Predict Fracture Initiation Research Articles

Fracture prediction under nonproportional loadings by considering combined hardening and fatigue-rule-based damage accumulation

To predict fracture initiation in ductile material under nonproportional loadings, an approach considering combined hardening and a fatigue-rule-based damage accumulation law was investigated. Eleven complex nonproportional experiments, including torsion-tension tests, compression-torsion tests, and uniaxial tension-compression cyclic tests were performed. Finite element analysis was conducted on the experimental data to extract information on essential local fractures, such as stress triaxiality and the Lode parameter. The Chaboche combined hardening model was tuned to capture the material hardening characteristics, ensuring a reliable description of the hardening behavior compared with the isotropic hardening assumption. In addition, the inclusion of fracture locus was proposed to determine the predeformation effect on damage evolution. Combined with the fatigue-rule-based damage accumulation laws, the damage evolution can consider the variable strain history. Thus, fatigue-rule-based damage accumulation, together with the combined hardening law, considerably improved the ability to predict fracture onset in cases of torsion-tension loading with respect to the integral-based damage accumulation law.

A through-thickness damage regularisation scheme for shell elements subjected to severe bending and membrane deformations

This research work proposes and validates a damage regularisation model for shell elements used in large-scale simulations. The model evaluates the ratio of bending to membrane loading in the elements based on the through-thickness gradient of the through-thickness plastic strain. The Cockcroft–Latham failure criterion is adopted, whose parameters are modified according to the length-to-thickness ratio of the shell elements in order to reduce the mesh dependency. This regularisation scheme is validated against experimental component tests on a double-chamber profile in an AA6005-T6 recrystallised aluminium alloy under quasi-static and impact loading conditions. The results show that the model is able to accurately predict fracture initiation under all tested conditions.

Ductile fracture prediction of high tensile steel EH36 using new damage functions

ABSTRACTThis study deals with ductile fracture prediction of a marine steel using new damage functions. The stress triaxiality and Lode angle are known to be governing parameters on the ductile fracture. Recent research has reported that loading path is also one of the important parameters. A three-dimensional fracture strain surface for EH36 steel was developed based on a series of tension/compression tests subjected to the tension, shear, shear-tension, and compression. To account for the non-proportional stress effects, the linear and non-linear damage evolution models were presented with a combination of the fracture strain surface. The material constants associated with the damage evolution were identified from calibration with the experimental data. Validation of the damage-based fracture models was conducted by comparing predicted fracture initiation with the results of structural test results. The fracture model with the non-linear damage evolution predicts slightly more improved predictions than the linear damage-based fracture model.

The analytical model of hydraulic fracture initiation for perforated borehole in fractured formation

Hydraulic fracturing has been the common method of stimulation treatment. Effective fracture network needs to be formed for having efficient hydraulic fracturing. The forming process of fracture network can be divided into 2 parts, i.e., initiation and propagation. In particular, the initiation part is the foundation of forming fracture network, and also basic parameter for analyzing fracture propagation. However, in fractured formation, it is very tricky to obtain initial fracture pressure due to the influence of rich natural fractures. Therefore, based on the rock mechanics and spatial geometry theory, this paper presents a mechanical model for predicting fracture initiation in fractured formation. Unlike conventional model, this new model considers multiple fractures conditions and the intersection method between natural fractures and perforated borehole. The model has been used to investigate influence factors of initial fracture pressure. These computed results reveal that the initial fracture pressure has complex distribution, and changes with variable occurrence of natural fracture. There are different kinds of intersections between natural fracture and perforated borehole. When fracture plane intersects through perforated borehole, influence of fracture plane on fracture initiation is relatively larger. Furthermore, with increasing number of natural fractures, initial fracture pressure decreases and the initiation mode tends to be tensile failure along natural fracture plane. Additionally, engineering parameters, like well trajectory and perforation azimuth, are related to fracture initiation pressure. Their optimizations can contribute to perform optimization of reservoir stimulation. Results from this model used in this paper aim at offering reference for field fracturing practice in fractured formation.

Determination of the Values of Critical Ductile Fracture Criteria to Predict Fracture Initiation in Punching Processes

Punching processes are widely used for producing automobile parts, mechanical components, and other parts. To produce highly accurate parts, it is important to estimate the ratio of the sheared surface to the cut surface. Many researchers have applied the finite-element method (FEM) to analyze the ratio of the sheared surface to the fracture surface on cut surfaces by using ductile fracture criteria. However, it is difficult to determine the fracture criteria on the cut surface by tensile tests or bending tests because the punching process involves many complicated steps. In this study, FEM was applied to the punching process to determine the values of critical fracture criteria (C) by using the ductile fracture criteria proposed by Cockcroft and Latham, Oyane, and Ayada. The ductile fracture criteria were compared with the boundary between the shear surface and the fracture surfaces using experiments performed with a simple punching system. The values of the ductile fracture criteria for the fracture initiation of the formed cut surface were predicted under various clearances between the punch and the die with various punch diameters. The influence of stress triaxiality and the effect of punch diameter on the sheared surface length are also discussed.

Prediction of fracture initiation in square cup drawing of DP980 using an anisotropic ductile fracture criterion

This paper deals with the prediction of fracture initiation in square cup drawing of DP980 steel sheet with the thickness of 1.2 mm. In an attempt to consider the influence of material anisotropy on the fracture initiation, an uncoupled anisotropic ductile fracture criterion is developed based on the Lou—Huh ductile fracture criterion. Tensile tests are carried out at different loading directions of 0°, 45°, and 90° to the rolling direction of the sheet using various specimen geometries including pure shear, dog-bone, and flat grooved specimens so as to calibrate the parameters of the proposed fracture criterion. Equivalent plastic strain distribution on the specimen surface is computed using Digital Image Correlation (DIC) method until surface crack initiates. The proposed fracture criterion is implemented into the commercial finite element code ABAQUS/Explicit by developing the Vectorized User-defined MATerial (VUMAT) subroutine which features the non-associated flow rule. Simulation results of the square cup drawing test clearly show that the proposed fracture criterion is capable of predicting the fracture initiation with sufficient accuracy considering the material anisotropy.

Survival Predictions of Ceramic Crowns Using Statistical Fracture Mechanics

This work establishes a survival probability methodology for interface-initiated fatigue failures of monolithic ceramic crowns under simulated masticatory loading. A complete 3-dimensional (3D) finite element analysis model of a minimally reduced molar crown was developed using commercially available hardware and software. Estimates of material surface flaw distributions and fatigue parameters for 3 reinforced glass-ceramics (fluormica [FM], leucite [LR], and lithium disilicate [LD]) and a dense sintered yttrium-stabilized zirconia (YZ) were obtained from the literature and incorporated into the model. Utilizing the proposed fracture mechanics–based model, crown survival probability as a function of loading cycles was obtained from simulations performed on the 4 ceramic materials utilizing identical crown geometries and loading conditions. The weaker ceramic materials (FM and LR) resulted in lower survival rates than the more recently developed higher-strength ceramic materials (LD and YZ). The simulated 10-y survival rate of crowns fabricated from YZ was only slightly better than those fabricated from LD. In addition, 2 of the model crown systems (FM and LD) were expanded to determine regional-dependent failure probabilities. This analysis predicted that the LD-based crowns were more likely to fail from fractures initiating from margin areas, whereas the FM-based crowns showed a slightly higher probability of failure from fractures initiating from the occlusal table below the contact areas. These 2 predicted fracture initiation locations have some agreement with reported fractographic analyses of failed crowns. In this model, we considered the maximum tensile stress tangential to the interfacial surface, as opposed to the more universally reported maximum principal stress, because it more directly impacts crack propagation. While the accuracy of these predictions needs to be experimentally verified, the model can provide a fundamental understanding of the importance that pre-existing flaws at the intaglio surface have on fatigue failures.

Modeling dynamic fracture growth induced by non-Newtonian polymer injection

Injection of viscous polymer solutions can lead to excessive wellbore pressure and dynamic fracture growth near wellbore which hence increases polymer injectivity. Polymer rheology can also play an important role in affecting the fracture growth due to the dramatic variation of fluid velocity. However, current reservoir simulations for chemical flooding generally overlook the induced fracture, which may lead to inaccurate modeling of polymer floods. In this paper, we developed a new polymer-injection-induced fracture model based on the GdK fracture model, and implicitly coupled it to a general-purpose chemical flood simulator. Mathematical description of the fracture mechanics is presented with consideration of different polymer rheology. The new model is used to simulate polymer floods in a five-spot field. The fracture growth predicted by the new model is verified against a semi-analytical numerical fracture model. The simulation results using the new model indicate that the fracture growth has positive effects on improving injectivity, pressure drop, oil recovery, etc. Also, the simulations illustrate for the first time how polymer rheology affects fracture growth during polymer injection, e.g., without consideration of polymer shear-thickening can postpone the prediction of fracture initiation. The simulations successfully explain why polymer can be economically injected in real polymer fields. The new simulator can be used to optimize polymer floods concerning polymer injectivity, impact of fracture growth on sweeping efficiency, flow out of zone, and others.

Experimental Investigation on Brazilian Tensile Strength of Organic-Rich Gas Shale

SummaryTensile strength is a critical parameter for hydraulic fracturing, predicting fracture initiation and propagation in reservoirs, especially in shale reservoirs with complex natural fractures and fissures. The tensile strength of conventional rocks, such as sandstone and limestone, has been well-studied and -documented. There are many studies of the tensile strength of laminated shale, which focus on scale effects, loading direction, and temperature effects; however, the studies on effects of mineralogy and the water content on tensile strength of organic-rich shales are very limited.The objectives of this paper are to (1) critically review the key parameters that affect the tensile strength of shale and 2) experimentally examine the effects of water content, mineralogy, and lamination on tensile strength. To do so, a rigorous workflow is followed: 1) Each 1-in.-long shale sample is cut into two subsamples, A and B, of similar length; (2) X-ray computed-tomography (CT) scan is performed to diagnose pre-existing cracks and defects inside the core plugs; (3) nuclear magnetic resonance (NMR) is then used to measure the air dry samples’ water content; (4) Sample A is placed on a load frame to measure the tensile strength; (5) Sample B is vacuumed and then saturated; (6) NMR is used to measure the water content after saturation; (7) tensile strength of the saturated Sample B is measured; and (8) after the sample fails, the pieces are used for X-ray diffraction (XRD), scanning electron microscopy (SEM), and pyrolysis to estimate the mineralogy and total organic content (TOC).A total of 70 Mancos and 48 Eagle Ford shale samples have been tested. The experimental results show that (1) bedding plane/lamination has a significant effect on Eagle Ford tensile strength, but no pronounced impact is observed for the Mancos shale; (2) the imbibed water significantly reduces the tensile strength by 4.4 to 51.7% as water content increases from 4.45 to 11.7%; (3) pre-existing detectable microfractures can significantly reduce the tensile strength by up to 66%; (4) Eagle Ford exhibits typical brittle hard-rock failure configuration, with primary fractures and secondary fractures being observed, whereas for the Mancos shale, only primary fractures are observed; (5) acoustic velocity-test results confirm that Eagle Ford is mechanically transversely isotropic, and Mancos is likely mechanically isotropic.

Numerical simulation for the determination of hydraulic fracture initiation and breakdown pressure using distinct element method

Hydraulic fracturing technique has been widely used in many cases to enhance well production performance. In particular, this technology is proven to be the most viable technique for the oil and gas production from unconventional reservoirs. Accurate prediction of fracture initiation and breakdown pressure is vital for successful design of Hydraulic Fracturing operation. Methods of predicting these pressures include analytical analysis, field experiments, laboratory experiments and numerical simulations. Despite great achievements in the area of analytical analysis, they often failed to represent the true reservoir case, and consequently are found to be erroneous. Field tests such as mini-frac test are the best method for prediction of initiation and breakdown pressure. However these tests are very limited due to their costs and are not very suitable for sensitivity analysis. Controlled laboratory tests seem to be the best option for predicting initiation and breakdown pressures. Test parameters such as fracturing fluid properties and principal stresses can be controlled with great precision to achieve accurate results. However, same as field tests, laboratory experiments are expensive. Core samples are limited and are expensive. Coring operation can take 4–5 days of rig time to take a 90 ft core. Geo-mechanical tests can take up to three days of a laboratory technician's time per sample. Consequently, this will limit the number of tests to be done, and as a result it causes limitations on the conclusions that can be drawn from these tests. Simulation studies on the other hand do not have these limitations and can be used for as many times as desired to perform sensitivity analysis.This paper presents a simulation model that is based on distinct element method. It is used to study the fracture initiation and breakdown pressure during hydraulic fracturing tests. The accuracy of the model was justified through comparison between laboratory experiments and numerical simulation. Four sandstone samples from two different sandstone types and a synthetic cement sample were used in the experimental studies. The tests were performed in True Tri-axial Stress Cell (TTC) with the capability to inject fluid into the samples. Simulation results demonstrate good agreement with experimental results. Fracture propagation path was found to be very similar. Fractures propagated in the direction of maximum horizontal stress.

Extracting ductile fracture toughness from small punch test data using numerical modeling

This paper presents a numerical method to extract ductile fracture toughness from notched small punch test data using FE ductile damage analysis based on the stress-modified fracture strain model. To validate the proposed method, analysis results are compared with series of mechanical (notched tensile, fracture toughness and small punch) test data for three different materials; low alloy steel SA508 Gr.3, TP316L austenitic stainless steel and CF8M cast austenitic stainless steel. Comparison with experimental fracture toughness data shows that the proposed method can predict fracture initiation values relatively well and tearing resistance conservatively.

Predicting failure of the Second Sandia Fracture Challenge geometry with a real-world, time constrained, over-the-counter methodology

An over-the-counter methodology to predict fracture initiation and propagation in the challenge specimen of the Second Sandia Fracture Challenge is detailed herein. This pragmatic approach mimics that of an engineer subjected to real-world time constraints and unquantified uncertainty. First, during the blind prediction phase of the challenge, flow and failure locus curves were calibrated for Ti–6Al–4V with provided tensile and shear test data for slow (0.0254 mm/s) and fast (25.4 mm/s) loading rates. Thereafter, these models were applied to a 3D finite-element mesh of the non-standardized challenge geometry with nominal dimensions to predict, among other items, crack path and specimen response. After the blind predictions were submitted to Sandia National Labs, they were improved upon by addressing anisotropic yielding, damage initiation under shear dominance, and boundary condition selection.

Prediction of Fracture Initiation in Hot Compression of Burn-Resistant Ti-35V-15Cr-0.3Si-0.1C Alloy

An important concern in hot working of metals is whether the desired deformation can be accomplished without fracture of the material. This paper builds a fracture prediction model to predict fracture initiation in hot compression of a burn-resistant beta-stabilized titanium alloy Ti-35V-15Cr-0.3Si-0.1C using a combined approach of upsetting experiments, theoretical failure criteria and finite element (FE) simulation techniques. A series of isothermal compression experiments on cylindrical specimens were conducted in temperature range of 900-1150 °C, strain rate of 0.01-10 s−1 first to obtain fracture samples and primary reduction data. Based on that, a comparison of eight commonly used theoretical failure criteria was made and Oh criterion was selected and coded into a subroutine. FE simulation of upsetting experiments on cylindrical specimens was then performed to determine the fracture threshold values of Oh criterion. By building a correlation between threshold values and the deforming parameters (temperature and strain rate, or Zener-Hollomon parameter), a new fracture prediction model based on Oh criterion was established. The new model shows an exponential decay relationship between threshold values and Zener-Hollomon parameter (Z), and the relative error of the model is less than 15%. This model was then applied successfully in the cogging of Ti-35V-15Cr-0.3Si-0.1C billet.

Fracture behaviour of a Fe–22Mn–0.6C–0.2V austenitic TWIP steel

The mechanical behaviour of a 22Mn–0.6C–0.2V austenitic TWIP steel has been extensively characterised for a variety of strain ratios (from shear to biaxial stretching) using smooth and notched specimens. A constitutive model involving a non-isotropic yield function together with isotropic and/or kinematic hardening satisfactorily represented the experimental database. It was used to estimate local stress and strain fields and to derive a fracture criterion based on the equivalent stress and Lode angle that were expressed to be consistent with the constitutive equations describing the plastic flow behaviour. A weak dependence on hydrostatic stress further improves prediction of fracture initiation, with an average standard error of less than 5% over ten different mechanical tests.

Mixed mode fracture analysis using extended maximum tangential strain criterion

Abstract The maximum tangential strain (MTSN) criterion is modified in this paper to predict fracture initiation in cracked specimens under mixed mode loading conditions. Theoretical investigations were performed to study the role of both first nonsingular strain term (T-strain) and Poisson's ratio on prediction of the crack propagation direction and fracture toughness. An extended version of the MTSN (EMTSN) criterion is proposed by taking into account the T-strain as well as the singular strain field. The EMTSN criterion is then examined using the experimental data reported for angled cracked plates made from polymethylmethacrylate (PMMA). It is shown that the EMTSN criterion provides more accurate estimation of the experimental results than conventional MTSN criterion.

Modeling of damage driven fracture failure of fiber post-restored teeth

Mechanical failure of biomaterials, which can be initiated by either violent force, or progressive stress fatigue, is a serious issue. Great efforts have been made to improve the mechanical performances of dental restorations. Virtual simulation is a promising approach for biomechanical investigations, which presents significant advantages in improving efficiency than traditional in vivo/in vitro studies. Over the past few decades, a number of virtual studies have been conducted to investigate the biomechanical issues concerning dental biomaterials, but only with limited incorporation of brittle failure phenomena. Motivated by the contradictory findings between several finite element analyses and common clinical observations on the fracture resistance of post-restored teeth, this study aimed to provide an approach using numerical simulations for investigating the fracture failure process through a non-linear fracture mechanics model. The ability of this approach to predict fracture initiation and propagation in a complex biomechanical status based on the intrinsic material properties was investigated. Results of the virtual simulations matched the findings of experimental tests, in terms of the ultimate fracture failure strengths and predictive areas under risk of clinical failure. This study revealed that the failure of dental post-restored restorations is a typical damage-driven continuum-to-discrete process. This approach is anticipated to have ramifications not only for modeling fracture events, but also for the design and optimization of the mechanical properties of biomaterials for specific clinically determined requirements.

AN EXPERIMENTAL AND NUMERICAL STUDY OF DUCTILE FRACTURE OF COPPER/STAINLESS STEEL CLAD SHEET IN DEEP DRAWING PROCESS

This work is an attempt towards employing ductile damage criterion and finite elementsimulations for prediction of fracture initiation and evolution in deep drawing of copper/stainlesssteel clad sheets. The material mechanical properties and ductile damage parameters weredetermined through standard and notched tensile tests. The effect of some important processparameters on damage evolution were examined through numerical modeling and the acceptablerange of variations for each parameter were introduced in order to prevent tearing of the blankduring the process. The numerical predictions of deformation and fracture behavior were in a goodagreement with experimental observations.

Ductile fracture prediction for welded steel connections under monotonic loading based on micromechanical fracture criteria

Micromechanics-based fracture criteria for structural steels can be used to predict ductile fracture initiation with large-scale yielding and no initial flaws. High-fidelity finite-element analyses were carried out on ten welded connection specimens between the square steel tube column and H-beam flange under monotonic tensile loading. The calibrated micromechanics-based fracture criteria, including the Stress Modified Critical Strain (SMCS) model and the Void Growth Model (VGM), were used to predict fracture initiation for each specimen. The predicted results demonstrated high accuracy compared to test results. Sensitivity analyses of fracture toughness parameters in the SMCS model and VGM to the predicted fracture results were conducted. The results show that, when the fracture toughness parameters were increased or reduced by twenty percent, fluctuations in the predicted fracture results were within the acceptable range. Therefore, calibrated micromechanics-based fracture criteria can be used to predict ductile fracture initiation of connections under monotonic loading. Subsequently, the user subroutine VUMAT, from ABAQUS software edited by the authors, was employed. Using the SMCS model and VGM as fracture criteria, the post-fracture load–displacement curves of eighteen notched round bar specimens and two welded connection specimens (between the square steel tube column and H-beam flange) that fractured at different locations were traced by deleting the failure elements one by one. The predicted results agree well with the test results. Consequently, these calibrated micromechanics-based fracture criteria and associated simulating techniques can be used for collapse analysis of steel structures during extreme events.

Modeling of Ductile Fracture at Engineering Scales: A Mechanism-Based Approach

ABSTRACTThis paper summarizes the work we conducted in recent years on modeling plastic response of metallic alloys and ductile fracture of engineering components, with the emphasis on the effect of the stress state. It is shown that the classical J2 plasticity theory cannot correctly describe the plasticity behavior of many materials. The experimental and numerical studies of a variety of structural alloys result in a general form of plasticity model for isotropic materials, where the yield function and the flow potential are expressed as functions of the first invariant of the stress tensor and the second and third invariants of the deviatoric stress tensor. Several mechanism-based models have been developed to capture the ductile fracture process of metallic alloys. Two of such models are described in this paper. The first one is a cumulative strain damage model where the damage parameter is dependent on the stress triaxiality and the Lode parameter. The second one is a modification to the Gurson-type porous plasticity models, where two damage parameters, representing void damage and shear damage respectively, are coupled into the yield function and flow potential. These models are shown to be able to predict fracture initiation and propagation in various specimens experiencing a wide range of stress states.

A nonlinear ductile damage growth law

One of the commonly used approaches for the prediction of ductile fracture is the continuum damage mechanics model, in which the void density is represented by a continuum variable called damage. For the prediction of fracture, an evolution law for the void growth, called the damage growth law, is needed. As per the continuum thermodynamics framework, the damage growth law is obtained from some proposed expression for the damage potential. The damage growth laws reported in the literature, based on some proposed expressions for the damage potential, have played a useful role in predicting fracture initiation in some engineering materials. However, some of them do not agree with the experimental trends of damage growth in other engineering materials. In almost all the laws, while expressing the material constants in terms of the measurable quantities in tension test, the triaxiality is assumed constant which does not happen after the necking. In the present work, an expression for the damage potential is proposed that leads to a nonlinear ductile damage growth law for steel. The material constants of the damage growth law are determined from the experimental measurement of void growth as well as triaxiality at various plastic strain levels in tension test conducted at IIT Kanpur. Thus, it avoids making any assumption about the triaxiality. It is shown that this law is consistent with the trends of damage growth of some other experimental results reported in the literature.