Fracture Of Metals Research Articles

Liquid metal embrittlement and fracture behavior of three Mo metals in liquid lead-bismuth eutectic

In this work, liquid metal embrittlement (LME) and the related fracture behavior of Mo, TZM and Mo-14Re were assessed in oxygen-saturated and -depleted LBE at 350°C by means of slow strain rate tensile (SSRT) tests. Neither tensile mechanical property data nor fractographic micrographs show strong evidence of LME. Nevertheless, the observation of deep secondary cracking at the fiber-like grain boundaries of Mo and Mo-14Re tested in LBE suggests occurrence of grain boundary wetting (GBW). Moreover, oxidation-assisted crack initiation and propagation were also identified. Both fracture modes may be a concern when Mo metals with a fiber-like grain structure are used in high-oxygen LBE environment.

EVALUATION OF TECHNOLOGICAL PLASTICITY OF HIGH BORON CORROSION-RESISTANT STEEL DURING HOT DEFORMATION OF PIPES USED FOR SPENT NUCLEAR FUEL STORAGE

Statement of the problem. The sustainable development of the global nuclear power industry in the future depends on how effectively the problems of radiation safety and nuclear proliferation are addressed. Such problems occur, among other things, at the final stage of the nuclear fuel cycle – spent fuel management. This task is currently on the agenda of both the international community and national governments of countries that develop or intend to develop nuclear energy [1−3]. Today, in connection with ensuring the safety of nuclear energy, boron-containing steels are widely used in the world practice as a material for biological protection and for the manufacture of special equipment parts. This is due to the fact that the B10 isotope in steels provides neutron capture [4]. Hexagonal pipes made of high-boron steel were needed to shield the support cover of the compacted spent fuel storage facility at nuclear power plants. Corrosion-resistant steels alloyed with boron are widely used in the nuclear power industry due to their special nuclear properties. During the operation of nuclear power plants, fuel assemblies that have served their useful life must be stored in special storage facilities using containers – hexagonal tubes made of 04Сr14Ti3B2V steel. To reduce costs during in the construction and operation of NPPs, part of the spent nuclear fuel management should be performed at Ukrainian enterprises. This will provide a significant import substitution effect. To manufacture hexagonal tubes from 04Сr14Ti3B2V steel according to the ingot-hot deformation-profiling-heat treatment scheme, it is necessary to develop the parameters of hot deformation and subsequent technological operations. The use of these pipes in compacted spent fuel storage facilities of NPPs will allow to increase the storage capacity by 2 times, which will give a significant national economic effect Material and methods of research. Material used for the study was steel 04Сr14Ti3B2V (CS-82), smelted using two variants (vacuum-induction – “VI” and vacuum-induction followed by vacuum-arc remelting – (“VD”). Steel grade 04Сr14Ti3B2V belongs to high-alloy, corrosion-resistant ferritic steels with a high boron content of up to 2 %. The method of differential thermal analysis was used to study the phase transformation temperatures in the steel. X-ray diffraction and micro-X-ray spectral analysis were used to assess the phase state. The steel samples were tested for weldability After cooling, the welded samples were subjected to X-ray transmission [5]. Hot twisting tests and mechanical tests at high temperatures were also performed. The macro- and microstructure of the metal was studied. Results. The macro- and microstructure of steel was evaluated depending on the smelting method. The phase composition of the steel was studied. The study of the plastic properties of steel ChS-82 by means of penetration and hot twisting tests showed that the temperature range of maximum plasticity is in a wide range (from 1 025 to 1 150 0C) with a rather low resistance to deformation. During hot deformation in the temperature range of 1 175 0C and above, metal fracture was observed along the grain boundaries at the melting points of the Cr−Fe−B boride phase. The optimal temperature range for hot deformation of steel ChS-82 is 1 000−1 050 ⁰С. Scientific novelty. For corrosion-resistant high boron steel used for spent nuclear fuel storage, the temperature range of hot deformation was selected and the structural and phase composition was estimated, which will allow deforming the metal on the piercing mill and manufacturing pipes.

An uncoupled ductile fracture criterion for a wide range of stress states in sheet metal forming failure prediction

In industrial production, sheet metal rupture will inevitably occur in the process of forming, especially in the shape of complex, relatively small thickness of the sheet. It is vital to accurately predict the failure of ductile metals under various working conditions. To deal with the problem, an uncoupled ductile fracture criterion (DFC) for a broad range of stress states is proposed based on the micro-mechanism of ductile fracture of metals, which redefines the relationship between the number of void nucleating and the equivalent plastic strain and takes into account the different deformation modes of the voids in the growth stage. Then, in order to verify the validity and advantages of the proposed DFC, the 3D fracture surface of AA 2024-T351 and AISI 1045 steel are constructed by the new criterion based on their fracture data under various stress states, and compared with the commonly used DF2016 criterion and Hu criterion. The prediction results show that the new criterion is better than them, both in terms of the maximum prediction error and the average prediction error. Furthermore, to further prove the effectiveness of the proposed model, five samples are designed for tension tests to calibrate the new DFC using a hybrid experimental–numerical approach, and the calibrated criterion is applied to cupping simulations of AA 6061 and compared with cupping experimental results. The results indicate that the Erichsen cupping number (IE) of the cupping simulation and experiment are 6.88 mm and 7.05 mm, respectively, and the error between them is only 2.411 %, and the location of the fracture is also basically the same. Therefore, all comparison results show that the proposed DFC can forecast the fracture problem in sheet forming more accurately under various stress states.

Electron Concept of Hydrogen Embrittlement and Hydrogen-Increased Plasticity of Metals

Based on theoretical and experimental studies of hydrogen effect on the electron structure of iron, nickel and titanium, an electron concept is proposed for hydrogen embrittlement as well as for hydrogen-improved plasticity of engineering metallic materials. This concept implies a hydrogen-caused redistribution of valence electrons across their energy levels and an increase in the density of electron states at the Fermi level, causing a softening of the crystal lattice and, thereby, leading to a decrease in the specific energy of dislocations with consequent increase in their mobility. Innate phenomena in metallic solid solutions, namely, short-range atomic order in its two versions, short-range ordering and decomposition, are shown to be a precondition for the localization of plastic deformation. Hydrogen enhances merely this effect resulting in pseudo-brittle fracture. The role of hydrogen-induced superabundant vacancies in hydrogen-caused localization of plastic deformation and grain-boundary fracture in pure metals is discussed. Using the temperature- and strain-dependent internal friction, the enthalpies of hydrogen diffusion and hydrogen–dislocation binding are studied, and their controlling effect on the temperature- and strain-rate-dependent hydrogen embrittlement is demonstrated. Finally, a physical rationale is proposed for using hydrogen as a temporary alloying element in the technological processing of titanium alloys, and for a positive hydrogen effect on the fatigue life and plasticity of austenitic steels.

Hydride induced embrittlement and fracture of non-hardening metals under hydrogen chemical equilibrium

The stress field in the hydride precipitation zone is examined, under conditions of hydrogen chemical equilibrium and constant temperature, in the case of non-hardening metals, by applying slip-line theory. It is proven that the hydride precipitation zone, in any geometry, is a constant stress area. In this area, the principal stresses are equal to the respective principal stresses, before hydride precipitation, minus the difference of hydrostatic stress before and after hydride precipitation. The general relations are applied to the case of a stationary sharp mode-I plane-strain crack and the deviations from Prandtl-field are derived, in the [-π/4, +π/4] sector ahead of the tip, where hydrides precipitate. In this case, the hydride precipitation sector is characterized by a constant hydride volume fraction. In addition, hydride precipitation is associated with the development of elastic sectors along the crack faces and the reduction of the centered fan sectors; the relation between hydride precipitation zone stress trace and the extent of the centered fan sector is presented. The mode-I plane-strain blunted crack is also considered and the deviations from the logarithmic spiral slip-lines is discussed together with the reduction of hydride volume fraction as the blunted crack-tip is approached. A general fracture criterion, based on the strength of hydride platelets, is derived, which indicates that fracture occurs, when a critical hydride precipitation zone stress trace dominates. The criterion is applied, under the condition of a dominant K-field annulus, surrounding the plastic zone, and the estimated threshold stress intensity factor of delayed hydride cracking correlates favorably with experimental measurements.

Effect of Welding Process on Microstructure and Mechanical Properties of Boron Containing Modified 9Cr-1Mo Steel

The influence of Shielded Metal Arc (SMA) welding and Activated Tungsten Inert Gas (A-TIG) welding on the microstructure and mechanical properties of boron containing Modified 9Cr-1Mo steel has been investigated. The tensile strengths of SMA weld joints at room temperature and 550℃ (730, 710 MPa) are higher than those of A-TIG weld joints (520, 465 MPa); whereas A-TIG welds (66, 18J) show better impact energies at room temperature and 0℃ than SMA welds (53, 12J). Creep resistances of both the weld metals, as estimated by impression creep technique are comparable; while, the A-TIG heat affected zone (HAZ) possesses higher creep resistance than the SMA HAZ. The fracture toughness of the A-TIG weld metal is inferior (64 kJ.m-2) to that of SMA weld metal (181 kJ.m-2) and brittle fracture is seen. It has been observed that the multiple passes of SMA process are advantageous in modifying the delta ferrite present in the weld metal although they cause severe damage to the HAZ microstructure.

Tensile deformation and failure of AlSi10Mg random cellular metamaterials

Cellular metamaterials are an emerging class of porous materials, in which the topology of the solid is precisely-engineered to achieve attractive combinations of properties. This work specifically examines the tensile response of a novel type of cellular metamaterials, in which pores of arbitrary elliptical shape are randomly-dispersed into an aluminum alloy matrix. Their porous mesostructure is generated numerically via a random sequential absorption algorithm, and is fabricated by laser powder bed fusion from AlSi10Mg powders. The results – obtained by means of digital image correlation combined with X-ray tomography – highlight the advantages offered by a random cellular topology. They are notably a low sensitivity to geometric imperfections (which inevitably result from manufacturing), coupled with the ability to delay long-wavelength strain localization, which is in turn responsible for failure. Structural disorder also leads to highly heterogeneous deformation patterns, which result from the interaction between geometric pores and promote void growth and coalescence during plastic straining. The presence of manufacturing defects exacerbates void interaction and promotes early plastic flow localization. Interestingly, experiments reveal that this class of porous solids effectively displays features of the ductile fracture of metals at the mesoscale. For example, void-sheeting is observed with increasing pore aspect ratio and is accompanied by large geometric distortions of the voids upon coalescence. Measured data for the pore strains are highly scattered and show a departure from McClintock’s model predictions for the voids within the fracture band. Collectively, this study highlights the potential offered by random porous metamaterials, which can be harnessed to revisit the complex mechanisms of ductile fracture at the mesoscale.

On the role of surface stress in environment-assisted fracture

Abstract Environment effects in plasticity and fracture of metals, well studied for several decades, still pose many unanswered questions. A micro-mechanics explanation of how dislocation activity is influenced by the material surface state, that can answer these questions, has been elusive. We build on a recently discovered effect in metal cutting – organic monolayer embrittlement (OME) – wherein metal surfaces are rendered brittle by long-chain organic adsorbates, to explore how material state variables influence surface plasticity and fracture. In particular, cutting experiments with Al containing Self Assembled Monolayers (SAMs), show that the OME is controlled by surface stress (f) induced by the adsorbates. This is contrary to many instances of environment-assisted fracture which are usually attributed to surface energy changes, and wherein f is largely ignored. Other contributions include (a) a cantilever technique to measure surface stress, (b) demonstration of strong effect of SAM molecule chain length on f, (c) characterization of how dislocation activity at crack-tips is affected by adsorbate-induced f, and (d) large improvements in machining processes enabled by controlled environment-assisted fracture. We make the case that surface stress, due to adsorbates, likely influences all environmentally assisted cracking (EAC) phenomena, warranting a revisit of extant models of EAC.

Experimental Characterization and Phase-Field Damage Modeling of Ductile Fracture in AISI 316L

(1) Modeling and characterization of ductile fracture in metals is still a challenging task in the field of computational mechanics. Experimental testing offers specific responses in the form of crack-mouth (CMOD) and crack-tip (CTOD) opening displacement related to applied force or crack growth. The main aim of this paper is to develop a phase-field-based Finite Element Method (FEM) implementation for modeling of ductile fracture in stainless steel. (2) A Phase-Field Damage Model (PFDM) was coupled with von Mises plasticity and a work-densities-based criterion was employed, with a threshold to propose a new relationship between critical fracture energy and critical total strain value. In addition, the threshold value of potential internal energy—which controls damage evolution—is defined from the critical fracture energy. (3) The material properties of AISI 316L steel are determined by a uniaxial tensile test and the Compact Tension (CT) specimen crack growth test. The PFDM model is validated against the experimental results obtained in the fracture toughness characterization test, with the simulation results being within 8% of the experimental measurements. (4) The novel implementation offers the possibility for better control of the ductile behavior of metallic materials and damage initiation, evolution, and propagation.

A direct analytical methodology for the assessment of ductile fracture in metals based on multiaxial tests

AbstractThe prediction accuracy of ductile damage models is subject to a sound calibration strategy, which normally involves the execution of complex multiaxial tests and requires dedicated facilities. In addition, finite element (FE) analysis is mandatory to retrieve the stress and strain states at the critical point, which cannot be directly measured from experiments. To overcome this complexity, a minimal set of simple multiaxial tests is selected, and an analytical‐numerical approach is proposed to evaluate, without resorting to FE, both the stress evolution with plastic deformation and the fracture strain, under any different loading condition of each test. This is achieved from the sole knowledge of the material bilinear stress–strain relation and of the applied test displacement at fracture. The obtained results are compared with a traditional testing and calibration methodology, and the robustness of the approach is proved on a 17‐4PH steel, an X65 steel, and a Ti6Al4V alloy.

Inhomogeneous Strain Behaviors of the High Strength Pipeline Girth Weld under Longitudinal Loading.

Unforeseen failures in girth welds present a significant challenge for the pipeline industry. This study utilizes 3D Digital Image Correlation (DIC) assisted cross-weld tensile testing to analyze the strain response of high-strength thick-walled pipelines, providing essential insights into the strain migration and fracture mechanisms specific to girth welds. The results reveal that the welding process significantly affects the mechanical distribution within the girth weld. The tested Shielded Metal Arc Welded (SMAW-ed) pipe exhibited undermatched girth welds due to high heat input, while Gas Metal Arc Welding (GMAW) introduced a narrower weld and Heat-Affected Zone (HAZ) with higher hardness than the base metal, indicative of overmatched girth welds. Strain migration, resulting from a combination of metallurgical heterogeneous materials and geometrical reinforcement strengthening, progressed from the softer HAZ to the base metal in the SMAW-ed sample with reinforcement, ultimately leading to fracture in the base metal. In contrast, the GMAW-ed sample shows no strain migration. Reinforcement significantly improves the tensile strength of girth welds and effectively prevents failure in the weld region. Sufficient reinforcement is crucial for minimizing the risk of failure in critical areas such as the weld metal and HAZ, particularly in SMAW-ed pipes.

Experimental and numerical study on extended endplate beam-column joints after high temperature to resist progressive collapse

This paper presents experimental and numerical investigations on the collapse-resisting behavior of extended endplate beam-column joints after high temperatures. The failure patterns and structural responses of extended endplate (EP) joints after high temperature to resist progressive collapse were first described, and then the effects of fire temperature and temperature duration on their collapse resistance mechanisms were also analyzed in detail. The test results demonstrate that all specimens withdrew from work owing to the serious deformation of endplate bolt holes or the brittle fracture of high-strength bolts, resulting in the catenary mechanisms of EP joints not being adequately developed. Compared with previous studies, it is shown that the fire temperature exerts the most significant effects on the carrying capacities and deformability of EP joints, and the collapse resistance of corresponding assemblies is most sensitive to the fire temperature. However, the temperature durations have an insignificant influence on the development of flexural mechanism resistance, especially for those after 600 °C and 800 °C. Moreover, the corresponding refined FE models in consideration of the damage and fracture of ductile metals have been established to reproduce the structural response of specimens after high temperature. To improve the post-fire collapse-resisting performance of assemblies, both the lateral plate reinforcement and welding reinforcement methods were proposed and compared with the assistance of validated FE models. From a practical viewpoint, the reinforcement with lateral plates is determined as a superior technique for post-fire EP joints against progressive collapse.

Improving interfacial strength at room and elevated temperatures in ultrasonic welding of pure titanium: Interlayer-induced phase transformation and concentrated plastic deformation

Ultrasonic welding (USW) of Titanium (Ti) sheets presents several challenges, notably crack formation due to sliding friction. To overcome this problem, the present study applied USW to pure α-Ti sheets both with and without different interlayer metals (Al, Ni, and Fe). Al interlayer improved the strength at room temperature significantly to cause base metal fracture (1700 N) by inducing concentrated plastic deformation, facilitating bonding with Ti while minimizing damage. However, its strength decreased significantly at 150–300 °C, suggesting the limitation of using Al interlayer at elevated temperatures. Conversely, the Ni and Fe interlayers led to a two-phase strength development. This enhancement was due to β-phase transformation, which reduces the interfacial defects and generates a more pronounced β-stabilization effect. The Ni interlayer, which has a higher β-transus point (765 °C) and lower molybdenum equivalency, required a higher welding energy (1800 J) and longer diffusion time into Ti, resulting in a gradual increase in strength due to a slower β-Ti transformation. On the other hand, the Fe interlayer with a lower β-transus point relative to Ti (595 °C) achieved peak strength at a lower welding energy of 1200 J. Both Fe and Ni interlayers displayed only a slight decrease in strength (1500–1600 N) at 300 °C with base metal fracture, revealing better joint performance at higher temperatures. These insights suggest a strategical interlayer selection based on the operation temperature, where Al interlayers were advantageous for low-energy welding at room temperature application, Fe and Ni interlayers offer sufficient strength at elevated temperatures by facilitating the β-Ti formation.

A unified model for deformation instability based on energy inflection

The critical state of plastic instability preceding the fracture of metals, such as necking under tension and shear banding under torsion, was carefully analyzed from thermodynamics considerations. A unified criterion based on energy inflection (EI criterion) was established which takes stress states and loading rates into account. Different from many other criteria that are based on a specified energy, the proposed criterion assumes deformation instability would occur at the point of energy inflection. Through calibrating the parameters that governed the entropy evolution, the EI criterion was shown to exhibit significant efficacy in predicting the critical strain at which instability occurred for Ti-6Al-4V, aluminum alloy, and pure copper under various loading conditions. Furthermore, a comparison of the proposed EI criterion was made with other criteria in the literature.

Relationship between weld rip diameter and fatigue fracture mode for friction stir spot welded tension‐peel joints

AbstractIn this study, the effects of variations in the weld rip diameter on fatigue properties were investigated based on the fatigue fracture mechanism using tensile peeling‐type friction stir spot welding (FSSW) joints. The results of the fatigue test showed that the high and low applied force ranges exhibited base metal fracture and weld area fracture, respectively, and the difference in the weld rip diameter hardly affected the fatigue life. In contrast, in the middle‐applied force range, the fracture morphology changes as the weld rip diameter decreases, and it is apparent that joints exhibiting weld area fracture decrease the fatigue life. The fatigue fracture morphology of the tensile peeling‐type FSSW joints can be described by a competing risk model between the weld rip diameter. Increasing the heat input during joining, thereby increasing the crack propagation resistance at the pressure fusion interface, can suppress weld area fracture.

Development of a method for computer processing of fractographic images to assess the cohesion of inclusions to the matrix in the weld metal after its operational degradation and hydrogenation

In this paper, the method for automated processing of digital fractographic SEM images based on the modified local Otsu approach was developed. The effectiveness of the developed method was demonstrated. The method is used to identify inclusions or their traces at the bottom of dimples in weld metal fractures with subsequent assessment of their quantity. It is assumed that a decrease in the cohesion of inclusions to the matrix is one of the signs of damage to the weld metal at the microstructural level. Consequently, the more inclusions were found at the bottom of ductile fracture dimples, the stronger their connection with the matrix and, accordingly, the less damage in the weld metal. The effect of operational degradation of the weld metal on a circumferential weld on thick pipes of thermal power plant steam pipelines is analyzed. For this purpose, the features of cohesion of inclusions with the matrix in the weld metal in the initial state and after operation on the main steam pipeline for 23 years were compared. The influence of hydrogen (absorbed by the weld metal during long-term operation, as well as during its additional electrolytic charging) on the cohesion of inclusions in its structure with the surrounding matrix is also considered. A decrease in cohesion between these two components of the weld metal structure under the influence of all analized factors is shown. Thus, a method for quantitative assessment of fractographic features caused by the decohesion of inclusions from the matrix developed on the basis of analysis and computer processing of scanning electron microscope images, can be used to assess the operational damage of the weld metal at the microscopic level.

Microstructure and mechanical properties of laser welded and post-weld heat-treated K439B superalloy

Laser welding of the new nickel-based alloy K439B applied for engine turbine was carried out. The effects of post-welding heat treatment (PWHT) on the microstructure and mechanical properties of the joints were investigated. Semi-quantitative statistical results show that after PWHT, γ’ phases in the whole joint grow up, and the radii of the primary and secondary γ’ phases are about 25 and 150 nm, respectively. The results of mechanical properties show that PWHT improves the microhardness in the weld by about HV 100, and the critical resolved shear stress provided by γ’ phases increases from 4.8 to 92 MPa. The tensile strength and yield strength of the heat-treated joint are comparable to those of the base metal. The presence of a large amount of carbides is the reason for the preferential fracture of the base metal during the tensile process.

A closed-form expression of a ductile Fracture Limit Surface (FLS) for general plane stress deformation paths

Ductile fracture in metals is a complex phenomenon caused by the nucleation, growth, and aggregation of micron-sized voids. Existing theoretical models for fracture use experimentally determined Fracture Limit Curves (FLCs) in two-dimensional strain space, which require multiple curves to account for anisotropic effects. This paper proposes a practical closed-form analytical function for a Fracture Limit Surface (FLS) which has a conical shape in the Elastic Distortional Deformation Space (EDDS). The EDDS is a space generated by the components of the elastic distortional metric of the microstructural vectors that characterize elastic distortional deformations and the orientations of anisotropy. The FLS is determined by simulating only four experimental FLCs using a large deformation Eulerian formulation of the microstructural vectors and it depends only on the current state of elastic distortional deformation. It is shown that the failure limit points predicted by the FLS agreed very well with those predicted using a popular evolution equation for damage-based ductile fracture. Path-independence of the FLS is also examined using experimental data. In addition, the example of a B-pillar forming process is presented to show that the FLS can be applied to practical engineering problems with material points that experience general loading paths.

A Modified DF2016 Criterion for the Fracture Modeling from Shear to Equibiaxial Tension.

This study introduces a modified DF2016 criterion to model a ductile fracture of sheet metals from shear to equibiaxial tension. The DF2016 criterion is modified so that a material constant is equal to the fracture strain at equibiaxial tension, which can be easily measured by the bulging experiments. To evaluate the performance of the modified DF2016 criterion, experiments are conducted for QP980 with five different specimens with stress states from shear to equibiaxial tension. The plasticity of the steel is characterized by the Swift-Voce hardening law and the pDrucker function, which is calibrated with the inverse engineering approach. A fracture strain is measured by the XTOP digital image correlation system for all the specimens, including the bulging test. The modified DF2016 criterion is also calibrated with the inverse engineering approach. The predicted force-stroke curves are compared with experimental results to evaluate the performance of the modified DF2016 criterion on the fracture prediction from shear to equibiaxial tension. The comparison shows that the modified DF2016 criterion can model the onset of the ductile fracture with high accuracy in wide stress states from shear to plane strain tension. Moreover, the calibration of the modified DF2016 criterion is comparatively easier than the original DF2016 criterion.

Surface stress can initiate environment-assisted fracture in metals.

Controlling environmental effects in surface plasticity/fracture of metals is of interest for areas as diverse as manufacturing processes, product performance, and structural safety. The key to controlling these effects is understanding the effect of adsorbates on surface energy (γ) and surface stress (f). While γ has been well studied, the role of surface stress has received much less attention. We characterize surface stress induced in metals by adsorption of organic monolayers. Linear alkanoic acids of varying chain length (3-18) are deposited by molecular self-assembly onto one side of an aluminum cantilever, several centimeters in length. The surface stress is estimated from in situ measurement of the cantilever deflection. We find that the organic adsorbates induce large surface stress of -4 to +30N/m. Furthermore, we show that f may be tuned by varying adsorbate-molecule chain length. The stress data explain beneficial embrittlement of metal surfaces by organic adsorbates in cutting and comminution processes, and point to a critical role, hitherto ignored, for f in environment assisted cracking (EAC) phenomena. Our results suggest opportunities for utilizing controlled environment-assisted fracture as an aid-fracture as a friend-to enhance material removal processes, apart from using surface stress itself as an experimental probe to explore various manifestations of EAC.