Fracture Propagation Mode Research Articles

Role of intermolecular potential in adhesive wear behaviors for elastic-plastic spherical microcontacts

This paper aims to investigate the effects of intermolecular potential on adhesive wear behaviors of microcontacts between a rigid flat and an elastic-plastic sphere, using a Finite Element (FE) model. In this model, a separation-dependent adhesive pressure, governed by the Lennard-Jones (LJ) potential, is applied to the surfaces of both the rigid flat and the sphere under combined normal loading and tangential displacement. To assess material damage, the Johnson–Cook (JC) failure and fracture energy criteria are utilized, allowing for element deletion upon reaching the damage threshold. The role of intermolecular potential in the adhesive wear behaviors of spherical microcontacts is evaluated, focusing on wear volume, wear rate, wear morphology and fracture propagation mode under various dimensionless normal loads and adhesive energies. Two fracture propagation modes, namely bidirectional-like and unidirectional-like modes, dependent on normal load and adhesive energy, have been identified. The transition from mild to severe wear regime is found to be closely related to the fracture propagation mode. Additionally, an interesting observation is that relatively high adhesive energy could result in decreased wear volume and wear rate under certain normal loads, which can be attributed to the change in fracture propagation mode caused by the adhesive pressure.

Advanced Deep Learning for microseismic events prediction for hydraulic fracture treatment via Continuous Wavelet Transform

Understanding hydraulic fracture propagation events would enable operators to estimate the fracture geometry, understand formation/fracture interactions, and estimate the total microseismic events cloud. This paper introduces a new method for understanding the dynamic fracture propagation events using a signal processing technique. Detection of the dynamic fracture event using continuous wavelet transform (CWT) is used to train a deep learning model with microseismic events to predict this cloud for every hydraulic fracture job. The deep learning model is used for the same formation. The CWT for treatment pressure is a convolution process that involves the application of a wavelet signal to the acquired pressure signal in a continuous manner. The wavelet is stretched, or compressed, and is moved along the acquired pressure signal, acting as a generic microscope to highlight small changes in the treating pressure signal. Using CWT as a mathematical microscope to discern diverse signals has been adopted by numerous engineering and medical applications since the 1990s.A normalized CWT scalogram for fracture-treating pressure is thus created as a unique representation of each fracture propagation mode in the hydraulic fracturing job. The new technique is calibrated with several fracture simulation runs and real field data using the previous techniques like moving reference point (MRP). That previous technique which has been previously validated through fracture simulation, is used as a calibration for the new normalized CWT scalogram technique. The technique was finally calibrated by comparing it with micro-seismic events recorded by the Marcellus Shale Energy and Environment Laboratory (MSEEL).The results of this comparison were exceptionally accurate. It can be used to train a deep learning model that can be used to estimate the cloud of microseismic events for hydraulic fracture jobs in Marcellus shale. The new deep learning framework uses CWT to convert the treating pressure during hydraulic fracture propagation to dimensionless representation (normalized CWT scalogram) which acts as a mathematical microscope for the fracture treating pressure on different wavelet stretch or compressions, enabling the detection of the major changes in the treating pressure then train the deep learning model to estimate microseismic events cloud.

Research on active load reducing optimization strategy for 6061Al low cycle fatigue cropping process based on crack propagation mode transformation

Low cycle fatigue cropping technology is a new type of cropping technology with advantages such as no chip waste and low load need. Numerous studies have shown that increasing the fatigue frequency or increasing the displacement load can improve the cropping efficiency, but at the same time, it will reduce the quality of the cross-section and cannot achieve a balance between high efficiency and high quality of cross-section. In response to this issue, this paper found the phenomenon of transition of fracture modes (from fatigue mode to tensile fracture mode) of 6061Al material in LCFC process, and proposed the Active Load Reducing Optimization Strategy for 6061Al in LCFC, which achieves a significant improvement in quality of cross-section with a slight loss of cropping efficiency. Firstly, this paper studied the crack fracture mechanism of 6061Al bar under different displacement loads in the LCFC process. It was found that the transition of crack propagation mode in 6061Al- LCFC process was the main reason for the increase in cross-section roughness. The crack propagation mode changed from fatigue propagation to tensile fracture propagation during the LCFC process, and the roughness of the area significantly increased under the tensile fracture propagation mode. Further research has found that the fundamental reason for the contradiction between the cropping efficiency and section quality of 6061Al is the change in time of crack propagation mode transition and tensile fracture zone area under different displacement loads. Finally, the LCFC process was modeled in a time dimension using acoustic emission(AE) technology, and it was divided into three stages according to the AE signals and the fracture mechanism. As a result, an active load reducing optimization strategy for LCFC process was developed to delay the occurrence of tensile fracture propagation modes. Experiments have shown that compared to the traditional LCFC cropping strategy using constant displacement loads, the active load reducing strategy proposed in this paper can reduce the proportion of tensile fracture area, achieve a significant improvement in quality of cross-section with a slight loss of cropping efficiency, thus achieving a balance between efficiency and section quality to some extent. At the same time, an LCFC process section quality efficiency evaluation index with dual weights of efficiency and section quality has been proposed, which can assist in selecting LCFC active load reducing strategy parameters with consideration of efficiency and section quality weights.

Numerical and Experimental Simulation of Hydraulic Fracture Propagation Mechanism in Conglomerate Formation Based on Hybrid Finite-Discrete Element Method

Hydraulic fracturing was the main technology to achieve the economic development of conglomerate reservoirs, knowing that the hydraulic fracture propagation mode was of great significance for improving the development of conglomerate reservoirs. This paper proposed a new method to understand the hydraulic fracture behavior based on a hybrid finite-discrete element method. The simulation indicated that a complex fracture network was created near the wellbore in the studied conglomerate reservoir, and hydraulic fracture propagation around the gravel layer was the main failure mode when the hydraulic fracture reached the gravel layer. From the simulations, it was shown that under small differences in horizontal stress and tensile strength, the hydraulic fracture propagated more easily around the gravel layer, while it could cross the gravel under large differences in horizontal stress and tensile strength. Greater tensile strength differences can reduce the complexity of the fracture network. In addition, higher pumping rates and viscosities of fracturing fluid contribute to the complex fracture network and also can produce more gravel crosses when the hydraulic fracture is met. The main reason was that a higher pumping rate and higher viscosity of fracturing fluid can obtain a higher net pressure, which can ensure the hydraulic fracture crosses the gravel layer.

Analysis of Optimal Loading Angle in Dynamic Flattened Brazilian Disc Splitting Test for Concrete

The mechanism of crack expansion in conventional Brazilian discs is clearly obtained with the help of numerical simulations, and the direction of crack expansion is related to the loading speed. Then the relationship between the stress field and the platform angle was given quantitatively based on the theoretical analysis. To obtain the accurate tensile strength of self-compacting concrete under the premise of numerical simulation and theoretical analysis, flattened Brazilian disc (BD) splitting tests with six loading angles were carried out by using the split Hopkinson pressure bar (SHPB) loading system. The crack initiation mode and fracture propagation mode of specimens under different loading angles and loading rates were analyzed through digital image correlation (DIC) and strain gauges. When the loading angle lies between 28° and 30°, the specimens easily meet the requirement of central cracking under high loading rates. The experimental results were well explained with theoretical analysis from a view of dimensionless Griffith’s equivalent stress. The dynamic tensile strength measured from the non-central crack mode in flattened BD splitting tests underestimates the inherent dynamic tensile strength.

Diagnosis model of shale gas fracture network fracturing operation pressure curves

Affected by reservoir heterogeneity, developed natural fractures, and bedding fractures, the fracturing pressure curves in fracturing of shale gas horizontal wells present complex shapes. A large amount of information contained in the fracturing curves is still not fully excavated. Based on the theory of shale gas fracture network fracturing, the calculation model of bottom hole net pressure is established by integrating the real-time data such as casing pressure, pump rate, and proppant concentration. Net pressure slope and net pressure index are constructed as key parameters, and the net pressure curve is divided dynamically to describe the mechanical conditions corresponding to the fracture propagation behavior during the fracturing process. Six fracture propagation modes were identified, including fracture network propagation, fracture propagation blockage, normal fracture propagation, fracture propagation long bedding, fracture height growth, and rapid fluid filtration, and then the operation pressure curve diagnosis and identification method were formed for shale gas fracture network fracturing in horizontal wells. The shortcomings of conventional operation curve diagnosis and identification methods are abandoned and the fracture network complexity index is presented. The higher index indicates more time of fracture network propagation and fracture propagation along bedding and the better reservoir stimulation effect. The model is applied to shale gas wells in the southeastern margin of Sichuan Basin, and the average fracture network complexity index of a single well is 0.3, which is in good agreement with the microseismic monitoring results. This proves the good reliability of the method developed. The method is helpful to improve the potential and level of fracturing stimulation of shale reservoirs and is of great significance for improving the post-fracturing evaluation technology of fracture network and guiding the real-time dynamic adjustment of field fracturing operations.

Fracture Propagation Modes of Lower Cambrian Shale Filled with Different Quartz Contents under Seepage-Stress Coupling

The content and spatial distribution of brittle minerals, such as quartz, are important factors in determining the fracture initiation mechanism induced by hydraulic fracturing in shale reservoirs. To further research the impact of quartz content in shales of the Lower Cambrian Niutitang Formation in northern Guizhou on the fracture expansion of its reservoir, 7 groups of randomly filling shale models with different quartz contents were established using rock failure process analysis (RFPA2D-flow) code for numerical test studies under seepage-stress coupling, and 5 samples were also subjected to uniaxial compression tests using the INSTRON 1346 electrohydraulic servo-controlled material testing machine (200T). The results show that the average growth rate of the compressive strength and the fracture proportion for a quartz content of 50% to 65% are 4.22 and 1.15 times higher than those for 35% to 50%, respectively. Fractures sprout, expand, and breakdown in the shale matrix or at the junctions of the shale matrix and quartz grains. The mechanical properties and pattern of the fracture extension of the shale in the physical tests are similar to those in the numerical tests, indicating the reliability of the numerical simulations. The fractal dimension curves at different stress levels are divided into three stages: flattening, increasing, and surging, and the fractal dimension value for a quartz content of 50%~65% at a 100% stress level is 1.02 times higher than that for 35%~50%. The high degree of natural fracture development in high quartz content formations in shale gas reservoirs is of some reference value for logging data. The research results provide a reference value for the content and spatial distribution of brittle minerals for the initiation mechanism and fracture propagation of hydraulic fracturing in shale reservoirs.

Numerical simulation on the initiation and propagation of synchronous perforating fractures in horizontal well clusters

Horizontal well perforation is one of the main factors affecting formation fractures around the wellbore and subsequent hydraulic fractures extending along the predetermined path. In this paper, the RFPA2D-Flow software is used to conduct a numerical simulation study on the initiation and propagation of parallel perforation fractures in the perforation cluster. Then influence of different perforation density and different stress on the initiation and propagation of perforation fractures are studied. The results show that there is a competitive relationship between initiation and propagation of perforated fractures. Three parameters, the initial initiation rate and the length of the hydraulic fracture, and the generated fracture number, are put forward as the indexes to measure the competitive relationship. Perforated fractures will eventually form a fracture propagation mode dominated by several main fractures and supplemented by micro fractures. Then it shows that an obvious spatial form associated with asymmetric main fractures and micro fractures derived around them. Parallel perforation fracture initiation presents four modes of competitive initiation. With the increase of perforation density, the fracture initiation rate of parallel perforation in the initial stage of fracturing decreases gradually, the number and the complexity of fractures also increase gradually; with the continuous increase of the stress difference, the initiation rate at the initial stage of hydraulic fracturing and the number of the main formed fractures and derived micro fractures all show a decreasing trend.

Experimental Investigation on Hydraulic Fracture Morphology of Inter-Salt Shale Formation

The inter-salt shale in the Qianjiang formation of Jianghan Basin in China is characterized by multiple salt-shale bedding planes, various rock strength, and high heterogeneity of rock mechanics. In this paper fracturing experiments under different conditions were carried out to study the effects of the injection velocity, type of fracturing fluid and interface strength on the propagation law of hydraulic fracture in the salt sedimentary rhythm there. In the meantime, Acoustic emission system and radial strain sensor were applied to monitor experimental process. The result indicates that 1) compared with the shale, there are four fracture propagation modes mainly being observed: passivating type (Mode I), “I”-type (Mode II), penetration type (Mode III) and mixed type ((Mode IV)), among which the mixed type is the relatively complex crack propagation mode. 2) With the increase of injection rate and viscosity of fracturing fluid, the hydraulic fracture will penetrate cementation surface more easily. 3) The increase of flow rate and viscosity will increase the breakdown pressure. The breakdown pressure of high strength cementation surface is 16.70% higher than that of low strength.

Experimental and Numerical Investigation on Basic Law of Dense Linear Multihole Directional Hydraulic Fracturing

Using the dense linear multihole to control the directional hydraulic fracturing is a significant technical method to realize roof control in mining engineering. By combining the large-scale true triaxial directional hydraulic fracturing experiment with the discrete element numerical simulation experiment, the basic law of dense linear holes controlling directional hydraulic fracturing was studied. The results show the following: (1) Using the dense linear holes to control directional hydraulic fracturing can effectively form directional hydraulic fractures extending along the borehole line. (2) The hydraulic fracturing simulation program is very suitable for studying the basic law of directional hydraulic fracturing. (3) The reason why the hydraulic fracture can be controlled and oriented is that firstly, due to the mutual compression between the dense holes, the maximum effective tangential tensile stress appears on the connecting line of the drilling hole, where the hydraulic fracture is easy to be initiated. Secondly, due to the effect of pore water pressure, the disturbed stress zone appears at the tip of the hydraulic fracture, and the stress concentration zone overlaps with each other to form the stress guiding strip, which controls the propagation and formation of directional hydraulic fractures. (4) The angle between the drilling line and the direction of the maximum principal stress, the in situ stress, and the hole spacing has significant effects on the directional hydraulic fracturing effect. The smaller the angle, the difference of the in situ stress, and the hole spacing, the better the directional hydraulic fracturing effect. (5) The directional effect of synchronous hydraulic fracturing is better than that of sequential hydraulic fracturing. (6) According to the multihole linear codirectional hydraulic fracturing experiments, five typical directional hydraulic fracture propagation modes are summarized.

Influence of the fracture process zone on fracture propagation mode in layered rocks

During the hydraulic fracturing, fractures are often kinked, blunted, or offset owing to the alternating, interbedded interfaces of diverse properties that render fracturing performance unsatisfactory. In this study, two cases (the artificial fracture perpendicular to and parallel to the bedding interfaces) were examined through three-point bending experiments on semi-circular bending (SCB) using shale specimens. The fracture process zone (FPZ) was studied based on the Digital Image Correlation (DIC) technique. The interbedded interfaces ahead of the artificial fracture tip affect the fracture propagation, with its extents are related to the bedding planes. In such conditions, the FPZ is composed of two parts: the FPZ ahead of the artificial fracture tip, and the zones of the deformed bedding interfaces distant from the artificial fracture tip. When the artificial fracture ran along the bedding interface, after peak loading, the interbedded interfaces on both sides of the main fracture undergone tensile failure. In turn, the deformation at the interbedded interfaces restricted the extension of the main fracture. The propagation mode of the hydraulic fracture accompanied by distant opening/slipping of interbedded interfaces is found remarkable in the layered reservoirs, and should be included in hydraulic fracturing designs or numerical fracturing modeling.

True triaxial hydraulic fracturing test and numerical simulation of limestone

Hydraulic fracturing, as a key technology of deep energy exploitation, accelerates the rapid development of the modern petroleum industry. To study the mechanisms of hydraulic fracture propagation and rock failure mode of the vertical well hydraulic fracturing, the true triaxial hydraulic fracturing test and numerical simulation are carried out, and the influence of the principal stress difference, water injection displacement, perforation angle and natural fracture on fracture propagation is analyzed. The results show that the fracture propagation mode of limestone is mainly divided into two types: the single vertical fracture and the transverse-longitudinal crossed complex fracture. Under high displacement, the fracturing pressure is larger, and the secondary fracture is more likely to occur, while variable displacement loading is more likely to induce fracture network. Meanwhile, the amplitude of acoustic emission (AE) waveform of limestone during fracturing is between 0.01 and 0.02 mV, and the main frequency is maintained in the range of 230–300 kHz. When perforation angle θ=45°, it is easy to produce the T-type fracture that connects with the natural fracture, while X-type cracks are generated when 0=30°. The results can be used as a reference for further study on the mechanism of limestone hydraulic fracturing.

Experimental study on fracture propagation induced by supercritical CO2 jet fracturing in artificial samples with prefabricated bedding planes

The propagation characteristic of fractures under the influence of bedding during SC-CO2 jet fracturing remains value to be studied. In order to avoid the large dispersion of natural rock property, seven types of artificial rock samples (Φ100 mm × 100 mm) with prefabricated different positions and numbers of bedding were prepared. A multifunction SC-CO2 jet system was used to conduct the SC-CO2 jet fracturing with ambient pressure. The main fractures on the surface were observed through vertical and side view after the experiment. In addition, five fracture propagation modes were summarized systematically. Finally, the fracture tortuosity parameters were obtained by quantitative analysis, and it was found that for horizontal bedding, the farther the vertical distance from the casing, the larger the fracture tortuosity formed by SC-CO2 jet fracturing; if a single fracture penetrates the bedding, the fracture tortuosity will increase.

The Propagation Behavior of Hydraulic Fracture in Rock Mass with Cemented Joints

The formation of the fracture network in shale hydraulic fracturing is the key to the successful development of shale gas. In order to analyze the mechanism of hydraulic fracturing fracture propagation in cemented fractured formations, a numerical simulation about fracture behavior in cemented joints was conducted based firstly on the block discrete element. And the critical pressure of three fracture propagation modes under the intersection of hydraulic fracturing fracture and closed natural fracture is derived, and the parameter analysis is carried out by univariate analysis and the response surface method (RSM). The results show that at a low intersecting angle, hydraulic fractures will turn and move forward at the same time, forming intersecting fractures. At medium angles, the cracks only turn. At high angles, the crack will expand directly forward without turning. In conclusion, low-angle intersecting fractures are more likely to form complex fracture networks, followed by medium-angle intersecting fractures, and high-angle intersecting fractures have more difficulty in forming fracture networks. The research results have important theoretical guiding significance for the hydraulic fracturing design.

Real-Time Formation-Face-Pressure Analysis Improves Acid-Fracturing Operations

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 194351, “Real-Time Analysis of Formation-Face Pressures in Acid-Fracturing Treatments,” by Vibhas Pandey, SPE, and Robert Burton Jr., SPE, ConocoPhillips, and Kay Capps, Capsher Technology, prepared for the 2019 SPE Hydraulic Fracturing Technology Conference and Exhibition, The Woodlands, Texas, USA, 5–7 February. The paper has not been peer reviewed. Knowledge of formation-face pressures is critical to the success of hydraulic-fracturing treatments, especially in multistage and multiple-cluster-type horizontal-well completions. This knowledge can help in evaluating the effectiveness of acid-fracturing treatments in dynamic mode and also can enable and improve real-time decision-making during treatment execution. This paper contains a detailed discussion of methods and a software tool that has been developed to generate information that predicts formation-face pressures in real time with the help of live bottomhole-pressure data. Introduction Acid-fracturing treatments combine the basic principles of hydraulic fracturing and acid-reaction kinetics to stimulate acid-soluble formations. Unlike propped fracturing treatments, in which a viscous pad is followed by slurry-laden fluid that assists in keeping the fracture propped open to create a conductive flow path for formation fluids, the fracture conductivity in acid fracturing is generated by dissolving the formation face to create an uneven surface on the fracture walls. Multiple pad and acid stages are pumped during the course of the treatment to create a high-conductivity fracture. The acid stage then fingers through the pad and the open fracture to create differentially etched patterns on the fracture walls. The uneven, etched surfaces prevent the fracture from closing completely once the hydraulic pressure is removed, leading to a highly conductive pathway to enhance hydrocarbon flow. Once the acid contacts the formation, the bottomhole treating pressure recedes quickly, and, as acid spending continues, the effective bottomhole pressures can fall below fracture-closure stresses. Not all loss of pressure is a result of acid spending alone. The fluid fingering among fluids of highly contrasting viscosities also can result in such drops. However, the stress relief that results from acid spending dominates this behavior. Pumping additional acid in these cases only results in near-well acid or mineral reactions, which do not extend fracture penetration and may increase wellbore-integrity risks. Unfortunately, without a good estimation of effective fracture pressures, determination of when the fracture-treatment pressure dips from above closure stress to below closure stress is difficult. Determination of this critical transition was a primary driver of the study that led to the creation of a tool that can assist in determining the real-time treating pressures at the entry of the fracture during stimulation treatments. Several acid-fracturing treatments were analyzed using the tool, leading to important conclusions related to fracture-propagation modes, acid-exposure times, and effectiveness of given acid types.

Experimental investigation on fracture propagation modes in supercritical carbon dioxide fracturing using acoustic emission monitoring

Supercritical carbon dioxide (SC-CO2) fracturing has broad application prospects due to its potential to increase production, to reduce water requirements, and to minimize environmental impacts. To date, lowering fracture breakdown pressure and inducing multi-fractures using SC-CO2 as fracturing fluid have been recognized, but the process of fracture propagation has been poorly studied. Therefore, acoustic emission (AE) system with 12 channels is used to monitor the SC-CO2 fracturing. The results show that energy release rate of SC-CO2 fracturing is 1–3 orders of magnitude higher and the cumulative energy is also much larger compared to hydraulic fracturing. Then much available energy can be converted into driving force to generate dynamic multi-fractures. Meanwhile, the curves of cumulative energy are observed as shape of sidestep or zigzag. The SC-CO2 fracturing evolution of AE events is discontinuous, suggesting that intermittent fracture propagation occurs. The process and modes of SC-CO2 fracture propagation are proposed, respectively. The process of SC-CO2 fracture propagation can be divided into 4 stages: damage zone evolution, fracture initiation, fracture post-instability and fracture arrest. And fracture propagation modes are classified into 3 types, including single curved fracture (Type -I), fracture band (Type -II) and fracture network (Type -III). This study is helpful for designing appropriate treatment to optimize CO2 sequestration or to enhance stimulated reservoir volume.

Nonlocal enrichment of a micromechanical damage model with tensile softening: Advantages and limitations

Upon crack propagation, brittle geomaterials such as concrete and rock exhibit a nonlinear stress/strain behavior, damage induced stiffness anisotropy, loading path dependent strain softening and hardening, unilateral effects due to crack closure and a brittle-ductile transition, which depends on the confining pressure. Challenges in theoretical and numerical modeling include the distinction between tensile and compressive fracture propagation modes, mesh dependency during softening, and lack of convergence when several critical points are expected on the stress/strain curve. To overcome these issues, we formulate a nonlocal micromechanics based anisotropic damage model. A dilute homogenization scheme is adopted for calculating the deformation energy of the Representative Elementary Volume due to the displacement jumps at open and closed micro-cracks. Tension (respectively compression) damage criteria are expressed in terms of non-local equivalent strains defined in terms of positive principal strains (respectively deviatoric strains). Constitutive parameters are calibrated against published experimental data for concrete and shale. We employ the arc-length control method to solve boundary-value problems with the finite element package OOFEM: the algorithm allows capturing softening, snap back and snap through. We simulate the development of the compression damage zone around a cavity under various stress levels at the wall and far field, and the softening behavior consequent to tensile fracture propagation during a three-point bending test. No mesh dependency is noted during softening as long as micro-cracks do not interact.

A study of Inconel 718 dependency on stress triaxiality and Lode angle in plastic deformation and ductile fracture

A numerical and experimental study of monotonic tensile tests on Inconel 718 with different stress states has been investigated. Focus was put to dependencies of stress triaxiality and Lode angle parameter on plastic behavior and ductile fracture. The constitutive plasticity model proposed by Bai and Wierzbicki (2008) and the modified Mohr–Coulomb (MMC) ductile fracture model (Bai and Wierzbicki, 2010) were adapted with suitable extensions. Experimental results were used to calibrate the models. By setting up parameter optimization for model calibration, the experimental results and numerical simulations were well correlated. Finally, the MMC fracture model well predicted both fracture initiation and fracture propagation modes.

On the mechanical response of woven para-aramid protection fabrics

The initial condition of the structure of the fabric's knitting form, before applying a load, is closely correlated with the performance of the fabric under loading. The present study focuses on the mechanical response of para-aramid protection fabrics under tensile loading and suggests a knitting angle as a considerable factor for the behavior of the fabric. A global and a local method are applied to monitor the effect of this angle on the fabric's performance and deformation mechanism. The experimental results indicate that this angle affects both the fracture propagation mode and the fracture toughness. A failure onset detection mode, based on this angle, is also suggested, followed by a calculation of the further feasible deformation of a permanently deformed zone until the next failure.

Experimental Studies and Models for Ductile Fracture in ASTM A992 Steels at High Triaxiality

AbstractThis study presents the results of experimental investigation and models for ductile fracture in the ASTM A992 structural steels subjected to high stress triaxiality. To this end, uniaxial tension tests on the notched tensile specimens are conducted, and subsequent scanning electron microscopy (SEM) studies are performed on the fractured surfaces. From this SEM analysis, the fracture initiation mechanism in ASTM A992 steels is found to be the ductile fracture; however, the fracture propagation mode can be either ductile or cleavage fracture. Nonlinear finite-element analysis of the notched specimens is carried out to obtain the stress triaxiality distributions, and the location of the peak stress triaxiality is found to be strongly correlated to the location of the ductile fracture initiation zones. Finally, two uncoupled ductile fracture prediction models, the void growth model (VGM) and the stress modified critical strain model (SMCS), are calibrated, and the model parameters are provided for A9...