最佳粘贴方式是:在裂纹延长线上或在裂尖并与裂纹垂直的线上,都距离裂尖3 mm左右,且粘贴方向与裂纹延长线垂直。给出了考虑贴片位置和试件厚度的起裂时间计算公式。     

NiTi合金动态断裂过程的实验和数值方法研究

为获得Ni Ti合金的动态起裂韧度和动态扩展速度与动态加载率之间的定量变化规律。利用分离式霍普金森压杆(SHPB)测试系统对单边三点弯曲试样(SENB)进行冲击加载试验,采用实验-有限元相结合的方法,获得动态断裂参数随时间的变化规律。SENB试样裂纹起裂时刻和裂纹扩展速度由粘贴在裂纹尖端的裂纹扩展计(CPG)测定。采用上述方法和数据获得Ni Ti合金的I型动态起裂韧度和动态扩展速度。实验结果表明:裂纹扩展计测得的起裂时刻与粘贴在同一试样上的监裂应变片测得的结果基本相符,因此可以利用裂纹扩展计代替传统的监裂应变片来监测裂纹起裂时刻,并获得Ni Ti合金的起裂韧度。同时,可以利用裂纹扩展计(CPG)获得裂纹动态扩展过程,绘制出裂纹扩展速度与时间的关系曲线,从而探讨Ni Ti合金的动态断裂韧度和裂纹扩展速度与动态加载率之间的定量变化规律。     

含单边缺口混凝土梁冲击破坏的近场动力学模拟

改进了近场动力学（peridynamics,PD）微观弹脆性（prototype microelastic brittle,PMB）模型中微观模量的计算方法,解决了PMB模型的“边界效应”问题。利用改进的PMB模型计算了冲击荷载作用下单边缺口混凝土梁的破坏全过程,得到了混合型裂纹扩展的角度、路径以及裂尖最大速度,并与试验和其他数值方法模拟结果进行了对比分析,验证了改进的PMB模型的有效性。研究结果表明,近场动力学方法在模拟破坏问题时不存在网格依赖性,其本构模型自然包含了损伤与断裂的描述,是求解结构冲击破坏问题的一种有效方法。     

含圆孔缺陷三点弯曲梁的裂纹扩展特性实验研究

采用动态焦散线实验系统,对含圆孔缺陷的PMMA材料进行冲击断裂力学实验,研究三点弯曲梁中不同直径和位置的圆孔缺陷对扩展裂纹的影响。实验结果表明:扩展裂纹与圆孔缺陷相互作用前,呈现Ⅰ型拉伸断裂,扩展路径平直;与圆孔缺陷相互作用后,贯通萌生的次裂纹沿直线继续扩展,未贯通的裂纹偏移扩展。扩展裂纹与路径上圆孔缺陷贯通过程中,裂纹扩展速度和动态应力强度因子快速降为零,裂纹的扩展受到抑制,且圆孔直径越大、距离越近,抑制作用越显著;贯通萌生次生裂纹的起裂速度和起裂韧度,随着圆孔缺陷直径的增大而变大。扩展裂纹与偏置圆孔缺陷相互作用过程中,当圆孔缺陷直径越大、偏置距离越小,裂纹起偏距离越短,最大偏移量越大,并且扩展裂纹动态应力强度因子和扩展速度局部小幅增大。研究结果为分析动态裂纹扩展特征和材料破坏模式提供借鉴。     

基于改进型近场动力学方法的多裂纹扩展分析

在非局部键型近场动力学理论基础上,提出了能够反映混凝土、岩石类材料力学特性和非局部长程力尺寸效应的改进型近场动力学微极模型,弥补常规微观弹脆性（Prototype Microelastic Brittle,PMB）键型近场动力学本构模型的应用范围限制和定量计算误差大等缺陷,并构建了相应的适合于模拟脆性多裂纹扩展问题的近场动力学算法体系。通过对不同核函数修正项对应的近场动力学定量计算结果进行比较,验证了改进型近场动力学模型和数值算法的计算精度并确定了精度最高的核函数修正项;模拟双裂纹脆性板受压和随机多裂纹脆性板受拉的裂纹扩展全过程并与已有结果对比,进一步验证了模型和算法在模拟多裂纹扩展问题时的可靠性。分析了含多裂纹三点弯梁的起裂和裂纹失稳扩展过程,并研究了裂纹初始倾角、初始长度等因素对构件破坏形式和破坏荷载的影响规律。     

基于近场动力学方法的混凝土板侵彻问题研究

由于近场动力学(Peridynamics)用统一空间积分-时间微分方程描述物体连续或不连续区域,改进常用微观弹脆性模型对势本构力函数,给出刚性体与变形体冲击问题接触算法;编制计算程序,验证经典简支梁变形及Kalthoff-Winkler试验;数值模拟刚性弹丸侵彻混凝土矩形板破坏过程,揭示损伤累积及裂纹扩展全过程与最终破坏形态。结果表明,改进的近场动力学模型及算法合理、可靠,能有效模拟混凝土结构冲击破坏及侵彻问题。     

冲击荷载作用下混凝土结构破坏过程的近场动力学模拟

混凝土在冲击、侵彻等动载荷作用下产生损伤和破坏的过程,其实质是力学模型从连续体到非连续体的转变过程。传统的连续介质理论基于连续性假设并运用偏微分方程求解问题,难以直接用于计算和模拟材料及结构发生破坏的整个过程。近场动力学(Peridynamics,PD)是一种新兴的基于非局部模型描述材料特性的数值计算方法。该方法假定位于连续体内的粒子通过有限的距离与其它粒子相互作用,通过积分计算在一定近场范围(horizon)内具有一定影响域的材料点之间的相互作用力,而不论位移场的连续与否,避免了传统的局部微分方程求解在面临不连续问题时的奇异性和现有多尺度算法的复杂性。该文概述了PD方法的理论基础,描述了其建模思路及计算体系,给出了用近场动力学方法模拟结构受冲击荷载的计算格式。算例结果表明:PD方法可以很好地刻画和模拟材料及结构的损伤累积与渐进破坏过程。最后讨论了PD方法在理论、计算和应用等方面有待进一步研究的问题。     

动态加载率对巷道内裂纹扩展速度及动态起裂韧度的影响

在巷道的钻爆法掘进过程中,巷道围岩周围将诱发较多的径向裂隙。为了研究不同冲击载荷作用下巷道内裂纹扩展速度及起裂韧度的变化规律,采用落锤冲击试验机对裂纹巷道模型试件进行冲击加载,模型试件选用青砂岩制作;试验中,采用裂纹扩展计进行裂纹扩展速度及起裂时间的测定,对不同冲击高度下裂纹扩展速度及起裂时间进行分析;随后采用ABAQUS有限元程序结合试验-数值法计算巷道模型试样在不同动态加载率作用下的动态起裂韧度,并对起裂韧度的变化规律进行统计分析。得出如下结论:①巷道内预制裂纹的扩展速度随着动态加载高度的增加而增大,随后逐渐趋于稳定,实测裂纹扩展速度略小于0.38倍砂岩纵波波速;②巷道内预制裂纹的起裂时间,随着动态加载率的增加呈缓慢下降的趋势,降低幅度范围约为50μs;③随着动态加载率的增加,巷道内裂纹的动态起裂韧度逐渐增大,上升趋势也逐渐加大。     

Determination of dynamic fracture parameters using a semi-circular bend technique in split Hopkinson pressure bar testing

Fracture initiation toughness, fracture energy, fracture propagation toughness, and fracture velocity are key dynamic fracture parameters. We propose a method to simultaneously measure these parameters for mode-I fractures in split Hopkinson pressure bar (SHPB) testing with a notched semi-circular bend (SCB) specimen. The initiation toughness is obtained from the peak load given dynamic force equilibrium. A laser gap gauge (LGG) is developed to monitor the crack surface opening displacement (CSOD) of the specimen, from which the fracture velocity and the fracture energy can be calculated. The feasibility of this methodology for coarse-grained solids is demonstrated with the SHPB-SCB experiments on Laurentian granite.     

冲击载荷下CFRP及GFRP层板断裂韧性的研究

利用Hopkinson杆加载装置, 对带有单边切口的炭纤维增强复合材料(CFRP)及玻璃纤维增强复合材料(GFRP)层板试件进行冲击拉伸加载实验。根据一维应力波理论求得作用于试件上的载荷P(t)和试件加载点的位移δ(t)。 根据试样中应力随时间的变化历史σ(t), 并基于断裂韧性测试原理, 建立了动态应力强度因子K_Ⅰ (t)响应曲线。利用柔度变化率方法确定起裂时间, 分别得到在两种加载速率下CFRP、 GFRP层板的动态断裂韧性。结果表明, 随着加载速率的提高, 这两种复合材料的断裂韧性降低。      

Study of mode I crack dynamic propagation behaviour and rock dynamic fracture toughness by using SCT specimens

To study crack dynamic propagation behaviour and rock dynamic fracture toughness, a single cleavage triangle (SCT) specimen was proposed in this paper. By using these specimens and a drop‐weight test system, impact experiments were conducted, and the crack propagation velocity and the fracture time were measured by using crack propagation gauges. To examine the effectiveness of the SCT specimen and to predict the test results, finite difference numerical models were established by using AUTODYN code, and the simulation results showed that the crack propagation path agrees with the test results, and crack arrest phenomena could happen. Meanwhile, by using these numerical models, the crack dynamic propagation mechanism was investigated. Finite element code ABAQUS was applied in the calculation of crack dynamic stress intensity factors (SIFs) based on specimen dimension and the loading curves measured, and the curves of crack dynamic SIFs versus time were obtained. The fracture toughness (including initiation toughness and propagation toughness) was determined according to the fracture time and crack speeds measured by crack propagation gauges. The results show that the SCT specimen is applicable to the study of crack dynamic propagation behaviour and fracture toughness, and in the process of crack propagation, the propagation toughness decreases with crack propagation velocity, and the crack arrest phenomena could happen. The critical SIF of an arrest crack (or arrest toughness) was higher than the crack propagation toughness but was lower than the initiation toughness.     

Quasi-static and dynamic fracture behaviour of rock materials: phenomena and mechanisms

An experimental investigation is conducted to study the quasi-static and dynamic fracture behaviour of sedimentary, igneous and metamorphic rocks. The notched semi-circular bending method has been employed to determine fracture parameters over a wide range of loading rates using both a servo-hydraulic machine and a split Hopkinson pressure bar. The time to fracture, crack speed and velocity of the flying fragment are measured by strain gauges, crack propagation gauge and high-speed photography on the macroscopic level. Dynamic crack initiation toughness is determined from the dynamic stress intensity factor at the time to fracture, and dynamic crack growth toughness is derived by the dynamic fracture energy at a specific crack speed. Systematic fractographic studies on fracture surface are carried out to examine the micromechanisms of fracture. This study reveals clearly that: (1) the crack initiation and growth toughness increase with increasing loading rate and crack speed; (2) the kinetic energy of the flying fragments increases with increasing striking speed; (3) the dynamic fracture energy increases rapidly with the increase of crack speed, and a semi-empirical rate-dependent model is proposed; and (4) the characteristics of fracture surface imply that the failure mechanisms depend on loading rate and rock microstructure.     

Determination of dynamic rock Mode-I fracture parameters using cracked chevron notched semi-circular bend specimen

Rock dynamic fractures are common in many geophysical processes and engineering applications. Characterization of rock dynamic fracture properties such as the initiation fracture toughness, the fracture energy, and the fracture velocity, is thus of great importance in rock mechanics. A novel method is proposed in this work to measure dynamic Mode-I rock fracture parameters using a cracked chevron notched semi-circular bend (CCNSCB) specimen loaded by a split Hopkinson pressure bar (SHPB) apparatus. A strain gauge is mounted on the sample surface near the chevron notch to detect the fracture onset, and a laser gap gauge (LGG) is used to monitor the crack surface opening distance (CSOD) during the dynamic test. With dynamic force balance achieved in the tests, the stable–unstable transition of the crack propagation crack is observed and the initiation fracture toughness is calculated from the dynamic peak load. The average dynamic fracture energy as well as the fracture propagation toughness are calculated based on the first law of thermodynamics. The measured dynamic fracture properties of Laurentian granite using CCNSCB method are consistent with those reported in the literature using other methods.     

Dynamic crack initiation toughness of 4340 steel at constant loading rates

Determination of fracture toughness for metals under quasi-static loading conditions can follow well-established procedures and ASTM standards. The use of metallic materials in impact related applications requires the determination of dynamic crack initiation toughness for these materials. There are two main challenges in experiment design that must be overcome before valid dynamic data can be obtained. Dynamic equilibrium over the entire specimen needs to be approximately achieved to relate the crack tip loading state to the far-field loading conditions, and the loading rate at the crack tip should be maintained near constant during an experiment to delineate rate effects on the values of dynamic crack initiation toughness. A recently developed experimental technique for determining dynamic crack initiation toughness of brittle materials has been adapted to measure the dynamic crack initiation toughness of high-strength steel alloys. A Kolsky pressure bar is used to apply the dynamic loading. A pulse shaper is used to achieve constant loading rate at the crack tip and dynamic equilibrium across the specimen. A four-point bending configuration is used at the gage section of the setup. Results are presented which show a monotonically increasing rate dependence of crack initiation toughness for 4340 high-strength steel.     

An experimental cum numerical technique to determine dynamic interlaminar fracture toughness

A method combining experimental and finite element analysis is developed to determine interlaminar dynamic fracture toughness. An interlaminar crack is propagated at very high speed in a double cantilever beam (DCB) specimen made of two steel strips which are bonded together by epoxy with a precrack of about 40 mm length. The face of the front cantilever is bonded to a large solid block and a special fixture is designed to apply impact load to the rear cantilever through a load bar. In the load bar, a compressive square shaped elastic stress pulse is generated by impacting it with a striker bar which is accelerated in an air gun. The rear cantilever is screwed to the load bar; when the incident compressive pulse reaches the specimen, a part of the energy is reflected into the load bar and the rest of it passes to the specimen. By monitoring the incident and the reflected pulses in the load bar through strain gauges, deflection of cantilever-end is determined. The crack velocity is determined by three strain gauges of 0.2 mm gauge length bonded to the side face of the rear cantilever. Further, the first strain gauge, bonded very close to the tip of the precrack, and the crack velocity determine the initiation time of crack propagation.

The experimental results are used as input data in a finite element (FE) code to calculate J-integral by the gradual release of nodal forces to model the propagation of the interlaminar crack. The initiation fracture toughness and propagation fracture toughness are evaluated for an interlaminar crack propagating with a velocity in the range of 850 to 1785 m/s. The initiation toughness and propagation toughness were found to vary between 90–200 J/m² and 2–13 J/m², respectively.     

二维C/SiC复合材料在冲击拉伸下的损伤过程

利用分离式霍普金森拉杆实验装置(SHTB)和超高速照相机,对二维C/SiC复合材料进行了冲击拉伸力学性能实验研究,同时结合其宏观力学行为,分析了在冲击拉伸载荷作用下的损伤破坏过程。结果表明:材料的应力-应变曲线呈明显的非线性特征,其内部损伤破坏和裂纹扩展过程分为四个阶段:损伤积累于第一阶段,裂纹起源于第二阶段,屈服失效于第三个阶段,快速扩展于第四个阶段。