Ductile Failure Criterion Research Articles

Quantitative Evaluation of Irradiated Ductility Degradation Using the Indentation Technique Combined with Numerical Experiments

The ductile properties of irradiated materials are among of the important indicators related to their structural integrity. These properties are generally determined by performing tensile tests on irradiated materials in the irradiation environment. Indentation tests are used for evaluating ductile properties easily and rapidly. Constants in the Swift-type material constitutive equation were identified via inverse analysis using the Kalman filter, such that the numerical experimental results reproduced the indentation test results. Numerical tensile experiments were conducted using stress–strain curves with the identified constants to obtain nominal stress and strain curves. The identified yield stress, work hardening coefficient, and exponent were 200–1000 MPa, 1100–1500 Ma, and 0.5–0.7, respectively. Furthermore, two methods were proposed for evaluating the total elongation. Method I was used to calculate the total elongation based on the relationship between the total and uniform elongations obtained from the tensile tests performed on irradiated materials. Method II was used to determine the total elongation from the ductile failure criterion based on the relationship between the stress and strain states in the tensile specimen model using the numerical tensile experiment and failure strain evaluated from actual tensile experiments. Evaluated minimum total elongation was 10%. The evaluation results for ion-irradiated materials were similar to the tensile test results for irradiated materials.

Mechanical and failure behaviors of adhesively bonded dissimilar materials joints incorporating bio-inspired morphological irregularities

Adhesive dissimilar material joints between stainless steel grade 316L and polyethylene terephthalate glycol (PETG) plastic were examined, in which irregular interface morphologies inspired by natural sutures were incorporated. The joint interfaces exhibited various teeth-like, non-repetitive zigzag patterns based on a pseudo-randomization method and sutural interdigitation index. Effects of their irregularities on mechanical and failure characteristics of adhesive butt joints were studied by means of experiments and FE simulations. The cohesive zone model, Drucker-Prager plasticity model and ductile failure criterion were applied for the interface and PETG, respectively. Predicted load-displacement curves of joints with different teeth profiles were well validated. The degree of irregularity became higher to 20 %, 40 %, and 60 %, the joint strengths decreased by 1.47 %, 5.39 %, and 11.95 %, while the total energies to failure increased by 110.74 %, 129.33 %, and 161.23 %, accordingly. More inhomogeneous morphologies led to earlier local damage initiation, but enhanced damage evolution range by impeding abrupt joint separation. Smaller teeth angles provided higher joint strength, whereas significant declines of bonding strength were observed by too acute teeth angle due to excessive stress localization and resulting teeth breakage. The teeth angle of 45° enabled the superior joint performance owing to a proper combination of interlocking and large adhesive force under shear stress state. Finally, a key insight for designing an optimal dissimilar joint with morphological irregularities was provided.

Prediction of micro/meso scale forming limit for metal foils using a simple criterion

Accurately forecasting the forming limit of metal foils is essential in their micro-forming due to the appearance of necking failure easily. In order to accurately and easily forecast the forming limit of metal foils influenced by size effect, an idea for failure modeling of metal foils using the mechanism that different fracture modes of metal foils are related to shear bands is presented in this work. Based on the idea, a simple ductile failure criterion that considers the maximum shear stress and grain size effect but excludes the stress triaxiality is proposed, motivated by the demand to decrease the complexity of parameter calculation tests in micro-forming failure prediction. The proposed criterion and another criterion called as the μ-DF2022 model are applied to capture the forming limit curves of various thick copper and 304 stainless steel foils with different grain sizes to verify its advantage and performance, and the corresponding mechanisms of their prediction capabilities are also elucidated. Furthermore, the proposed and μ-DF2022 models are applied to forecast the limit drawing ratio of a 304 stainless steel foil to further show its performance and advantage in real micro-forming. The applications show that the proposed failure criterion effectively forecasts the forming limit of metal foils between equi-biaxial tension and uniaxial tension influenced by size effect. Moreover, the prediction accuracies using the proposed model and the μ-DF2022 model are comparable, whereas the proposed model does not require complex micro equi-biaxial tension tests, which are typically unavailable to many engineers and researchers, to calibrate its material parameters. Therefore, it is recommended to utilize the proposed criterion to capture the forming limit of metal foils in micro-forming. This work advances the forming limit modeling method of metal foils and provides insight into the application of uncoupled ductile failure criteria in the micro-forming of metal foils, which contributes to the development of their optimal micro-forming processes.

Modeling and simulation of the negative angle cutting with a combined constitutive model for U71Mn rail steel

Due to the increasing train axle load, running speed and traffic volume, rails are prone to severe rolling contact fatigue (RCF) defects. Repairing the rail profile by milling is the latest technology to solve the above hidden problems, but there are few related studies in recent years. In this paper, the milling maintenance process of rails is studied by using finite element method(FEM). This study examines the fracture behavior of U71Mn steel rail under various stress triaxiality conditions, temperatures, and strain rates. An electronic universal testing machine is utilized to conduct the experiments, and a Johnson-Cook (J-C) failure model for U71Mn steel rail is developed. A 3D cutting simulation model was developed using the Johnson-Cook (J-C) combined constitutive model. The accuracy and reliability of the numerical simulation model were confirmed by comparing the cutting force and chip morphology obtained from both simulation and experimental results. The cutting simulation model had an error rate of within 10 % when predicting the cutting force. Following that, the fracture zone and chip portion of the tensile specimen were metallographically analyzed. The applicability of the ductile failure criterion in describing the observed failure in the U71Mn rail has been proven.

Cortical and Trabecular Bone Stress Assessment during Periodontal Breakdown-A Comparative Finite Element Analysis of Multiple Failure Criteria.

Background and Objectives: This numerical analysis investigated the biomechanical behavior of the mandibular bone as a structure subjected to 0.5 N of orthodontic force during periodontal breakdown. Additionally, the suitability of the five most used failure criteria (Von Mises (VM), Tresca (T), maximum principal (S1), minimum principal (S3), and hydrostatic pressure (HP)) for the study of bone was assessed, and a single criterion was identified for the study of teeth and the surrounding periodontium (by performing correlations with other FEA studies). Materials and Methods: The finite element analysis (FEA) employed 405 simulations over eighty-one mandibular models with variable levels of bone loss (0-8 mm) and five orthodontic movements (intrusion, extrusion, tipping, rotation, and translation). For the numerical analysis of bone, the ductile failure criteria are suitable (T and VM are adequate for the study of bone), with Tresca being more suited. S1, S3, and HP criteria, due to their distinctive design dedicated to brittle materials and liquids/gas, only occasionally correctly described the bone stress distribution. Results: Only T and VM displayed a coherent and correlated gradual stress increase pattern for all five movements and levels of the periodontal breakdown. The quantitative values provided by T and VM were the highest (for each movement and level of bone loss) among all five criteria. The MHP (maximum physiological hydrostatic pressure) was exceeded in all simulations since the mandibular bone is anatomically less vascularized, and the ischemic risks are reduced. Only T and VM displayed a correlated (both qualitative and quantitative) stress increase for all five movements. Both T and VM displayed rotation and translation, closely followed by tipping, as stressful movements, while intrusion and extrusion were less stressful for the mandibular bone. Conclusions: Based on correlations with earlier numerical studies on the same models and boundary conditions, T seems better suited as a single unitary failure criterion for the study of teeth and the surrounding periodontium.

Ductile failure of Inconel 718 during flow forming process and its numerical investigation

Understanding the fracture mechanisms and predicting failure in forming processes is of great interest to increase formability and numerical methods have been proven effective. Incremental forming processes such as flow forming possess challenges due to material experiencing complex stress states and highly non-linear deformation histories during forming. Although some of the ductile failure criteria integrated into finite element (FE) simulations are found to be in better correspondence with the experimental findings, they mostly lack sufficient accuracy in predicting formability limits for flow forming and spinning processes. In this context, this work is concerned with the analysis and prediction of ductile failure of nickel-based super alloy Inconel 718 (IN718) in the flow forming process through FE analysis. Failure is modeled with three different models, namely Cockcroft–Latham (CL), Johnson–Cook (JC) and modified Mohr–Coulomb (MMC), implemented as a user subroutine in explicit FE solver Abaqus. Tensile tests are conducted with four different specimen geometries in order to identify parameters of the plasticity and damage models. Then, the developed framework is applied to a FE simulation of the flow forming process to predict formability limits and perform process parameter optimization to improve formability. Flow forming tests conducted at three thickness reduction values reveal that the forming limit of the material is at about 50% thickness reduction where radial cracks are observed on the outer surface. It is found that the CL model is able to predict formability limits and crack initiation locations while the MMC and JC models failed in both with significant over prediction of damage. Furthermore, the effect of temperature and process parameters such as the ratio of feed rate to revolution speed and roller axial and radial offsets are further examined with the developed FE frameworks. It has been shown that process parameter optimization requires accurate modeling of ductile damage since the behavior of different models is diverse.

Effect of fluid pressure on adhesive wear of spherical contact

Lubrication with fluid between two rough surfaces has been extensively studied, while the present work provides an additional principle for its effect on adhesive wear reduction associated with the fluid pressure. In this work, a finite element model for elastoplastic spherical contact with fluid pressure applied on surface is established, considering the realistic ductile failure criteria. It is found that as the fluid pressure increases, the location of fracture nucleation in the front contact region moves towards the contact surface. Two wear modes, namely the detaching and scratching modes, are identified by defining a critical dimensionless fluid pressure. Accordingly, the wear coefficient decreases as the fluid pressure increases, serving as the additional principle for wear reduction in lubrication systems.

A new ductile failure criterion with stress triaxiality and Lode dependence

An accurate failure criterion is crucial for mechanical and structural design. In this paper, the effects of three stress invariants on material failure are studied based on microscopic mechanisms. A new ductile failure criterion that accounts for the influence of stress triaxiality and normalized third invariant of deviatoric stress tensor is proposed. The parameters in the new failure criterion are analyzed to determine their effects on the failure loci. The new proposed failure criterion is calibrated with a series of test results on 2024-T351 aluminum alloy, TRIP690 steel and additively manufactured Ti-6Al-4V. The results demonstrate that the new criterion can predict the failure loci of these materials with high accuracy. Comparisons with three criteria indicate that the new failure criterion has a broader application than Lou-Huh criterion and extended Mohr–Coulomb criterion for the above three materials.

Predicting ductile failure of aluminum components under general loading conditions: Computational implementation, model verification and experimental validation

Predicting ductile failure in engineering structural components has been an active research topic for decades. Although the micro-mechanical phenomena that govern ductile failure are well understood, and many failure criteria proposed, a leading criterion has yet to be agreed upon. In the current study, four different ductile failure criteria were implemented in the finite element framework and used to study the ductile failure process of Al2024 and Al7075 specimens. The failure criteria included stress based, energy based and strain based formulations. Several different sets of experiments were first used to determine the different model parameters required to predict failure initiation. Then these same parameters were used to predict failure initiation for a different set of experiments with different geometries and under a complex stress and strain state. A critical analysis of the performance of the different ductile failure criteria was then conducted. It is shown that, while the stress based criteria fail to provide accurate predictions for the general loading case, the energy based and strain based criteria perform well and predict both the failure load and crack location.

Modeling contact of Au-coated sphere with rigid flat: Electrical contact resistance, adhesive wear and friction

The electrically conductive coating is an effective means to reduce electrical contact resistance (ECR) of sliding electrical contact, however, the wear resistance of the coated surface influences its reliability and lifetime. The electrical contact performance of coated systems is assessed in this work using the Au-coating/Cu-substrate system. An elastic-plastic coated spherical contact is established with the realistic ductile failure criterion to allow material damage, and the effects of coating thickness and normal loading are investigated. As coating thickness increases, it is found that: the ECR decreases and then increases, presenting minimum value at a thickness independent of normal loading; the static friction coefficient increases which is related to the locations of damage occurrence at sliding inception; the wear coefficient increases to a peak and then decreases. Two wear modes of coated sliding electrical contact are observed, namely debonding mode and detaching mode, where wear particle forms by coating debonding and fracture inside coating, respectively. A critical dimensionless coating thickness, which is the minimum thickness able to protect the coating from debonding, is proposed.

A new ductile failure criterion for micro/meso scale forming limit prediction of metal foils considering size effect and free surface roughening

For the establishment of reliable micro/meso scale forming processes, it is essential to accurately predict the forming limits of metal foils under different deformation paths. However, existing uncoupled ductile failure criteria cannot accurately forecast the micro/meso scale forming limits of metal foils affected by size effect in the whole strain path range of forming limit curve (FLC). To deal with the issue, a new ductile failure criterion is proposed in this paper based on the theoretical analyses of size effect affected void nucleation, growth, and coalescence, as well as free surface roughening affected critical damage of metal foils. A method for identifying the material parameters of the proposed ductile failure criterion coupled with a modified Yld2000 yield criterion is provided, and the size effect on micro/meso scale FLCs forecasted by the proposed ductile failure criterion is discussed. In addition, the forming limit strains of SUS304 and pure copper foils with different thicknesses and grain sizes are employed to reveal the prediction capability of the new ductile failure criterion. Comparisons between the experimental results and the FLCs constructed by the proposed failure criterion demonstrate that the proposed failure criterion can accurately predict the forming limits of metal foils affected by size effect between uniaxial tension (UT) and equi-biaxial tension (EBT). To further show the prediction capability of the proposed failure criterion in actual micro/meso scale forming processes, the limit drawing ratio of a SUS304 foil and local maximum forming heights in micro-channel hydroforming for pure copper foils are predicted using the new failure criterion combined with the finite element method, and the predicted results are compared with experimental results. Comparative research demonstrates that the proposed ductile failure criterion can accurately forecast the forming limits of metal foils affected by size effect in actual micro/meso scale forming processes.

Plasticity-induced damage and material loss in oscillatory contacts

Knowledge of plasticity-induced damage leading to the loss of material in oscillatory contacts is of paramount importance to various electromechanical systems comprising contact-mode elements exposed to high-frequency vibrations. However, experimental investigation of the wear behavior of devices experiencing microscopic oscillatory contact (fretting) is complex, time consuming, and expensive. More importantly, the progression of critical damage processes, such as the decrease of the material’s strength with the accumulation of plastic deformation in the vicinity of the contact interface and the removal of material in the form of microscopic wear debris, cannot be tracked in real time, necessitating cumbersome and costly post-testing microanalysis. Alternatively, computational wear modeling is more effective than experiments and can provide valuable insight into the effect of important parameters, such as load, coefficient of friction, oscillation amplitude, and material behavior, on the loss of material during oscillatory sliding contact. Accordingly, the principal objective of this study was to introduce a computational approach, which can be used to analyze the loss of material due to plasticity-induced damage in oscillatory mechanical components. To achieve this aim, a plane-strain finite element model of a rigid cylinder in reciprocating sliding contact with an elastic-plastic half-space exhibiting isotropic strain hardening was used to study how the evolution of damage due to the progression of plasticity leads to the loss of material. A quasi-static, isothermal damage model based on a ductile failure criterion was implemented in the finite element analysis to simulate the removal of the fully damaged elements. Numerical results illuminate the effects of the load and coefficient of friction on the development of plasticity, cumulative damage, and loss of material with accruing oscillation cycles. The deviation of the wear behavior from classical wear theory in the high-load simulations is explained by the plastic shear strain distribution and less slip at the contact interface encountered at high loads. The novelty of this study is the development of a computational methodology, which sheds light into the evolution of plasticity, damage, and loss of material in reciprocating sliding contacts and provides an effective computational capability for assessing the effects of load, friction, and material behavior on the mechanical performance of components operating in oscillatory contact mode.

Ballistic Impact of Structural Steels at Low Temperatures

Abstract Steels are usually stronger at low temperatures than at high temperatures. But low temperatures are, particularly in combination with high strain rates and high stress triaxiality ratios, known to cause embrittlement. The common understanding is that the ductility of steels decreases dramatically below a threshold temperature known as the ductile-to-brittle transition temperature. This study explores the ballistic performance of Strenx 960 Plus steel plates at both low temperatures and room temperature. We describe a ballistic setup where target plates were cooled down to as low as −60 °C before we present results from ballistic impact tests with three different projectile types. The ballistic limit velocities from tests at low temperatures were higher than the ballistic limit velocities from tests at room temperature, indicating that brittle fracture does not take place. An analytical approach based on the Johnson–Cook constitutive relation, the Cockcroft–Latham ductile failure criterion, and a simple brittle fracture criterion is presented. The model suggests that ductile fracture prevails for most realistic material state histories, both in the ballistic impact tests as well as for quasi-static and dynamic tensile tests. This supports previous observations that brittle fracture is unlikely to occur in modern steels even when subjected to rapid loading and low temperatures.

Solution to problems caused by associated non-quadratic yield functions with respect to the ductile fracture

Accurate material behaviour and its response to loading are needed for a reliable calibration of ductile failure criteria. Non-quadratic yield functions are often necessary for such a description. Nevertheless, it leads to the prediction of stress states that are inconsistent with the expected behaviour and deformations that are in contradiction with the experiments when the associated flow rule is adopted. These problems may be solved by applying a less restrictive non-associated flow rule. Therefore, an isotropic non-associated non-quadratic phenomenological plasticity model was developed as a function of the second and third invariants of deviatoric stress tensor. The yield function that is convex was proposed to allow for different yield stresses in tension, compression and shear. For simplicity, the same function was used for the plastic potential. Each of these functions is described by seven material parameters. The scheme of the so-called next increment corrects error method was adopted for the implementation of the proposed model within finite elements. Above that, three additional plasticity models were calibrated for 2024-T351 aluminium alloy. It is shown how the non-associated flow rule resolves the above-mentioned problems. Moreover, the possible extensions of the proposed yield function are described to include the hydrostatic stress dependence.

Topologically optimal design and failure prediction using conditional generative adversarial networks

AbstractAmong the various structural optimization tools, topology optimization is the widely used technique in obtaining the initial design of structural components. The resulting topologically optimal initial design will be the input for subsequent structural optimizations such as shape, size and layout optimizations. However, iterative solvers used in conventional topology optimization schemes are known to be computationally expensive, thus act as a bottleneck in the manufacturing process. In this paper, a novel deep learning‐based accelerated topology optimization technique with the ability to predict ductile material failure is presented. A Conditional Generative Adversarial Network (cGAN) coupled with a Convolutional Neural Network (CNN) is used to predict the optimal topology of a given structure subject to a set of input variables. Subsequently, the same cGAN is trained to predict the Von‐Mises stress contours on the optimal structure by means of color transformed image‐to‐image translations. The ductile failure criterion is evaluated by comparing the cGAN predicted maximum Von‐Mises stress with the yield strength of the material. The proposed novel numerical method is proven to arrive at the topologically optimal design, accompanying the material failure decision within a negligible amount of time but also maintaining a higher prediction accuracy.

Failure probability assessment of SNF cladding transverse tearing under a hypothetical transportation accident

This paper presents a structural assessment of spent nuclear fuel (SNF) rod under drop accident of a transport cask using deterministic and probabilistic approaches. The performed case study is a baseline 9-m side-drop scenario during the first 30 years of dry storage. A hypothetical database is generated for five variables related to operating conditions to predict the changes in the mechanical properties of SNF rod using Monte Carlo Simulation and material constitutive models. Finite-element analyses are conducted including a sophisticated transport cask with simplified fuel assemblies to obtain the acceleration response and a detailed full-length single fuel rod to evaluate the deformation response of fuel cladding with various representative material properties. Failure probabilities of a fuel rod subjected to a transverse inertia loading due to side-drop are estimated based on ductile tearing failure criteria. The findings from the dynamic response analysis show that the cladding might yield owing to the 9-m side-drop, but the structural integrity of SNF rod is generally maintained. Yet the probabilistic assessment highlights the importance of conducting further structural integrity evaluation for high burnup fuel under hypothetical accidents as it initially shows concerned failure probabilities comparing to low and intermediate burnup fuel. The modeling technique and analysis methodology outlined in this study have a potential advantage to consider several geometric and material characteristics of SNF rod, hence improving the structural integrity assessment of SNF further.

Measurement of Local Material Properties and Failure Analysis of Resistance Spot Welds of Advanced High-Strength Steel Sheets

afety evaluation of resistance spot welds necessitates the accurate measurement of local constitutive properties. This study employed miniature mechanical tests to investigate the deformation and failure behaviors of nugget, heat affected zone (HAZ), and corona bond of resistance spot welded JSC980YL steel. A novel mini-peel test was developed to enable local fracture in HAZ for numerical inverse calibration of constitutive parameters. The fracture constants of weld zones calibrated using Cockcroft-Latham ductile failure criterion were incorporated in finite element models to predict the failure modes of spot welds in tensile-shear and cross-tension coupon tests. The result indicates that the ultimate tensile strengths of the nugget and the corona bond were 37.6% higher and 5.8 % lower, respectively, than that of the base material. The nugget and HAZ exhibited ductile fracture, whereas the corona bond was brittle fracture with only 1.2% elongation. In the coupon tests, the increase of nugget diameter slowed down the damage accumulation rate in the nugget and accelerated that in the HAZ, resulting in the transition of failure mode from interfacial to pullout. The failure load of corona bond in coupon tests increased with the increase of nugget diameter while its contribution to the peak load decreased.

Numerical study of chip formation and cutting force in high-speed machining of Ti-6Al-4V bases on finite element modeling with ductile fracture criterion

This paper suggests a novel numerical model to accurately simulate the chip formation for a wide range of high cutting speeds. It consists of finite element (FE) modeling of orthogonal machining of titanium alloy (Ti-6Al-4V) in which the Johnson–Cook (JC) material law which can reflect the strain rate hardening and thermal softening influences, and the JC damage law coupled with the displacement-based ductile failure criterion are implemented during the chip formation. Orthogonal machining simulations are performed in a conventional high cutting speed range of 170 to 250 m/min and at the extreme high cutting speeds ranging from 1200 to 4800 m/min, and saw-tooth chips are occurred. The development of chip serration and cutting force are analyzed. It is found that saw-tooth chip formation in high-speed machining of Ti-6Al-4V is the result of ductile fracture. When the cutting speed is increased from conventional to extreme high speeds, the chip morphology changes with varying the fracture behavior. The numerical model is also verified by comparing predicted results with available experimental data in the literature. The results indicate that chip morphology and cutting force can be accurately acquired using the ductile failure criterion in high-speed machining of Ti-6Al-4V.

Ductile failure prediction of pipe-ring notched AISI 316L using uncoupled ductile failure criteria

The goal of this work is to predict ductile failure of pipe-ring notched AISI 316L specimens, where notches mimic the geometry of corrosion defects. Uncoupled damage models are used to that end. The Johnson-Cook and Lou-Huh criteria are calibrated.The uncoupled damage models are calibrated using experimental results from testing pipe-ring notched specimens. Calibration was achieved using a hybrid experimental-numerical approach. Six notch shapes are studied. Experimental matrices are designed to determine these shapes, and the pipe-ring notched specimen is inspired from the literature.Calibration of the uncoupled damage models require a ductile crack initiation indicator. The first indicator was based on the derivative curves of the force versus the connectors displacement curve. The second was based on the raw images of the gage section. The third was based on a percentage of the connectors displacement at fracture.Results show that ductile fracture depends on the Lode paramater. As a consequence, the Lou-Huh criterion is more effective at predicting ductile fracture than the Johnson-Cook criterion.

Measurement of local material properties and failure analysis of resistance spot welds of advanced high-strength steel sheets

Safety evaluation of resistance spot welds necessitates the accurate measurement of local constitutive properties. This study employed miniature mechanical tests to investigate the deformation and failure behaviors of nugget, heat affected zone (HAZ), and corona bond of resistance spot welded JSC980YL steel. A novel mini-peel test was developed to enable local fracture in HAZ for numerical inverse calibration of constitutive parameters. The fracture constants of weld zones calibrated using Cockcroft-Latham ductile failure criterion were incorporated in finite element models to predict the failure modes of spot welds in tensile-shear and cross-tension coupon tests. The result indicates that the ultimate tensile strengths of the nugget and the corona bond were 37.6% higher and 5.8% lower, respectively, than that of the base material. The nugget and HAZ exhibited ductile fracture, whereas the corona bond was brittle fracture with only 1.2% elongation. In the coupon tests, the increase of nugget diameter slowed down the damage accumulation rate in the nugget and accelerated that in the HAZ, resulting in the transition of failure mode from interfacial to pullout. The failure load of corona bond in coupon tests increased with the increase of nugget diameter while its contribution to the peak load decreased.