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。     

Characterization and measurement of the mode 1 dynamic initiation of cracks in metals at intermediate strain rates—A review

Dynamic initiation of cracks in metals occurs as a result of impact loading. In general, for the case of small scale yielding, the fracture toughness of the material reduces with elevated strain rate, whereas for large scale yielding fracture toughness increases with strain rate. The review concentrates on the modelling of the crack tip stress field at intermediate strain rates ( <100 s⁻¹) and on the micromechanical causes of the rate dependency of dynamic initiation fracture toughness. The validation of developed fracture criteria requires careful experimental testing, and problems associated with instrumented impact testing are discussed. Some results are given for 150M12 and 817M40/En24 structural steels.     

Partially stiffened elastic half-plane with an edge crack

A technique, using the Brazilian disk specimen, for measuring the fracture toughness of unidirectional fiber-reinforced composites, over the entire range of crack-tip mode mixities, was developed. The fracture toughness of a graphite/epoxy fiber-reinforced composite was measured, under both mode-I and mode-II loading conditions. We found that for certain material orientations the mode-II fracture toughness is substantially higher than the mode-I toughness. The complete dependence of the fracture toughness on the crack-tip mixity was determined for particular material orientations and the phenomenological fracture toughness curves were constructed. Using the Brazilian disk specimen, together with a hydraulic testing machine, the fracture toughness of the composite under moderate loading rates was measured. We observed that the mode-I fracture toughness was not sensitive to the loading rate at the crack tip, K, while the mode-II ‘dynamic’ fracture toughness increased approximately 50 percent over the quasi-static fracture toughness. A qualitative explanation of the dependency of fracture toughness on crack-tip loading rate is discussed. Finally, a mechanical fracture criterion, at the microscopic level, which governs the crack initiation under mixed-mode loading conditions is presented; these theoretical predictions closely follow the trend of experimental measurements. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Effect of loading rate on dynamic fracture initiation toughness of brittle materials

Numerical simulation is carried out to investigate the effect of loading rate on dynamic fracture initiation toughness including the crack-tip constraint. Finite element analyses are performed for a single edge cracked plate whose crack surface is subjected to uniform pressure with various loading rate. The first three terms in the Williams’ asymptotic series solution is utilized to characterize the crack-tip stress field under dynamic loads. The coefficient of the third term in Williams’ solution, A ₃, was utilized as a crack tip constraint parameter. Numerical results demonstrate that (a) the dynamic crack tip opening stress field is well represented by the three term solution at various loading rate, (b) the loading rate can be reflected by the constraint, and (c) the constraint A ₃ decreases with increasing loading rate. To predict the dynamic fracture initiation toughness, a failure criterion based on the attainment of a critical opening stress at a critical distance ahead of the crack tip is assumed. Using this failure criterion with the constraint parameter, A ₃, fracture initiation toughness is determined and in agreement with available experimental data for Homalite-100 material at various loading rate.     

Some studies on the impact behavior of banded microalloyed steel

Microalloyed steels are used in automobile industries, offshore platforms and in structural applications. It is essential to establish a relation between service consition such as temperature, loading rate and fracture behavior of the steel. Impact study on new material is very handy to understand the mechanicl properties in a rapid and inexpensive way. The present investigation aims to assess impact toughness (CVN), ductile brittle transition temperature (DBTT, 25J), and initiation dynamic fracture toughness (J_ld*) of the indigenously developed microallayed seel. The steel has shown banding with alternate layers of ferrite and pearlite. The banding concentration (ferrite bands per mm) has been altered by heat treatment. Presence of banding has given spikes and splits in impact fracture. Change in banding concentration has affected DBTT of the steel, upper shelf energy and the extent of splitting. A model of crack divider with respect to the present microstructure has been analyzed. Banding in divider orientation improves the impact as well as initiation dynamic fracture toughness of the steel. The effect of temperature on splitting is also discussed. Splits in fractured surface disappear with decreasing temperature and higher numbers of splits yield lower toughness. Further, initiation dynamic fracture toughness is calculated for all temperatures and correlated with impact toughness.     

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。     

Rate effects on the mechanical response of magnesium aluminate spinel

The uniaxial compressive, biaxial flexural strength and fracture toughness of a polycrystalline transparent MgAl₂O₄ spinel were characterized over a wide range of loading rates. The flexural tests were carried out by means of ring-on-ring equibiaxial bending, while the fracture toughness was determined by four-point bending on samples with Chevron notch (CN) configuration. The surface crack (SC) method was also attempted in determining the fracture toughness. Quasi-static experiments were conducted on a servohydraulic testing machine, while the high-rate experiments were performed on a modified Kolsky bar. Results showed that both the failure strength and fracture toughness of the spinel were rate sensitive. Edge beveling in sample preparation did not affect the ring-on-ring flexural strength significantly, and the failure initiation sites were found to be inside the loading ring area regardless of edge conditions. Fracture toughness tests following ASTM standard were largely affected by the inherent coarse microstructure of this material.     

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.     

Behavior of particle-filled polymer composite under static and dynamic loading

Experimental results of quasi-static and dynamic fracture of particle-filled polymer composite (PFPCM) “ALTUGLAS EI CH25” with a matrix of polymethylmethacrylate (PMMA) are reported in this paper. PMMA matrix is filled with rubber particles, as result a shock-resistant transparent composite is produced. The main task was to investigate experimentally and theoretically the fracture toughness of this composite under static and dynamic loading. A high-rate loading has been created by impulse magnetic field. Analysis of fracture process and its relation with the load parameters and material microstructure have been established. Application of the original testing method enabled determination of fracture toughness at very short loading times and comparison of the results with material dynamic properties. Theoretical analyses are based in general on the notion of delayed fracture. More specifically, the theoretical analysis is based on experimental results and on the hypothesis of fracture incubation time, or delay time.     

Comparison of fracture toughness temperature curves determined at impact and rapid loading on small and large specimens

Comparing the fracture toughness temperature curves evaluated at static and rapid loading on larger (SENB, 1CT) specimens with the fracture toughness curve determined on precracked Charpy specimens at impact loading, the following conclusions can be drawn:

Evaluation on Fracture Toughness at Dynamic Loading for Welded Joint Based on the Local Approach

The changes in mechanical properties and fracture toughness by dynamic loading were investigated with experiments. The parameter R, which can reflect the effect of the loading rate and the temperature rising during the high loading rate, could be employed to describe the constituent relation for the typical structure steel and its weld metal. The dynamic loading effect on the stress/strain fields and the temperature variation in the vicinity of the crack tip was analyzed by the finite element method, the dynamic fracture behavior was evaluated based on the local approach. It has been found that the Weibull stress is an effective fracture parameter, independent of the temperature and the loading rate.     

An analysis of the crack tip fields in a ductile three-point bend specimen subjected to impact loading

Analyses of an impact fracture test of a precracked, three-point beam of HY100 steel were performed to determine the dynamic fracture toughness. During impact, the crack tip opening displacement (CTOD) 100 μm behind the crack tip was measured using an optical measuring device called the interferometric strain/displacement gage. Since fracture initiates when stress wave effects dominate, a numerical simulation of the fracture event was conducted to obtain relevant near crack tip field parameters. The specimen was modeled by a plane stress finite element simulation using a rate sensitive elastoplastic material law. Since the simulated CTOD was to be compared with the measured CTOD in a region of residual strains due to crack closure, this effect was included in the model. The simulation produces a CTOD versus time response within 10% of the observed response, indicating that the other field quantities (such as the J-integral) should also be reliable. The loading rate /.K₁ was approximately 8 × 10⁶MPa√m/sec. If the fracture initiation time is assumed to coincide with the time at which the simulated and observed CTOD curves diverge, then the impact fracture toughness is 56% higher than the static fracture toughness.     

温度与加载速率对X70级管线钢韧脆转化现象的影响

在不同温度下测试了X70级管线钢的动态断裂韧度。研究表明:加载速率对动态断裂韧度的影响与温度对其的影响同样重要;在恒温下,增大加载速率可以诱发韧脆断裂转变;当温度由298 K向193 K逐渐降低,或加载速率从0.01 mm/s向1 000 mm/s增大时,均将导致材料的韧脆断裂转变。     

Statistical scatter in the fracture toughness and Charpy impact energy of pearlitic steel

The effect of microstructure on the fracture characteristics of high carbon hypo eutectoid steel was studied under conditions of quasistatic and dynamic loading. Experimentally determined sets of fracture toughness and Charpy impact energy values were statistically treated. A relationship was found between fracture toughness and Charpy impact energy. In the very brittle domain, the fracture toughness increases slightly with increasing Charpy impact energy. In the domain where the fracture toughness is higher, the rise in fracture toughness with increasing Charpy impact energy is more pronounced. Detailed SEM examination of fractured compact tension (CT) and Charpy V-notch (CVN) specimens showed that the fracture at ambient temperature occurred almost exclusively by cleavage. There were no visible differences in the morphology of cleavage facets on the fracture surfaces of Charpy and CT specimens. Mechanisms of cleavage initiation were revealed by the fractographical investigation of fracture surfaces. Whereas the fracture surfaces of broken CT specimens exhibit a number of cleavage origins, the fracture surfaces of CVN specimens usually show only one.     

Effect of temperature on the mode I and mixed mode I/III fracture toughness of SA333 steel

The effect of temperature on tensile properties, mode I and mixed mode I/III fracture toughness of SA333 Grade 6 steel was investigated. The variation of ultimate tensile strength and strain hardening exponent with temperature as well as the appearance of serrations in the stress-strain plots indicated that dynamic strain aging regime in this steel is in the temperature range 175-300 °C at a nominal strain rate of 3 × 10⁻³ s⁻¹. Both mode I and mixed mode I/III fracture toughness values were found to exhibit a significant reduction in the DSA regime. The mixed mode I/III fracture toughness was found to be significantly lower than the mode I fracture toughness at all temperatures. However, the difference between the two toughness values was much higher prior to the onset of DSA. The results are explained on the basis of the nature of deformation fields under mode I and mixed mode I/III loading as well as the fracture mechanism prevalent in these steels at different temperatures.     

钢结构防动载断裂选材的定量判据

分析了目前钢结构防断选材中应用的脆性转变温度方法的局限性,扼要介绍了材料断裂韧度的温度效应和加载速率效应.在钢材断裂特性分析的基础上提出了平面应变型断裂临界温度的概念,构建了断裂特征分析图,从而为钢结构防动载断裂选材提出了一个新的定量的判据,并以船只设计选材为例,阐明了该判据的应用.     

Dynamic fracture of bovine bone

True clinical fracture of bones in bovine, race horses or humans occur predominantly during impact loading (e.g. car accidents, falls or physical violence). Although static fracture tests provide an estimate of fracture toughness or R-curve behavior in bones, the static toughness values may be ill suited for predicting failure under dynamic loading conditions due to the visco-elastic response of bone (i.e. strain rate dependent properties). Despite decades of the study on deformation rate dependency of bone properties such as compression and fracture toughness, high-quality dynamic fracture data remain limited. Preliminary tests (compression and fracture toughness) have been conducted on dry and wet bovine bone under both static and dynamic loading conditions. While compression tests have been conducted with loading direction parallel and perpendicular to the bone axis (longitudinal and transverse, respectively), fracture tests were performed only in the transverse direction. The strain rate in compression tests varied between 10^− 3 and 10³ s^− 1, and the stress intensity rate varied between ∼10^− 3 and 10⁵ MPa√m/s. While low strain rate tests were conducted on conventional mechanical testing machines, high strain rate experiments were conducted on a split-Hopkinson bar under compression and a novel three-point bend configuration. The fracture morphology and the extent of damage of bone in each case were characterized using SEM, and an attempt is made to relate these to the rate dependent fracture toughness of the bone. It is believed that such understanding is crucial for mechanistic interpretation of bone fracture phenomenon and eventually for predicting bone failure reliably.     

Experiments and predictions of the effects of load history on cleavage fracture in steel

A series of experiments conducted on two steels, A533B and A508, are summarised. Tests were conducted to explore the influence of different room temperature pre-loading cycles on subsequent low temperature (−150 °C and −170 °C) cleavage fracture. In all cases the low temperature fracture toughness was modified, with tensile pre-loading increasing the toughness and precompression reducing the toughness.Results from finite element simulation of the pre-loading cycles are illustrated. Tensile pre-loading created compressive residual stresses and precompression generated tensile residual stresses. The residual stresses were adopted in a stress based local approach to fracture model using Weibull statistics and applied to the experimental results. The parameters in the Weibull model were calibrated for the virgin steels prior to its application to prior loading cases. The model is found to be successful in predicting the change in toughness relative to the virgin material for pre-loading in tension of A533B steel. The model underestimated the change in toughness for tensile pre-loading of A508 steel and overestimated the toughness change for precompression of both steels.     

Impact and dynamic fracture of tough polymers by thermal decohesion in a Dugdale zone

At low temperatures and high loading rates, normally tough crystalline thermoplastics may undergo a transition from ductile tearing to brittle rapid crack propagation (RCP). It is proposed here that RCP — characterised by low toughness, high crack speed (>100 m/s) and a macroscopically smooth fracture surface — occurs by self-sustained melting of a layer, one chain length thick, at each cohesive surface of a crack-tip craze, due to adiabatic heating. Initiation of RCP from a rapidly loaded sharp notch, i.e. impact fracture, requires both the formation of this melt layer, and sufficient crack extension force to propagate it. A schematic linear-elastic analysis based on the Dugdale model accounts both for the measured dynamic fracture resistance, and for the variation of impact fracture resistance with impact speed, in two pipe-grade polyethylenes and in a neat and a rubber-toughened polyacetal. It is concluded that crack initiation resistance, unlike dynamic fracture resistance, cannot be defined as a geometry-dependent material property.