Crack Energy Research Articles

Air assisted in situ upgrading via underground heating for ultra heavy oil: Experimental and numerical simulation study

In situ upgrading (ISU) via underground heating and thermal cracking is regarded as an effective technique for exploiting ultra heavy oils, in which the heavy oil components can be cracked into light oils and gases for production. However, the slow heating rate via conduction from wellbore to oil formation is the main issue concerned in the conventional ISU process. Herein, an air injection assisted ISU technique (AAISU) is proposed to improve heat transfer by gas convection and along with the thermal effect of oil oxidation due to the oxygen in the injected air. Air injection can also provide extra energy for oil production and reservoir pressure maintenance. To illustrate the advantage of the proposed AAISU technique, thermal cracking experiments of ultra heavy oil samples in the presence of air under high pressure were conducted to investigate the reaction mechanisms and to establish the kinetics models of the oxidation and cracking reactions. Moreover, reservoir numerical simulation studies are performed to evaluate the effects of air injection during the ISU process. The experiment results indicate that the ultra heavy oil can be effectively cracked into gases, light oils and coke-like substances in the presence of air at temperature over 350 °C. The activation energy of the thermal cracking derived is around 248 kJ/mol. The numerical simulation results show that the heat transfer rate can be effectively enhanced by air injection because of the heat convection of the air flow and the thermal effect of the low temperature oxidation reactions of oil components. The total oil recovery factor can be increased via air injection with improved energy conversion efficiency from 6.51 GJ/GJ to 8.42 GJ/GJ.

Relationships among Charpy impact toughness, microstructure and fracture behavior in 10CrNi3MoV steel weld joint

Charpy impact toughness of 10CrNi3MoV steel weld joint was investigated, and microstructure and fracture behavior were considered as reasons for differences in Charpy impact toughness. Charpy impact toughness of fusion zone (FZ) was lower than that of heat affected zone (HAZ) due to combinations of microstructure and fracture behavior. Acicular ferrite (AF) and grain boundary ferrite (GBF) with granular bainite (GB) in columnar dendrite pattern of FZ and a mixed mode of quasi cleavage fracture and ductile fracture was responsible for the low toughness; while tempered martensite with some bainite in fine dispersed equiaxed dendrite pattern of HAZ was contributed to absorb crack energy and improve the toughness. Tortuous crack propagation path of zigzag pattern confirmed the crack deviated at large angle grain boundary and depressed crack growth.

Poling angle effect on two mode-III semi-permeable collinear cracks in a piezoelectric strip: Strip-saturation model

A mathematical strip-saturation model is proposed to analyze the influence of changing poling direction on two hairline electrically semi-permeable collinear cracks embedded in a piezoelectric strip. The remote boundaries of the piezoelectric strip are subjected to anti-plane mechanical and in-plane electrical loads. Fourier series method is applied to transform the model into triple series equations which are then reduced to singular integral equations using the integral equation technique. The closed-form analytical expressions are derived for saturation zones length, crack-sliding displacement (CSD), crack opening potential drop (COPD), intensity factors (IFs), and energy release rate (ERR). Influence of the poling angle on CSD, COPD, and ERR are shown graphically for various cracked piezoelectric ceramics. In addition, the numerical study is also presented for analyzing the effect of inter-crack distance and prescribed electrical and mechanical loadings on the fracture parameters. All graphical results are analyzed and compared.

Cracking energy density for rubber materials: Computation and implementation in multiaxial fatigue design

AbstractThis article proposes a heuristic model to predict the fatigue life of rubber parts. The developed model is mainly based on the Cracking Energy Density (CED), originally developed by Mars, for the study of rubber parts fatigue. The main contribution consists in the integration of the theoretical framework of the critical plane analysis proposed by Mars with Saintier's experimental research carried out in tension and torsion modes. The CED parameter, derived from the Strain Energy Density (SED), can predict the occurrence of the first crack and its possible orientation depending on the type of loading path. In this context, an analytical analysis of this parameter is carried out for typical loading cases. Furthermore, the variation of CED by SED ratio (WC/W), as a function of the probable crack angle θ and the principal stretch λ1 is investigated. A finite element (FE) model is, then, developed to measure the performance and efficiency for some basic criteria used to investigate rubber fatigue life in literature. The pertinence of such criteria is evaluated in terms of a determination coefficient. The CED parameter can be considered as the most effective criterion for describing the multiaxial fatigue life of rubbers.

Crack Growth and Energy Release Rate for an Angled Crack under Mixed Mode Loading

The evaluation of energy release rate with angle is still a challenging task in metal crack propagation analysis, especially for the mixed Mode I-II-III loading situation. In this paper, the energy release rate associated with stress intensity factors at an arbitrary angle under mixed mode loadings has been investigated using both a numerical method and theoretical derivation. A relatively simple and precise numerical method was established through a series of spatial-inclined ellipses in Mode I-II and ellipsoids in Mode I-II-III, with different propagation angles computed from simulation. Meanwhile, a theoretical expression of the energy release rate with angle for a crack tip under a I-II-III mixed mode crack was deduced based on the propagation mechanism of the crack tip under the influence of a stress field. It is confirmed that the theoretical expression deduced could provide results as accurately as the present numerical method. The present results were confirmed to be effective and accurate by comparison with experimental data and other literature.

Fracture Behavior of Cracked Ring Specimen at Different Crack Positions

Abstract The previous research review of piping systems revealed that the plastic pipe companies suffered from many problems in natural gas pipeline systems. One of the most significant problems that appeared in the piping systems are external cracks due to manufacturing processes, welding technique, and installation processes. The principal goal of the present experimental study is to predict the crack growth behavior and energy release rate of cracked ring specimens made from high-density polyethylene (HDPE) under different crack position angles and various crosshead speeds. The effect of loading rate on the external radial crack at different crack position angles plays an important role in the prediction of fracture behavior of plastic pipe materials. For this reason, it is necessary to conduct a study for the fracture analysis of pipe ring specimens under tension loading with double external cracks at constant radial crack length to width ratio equal a/W = 0.5. A precracking machine is designed especially in the present experimental study to simulate the actual radial cracks at outer surface of pipe ring specimens. The effects of crosshead speed and crack position angle revealed a significant effect on the energy release rate and maximum applied load under tensile load.

Retrofitting of Reinforced Concrete Beams with Steel Fiber Reinforced Composite Jackets

In the present study, a new method for retrofitting reinforced concrete beam is introduced in which steel-concrete composite jackets containing steel fiber is used. For this purpose, 75% of the peripheral surface of reinforced concrete beams was initially reinforced using steel plates and bolts, and steel fiber reinforced concrete was used between the steel plates and the peripheral surfaces of the beam. Thus, due to the relatively high tensile strength of concrete reinforced with steel fibers, not only the cross-section and moment of inertia of the beam will increase, but the tensile strength of the beam will also increase. The variables studied were steel fiber value (0, 1 and 2% by volume of concrete) and type of retrofitting (concrete jacket, steel-concrete composite jacket, and carbon fiber reinforced polymer (CFRP) sheet). Thus, 8 reinforced concrete beams were constructed and their response to four-point loading was compared by examining the parameters such as crack load, yield load, ultimate load, ductility, stiffness, and energy absorption capacity. The results showed that steel fiber-reinforced composite jackets delay the formation of the first crack in concrete and the yield of steel rebars with confinement and they result in an increase in energy absorption capacity of the beams by 89 to 129% depending on the amount of steel fiber. On the other hand, the use of steel-concrete composite jackets, due to their higher flexural stiffness, exhibits higher flexural capacity compared to steel-reinforced concrete jackets and CFRP sheets. They have a much better performance in terms of ductility.

Size distribution measurement of coal fragments using digital imaging processing

This paper focuses on the size distribution measurement of coal fragments by digital imaging processing. The fast and precise measurement of coal fragments, which is important to understand the crack propagation and energy dissipation process of coal failure, has not been achieved by previous research. In this paper, an image analysis method using MATLAB is proposed to measure fragment size distribution of coal fragments. The acquisition setup, analysis step and coding process for fragment size distribution measurement by digital imaging processing are introduced in detail. The statistical size distribution of coal fragments measured by image processing is compared with the theoretical distribution function and manual sieving results. This paper provides an innovative and efficient method for size distribution measurement in the study of coal failure process.

Computing critical energy release rates for fracture in atomistic simulations

We describe a method for computing critical energy release rates for crack propagation in atomistic models and apply it to simulations of fracture in Ni. Our method relies on independent calculations of crack surface area and energy dissipated during fracture. We show that the critical energy release rate for fracture in Ni increases linearly with model size due to increasing energy dissipated through plastic deformation. We conclude with a discussion of prospects for direct comparisons of critical energy release rates computed from atomistic simulations to ones obtained from experiments.

Elastomeric Component Fatigue Analysis: Rubber Fatigue Prediction and Correlation Comparing Crack Initiation and Crack Growth Methodologies

<div class="section abstract"><div class="htmlview paragraph">Many elastomeric components in automotive industry applications are subjected to dynamic service loads. Fatigue must be a consideration in the design of these components.</div><div class="htmlview paragraph">The two approaches in rubber fatigue analysis discussed in this study are the conventional crack initiation approach, based on continuum mechanics parameters versus life, and the crack growth approach, based on fracture mechanics parameters. These methodologies are demonstrated for a passenger vehicle engine mount here. Temperature effects are not considered since testing was performed at ambient room temperature and with a low frequency, typically less than 5 Hz.</div><div class="htmlview paragraph">The promising methodology for fatigue life prediction, discussed in this paper, is a critical plane approach based on crack energy density. Rainflow cycle counting method and Miner’s damage rule are used for load cycle characterization and damage accumulation respectively. A fracture mechanics approach is implemented based on specimen crack growth data.</div><div class="htmlview paragraph">This methodology is validated with component testing under constant amplitude experiments. Results are investigated through analysis of critical stress locations hot spots, life values, and strain states. Comparative results are presented. Numerical results show reasonable correlation with experimental data.</div></div>

Adhesion enhancement of conductive graphene/PI substrates through a vacuum plasma system

Flexible electronics is regarded as a promising solution to stretchable/wearable devices given its light weight and high deformation ability. As a conducting layer, a widely utilized transparent conducting film, namely, indium tin oxide (ITO), is approaching its limitation on flexible applications given its high stiffness and brittle characteristics. Thus, this study proposes a highly flexible bi-layered architecture composed of polyimide (PI) substrate and coated graphene ink. This architecture is considered to replace the foregoing ITO film. A vacuum plasma system with different treatment parameters is taken into account to enhance the interfacial adhesion of graphene/PI substrates. Furthermore, a fracture-based finite element model of patterned graphene/PI substrates with various line widths are constructed to ensure the applicability of the presented stacked structure to actual flexible applications. This model aims to analyze the interfacial cracking energy under several designed bending radii of curvature through a modified virtual crack closure technique. To determine the fractured criterion of graphene/PI interfacial adhesion, a series of pull-off and lap shear tests are performed while requiring a comparison with simulated predictions. Results show that the peeling mode dominates the interfacial delamination of graphene/PI substrates, and the critical pull-off energy is appropriated to judge the fractured occurrence in accordance with the corresponding results of experimental bending tests. The adoption of vacuum plasma treatment contributes to enhancing the interfacial adhesion of the concerned interface and preventing the delamination failure occurrence during operating flexural loads.

Assessment of performance criteria of reinforced concrete beams retrofitted with FRP fabrics subjected to extreme impulsive loading

The study performed in this paper is limited to the reinforced concrete structures, in which the behavior of reinforced concrete beams retrofitted with FRP fabrics subjected to extreme loading is investigated. In this regard, numerical modeling using finite element software LS-DYNA is carried out. Numerical results are validated against the results of laboratory experiments. Strengthening of beams with an external composite layer of carbon, glass and Kevlar fibers are considered. Common performance criteria of concrete members such as deflection ratio, support rotation, and ductility ratio as well as crack pattern and energy absorption are investigated. Results show full wrapping mode with carbon fibers leads to the most reductions in the growth of flexural cracks as well as flexural-shear cracks along the free span of the beam. The ductility of the beam is reduced in different states of retrofitting, which the greatest amount of ductility reduction is obtained when reinforced with carbon fibers. Elastic and plastic energies absorbed by the retrofitted beam are, respectively, increased and decreased in all of the investigated cases. Retrofitting with carbon fibers in different states shows the largest increase in the absorption of elastic energy and also the greatest reduction in the plastic energy absorption.

Multi-scale modeling and mechanical performance characterization of stingray skeleton-inspired tessellations

Sharks and rays have distinctive skeletons among vertebrate animals, consisting primarily of unmineralized cartilage wrapped in a surface tessellation of minute polygonal tiles called tesserae, linked by unmineralized collagenous fibers. The discrete combination of hard and soft tissues is hypothesized to enhance the mechanical performance of tessellated cartilage (which performs many of the same functional roles as bone) by providing either rigidity or flexibility, depending on the nature of the applied load. These mechanisms and the effect of tesserae ultrastructure on cartilage mechanics, however, have never been demonstrated in the actual tissue, nor in bio-accurate models. Here, we develop bio-inspired three-dimensional tesserae computer models, incorporating material properties and ultrastructural features from natural tessellated cartilage. The geometries of ultrastructural features were varied parametrically, and the effective modulus of whole tesserae was evaluated using finite element analysis to determine the roles of ultrastructural features in mechanics. Whereas altering some structural features had no effect on the macroscopic in-plane modulus of tesserae, a three-fold increase in the contact surface area between two adjacent tesserae increased the effective modulus of tesserae by 6%. Modeled stress distributions suggest that tesseral ‘spokes’ (distinct hypermineralized features in tesserae) bear maximum stresses in the skeleton and serve to funnel stresses to particular populations of cells in tesserae, while spokes’ lamellated structure likely helps dissipate crack energy, making tesserae more damage-tolerant. Simulations of multi-tesseral arrays showed that maximum stresses in tension and compression are borne by different tissues, supporting hypotheses of multi-functional properties of tessellated cartilage. Further, tesseral array models showed that minor alterations to tesserae/joint shape and/or material properties can be used to tune the mechanical behavior of the whole tiled composite. Our models provide the first functional understanding of the distinct morphologies of spokes and of ‘stellate’ tesserae (a tesseral shape observed first over 150 years ago), while also being useful drivers for hypotheses of growth, mechanics, load management, and the prevention and ‘directing’ of cracks in tessellated cartilage, as well as other biological composites. Additionally, these results establish guidelines and design principles for bio-inspired, tunable tiled materials.

Test Methods for the Characterization of Polypropylene Fiber Reinforced Concrete: A Comparative Analysis

Fiber reinforced concrete (FRC) has been used for many different construction applications and many testing protocols have been developed to determine their mechanical behavior. The present paper focus on the comparisons between four major mechanical tests used for FRC: EN 14651, ASTM 1609, ASTM 1550 and EN 14488. Polypropylene fibers were used as reinforcement in volume ratios of 3 kg/m3, 6 kg/m3 and 10 kg/m3 in a self-consolidating high-strength concrete. The flexural beam tests according to ASTM 1609 and EN 14651 standards were carried on through a closed loop type of control. Comparisons include crack post-peak strength and energy absorption values. The EN 14651 showed to be more precise for low ductile concretes (i.e., low fiber volume fractions). This is due to the notch in the EN standard and different methods used for instrumenting the specimen. The EN 14488 due to its boundary conditions showed much higher results with a deflection-hardening type of behavior. Since the EN 14488's frame comes into full contact with the specimen, that creates a more rigid material response. This response differs from the other three tests since their support conditions allow rotations and induces cracks to appear in determined locations.

Formula omitted]-convergence for high order phase field fracture: Continuum and isogeometric formulations

We consider high order phase field functionals introduced in Borden et al. (2014) and provide a rigorous proof that these functionals converge to a sharp crack brittle fracture energy. We take into account three dimensional problems in linear elastic fracture mechanics and functionals defined both in Sobolev spaces and in spaces of tensor product B-splines. In the latter convergence holds when the mesh size vanishes faster than the internal length of the phase-field model. On the theoretical level, this condition is natural since the size of the phase field layer, around the crack, itself scales like the internal length; on the numerical level, it should be satisfied by local h-refinement.Technically, convergence holds in the sense of Γ-convergence, with respect to the strong topology of L1, while the sharp crack energy is defined in GSBD2. The constraint on the phase field to take values in [0,1] is taken into account both in the Sobolev setting and in the iso-geometric setting; in the latter, it requires a special treatment since the projection operator on the space of tensor product B-splines is not Lagrangian (i.e., interpolatory).

Experimental investigation of the contributions of CFRP and externally collar strengthening to the seismic performance of RC columns with different cross-sections

In this study, the performance of nine reinforced concrete (RC) short columns under cyclic lateral loading effect was investigated experimentally. The columns are divided into three series with cross-sectional dimensions of 300 × 400, 300 × 550 and 300 × 700 mm2. Each series includes three columns that are strengthened with one Reference column, two CFRP Wrap and Externally Collar columns. Aspect ratios (αs) of all columns in the series are 1.5. Therefore, the columns were tested under the same behavior demand according to the cross-section dimension. Reference columns in each series are perfect columns with sufficient stirrup spacing as required by regulations. The performance of RC columns was evaluated in terms of crack development, hysteretic character, stiffness, ductility, and energy consumption. The results showed that CFRP wrap and collar strengthening significantly improved the performance of the columns.

Thermal and mechanical properties of ferroelastic RENbO4 (RE = Nd, Sm, Gd, Dy, Er, Yb) for thermal barrier coatings

A new series of rare-earth niobates (RENbO4) are synthesized and investigated for potential thermal barrier coating (TBC) application. The thermal conductivities of RENbO4 are much lower than yttria stabilized zirconia (YSZ) mainly due to the chemical inhomogeneity. The lattice distortion contributes to the higher coefficients of thermal expansion (CTEs, 12.8 × 10−6 K−1) of RENbO4 below the critical temperature. Especially, RENbO4 exhibits high fracture toughness (1.93–2.77 MPa m0.5) comparable to YSZ, owning to the absorption of crack energy by the ferroelastic domain switching. The excellent thermal and mechanical properties give RENbO4 great potential as new durable TBC materials.

Mesoscale fracture behavior of Longmaxi outcrop shale with different bedding angles: Experimental and numerical investigations

Abstract The mechanical properties and fracturing mechanism of shale containing beddings are critically important in shale gas exploitation and wellbore stability. To investigate the effects of shale bedding on crack behavior and fracturing mechanism, scanning electron microscope (SEM) with a loading system was employed to carry out three-point bending tests on Longmaxi outcrop shale. The crack initiation and propagation of Longmaxi shale were observed and recorded by taking photos during loading. The cracking paths were extracted to calculate the crack length through a MATLAB program. The peak load, fracture toughness and fracture energy all increase with the bedding angle from 0° to 90°. The crack length and energy were also found to increase with the bedding angle in the range of 0°–60° and then drop slightly. The fracturing mechanism of shale includes the main crack affected by the bedding angle and disturbed by randomly distributed particles. The main cracking path was accompanied by several microcrack branches which could form an interconnected crack system. When the main crack encounters larger sedimentary particles, it will deflect around the particles and then restore to the initial direction. A numerical technique using extended finite element method (XFEM) coupled with anisotropic cohesive damage criteria was developed, which is able to capture the dependence of crack propagations on bedding angle and sedimentary particles. This study sheds light on understanding and predicting mesoscale fracture behavior of shale with different bedding angles.

Mechanical and Acoustic Emission Characteristics of Sandstone through Triaxial Unloading Test after Cyclic Freezing‐Thawing Treatment

The dynamic failure behaviour of tunneling rock in the cold region where freezing‐thawing frequently occurs is unclear. This study aimed to test and understand the damage characteristics of tunneling sandstone samples in the cold region via triaxial unloading test and acoustic emission (AE) technique. The sandstone samples were first subject to different cycles of freezing‐thawing. Their stress‐strain curves, deformation modulus, and the AE characteristics were then measured under triaxial unloading conditions and through the AE test. The results showed that the freezing‐thawing treatment with less than 60 freezing‐thawing cycles caused rather less damage compared to the triaxial unloading condition. For the samples subject to more severe freezing‐thawing treatment, more cracks were produced. These cracks were not closed under small confining pressure during the triaxial test, causing weaker mechanical properties of samples. We also found that the freezing‐thawing treatment had a significant deterioration on the mechanical properties of the sandstone samples when the number of freezing‐thawing cycles exceeded a certain threshold (between 60 and 80 in this study). As the AE characteristics matched well with the key stages of the measured axial stress‐strain curves and the deformation modulus that varied with the decreasing confining pressure, the AE characteristics can be potentially used to quantify the released energy of rock cracking and identify the critical damage phases during the tunneling engineering process.

Seismic behaviour of concrete columns with high-strength stirrups

The seismic behaviour of reinforced concrete (RC) columns made from high-strength materials was investigated experimentally. Six high-strength concrete specimen columns (1:4 scale), which included three with high-strength stirrups (HSSs) and three with normal-strength stirrups (NSSs), were tested under a combination of high axial and reversed cyclic loads. The effects of stirrup strength and the ratio of transverse reinforcement on the cracking patterns, hysteretic response, strength, stiffness, ductility, energy dissipation and strain of transverse reinforcement were studied. The results indicate that good seismic behaviour of an RC column subjected to high axial compression can be obtained by using a well-shaped stirrup. Stirrup strength had little effect on the lateral bearing capacity. However, the ductility was significantly modified by improving the stirrup strength. When loaded with a large lateral displacement, the strength reduction of NSS specimens was more severe than that of those with HSSs, and increasing the stirrup strength had little effect on the stiffness reduction. The ductility and energy dissipation of specimens with HSSs were superior to those with NSSs. When the ultimate displacement was reached, the core concrete could be effectively restrained by HSSs.