Engineering Fracture Mechanics最新文献

Creep fracture presents a major concern for structural materials and critical components operating at elevated temperatures, thus requiring effective computational models. This study presents a phase-field framework for modelling creep crack growth and fracture behaviour of modern high-temperature materials such as Creep Strength Enhanced Ferritic (CSEF) martensitic steels. The model was formulated using the thermodynamic principles of the variational phase field theory of fracture and considering some physical aspects of creep fracture. Within the modelling framework, a dissipation potential dependent on creep damage is introduced to capture the effect of creep cavitation on the fracture energy and the creep crack growth resistance of the solid in a phenomenological manner. An elastoplastic power-law creep model is coupled to the phase-field formulations to account for the non-linear deformation processes ahead of the crack tip due to inelastic deformations, as well as their contribution to fracture at high temperatures. The capability of the proposed model is assessed against experimental data from compact tension (CT) creep tests conducted on P91 and P92 steels. Good agreement was obtained between the FE-predicted creep crack growth behaviour and the experimental measurements, showcasing the model’s feasibility. Further, numerical experiments were conducted using the proposed model to elucidate some key aspects influencing the fracture behaviour of martensitic steels. The proposed computational framework not only demonstrated good capability but was also able to offer improved mechanistic insights into the influence of material tendency to develop creep cavities on crack growth behaviour. This work contributes valuable insights into understanding the fracture process of CSEF steels at elevated temperatures and further demystifies the role of creep ductility.

The prediction of crack propagation in materials is a crucial problem in solid mechanics, with many practical applications ranging from structural integrity assessment to the design of advanced materials. The phase-field method has emerged as a powerful tool for modeling crack propagation in materials, due to its ability to accurately capture the propagation of cracks. However, current phase-field algorithms suffer from the elevated computational cost associated with the so-called staggered solution scheme, which requires extremely small time increments to advance the crack due to its inherent conditional stability. In this paper, we present, for the first time, a quantitative analysis detailing the numerical implementation and comparison of two common solution strategies for the coupled large-deformation solid-mechanics-phase-field problem, namely the quasi-Newton based Broyden–Fletcher–Goldfarb–Shanno algorithm (BFGS) and the full Newton based alternating minimization (or staggered) (AM/staggered) algorithm. We demonstrate that the BFGS algorithm is a more efficient and advantageous alternative to the traditional AM/staggered approach for solving coupled large-deformation solid-mechanics-phase-field problems. Our results highlight the potential of the quasi-Newton BFGS algorithm to significantly reduce the computational cost of predicting crack propagation in hyperelastic materials while maintaining the accuracy and robustness of the phase-field method.

Ensuring the continuity and stability of the oxide scale to separate T91 steel from lead-bismuth eutectic (LBE) is an effective method for limiting corrosion in the lead-based fast reactor system. In this study, we specifically investigated the impact of cooling thermal stress on the stability and damage of the oxide scales of T91 steel when exposed to oxygen-saturated liquid lead–bismuth eutectic at high temperatures. A corrosion test was conducted on T91 steels exposed to stagnant oxygen-saturated LBE at 500 °C. Experimental results indicated that oxide damage exists without any external force, including interface cracks and through-scale cracks perpendicular to the interface. We developed a three-layer peridynamic model of T91-(Fe,Cr)₃O₄-Fe₃O₄ to simulate the deformation and failure of oxides under a cooling process from high temperature to room temperature. The simulation results show that a temperature drop of about 200 °C from 500 °C can cause through-oxide cracks in the oxide scale, and subsequent cooling can lead to the propagation of interfacial cracks. Additional modifications were introduced in the model to account for the porosity of the oxide scale. Parametric investigations illustrate the effects of oxide scale thickness and porosity on the resulting crack patterns.

Natural folded fissures commonly exist rather than simple straight fissures in rock masses surrounding tunnels. The presence of folded fissures significantly affects the fracture processes and failure modes of tunnel structures, thereby affecting their safety and stability. However, the research in this area is limited. Given this, this study fabricates city-gate section tunnel specimens containing folded fissures of various dip angles (α) and orientation angles (β), utilising 3D sand printing with sand and furan resin as matrix materials. Uniaxial compression fracture tests were conducted at a loading rate of 0.3 mm/min on these specimens using a digital image correlation technique to assess the impact of folded fissures on the mechanical properties and failure modes of the tunnel structures. Additionally, the improved smoothed particle hydrodynamics (SPH) method was used for damage evolution simulations during interactions between folded fissures and tunnels, and the simulation results were compared with the experimental results to verify the correctness of the method. The results show that for folded fissures, wing cracks and anti-wing cracks initiate not only from the ends but also from the bends of the fissures, whereas for straight fissures, they appear only at the fissure ends. Except for β = 0°, the interaction between folded fissures and tunnels generally results in the formation of a crack connecting the folded fissure upper end with tunnel, and the connection position varies with β. Different β values of folded fissures also influence the appearance and morphologies of top major cracks, underside major cracks, side cracks, and corner cracks around tunnels. As α increases, the overlap point of the crack and the tunnel moves from the tunnel corner to the tunnel crown. Moreover, folded fissures significantly affected the peak strength of the tunnel structures. As β increases, the peak strength first decreases, then increases, and finally decreases, reaching a minimum of 3.58 MPa at β = 90°. However, the peak strength differences are not evident under different α values. Finally, the influence of folded fissures on the cracking mechanism of the tunnel models is discussed in detail. This study provides insights into the impact of folded fissures on tunnel fracture modes and offers a reference for the application of SPH to reveal the underlying failure mechanisms of tunnel structures.

A thorough understanding of the mechanical properties and fracture mechanisms of coal under high-temperature conditions is crucial for preventing deep coal and rock dynamic disasters. Utilizing the Hopkinson pressure bar experimental system, this study conducts an in-depth analysis of the strength characteristics, failure modes, microscopic properties, and energy consumption effects of coal between 0 and 250℃. The findings reveal that during the heating process, coal’s mass loss increases with temperature, but at a decreasing rate. Significant changes in the coal samples’ dynamic strength, fragmentation, microscopic features, energy evolution, and fracture mechanisms occur at 100℃, marking a turning point in their dynamic behavior. Both dynamic strength and elastic modulus experience a transient increase at 100℃, while the fractal dimension experiences a brief decrease. At 100℃, the thermal expansion of coal particles predominates over the thermal damage from high temperatures, resulting in an increase in the coal samples’ dynamic strength. As the temperature further rises, thermal damage to the coal samples intensifies, leading to a decrease in dynamic strength. Similarly, the absorption and dissipation energy index K of the coal samples experiences a brief increase at 100℃, signifying a sudden change in the energy evolution pattern. Observations of the coal samples’ failure modes upon impact reveal a transition from tensile failure to a combined shear-tensile failure with increasing temperature.

Elucidating the effects and fundamental mechanisms of microwave-assisted mechanical excavation under high initial stress conditions is of paramount importance for enhancing the efficiency of deep resource extraction. In this study, indentation experiments were conducted on microwave-damaged rock under initial stress conditions using a tunnel boring machine (TBM) for the first time. By integrating acoustic emission, digital image correlation, and the discrete element method, we conducted a comprehensive analysis of the multifaceted effects of microwave irradiation and initial stress on rock fracturing. The rock-breaking efficiency was evaluated based on the volume of broken rock and the energy consumption. The indentation failure of the sample can be divided into three stages: microfracture closure, elastic deformation, and unstable crack propagation. The microwave irradiation reduced the peak load during the indentation process and simultaneously reduced the brittleness of the specimen. The experimental and simulation results jointly demonstrated the existence of an initial stress threshold in the rock fracturing process. When the initial stress is below the threshold, it suppresses the extension of rock fractures, which is unfavorable for rock fragmentation. When the initial stress exceeds the threshold, stress-induced rock failure occurs, which promotes rock fragmentation. A notable observation is that microwave irradiation alters the initial stress threshold of the rock, where a higher microwave power correlates with a lower initial stress threshold. This indicates that the optimal parameters for microwave equipment must be reconsidered when the initial stress changes. Methods for optimizing rock breakage at initial stress were suggested and examined.

Deep underground projects are in a complex in-situ stress environment, and rock burst disasters induced by the hard and brittle failure of the rock mass are prone to occur around the tunnel openings. In this study, we aim to investigate the impacts of geological weak surfaces in rock masses on rockbursts and the spatiotemporal evolution of microcracks during the development of rockbursts. To achieve this objective, two sets of cubic specimens containing a circular through-hole were designed. One set of specimens had prefabricated unfilled cracks around the openings. Subsequently, the rockburst development process within the deep tunnel was replicated by subjecting both groups of specimens to identical gradient loading using a true triaxial test system. During the test, a micro camera was used to record the failure of the borehole wall in real-time, and an acoustic emission monitoring system was utilized to capture the stress waves released during structural damage. Finally, a multi-level synergistic analysis of the rockburst damage mechanism was carried out based on the macro-imaging data and micro-acoustic emission data. The results show that failure in high-stress environments within tunnels mainly includes microcrack initiation, particle ejection, crack propagation, local cracking, slabbing spalling, damage penetration, and rockburst damage. The time–frequency domain information of acoustic emission signals is highly perceptive and representative of the structural damage state of the specimens. The mutation characteristics observed in time-domain parameters, such as acoustic emission amplitude, event rate, ringing, and cumulative energy, correspond to drastic alterations in the internal structure of the rock. The distribution characteristics of frequency-domain parameters, such as acoustic emission peak frequency, dominant frequency energy, and frequency centroid, reflect different crack scales and damage modes. The diffusion of frequency centroid indicates that the damage and failure patterns within the rock are evolving towards a more complex direction. The energy magnitude of acoustic emissions represents the damage intensity during the development of rockburst. The root causes of induced structural-control type rockbursts in deep hard brittle rock masses are the combined effects of localized high-stress concentrations and large-scale discontinuous geological structures, such as natural joints, fissures, and structural planes. The naturally occurring large-scale through-type geological weak surfaces within the rock mass reduce the strength of the surrounding rock, alter the location of stress concentration, and change the initial damage characteristics. Moreover, they promote the evolution of rockbursts, thereby increasing their destructive intensity.

Soft–hard interbedded rocks are widely distributed at the Earth’s surface, and their mechanical properties and failure behavior directly affect the stability of local tunnel and slope engineering projects. Previous studies have rarely considered the influence of size effects on the mechanical properties and failure behavior of such rocks. Therefore, this paper used the combined finite–discrete element numerical method (FDEM) to study the mechanical properties and failure behavior of soft–hard interbedded rock samples considering the size effect. First, appropriate input parameters were calibrated and verified by using a new parameter calibration method. Second, the effects of the element size and loading rate were studied to obtain appropriate model parameters. Finally, the effects of the layer number, sample size, and height–diameter ratio of composite rock samples on their mechanical properties and failure behavior at different layer dip angles and layer thickness ratios were investigated. The results support the following findings: (1) For the composite rock samples with a layer thickness of 10 mm, reliable simulation results can be obtained by using a 2.2 mm element size, and the loading rate should not exceed 0.2 m/s in FDEM numerical modeling. (2) The number of layers in the sample should be at least 5, and when the height–diameter ratio is a constant 2.0, the height of the sample should not be less than 110 mm. (3) As the height–diameter ratio of the composite rock samples increases, both the compressive strength and elastic modulus decrease for all layer dip angles considered, but the rock failure mode changes for layer dip angles of 15–75°; in addition, the sample size effect is most significant for layer dip angles of 60° and 75°. (4) Taking horizontally layered composite rock samples as examples, both the compressive strength and elastic modulus of samples with different layer thickness ratios decrease with increasing height–diameter ratio and their failure modes also depend on the height–diameter ratio.

This study aims to identify specimens that are more suitable for examining the failure properties of I/III mixed-mode cracks under static loads. We employed single-edge notched bending (SENB) and edge-notched disc bending (ENDB) specimens, and used ABAQUS finite element method software to calculate the stress intensity factors for modes I and III at various crack inclination angles θ. Fracture toughness was tested using three-point bending experiments on two types of sandstone specimens. The evolution of mixed-mode I/III fracture morphology and failure patterns was investigated with a monocular microscope. Additionally, the evolution of the strain field and crack tip opening displacement was analyzed using the digital image correlation method. Our findings indicate that the fracture toughness of the ENDB specimens surpassed that of the SENB specimens. The effective fracture toughness of both SENB and ENDB specimens initially increased with crack inclination angle θ but then decreased. The fracture mode correlated with energy release; the energy release rate of the ENDB specimens exceeded that of the SENB specimens, making the former more brittle. For SENB specimens, crack initiation was primarily intergranular, while ENDB specimens exhibited predominantly transgranular fractures. The crack initiation moments for both specimen types increased with higher crack inclination angles. Furthermore, at the same crack inclination angle, ENDB specimens cracked earlier than SENB specimens.

Internal temporary plugging fracturing technology improves the stimulated volume by forming a more complex fracture network, significantly enhancing the reservoir stimulation effect and effectively developing unconventional oil and gas resources. However, the current understanding of the fracture propagation mechanism on internal temporary plugging fracturing is still unclear, making it difficult to determine the main controlling factors that affect the shape of the fractures. In addition, the lack of practical numerical simulation methods for temporary plugging fracturing makes it challenging to provide guidance for field scheme design. Utilizing the finite element cohesive zone method, which incorporates globally embedded cohesive elements, this study constructs a comprehensive numerical model to investigate the propagation behavior of temporarily plugging fracturing fractures. The model delves into the impact of various geological and construction parameters on the opening conditions of branch fractures during the temporary plugging fracturing process, as well as the propagation patterns of fractures within naturally fractured reservoirs. The research findings indicate that the construction factors have the following impacts: When the pumping rate and viscosity of the fracturing fluid are comparatively high, the resulting fluid pressure within the fracture escalates, resulting in the creation of numerous branch fractures within the naturally fractured reservoir. This, in turn, augments the fracture’s complexity. However, these two factors have little influence on the maximum deflection distance of the deflected fracture. As for the geological factors, an increase in the horizontal stress difference will decrease the maximum deflection distance of the deflected fracture, reducing the number of natural fractures intersected and shortening the length of the deflected fracture. Conversely, an increase in the approach angle will increase the maximum deflection distance of the deflected fracture, thereby expanding the affected area. Additionally, the influence of Young’s modulus and Poisson’s ratio on fracture propagation is very slight. A decrease in the tensile strength of natural fractures leads to more natural fractures being intersected during the process, resulting in increasing the length of the fracture and an improvement in the complexity of the fracture grid.