Fracture Of Metals Research Articles

Microstructure, mechanical properties and tear toughness of laser-welded DP980 dual phase steel

In this study, DP980 steel sheets were laser welded with a laser power of 4.5 kW and a welding speed of 4.5 m min−1. The microstructural evolution and mechanical properties of the welded joint and the effect of notch location on tear toughness were evaluated. Results show that the fusion zone (FZ) was composed of lath martensite, the hardness (276 HV) of the heat-affected zone (HAZ) was lower than that of the base metal (305 HV) resulting from martensite tempering. The welded specimens failed at the soft HAZ with a 98.4% joint efficiency during the tensile test, and the ultimate tensile strength of the as-recieved steel and welded joint was 1026 MPa and 1010 MPa, respectively. The tear energy of the FZ and HAZ was lower than DP980 base metal (BM). Thus, it is considered that the fracture toughness of the joint decreased after welding. The crack growth path of the FZ gradually deviated toward the HAZ during tearing due to the asymmetrical plastic zone at the crack tip. Compared with the ductile fracture of the base metal, the significant decrease in the fracture toughness of the welded joint is due to the weak deformation resistance of tempered martensite.

Interfacial microstructure and mechanical properties of the Al/Ti joint by magnetic pulse welding

Magnetic pulse welding (MPW) had attracted widespread attention due to its advantages on joining dissimilar metals. In this study, the joints of Al 1060/Ti-6Al-4 V dissimilar at different discharge energies were prepared to characterize the interface microstructure and to evaluate the tensile shear properties. The MPW joint consists of an annular welded zone and a central non-welded zone. The low discharge energies lead to joints with poor quality. A small amount of intermetallic compound (IMC) TiAl3 was formed at the welding interface due to mutual diffusion. With the increase of discharge energy, the failure mode changed from welding zone fracture to base metal fracture at the weaker side (Al side). The welding interface consists of four regions: transition zone, shear wave interface zone, sine wave interface zone and flat interface zone. Element diffusion is accompanied by the formation of dislocations, stacking faults (SFs) and deformation twins, which provides sufficient strain hardening ability for the interface. According to the element distribution and phase composition analysis results in the interface, the sound Al/Ti dissimilar joints are attributed to the wavy pattern in the interface.

Microstructure, tensile properties and fracture toughness of friction stir welded AA7075-T651 aluminium alloy joints

Abstract The main objective of this investigation is to study the microstructure, tensile properties and fracture toughness of friction stir welded (FSW) butt joints of 10 mm thick AA7075-T651 plates. The microstructural features of stir zone (SZ), thermos-mechanically affected zone (TMAZ), heat affected zone (HAZ) were analyzed using optical microscopy technique. The tensile properties were evaluated using smooth and notch tensile specimens and compared to base metal properties. The microhardness survey was done across the weld cross section and correlated to the failure of tensile specimens. Compact tension (CT) specimens were used to evaluate the fracture toughness of welded joints. The fractured tensile and CT specimens were analyzed using scanning electron microscopy (SEM). Results showed that the FSW AA7075-T651 specimens welded using axial load of 12 kN, tool rotation speed of 750 rpm and welding speed of 30 mm/min exhibited 412 MPa tensile strength and 9% elongation. It showed 88 and 89% of base metal strength elongation. The joints showed fracture toughness of 23 MPa m1/2 which is 80% of base metal fracture toughness. The superior tensile and fracture toughness properties of joints are mainly attributed to the evolution of finer grains in SZ due to the stirring action of FSW tool.

Experimental studies of fracture and fatigue crack growth in dissimilar metal welded joints of nickel-based superalloys: Cracks oriented along the weld

Nickel-based superalloys, Inconel 718 and Nimonic 80A, were welded without any interlayer for Blisk manufacturing in aerospace industries. Fracture toughness and fatigue crack growth rate of the dissimilar metal welded joints were experimentally studied to confirm their structural integrity. Cracks were oriented in and along the weld. Base metals, Inconel 718, and Nimonic 80A differ in yield strength but have the same modulus of elasticity. Electron beam welding was used to butt weld Inconel 718 to Nimonic 80A. J resistance curve ( J vs. crack extension) was experimentally found to determine the elastic–plastic fracture toughness. The welded specimens showed lesser fracture toughness and higher crack growth rates than base metals. An increase in load ratio resulted in an increase in fatigue crack growth rate in the high Δ K region and a decrease in Δ Kth. Fractography studies confirmed linear elastic and plane strain fracture all along the fatigue crack growth rate path except for the final fracture with plane stress.

Void-Induced Ductile Fracture of Metals: Experimental Observations

The paper presents a literature review on the development of microvoids in metals, leading to ductile fracture associated with plastic deformation, without taking into account the cleavage mechanism. Particular emphasis was placed on the results of observations and experimental studies of the characteristics of the phenomenon itself, without in-depth analysis in the field of widely used FEM modelling. The mechanism of void development as a fracture mechanism is presented. Observations of the nucleation of voids in metals from the turn of the 1950s and 1960s to the present day were described. The nucleation mechanisms related to the defects of the crystal lattice as well as those resulting from the presence of second-phase particles were characterised. Observations of the growth and coalescence of voids were presented, along with the basic models of both phenomena. The modern research methods used to analyse changes in the microstructure of the material during plastic deformation are discussed. In summary, it was indicated that understanding the microstructural phenomena occurring in deformed material enables the engineering of the modelling of plastic fracture in metals.

Seismic performance study on Q460 high-strength steel welded cruciform beam-column connections

Four Q460 high-strength steel welded cruciform beam-column connection specimens were designed. Among them, different forms of weld access holes were adopted for three specimens, and the locally expanded beam flange configuration was adopted for the other one. Quasi-static tests were carried out on the specimens. Seismic performance of the specimens was studied. The test results show that the specimens have good seismic performance. The ultimate failure modes of each specimen are all shown as the fracture of the beam flange steel base metal or welding material near the weld access hole. Ductility of high-strength steel welded connection can be improved by changing the form of weld access hole. Seismic performance of the expanded beam flange connection specimen was relatively better, which was mainly reflected in the fact that the failure at the weld access hole was delayed in late loading period. Finite element analysis (FEA) was conducted on each specimen. The FEA results were almost consistent with the test results. Analysis shows that the maximum von Mises stresses around the weld access holes of each specimen occurred near the weld access hole toes. The von Mises stress value of each specimen in the middle part of the beam flange width was almost the same. The difference of the von Mises stress values in the beam web height direction of the specimens with different configurations was mainly manifested in the range of three times of the weld access hole height below the upper weld access hole.

Atomistic simulation and continuum modeling of the dynamic tensile fracture and damage evolution of solid single crystalline Al with He bubble

The dynamic tensile fracture mechanism and damage evolution characteristics of solid single crystalline Al with He bubble are investigated by molecular dynamics (MD) simulations and continuum modeling. The physical mechanism leading to fracture is found to be dominated by He bubble growth, in wide ranges of strain rate and initial bubble volume fraction. The bubble growth process can be divided into three stages and the microscopic mechanism is related to the plastic deformation of surrounding metallic material predominated by dislocation growth and movement. It is further revealed that the evolution characteristics of He bubble are independent on strain rate or initial bubble volume fraction, but the growth rate increases significantly with increasing strain rate or decreasing bubble fraction. Additionally, the critical stress for bubble growth and dynamic tensile strength at the strain rates from 106 /s to 1010 /s are obtained, showing strain rate hardening effect. Finally, a continuum model based on the understanding derived from MD simulations is proposed to describe the damage evolution of the metal with He bubble, including the new strength softening effect induced by bubble growth. The dynamic tensile processes at the strain rates from 104 /s to 1010 /s are calculated by this model, and the obtained pressure and bubble evolutions and dynamic tensile strength are in good agreement with available MD results. We suppose that the present study sheds some light on the dynamic fracture of solid metals containing He bubble.

Flexural behavior of clad rack beam-to-column bolted connections at high temperatures

The fire resistance of beam-to-column connections affects the safety of clad racks under fire conditions. However, existing research lacks studies on the behavior of these special connections at high temperatures. This study investigates the fire resistance of clad rack beam-to-column bolted connections (CRBCs). Firstly, single cantilever tests of CRBCs at temperatures of 20 °C, 300 °C, 400 °C, 500 °C, 600 °C, and 700 °C are carried out. The failure modes of the CRBCs at 600 °C and below are identical to that at ambient temperature, while the failure mode at 700 °C is different. The stiffness and strength of the CRBCs decrease with the increasing temperature. Subsequently, an FE model considering the high-temperature metal fracture is established, which can accurately simulate the typical failure mechanisms and satisfactorily predict the full range moment-rotation behavior of CRBCs. Furthermore, parametric analyses are conducted to investigate the influences of related parameters on the failure mechanism, initial rotational stiffness, and flexural capacity of CRBCs. Finally, theoretical models based on the component method are established for calculating the initial rotational stiffness and flexural capacity of CRBCs at high temperatures, respectively. The comparison of the theoretical model, experimental results, and numerical results indicates that the theoretical model can accurately calculate the mechanical properties of CRBCs at high temperatures. The findings of this study can provide a reference for the analysis and design of the fire resistance of clad rack structures.

Molecular dynamics study on spallation fracture in single crystal and nanocrystalline tin

Spallation fracture in ductile metals with low melting points is an important scientific concern of dynamic fracture. Classical spallation and micro-spallation simulations of single crystal (SC) and nanocrystalline (NC) tin were carried out using non-equilibrium molecular dynamics at shock pressures of 13.5–61.0 GPa. The shock wave velocity had no effect on the waveform evolution in the SC Sn but not in the NC Sn. The front width of the stress wave in the classical spallation of the NC Sn was predominantly affected by grain boundary sliding. The atomic trajectory technique was first introduced to reproduce the evolutionary processes of void growth and coalescence quite effectively. In the classical spallation, the differences in void evolution behavior of SC and NC Sn were mainly reflected in nucleation position, spatial distribution, and growth zone, while their evolutionary behaviors were shared in the micro-spallation. In the NC model, for the classic spallation, voids mostly nucleated at grain boundaries and grew along grain boundaries, resulting in intergranular fractures; for the micro-spallation, voids nucleated at the grain boundary and inside the grain, resulting in intergranular, intragranular, and transgranular fractures. Furthermore, the void volume fraction followed the bilinear rise at the early nucleation and growth stages, and the critical transition point fundamentally signified the initiation of void nucleation to growth.

A Metallic Fracture Estimation Method Using Digital Image Correlation

This paper proposes a metallic fracture estimation method that combines digital image correlation and convolutional neural networks, based on a proven theory that the strain distribution of a component changes when a crack occurs in a structure. By using digital image correlation, the method achieves noncontact and nondestructive sensing, as well as high interference immunity. We utilize a digital image correlation system to produce strain distribution graphs that reflect occurrences and propagations of fractures during fatigue processes. A deep residual network (ResNet) regression model is trained by correlating strain distribution graphs with the corresponding fracture lengths, so that the fracture propagation condition can be estimated by data from digital image correlation. In the experiment, according to the American Society for Testing Materials (ASTM) standards, we fabricate a set of aluminum specimens and perform fatigue tests with data acquisition by digital image correlation. Finally, we obtain a crack length estimation mean absolute error of 0.0077 mm, or 0.26% of the measuring range. The results show the precision, as well as the practicality, of the proposed method.

Anti-collapse performance assessment of steel beam-column substructures with all-welded connections after exposure to fire

This paper presents both the experimental and numerical studies on the collapse resistance of beam-column substructures with all-welded connections under a middle-column removal scenario induced by fire scenarios. Extensive structural responses and key failure phenomena of specimens were monitored and recorded during push-down tests. The collapse resistance of specimens after exposure to fire deteriorated by comparison with those without fire exposure. In general, as the fire temperatures rose, the potentials of bending mechanisms against the progressive collapse were gradually degenerated, and the corresponding potentials of catenary mechanisms were strengthened instead. Nevertheless, the effect of temperature durations on the development history of resistance mechanisms was almost negligible. Additionally, an abundance of elaborated numerical models that considers the damage and fracture of ductile metals was developed to understand the structural responses of post-fire specimens. The simulated results showed that the vertical capacity and deformability of post-fire specimens were greatly enhanced with increasing beam heights. The span-to-depth ratios intrinsically influenced the contribution proportions of resistance mechanisms in beam-column substructures. With increased span-to-depth ratios, the vertical capacity provided by the bending mechanisms decreased obviously and deactivated works in the early stage, whereas that provided by the catenary mechanisms entered the stages of rapid development. Furthermore, two typical strengthening solutions after the fire events for post-fire substructures were comprehensively compared in terms of the structural behaviors against progressive collapse. From a practical viewpoint, strengthening with welded ribbed plates was considered a superior solution for the post-fire all-welded connections.

Increasing the crack resistance when pressing pipes made of 08Kh13N4M1F steel

The main factor determining the possible appearance of continuity defects in hotpressed pipes is the physical nature of the material subjected to deformation, namely, its plasticity. Alloying of a metal contributes to a decrease in its plasticity and an increase in the resistance to plastic deformation, which, on the one hand, increases the stress level during hot deformation, and, on the other hand, reduces the threshold stress value corresponding to the onset of metal fracture. The introduction of computer simulation tools, in particular software products based on the finite element method, made it possible to perform an analytical study of the features of the stress-strain state of the metal during hot pressing of steel pipes. Particular attention in the modeling is paid to the deformation conditions of the surface layers of the metal, since one of the characteristic surface defects of hot-pressed pipes is cracks and ruptures. The dependences of the stress-strain state on the technological parameters of the pressing process are revealed, namely, the nature of the change in the stress-strain state of the surface layers of the pressed shell along the deformation zone length, the effect of temperature, elongation coefficient and friction coefficient on the change in the stress-strain state are determined, the degree of the plasticity resource use by the Kolmogorov failure criterion is assessed. On the basis of the results obtained, a set of theoretical and experimental studies was carried out to investigate the mechanisms of the formation of surface defects in hot-pressed pipes, technological parameters and technological tool parameters that contribute to the elimination of surface defects were determined.

A new ductile fracture model for structural metals considering effects of stress state, strain hardening and micro-void shape

In this study, a new micromechanically-motivated phenomenological uncoupled fracture model is proposed for predicting the ductile fracture of structural metals considering the combinative effects of stress state, strain hardening and micro-void shape. The Lee–Mearvoid growth theory, which is used for characterizing the growth of ellipsoidal voids in a hardening matrix, is modified to derive the influence function of stress triaxiality, void shape and strain hardening on material’s ductility. Meanwhile, a J2-J3 based Bai–Wierzbicki’s yield function is used as a failure criterion and incorporated into the power-law hardening function to derive the Lode dependence of fracture strain. Further, a detailed parametric study is performed to demonstrate the influence of the material parameters on the 3D fracture envelope and 2D fracture locus of the new model. It is shown that the unique feature of the new fracture model is that it constructs an explicit and quantitative relationship between the ductility, strength and microstructure of metals. All parameters in the new model have intuitive physical meanings which correspond to specific material property indies controlling the material strength and evolution of micro-void defects. In the meantime, these model parameters can be easily calibrated via conventional tests, which greatly reduces the difficulty of the calibration work and makes the model being more friendly for engineering applications. Finally, test data of three structural metals, i.e. 2024-T351, 6061-T6 aluminum alloys and Q460 high strength structural steel, obtained under two loading conditions, i.e. proportional and non-proportional loading conditions, are used to calibrate and validate the proposed model. The accuracy of the new fracture model is then investigated by comparing its prediction result with that obtained from the modified Mohr–Coulomb and Hosford–Coulomb model. It is shown that the proposed new fracture model can predict the ductility limit of the investigated structural metals accurately under a wide range of stress states and provides the same good prediction accuracy as the other two typical models, which confirms the promising prospect of the new model in practical applications.

Study on microstructures and fatigue behavior of dissimilar Al/steel resistance spot welded joint

AbstractThe microstructures and fatigue behavior of dissimilar resistance spot welded joint between 5083 aluminum alloy and DP980 high strength steel are studied. Al/steel joint has fusion‐brazing characteristics composed of Al nugget and intermetallic compound (IMC) layer. The microstructure, microhardness, static tensile/shear property, and fatigue test of welded joints are carried out. The number of fatigue cycles increases with the decreasing of load level, and the fatigue limit load is 1.08 kN about 21% of static tensile/shear force. The fatigue test results reveal that different fatigue load levels lead to different fracture mode. It can be summarized as nugget pullout failure (fatigue load > 3.5 kN), interfacial failure (1.5 kN < fatigue load < 3.5 kN), and aluminum alloy base metal fracture (fatigue load < 1.5 kN). Furthermore, the fatigue fracture mechanism under different load levels is analyzed.

A damage evolution model considering void shape effect and new damage level definition

Metal fracture under different stress states is one of the most concerned problems in engineering field. Damage evolution model is of great value to intensive comprehension on fracture mechanism and its engineering application. In this paper, considering that fracture surfaces are formed by void evolution, a new damage level with clear physical significance was defined as the ratio of the maximum statistical cross-sectional area of voids to the cross-sectional area of the representative elements. Combined with this new damage level, a novel damage evolution model was established based on void nucleation, uniform growth and shape change. Subsequently, two groups of quasi-static tensile tests were conducted. One group was called fitting group and used to fit numerical model parameters. Another one was called analysis group and chosen to investigate fracture and void evolution. DIC analyses and SEM tests were also carried out to analyze the influence of specimen shapes on strain distribution and void evolution. Moreover, the damage evolution model was applied to the numerical simulation to verify its reliability. The numerical results were basically consistent with the experimental results by comparing the force-displacement relationships, fracture strains, void number density distributions and void ellipticity distributions. The effects of specimen shapes on damage and void evolution were specifically investigated by numerical simulation. All results illustrated the proposed model provides a tool for predicting quasi-static ductile fracture under different stress states and has guiding value for engineering design.

Dissimilar welding of Inconel 713 superalloy and AISI 4140 steel using Nd:YAG pulse laser: An investigation on the microstructure and mechanical properties

In the present research, welding of Inconel 713 superalloy and high strength AISI 4140 steel was successfully implemented by using pulsed Nd:YAG laser. The effect of heat input on the microstructural and mechanical properties of the weld was studied to find the best conditions for achieving a high quality weld. The microstructure characteristics of different weld regions including weld metal and heat affected zone were examined using the optical and scanning electron microscopies. It was observed that a columnar dendritic structure was formed at the edge of weld metal due to higher gradient temperature, while the reduction of gradient at the center of weld metal produced a cellular dendritic structure. The increase of heat input encouraged the formation of coarse dendritic structure which resulted in a reduction in the hardness of weld metal. The HAZ of 4140 steel side showed the maximum hardness for different values of heat inputs due to formation of martensitic microstructure, while the hardness in HAZ of Inconel 713 side was not showed a significant change. The higher value of heat input led to the segregation of Nb, Mo and Ti elements in the inter-dendritic regions and formation of brittle laves phase which resulted in a reduction in the tensile strength. However, the weld joint indicated a maximum tensile strength of 1068 MPa at lower heat input (1875 J/mm). The fracture surface of the tensile specimens showed a brittle fracture in the weld metal at different heat inputs.

Damage evolution during fracture by correlative microscopy with hyperspectral electron microscopy and laboratory-based microtomography.

Damage evolution during fracture of metals is a critical factor in determining the reliability and integrity of the infrastructure that the society relies upon. However, experimental techniques for directly observing these phenomena have remained challenged. We have addressed this gap by developing a correlative microscopy framework combining high-resolution hyperspectral electron microscopy with laboratory x-ray microtomography (XMT) and applied it to study fracture mechanisms in a steel inclusion system. We observed damage nucleation and growth to be inhomogeneous and anisotropic. Fracture resistance was observed to be controlled by inclusion distribution and the size scale of an inclusion-depleted zone. Furthermore, our studies demonstrate that laboratory XMT can characterize damage to the micrometer scale with a large field of view in dense metals like steel, offering a more accessible alternative to synchrotron-based tomography. The framework presented provides a means to broadly adopt correlative microscopy for studies of degradation phenomena and help accelerate discovery of new materials solutions.

Experimental and numerical investigation of ductile fracture for AA6061-T6 sheets at room and elevated temperatures

Warm forming is largely employed to enhance the poor formability of aluminum alloys at room temperature. The knowledge of the effect of stress state and temperature on the forming limits is an important aspect for the control of warm forming operations. Therefore, the objective of this work is to investigate the influence of temperature on the ductile fracture of AA6061-T6 aluminum alloy sheet metal under different stress states, ranging from shear to biaxial tension. For this purpose, experiments and numerical simulations of uniaxial tensile tests on dog-bone shaped specimen, notched specimens with different radius, specimens with a central hole and shear specimens are conducted at room and elevated temperatures. The hybrid experimental–numerical approach is used to identify the fracture strain and the corresponding stress state parameters (i.e. stress triaxiality and Lode parameter) to derive the fracture loci at room temperature, 150∘C and 200∘C. To accurately model the material behavior, the positive strain rate sensitivity in the flow stress response at elevated temperature is considered and attention is paid to constrain the finite element model by using the real experimental boundary conditions. The strain rate dependence of fracture strain is not considered in this work. A strong dependency of the ductile fracture on the temperature and stress state is evidenced. Also, an extension of Lou’s ductile fracture criterion which includes the impact of temperature on ductile fracture prediction is proposed. The prediction accuracy of the original and extended criteria is evaluated. It is found that Lou’s ductile fracture criterion is well suited to predict accurately ductile fracture at elevated temperatures under isothermal conditions whereas the extended form is very useful to predict ductile fracture initiation under non-isothermal conditions.

Characterization of Ti-6Al-4V alloy in the temperature range of warm metal forming and fracture analysis of the warm capping process

The flow behavior of a Ti–6Al–4V alloy during warm forming was characterized, and the results were analyzed to reveal the causes of fracture during warm forging of a small Ti–6Al–4V alloy bolt. The experimental flow curves obtained by compression test of cylindrical specimens at the warm forming conditions were fitted by a general extended C–m model after post-processed by a high-accuracy temperature correction scheme. Finite element analyses of the compression tests were conducted at various sample temperatures and strain rates using the flow model, revealing that the predicted compression loads were consistent with the experimental results. Flow behavior was also analyzed to reveal the metallurgical and macroscopic instabilities of the Ti–6Al–4V alloy. The flow behavior, macroscopic instability indices, and predicted forming load-stroke curves of Ti–6Al–4V alloy and SCr420H alloy steel were compared. The results study revealed that fracturing of the material in the capping stage during the automatic multi-stage warm forging of the small Ti–6Al–4V alloy bolt was caused by the sharp decrease in the forming load in an unpredictably early stroke from macroscopic instability, as indicated quantitatively by its macroscopic instability index.

Evaluating the J-integral of mode I crack in gradient nanocrystalline metals with residual stress induced by surface mechanical treatment

In gradient nanocrystalline metals, the grain size variation usually comes with the compressive residual stress induced by mechanical surface treatments such as surface mechanical attrition and severe shot peening. This paper focused on the fracture of gradient nanocrystalline metals with the residual stress that is introduced during the mechanical surface treatment. The J-integral of a mode I crack in the gradient nanocrystalline metal with different residual stress was calculated using finite element method. ABAQUS/Python scripts were developed to automatically create the gradient material, exert the residual stress and calculate the J-integral. By employing the empirical residual stress formula of sinusoidal decay type, a parametric study was carried out to investigate the effects of two characteristic features of residual stress, that is, the magnitude and the compressive zone depth, on the calculated J-integral. The results reveal that J-integral is significantly influenced by the residual stress. Specifically, the J-integral will decrease as the magnitude of residual stress increases. As the compressive zone depth increases, the J-integral first decreases and then increases. This indicates that the induced residual stress, along with the surface grain size variation, contributes jointly to the enhanced toughness of the gradient nanocrystalline metals.