Stable Crack Propagation Research Articles

Prediction of Sandstone Dilatancy Point in Different Water Contents Using Infrared Radiation Characteristic: Experimental and Machine Learning Approaches

Abstract In rock mechanics, the dilatancy point is always occurring before rock failure during loading process. Water content plays a significant role in the rock physiomechanical properties, which also impact the rock dilatancy point under loading process. This dilatancy point significantly plays a warning role in the rock engineering structures stability. Therefore, it is essential to predict the rock dilatancy point under different water contents to get an early warning for effective monitoring of engineering projects. This study investigates the water contents effects on sandstone dilatancy point under loading in the presence of infrared radiation (IR). Furthermore, this IR was used for the first time as an input parameter for different artificial intelligence (AI) techniques to predict the dilatancy point in the stress-strain curve. The experimental findings show that the stress range in stress-strain curve stages (crack closure and unstable crack propagation) increases with water content. However, this range for deformation and stable crack propagation stages decreases with water content. The dilatancy stress, crack initiation stress, and elastic modulus are negatively linearly correlated, while peak stress and stress level are negatively quadraticaly correlated with a high (R2). The absolute strain energy rate, which gives a sudden increase at the point of dilatancy, is used as the dilatancy point index. The stress level is 0.86 σmax at the dilatancy point for dry rock and decreases with water content. This index is predicted from IR data using three computing techniques: artificial neural network (ANN), random forest regression (RFR), and k-nearest neighbor (KNN). The performance of all techniques was evaluated using R2 and root-means-square error (RMSE). The results of the predicted models show satisfactory performances for all, but KNN is remarkable. The research findings will be helpful and provide guidelines about underground engineering project stability evaluation in water environments.

Research on crack propagation law of rotating bending fatigue cropping process based on cusp catastrophe theory

A large number of experimental results show that in variable load low-stress fatigue cropping there are two forms of fatigue crack propagation, stable crack propagation and sudden-jump crack propagation, but sudden-jump crack propagation seriously affects cropping section quality. In this paper, the process of crack propagation under cropping is analyzed, and the condition of alternate stable-jump crack propagation under circumferential rotating bending fatigue loading is proposed from the fracture mechanism. Based on the energy method, the cusp-catastrophe model potential function of cropping process is constructed, and with XFEM (extended finite element) and total potential energy catastrophe theory, the multiple catastrophe positions in the cropping process are obtained. The results of the AE (acoustic emission) measurement experiment in cropping confirm the phenomenon of multiple catastrophe in the process of fatigue crack propagation. The loading curve based on catastrophe theory is obtained by fatigue strength theory. Compared with current common loading curves, the obvious sudden-jump crack propagation phenomenon is eliminated basically, no obvious catastrophe trace is observed in the propagation area, and the fracture area is reduced by 54.65%, which significantly improves the quality of cropping section.

Fracture toughness of the shot-earth 772

In the present research work, the fracture behaviour of a specific earthen material, that is, the shot-earth 772, is experimentally investigated. In particular, three-point bending tests on single edge-notched prismatic specimens of shot-earth are performed according to the RILEM Recommendations. Then, by exploiting the experimental results, the fracture toughness is analytically determined by means of the Modified Two-Parameter Model, which allows us to take into account the possible crack deflection, typically observed during the stable crack propagation in quasi-brittle materials.

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

Improvement of boundary effect model in multi-scale hybrid fibers reinforced cementitious composite and prediction of its structural failure behavior

The inclusion of multi-scale hybrid fibers in cement-based materials exhibits excellent crack resistance performance at multi-level than that of single type or size of fiber. Multi-scale hybrid fiber reinforced cementitious composites (MHFRCCs) can be served as a building material with stringent crack resistance requirements for civil structure such as pipeline, river levee, nuclear reactor, water tower , and sewage treatment sedimentation tank. The tensile strength ( f t ) and fracture toughness ( K IC ) are two important size independent material parameters for guiding structural safety design and stability assessment. In this study, the real f t and K IC of MHFRCCs were determined by three-point bending test based on the boundary effect model. The average aggregate particle size with a large proportion was chosen as the “representative aggregate ( d 1 = 0.45 mm)" by using grading curve. The fiber long axis dimensions that appear most frequently on the cross section were selected as the “representative fiber ( d 2 = 0.28 mm for steel fiber and d 3 = 0.0794 mm for polyvinyl alcohol fiber)" according to the statistical analysis of backscattered electron imaging. The fracture failure behaviors of MHFRCC structure were predicted based on the determined f t and K IC values, and the structural fracture failure bands with ±15% variations were established. In addition, the microstructure analysis of MHFRCC revealed multi-scale enhancement mechanism in the cement matrix . Finally, the complete fracture process images and P-CMOD curve were divided into three stages: pre-cracking stage, crack stable propagation stage, and unstable failure stage. This analysis would be helpful to understand the fracture crack resistance effect of multi-scale hybrid fibers at multi-level within cement matrix. • Backscattered electron imaging was used to determine hybrid fiber morphologies based on statistical analysis. • The relationship between fictitious crack growth and the fibers-aggregates system was established. • Size independent fracture parameters of MHFRCC were determined using boundary effect model. • Structural failure curves/bands were constructed to predict the fracture behavior of MHFRCC. • Microscopic analysis and complete fracture process images confirmed the multi-scale crack resistance mechanism for MHFRCC.

Influence of bedding planes on fracture characteristics of coal under mode II loading

Due to bedding planes and natural fractures, the mechanical behavior of coal under different loading modes is usually anisotropic. The anisotropy usually influences the strength and stability of coal and rock mass in engineering. In this research, we used the asymmetric semi-circular bend (ASCB) method to carry out fracture tests of coal specimens with various bedding angles (0°, 22.5°, 45°, 67.5°and 90°) between loading direction and bedding planes under mode II loading. The influence of the bedding angle on the fracture toughness and fracture pattern of coal specimens under mode II loading was investigated. Moreover, based on the acoustic emission (AE) technology and digital image correlation (DIC) technique, the influence of the bedding plane on the failure process of coal specimens was studied. The research results indicate that the effect of the bedding plane is significant. The apparent mode II fracture toughness decreases and then gradually increases with the increase of bedding angle, picturing a check mark shape. The minimal value was obtained at the bedding angle of 22.5°. The obtained fracture patterns can be classified into fracture growth along the bedding and across the bedding. The influence of the bedding plane on the crack propagation is the most obvious at the bedding angle of 22.5° and 45°, where the cracks mainly grow along the bedding planes. The failure process of ASCB coal specimens can be divided into the compaction stage, the stable crack propagation stage, the rapid crack propagation stage, and the unstable crack propagation stage. The bedding angle has more influence on the crack initiation load while has less influence on the crack damage load between 80 %Pmax to 90 %Pmax.

Mesh refinement procedures for the phase field approach to brittle fracture

Two refinement procedures for phase field approach are proposed and their numerical performances are investigated in the solution of fracture problems. Starting from a coarse discretization of the domain, an energetic criterion is used to determine the active zones where damage initiates, evolves and mesh refinement is mandatory to accurately approximate crack topology. Moreover, the extension of the refined regions is strictly correlated with the size of the process zone defined by the optimal profile. Global and global/local refinement strategies are investigated. In particular, the global/local technique solves the displacement and phase field problems on a local mesh dynamically updated and adaptively refined during the computation. Once the solutions of the local problems are obtained, the phase field is interpolated back onto the original mesh, adequately refined where a damage threshold is exceeded. The performances of the refinement techniques are analyzed and compared in representative examples with stable and brutal crack propagation.

Development of a fracture-mechanics based fatigue testing method for epoxy/electrical steel laminates with thin adhesive layer

A fatigue fracture mechanics method was developed, implemented and used to characterize the crack kinetics of laminated electrical steel. Tests were carried out on double cantilever beam specimen based on electrical steel sheets and an epoxy adhesive layer with a thickness of just 10 µm. Two different crack length measurement methods based on optical measurement or compliance calculation were evaluated. As validation method specimens with different artificial initial crack length were used. The accuracy is limited to thin and stiff adhesives ensuring a small plastic zone size compared to the crack length. The accuracy of the crack length evaluation methods was higher for the compliance based approach meeting standard specifications at low to intermediate crack length values. The compliance based method was applied to investigate a specimen series varying the pre-curing temperature of the epoxy adhesive systematically. The fatigue tests were performed at ambient condition and 60 °C. At ambient temperature, laminates based on coated electrical steel pre-cured at low to intermediate temperatures revealed rather slow crack propagation rates in comparison to laminates made from coated steel pre-cured at 250 °C. The differences in the stable crack propagation rate of the investigated laminates at ambient conditions were up to two orders of magnitude higher. At 60 °C, the worst performance was also obtained for laminates based on coated steel sheets pre-cured at 250 °C. The failure mode was similar for laminates tested at 23 and 60 °C. While interfacial failure was ascertained for laminates based on coated steel sheets pre-cured at 190 and 250 °C, the better performing laminates made from sheets with epoxy coating pre-cured at 210 or 230 °C exhibited mainly cohesive failure. Hence, adequate pre-curing conditions are of utmost importance for the crack kinetics of adhesively bonded electrical steel.

Study on Acoustic Emission Characteristics and Damage Evolution of Shale under Uniaxial Compression

In this study, uniaxial compression tests were conducted and acoustic emission monitoring technology was used on shales from the Liutang Village, Shizhu County, Chongqing City. The ringing count and energy evolution law during the compression process of shale samples were analyzed, and their damage evolution characteristics are discussed. The results show that the cumulative ringing count and cumulative energy curves of acoustic emission can be used to distinguish among crack closure, stable crack propagation, and unstable crack propagation. According to the change process of acoustic emission events, the damage evolution process of shale samples is divided into damage maintenance, initiation, and acceleration. The constitutive model established based on the acoustic emission ringing count can better describe the uniaxial compression process of shale samples.

Modification of Coal Samples with Bursting Liability Subjected to Microwave Irradiation

This paper introduces the innovative technique to release the bursting liability of coal seam via microwave irradiation. To verify the feasibility of this environment-friendly technique, a series of laboratory tests incorporating acoustic emission (AE) investigation were carried out. Test results indicated that both the uniaxial compressive strength (UCS) and bursting energy index of raw and water-soaked coal samples were significantly reduced. In particular, the bursting liability was reduced by one level when the values of UCS were compared, the evidence of which is the variation of wave velocities of tested coal samples. It can also be found from the events and hits in the complete stress-strain curve and the cumulative curve of acoustic emission that the elastic modulus of the raw and water-soaked coal samples subjected to microwave irradiation decreased by 58.42% and 29.63%, respectively. This facilitates the entry into the stage of stable crack propagation more quickly, the growth rate and size of the cracks were slower and more uniform, and there were no smaller coal fragments ejecting during the failure process of the coal samples. Meanwhile, the proportion of high-energy events released in coal samples experienced a decline after the treatment of the microwave. Moreover, microwave heating principally promoted the initiation and expansion of microcracks in coal samples under the influence of microwave power of 1 kW and a heating time of 120s, which may cause the overall damage of large fractures to break into multiple small and medium cracks. Based on the experimental results, the conceptual process of using microwave in weakening the bursting liability of coal seam was then proposed, which will be the meaningful reference for microwave-assisted oil recovery and coal bed methane production.

Experimental study on Mode-I fracture toughness using Chevron Straight-notched Semi-circular Bend (CSNSCB) method

Rock fracture toughness is an important parameter for the engineering geological design. In this present study, chevron straight-notched semi-circular bend (CSNSCB) specimens with wide-flat liner chevron ligament (Type-A) and long-narrow liner chevron ligament (Type-B) were designed and prepared using wire cutting method. Mode-I fracture toughness KIC values and dimensionless stress intensity factor Y* were calculated. Moreover, these two types rock failure properties and KIC values variation features were both analyzed. The obtained results indicate that the width of precast artificial crack is small which can ensure the flatness and symmetry of cutting plane and then the tested Stress Intensity Factor (SIF) which has smaller deviation is more accurate and reliable. The Y* values for each type of configuration were calculated using Finite Element Method (FEM) and Slice Synthesis Method (SSM) and they firstly had a decreased trend and then an increased trend with the increasing of normalized crack length α, which represent the transition from stable crack propagation (α > αm) to unstable crack propagation (α < αm). Compared to Type-A specimens, Type-B specimens had the calculated minimum dimensionless stress intensity factor Y*min with smaller deviation and average KIC values with smaller variation coefficients. Moreover, Type-A specimen has the average fractal dimension D value of 1.2209 which is 4.79% larger than that of 1.1651 for Type-B specimen, indicating its more complex crack generation and rougher fracture surfaces. That is to say, Type-B specimen has the advantages of small crack FPZ and crack propagation without deviation as well as smooth fracture surface. Since it can offset the harmful influence of rock anisotropy and good crack guided effect, its KIC value is much satisfactory and closer to the true fracture toughness of rock-like materials.

Theoretical Limits in Detachment Strength for Axisymmetric Bi-Material Adhesives

AbstractReversible dry adhesives rely on short-ranged intermolecular bonds, hence requiring a low elastic modulus to conform to the surface roughness of the adhered material. Under external loads, however, soft adhesives accumulate strain energy, which release drives the propagation of interfacial flaws prompting detachment. The trade-off between the required compliance, for surface conformity, and the desire for a reduced energy release rate, for better strength, can be achieved with a bi-material adhesive having a soft tip and a rigid backing (RB). This design strategy is widely observed in nature across multiple species. However, the detachment mechanisms of these adhesives are not completely understood and quantitative analysis of their adhesive strength is still missing. Based on linear elastic fracture mechanics (LEFM), we analyze the strength of axisymmetric bi-material adhesives. We observed two main detachment mechanisms, namely (i) center crack propagation and (ii) edge crack propagation. If the soft tip is sufficiently thin, mechanism (i) dominates and provides stable crack propagation, thereby toughening the interface. We ultimately provide the maximum theoretical strength of these adhesives obtaining closed-form estimation for an incompressible tip. In some cases, the maximum adhesive strength is independent of the crack size, rendering the interface flaw tolerant. We finally compare our prediction with experiments in the literature and observe good agreement.

A novel casting procedure for SFRC piles without shear reinforcement using the centrifugal forming technique to manipulate the fiber orientation and distribution

The structural performance of steel fiber-reinforced concrete (SFRC) members primarily depends on the distribution and orientation of the fibers. Previous experimental studies revealed that the distribution and orientation of fibers are random and hardly controlled during the fabrication process. The centrifugal forming technique may improve the orientation and distribution of steel fibers in hollow circular SFRC piles and increase the shear strength of the piles. Accomplishing this task can lead to the possible replacement of shear reinforcements with steel fibers in piles, thereby improving the demanding productivity that is desired in the pile construction industry by saving considerable time and labor. This paper presents a novel experimental program to (a) develop the casting procedure for SFRC piles and (b) investigate the shear strength of SFRC piles without shear reinforcement compared to that of conventional reinforced concrete (RC) piles with shear reinforcement. With the aid of X-ray technology, different mix proportions and casting methods were investigated to determine the optimum procedure, which produced fiber distributions and orientations in SFRC piles superior to those of RC piles. The centrifugal forming method influenced the orientation of the fibers so that they were oriented in the shear stress direction (i.e., circumferential direction). The loading test demonstrated that the steel fibers effectively worked as shear reinforcement and that the shear capacity increased with the steel fiber content. Moreover, the SFRC piles exhibited stable crack propagation and improved post-cracking stiffness without severe concrete damage compared to that of the RC piles.

Evaluation of Monkman–Grant strain as a key parameter in ductility exhaustion damage model to predict creep rupture of grade 92 steel

Conventional strain-based numerical prediction assumes that failure occurs when ductility is exhausted or accumulation of creep strain reaches the critical failure strain. Due to instability at the onset of rupture, the failure strain value appears to be scattered and leads to the erroneousness in prediction. In this paper, a new local constraint-based damage model incorporating the Monkman–Grant ductility, as a measure of strain during uniform creep deformation stage, was implemented into a Finite Element (FE) model to predict the creep damage and rupture of Grade 92 steel under uniaxial and multiaxial stress states. The prediction was applied on plain and notched bar specimens with various notch acuities. The uniaxial stress-dependent Monkman–Grant (MG) failure strain was adopted in the FE to simulate the influence of the constraints which were induced by the creep damage. The implication of reduced failure strain in long-term creep time on the rupture prediction is discussed. The multiaxial MG failure strain of the notched bar, which has a lower value than uniaxial failure strain due to the geometrical constraint, was estimated based on the linear inverse relationship between normalised MG failure strain and normalised triaxiality factor. It was found that the results obtained from the proposed technique were in good agreement with the experimental data within the scatter band of ± factor of 2. It was shown that MG failure strain can be used as an alternative to strain at fracture. MG strain outweighed strain at fracture because the determination of its value only required short-term testing to be performed. In most cases considered in the present investigation, the rupture-type fracture was predicted, however, there was evidence that under high constraint and low stress, stable crack propagation occurred before fracture. The location of the maximum creep damage was found to be dependent on the creep time, geometry or acuity level of the specimen. For sharp notch specimen, the failure was initiated near the notch root, however, as the notch radius increased, the initiation location moved further away towards the specimen centre.

Use of linear amplitude sweep test as a damage tolerance or fracture test to determine the optimum content of asphalt rejuvenator

A procedure based on the linear amplitude sweep (LAS) test has been developed to determine the optimum rejuvenator content for restoring the properties of aged asphalt binders. The LAS test was employed to assess the damage tolerance, fatigue resistance, and fracture behavior of rejuvenated aged binders. In case of fracture analysis, a crack-based indicator to govern the stable crack propagation was defined to characterize fracture resistance of asphalt binders. While rejuvenators improved all of these properties, higher contents were required to restore the cracking behavior of the original binder. The linear relationship between the integrated damage-fatigue-fracture indicator and rejuvenator content was used to determine the optimum content. Unlike the similar permissible range based on restoration of the low and high-temperature performance, the LAS-based approach demonstrated superior performance of the aromatic extract rejuvenator over the waste cooking oil at higher contents.

Mode I and Mode II interfacial fracture energy of SiC/BN/SiC CMCs

Quantifying the mixed mode fracture toughness of interfaces in ceramic matrix composites (CMCs) is crucial for understanding their failure. In this work we use in situ micromechanical testing in the scanning electron microscope to achieve stable interfacial crack propagation in Mode I (Double Cantilever Beam) and Mode II (Push out) and measure the corresponding fracture resistances. We use this approach to measure the interfacial fracture resistance in SiC/BN/SiC CMCs and compare it to the fracture energy of the fibres. During in-situ testing, fracture paths can be observed while data is acquired simultaneously. We clearly observe debonding at the BN-fibre interface (i.e. inside adhesive debonding). The critical energy release rate of the BN-fibre interface for Mode I and II (GIc ≈ 2.1 ± 1.0 J/m2 and GIIc ≈ 1.2 ± 0.5 J/m2) are equivalent and is lower than that measured for the fibre using microscopic DCB tests (GIc ≈ 6.0 ± 2.0 J/m2). These results explain the generalized fibre debonding and pull out observed in the fracture of these CMCs. By enabling direct observation of crack paths and quantifying the corresponding fracture energies, we highlight possible routes for the optimisation and modelling of the new generation of CMC interphases.

An assessment of phase field fracture: crack initiation and growth.

The phase field paradigm, in combination with a suitable variational structure, has opened a path for using Griffith's energy balance to predict the fracture of solids. These so-called phase field fracture methods have gained significant popularity over the past decade, and are now part of commercial finite element packages and engineering fitness- for-service assessments. Crack paths can be predicted, in arbitrary geometries and dimensions, based on a global energy minimization-without the need for ad hoc criteria. In this work, we review the fundamentals of phase field fracture methods and examine their capabilities in delivering predictions in agreement with the classical fracture mechanics theory pioneered by Griffith. The two most widely used phase field fracture models are implemented in the context of the finite element method, and several paradigmatic boundary value problems are addressed to gain insight into their predictive abilities across all cracking stages; both the initiation of growth and stable crack propagation are investigated. In addition, we examine the effectiveness of phase field models with an internal material length scale in capturing size effects and the transition flaw size concept. Our results show that phase field fracture methods satisfactorily approximate classical fracture mechanics predictions and can also reconcile stress and toughness criteria for fracture. The accuracy of the approximation is however dependent on modelling and constitutive choices; we provide a rationale for these differences and identify suitable approaches for delivering phase field fracture predictions that are in good agreement with well-established fracture mechanics paradigms. This article is part of a discussion meeting issue 'A cracking approach to inventing new tough materials: fracture stranger than friction'.

Intrinsic toughening and stable crack propagation in hexagonal boron nitride.

If a bulk material can withstand a high load without any irreversible damage (such as plastic deformation), it is usually brittle and can fail catastrophically1,2. This trade-off between strength and fracture toughness also extends into two-dimensional materials space3-5. For example, graphene has ultrahigh intrinsic strength (about 130gigapascals) and elastic modulus (approximately 1.0terapascal) but is brittle, with low fracture toughness (about 4megapascals per square-root metre)3,6. Hexagonal boron nitride (h-BN) is a dielectric two-dimensional material7 with high strength (about 100gigapascals) and elastic modulus (approximately 0.8terapascals), which are similar to those of graphene8. Its fracture behaviour has long been assumed to be similarly brittle, subject to Griffith's law9-14. Contrary to expectation, here we report high fracture toughness of single-crystal monolayer h-BN, with an effective energy release rate up to one order of magnitude higher than both its Griffith energy release rate and that reported for graphene. We observe stable crack propagation in monolayer h-BN, and obtain the corresponding crack resistance curve. Crack deflection and branching occur repeatedly owing to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which intrinsically toughens the material and enables stable crack propagation. Our in situ experimental observations, supported by theoretical analysis, suggest added practical benefits and potential new technological opportunities for monolayer h-BN, such as adding mechanical protection to two-dimensional devices.

High damage-tolerance bio-inspired B4C/2024Al composites with adjustable mechanical performance by tuning ceramic thickness

Freeze casting is a promising approach for assembling lamellar metal-ceramic composites with an exceptional combination of strength and toughness. Although these mechanical properties can be optimized by regulating the microstructure of porous ceramic structures, the effect of lamellar thickness is rarely mentioned, especially for B4C/Al composites. Herein, by controlling the freezing temperature, we used freeze casting to create nacre-like B4C scaffolds with identical ceramic content yet different lamellar thicknesses and then infiltrated them with 2024Al alloy. The effects of lamellar thickness on the damage-tolerance behavior and toughening mechanisms are discussed. The refinement of lamellae decreases the probability of observing catastrophic flaws in ceramic layers, increasing strength from 534 ± 14 to 578 ± 15 MPa and increasing crack-initiation toughness (KIc) from 9.2 ± 0.6 to 11.4 ± 0.2 MPa m1/2. These composites exhibit higher damage tolerance resulting from several toughening mechanisms, such as plastic deformation, crack deflection and blunting, and the uncracked-ligament bridging of ductile metal layers, which is reflected in the stable crack propagation during fracture and rising R-curve behavior. Importantly, coarsening of the structure of composites allows the fracture behaviors transform from single to multiple crack propagation, thus absorbing much more fracture energy, with the valid crack-growth toughness (KJc) enhancing markedly from 17.1 ± 2.4 to 29.8 ± 2.3 MPa m1/2.

Effect of sub-/super-critical CO2 and brine exposure on the mechanical and acoustic emission characteristics of shale

The effect of fluid-shale interaction on the mechanical behavior of shale is of great significance for the safety of long-term CO2 sequestration in shale formations. In this paper, uniaxial compression and Brazilian splitting tests were conducted to the examine the mechanical alterations of shale caused by various fluids (brine, subcritical CO2, supercritical CO2, subcritical CO2+brine and supercritical CO2+brine) soak. After soaking in various fluids at 45 °C for 10 days, the uniaxial compression strength (σc), tensile strength (σt), Young's modulus (Ec) and tensile modulus (Et) of shale were decreased. Supercritical CO2 induced more significant mechanical weakening in shale than subcritical CO2 as its much higher adsorption affinity in shale. Compared with the brine and corresponding single CO2 conditions, further mechanical weakening in shale was observed in the Sub/-or supercritical CO2+brine soak conditions. The weakening effect was further verified by acoustic emission (AE) analysis results, as the crack damage stress was decreased and the stable crack propagation was shortened, and the reduction and shortening were enhanced for the CO2+brine soaked samples. The fractal dimensions obtained by AE signals were increased, and show a positive correlation with the damage variables, which indicated that the fractal dimension of AE signals can be used to describe the damage in shale.