Ductile Fracture Behavior Research Articles

Dynamic Mechanical Behavior and Energy Release Effect of a Novel Nb17Zr33Ti17W33 High-Entropy Alloy under Impact Load

The present study successfully demonstrates the fabrication of a novel class of high-entropy alloy, namely Nb17Zr33Ti17W33, through suspension melting and casting technique. To investigate the dynamic mechanical behavior and energy release effects of the alloy under high-speed impact loads, various techniques were employed, including split Hopkinson pressure bar (SHPB), X-ray diffractometer (XRD), scanning electron microscope (SEM), and high-speed photography. These methods were utilized to acquire crucial data, such as crystal structure analysis, stress–strain curves, and microstructural examination of failed specimens. The modified Johnson–Cook (J-C) model was employed to elucidate the dynamic flow behavior of the alloy, while investigating the failure mechanism and energy release phenomenon during the process of dynamic compression. The experimental results demonstrate that the alloy material exhibits a dual-phase (BCC1 + BCC2) structure, exhibiting ductile fracture behavior under dynamic compression conditions. On the fracture surface, typical dimple structures along with evidence of shear slip and melting traces were observed, indicating an energy-releasing failure process. The newly developed alloy exhibited exceptional strength, high density, remarkable plasticity, and outstanding energy release properties, rendering it highly promising for applications under extreme loads.

Interfacial microstructure evolution and mechanical properties of TIG welding joints of AISI 304/Q235B rolled composite plates

This study presents a comprehensive characterization of the microstructure evolution and distribution of interfacial bonding properties in the original rolled plates and a welded AISI 304/Q235B clad plates. The findings reveal that an uneven diffusion layer, rich in chromium (Cr) and nickel alloy elements, is formed at the interface of the heat-affected zone during the welding process. The thickness of the diffusion layer reaches 10 μm in the overheated region. This nonuniformity results from the dilution of alloying elements from stainless steel into carbon steel. The presence of the diffusion layer contributes to the highest microhardness value at the weld metal interface. Tensile-shear experiments indicate a 22% increase in the tensile-shear strength of the overheated zone interface compared to the base metal interface, accompanied by a 30% decrease in elongation. While the diffusion layer enhances the interface shear strength, it reduces the interface plasticity, thereby increasing the risk of fracture. Fracture analysis of the overheated zone interface reveals brittle fracture characteristics, with a small amount of Cr present. In contrast, the interface of original rolled plates displays characteristics of ductile fracture, which emphasizes the intricate interaction between brittle and ductile fracture behaviors at the interface. These observations stem from the nonuniform diffusion layer, distinct microstructure distribution, and varying rates of crack propagation.

Microscale modeling of the ductile fracture behavior of thin stainless steel sheets

This study aims to model the fracture behaviour of thin stainless steel sheets in the microscale, which are widely used in the manufacturing of thin-walled structures such as bipolar plates, while considering the effects of geometry and grain size. To achieve this, 304 austenitic stainless steel with two different thicknesses is heat-treated to obtain samples with distinctive grain sizes. Uniaxial tensile tests and cup drawing tests are performed on the resulting samples, and the fracture strains are measured using a digital image correlation system. The morphology of fracture surfaces is also analysed to understand fracture mechanisms in the microscale. A new ductile fracture model based on the normalized Cockcroft–Latham criterion is developed to take the size effect into account, which is then applied in finite element analysis to predict damage evolution and fracture initiation during the tests. The results reveal a significant reduction in the fracture strain with decreasing sheet thickness and increasing grain size. Furthermore, the fracture mode changed from tensile fracture of a polycrystalline metal to shear fracture of a single-crystal metal as the number of grains across the thickness decreased. It is confirmed that the proposed model accurately replicates the decrease of the fracture strain as the plastic deformation scaled down to the microscale and successfully predicts the displacement at the onset of fracture under different loading conditions. Based on these results, it can be concluded that the proposed model has great potential for predicting fracture in microforming processes.

Experimental and PDDO-based meshfree numerical study on ductile fracture behavior of steel plate

This paper presents a meshfree approach based on the coupling of peridynamic differential operator (PDDO) and continuum mechanics to predict the ductile fracture of the steel plates. The interpolation function of particles for a shell model is established by applying the PDDO in the 2D case. The weak form of the kinetic equations is derived by discretizing the motion variables. The Gurson-Tvergaard-Needleman (GTN) constitutive model is adopted to characterize the plasticity and ductile fracture behavior of material, in which the material parameters are determined by conducting uniaxial tensile tests and scanning electron microscope analysis. The ductile fracture test of the EH36 steel plate with a prefabricated crack is conducted to validate the proposed numerical approach. The results show that the established meshfree shell model is able to accurately simulate the bearing capacity, crack propagation rate, and ductile fracture characteristics of the structure.

Numerical analysis of non-proportional biaxial reverse experiments with a two-surface anisotropic cyclic plasticity-damage approach

This paper deals with the numerical analysis of ductile damage and fracture behavior under non-proportional biaxial reverse loading conditions. A two-surface anisotropic cyclic elastic–plastic-damage continuum model is adequately presented, which takes into account the Bauschinger effect, the stress-differential effect, and the change of hardening rate after reverse loading. An efficient Euler explicit numerical integration algorithm, based on the inelastic (plastic or plastic-damage) predictor-elastic corrector approach, is utilized to analyze the stress and finite strain loading histories. Detailed discussions are provided on different numerical integration-related consistent tangent operators that achieve convergence within the global Newton–Raphson scheme. The proposed continuum model is implemented into the commercial software Ansys as a user-defined subroutine (UMAT). Furthermore, the novel non-proportional biaxial tensile reverse experiments are performed to validate the proposed continuum model. The associated numerical simulations investigate the stability and accuracy of the proposed algorithm and material model.

Achieving an improved performance in double-sided friction stir weld joints by adjusting the welding conditions during the passes of AA6061-T6 aluminium alloy

The present study aims at investigating the effect of interaction between the process parameters of each weld pass (1st pass and 2nd pass) to improve the joint performance during the double-sided friction stir welded joints of AA6061-T6 aluminium alloy. Experimental designs were conducted following a design matrix developed using response surface methodology to investigate the impact on the microstructural changes and tensile properties. Pronounced interaction between the process parameters of each weld pass of the joint was observed and indicated a strong interdependency between them. The detailed microstructural study conducted through EBSD insights into the extent of homogenization, and abnormal grain refinement and reflects a vast transformation of low-angle grain boundaries into high-angle grain boundaries within the stir zone. The profound examination through scanning electron microscopy and transmission electron microscopy analysis validated the uniform homogeneous distribution of Al-Fe and Al-Si-rich precipitates. The significant improvement in the grain refinement and distribution of the strengthening precipitates within the stir zone led to a substantial enhancement in the microhardness, along with achieving a maximum ultimate tensile strength of 223 MPa and a percentage elongation of 14%. A study of the fracture morphology ensures the ductile fracture behavior of the tensile fracture specimen by observing the presence of numerous dimples within a well-bonded joint.

Effect of nickel interlayer on laser welding of copper/titanium dissimilar metal joint

Cu/Ti dissimilar metal welding components have significant potential applications in aerospace, nuclear industries, and other fields. To achieve a robust connection between titanium (TA2) and copper (T2), this study conducted laser-welding experiments using nickel as an intermediate layer. The influence of a nickel intermediate layer on the joint formation, microstructure, mechanical properties, melting, and element diffusion behavior were investigated. The results revealed that without the addition of a nickel intermediate layer, when the laser beam was biased toward the titanium side during welding, a 17 μm thick layer of intermetallic compounds formed in the copper-side weld region. In this area, a notable concentration of fragile Ti–Cu intermetallic compounds forms, rendering it the weakest point in the joint. As a consequence, the ultimate tensile strength of the joint measures at 131 MPa, which is roughly 55 % of the tensile strength of the copper base material. The introduction of a nickel intermediate layer led to a change in the microstructure of the fusion zone, with Cu–Ti intermetallic compounds dispersed within, preventing the formation of brittle Ti–Cu phases at the copper interface. The addition of the nickel interlayer increased the tensile strength of the TA2-T2 joint to 224 MPa, which is approximately 94 % that of the copper parent material, and the joint exhibited ductile fracture behavior in the copper parent material's heat-affected zone.

Investigation on Various Ceramic Nanoparticles Reinforced Aluminum Matrix Composites via Accumulative Roll Bonding and Cryorolling

Accumulative roll bonding (ARB) and cryorolling (CR) processes are used to fabricate three kinds of composites of AA1050 reinforced with SiCp and TiCp, to become the Al‐SiCp, Al‐TiCp, and Al‐SiCp‐TiCp composites. The effects of reinforced particles and different passes of ARB and CR on mechanical properties, microstructure, and particle distribution are studied. The microstructure shows the uniform distribution of particulates, decreased porosity, and excellent bonding between particle and matrix as the rolling cycles increased. Mechanical properties of the ARBed and cryorolled sheets are examined, and samples are compared with matrix alloy and composites. The findings demonstrate that the Al‐SiCp, Al‐TiCp, and Al‐SiCp‐TiCp composites improve the elastic modulus, yield strength, and hardness. After comparing the results of composites, the best combination of hardness, yield strength, ultimate tensile strength, toughness, and young modulus base composite has been chosen. Al‐TiCp and Al‐SiCp composites demonstrate ductile fracture behavior, as observed through SEM testing, while Al‐SiCp‐TiCp composite sample exhibits a bimodal fracture. This composite should aid in the creation of lightweight composites for use in aeronautical applications.

Effect of interfacial coatings on mechanical properties of 2.5D Nextel™ 720 fiber toughened zirconia-mullite matrix composites

In this study, 2.5D bent shallow cross-linked structure Nextel™ 720 (N720) fiber-reinforced yttria-stabilized zirconia-mullite composites with pyrolytic carbon (PyC) and fugitive pyrolytic carbon (FC) interfacial coatings were prepared by using a precursor infiltration and pyrolysis approach (called as N720f/PyC/YSZ-Mu and N720f/FC/YSZ-Mu, respectively). The effects of PyC and FC coatings on the failure mechanisms and mechanical properties of the composites were investigated. The N720f/PyC/YSZ-Mu composite shows ductile fracture behavior while the N720f/FC/YSZ-Mu composite shows brittle fracture behavior. The fracture toughness and flexural strength of the N720f/PyC/YSZ-Mu composite reach ∼7.00 MPa·m1/2 and ∼ 120.79 MPa, respectively. In contrast, the corresponding properties of the N720f/FC/YSZ-Mu composite are ∼3.66 MPa·m1/2 and ∼ 59.02 MPa, respectively. Microstructure analysis indicates that the weak PyC interfacial layer contributes to toughening by promoting fiber pull-out and debonding, thus enhancing the mechanical properties of the N720f/PyC/YSZ-Mu composite. The retrogression of the mechanical properties of the N720f/FC/YSZ-Mu composite originates from the damage to N720 fibers during the preparation procedure. This paper provides a new idea for the development of high-performance oxide CMCs with a 2.5D structure.

Characterization of ductile damage and fracture behavior under shear reverse loading conditions

AbstractThis paper deals with experimental and numerical analysis of the ductile damage and fracture behavior undergoing biaxial non‐proportional shear reverse loading conditions. In the numerical analysis, an anisotropic cyclic plastic‐damage constitutive model is considered to characterize the damage and fracture behavior. Additionally, the change in the hardening rate is observed after shear reverse loading. Thus, a newly modified non‐hardening strain region method is introduced to capture the change in the hardening ratio between isotropic and kinematic hardening. Furthermore, the damage surface is assumed to translate in the direction of the damage strain increment. Hence, a softening law based on the damage strain is proposed. The modified constitutive model predicts macroscopic force‐displacement and microscopic evolution of plastic and damage behaviors under shear reverse loading with a tensile preload. Through digital image correlation technique and scanning electron microscopy, numerical results can be validated by experimental ones. Furthermore, scanning electron microscopy pictures also reveal different damage and fracture mechanisms on the micro‐level.

Effect of welding parameters on the microstructure and mechanical behavior of dissimilar AISI 304/AISI 430 thin plates welded by gas Tungsten arc welding

In this work, the microstructure and mechanical behavior of dissimilar AISI 430 and AISI 304 thin sheets welded by Gas Tungsten Arc Welding using AISI 304 and AISI 316L as filler metals were studied. The obtained joints were characterized by optical and scanning electronic microscopy, Energy-dispersive X-ray spectroscopy and fluorescence, tensile tests and the Vickers microhardness test. Measurements carried out by energy dispersion X-ray spectrometry in the melting zone indicated a slightly heterogeneous distribution of nickel and iron. It was observed that the beginning of solidification of the zone fusion occurred through epitaxial growth. The WM solidification morphology was basically cellular influenced by temperature gradient, solidification rate and chemical composition. Variations in chemical composition and solidification morphology did not significantly alter the Vickers microhardness values in the melting zone. Results obtained in tensile tests indicated adequate mechanical behavior for the joints using AISI 304 as filler rod; those exhibited a ductile fracture behavior with the presence of dimples. However, the AISI 316L joints failure prematurely resulting in a fragile behavior.

Laser beam powder bed fusion of novel biomedical titanium/niobium/tantalum alloys: Powder synthesis, microstructure evolution and mechanical properties

The synthesis of spherical titanium/niobium/tantalum (TNT) alloy powders, namely Ti-20Nb-6Ta, Ti-27Nb-6Ta, Ti-35Nb-6Ta, and Ti-22Nb-19Ta (in wt-%) by electrode induction melting gas atomization is reported. The powder materials are characterized in detail using X-ray diffraction and scanning electron microscopy. Their processability via laser beam powder bed fusion (PBF-LB/M) is proven, and microstructure as well as mechanical properties of the additively manufactured specimens are assessed. All powders feature a dendrite-type microstructure with Nb/Ta-rich dendritic and Ti-rich inter-dendritic phases. Crystal structures of the powders are strongly composition-dependent. Nb- and Ta-rich Ti-35Nb-6Ta and Ti-22Nb-19Ta feature a body-centered cubic lattice, whereas Ti-rich Ti-20Nb-6Ta and Ti-27Nb-6Ta powders are characterized by multi-phase microstructures, consisting of non-equilibrium martensitic phases. Processing by PBF-LB/M causes significant changes in their microstructures: the dendrite-type morphologies vanish, and the formation of microstructures with a homogeneous element distribution can be observed in all additively manufactured parts. Ultimate tensile strength (UTS) as well as elongation at fracture are assessed by tensile testing. UTS values are found to be in a range from 651 MPa (Ti-35Nb-6Ta) to 802 MPa (Ti-20Nb-6Ta); strain-to-failure is between 21.3 % (Ti-35Nb-6Ta) and 31.7 % (Ti-22Nb-19Ta). Ductile fracture behavior is seen for all TNT alloys investigated.

Numerical parametric evaluation of ultimate resistance of high-strength bolts

High strength bolts are widely used in the engineering field due to their good compressive properties. Accurate assessment of the ultimate capacity performance of high strength bolts under combined loading is essential to ensure the safety of steel structures in the connection zone. Existing studies are not sufficiently clear on the economic and condition-specific limits on the effects of various factors on high-strength bolts under complex stress conditions. In response to these problems, the aim of this paper is to analyse the effects of different factors on the load-bearing performance of high-strength bolts on the basis of numerical simulations. The mesoscale critical plastic strain (MCEPS) method was used on the model fracture and the accuracy of the model evaluation was verified. The effects of grade, type, diameter and bolt hole clearance on the ductile fracture behaviour of high-strength bolts were investigated. It was found that fully threaded bolts and through-hole clearances can mainly affect the load-bearing properties of high-strength bolts and increase the bolt's fracture-deformation capacity. The load-bearing performance of high-strength bolts under the influence of different factors was also compared with the Eurocode design (EC3) predictions.

Ductile fracture of anisotropic QP980 steel sheet by using the isotropic/anisotropic modified Mohr-Coulomb models

The modified isotropic Mohr-Coulomb (iso-MMC) fracture model, which utilizes the Mises yield function and isotropic hardening law, was calibrated through tensile tests on various specimens. The resulting calibrated model was constructed in (η, L, ε¯f) space to represent the isotropic fracture locus of QP980 steel sheet. Subsequently, this calibrated model was utilized to predict the ductile fracture behavior of QP980 steel sheet loaded along rolling direction under different stress states. To comprehensively consider the impact of anisotropic properties on fracture evolution, an anisotropic MMC (aniso-MMC) ductile fracture model was developed in terms of strain space. Under the assumption of plane stress, the Mohr-Coulomb (MC) criterion was converted from stress space to strain space through an anisotropic constitutive model in order to characterize ductile fracture in QP980 steel sheet. The quadratic yield function of Hill's 48 yield function with the isotropic hardening law was employed to characterize the plastic behavior during loading. The anisotropic fracture loci of QP980 steel sheet were obtained in various loading directions. The scanning electron microscope (SEM) was utilized to characterize the microscopic morphology of the fracture surfaces of QP980 steel sheet, investigating the fracture mechanism of anisotropic ductility under different stress states. The results of both tensile experiments and simulations have confirmed that the plastic anisotropy of materials plays a critical role in determining ductile fracture behavior. The aniso-MMC fracture model was ultimately validated through hole expansion tests and equi-biaxial tensile tests, demonstrating its capability to predict ductile fracture behavior under both proportional and non-proportional loading conditions.

Ductile fracture behavior of in situ TiB2 particle reinforced 7075 aluminum matrix composite in various stress states

The fracture behavior of an in situ TiB2 particle reinforced 7075 aluminum matrix composite in various stress states was investigated by mechanical tests, microscopic characterization, and numerical simulations. Four sets of tensile specimens, one group of shear specimens, and one group of compression specimens were designed, and mechanical experiments were conducted on these six groups of specimens. The strain distributions of the six groups of specimens during deformation were investigated by in situ strain testing and finite element simulation. The fracture morphology of the specimens was characterized to analyze the damage mechanism in different stress states. It was found that the fracture mechanism of the material is mainly interfacial debonding and particle fracture, manifested as tensile fracture under high stress triaxiality and shear fracture under low stress triaxiality. For the tensile fracture, the nucleated voids grew with the maximum principal stress; for the shear fracture, they showed significant shape change with the maximum shear stress but little volume change. Based on the fracture mechanisms uncovered by the experiments, a modified ductile fracture criterion, considering the influence of both the maximum principal stress and maximum shear stress, was developed for the composite. Comparison with the modified Mohr–Coulomb, Lou–Yoon–Huh, Hu, and Mu models shows that the proposed model can predict the ductile fracture behavior of the aluminum matrix composites more accurately.

In-situ EBSD observation and simulation of free surface roughening and ductile failure in the ultra-thin ferritic stainless steel sheet

Surface roughening is one of the most fundamental issues which greatly affects the plastic deformation and ductile fracture behavior of ultra-thin sheet materials. The present study investigated the influence of free surface roughening on the micromechanical failure in the ultra-thin ferritic stainless steel (FSS) sheet for bipolar plate in proton exchange membrane (PEM) fuel cell under in-situ tensile deformation. The microstructural evolution during uniaxial tension was monitored using in-situ electron backscatter diffraction (EBSD) technique. Then the initial microstructure was directly mapped onto the finite element mesh of representative volume element (RVE) in the crystal plasticity finite element (CPFE) simulation. The CPFE model was incorporated with the microscopic ductile fracture criterion in terms of plastic energy dissipation in order to simulate the initiation of ductile fracture caused by the free surface roughening. The in-situ EBSD observations reveal that the microstructure of the ultra-thin FSS sheet exhibits heterogeneous plastic deformation and strain localization on the surface at the large deformation owing to the considerable free surface roughening. The CPFE simulations well reproduce the stress/strain heterogeneity and the local hotspots of the ductile fracture initiation. It reveals that the increase of surface roughness with the deformation results in the increased inhomogeneity of thickness which accelerates the strain localization, consequently the premature necking and failure in the ultra-thin steel sheet. Due to the free surface roughening which can facilitate strain localization, the initiation of ductile fracture is obviously observed in both the ridges and valleys in the free surface and its propagation along the pronounced localization band through the thickness, leading to material failure. The good correlation of free surface roughening to the onset of plastic instability and ductile fracture further advances the insight into the micromechanical failure during plastic deformation of the ultra-thin FSS sheet for bipolar plate in PEM fuel cell.

Investigation of the Microstructure and Mechanical Properties in Friction Stir Welded Dissimilar Aluminium Alloy Joints via Sampling Direction

This research study investigates the influence of sampling direction on the microstructure and mechanical properties of dissimilar joints formed by friction stir welding (FSW). The specimens were cut in two directions: perpendicular (transverse) and parallel (longitudinal) to the FSW joint. The tests conducted included X-ray diffraction (XRD), macrostructure, microstructure, tensile, microhardness, and fractography analysis. Different phases were noted in the XRD patterns and explained, with the aluminum phase being the dominating one. The results further showed that the transverse dissimilar joint exhibited higher microhardness compared to the longitudinal dissimilar joint, which is consistent with the respective grain sizes. Moreover, the ultimate tensile strength of the longitudinal joint exceeded that of the transverse joints, showing a substantial 47% increase. Similarly, the elongation of the joints followed a similar trend, with the longitudinal joint displaying a significant 41% increase in elongation compared to the transverse joint. Fractographic analysis revealed ductile fracture behaviour in all joints.

Effects of mixed corrosion damage on constitutive relation and ductile fracture behavior of butt-welded joints under cyclic loading

The corrosion damage of welded connection regions posed a threat to the reliability and durability of welded steel structures. In this paper, the morphology scanning and cyclic loading tests of butt-welded joints exposed to the salt spray corrosion environment were conducted to investigate the effects of corrosion damage on surface characteristics and hysteresis behavior of butt-welded joints. Three-dimension scanning test results showed that the butt-welded joint under the salt spray environment suffered a kind of mixed corrosion damage. Compared with the thickness loss caused by uniform corrosion, pitting damage tended to cause serious local geometric defects, aggravating the geometric discontinuity problem. For the corroded butt-welded joint, the hysteresis loop area and cycles decreased significantly with the increase of corrosion damage, and the ductile fracture behavior under the cyclic loading gradually disappeared. The numerical comparative analysis of models with two different patterns of corrosion damage showed that the pitting damage caused a severer hysteresis behavior degradation of butt-welded joints compared to the uniform corrosion damage under the same volumetric loss ratio. Particularly, the pitting damage had a significant effect on reducing cyclic deformation and energy dissipation. According to the above experimental and numerical analysis, a modified cyclic plastic fracture criteria considering pitting damage and a nonlinear cyclic constitutive model considering mixed corrosion for the corroded butt-welded joint were proposed respectively to equivalently consider the influence of corrosion geometric damage by weakening the material properties.

Influence of base metal temper conditions on AA6082 and AA7050 dissimilar friction stir weld properties

In this work, the effect of base metal temper conditions was studied on joint properties of various dissimilar AA6082/AA7050 weld combinations: (i) 6082-T6/7050-T7, (ii) 6082-T6/7050-O, (iii) 6082-O/7050-T7, and (iv) 6082-O/7050-O. The weld macrostructure showed high susceptibility to onion ring formation when either side base metal was in O-temper condition. Heterogeneity in weld nugget microstructure was found to be minimum for dissimilar joints produced with different tempered base metals having the largest strength difference. The weld nugget average hardness was highest (120.86 HV) for the hardest base metals combination and lowest (97.57 HV) for the softest base metals combination dissimilar joints. The highest joint tensile strength was 218.5 MPa for weld fabricated from base metals having the smallest difference in tensile strength. In contrast, the lowest joint tensile strength was 120.4 MPa for weld produced from the combination of base metals having the greatest difference in tensile strength. All welded joints were fractured from the advancing side and exhibited ductile fracture behavior.

A new ductile fracture model for Q460C high-strength structural steel under monotonic loading: Experimental and numerical investigation

Tensile fracture static tests on nine specimens covering different stress states were performed to investigate the ductile fracture behavior of Chinese Q460C high-strength structural steel (HSS). Based on combining the Rice-Tracey model and Xue model, the new model considering the effect of stress triaxiality and Lode angle was proposed to predict ductile fracture of Q460C HSS. Furthermore, the stress softening criterion was incorporated into the new fracture model to simulate the fracture propagation of Q460C HSS better. The material parameters of the proposed fracture model for Q460C HSS were identified using MATLAB optimization code. The VUMAT subroutine was developed to perform ductile fracture simulations for each specimen, and the fracture simulation results show satisfactory agreement with the experimental results. In addition, the five existing ductile fracture models were also selected to compare the prediction accuracy of the new model. The comparative results show that the new model has better accuracy and is more suitable for predicting fracture initiation and propagation of Q460C HSS. Finally, the applicability of the new model in predicting the fracture of Q460 HSS was furtherly verified by successfully simulating the existing fracture tests of the notched round bar specimens and the bolted connections.