Fracture Mechanics Studies Research Articles

An experimental study of fracture mechanism and morphology of granite specimens under various dynamic loading rates

Numerous studies have shown that dynamic fracture toughness (DFT) of rock is dependent on loading rate. This paper quantitatively studied the effect of loading rate on DFT from perspective of mesoscopic fracture morphology. First, notched semi-circle bend (NSCB) and short-core-in-compression (SCC) samples of granite were prepared for dynamic mode I and mode II fracture tests. Then, the DFT values of NSCB and SCC specimens at various loading rates were calculated and analyzed. After that, mesoscopic morphologies of failure surfaces of NSCB and SCC specimens under various loading rates were obtained by scanning electron microscope (SEM). The fracture morphology features of specimens were quantitatively characterized by a method combining deep learning and SEM images. The analytical results suggested that as loading rates rose, the increase in the percentage of mesoscopic fracture morphology caused by shear stress (MFM-S) on the fractured surface was the primary reason for the increase of DFT. When dynamic loading varied from 40 to 120 GPa m1/2 s−1, there was a linear relationship between the DFT and the proportion of MFM-S. Additionally, with an increasing loading rate, the proportion of MFM-S on the fracture surface of NSCB specimens varied slightly more, which can explain why the dependence of DFT on loading rate in NSCB specimens was somewhat more obvious than in SCC specimens.

High-performance bamboo-wood composite materials based on the natural structure and original form of bamboo: Fracture behavior and mechanical characterization

Reducing the destruction of bamboo's original structure to produce high-utilization and high-performance engineered bamboo products has long been a goal in the industry. This study developed a new natural arc-shaped laminated bamboo-wood lumber (NALBWL) based on the natural structure and original form of bamboo, using equal arc-shaped bamboo split (EASB) and poplar veneer. A multi-scale analysis of the bending performance and crack propagation dynamics of EASB and NALBWL was conducted. EASB's modulus of elasticity (MOE) and rupture (MOR) were 1.06 and 1.17 times higher than those of air-dried bamboo, respectively, exhibiting excellent mechanical properties. Additionally, its toughness in the full and elastic-plastic stages were 1.24 and 1.38 times higher than those of air-dried bamboo. NALBWL's MOE and MOR were 12.73 GPa and 174.41 MPa, fully meeting structural material requirements and even surpassing most engineered bamboo products. Moreover, NALBWL's radial bending load values were 2.29 and 2.24 times higher than that of EASB and air-dried bamboo, respectively, and it also demonstrated excellent water heat and drying resistance in impregnation and peeling performance. Real-time crack propagation analysis indicated that NALBWL's superior mechanical properties benefited from the respective advantages of bamboo and poplar veneer. This work showcased the application of "natural form-inspired design" in developing bamboo-wood composite materials, and the conclusion provided a reference for the sustainable development of engineered bamboo products and the study of biomass material fracture mechanisms.

Proportioning optimization of transparent rock-like specimens with different fracture structures

Clarifying the principles of proportioning optimization for brittle transparent rock-like specimens with differential fracture structures is crucial for the visualization study of the internal fracture and seepage evolution mechanisms in rock masses. This study, utilizing orthogonal experimental methods, uncovers the influence mechanisms, extents, and patterns by which the ratios of resin, hardener, and accelerator, along with the freezing duration, impact the mechanical characteristics of transparent rock-like specimens. Notably, it was observed that as the accelerator ratio and freezing time are increased, there’s a general decline in the uniaxial compressive strength, tensile strength, and elastic modulus of the specimens. In contrast, an increase in the hardener ratio initially leads to an enhancement in these mechanical properties, followed by a subsequent decrease. Under uniaxial compressive loading, the specimens exhibit four typical modes of failure: bursting failure, splitting failure, single inclined plane failure, and bulging failure. As the hardener and accelerator ratios increase, the mode of failure gradually shifts from bulging to bursting, with freezing time having a minor overall impact on the evolution of failure modes. The study proposes a method for inducing random three-dimensional closed fractures within the specimens and further clarifies the principles for optimizing the proportions of specimens with different fracture structures, such as intact, embedded regular, and random three-dimensional fractures. This research facilitates the in-depth application of transparent rock-like materials in various scenarios and provides theoretical guidance and technical support for visualizing the evolution of fracture and seepage characteristics within the fractured rock mass.

A fracture mechanics study of stress resultant intensity factors in stiffened panels employing RKPM meshfree FSDT modeling

Panel cracking for stiffened panels is investigated based on linear elastic fracture mechanics. Meshfree flat shell formulation and fracture modeling are employed. After investigating stress distribution of the intact stiffened panel, the mode-I and mixed-mode crackings are studied using the stress resultant intensity factors (SRIFs). When the crack tip approaches to the stiffener, deformation around the crack is suppressed due to presence of the stiffeners. The mechanical behaviors are carefully examined by changing crack length, loading conditions and thickness of the main panel and stiffeners. Quantitative evaluation of SRIFs is addressed by comparing with the reference solutions. An in-depth discussion is provided for the meshfree modeling of the panel cracking in the stiffened panels. This series studies are useful for examining crack arrest effect of the stiffened panel due to presence of the stiffeners.

Fracture Mechanics Induced by Drilling and Blasting in Underground Openings: Prevailing Practices and Studies

This article presents the prevailing practices and literature review on the study of fracture mechanics of rock mass, during the construction of underground openings, by drill and blast method (DBM). The paper makes an introductory discussion on basic fracture and failure mechanics on the rock, followed by a review due to the blast-induced failure mechanism. The damage zone in drill and blast openings is discussed which is followed by the conclusions of the literature. DBM is a widely acceptable and broadly applicable approach in the underground construction method. The blasting damaged zones are classified as the borehole expansion zone, crushed zone, nonlinear fracture zone, and radial crack propagation zone. Blasting activities create stress waves in construction work, which fundamentally influence the fracture and failure mechanics of rock with pre-existing discontinuities, in-situ stress, and groundwater conditions. On the other hand, the cumulative action of subsequent explosion gas and stress waves defines the final damage limit and crack pattern map. Thus, it is of utmost importance to investigate the blast-induced fracture and failure mechanism to mitigate the instability in the underground openings.

Study of High-Quality GaN Growth and Fracture Mechanisms: Insights from Molecular Dynamics

Study of High-Quality GaN Growth and Fracture Mechanisms: Insights from Molecular Dynamics

Study on the Deformation and Fracture Mechanisms of Plastic Metals Considering Void Damage

Fracture initiation in plastic metals is attributed to the development of voids. Analyzing the nucleation and growth processes of voids facilitates the study of plastic deformation and fracture mechanisms in metal materials. Uniaxial tensile tests were conducted on two high-quality carbon structural steels, and the microfracture surface morphology of the tensile specimens was observed by using a scanning electron microscope (SEM). From the perspective of vacancy condensation, the nucleation mechanism of voids in the absence of inclusions or particles was analyzed. Based on the continuum damage mechanics theory and the Rice–Tracy (R-T) model, a damage parameter considering the void volume fraction was derived, and a plastic potential function, hardening curve, and constitutive model for the plastic deformation process of the plastic metal material were established. Based on the uniaxial tensile test data of the two sheets of high-quality carbon steel, the strain range data in the hardening stage were converted into true stress–plastic strain data, and the established hardening curve was used to fit the true stress–plastic strain data. The results showed good agreement between the established hardening curve and the experimental results, which effectively reflected the deformation process of ductile fractures in plastic metal materials.

Numerical and experimental investigation of impact strength and fracture mechanism of Kevlar and Hemp elastomeric thin biocomposite laminate under high-velocity impact: A comparative study

Thin composite structures reinforced with woven fabrics are widely used in many industrial sectors due to their lightweight, easy access, and high specific performance. Due to safety, the study of fracture mechanisms and energy absorption of thin composite structures is significant. In this study, Kevlar and Hemp elastomeric thin biocomposite laminates with an elastomeric matrix filled with natural lignin filler with a low carbon footprint and with the aim of the sustainability concept of environmental protection have been designed experimentally and numerically for testing under high-velocity impact. The impact resistance of elastomeric composites reinforced with natural fillers is less reported. Good interaction of lignin-filled elastomeric matrix with woven hemp fabric is expected to show impact performance comparable to Kevlar-reinforced composites. For this purpose, Kevlar and hemp thin elastomeric biocomposites are tested under high-velocity impact with almost the same surface density. Also, tensile and dynamic compression tests (Hopkinson test) were performed to check the mechanical properties of the elastomer layer. The proposed laminates' penetration resistance and fracture behavior during an impact according to the Kevlar and hemp fabric constituent material model and the user-defined material model (VUMAT) for the nonlinear behavior of elastomeric materials considering the damage criterion in ABAQUS/Explicit confirmed. The results show, In the thin elastomeric hemp biocomposites, the tensile failure and stress transfer to the surrounding bundles cause more yarns to participate in the load-carrying process. As a result, the low penetration depth and more significant protection margin in fully green hemp composites provide impact performance comparable to Kevlar elastomeric composites.

An experimental fracture mechanics study of the combined effect of hydrogen embrittlement and loss of constraint

This work presents a systematic investigation of the combined effect of hydrogen embrittlement and loss of constraint. The fracture mechanics experiments are performed on an advanced martensitic high strength steel using a single-edge-notch bend specimen, with different crack over height ratio, subjected to electrochemical in-situ hydrogen charging at various loading rates. It is found that the environmentally driven ductile-to-brittle transition region in fracture toughness is obtained for both the high and low constraint specimen configurations. This region is characterized by a change from transgranular dimple rupture to an intergranular mode of fracture. The transition region for the low constraint specimen is shifted towards longer hydrogen exposure times, which is an effect of the reduced hydrostatic stress ahead of the crack front compared to the high constraint specimen. The low constraint specimen exhibits significant plastic straining, which is reflected in a significant decrease in the fracture toughness due to hydrogen assisted transgranular dimple rupture.

Comparative Experimental Study of Fracture Mechanisms and Characteristics of Coal-like Materials under Quasi-Static and High-Pressure Air Blasting Fracturing

High-pressure air blasting is a burgeoning technique for anhydrous fracturing. A crucial prerequisite for the application of air blasting is to comprehend the distinctions between air blasting and quasi-static fracturing, as well as the change in fracture control factors with variations in blasting pressure. This research undertook experiments on coal-like material specimens with quasi-static fracturing and blasting fracturing with blast pressures ranging from 5 to 10 MPa, by utilizing a self-designed air fracturing experimental system. The fracturing mechanism and characteristics of specimens subjected to different loading conditions were analyzed using a pressure sensor, high-speed camera, and fractal theory. The following primary findings were obtained: The average fracture pressure under quasi-static loading was 3.269 MPa, exhibiting similar absolute deviations. The pressure–time curve of blast fracturing was found to undergo four distinct stages: pressure surge, pressure fallback, pressure maintenance, and pressure plummet. The duration of the pressure maintenance stage exhibited a negative correlation with the blast pressure, ceasing to exist at pressures above 9 MPa. This suggests that the dynamic impact effect gradually supplants the dominant role of the air wedge effect. Quasi-static fracturing formed single fractures, whereas blast fracturing tended to generate complex fracture networks. The increase in blast pressure could significantly enhance the complexity of the fracture network in a logarithmic fashion. The fractal dimension of quasi-static fracturing was slightly higher than that of 6 MPa blast fracturing, indicating the prevalence of quasi-static action under low blast pressure. The smash district increased exponentially as the blast pressure increased, and the average particle size decreased gradually. These investigations provide valuable insights for designing high-pressure air blasting fracturing procedures.

Comprehensive study of hot deformation behavior and fracture mechanism of Al/Cu laminated composite

The hot deformation behavior of Al/Cu laminated composite was studied by isothermal tensile experiments in the temperature range of 200–500 °C and strain rate range of 0.001–1/s. The Arrhenius constitutive model was established to predict the flow stress by determining the constants of composites. In addition, based on the dynamic material model (DMM), the hot working map is established. The results show that the power dissipation efficiency is high under the conditions of high temperature, medium and low strain rate. When the strain is 0.25, there are few areas of stability due to the increasing frictional resistance at the interface. It is found that the dynamic recrystallization (DRX) is promoted by hot tensile and laminate structure. The softening mechanism of Al side is mainly dynamic recovery (DRV) and discontinuous dynamic recrystallization (DDRX), while that of Cu side is DRX. During the tensile process, the Al side grains always maintain stable Brass, Copper and S textures; the Cu side grains always maintain the Cube texture with stable-end orientation and when the deformation exceeds 60 %, the Goss unstable texture appears. It is conducive to dislocation movement and increases the recrystallization ratio. Then, the fracture mechanism of Al/Cu laminated composite is revealed. Due to the presence of the interface, the Cu side shows brittle fracture at room temperature. The snake-like sliding occurs on both sides of Al and Cu under hot deformation, showing ductile fracture, and the defects on Cu side are more serious than those on Al side. These results lay a foundation for optimizing the hot working parameters, improving the performance of the forming parts and clarifying the fracture damage mechanism of Al/Cu laminated composite.

Numerical simulation the fracture of rock in the framework of plastic-bond-based SPH and its applications

In this paper, a two-dimensional numerical approach named plastic-bond-based smoothed particle hydrodynamics (PB-SPH) is proposed to simulate the fracture of rock with pre-existing flaws. Under the PB-SPH framework, interaction particles are interconnected through bonds. The Drucker–Prager criterion of a tensile zone cut-off coupled maximum tensile stress criterion is applied to judge shear failure and tensile failure. To verify the capability of the PB-SPH method in simulating rock fracture behavior, two-dimensional numerical simulations of a single flaw, two coplanar flaws, and multiple flaws of uniaxial compression are carried out. Compared with previous experimental and numerical results, the present numerical results show good agreements on crack initiation positions, crack morphology and crack patterns. The numerical simulation of the progressive failure process of rock slope with multiple flaws is carried out, which shows that the PB-SPH method can be well applied to rock mechanics engineering. The present results show that the PB-SPH method has a certain reference in the study of rock fracture mechanics and the application of rock mechanics engineering.

A 2D numerical modelling of plasticity induced crack closure on MT specimens

In many engineering fields, structures are often subjected to fatigue loading that initiate and propagate cracks. The development of fracture mechanics studies has enabled the tolerance of a substantial amount of damage and the ability to predict failure in fatigue damaged structures, extending their service life safely. This can bring social impacts from economics, public-safety, or an environmental point of view. These damage tolerance analyses can be performed using linear elastic fracture mechanics (LEFM), where the Stress Intensity Factor (SIF) is a chief key, or under elastoplastic fracture mechanics (EPFM), where plasticity induced crack closure (PICC) plays a major role on fatigue crack growth (FCG). This study deals with the computational modelling developed consisting in the simulation of a two-dimensional model to predict FCG under PICC conditions. Extremely fine meshes were generated around the crack tip, in order to predict PICC loads, SIF and plastic zone size. The results showed good agreement with the experimental model, allowing for accurate damage tolerance assessments.

Crack formation in the tungsten armour of divertor targets under high heat flux loads: A computational fracture mechanics study

In the framework of EUROfusion, the thermally induced stress and the consequential fracture of the design of DEMO-divertor with copper alloy heat sink have been analyzed, to assess the structural integrity of tungsten armor of the plasma facing component (PFC). With finite element method (FEM), the influence of mesh and plasticity of tungsten has been evaluated. Due to the lack of material data, especially those of irradiated tungsten under operation conditions, conservative assumptions have been made in terms of plasticity, fracture strength and fracture toughness. The crack initiation and propagation with assumed plasticity and fracture toughness for heat fluxes ranging from 10MW/m2 up to 30 MW/m2 have been analyzed. Thermal stress analysis has been performed with various plasticity of tungsten, to locate the peak stress, which leads to the highest possibility of crack initiation. Extended finite element method (XFEM) analysis has been performed afterwards with various fracture strength toughness. The influence of plasticity and fracture toughness from almost zero up to Kic = 20MPa∙m1/2 has been evaluated. The predicted crack propagation has been verified by results of J-integral calculation.

Study of Blast Mitigation Performance and Fracture Mechanism of Polyurea under Contact Explosion.

In order to further study the blast mitigation performance of polyurea and to investigate the protection mechanism and damage characteristics of polyurea-protected structures under contact explosion loads, based on earlier work, this paper investigated the response and energy absorption performance of polyurea under various frequency loads. Qtech T26 blast mitigation polyurea (T26 polyurea) was adopted to protect the reinforced concrete (RC) slab and damage analysis of the post-explosion specimens was carried out at micro and macro levels. The response and energy absorption capacity of the material towards different frequency loads were investigated by dynamic mechanical analysis (DMA). Protective performance of T26 polyurea on RC slab was examined with a 10 kg TNT contact explosion test. Scanning electron microscopy (SEM) was employed to analyze the microscopic fracture morphology of the typical areas of the coating after the explosion. The chemical structure changes of the blast-face coating before and after the explosion were compared by Fourier transform infrared spectroscopy (FTIR). The results show that the glass transition region of T26 polyurea is −40 °C to 10 °C, which is a large temperature range, and the microphase separation of T26 polyurea is low. It is significantly influenced by the ambient temperature and loading frequency. The energy absorption of T26 polyurea is realized through the interaction between the hard and soft segments. When the frequency is between 102 Hz and 106 Hz, the loss factor of T26 polyurea is between 0.20 and 0.31, which exhibits a good energy dissipation performance. In the contact explosion of 10 kg TNT, the fragmentation rate of the coated specimen decreased significantly compared with that of the unprotected specimen, realizing the zero fragmentation protection effect on the back-blast face. The maximum deformation area and the main energy absorption area of T26 polyurea under contact explosion is the ring area outside the longitudinal deformation area. The chemical structure of T26 polyurea changed significantly after the explosion; typically the N-H bonds, etc., were broken and the percentage of hydrogen bonding was reduced. T26 polyurea has realized the protection effect of zero fragmentation of large-equivalent contact explosion, which has a high application value for blast mitigation and blast-fragmentation prevention in actual engineering.

Mechanical Behavior and Fracture Characteristics of Rock with Prefabricated Crack under Different Triaxial Stress Conditions

In this study, the compression failure test of rock with prefabricated fractures under different true triaxial conditions is carried out by using the true triaxial electro-hydraulic servo test system. The traditional large number of fracture laws with prefabricated fissures are merged and attributed to the induction of intermediate principal stress. The test results show that the direction of σ2 has a significant effect on the deformation characteristics of the prefabricated fractured rock. The internal crack expansion direction is more random and the crack distribution is more extensive and complex under uniaxial and conventional triaxial conditions. Under biaxial and true triaxial conditions, the crack propagation direction is clearly along the σ2 direction. This shows that the development process of internal cracks in rocks tend to the direction of σ2. Further, the failure mechanism of rock with prefabricated cracks is analyzed based on theory. It is found that the intermediate principal stress direction plays a very important role in inducing the direction of rock crack propagation. According to the unified idea, the fracture analysis of fractured rock is summed up as true triaxial theory, and the results are consistent with the experimental results. This provides a new perspective for the study of rock fracture mechanics, and provides an important basis for the analysis of surrounding rock fracture mechanism of underground engineering.

Identification of fracture damage characteristics in ultra-high performance cement-based composite using digital image correlation and acoustic emission techniques

The study of fracture mechanism and damage of ultra-high performance fiber reinforced cement-based composite (UHPFRCC) is of great significance for material performance evaluation and nondestructive structural health monitoring. In this paper, three-point bending fracture tests of UHPFRCC with fiber content of 1% and 2% were carried out, and the digital image correlation (DIC) and acoustic emission (AE) technique were used to monitor the fracture simultaneously. The microcrack area of UHPFRCC1 (UHPFRCC with 1% fiber content) decreases gradually with the loading process, and the microcrack area of UHPFRCC2 (UHPFRCC with 2% fiber content) increases first and then decreases with the loading process. The macrocrack area of UHPFRCC1 and UHPFRCC2 increases gradually with the same growth trend. The matrix failure and fiber/matrix debonding behavior of UHPFRCC were identified effectively based on the unsupervised k-means clustering method, and the fiber/matrix debonding behavior was obvious when the AE amplitude was greater than 60 dB. Meanwhile, the clustering results are also verified by the evolution of FPZ length. Finally, an efficient identification model of UHPFRCC matrix failure and fiber/matrix debonding behavior based on supervised KNN algorithm is proposed, which provides theoretical support for nondestructive structural health monitoring in practical engineering.

Mechanochemiluminescent Hydrogels for Real-Time Visualization of Chemical Bond Scission

AbstractQuantitative and real-time characterization of mechanically induced bond-scission events taken place in polymeric hydrogels is essential to uncover their fracture mechanics. Herein, a class of mechanochemiluminescent swelling hydrogels have been synthesized through a facile micellar copolymerization method using chemiluminescent bisacrylate-modified bis(adamantyl)-1,2-dioxetane (Ad) as a crosslinker. This design and synthetic strategy ensure intense mechanochemiluminescence from Ad located in a hydrophobic network inside micelles. Moreover, the mechanochemiluminescent colors can be tailored from blue to red by mixing variant acceptors. Taking advantages of the transient nature of dioxetane chemiluminescence, the damage distribution and crack evolution of the hydrogels can be visualized and analyzed with high spatial and temporal resolution. The results demonstrate the strengths of the Ad mechanophore and micellar copolymerization method in the study of damage evolution and fracture mechanism of swelling hydrogels.

An experimental study of ultra-high pressure water jet-induced fracture mechanisms and pore size evolution in reservoir rocks

Ultra-high pressure water jets (UHP-WJ) rock breaking is a promising technology for developing unconventional natural gas reserves . In this study, rock-breaking experiments were conducted using typical reservoir rock samples such as sandstone , shale, and coal. The failure patterns in rocks after impact by UHP-WJ were analyzed. Moreover, the 3D damage field model of the rock was reconstructed based on CT scanning, and the internal crack distribution was visualized and examined. Furthermore, the fracture mechanisms of three types of reservoir rocks under the UHP-WJ impingement were studied. The results indicate that intergranular fractures produced due to shear stress are the dominant type of fracture in sandstones, tensile failure due to stress wave was the main type of fracture observed in shale, and coal exhibited the double breaking characteristics of stress wave effect and water wedge effect. The fragments size distribution of samples shows the largest fragments in shale, followed by coal, and the smallest fragments are observed in sandstone. Finally, the evolution of rock pore structure before and after the impact of UHP-WJ was monitored by nuclear magnetic resonance (NMR) methods. The results reveal that the micropores transform into mesopores and macropores in the sandstone, where the total porosity increased by 14.3%–24.8%. As the jet pressure exceeds 100 MPa, the porosity of shale increases significantly, such that the total porosity of the shale sample increased by 7.6%–37.1%. The mesopores transform into macropores and fractures with the influence of the water wedge effect in coal samples, resulting in the reduction of mesopores and the increase of macropores, where the total porosity of the coal sample increased by 14.6%–40%. The complexity and heterogeneity of pores were reduced under UHP-WJ with the increasing jet pressure. The findings provide a theoretical reference for optimizing the recovery from unconventional natural gas reservoirs .

Design aspects of femur bone fractures: A review

Bones are the principal structural elements in the body to defend vital organs and provide a rigid skeleton for locomotion. The main reason for studying the mechanics of bone is to improve our understanding of how and why bone fractures. From seeing engineering point of view, the main reason for bone fracture representing that is structural failure. This will be occurring when the force or moments acting on bone are exceeded bone load-bearing capacity. In previous Technologies, they used metal rods with the help of nuts and bolts for connecting the bone fracture with the help of bone grafts. The bone grafts are helpful for fast growth in bones at fracture places. During this process by drilling the holes further weakens the bone for fixing the fractured bone. The technology adopted uses materials, couplings through bolts, rods, and screws implanted on bone fracture. Further, the bone is so assumed that transverse fracture failure due to stress concentration. Thus, the study of fracture mechanisms has importance in curing bone fracture. This paper reviews the equation on complete femur bone mechanism and its internal fixtures to repair the fracture and the importance of Bio-degradable materials.