Ductile Fracture Behavior Research Articles

Plasticity and Ductile Fracture of High-Strength Steel Center-Holed Plates under Tension

The mechanical behavior of bolted connections in high-strength (HS) steel structures is a matter of concern, since the less ductility compared to mild steels may affect the load transfer and stress distribution of bolted connections. Conducting advanced finite element (FE) analysis incorporating validated material plasticity and ductile fracture criterion allows for a deeper understanding for the load transfer mechanism of HS steel joints. This paper focused on the plasticity and ductile fracture behaviors of HS (S700MC and S960Q) center-holed (CH) plates under tension, which are the essential component in bolted connections. Firstly, a combined linear and power law was used to calibrate and describe the post-necking stress–strain relations of the investigated HS steels. Then, a void-growth-model based on Rice–Tracey criterion was used to simulate the ductile fracture of five groups of CH plates with different hole diameters. The tensile strength and deformation capacity of the CH plates were discussed. The results showed that S700MC had a strain-hardening plasticity until the tensile fracture, while S960Q had a strain-hardening plasticity with a strain-softening behavior followed near the tensile fracture. Incorporating the calibrated plasticity and ductile fracture criterion in FE analyses generated close force-displacement curves as the experimental results. The equation specified in Eurocode produced conservative predictions for the net cross-sectional tensile strength of the investigated HS steels.

Ductile Fracture Behavior of ASTM A516 Gr.70 Pressure Vessel Steel by ASTM and ISO Fracture Toughness Standards

Fracture toughness determination is crucial for the design phase of pressure vessels, and, although ASTM E1820 and ISO 12135 fracture toughness standards have existed for some time, some differences have been reported in the determination of this property. This study investigates the ductile fracture behavior of ASTM A516 Gr.70 pressure vessel steel and assesses the differences in estimating both standards. The steel’s tensile properties and initiation fracture toughness (JIC) were evaluated, taking into account the parallel and perpendicular orientations to the rolling direction. The results reveal the properties’ dependence on the rolling direction, mainly attributed to perlite banding. Additionally, as for the JIC determination, the differences were associated with the different blunting line slope estimations on each standard, reinforcing the necessity of a work-hardening-based blunting line for each material assessed.

RETRACTION NOTICE: Investigation of ductile fracture behavior of lap-welded joints with 460 MPa steel

RETRACTION NOTICE: Investigation of ductile fracture behavior of lap-welded joints with 460 MPa steel

Cross-linker Mobility Governs Fracture Behavior of Catch-Bonded Networks.

While most chemical bonds weaken under the action of mechanical force (called slip bond behavior), nature has developed bonds that do the opposite: their lifetime increases as force is applied. While such catch bonds have been studied quite extensively at the single molecule level and in adhesive contacts, recent work has shown that they are also abundantly present as crosslinkers in the actin cytoskeleton. However, their role and the mechanism by which they operate in these networks have remained unclear. Here, we present computer simulations that show how polymer networks crosslinked with either slip or catch bonds respond to mechanical stress. Our results reveal that catch bonding may be required to protect dynamic networks against fracture, in particular for mobile linkers that can diffuse freely after unbinding. While mobile slip bonds lead to networks that are very weak at high stresses, mobile catch bonds accumulate in high stress regions and thereby stabilize cracks, leading to a more ductile fracture behavior. This allows cells to combine structural adaptivity at low stresses with mechanical stability at high stresses.

Evaluation of the Wear and Mechanical Properties of Titanium Diboride-Reinforced Titanium Matrix Composites Prepared by Spark Plasma Sintering.

The synthesis of x-wt.% (where x = 2.5, 5, 7.5, and 10) TiB2-reinforced titanium matrix was accomplished through the spark plasma sintering technique (SPS). The sintered bulk samples were characterized, and their mechanical properties were evaluated. Near full density was attained, with the sintered sample having the least relative density of 97.5%. This indicates that the SPS process aids good sinterability. The Vickers hardness of the consolidated samples improved from 188.1 HV1 to 304.8 HV1, attributed to the high hardness of the TiB2. The tensile strength and elongation of the sintered samples decreased with increasing TiB2 content. The nano hardness and reduced elastic modulus of the consolidated samples were upgraded due to the addition of TiB2, with the Ti-7.5 wt.% TiB2 sample showing the maximum values of 9841 MPa and 188 GPa, respectively. The microstructures display the dispersion of whiskers and in-situ particles, and the X-ray diffraction analysis (XRD) showed new phases. Furthermore, the presence of TiB2 particles in the composites enhanced better wear resistance than the unreinforced Ti sample. Due to dimples and large cracks, ductile and brittle fracture behavior was noticed in the sintered composites.

Analysis of ductile damage evolution and failure mechanisms due to reverse loading conditions for the aluminum alloy EN‐AW 6082‐T6

AbstractThis paper deals with experiments and numerical simulations for reverse uniaxial tension‐compression and shear tests to investigate the ductile damage and fracture behavior of the aluminum alloy EN‐AW 6082‐T6. For this purpose, experiments with different loading sequences and number of loading cycles with uniaxial tensile and new shear specimens have been performed. Furthermore, the digital image correlation (DIC) technique captures the deformations and strain fields during the experimental processes. A modified anisotropic stress‐state‐dependent elastic‐plastic‐damage continuum model is proposed and is validated by experimental strain fields measured by DIC. Moreover, numerically predicted distributions of the principal damage strains are compared with fracture images taken by scanning electron microscopy (SEM).

Modelling of ductile fracture in pure iron considering the ductile–brittle transition at very high strain rate

At different deformation rates, a ductile material may fail by isothermal deformation fracture, thermoplastic shear instability, or even brittle fracture. The failure mode transition at sound speed deformation rate has not been well described. This paper aims to establish a new fracture model which considers the strain rate embrittlement effect (SREE) at the sound speed deformation rate, and can be used to predict the failure of the material in a large range of strain rate. Taking pure iron DT 8 as the experimental material, impact tests of pure iron specimens under different stress states, strain rates, and temperatures were carried out. Fracture strains under different stress states, strain rates, and temperatures were obtained. The strain rate was increased to 227450/s (5.36/s in log10) to obtain the ductile–brittle transition point. A new fracture model is proposed to describe the evolution of fracture strain over the wide range of strain rate. The new proposed model is experimentally validated and compared with the J-C failure model. It shows the new proposed fracture model can predict the ductile fracture behavior of a ductile material in the full strain rate range, and capture the failure mode transition at the sound speed deformation rate of a material.

Molecular Dynamics Study on Mechanical Properties of CO2–N2 Heteroclathrate Hydrates

One of the key issues in the exploitation of natural gas hydrates is the geo-mechanical stability of hydrate-bearing sediments. Herein, based on classical molecular dynamics simulations, we have investigated the mechanical stability of CO2–N2 heteroclathrate hydrates (CNHHs) under uniaxial loading. The results show that the ratio of CO2 to N2 in the large and small water cages has a crucial effect on the mechanical properties and fracture behaviors of CNHHs. In most cases, CNHHs show brittle fracture behavior, and their ultimate tensile strength, failure strain, and Young’s modulus decrease significantly with the increase of CO2 ratio in small cages. However, when the small cages are all occupied by CO2, CNHHs display ductile fracture behavior. In particular, the CNHHs with a CO2 fraction of 0.75 have their hydrated cages broken by amorphization with increasing strain. The effect of CO2 molecules occupying the different types of water cages on fracture behavior has been discussed in detail. The results indicate that the occupancy of small cages by CO2 plays a decisive role in the fracture position. In the case of the nearest-neighbor large cages, the fracture positions are consistent with the positions of CO2 molecules with a high probability. The lattice distortion caused by the occupancy of the water cage by CO2 is the key factor affecting fracture behavior. These results should be helpful in understanding the deformation and fracture mechanisms of heteroclathrate hydrates as well as in assessing the impact of CH4–CO2/N2 replacement and CO2 sequestration on the geological stability of gas hydrate reservoirs.

A method for determining the ductile damage parameters of high strength steels and weld metal

The microvoid based Gurson-Tvergaard-Needleman (GTN) model is a powerful tool for predicting ductile fracture behavior, and application of such model to steels and welds needs the identification of microvoid related damage parameters. Currently, there is no standard damage parameter identification method available. In this study the previously proposed complete Gurson model (CGM) where a physical void coalescence mechanism is incorporated into the GTN model is revisited. According to the CGM, the void nucleation process dictates ductile fracture. By adopting the cluster nucleation model with an effective initial void volume fraction as the only controlling parameter, a method is proposed to explicitly determine the effective initial void volume fraction from the strain at maximum load and strain at fracture of a specially designed notched tensile specimen. The proposed equation has been experimentally verified by applying to three high strength materials, including a X80 pipeline steel and associated weld metal, and a 15CrMo steel. A general procedure for damage parameter identification is also suggested. It is argued that the obtained effective initial void volume fraction can be treated as a type of material ductility indicator.

Experimental and numerical investigation on mechanical behavior of Q355 steel and connections in fire conditions

Understanding the temperature-dependent mechanical behavior of steel materials is the basis for evaluating collapse resistance of a steel structure and assessing its after-fire safety. Tensile experiments are conducted on Q355 steel to study the influence of heating histories and stress triaxialities on mechanical behavior in the whole process of fire including heating, heating-cooling and after-fire stages. The true stress-strain curves are measured, and microscopic failure mechanism is investigated. A SMCS model is used to simulate fracture behavior of steel and the model parameters are calibrated based on experimental and numerical results. A parametric study is conducted to study mechanical performance of steel connections under and after fire. The experimental results show that Q355 steel exhibits ductile fracture behavior under and after fire. The higher the tensile and peak temperatures, the greater the fracture strain and the better the ductility. The stress triaxiality may affect the sensitivity of fracture behavior of steel to the temperature condition. The SMCS model is applicable to evaluating fracture performance of Q355 steel in fire, with errors less than 10%. The fracture model of steel materials has a great influence on the mechanical performance of connections in fire. The ductility coefficient of steel connections in fire can be increased to 1.8 times the ambient-temperature value.

Constitutive relationship and characterization of fracture behavior for WE43 alloy under various stress states

The plasticity and fracture behavior of the WE43 alloy were investigated under various stress states. Mechanical experiments were conducted with special designed specimens for tension, compression and shear. The testing process was recorded and handled by digital image correlation technology. Experimental results show that WE43 alloy possesses the low tension−compression asymmetry and the fracture mechanism belongs to the ductile fracture. The plastic deformation behavior under uniaxial tension and compression was simulated with different hardening laws based on the Drucker yield function. The stress state and fracture strain were obtained by the numerical simulation. The ductile fracture behavior was numerically predicted by Brozzo, Oh, Ko-Huh and DF2016 criteria to compare with the experimental results. The results suggest that the plastic deformation can be reasonably modeled by the Swift−Voce hardening law and Drucker yield function. It is also demonstrated that the DF2016 criterion can accurately predict the fracture behavior of the alloy under various stress states.

Plastic deformation and ductile fracture of L907A ship steel at increasing strain rate and temperature

A combined theoretical, numerical, and experimental approach is adopted to systematically characterize the plastic deformation and ductile fracture behaviors of L907A low-alloy ship steel widely used in ship construction, with the effects of strain rate and thermal softening duly accounted for. A mixed Swift-Voce hardening model coupled with strain rate and temperature-dependent terms is proposed, while the Hosford-Coulomb model of ductile fracture is modified by introducing temperature-dependent term to account for the effect of thermal softening. To calibrate the parameters appearing in the constitutive models, dynamic uniaxial tensile tests at varying strain rates (0.001 ∼ 5200 s−1) and quasi-static uniaxial tensile tests at varying temperatures (20 ∼ 800 °C) are separately performed, with the coupling between strain rate and temperature effects ignored. The calibrated plasticity and ductile fracture models are numerically implemented with the method of finite elements (FE), and their applicability to practical applications is validated against experimental results of ballistic impacts. Normal and oblique impacts on L907A target plates with conical, hemispherical, and blunt projectiles are considered. Good agreement between FE simulation results of plastic deformation and ductile fracture modes and those measured experimentally is achieved, thus demonstrating the viability of the established plasticity and fracture models of L907A steel for applications in complex ballistic impact scenarios.

Phenomenological 2D and 3D Models of Ductile Fracture for Girth Weld of X80 Pipeline

Welding is the main method for oil/gas steel pipeline connection, and a large number of girth welds are a weak part of the pipeline. Under extremely complex loads, a steel pipeline undergoes significant plastic deformations and eventually leads to pipeline fracture. A damage mechanics model is a promising approach, capable of describing material fracture problems according to the stress states of the materials. In this study, an uncoupled fracture 2D model with a function of fracture strain and stress triaxiality, two uncoupled 3D fracture models, a consider the effect of Lode parameter stress-modified critical strain (LSMCS) model, and an extended Rice–Tracey (ERT) criterion were applied to X80 pipeline girth welds. Comprehensive experimental research was conducted on different notched specimens, covering a wide range of stress states, and the corresponding finite element models were established. A phenomenon-based hybrid numerical–experimental calibration method was also applied to determine the fracture parameter for these three models, and the stress triaxiality of the influence law of the tensile strength was analyzed. The results showed that the proposed fracture criterion could better characterize the ductile fracture behaviors of the girth welds of the X80 pipeline; however, the prediction accuracy of the 3D fracture model was higher than that of the 2D fracture model. The functional relationship between the tensile strength and stress triaxiality of the X80 pipeline girth welds satisfied the distribution form of the quadratic function and increased monotonically. The research results can be used to predict the fracture of X80 pipeline girth welds under various complex loads.

Plasticity, ductile fracture and ballistic impact behavior of Ti-6Al-4V Alloy

The strength and fracture behavior of a metal are crucial to its response in impact events. In the present paper, a hybrid experimental-numerical study was conducted to characterize the plasticity and ductile fracture behavior of Ti-6Al-4V alloy. To this end, ten types of specimens were utilized to cover a range of stress states from uniaxial tension to high stress triaxiality, shear, plane strain as well as elevated temperatures and strain rates. Especially, a slightly modified in-plane shear specimen suggested by Roth and Mohr was proposed and adopted to prevent premature fracture initiation from the free boundaries at a tension stress state. Aside from a Split Hopkinson Pressure Bar (SHPB) system, a high-speed servo-hydraulic testing machine was applied to conduct high-rate tests. A Modified Johnson-Cook (MJC) plasticity model and an extended Xue–Wierzbicki (XW) fracture criterion with strain rate and temperature effects were used and calibrated using a combined experimental-numerical approach. It was found that the fracture strain of the metal is sensitive to both the stress triaxiality and the deviatoric state parameter, and reduced ductility is reached at increasing stress triaxialities and a plane strain as well as shear stress states. The yield strength increases with increasing strain rate and decreasing temperature. An enhanced ductility is achieved at elevated temperatures and lower strain rates, except for at the highest strain rate of 750 /s, at which a higher ductility is observed compared with that at slightly lower strain rates. Ballistic impact tests on 4 mm thick Ti-6Al-4V alloy targets were conducted using 5.95 mm diameter blunt rigid projectiles in an impact velocity range of 167.9 – 321.6 m/s. Parallel finite element simulations were performed for the ballistic tests and the validity of the calibrated material models was confirmed by comparing the predicted ballistic resistance and failure pattern of the target with the experimental ones. In addition, it was found that a finite element prediction on the ballistic resistance of the Ti-6Al-4V alloy targets struck by blunt projectiles is highly mesh size sensitive and a converged solution was obtained by using a mesh size of 20 μm.

Strong and ductile medium manganese steel processed by low-temperature partitioning

Conventional medium Mn steels processed by intercritical annealing (IA) demonstrate low yield strength. The present work investigates the effect of low-temperature partitioning (LTP) on the microstructure and tensile properties of medium Mn steel. It is found that medium Mn steel processed by LTP at 300°C possesses ultra-high yield strength of 1683 MPa and proper uniform elongation of 11.3%. The LTP process does not reduce the yield strength but substantially enhances the tensile ductility. The increase of the tensile ductility after LTP treatment can be ascribed to the reduction of the residual stress, martensitic transformation, and the well-developed ductile fracture behaviour. The present LTP process provides an alternative pathway to develop strong and ductile medium Mn steel.

Low-Temperature Deformation and Fracture of Cr-Mn-N Stainless Steel: Tensile and Impact Bending Tests

The paper presents the results of tensile and impact bending tests of 17%Cr-19%Mn-0.53%N high-nitrogen austenitic stainless steel in temperatures ranging from −196 to 20 °C. The steel microstructure and fracture surfaces were investigated using transmission and scanning electron microscopes, as well as X-ray diffraction analysis. The steel experiences a ductile-to-brittle transition (DBT); however, it possessed high tensile and impact strength characteristics, as well as the ductile fracture behavior at temperatures down to −114 °C. The correspondence between γ–ε microstructure and fracture surface morphologies was revealed after the tensile test at the temperature of −196 °C. In this case, the transgranular brittle and layered fracture surface was induced by ε-martensite formation. Under the impact bending test at −196 °C, the brittle intergranular fracture occurred at the elastic deflection stage without significant plastic strains, which preceded a failure due to the high internal stresses localized at the boundaries of the austenite grains. The stresses were induced by: (i) segregation of nitrogen atoms at the grain boundaries and in the near-boundary regions, (ii) quenching stresses, and (iii) reducing fcc lattice volume with the test temperature decrease and incorporation of nitrogen atoms into fcc austenite lattice. Anisotropy of residual stresses was revealed. This was manifested in the localization of elastic deformations of the fcc lattice and, consequently, the stress localization in <100>-oriented grains; this is suggested to be the reason of brittle cleavage fracture.

Ductile fracture prediction of ZK61M high-strength magnesium alloy sheet during hot deformation process

To predict the ductile fracture behavior of a ZK61M high-strength magnesium alloy sheet during hot deformation, experiments and numerical simulations were performed under specific conditions. According to the stress state parameters and fracture strain data obtained for different specimens, the undetermined coefficients of four fracture models were solved using the particle swarm optimization algorithm. According to the error evaluation results, the modified Wilkins model was the most accurate. This model was introduced into a finite element, and the results were compared with experimental results. The results indicated that the mean relative errors of the fracture displacement and fracture load of the tensile parts were <15%, and the mean relative error of the forming depth of the cylindrical parts was 13.88%. The established finite element model can accurately simulate the experimental process and predict the fracture position.

Quenching of Carbon Steel Plates with Water Impinging Jets: Differential Properties and Fractography

The demand for steel components with tailored properties is constantly growing. To obtain a specific variation of microstructures and mechanical properties along the component it must undergo a controllable cooling. One way to control the cooling rates along the component is by using different simultaneous water jet impingements on a hot austenitized surface. This can be done by a newly developed test rig for water Impinging Jet Quenching Technique (IJQT). This work discusses the effect of IJQT on mechanical properties and fracture behavior of 15 mm steel plates containing 0.27 and 0.38 mass-% carbon. The samples were cooled in a specifically designed setup of the technique to obtain simultaneous water and air cooling resulting in diverse microstructures. The mechanical property gradients of both steels were analyzed through hardness measurements and tensile tests. The fracture surfaces and the near fracture regions were observed using scanning electron microscope and light optical microscope respectively.The results from tensile tests showed that the larger part of the sample with higher carbon content was fully hardened, however smoothly transitioning to a more ductile region. The sample with lower carbon content combined various degrees of hardening and transitioned from higher to lower ultimate tensile strength values. Fracture behavior of higher carbon steel was predominantly brittle transitioning to a ductile, while the lower carbon steel had a small region showing brittle fracture transitioning to a larger region of predominant ductile fracture behavior.

Influence of stress triaxiality on hydrogen assisted ductile damage in an X70 pipeline steel

The presence of hydrogen in steel components affects their structural integrity through a phenomenon called hydrogen embrittlement. While it is known that hydrogen affects the mechanical damage development upon loading, the specific mechanisms are still unclear. An experimental study is presented that investigates the plastic anisotropy and ductile fracture behavior of API 5L X70 pipeline steel with and without hydrogen charging at multiple scales. Three different tensile test specimen geometries (smooth and notched axisymmetric) are employed to investigate the influence of stress triaxiality thereon. The macromechanical responses during tensile tests are analyzed, along with micromechanical features of the resulting damage obtained using High-Resolution X-ray Computed Tomography. For all stress triaxialities tested, the presence of hydrogen does not affect the macroscopic plastic anisotropy, but accelerates ductile damage development and fracture, which is in agreement with the plasticity dominated hydrogen embrittlement mechanisms HELP and HESIV. Increasing stress triaxiality leads to a larger susceptibility to hydrogen embrittlement. Hydrogen accelerated void nucleation and enhanced lateral void growth are observed and quantified. The presented results can aid the development of numerical models describing hydrogen embrittlement in high-strength low-alloy steel.

Effects of particle size in silica sol on the mechanical and thermal properties of SiO2f/SiO2 composites

AbstractIn this paper, quartz fiber‐reinforced silica matrix SiO2f/SiO2 composites were prepared by the precursor impregnation‐heat treatment method using quartz fiber needle felt as the reinforcement and silica sol as the precursor. The effects of particle size in silica sol (10, 50, and 100 nm) on the density, apparent porosity, mechanical properties, and thermal properties of SiO2f/SiO2 composites were investigated. The phase composition and microstructure of the composites were characterized by X‐ray diffraction and scanning electron microscopy, respectively. The thermal expansion coefficient and thermal conductivity of composites were measured by a push rod method and the laser method. The results show that the density, apparent porosity, and mechanical strength of the specimens firstly increase and then decrease with the increase in the particle size in silica sol. The sample using silica sol with particle size 50 nm has the optimum overall performances (i.e., the flexural strength of 13.7 MPa and the compressive strength of 59.8 MPa), and shows a ductile fracture behavior. At 300°C–700°C, the average thermal expansion coefficient of the optimal sample is .783 × 10−6/°C. And the thermal conductivity of the samples increases with the increase in temperature, and it reached the highest value of .810 W/(m·K) at 700°C. The SiO2f/SiO2 composites show obvious advantages in the application of load‐bearing and thermal insulation integration, and they are expected to meet the demanding requirements of hot‐pressing sintering and non‐ferrous metallurgy industries.