Ductile Fracture Research Articles

Largely enhanced damping performance of nitrile rubber/ethylene–vinyl acetate blends via phenolic resin-induced phase separation

The low glass transition temperature (Tg) and narrow effective damping temperature range (EDTR) greatly restrict the application of common vulcanized rubbers in fields relating to high temperature environmental condition due to the greatly reduced damping performances. In this work, phenolic resin (PR) was incorporated into nitrile rubber/ethylene–vinyl acetate (NBR/EVM) blends through melt-compounding processing, and the microstructure and damping performance changes induced by PR were comparatively investigated. The results show that PR induces the phase separation of the blends and most of PR particles selectively locate in the NBR component. Molecular dynamics simulation confirms that more hydrogen bonds form in the ternary blends, and reduced free volume fraction is also achieved. The ternary blends show good high-temperature damping performances, and apparently enhanced Tg (up to 17.7 °C) and EDTR (82.6 °C, from −2.6 to 80 °C) are also achieved. Additionally, the ternary blends exhibit good mechanical properties, including higher ductility and larger fracture energy, and better energy dissipation ability. This work provides an alternative resolution way to enhance the damping performances of the common vulcanized rubbers at high temperatures.

Disclosing Connection Links between Microstructure and Mechanical Performance in Pulsating Current Gas Tungsten Arc Welding of Hastelloy B-2 Superalloy

In this study, welding a Ni-Mo-based superalloy (Hastelloy B-2) was examined in order to characterize the microstructure and mechanical performance of joints along with assessing the effects of current intensity on the microstructure and mechanical responses of different weld zones. The gas tungsten arc welding (GTAW) process was used to weld the samples using ERNiCrMo-2 filler metal. The pulsed current GTAW process was used to weld the superalloy sheets of thickness of 1 mm with background current (Ib) of 20 A and 40 A and peak current (Ip) of 80 A and 60 A. Tensile and Vickers microhardness tests were conducted to evaluate the effect of pulsed current on mechanical properties of the welds along with chemistry and microstructure characterizations. Finally, the fracture surfaces after the tensile test were studied using SEM fractography analysis. The results indicated that increasing Ib and decreasing Ip led to low heat input and high cooling rate resulting in a high thermal gradient. This caused microstructure transition from the columnar dendrites to the coaxial ones in the weld zone; molten metal convection in the fusion zone led to fine grains in the weld zone during welding time. Moreover, a significant decrease in the amount of molybdenum carbides at the interdendritic regions of the weld metal was observed under these conditions. The tensile strength of the weld metal was higher than that of the base metal resulting in the fracture of all welds from the base metal. Additionally, the microhardness results indicated a significant increase for the weld metal compared to both heat-affected zone (HAZ) and base metal. The higher mechanical properties of the weld metal is attributed to the increase in background current and decrease in peak current leading to a fine grain microstructure. Fractography following the tensile test showed a completely ductile fracture.

Comparative study of dynamic cyclic fatigue resistance of two files with different kinematics

Objectives: The aim of the study was to determine the dynamic cyclic fatigue resistance (DCFR) of R-motion file and Protaper Next file (PTN) by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Methods: R-motion file (25/.06), PTN file (25/.06) were used and divided into 2 groups (n = 28) according to the file type. DCFR was performed using an artificial canal. Files were allowed to work until separation. The numbers of cycles to failure (NCF) and the time to fracture (TTF) were calculated. The files were subjected to DSC and SEM. Results: R-motion files had higher NCF and TTF compared to PTN files. SEM examination showed that all files showed ductile fracture. DSC of the R-motion file displayed Austenitic phase at body temperature) 37 ˚C) while PTN file failed to demonstrate any peaks within the applied heat range. Conclusions: R-motion files had higher CFR and TTF than PTN files. Conclusions: R-motion files had higher CFR and TTF than PTN files.

Microstructural Insights and Composition Analysis of a Fractured Brass Alloy of Unknown Composition Using Advanced Characterization Techniques

This study aims to investigate the microstructural and compositional characteristics of a fractured brass alloy using energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The brass alloy, initially of unknown composition, was determined to consist of 63.65 at% Cu, and 36.35 at% Zn through EDS analysis. Microstructural analysis via SEM revealed a typical ductile fracture pattern with necking and dimple formation, indicative of significant plastic deformation prior to failure. The alloy exhibited a grain size ranging from 10 µm to 80 µm and displayed both α-phase (FCC structure) and possible traces of β-phase, though XRD detected only the α-phase due to the low concentration of β-phase (<5%). A comparison between EDS and XRD results for analyzing chemical composition showed that good assumptions could be made from the lattice parameters of the binary alloy following Nelson-Relay function to predict an approximate composition in binary alloy systems, which was found to be 34 at% Zn in our analysis. Furthermore, the sample’s crystal structure was confirmed as FCC, which supported its observed ductility and plastic deformation prior to fracture. The SEM analysis also confirmed that the material went through an annealing process. The fracture morphology further showed that the FCC structure and the annealing process both contributed to the material’s increased ductility. The analysis further confirms that EDS is more reliable for chemical composition determination in an alloy system. Evidence of annealing twinning in the SEM images suggests the sample underwent an annealing process, contributing to its enhanced ductility.

Test-and-FE-based method for obtaining complete stress-strain curves of structural steels including fracture

Ductile fracture is a common limit state of frameworks of steel structures and can be predicted by incorporating models for fracture initiation and propagation in finite element (FE) analyses. However, accurate prediction of fracture relies on unique fracture parameters for a given material, requiring precise predictions of fracture strain and stress states obtained through coupon tests and accompanying FE data analysis. In recent decades, various methods have been proposed to predict ductile fracture initiation, including various design geometries of coupons, test setups and FE analyses, which may lead to inconsistent fracture strains and stress states. Hence, there is a need for proposing a standard procedure for the design, test and accompanying FE-based calibration of fracture parameters, as pursued in this paper. Given that ductile fracture initiates under significant plastic strain, a constitutive model for the full strain range is required, as also proposed in this paper. Moreover, to reduce the experimental and data analytical efforts, an empirical method is proposed that uses only a small number of coupon tests to calibrate the fracture parameters. The method encompasses three levels wherein one, two, or three coupon tests are required. All in all, the paper presents a methodology for determining the full-range true stress-strain curve and the initiation of fracture, including a new post-necking constitutive model and methods for determining the free parameters of the post-necking and fracture initiation models. While applied to steel in this paper, the proposed methodology is potentially also applicable to other metals.

Extracting ductile cast iron microstructure parameters from fracture surfaces: A deep learning based instance segmentation approach

This study investigates the deep-learning based microstructural analysis from SEM images of ductile cast iron fracture surfaces. A Mask R-CNN model was trained, achieving 70% precision and 75% recall in graphite particle detection. Combined with a fracture surface reconstruction using the.4-quadrant backscattered electron signal, key parameters, including the particle size, shape and distance were extracted accurately. Compared to micrograph analysis, following probabilistic simulations showed the impact of the higher microstructural variance for the fracture surfaces on crack initiation, leading to higher scatter and elevated crack resistance curves. This highlights the potential of deep-learning based analysis for comprehensive microstructural characterization.

Causes and analysis of failure of bearing cage at non-driven end of wind turbine generator

The non-driving end bearing cage of a wind turbine generator experienced a fracture and subsequent failure. In order to understand the reasons behind this failure, various analyses were conducted including examination of the metallographic structure, mechanical properties, fracture surface morphology, bearing vibration, and operating temperature of the cage. Through this analysis, the fracture process of the cage was deduced. The findings indicate that the fracture of the bearing cage was directly caused by the abnormal installation of the spacer. The eccentricity of the axial component resulted in ductile fracture, while the eccentricity of the radial component led to brittle fractures.

Assessment of the impact behavior of CLF-1 steel with Charpy V-notch testing and miniature Charpy V-notch testing

China Low-activation Ferrite steel (CLF-1), as one of the structural materials developed in China for fusion reactors, is a Reduced activation Ferritic/Martensitic (RAFM) steel modeled after the traditional T91 steel (9Cr-1Mo-0.2V-0.08Nb). For fusion reactor materials, the Charpy impact testing is an important method to assess their notch sensitivity. In this study, Charpy impact tests were conducted on CLF-1 steel using Charpy V-notch specimens (CVN) and Miniature Charpy V-notch specimens (Kleinstprobe, KLST) to assess the impact behavior of CLF-1 steel, and data normalization was performed on the KLST specimen. The results show that as the specimen size of CLF-1 steel decreased, the curves of impact absorbed energy, lateral expansion, and shear fracture appearance shifted towards lower temperatures, and the impact absorbed energy significantly decreased. Both the macroscopic and microscopic characteristics of the specimens obtained from the experiments indicated that KLST specimens could achieve the same fracture characteristics as CVN specimens. Additionally, at the same experimental temperature, KLST specimens exhibited a higher proportion of ductile fracture regions.

Microstructure and mechanical properties of thin-walled TA1 titanium pipes fabricated by high-frequency induction welding

High frequency induction welding (HFIW) was effectively employed for the high-speed fabrication of thin-walled TA1 titanium pipes (TWTPs) with a nominal wall thickness of ∼0.6 mm. The microstructure and mechanical properties of TWTPs manufactured under varying welding parameters were investigated. The weld zone (WZ) exhibits a waist shape measuring ∼622 μm in width, and the width of the heat-affected zone (HAZ) spans between 763 and 864 μm. Both the WZ and HAZ are composed of a mixture of coarse serrated α grains with fine acicular and twins, while the BM retains equiaxed grains. This unique microstructure was resulted from the thermal cycling during HFIW, contributing to a notable increase in microhardness within the WZ compared to both the HAZ and the BM. Optimal manufacturing conditions were identified at a welding power of 14.4 kW, a welding speed of 60 m/min, an opening angle of 6°, and a squeeze displacement of 0.2 mm, yielding the TWIP with a tensile strength of ∼307 MPa and tensile elongation of ∼27%. Tensile fracture analysis revealed that failure predominantly occurred within the BM, underlining a ductile fracture mode characterized by pronounced dimple formations. The enhanced mechanical performance of the weld joints can be attributed to the heterogenous microstructure in the WZ, where the presence of large serrated α-grains enhances ductility, and the fine martensite and twins contribute to the high strength.

Phase Field Coupled Finite Deformation Plasticity Formulation of Ductile Fracture With Nonlinear Kinematic Hardening and Modified Energy Release Function

ABSTRACTA ductile damage theory is presented by coupling the covariant formulation of finite deformation plasticity with the phase field modeling of fracture, including kinematic hardening for the ductile response of the materials. A phase field coupled nonlinear kinematic hardening equation is proposed in the reference configuration having equivalent representation through the Lie derivative of the kinematic hardening tensor by push‐forward operation in the spatial configuration, thus ensuring the satisfaction of frame invariance. To capture the correct physical response of the material by the phase field evolution equation, the fracture driving function, that is, the difference between the sum of elastic and plastic energies and threshold energy, after the damage initiation is modified by an energy release controlling function, which is an empirical relation of equivalent plastic strain. In defining the energy release controlling function, well‐defined points with physical significance in the experimental load versus displacement curve are used. To simulate the response of the material in relatively large time steps, a modified staggered scheme is presented, evaluating the fracture driving and energy release controlling functions from the previous converged step and using the updated phase field variable in the weak form of the momentum balance equation. To quantify different material parameters from available experimental results in the literature, the developed phase field coupled elasto‐plastic model uses a neural network optimization procedure consisting of neural network training together with optimization in MATLAB and finite element model evaluation in Abaqus user element subroutine UEL. Model capabilities are demonstrated by simulating the crack propagation in complex 3D geometries such as the second and third Sandia Fracture Challenges.

Optimizing Transformation-Induced Plasticity to Resist Microvoid Softening

AbstractMany high-performance steels that are critical for energy-efficient, lightweight designs rely on transformation-induced plasticity (TRIP) to achieve superior combinations of strength and ductility/toughness. Further development of these alloys will require greater optimization of the metastable (retained) austenite phase responsible for TRIP. Considering the complex nature of TRIP and its effects on ductile fracture, an integrated computational materials engineering (ICME) approach to materials optimization is desired. In this work, we report the results of a large series of micromechanical finite element calculations that probe the interaction of TRIP and void-mediated ductile fracture mechanisms. The simulations identify the optimal austenite stability for maximizing the benefit of TRIP across a wide range of stress states. The applied stress triaxiality significantly influences the microvoid growth rate and the computationally determined optimal stability. The simulation results are compared with existing experimental data, demonstrating good agreement.

Enhanced Durability of Wood Cutting Tools through Thermal Cycling.

This study investigates the impact of multi-step austenitization heat treatment on the in-service life of modified AISI A8 cold work tool steel knives used in wood cutting. The knives were subjected to two treatment methods: single quenching and double tempering (SQDT) and double quenching and double tempering (DQDT). Both treatments were followed by physical vapor deposition (PVD) coating to enhance surface properties. The DQDT treatment resulted in a finer microstructure and more uniform carbide distribution. Field tests on 24 knives over 124 h demonstrated up to 130% improvement in wear resistance for DQDT knives, along with superior edge stability and better PVD coating preservation. DQDT knives exhibited ductile fractures characterized by dimples, contrasting with the brittle fracture and cleavage facets in SQDT knives. Residual stress measurements showed higher compressive stresses in DQDT knives (-280 MPa) compared to SQDT knives (-30 MPa), which increased further after field testing. The enhanced performance of DQDT knives is attributed to their refined microstructure, improved carbide distribution, and higher compressive residual stresses, offering significant potential for improving wood cutting tool efficiency and durability.

Influence of heat treatment on metallurgical and mechanical properties of aluminium Al6061 hybrid metal matrix composites

Abstract The current investigation explores the metallurgical and mechanical properties of hybrid metal matrix composites (HMMC) with aluminum 6061 as the base material, reinforced with ceramic of tungsten carbide (WC) nanoparticles, graphite (as a solid lubricant) and redmud (RM). Four different composites with tungsten carbide nanoparticles (1.5 wt.%) and graphite (2 wt.%) with a varying proportions of redmud (0, 1, 3 and 5 wt.%) are fabricated by stir casting. Microscopic analysis reveals grain boundary with segregations, some of which dissolve to form discontinuous grain boundaries after heat treatment. FESEM examination shows the formation of intermetallic, some of which dissolve on heat treatment to form precipitate that are distributed evenly along the grain boundary and also inside grains. EDX mapping further validated the uniform distribution of reinforcements without any agglomeration. The addition of reinforcements and heat treatment (T6) resulted in substantial improvements in hardness, ultimate tensile strength, yield strength, and compressive strength compared to unreinforced Al6061 and untreated HMMC, respectively. Fractographic analysis of tensile tested samples reveals the presence of dimples, tearing edges and small voids, indicating ductile fracture.

An alternative finite element formulation to predict ductile fracture in highly deformable materials

Abstract An alternative finite element formulation to predict ductile damage and fracture in highly deformable materials is presented. For this purpose, a finite-strain elastoplastic model based on the Gurson-Tvergaard-Needleman (GTN) formulation is employed, in which the level of damage is described by the void volume fraction (or porosity). The model accounts for large strains, associative plasticity and isotropic hardening, as well as void nucleation, coalescence and material failure. To avoid severe damage localization, a nonlocal enrichment is adopted, resulting in a mixed finite element whose degrees of freedom are the current positions and nonlocal porosity at the nodes. In this work, 2D triangular elements of linear-order and plane-stress conditions are used. Two systems of equations have to be solved: the global variables system, involving the degrees of freedom; and the internal variables system, including the damage and plastic variables. To this end, a new numerical strategy has been developed, in which the change in material stiffness due to the evolution of internal variables is embedded in the consistent tangent operator regarding the global system. The performance of the proposed formulation is assessed by three numerical examples involving large elastoplastic strains and ductile fracture. Results confirm that the present formulation is capable of reproducing fracture initiation and evolution, as well as necking instability. Convergence analysis is also performed in order to evaluate the effect of mesh refinement on the mechanical response. In addition, it is demonstrated that the nonlocal parameter alleviates damage localization, providing smoother porosity fields.

First-principles study on the failure and doping strengthening mechanisms at the interface of high-alumina ferritic heat-resistant steel/oxide film

This study employs first-principles calculations based on density functional theory to explore the failure mechanisms and enhancement strategies for the interface between high-aluminum ferritic heat-resistant steel and oxide film (α-Fe/Al₂O₃ interface). Establishing an interface model at the lowest-energy densely packed plane and performing tensile tests determined that fracture behavior manifests as ductile fracture at the interface. Further analysis of the charge density and differential charge density maps for various doped models confirmed the feasibility of strengthening the α-Fe/Al₂O₃ interface structure through alloy element doping. The results demonstrate that effective dopant elements (such as V, Mn, and Mo) can induce more electrons to transfer to the interface, increase electron concentration, and alter the chemical bonding nature of the interface, thereby significantly enhancing its bonding performance. This study proposes a method to enhance the interface of high-aluminum ferritic heat-resistant steel through doping, providing a theoretical foundation for further research on similar heat-resistant steel surfaces with Al₂O₃ films.

Design of flat hole-clinching for joining polymer and metal sheets

There is a growing need for joining processes capable of joining dissimilar materials, specifically lightweight materials. In this study, flat hole-clinching (FHC) is developed for joining polycarbonate and AZ31 magnesium alloy sheets. A ductile fracture criterion is calibrated using tensile tests and employed in the finite element analysis of the process to investigate the effect of geometric parameters on the joint characteristics and strength. A tool concept is also proposed for the FHC process. Results show that the FHC process enables to successfully connect these sheets without fracture. The flat hole-clinched joint resisted the maximum force of 0.9 kN in the pull-out test, and failed with bottom separation mode.

The effects of tool rotational speed and post-weld heat treatment of friction stir welded AZ31-B magnesium alloy

Abstract This research article examines the effect of increasing the tool rotational speed and post-weld heat treatment behavior of friction stir-welded AZ31-B magnesium alloy. The variable tool rotational speeds of 1000 rpm, 1200 rpm, 1400 rpm, and 1600 rpm with fixed tool traverse speeds of 30 mm min−1 were chosen based on the trial welding method. Results reveal that the as-welded tensile strength values of 133 MPa, 209 MPa, 215 MPa, and 213 MPa for the corresponding tool rotation speeds and post-weld heat-treated tensile strength values of 140 MPa, 213 MPa, 223 MPa, and 227 MPa significantly increased joint strength when compared to the as-welded process. The measured grain size values are reduced when the tool rotational speed increases due to material dynamic recrystallization. Al-Mg brittle intermetallic compounds significantly reduced the mechanical strength during the increase of the tool’s rotational speed. Fractography analysis reveals all welded samples fractured at the center of the stir zone with a ductile fracture angle of 45°.

Effect of torsional angle on the fretting wear behavior and fracture failure mechanism of steel wires with spiral contact structure

The fretting wear of steel wire will reduce the service life of wire rope and severely threaten the safety of mine hoisting. To investigate the influence of torsion on the fretting wear behavior of steel wire, the fretting wear tests of steel wires with spiral contact structure under the action of tensile-torsional coupling forces were conducted, and the fatigue fracture failure mechanism was revealed. The result indicates that the CoF increases with increasing torsional angle. As the cycles increase, the sliding distance between the wires decreases, while the adhesion increases. The fretting area gradually transitions from gross sliding status to partial sliding status. The sliding distance and energy loss between the wires increase with increasing torsional angle. The wear depth and coefficient increase with increasing torsional angle, and the wear level of peak contact structure is more serious than that of valley contact structure. The primary wear mechanisms of steel wire are abrasive wear, adhesive wear and fatigue wear. The fatigue fractograph of steel wire is obviously divided into fatigue source zone, crack growth zone and final fracture zone. Abundant secondary cracks and dimples exist in the final fracture zone, and the fatigue fracture failure mechanism is mainly ductile fracture.

Mesoscopic coupled plasticity-damage model of rough surface considering size-dependent plasticity

Metallic materials exhibit significant size effect at mesoscopic scale have been revealed by many experiments. Thus, it is essential to consider size effect when conducting mesoscopic scale studies in tribology. However, there is no constitutive model considering both ductile fracture and size effects of rough surface in the literature. At the same time, the simulation of friction and wear at the mesoscopic scale suffer from computational inefficiency using the conventional theory of mechanism-based strain gradient (CMSG) plasticity theory. To address the aforementioned challenges, we propose a mesoscopic coupled plasticity-damage model by combining the coupled plasticity-damage model and the simplified CMSG plasticity theory. This model can take into account not only the effect of stress state, temperature, strain rate on plasticity and fracture, but also the effect of the effective plastic strain gradient. Meanwhile, the model is able to solve the effective plastic strain gradient by a simplified method, thereby enhancing the computational efficiency. Then, model parameters for bearing bushing material of an engine are calibrated. Finally, scratch simulation and scratch tests of bearing bushing material at the mesoscopic scale are conducted to validate the effectiveness of the mesoscopic constitutive model. The residual scratch depth and width, coefficient of friction at different normal loads are investigated by experiment and simulation, respectively. Results proved the effectiveness of mesoscopic coupled plasticity-damage model, which is applicable for investigating issues related to wear, friction, and contact.

Temperature dependence of the deformation behavior and mechanical properties of a Fe–Mn–Al–C low-density steel for cryogenic application

The temperature dependence of the deformation behavior and mechanical properties of a Fe–20Mn–6Al-0.6C-0.15Si austenitic low-density steel was studied by tensile test and Charpy impact test at −196 °C, −77 °C and room temperature (RT), followed by microstructure analyses. The results indicate that the decreased tensile temperature, which is corresponding to reduced stacking fault energy (SFE), promotes the planar glide of dislocations. This led to larger fraction of low-angle grain boundaries (LAGBs) at lower tensile temperature. Consequently, both the tensile strength and elongation were improved as temperature decreased. At lower impact test temperature, the load and energy of crack initiation were larger, indicating a higher resistance to crack initiation. However, the transition from stable to unstable crack propagation occurred more rapidly at lower temperature, resulting in lower crack propagation energy and reduced resistance to crack propagation. This is because less region experienced planar glide. Due to the rapid unstable crack propagation, the Charpy absorbed energy at −196 °C was the lowest. The smaller fracture surface area, larger fibrous zone and more obvious ductile fracture characteristics in radial zone suggest that the toughness is better at higher temperature. Additionally, the presence of secondary cracks at RT delayed fracture, therefore benefitting toughness.