Stable Crack Research Articles

RILEM Standard: testing methods for determination of the double-K criterion for crack propagation in concrete using wedge-splitting tests and three-point bending beam tests, recommendation of RILEM TC265-TDK

In this recommendation, standard testing methods for determination of the double-K criterion for Mode I crack propagation in concrete using wedge-splitting tests and three-point bending beam tests are specified for the fracture parameters of the initial cracking toughness K_{Ic}^{ini} and the unstable fracture toughness K_{Ic}^{un}. Along with the recommendation of the standard testing methods, the theoretical background of the double-K criterion, the calculation methods and the results of round-robin testing for determining the double-K fracture parameters are presented in technical reports [1, 2]. The recommendation of the standard testing methods includes geometry for specimens, fabrication of specimens, testing machine, load transmission system and supports, measuring instruments, determination of initial cracking load Pini, determination of measured maximum load Pmax and initial compliance ci, calculation methods for wedge-splitting test and three-point bending beam test, as well as the testing results and testing report forms. According to these methods, the measured fracture parameters of double-K criterion can be used to describe the onset of cracking, and the onset of unstable cracking or failure for predicting crack initiation, structure failure and crack stability in concrete structures.

Enhancements for technique of stable low cycle fatigue crack propagation simulation for gas turbine engine

Enhancements for technique of stable low cycle fatigue crack propagation simulation for gas turbine engine

Crack Propagation Analysis of Damaged Solid Propellant under Impact Overload

The ammunition safety problem is particularly prominent when the storage and transportation launch box is airdropped and landed. A safety evaluation method of rocket air drop based on propellant damage evaluation is proposed. Based on the theory of fracture mechanics and the evaluation method of structural integrity of rocket engine, established a local finite element model of rocket engine with initial damage, the crack propagation analysis is carried out by using the propagation finite element method (XFEM). The results show that when the landing impact overload is 30g (25ms) that the airdrop equipment should be able to withstand, the modified double base propellant has produced the phenomenon of crack instability propagation. When the initial crack direction and load direction are 120 °, the propagation is the most serious and there are safety problems; when the solid propellant is airdropped, it is necessary to increase the buffer to reduce the overload.

Temperature Effect on Crack Evolution of Red Clay in Guilin

As temperature changes, red clay is prone to shrink and generate cracks, which weaken the structure and the stability of soil mass, leading to various engineering problems, such as damage and instability in engineering structures. To study the effect of environmental temperature on the crack evolution of red clay, Guilin Red Clay was taken as the research object, and the saturated mud samples were dried at 23, 40, and 60 °C respectively. During the drying process of the samples, the change of moisture content and the evolution process of surface cracks were monitored by high-definition automatic photographing and a weighing device, which were also improved. We used PCAS software to process the crack image, extract various geometric elements, observe, and analyze the change rule of the cracks during the drying process of red clay at different temperatures. The test results show that the cracking evolution of red clay at different temperatures is mainly divided into three stages: (i) the initiation of micro cracks; (ii) crack progress; and (iii) crack stability. With the increase of environmental temperature, stage (i) took less time. Meanwhile, the growth rate of the crack area increased. The number of final crack blocks of soil is significantly reduced. Moreover, the final crack rate is obviously increased. When the temperature is either 23 °C or 40 °C, the initial cracks almost happen at the same time in the samples with different diameters. While the temperature is higher than 60 °C, the cracking time will delay with the increase of the diameter. In addition, the decrease in water content leads to a decrease in the curvature radius of soil particles. Under the joint action of the surface tension and the matrix suction, the distance between red clay particles becomes shorter, so the time for red clay to start to generate cracks will be shorter, and the final crack rate will increase with the increase in temperature.

Real-time monitoring of the development of brittle fracture in hard rock tunnels based on physical model test

Brittle fracture in deep hard rock tunnels in the Jinping laboratory, China, can often endanger the stability of engineering structures. This study describes physical model testing and real-time observations on the evolution process of v-shaped brittle fracture, failure characteristics, onset conditions, and influencing factors in hard rock tunnels. Real-time observations are presented integrated a high-speed camera, acoustic emission system, a digital image correlation, and distributed fiber optic strain sensing technologies. The results show that the process of spalling failure can be divided into two main stages, namely, the instantaneous cracking process in the sidewall surface, and the spalling process of the thin rock chips developing progressively from the excavation boundary to the far field, finally forming stable v-shaped notches. The deviatoric stress of spalling failure in the physical model test satisfies the constant deviatoric stress criterion. The critical strain at spalling failure is consistent with that calculated by the Hoek–Brown strength criterion. Spalling is a form of brittle failure caused by deferential tensile strain. The working face effect is observed in the physical model excavation process. The highest strain caused by overloading occurs near the free surface of the sidewall. The relationship between stress and geometric similarity constants in deep rock engineering is derived. The failure characteristics revealed in the physical model tests are verified by numerical simulations based on cellular automaton theory, which agree well with the experimental results. The research findings can provide theoretical support for the prevention and control of spalling disasters in deep engineering practice.

Numerical Study of Thermal Shock Damage Mechanism of Polycrystalline Ceramics

A dual-scale model is proposed to study the effect of microstructure parameters (grain size and grain boundary fracture energy) on the thermal shock damage mechanism on an example of alumina. At microscale, representative volume element (RVE) models generated by Voronoi tessellation are simulated to obtain the mechanical parameters for macro models. At macroscale, a coupled thermomechanical model based on the finite–discrete element method (FDEM) is applied to simulate the crack nucleation and propagation. Energy dissipation (ALLDMD) is introduced to investigate the thermal shock cracking mechanism by combining crack patterns and crack density, which indicates that decreasing grain size and increasing grain boundary fracture energy have a positive effect on thermal shock resistance. The proposed models not only predict the critical stress temperature which is well consistent to the theoretical thermal shock resistance factor, but also quantify the two previously unconsidered stages (crack nucleation and crack instability stage). Our models suggest the crack nucleation and instability will not occur immediately when the model reaches critical stress, but the models can sustain for higher temperature difference. The thermal shock damage mechanism and the influence of microstructural parameters on thermal shock resistance have also been discussed in detail.

Optimization of Die Design to Abolish Surface Defect on Telescopic Front Fork (TFF) Tube

Background: The telescopic fork is mainly used for suspension purposes in different devices to absorb the vibration and disturbances which come from the road or mechanical devices. Factors such as die angle, drawing velocity, lubrication, and area reduction per pass significantly affect the drawing loads and residual stresses formed in the drawn tube during the tube drawing process. Objective: An instantaneous transverse crack was found on the pipe's outer surface during the drawing process in the current work, and the key challenges were to reduce the percentage of pipe rejection. Methods: In this work, optimum drawing die designs were proposed by using the Finite Element Method (FEM). The FEM solving tool is called Abaqus, and it has been used for simulating and solving the cold-rolling process. The FEM model of cold drawing process is generated in Abaqus with the same boundary condition (Axial load and constrain) as used on the actual wire drawing machine. Result: With current die geometry, there was a substantial reduction in area, axial stress (Tensile) along the die side is 672 MPa, which is 23 % lower than the current die axial stress value 877 MPa. There was 48 % plastic strain found along the purposed die side, which was 17 % lower than the existing strain of 64%. Finally, the area was reduced by changing the die geometry from the current level of ~52% to 35 to 40 %. Conclusion: By reducing the area (35 to 40 %) in the output tube, and strain (17 %), it was possible to abolish the transverse crack on the pipe's outer surface. As part of the optimisation of the FEM work process, this work gives us encouraging results. Further research will be considered for future work.

Finite element-based analysis of buckling-induced plastic deformation

In recent years, the investigation of adhesion properties of thin films on brittle substrates is receiving more attention due to the spread of thin film applications (coatings, protective layers, conductive layers in microelectronics, etc.). However, a basic approach for adhesion measurements proposed by Hutchinson and Suo almost three decades ago is still widely used due to its easy applicability. This approach uses thin film spontaneous buckling using residual compressive stresses in the film, however, it is based on Euler beam theory and accounts only for elastic behavior. Modern material combinations may lead to permanent plastic deformations, even under the controlled experimental conditions according to the elastic model. This work aims at investigating the plastic deformation influence on the experimental procedure according to Hutchinson and Suo by the means of the finite element analysis on the specific Mo-Cu-glass system. The deformations of spontaneously formed buckles are compared between purely elastic, (more realistic) elastic-plastic model and previously published experimental results together with quantification of the dissipated energy via the current finite element model. The comparison of the two numerical approaches shows non-negligible plastic deformations of the spontaneously formed buckles which has an effect on the film deformed shape (corresponding to the experimental observations) as well as on the portion of the shear loading at the delamination crack tip. This indicates that the plastic deformation during the buckling delamination may have some impact on the stability of the delamination crack.

Research on low temperature brittleness of 30Cr1Mo1V high-temperature rotor and its life optimization in rapid start-up and warm-up process

Recently, there is the serious fracture for two high-temperature steam turbine rotors made of 30Cr1Mo1V steel in different power plants, characterized by brittle fracture. This steel has the disadvantage of low temperature brittleness. Currently, a large number of 30Cr1Mo1V rotors are in service. Meanwhile, these rotors are required to start up and warm up faster due to the demand for operation flexibility of the power system. In order to ensure the safety of the 30Cr1Mo1V rotors under flexible operation, it is necessary to study the low temperature brittleness of the rotors and its influence on the fatigue life in rapid start-up and warm-up processes. In this paper, the fracture causes of the 30Cr1Mo1V rotor were analyzed by the mechanical property tests, microstructure examination and fracture analysis. The test results showed that the toughness of rotor decreased significantly after service. Secondly, the temperature and stress changes of the high-temperature rotors during the rapid start-up and warm-up process were calculated by the finite element method. The simulation results showed that if the speed rose too fast, the thermal stress in the rotor inlet area would increase significantly and the temperature was lower than the fracture appearance transition temperature when reaching the full speed. The high stress will easily lead to a crack initiation under low-cycle fatigue. Then, crack instability propagation will soon occur due to the low temperature and poor toughness. Thirdly, the life of the turbine in the rapid start-up was optimized by controlling the startup time. In addition, the life under different warm-up conditions was quantitatively analyzed. It not only reveals the necessity of the warm-up process for the center hole rotor, but also successfully demonstrates that the warm-up process of the solid rotor can be cancelled, which improves the economy of the power plant.

Thermal shock resistance of Cr2O3–Al2O3–ZrO2 bricks with the microstructure of Cr2O3 aggregates coated by ZrO2

Zirconia-coated Cr2O3 aggregates synthesized by mixing ZrO2 powders and Cr2O3 aggregates with a phenolic resin binder followed by thermosetting treatment were employed as modified Cr2O3 aggregates to obtain Cr2O3–Al2O3–ZrO2 bricks (high-chromia bricks). The elastic modulus (E) and cold modulus of rupture (CMOR) of these high-chromia bricks before and after thermal shock cycles were systematically investigated, and the residual ratios of CMOR and E were calculated. The thermal shock resistance of the high-chromia bricks was significantly improved by the factor of modification of Cr2O3 aggregates. The mechanism of the improved thermal shock resistance of these high-chromia bricks was studied via microstructure analysis, and the crack propagation behavior was analyzed by scanning electron microscopy (SEM). The fracture work (γWOF), thermal shock damage factor (R′′′′), and thermal stress crack stability parameter (Rst) were measured and calculated using the wedge splitting test (WST). The results indicate that the porous ZrO2 coating layer wrapped the Cr2O3 aggregates, forming modified Cr2O3 aggregates that can increase crack deflection, free path of crack propagation, and fracture work, thus improving the thermal shock resistance of high-chromia bricks. The thermal shock resistance of the fabricated high-chromia bricks was highly correlated with the thickness of the ZrO2 coating layer surrounding the Cr2O3 aggregates. The variation trend of Rst is well consistent with the experimental results, which is suitable to evaluate the thermal shock resistance of high-chromia bricks.

On the significance of diffuse crack width self‐evolution in the phase‐field model for residually stressed brittle materials

The Phase-Field method is an attractive numerical technique to simulate fracture propagation in materials relying on Finite Element Method. Its peculiar diffuse representation of cracks makes it suitable for a myriad of problems, especially those involving multiple physics and complex-shaped crack patterns. Recent literature provided linear relationships between the width of the diffuse crack and the material intrinsic fracture toughness, through a material characteristic length. However, lately, it was shown how the existence of a residual stress field can affect the represented crack width even for fully homogeneous materials, in terms of toughness. In this short note, the authors tried to shed some light on the factors influencing the width of the diffuse crack representation. By simulating crack propagation in several residually stressed brittle materials, it was shown how the width of the diffuse crack is affected by the ratio between the driving force - due to the externally applied load - and the driving force required to propagate the crack. In other words, the diffuse crack extent can be linked to the degree of crack propagation stability/instability. Monitoring the evolution of the studied quantity can be of great interest to rapidly assess crack instability circumstances, under displacement control.

Transfer station cracks induced by cutting anchor cables and crack stabilization: A case study

This study reports an investigation on transfer station cracking induced by cutting anchor cables at North Yuehu Park station in Changsha city through both field monitoring and numerical simulation. The incident was triggered by cutting part of the original anchor cables while excavating a deep groove for an entrance diaphragm wall. When part of these support structures were removed, the stress condition of the station changed significantly, which eventually resulted in a large area of cracks. Crack repair and foundation pit bottom grouting were applied to prevent further cracking and resume the stability of the station, which achieved the desired result. The mechanisms of station cracking and the effectiveness of stabilization are investigated through both numerical simulation and field monitoring data analysis. In addition, an optimization scheme for station design is proposed and verified through numerical analysis. The lesson to be learned is that cutting the original anchor cables and structures of a sharp corner should be avoided during station construction. A reinforcement and field monitoring system should be implemented in advance to guarantee the safety of the station if cutting the original anchor cables is unavoidable.

Deep learning model to predict fracture mechanisms of graphene

Understanding fracture is critical to the design of resilient nanomaterials. Molecular dynamics offers a way to study fracture at an atomistic level, but is computationally expensive with limitations of scalability. In this work, we build upon machine-learning approaches for predicting nanoscopic fracture mechanisms including crack instabilities and branching as a function of crystal orientation. We focus on a particular technologically relevant material system, graphene, and apply a deep learning method to the study of such nanomaterials and explore the parameter space necessary for calibrating machine-learning predictions to meaningful results. Our results validate the ability of deep learning methods to quantitatively capture graphene fracture behavior, including its fractal dimension as a function of crystal orientation, and provide promise toward the wider application of deep learning to materials design, opening the potential for other 2D materials.

Energy Release Rate Formulations for Non-conventional Fracture Test Geometries

Non-conventional fracture test geometries such as single cantilever bending, clamped beam bending and clamped wire bending have been developed over the years to suit the needs of micro- and nano-scale testing. The same geometries can also be used at the macro-scale if testing standards are well established. Development of length scale compatible geometries helps the materials engineer to seamlessly extract material properties across multiple length scales. This article proposes the crack driving force (or energy release rate G) criteria for the above three geometries. These geometries have found widespread use at the micro-scale and can be adapted to the macro-scale. Use of a global energy-based criterion instead of the local stress-based criteria (stress intensity factor K) has its own advantages, especially in multi-phase materials and interface-dominated structures which display an R-curve. Aspects of crack stability even under load control arising because of the geometric factors, especially the beam or wire aspect ratio, are discussed in this context. Validation of the compliance approach is carried out by comparing it to the J-integral extracted directly for a linear elastic material using extended finite element modeling and also using the analytical formulation for a double-cantilever beam specimen. Experimental evidence of the validity of the solutions in determining the fracture energy of a linear elastic brittle material, PMMA, as a homogeneous model system is shown.

Effect of Short Fiber Reinforcements on Fracture Performance of Cement-Based Materials: A Systematic Review Approach.

Fracture characteristics were used to effectively evaluate the performance of fiber-reinforced cementitious composites. The fracture parameters provided the basis for crack stability analysis, service performance, safety evaluation, and protection. Much research has been carried out in the proposed study field over the previous two decades. Therefore, it was required to analyze the research trend from the available bibliometric data. In this study, the scientometric analysis and science mapping techniques were performed along with a comprehensive discussion to identify the relevant publication field, highly used keywords, most active authors, most cited articles, and regions with largest impact on the field of fracture properties of cement-based materials (CBMs). Furthermore, the characteristic of various fibers such as steel, polymeric, inorganic, and carbon fibers are discussed, and the factors affecting the fracture properties of fiber-reinforced CBMs (FRCBMs) are reviewed. In addition, future gaps are identified. The graphical representation based on the scientometric review could be helpful for research scholars from different countries in developing research cooperation, creating joint ventures, and exchanging innovative technologies and ideas.

Real-Time Monitoring and Early Warning of Water Inrush in a Coal Seam Floor: A Case Study

Monitoring and early warning systems are an efficient way to avoid and mitigate the significant loss of properties and people’s lives caused by water inrush via the coal seam floor, a frequent problem in north China’s deeper coal mines. This paper discusses the basic technical problems, such as the monitoring conditions, indexes and technical system of monitoring and early warning of water inrush in coal seam floor, and selection of monitoring positions. Double criteria for water inrush monitoring and early warning were established based on three factors: crack instability extension, water temperature, and water pressure. Accordingly, a new monitoring and early warning system of the water inrush in coal seam floor was developed using a fiber Bragg grating sensor. This was applied at coal face 2113 of the Chengjiao mine, Henan Province, which lowered the water inrush probability, and so no early warning was necessary. The rationality and effectiveness of this technique were thus verified.

Temperature and radiation effects on brittle versus ductile fracture behavior in miscible phase boundaries: insight from atomistic simulations

Temperature- and irradiation-assisted failure mechanisms in miscible phase boundaries are clarified via atomistic calculations. We first establish the temperature-dependent brittle-to-ductile transition in U–Zr miscible phase boundaries. Our results confirm that these boundaries are mostly brittle at low temperatures and ductile at elevated temperatures. We then investigate the changes induced by irradiation on the fracture mechanisms in such phase boundaries. The irradiation-induced defect accumulation follows a multi-stage process that starts with the accumulation of isolated small dislocation loops before transitioning to the saturation and growth of larger dislocation loops and end up with a reorganization into forest dislocations. The accumulation of loops is the primary feature to participate in the delineation between brittle and ductile interfacial fracture in irradiated phase boundaries. At low damage levels, radiation defect interactions with the crack tip are limited and U–Zr miscible boundaries fail through the emission of dislocations ahead of the crack tip followed by brittle cleavage in agreement with the classical Griffith’s criterion for crack stability. At higher damage levels, the failure mode transitions from brittle crack growth to ductile void growth. In this case, the microcrack tip is blunted by the high density of pre-existing, radiation-induced defects in the vicinity of the crack. This interaction leads to the development and growth of a cavity at the interface as opposed to interfacial cleavage.

Back-analysis of Donghekou landslide using improved DDA considering joint roughness degradation

Currently, the dynamic behavior of high-speed long-runout landslides triggered by earthquakes remains unclear due to the complexity of such phenomena. Degradation in the shear strength of rock masses is considered a primary factor contributing to the high velocities and long runout distances of these landslides. In this study, the strength degradation induced by changes in rock joint morphology is analyzed considering a numerical model of the Donghekou landslide, by using discontinuous deformation analysis (DDA). First, an exponential joint roughness degradation function (i.e., JRCm) that depends on displacement is incorporated with the nonlinear Barton–Bandis (B–B) shear strength criterion, and this modified B–B criterion is combined with the DDA. Moreover, a virtual joint is introduced in the DDA to model the propagation of cracks in intact rock masses, in order to simulate the dynamic phenomenon of earthquake-induced landslides. Subsequently, this improved DDA (IDDA) is validated via simulations of a shear test and a sliding block over an inclined plane. Finally, the IDDA is employed to simulate the Donghekou landslide, which was induced by the 2008 Wenchuan earthquake in China. Furthermore, the entire dynamic process of the landslide, including crack propagation, instability, initiation, and motion, is depicted. The simulated deposit patterns obtained via the IDDA are in good agreement with the results of field investigations. The simulation results also prove that the IDDA can reflect key movement behaviors of Donghekou landslide and that the degradation of rock joints significantly influences the long runout distance and high velocity of sliding mass.

Progressive Failure and Acoustic Emission Characteristics of Red Sandstone with Different Geometry Parallel Cracks under Uniaxial Compression Loading

The geometric distribution of initial damages has a great influence on the strength and progressive failure characteristics of the fractured rock mass. Initial damages of the fractured rock were simplified as parallel cracks in different geometric distributions, and then, the progressive failure and acoustic emission (AE) characteristics of specimens under the uniaxial compression loading were analyzed. The red sandstone (brittle materials) specimens with the parallel preexisting cracks by water jet were used in the tests. The energy peak and stress attenuation induced by the energy release of crack initiation were intuitively observed in the test process. Besides, three modes of rock bridge coalescence were obtained, and wing crack was the main crack propagation mode. The wing crack and other cracks were initiated in different loading stages, which were closely related to the energy level of crack initiation. The propagation of wing crack (stable crack) consumed a large amount of energy, and then, the propagation of shear crack, secondary crack, and anti‐wing crack (unstable crack) was inhibited. The relationship between the crack propagation mode and the geometric distribution of existing cracks in the specimen was revealed. Meanwhile, the strength characteristic and failure mode of fractured rock with the different geometric distributions of preexisting crack were also investigated. The energy evolution characteristics and crack propagation were also analyzed by numerical modeling (PFC2D).

The Research on Mechanical Performance of Portal Crane Main Steel Structure Based on Sustainable Development

This paper introduces an evaluation method of mechanical performance of portal crane main steel structure. The risk factors of portal crane are investigated from five aspects of strength, stiffness, corrosion, crack and local stability. Through quantification, each evaluation index is transformed into a quantitative value that is easy to identify, which directly reflects the state of the steel structure of the crane. Scientific and reasonable evaluation results are beneficial to maintenance and use of portal crane, and it can save energy and reduce emission, reduce cost and increase efficiency.