预加载下岩石的动态力学性能研究

动态和静态载荷共同作用下的岩石力学特性是深部地下岩石工程的关键问题。设计了用于测试静态预加载下岩石动态力学性能的分离式霍普金森压杆系统,并详细介绍了具有预加载装置的分离式霍普金森压杆系统的原理、数据分析和应力波的传播过程。通过具有预加载装置的分离式霍普金森压杆系统研究了岩石在不同预拉伸应力下的拉伸强度。结果表明:动态拉伸强度和总拉伸强度随着加载率的增加而增加,同时,在相同加载率下,动态拉伸强度随着预拉伸载荷的增加而减小,而总拉伸应力与预拉伸载荷的大小无关。此外,对不同预加载条件下岩石的动态断裂韧度也进行了研究,实验结果说明岩石的动态断裂韧度和总断裂韧度随着加载率的增加而增加。在相同加载率下,动态断裂韧度随着预加载荷的增加而减小,而总断裂韧度随着加载率的增加而增加。     

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。     

低温/饱水耦合作用下红砂岩静动态Ⅱ型断裂特性试验研究EI北大核心CSCD

利用伺服压力机、分离式霍普金森压杆对干燥室温、饱水室温、-40℃干燥冻结和-40℃饱水冻结四种工况下的红砂岩试件分别进行了静、动态短芯压剪试验;通过分析对比静、动态以及动态范围内不同加载率下的断裂韧度、断裂能等参数变化规律,研究了低温和水饱和效应对红砂岩的Ⅱ型断裂力学特性的影响并探讨了二者的作用机制。结果表明:(1)水对砂岩Ⅱ型断裂韧度与断裂能产生弱化效应,水饱和红砂岩静态Ⅱ型断裂韧度为干燥下的77%;(2)冻结作用使红砂岩静、动态断裂韧度分别提升36%和18%以上,静态荷载下干燥冻结砂岩Ⅱ型断裂韧度最高,动态荷载下饱水冻结砂岩具有最大的Ⅱ型断裂韧度;(3)冻结与干燥室温红砂岩断裂韧度率敏感性近似相同,而饱水室温红砂岩具有最高的Ⅱ型断裂韧度率敏感性和动态增强因子。     

冲击载荷下环氧树脂动态压缩力学响应实验研究

为了从细观角度研究碳/环氧复合材料的动态力学性能,利用分离式霍普金森压杆实验装置对其组分材料TDE86#环氧树脂体系进行动态冲击压缩实验,获得不同应变率加载条件下环氧树脂的应力一应变曲线,基于应力-应变曲线分析了应变率对环氧树脂动态压缩力学性能的影响.研究结果表明:环氧树脂具有明显的应变强化效应,随着应变率的增加,环氧树脂的强度基本没有变化,最大应力时的应变逐渐减小,动态模量和压缩刚度有很大程度的提高.     

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。     

不同强度混凝土高温下动态劈拉性能研究

采用大直径分离式霍普金森压杆(SHPB)装置和加温装置,在不同温度(20 ℃、200 ℃和400 ℃)下对3种不同强度(C20、C45和C70)混凝土材料开展不同应力率的动态劈裂拉伸实验,得到了温度和应力率耦合作用下混凝土材料的动态劈裂强度及相应的破坏形态。实验结果表明,混凝土材料的动态劈裂强度随应力率的增加而增加,且应力率相近时,其动态劈裂强度随温度的增加而明显降低。在此基础上给出了描述混凝土材料在不同温度下的动态劈裂强度与应力率关系的表达式,并确定了相关材料参数。通过对不同应力率和温度耦合作用下混凝土材料的动态劈裂强度进行横向对比,发现混凝土材料动态劈裂强度的温度敏感性随应力率的增加逐渐减小,并且随着混凝土材料强度的增加其温度敏感性也逐渐变小,但是其动态劈裂强度的应力率敏感性却随着温度的升高逐渐增大。     

基于键基近场动力学理论的单裂纹圆孔板冲击破坏研究

基于键基近场动力学理论,建立分离式霍普金森杆冲击单裂纹圆孔板动力学模型,其中霍普金森杆用一维PD模型、单裂纹圆孔板用二维PD模型描述,采用短程斥力模型描述碰撞过程,模拟冲击压缩条件下单裂纹圆孔板动态破坏行为。通过试样端面受力分析得到端面载荷的V形分布规律,解决了传统实验-数值研究法在端面加载上的局限性。研究不同入射速度下试件裂纹的扩展过程和破坏模式,准确捕获了裂纹起裂、止裂及二次起裂时间。根据键基近场动力学理论下材料动态应力强度因子计算方法,求得裂纹的起裂韧度,为材料动态断裂韧度计算提供了新的途径。     

有机玻璃纯Ⅰ型和纯Ⅱ型动态断裂行为的实验研究EI北大核心

采用尖槽式中心切口圆盘试件,在分离式霍布金森压杆试验装置上对有机玻璃纯Ⅰ型和纯Ⅱ型加载条件下的动态断裂行为进行了实验研究。结果表明,加载速率对有机玻璃的断裂行为有显著的影响,有机玻璃纯Ⅰ型和纯Ⅱ型断裂韧度的测试结果均表现出明显的加载速率相关性,且随着加载速率的增加而增大。     

有机玻璃纯Ⅰ型和纯Ⅱ型动态断裂行为的实验研究

采用尖槽式中心切口圆盘试件,在分离式霍布金森压杆试验装置上对有机玻璃纯Ⅰ型和纯Ⅱ型加载条件下的动态断裂行为进行了实验研究。结果表明,加载速率对有机玻璃的断裂行为有显著的影响,有机玻璃纯Ⅰ型和纯Ⅱ型断裂韧度的测试结果均表现出明显的加载速率相关性,且随着加载速率的增加而增大。     

三维四向编织复合材料动态压缩性能实验研究

利用分离式霍普金森压杆(SHPB)研究了三维四向编织碳纤维环氧树脂复合材料在动态压缩载荷作用下的力学性能。在横向对复合材料进行了动态压缩实验,得到了应变率从900/s―1500/s下的应力-应变曲线,并且与准静态压缩下的结果进行了对比。分析比较了应变率对三维编织复合材料横向压缩强度和模量的影响。实验中根据SHPB理论假设,采用波形整形技术,使得试件在加载过程中处于应力平衡和均匀变形状态。实验结果表明:压缩强度和模量具有一定的应变率强化效应;与准静态结果相比,在高应变率下的复合材料的强度和模量有明显的增大,并表现出明显的脆性。还分析了应变率对复合材料破坏模式的影响。     

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material under thermo-mechanical loading

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material (FGM) is investigated using a three point bend specimen. The modified split Hopkinson pressure bar (SHPB) apparatus in conjunction with induction coil heating system is used during elevated temperature dynamic loading experiments. A simple and accurate technique has been developed to identify the time corresponding to the load at which the fracture initiates. A series of experiments are conducted at different temperatures ranging from room temperature to 800 °C, and the effect of temperature and loading rate on the fracture initiation toughness is investigated. The material fracture toughness is found to be sensitive to temperature and the fracture initiation toughness increases as the temperature increases. Furthermore, the fracture initiation toughness is strain rate sensitive and is higher for dynamic loading as compared to quasi-static loading.     

岩石在动载作用下破坏模式与强度特性研究

岩石在高应变率下的破坏类型及动态强度理论是工程爆破中的一个重要的基本问题。以往研究岩石的破坏类型只停留在静载作用下,而其强度理论也多采用静态强度理论。作者利用ＳＨＰＢ装置对常见的四种岩石进行了大量冲击试验。结果表明,岩石在冲击载荷作用了下破坏分为四种类型：压剪破坏,拉应力破坏,拉应变破坏和卸载破坏,而且其破坏强度随冲击速度的提高而提高。最后对强度理论作了讨论。     

Bonded particle modeling of grain size effect on tensile and compressive strengths of rock under static and dynamic loading

The effect of grain (particle) size on the strength is an interesting subject in the rock engineering. Some investigations about the impact of particle size on static strength of rock have been conducted and reported in the literature. However, this issue has not received enough attention when high loading rates are involved. In this work, by utilizing the CA3 bonded particle - finite element computer program, the combined influence of loading rate and particle size on the compressive and tensile strengths of rock is examined. The bonded particle model is used to simulate the crack initiation and failure of the rock specimen and the finite element is utilized to model the elastic bars in the Split Hopkinson Pressure Bar (SHPB) apparatus employed for the dynamic testing. Specimens with four different particle sizes were prepared. The results suggest that the particle size does not affect the rock strength under static and dynamic loading. However, the particle size modifies the nominal tensile strength of the notched Brazilian specimens. For the intact Brazilian specimens under high stress rates, the particle size contributes to the tensile strength and this contribution can be justified based on the principles of fracture mechanics. The theoretical reason for these observations is derived for a 3D bonded particle system and discussed.     

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.     

Effect of strength mis-match and dynamic loading on ductile fracture initiation

It has been well known that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using a two-parameter criterion based on equivalent plastic strain and stress triaxiality.The present study focuses on the effects of geometrical discontinuity, strength mis-match, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on the critical condition for ductile fracture initiation using a two-parameter criterion. Fracture initiation testing has been conducted under static and dynamic loading using circumferentially notched round-bar specimens. In order to evaluate the stress/strain state in the specimens, especially under dynamic loading, a thermal elastic-plastic dynamic finite element (FE) analysis considering the temperature rise due to plastic deformation has been carried out.The tensile tests on specimens with an undermatching interlayer showed that the relationship between the critical equivalent plastic strain to initiate ductile fracture and stress triaxiality was equivalent to that obtained on homogeneous specimens under static loading. Moreover, the two-parameter criterion for ductile fracture initiation is shown to be independent of the loading rate. It was demonstrated that the critical global strain to initiate ductile fracture in specimens with strength mis-match under various loading rate can be estimated based on the local criterion, that is two-parameter criterion obtained on homogeneous specimens under static tension, by mean of FE-analysis taken into account accurately both strength mis-match and dynamic loading effects on stress/strain behaviors.     

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.     

三轴SHPB加载下砂岩力学特性和破坏模式的试验研究

利用改造的三轴SHPB动静组合加载实验装置,对均质砂岩进行了不同围压和不同应变率下的三轴冲击压缩试验,作为对比利用RMT-150C试验机也进行了部分准静态下的三轴压缩实验。根据实验结果,分析了围压对砂岩动态冲击性能的影响,并重点讨论了冲击过程中岩石的破坏模式。研究结果表明,在围压一定的情况下,岩石的动态压缩强度随应变率的提高而提高;在应变率相同的情况下,岩石的动态压缩强度和弹性模量会随着围压的增大而增大。岩石发生破坏的临界入射能,随着围压的增大而增大。岩石单位体积吸收能与应变率之间呈线性递增关系,而且递增的程度随着围压的增加而增加。三轴冲击加载下,应变率较低时岩石内部形成压剪破裂面但整体不失稳,应变率很大时岩石破碎形成锥形块体形式。     

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.     

实时高温下与热处理后砂岩动态拉伸特性研究

为了开展实时高温下及加热冷却后岩石动态力学性质的对比研究,利用自行设计的加热装置结合SHPB系统对两种温度状态砂岩的动态拉伸特性进行试验研究,处理温度为常温(25℃)～600℃共7组。结果表明:砂岩的率效应在两种温度状态下都存在,热处理后的砂岩拉伸强度除100℃外都比常温状态小,且随着热处理温度的增大,拉伸强度减小,温度会影响试样的破坏形态。高温下砂岩的拉伸强度除600℃外都比常温状态大,且拉伸强度随着温度的升高先增大后减小,加载率对破坏模式起主导作用。高温下的热膨胀作用会抑制裂纹相互连接,使得岩石结构更加密实,导致砂岩在实时高温下的拉伸强度相比加热冷却后有很大的区别。