The role of chain length and conformation in stress-transmission and fracture of thermoplastic polymers

In this paper, a review of the molecular aspects of fracture is given, a subject that was pioneered by S. N. Zhurkov and his colleagues. Particular attention will be paid to the mechanisms of stress transfer onto straight chain segments, the role of chain interpenetration in establishing interfacial strength during crack healing, to the concept of taut tie molecules, to stress distribution in UHMWPE fibers, and to the possible role of chain ruptures in the deformation process of fibres. Using Raman microscopy, it is observed that some chains are exposed to stresses of up to 10 GPa, which is close to their estimated strength. From these experiments, a mechanical model of the organization of almost fully oriented UHMWPE fibers is developed accounting also for the presence of numerous and dispersed defects. The principal deformation mechanisms are chain slippage, crystal plasticity, and intra-and intermicrofibrillar slippage.     

XFEM modeling of hydraulic fracture in porous rocks with natural fractures

Hydraulic fracture (HF) in porous rocks is a complex multi-physics coupling process which involves fluid flow, diffusion and solid deformation. In this paper, the extended finite element method (XFEM) coupling with Biot theory is developed to study the HF in permeable rocks with natural fractures (NFs). In the recent XFEM based computational HF models, the fluid flow in fractures and interstitials of the porous media are mostly solved separately, which brings difficulties in dealing with complex fracture morphology. In our new model the fluid flow is solved in a unified framework by considering the fractures as a kind of special porous media and introducing Poiseuille-type flow inside them instead of Darcy-type flow. The most advantage is that it is very convenient to deal with fluid flow inside the complex fracture network, which is important in shale gas extraction. The weak formulation for the new coupled model is derived based on virtual work principle, which includes the XFEM formulation for multiple fractures and fractures intersection in porous media and finite element formulation for the unified fluid flow. Then the plane strain Kristianovic-Geertsma-de Klerk (KGD) model and the fluid flow inside the fracture network are simulated to validate the accuracy and applicability of this method. The numerical results show that large injection rate, low rock permeability and isotropic in-situ stresses tend to lead to a more uniform and productive fracture network.     

论地震发生机制

地震发生的物理机理和过程是还没有认识清楚的问题. 此前人们将浅源地震归因于弹性回跳,根据这一观点和岩石实验结果计算得到的地震能量与实际观测结果有很大矛盾,被称之为“热流佯谬”. 中源和深源地震发生在地幔区域,其成因也没有合理的解释. 考虑到地壳和地幔是离散集合态物质体系及其慢动力学运动行为的基本特点,本文根据物理学原理,特别是近年凝聚态物理发展出来的相关新观念,并依据已有观测事实,从新的视角探究地震发生的物理机制. 1) 关于地壳岩石层中的应力分布:在不考虑构造力时,依据万物皆流的流变学原理,原始地壳岩石在自重压强长时间作用下,纵向和横向应力相同,没有差应力. 大地构造力推动岩块滞滑移动挤压断层泥,施加于其他岩块,逐渐传递和积累. 这种附加的横向构造力与原始岩石中应力叠加,形成地壳岩石层中的实时应力. 由于断层泥属于颗粒物质体系,具有与岩石不同的力学特征,其弹性模量比岩石小得多,且随压强而增大,导致构造作用力随深度非线性增大. 给出了地壳中构造应力分布及其变化规律. 2) 关于地壳岩石层强度:地壳岩石的自重会使岩石发生弹性–塑性转变. 通过对弹性–塑性转变深度的计算,并根据实际情况分析,给出了地壳岩石弹性、部分塑性和完全塑性三个区域的典型深度范围. 在部分塑性区,塑性体比例达到约10%以上时,发生塑性连通,这时岩石剪切强度由塑性特征决定. 塑性滑移的等效摩擦系数比脆性破裂小一个数量级以上,致使塑性滑移时岩石剪切强度比脆性破裂小得多. 同时,随深度增大,有多种因素使得岩石剪切屈服强度减小. 另一方面,地震是大范围岩石破坏,破坏必然沿薄弱路径发生. 因此,浅源地震岩石的实际破坏强度必定比通常观测到的岩石剪切强度值低. 给出了地壳岩石平均强度和实际破坏强度典型值随深度的分布规律. 3) 关于地震发生的条件和机制:地震发生必定产生体积膨胀,只有突破阻挡才可膨胀. 地震发生的条件是:大地构造力超过岩石破坏强度、断层边界摩擦力以及所受阻挡力之和. 因此,浅源地震是岩石突破阻挡发生的塑性滑移. 在此基础上提出了浅源地震发生的四种可能模式. 深源地震是冲破阻挡发生的大范围岩块流. 浅源地震和深源地震都是堵塞–解堵塞转变,是解堵塞后岩石层块滑移或流动造成的能量释放. 4) 关于地震能量和临震前兆信息:地震能量即为堵塞–解堵塞转变过程释放的动能. 以实例估算表明,地震岩石滑移动能与使岩块剪切破坏和克服周围摩擦阻力所需做的功相一致,不会出现热流佯谬. 同时指出,通过观测地震发生前构造力的积累过程、局域地区地质变迁以及岩石状态变化等所产生的效应,均可能获得有价值的地震前兆信息. 关键词： 地震发生机制 热流佯谬 地壳岩石应力和强度 堵塞–解堵塞转变     

Direct observation of plasticity and quantitative hardness measurements in single crystal cyclotrimethylene trinitramine by nanoindentation

Investigation of deformation beginning with elasticity and continuing through the elastic–plastic transition to incipient cracking has been conducted for (210), (021) and (001) oriented single crystals of the explosive cyclotrimethylene trinitramine, commonly known as “RDX”. Nanoindentation was performed with a conical tip over a range of loads. The resulting load–depth data exhibited distinct, reproducible, orientation-dependent load excursions demonstrating elastic–plastic transitions. Indent impressions were imaged by atomic force microscopy revealing deformation features consistent with slip on six planes. Impressions on the (210) and (001) planes showed deformation pile-up features associated with the zone axes of slip planes. Slip traces were evident on the (210) plane indicating slip on four planes and suggesting cross-slip. Height data, for impressions formed by progressively increasing loads, indicated one additional slip system consistent with (010) slip. All of the orientations exhibited cracking thresholds at very low loads. The reduced elastic moduli were anisotropic and the hardness values were isotropic indicating limited plasticity. Maximum shear stresses estimated from a Hertzian model, at load excursions, were within 1/15 to 1/10 of published shear moduli, indicating deformation initiated near the theoretical yield strength, presumably by homogeneous nucleation of dislocations. The material strength parameters and deformation pathways inferred from this work are compared to previous microhardness investigations in which the ambiguity of results can be attributed to the effects of cracking and simultaneous slip on multiple systems. A mechanistic explanation for the hindered plasticity, and cracking, observed for RDX is offered in terms of compatibility conditions.     

Climb via vacancy diffusion of edge dislocations in 2D dislocation microstructures

The prevention of strength degradation of components is one of the great challenges in solid mechanics. In particular, at high temperatures material may deform even at low stresses, a deformation mode known as deformation creep. One of the microstructural mechanisms that governs deformation creep is dislocation motion due to the absorption or emission of vacancies, which results in motion perpendicular to the glide plane, called dislocation climb. However, the importance of the dislocation network for the deformation creep remains far from being understood. In this study, a climb model that accounts for the dislocation network is developed, by solving the diffusion equation for vacancies in a region with a general dislocation distribution. The definition of the sink strength is extended, to account for the contributions of neighbouring dislocations to the climb rate. The model is then applied to dislocation dipoles and dislocation pile-ups, which are dense dislocation structures and it is found that the sink strength of dislocations in a pile-up is reduced since the vacancy field is distributed between the dislocations. Finally, the importance of the results for modelling deformation creep is discussed.     

Numerical simulation of deformation and fracture of a material with a polysilazane-based coating

The paper studies the localization of plastic deformation and fracture in a material with a porous coating. A dynamic boundary value problem in the plane strain formulation is solved. The numerical simulation is performed by the finite difference method. The composite structure corresponds to the experimentally observed one and is specified explicitly in the calculation. A generation procedure of the initial finite-difference grid is developed to describe the coating structure with adjustable porosity and geometry of the substrate-coating interface. Constitutive equations for the steel substrate include an elastic-plastic model of an isotropically hardening material. The ceramic coating is described by a brittle fracture model on the basis of the Huber criterion which accounts for crack nucleation in triaxial tension zones. It is shown that the specific character of deformation and fracture of the studied composite results from the presence of local tensile regions in the vicinity of pores and along the coating-substrate interface, in both tension and compression of the coated material. The interrelation between inhomogeneous plastic flow in the steel substrate and crack propagation in the coating is studied.     

Microstresses in textured polycrystals studied by the multireflection diffraction method and self-consistent model

A new version of the X-ray diffraction method for determining macrostresses and microstresses in textured polycrystalline material is presented. In this method the lattice strains for various orientation of the scattering vector as well as for various crystallographic planes {hkl} are measured. The interpretation of the experimental data is based on the least-squares fitting procedure, in which the diffraction elastic constants and theoretical values of microstresses are used. The diffraction elastic constants and the microstresses are calculated by the self-consistent model. The new method was successfully applied for stress determination in one- and two-phase steels subjected to elastoplastic deformation, and the significant anisotropy of the incompatibility stresses was observed in textured samples.     

Energy monitoring and analysis during deformation of bedded-sandstone: Use of acoustic emission

This paper investigates the mechanical behaviour and energy releasing characteristics of bedded-sandstone with bedding layers in different orientations, under uniaxial compression. Cylindrical sandstone specimens (54 mm diameter and 108 mm height) with bedding layers inclined at angles of 10°, 20°, 35°, 55°, and 83° to the minor principal stress direction, were produced to perform a series of Uniaxial Compressive Strength (UCS) tests. One of the two identical sample sets was fully-saturated with water before testing and the other set was tested under dry conditions. An acoustic emission system was employed in all the testing to monitor the acoustic energy release during the whole deformation process of specimens. From the test results, the critical joint orientation was observed as 55° for both dry and saturated samples and the peak-strength losses due to water were 15.56%, 20.06%, 13.5%, 13.2%, and 13.52% for the bedding orientations 10°, 20°, 35°, 55°, and 83°, respectively. The failure mechanisms for the specimens with bedding layers in 10°, 20° orientations showed splitting type failure, while the specimens with bedding layers in 55°, 83° orientations were failed by sliding along a weaker bedding layer. The failure mechanism for the specimens with bedding layers in 35° orientation showed a mixed failure mode of both splitting and sliding types. Analysis of the acoustic energy, captured from the acoustic emission detection system, revealed that the acoustic energy release is considerably higher in dry specimens than that of the saturated specimens at any bedding orientation. In addition, higher energy release was observed for specimens with bedding layers oriented in shallow angles (which were undergoing splitting type failures), whereas specimens with steeply oriented bedding layers (which were undergoing sliding type failures) showed a comparatively less energy release under both dry and saturated conditions. Moreover, a considerable amount of energy dissipation before the ultimate failure was observed for specimens with bedding layers oriented in shallow angles under both dry and saturated conditions. These results confirm that when rock having bedding layers inclined in shallow angles the failures could be more violent and devastative than the failures of rock with steeply oriented bedding layers.     

裂缝性应力敏感油藏流固耦合的数值模拟

以裂缝各向异性全张量渗透率动态耦合渗流模型为基础,建立三维三相裂缝各向异性特低渗透油藏流固耦合数学模型,并运用模块化显示耦合迭代求解建立流固耦合渗流数值模拟方法,编制耦合求解程序.研究表明:对于裂缝性油藏,随着油藏开采过程中压力的变化,岩石变形和储层介质的各向异性应变特性使开发过程中多组裂缝的变形程度不同,引起裂缝渗透率的非线性变化,进而使油藏裂缝渗透率张量主值发生旋转,改变液流方向、影响油藏开发效果.     

Influence of nanotwin generation near crack twins on the fracture toughness of nanomaterials

A theoretical model of microscopic mechanisms of the nucleation and development of deformation twins in nanocrystalline materials has been developed. Within the model, we have studied the generation of deformation twins near crack tips, which occurs through multiple nanoscopic shears that represent nanoscopic regions of an ideal plastic shear. It has been shown that the nucleation of such nanotwins near crack tips reduces the high local stresses that arise near these tips. Thus, the generation and development of nanotwins near crack tips increases the fracture toughness of brittle nanocrystalline materials and serves as an efficient mechanism of improving the crack resistance of deformed nanocrystalline materials.     

Anisotropic surface stress in polymer crystals: Example of polyethylene

The model of sharp folds at the surface of lamellar single crystals of polymers lead to high surface stress. The layer of folds is analogous to an epitaxial layer. Weak strains of the crystalline layers, when they are possible, relax these surface stresses. The principal properties of the most studied polymer, polyethylene, which are not observed in nonpolymeric materials, are shown to be deformation modes for relaxing the surface stress. From experimental observations it is inferred that in polyethylene, in the (110) and (100) fold domains, there are, respectively, an extension and a contraction along the growth faces. The observed twinning and martensitic modes appearing in polyethylene single crystals are the modes activated by the tensile surface stress due to the intrafold interaction. The idea proposed in this paper that surface stress activates some deformation modes can be easily extended to other lamellar polymer crystals.     

Structural mechanisms of fracture of nanocrystalline materials

Structural mechanisms and features of brittle and quasi-brittle fracture of nanocrystalline materials are theoretically analyzed. The role of size effects and internal stresses caused by a nonequilibrium structure during brittle trans-and intercrystallite fracture is studied. The dependence of the nanocrystalline material durability on the working stress and grain size is calculated. The conditions for certain mechanisms of plastic deformation to be operative in nanocrystalline materials are analyzed. The influence of the grain-boundary and dislocation mechanisms of plastic deformation on the conditions of nanocrack formation is studied. The dependence of the fracture toughness of nanomaterials on structure parameters is calculated.     

Size effects and stochastic behavior of nanoindentation pop in

A statistical model for pop in initiated at preexisting dislocations during nanoindentation is developed to explain size-dependent pop-in stresses. To verify theoretical predictions of this model, experiments were performed on single-crystal Mo, utilizing indenter radii that vary by over 3 orders of magnitude. The stress where plastic deformation begins ranges from the theoretical strength in small volumes, to 1 order of magnitude lower in larger volumes. An intermediate regime shows wide variability in the stress to initiate plastic deformation. Our theory accurately reproduces the experimental cumulative probability distributions, and predicts a scaling behavior that matches experimental behavior.     

Self-consistent tilted-axis-cranking study of triaxial strongly deformed bands in 158Er at ultrahigh spin

Stimulated by recent experimental discoveries, triaxial strongly deformed (TSD) states in (158)Er at ultrahigh spins have been studied by means of the Skyrme-Hartree-Fock model and the tilted-axis-cranking method. Restricting the rotational axis to one of the principal axes--as done in previous cranking calculations--two well-defined TSD minima in the total Routhian surface are found for a given configuration: one with positive and another with negative triaxial deformation γ. By allowing the rotational axis to change direction, the higher-energy minimum is shown to be a saddle point. This resolves the long-standing question of the physical interpretation of the two triaxial minima at a very similar quadrupole shape obtained in the principal-axis-cranking approach. Several TSD configurations have been predicted, including a highly deformed band, which is a candidate for the structure observed in experiment.     

On mechanical stability of molybdenum

Investigations of the strength properties of materials under different loading conditions are of practical importance in many engineering applications. The knowledge of elastic moduli as a function of strain is required for determination of strength properties. In the present work, we have determined the elastic moduli of molybdenum through first principles study of the energy changes under three different loading conditions namely ‘uni-axial tensile deformation’ along [0 0 1] direction, ‘uni-axial tensile loading’ along [0 0 1] direction and ‘hydrostatic tensile loading’. The stability conditions for the system are expressed in terms of the elastic moduli and analyzed along the deformation paths corresponding to these three loading modes. The theoretical spall strength (σ_S), tensile strength (σ_T) along [0 0 1] direction and hydrostatic tensile strength (σ_H), are evaluated as a stress at the first onset of the instability for three loading conditions, respectively. The calculated equilibrium volume and elastic moduli are compared with that reported from experimental and other theoretical works.     

Mechanism of recovery of the mechanical properties of neutron-irradiated metals in the course of thermocycling

The mechanism of recovery of the strength and strain characteristics of neutron-irradiated metals and alloys (up to their initial values observed prior to irradiation) during periodic quenching in the temperature range below the irradiation temperature is considered. It is assumed that the removal of radiation-induced defects (interstitial and vacancy loops) from radiation-hardened metals is associated with the formation of defect-free channels along the slip planes (the phenomenon of dislocation channeling) under the thermal stresses arising in each cycle of quenching. The relationships describing both the kinetics of the decrease in the yield strength σ_Y and in the ultimate strength σ_U and the kinetics of the increase in the uniform strain ε_U of a preliminarily irradiated material with increasing number of quenching cycles are derived using the equations of dislocation kinetics. The theoretical results obtained are compared with experimental data on the kinetics of recovery of the mechanical properties of neutron-irradiated samples (the austenitic FeNiCr and ferritic FeCrMo structural steels and the titanium alloy TiAlZr) in the course of periodic quenching.     

Dragon-Kings in rock fracturing: Insights gained from rock fracture tests in the laboratory

In order to shed some lights to the “dragon-kings” concept, this paper re-examines experimental results on rock fracture tests in the laboratory, obtained from acoustic emission monitoring. The fracture of intact rocks as well as rocks containing natural structures (joints, faults, foliations) under constant stress rate loading or creep conditions is generally characterized by typical stages with different underlying physics. The primary phase reflects the initial rupture of pre-existing microcrack population in the sample or in the fault zone. Sub-critical growth dominates the secondary phase. The third phases termed nucleation phase corresponds to the initiation and accelerated growth of the ultimate fracture. The secondary and nucleation phases in both intact rock and faulted rock show power-law (of time-to-failure) increasing event rate and moment release. Samples containing planar structures such as foliations and faults demonstrate very similar features to natural earthquakes including: 1) small number of immediate foreshocks by which fault nucleation zones could be mapped; 2) the critical nucleation zone size is normally a fraction of the sample dimension; 3) a lot of aftershocks concentrated on the fault ruptured during the main event; 4) stress drop due to the main rupture is of the order from a few tens to a few hundreds MPa; 5) b-value drops during foreshocks and recovers during the aftershocks. All these results agree with the suggestion that laboratory measurements require no scaling but can be applied directly to the Earth to represent local fault behavior. The ultimate failure of the sample, or fracture of major asperities on the fault surface, normally lead to extreme events, i.e., dragon-kings, which has a magnitude significantly greater than that expected by the Gutenberg-Richter power-law relation in the magnitude-frequency distribution for either foreshocks or aftershocks. There are at least two mechanisms that may lead to dragon-kings: 1) The power-law increasing event rate and moment release; and 2) Hierarchical fracturing behavior resulting from hierarchical inhomogeneities in the sample. In the 1st mechanism, the final failure corresponds to the end point of the progressive occurrence of events and thus the resulted dragon-king event can be interpreted as a superposition of many small events. While for the 2nd mechanism an event of extreme size is the result of fracture growth stepping from a lower hierarchy into a higher hierarchy on fault surface having asperities characterized by hierarchical distribution (of size or strength) rather than simple fractal distribution. In both mechanisms the underlying physics is that fracture in rocks is hard to stop beyond certain threshold corresponding to the critical nucleation zone size.     

Tilted rotation of triaxial nuclei

The Tilted Axis Cranking theory is applied to the model of two particles coupled to a triaxial rotor. Comparing with the exact quantal solutions, the interpretation and quality of the mean field approximation is studied. Conditions are discussed when the axis of rotation lies inside or outside the principal planes of the triaxial density distribution. The planar solutions represent ΔI = 1 bands, whereas the aplanar solutions represent pairs of identical ΔI = 1 bands with the same parity. The two bands differ by the chirality of the principal axes with respect to the angular momentum vector. The transition from planar to chiral solutions is evident in both the quantal and the mean field calculations. Its physical origin is discussed.     

Crack growth on the basal plane in single crystal zinc: experiments and computations

Crack propagation on the basal planes in zinc was examined by means of in situ fracture testing of pre-cracked single crystals, with specific attention paid to the fracture mechanism. During quasistatic loading, crack propagation occurred in short bursts of dynamic crack extension followed by periods of arrests, the latter accompanied by plastic deformation and blunting of the crack-tip. In situ observations confirmed nucleation and propagation of microcracks on parallel basal planes and plastic deformation and failure of the linking ligaments. Pre-existing twins in the crack path serve as potent crack arrestors. The crystallographic orientation of the crack growth direction on the basal plane was found to influence both the fracture load as well as the deformation at the crack-tip, producing fracture surfaces of noticeably different appearances. Finite element analysis incorporating crystal plasticity was used to identify dominant slip systems and the stress distribution around the crack-tip in plane stress and plane strain. The computational results are helpful in rationalizing the experimental observations including the mechanism of crack propagation, the orientation dependence of crack-tip plasticity and the fracture surface morphology.     

A review on the research progress of push-out method in testing interfacial properties of SiC fiber-reinforced titanium matrix composites

Experimental analysis of single-fiber push-out for SiC fiber-reinforced titanium matrix composites (TMCs) is complicated by the incorporation of large thermal residual stresses, strong chemical bond of the fiber/matrix interface and matrix plastic deformation. This paper summarizes the development of push-out test and the characteristics of push-out test for TMCs such as crack initiating at the bottom face and theoretical analysis of the test. Moreover, it deeply analyzes the progresses of interfacial shear strength and fracture toughness, and work focus is pointed out in future.