Crack Propagation Patterns Research Articles

Study on Crack Propagation in Proton Exchange Membranes with Doped Microcapsules

Crack defects produced during the operation of proton exchange membrane fuel cells (PEMFCs) have a significant impact on their service life. One common approach to extend their working time is to embed self-healing microcapsules in the membrane to repair the defects. In this study, we use the extended finite element method (XFEM) to investigate the capture and control of cracks by doped microcapsules and numerically simulate the crack propagation patterns. The results show that a two-layer microcapsule distribution along the membrane cross-section can achieve complete control of cracks. In addition, the two-material microcapsules with layered, staggered arrangement exhibit the maximum distance between cracks they can attract, while minimizing their impact on the mechanical properties of the membrane. Based on these findings, we determine the theoretical doping amount of microcapsules per unit volume, which has important implications for future experimental design and engineering applications.

Implementation of a phase field damage model with a nonlinear evolution equation in an FFT-based solver

This paper focuses on the numerical implementation of phase-field models of fracture using the Fast Fourier Transform (FFT) based numerical method. Recent studies on phase-field models focus on the discussions of the choice of the value of regularization length proposed to smear the discontinuity of the sharp crack. Some studies argue that it should be considered as a material property because it has a significant impact on the mechanical behavior of a material in some phase-field models, for instance, in the model proposed by Miehe. However, our results in this study for heterogeneous materials have shown that the choice of regularization length not only affects the macroscopic mechanical behavior but also the local crack propagation patterns. As a result, it can be challenging to select an appropriate value that produces both accurate macroscopic responses and local crack patterns for certain phase-field models, such as Miehe’s model. Thus, the phase-field model proposed by Wu, which has been proven to reduce the length sensitivity for homogeneous material, has been successfully implemented in an FFT-based solver with the application of Newton–Krylov algorithm.The length sensitivity of Wu’s phase-field model for heterogeneous materials is also investigated in this paper. Our tests show that Wu’s phase-field model has partial length sensitivity for heterogeneous materials. However, the main reason for this sensitivity is that one phase enters the damage zone of other phase, which is different from Miehe’s model. As a result, a set of criteria for accurately choosing the regularization value has been established for Wu’s model in this work. Meanwhile, it has also been found that Wu’s model may be more suitable than Miehe’s model for brittle failure due to the introduction of an elastic stage.

Hybrid phase-field modeling of mesoscopic failure in concrete combined with Fourier-Voronoi stochastic aggregate distribution modelling approach

Concrete crack propagation is significantly affected by its complex mesoscopic structure, tracking which is of great importance for revealing the fracture failure modes and crack propagation patterns of concrete. This study presents a stochastic aggregate distribution modeling approach combining the Fourier-Voronoi method and techniques with XCT images. it enables the generation of stochastic aggregate distribution models that mimic real concrete's morphological characteristics. Meanwhile, with the hybrid phase-field theory and finite element method, a multi-field coupling model is established to simulate the crack propagation. In contrast to the isotropic or anisotropic phase-field theory, with the hybrid phase-field theory the stiffness degradation of concrete material in all directions during crack propagation are taken into consideration. With the proposed methodology, the effects of volume fraction, shape, roughness and pre-notch depth on the fracture behavior and strength of concrete are investigated. Results show that both the tensile and shear strength of concrete grows with the increase of aggregate volume fraction. Benefited from the application of hybrid phase-field theory, two shear fracture failure modes of concrete with distinct shear strengths are revealed, influenced by aggregate interaction during the shear fracture process.

Experiment Study on Damage Properties and Acoustic Emission Characteristics of Layered Shale under Uniaxial Compression.

The gently tilt-layered shale displays anisotropic behavior and includes structural planes that cause the rock to exhibit weakened features. As a result, the load-bearing capacity and failure mechanisms of this type of rock differ significantly from those of other rock types. A series of uniaxial compression tests were performed on shale samples from the Chaoyang Tunnel to investigate damage evolution patterns and typical failure characteristics of gently tilt-layered shale. An acoustic emission testing system was incorporated to analyze the acoustic emission parameters of the shale samples during the loading process. The results indicate that the failure modes of the gently tilt-layered shale are significantly correlated with the structural plane angles and water content. The shale samples gradually transition from tension failure to tension-shear compound failure as the structural plane angles and water content increase, with an increasing level of damage. The maximum values of AE ringing counts and AE energy for shale samples with diverse structural plane angles and water content are reached near the peak stress and serve as precursors to rock failure. The primary factor influencing the failure modes of the rock samples is the structural plane angle. The precise correspondence between the structural plane angle, water content, crack propagation patterns, and failure modes of gently tilted layered shale can be captured by the distribution of the RA-AF values.

Study on the Mechanisms of Rock Mass Watering for Rockburst Prevention in Phosphorite Mines from Laboratory Results

Owing to the continuous increase in mining depth, Yichang phosphorite mines in China have entered the field of deep mining. The frequency of rockburst disasters is increasing. In situ experience indicates that the practice of spraying water onto a working face after blasting is an effective method of rockburst prevention. In order to investigate the underlying mechanisms of rockburst prevention by watering in phosphorite mines, a series of uniaxial compression laboratory experiments was carried on phosphorite samples under dry and water-saturated conditions with an acoustic emission (AE) monitoring system. A high-speed camera was used to record the failure process and pattern of a given rock sample prior to rockburst. The effects of water on the mechanical properties and fracturing characteristics of phosphorite failure were determined. Experimental results indicate that water reduces the uniaxial compressive strength and Young’s modulus. Saturated phosphorite causes more small fragments after it fractures. A Gaussian mixture model (GMM) clustering algorithm was utilized to analyze the crack propagation patterns of rock samples during the entire process. It was determined that during the unstable crack propagation phase, the presence of water makes the shear characteristics become more obvious. Water reduces releasable strain energy which is consumed by internal damage and plastic deformation of the rock sample. Moreover, the mechanism of watering for rockburst prevention is discussed from both macro and micro perspectives. The primary reasons for this are the transfer of stress concentration zones and stress-releasing effects via microcrack propagation on the working face.

Study on the coupling mechanism of shaped blasting and empty hole to crack coal body

In this study, the mechanism of cracking under the coupling effect between the use of shaped blasting and deployment of empty holes was explored through theoretical analyses, numerical simulations, and experiments to determine the effects of empty holes on the traveling characteristics of blast-induced stress waves, the propagation patterns of detonation cracks, and the evolution characteristics of blast-induced damage. The results indicated that shaped blasting concentrated energy in a fixed direction to enhance the efficiency of explosive energy use, and the resultant shaped jet created a directional, cut-like crack in the coal layer of specimens. The additional free face provided by empty holes increased the tensile stress, the level of damage in areas located far away from the blast hole, and the propagation capacity of the crack. The levels of specimen damage in shaped blasting that were determined using the resistivity and ultrasonic wave velocity were 1.2 and 1.7 times higher, respectively than those in ordinary blasting. The tensile strain at the empty holes was 2.73 times higher and the static loading phase was 0.9 μs longer in shaped blasting than in blasting sites without empty holes. The coupling effect between shaped blasting and empty holes substantially improved the effectiveness of the blasting operation in creating directional cracks. This study provides a theoretical and experimental basis for directional blasting techniques; reveals an effective approach that enhances the efficiency of blasting, improves the effectiveness of blast holes, and limits blasting disturbance; and offers socially valuable insights into safe and efficient production practices.

The Modelling of Rock Fragmentation Mechanisms by Carbide Buttons Using the 3D Discrete Element Method

Button cutters are commonly used in hard rock drilling because the inserted carbide buttons provide exceptional wear resistance, impact resistance, and high strength in challenging geological formations. One of the most pressing issues in designing a button cutter is to study the rock breaking mechanisms of carbide buttons. In this study, the three-dimensional discrete element method (DEM) was employed to investigate the rock breaking mechanism and cutting performance of five widely used carbide buttons, i.e., spherical, saddle, wedge, conical, and parabolic buttons. The simulation results were compared with laboratory tests to reveal the rock indentation process. The crack propagation pattern, energy dissipation, and damage evolution associated with the force–penetration depth curve were investigated. Tensile damage was the primary determinant for crack propagation and coalescence. By systematically exploring the penetration index, specific energy, and crack propagation characteristics, the conical button had a high rock breaking efficiency when the penetration depth was low, and the saddle button had a high rock breaking efficiency when the penetration depth was high. The findings can provide references for the design of a button cutter.

In situ synchrotron radiation µCT indentation of cortical bone: Anisotropic crack propagation, local deformation, and fracture

The development of treatment strategies for skeletal diseases relies on the understanding of bone mechanical properties in relation to its structure at different length scales. At the microscale, indention techniques can be used to evaluate the elastic, plastic, and fracture behaviour of bone tissue. Here, we combined in situ high-resolution SRµCT indentation testing and digital volume correlation to elucidate the anisotropic crack propagation, deformation, and fracture of ovine cortical bone under Berkovich and spherical tips. Independently of the indenter type we observed significant dependence of the crack development due to the anisotropy ahead of the tip, with lower strains and smaller crack systems developing in samples indented in the transverse material direction, where the fibrillar bone ultrastructure is largely aligned perpendicular to the indentation direction. Such alignment allows to accommodate the strain energy, inhibiting crack propagation. Higher tensile hoop strains generally correlated with regions that display significant cracking radial to the indenter, indicating a predominant Mode I fracture. This was confirmed by the three-dimensional analysis of crack opening displacements and stress intensity factors along the crack front obtained for the first time from full displacement fields in bone tissue. The X-ray beam significantly influenced the relaxation behaviour independent of the tip. Raman analyses did not show significant changes in specimen composition after irradiation compared to non-irradiated tissue, suggesting an embrittlement process that may be linked to damage of the non-fibrillar organic matrix. This study highlights the importance of three-dimensional investigation of bone deformation and fracture behaviour to explore the mechanisms of bone failure in relation to structural changes due to ageing or disease. Statement of significanceCharacterising the three-dimensional deformation and fracture behaviour of bone remains essential to decipher the interplay between structure, function, and composition with the aim to improve fracture prevention strategies. The experimental methodology presented here, combining high-resolution imaging, indentation testing and digital volume correlation, allows us to quantify the local deformation, crack propagation, and fracture modes of cortical bone tissue. Our results highlight the anisotropic behaviour of osteonal bone and the complex crack propagation patterns and fracture modes initiating by the intricate stress states beneath the indenter tip. This is of wide interest not only for the understanding of bone fracture but also to understand other architectured (bio)structures providing an effective way to quantify their toughening mechanisms in relation to their main mechanical function.

Phase-field ductile fracture analysis of multi-materials and functionally graded composites through numerical and experimental methods

Despite extensive studies on material models for fracture applicable to homogenous materials, the demand for advanced numerical methods to predict the failure in multi-materials and functionally graded materials (FGMs) remains substantial. This study aims to address this gap by using a phase-field approach for analyzing crack development and ductile fracture in FGMs via numerical and experimental methods. To account for the failure induced by material plastic deformations, we introduce the elastoplastic material framework within the damage driving force. This framework enables us to analyze fracture in the FGM setting, whereby the gradual spatial changes of the elastoplastic and fracture properties across the functionally graded medium are modelled by considering the effective property values calculated via rule of mixtures. The influence of the gradation profiles and orientations on the problems of crack initiation, propagation, and fully-developed crack pattern is elucidated via mixed-mode crack analyses on a representative numerical example. In particular, the fracture resistance changes resulting from the property mismatches between the constituent FGM materials are assessed. To determine the efficacy of the numerical model in predicting the fracture behavior, it is evaluated against the experimental tensile test data obtained from miniaturized tensile test specimens excised from an FGM block consisting of 316L and IN718 powders, deposited via Laser powder blown Directed Energy Deposition (LDED).

Water-bearing effect on mechanical properties and interface failure mode of coal–rock combination samples

The coal and rock mass in water-rich mines with underground waterproof coal-and-rock pillars will be inevitably eroded by groundwater. The mechanical properties and interface failure mode of coal–rock mass under high-moisture conditions directly affect the mine stability and operation safety. This study performed uniaxial compression tests of samples from coal, rock, and their combination with different water contents, the evolution law of mechanical properties and acoustic emission (AE) damage characteristics of coal–rock combinations (C-RC) with different water contents were studied. The C-RC interface failure modes at different water contents were clarified. The results showed that the deterioration of average peak strength of coal, rock, and C-RC was obvious with increased water content. The deterioration degree in the descending order was as follows: C-RC > coal mass > rock mass. The maximum AE ringing number first increased rapidly and then sharply dropped, while the AE cumulative ringing number dropped slowly and then rapidly, reflecting the internal crack propagation patterns in C-RC. The failure mechanism of water-bearing C-RC was related to coal–rock strength ratio and the water-bearing effect of coal–rock interface. When the water-bearing state of C-RC changed from dry to saturated, the macro-failure mode showed the law of only coal fracture (0%)—minor fracture at the coal–rock interface (4%)—both coal and rock fracture (8%)—only coal fracture (12%). With increased water content, the water-bearing effect of C-RC interface gradually prevailed in the C-RC failure pattern.

Finite element-based phase field simulation of complex branching crack propagation under different loads

The tensile failure of complex branching cracks will cause irreversible damage to the structures. In this paper, the effects of the crack orientations, the local branch lengths, and the different load forms on the crack propagation pattern and the bearing capacity of structures are investigated based on the phase field method and experimental research method. The results show that the bearing capacity and propagation patterns of the specimens change significantly with the change of the orientation of the Y-shaped crack under tensile load. The local branch length of the crack only affects the sequence in which the regional crack reaches the final failure and the bearing capacity of the specimen. Under the dynamic impact load, the bifurcation phenomenon occurs, and with the increase of the dynamic impact load, the number of bifurcations increases. The crack propagation pattern in the numerical simulation is completely consistent with the experiment, and the difference in the maximum reaction force is 2.5%.

Shake table tests on 1:2 reduced scale masonry house with the application of horizontal seismic bands

The safety of people's lives is crucial in structures that provide shelter. Earthquakes are a major natural disaster that have claimed thousands of lives over the years, but other factors can also cause damage to these structures. Building collapses are often due to their inability to endure the seismic loading, not the earthquake itself. Most residential buildings are masonry structures. There are many strategies for enhancing structural behavior, but very little research has been done on masonry structures, which house about 1/3rd of the world's population. This study focuses on characterization of seismic response using shake table tests to assess the dynamic behavior of masonry structures when horizontal seismic bands are applied. Horizontal seismic band is one of the traditional techniques used in masonry structure. Three reduced scale models were constructed with one using a reinforced concrete band, one with a timber band, and one without a band. All models were constructed using extruded earth block and mud mortar, common building materials in developing nations. The models were tested on a shake table using the same loading signal to compare their response behavior. A high-speed camera was also used to capture images; accelerometers and displacement sensors were installed to record the response. Digital Image Correlation (DIC), which provides non-contact optical measurement, has been essential in obtaining full-field measurement. The results showed significant improvement in the seismic response of structures with horizontal seismic bands. The behaviors are compared in terms of natural frequency, damping, energy dissipation, and crack propagation patterns using two types of materials as seismic bands.

Fracture features of brittle coal under uniaxial and cyclic compression loads

Under the effects of complex geological and stress environments, burst hazards continue to be a major challenge for underground space utilization and deep resources exploration as its occurrence can lead to personnel causalities, equipment damage and structural collapse. Considering the stress path experienced by in-situ coal body, cyclic loading appears in quite various forms for instance shearer cutting, overlying strata breakage, hydro-fracturing and blasting, during tunnel, mining and underground space utilizing process. The stability of the underground coal body subject to periodic loading/unloading stress is extremely important for maintain the function of designed engineering structure for waste storage, safe mining, roadway development, gas recovery, carbon sequestration and so on. The mechanical properties of hard rock subject to cyclic fatigue loads has been intensively investigated by many researchers as the rock burst induced by supercritical loads has long been a safety risk and engineering problems for civil and tunneling engineering under deep overburden. More recently, the mechanical properties of coal samples under cyclic fatigue loads is investigated from the aspect of hysteresis, energy dissipation and irreversible damage as the burst hazards of brittle coal is rising in many countries. However, the crack propagation and fracture pattern of brittle coal need more research to understand the micro mechanism of burst incubation subject to cyclic fatigue loads as brittle coal can store more elastic strain energy and rapidly release the energy when its ultimate strength once reached. This research studied the internal crack status corresponding to different cyclic fatigue loading stage of brittle coal samples. The AE monitoring was applied during the uniaxial and cyclic loading process of brittle coal samples to record the crack intensity of samples at different loading stages. The damage evolution curve corresponding to loading status was then determined. The fracture pattern of coal samples determined by micro-CT scan was observed and discussed. It has been found by this paper that brittle coal of uniaxial compression tests demonstrated sudden failure caused by major splitting fracture while that of cyclic fatigue tests experienced progressive failure with mixture fracture network.

Subcritical crack propagation in glacial quarrying during subglacial water pressure variation

AbstractGlacial quarrying stems from the fracturing of subglacial bedrock. Much evidence shows that subcritical crack propagation of bedrock is closely related to subglacial water pressure fluctuations. Here we employ a model that assesses the impact of subglacial water pressure fluctuation on cavity length and subcritical crack propagation, while analyzing the effect of a pre-existing crack location using a phase-field model (PFM). Our results indicate that the cavity length is reduced during diurnal fluctuations in water pressure. There are two patterns of subcritical crack propagation on the corner of the step. The first stems from a rapid drop in water pressure. The second occurs after the water pressure recovers from the fluctuation to the initial steady state. This pattern is a consequence of enhanced stress concentration on the step since the modeled cavity length exceeds its steady value and has higher efficiency in promoting subcritical crack propagation. Additionally, based on the PFM results, we speculate that the subcritical crack initiation and propagation happen on a broader scale, including the ice-bed contact region and its adjacent region. Our findings imply that the duration of subcritical crack propagation is short and typically ceases once the cavity length adjusts to reduced water pressure levels.

Research progress on crack propagation behavior of solid propellant

Crack sprouting will lead to the mechanical properties of propellant degradation, and the additional combustion surface formed by crack propagation may cause over-pressurization, resulting in uneven combustion of propellant, significantly changing the thrust characteristics of the engine, reducing the range of the vehicle, and may even cause catastrophic accidents such as explosions. Therefore, the study of crack propagation law of solid rocket propellant pillar is an important basis to ensure the structural integrity and safety of the engine. In order to study the crack propagation pattern of propellant pillar under different working conditions, three aspects of propellant crack initiation criterion, crack propagation model, and crack propagation influencing factors are systematically reviewed. On this basis, the next research focus is sorted out, and it is considered that the construction of crack propagation model by establishing a multi-scale model, the characterization of crack propagation by using various test characterization means, and the analysis of damage evolution under the coupling of multiple working conditions will be the next research focus.

Flexural Behaviour of the Two-Way Spanning Reinforced Concrete Slab Using Spherical Plastic Bubble Balls

The use of conventional reinforced concrete in the construction industry increases each year, especially in developing countries. However, the concrete content, particularly cement production contributed to the greenhouse gas emission subsequently increase to climate change. Thus, the reinforced concrete slab containing high-density polyethene (HDPE) hollow spherical plastic bubble balls also known as bubble deck slabs were introduced for sustainable construction. This type of slab forms a slab that has less concrete volume compared to the normal solid reinforced concrete slab. Although this unique system can facilitate up to a 50% longer span compared to the conventional reinforced concrete solid slab, yet, it can cause the performance of the slab structure such as flexural and shear capacity may be affected due to the thirty to forty per cent of fewer concrete volumes. Hence, this paper studies the comparison of the performance of the two-way supported slabs; reinforced bubble deck slab and normal solid reinforced concrete slab after being subjected to the area loading. The square slabs are 1200mm by 1200mm in width and length with a thickness of 235mm. The investigations of the experiments included flexural strength, bending stiffness and load-deflection behaviour due to the impact of the area loading. Also, the crack propagation and crack pattern which differs also was shown for each type of slab system, especially in shear strength.

Experimental study on the ultimate bearing capacity of damaged RC arches strengthened with ultra-high performance concrete

With the deterioration of the infrastructure and the increasing traffic volume, the service performance of bridge structures dramatically dropped. Strengthening and retrofitting these bridges with ultra-high-performance concrete (UHPC) can effectively improve their mechanical properties and durability. In this paper, the experimental investigation of the behavior of damaged RC arches strengthened with UHPC is presented. Three strengthened arches, including one arch strengthened with C50 (RC1-C50) and two arches strengthened with UHPC (RC2-UHPC, RC3-UHPC), were tested under asymmetric load. Load-deflection behavior, crack propagation pattern and hinge formation mechanism are extensively studied. Results indicated that the initial crack load and ultimate bearing capacity of composite arches strengthened by UHPC were respectively enhanced by 185% and 186%, showing good effects of strengthening and toughening on the original arch. On the premise of closing the cracks in the original arch ring, the stiffness and ductility of the composite arch strengthened with UHPC (RC2-UHPC) were improved by 58% and 34%, respectively. Different from the arch strengthened with C50 (RC1-C50), no obvious cracking and separation phenomena were observed in the interface of composite arches strengthened with UHPC in the whole process of eccentric loading. The combination of keyway and planting bar (rigid and flexible) can provide reliable bonding between the UHPC and the original structure. Finally, the calculation model for predicting the cracking load and ultimate capacity of the composite arches were proposed, and the calculation results are in good agreement with the experimental results.

Microstructures evolution and chloride migration characteristics of concrete under ultra-deep underground environment

Understanding the microstructures evolution and chloride migration characteristics of concrete is essential to solve the durability problem of reinforced concrete structures in ultra-deep buried rail transit projects (40 m–100 m). Herein, we developed a novel experimental system that can simulate the corrosion process under high hydraulic pressure and stray current coexistence environment. The changes in microstructures, phases, elements, minerals, and the Ca-leaching were comprehensively investigated. Further, the mechanisms of pores and cracks propagation, and migration characteristics of chloride ions were revealed. The results showed that stray current and hydraulic pressure significantly altered the microstructures of concrete, both of which led to the increase of specific surface area, porosity, pore diameter and more ink-bottle-shaped pores. The development of dominant microfractures and three types of crack propagation patterns caused by hydraulic pressure were found, and the mechanical framework for “local fracturing” was established. Migration characteristics in micro-nano scale showed chloride ions were mainly transported along the pores, and the decrease of adsorption sites was the main reason.

AE Characteristic and Mechanical Behaviors of Red Sandstone with Two Prefabricated Close-Collinear-Equal Length Cracks under Compression.

Preexisting cracks are of great significance for the stability of rocks, they can influence the growth and propagation pattern of cracks. For revealing the failure characteristics of rocks with prefabricated cracks, the mechanical behaviors of red sandstone with two close-collinear-equal length cracks were studied by acoustic emission (AE) monitoring experiment. The results indicated that the stress of red sandstone specimens showed an obvious stepwise growth phenomenon, and AE ring counts can better characterize the growth of microcracks of rocks. Moreover, AE events were mainly distributed near the prefabricated cracks of rocks. Then, a novel modified Kachanov method for analyzing collinear-close cracks was proposed. The influences of crack length and crack spacing on the interaction force of cracks were discussed. Then, it can be found that this modified method had a good applicability for two close-collinear-equal length cracks of rocks under compression.

A Study on Aging Fracture Behavior of Sintered Joint by Bi-Modal Cu Paste

Power semiconductors for automobiles were joined by high Pb soldering to joint properties at high temperatures during operations. However, this process is being replaced by low-temperature sintering. In the low-temperature sintering bonding method, Cu powder is low-cost and exhibits high-performance material and excellent bonding properties. In this study, the effect of high-temperature aging on Cu paste sintered joints and the failure mechanism of the shear test were investigated. Cu monomodal and bimodal pastes were manufactured, and electroless nickel immersion gold (ENIG) surface treatment was applied on the Cu chip side. In the ENIG-treated joints, a significant number of Kirkendall voids were observed at the interface under Au after 2,000 h at 200 °C, and Cu2O was also formed. This is attributed to the difference between the interdiffusion coefficients of Au and Cu, which decreases the shear strength of the junction and rapidly increases the electrical resistance. After the shear strength tests, the crack propagation pattern at the interface of the ENIG-treated joint was confirmed via cross-section scanning electron microscopy. Future studies on surface treatment are required to clarify the long-term stability of the joints.