Formation Of Brittle Cracks Research Articles

Features of siliconizing of pipe steel X12CrNi25-20 with various term of operation in the conditions of the reactionary furnace

Most refinery and petrochemical reaction furnaces are operated under harsh conditions characterized by high pressures, temperature, and aggressiveness of the process environment. The high temperature and features of the heated raw material contribute to the formation and deposition of coke on the surface of the furnace pipes, which through adhesion and diffusion phenomena has a negative effect and reduces the operational reliability of the entire furnace, causing such a negative phenomenon as carburization of the metal. Carburization contributes to a significant limitation of the ductility of the metal and increases brittleness, which leads to the formation of various unacceptable defects, in particular the formation of brittle cracks. The occurrence of defects of this type inevitably leads to the depressurization of the coil and, as a result, to the occurrence of emergency situations and emergency shutdown for repair. The problem of carburizing a metal can help solve the silicization of its surface layers, since silicon can serve as an effective protection for active saturation of the metal with carbon. In this regard, a relevant topic is the study of the features of the formation of a modified silicon-based surface layer in the coil tubes of a reaction furnace of steel X12CrNi25-20 with a different operating life in order to increase the reliability and reliability of the furnace during operation.

Effect of electron beam treatment on the structure and properties of AlCoCrFeNi high-entropy alloy

In the last decade the attention of scientists in the field of physical material science has been drawn to the study of high entropy alloys. Using wire arc additive manufacturing (WAAM) technology the high-entropy alloy (HEA) AlCoCrFeNi of nonequiatomic composition has been produced (wt. %: 15.64 Al; 7.78 Co; 8.87 Cr; 22.31 Fe; 44.57 Ni). It is shown by methods of modern material science that the HEA is a polycrystalline material with a grain size 4–15 μm along whose grain boundaries the second-phase particles are detected. It is shown by the mapping methods that bulks of grains are enriched in aluminium and nickel while grain boundaries contain chromium and iron. Cobalt is quasi-uniformly distributed in crystal lattice of the manufactured HEA. The material ultimate strength in testing for compression depends on production mode and varies within the interval from 1300 to 1800 MPa. The HEA wear parameter amounts to 1.4∙10−4 mm3/N⋅m, friction coefficient — 0.65. In testings for tension the material failure occurred by the mechanism of intragrain cleavage. The formation of brittle cracks along boundaries and in grain boundary junctions i.e. in sites containing the inclusions of second phases is revealed. One of the reasons of HEA increased brittleness is the revealed nonuniform distribution of elements in alloy structure. The HEA’s irradiation by pulsed electron beam with energy density of 10–30 J/cm2 (pulse duration 200 μs, number of pulses 3) results in material homogenization. High-velocity crystallization of molten surface layer of HEA samples is accompanied by the formation of columnar structure having a submicro-nanocrystalline structure. The HEA irradiation results in the increase in hardness and plasticity of the material. The largest increase (1.6 times) in ultimate strength is obtained in the alloy after electron beam processing with energy density of 30 J/cm2. Electron beam processing leads to decrease in microhardness of alloy surface layer up to 90 μm thick. The research was supported by Russian Research Foundation grant (RSF) (project № 20-19-00452).

Modification of high-entropy alloy AlCoCrFeNi by electron beam treatment

In this study, a high-entropy alloy (HEA) of Al–Co–Cr–Fe–Ni system was fabricated via wire-arc additive manufacturing technology (WAAM) in the atmosphere of pure Ar followed by high-intensity electron beam treatment. SEM, TEM, and X-ray diffraction methods were used to characterize the manufactured material's microstructure. The HEA has the following composition: Al (36.5 at. %), Ni (33.7 at. %), Fe (16.4 at. %), Cr (8,6 at. %) and Co (4.9 at. %). The obtained material without electron beam treatment has a polycrystalline structure with a grain size of 4–15 μm. Bulks of grains are enriched in Al and Ni, while grain boundaries contain Cr and Fe. Co is quasi-uniformly distributed in the crystal lattice of the manufactured HEA. The ultimate material strength in testing for compression depends on production mode and varies within the interval from 652 to 1899 MPa. The HEA wear parameter amounts to 1.4∙10−4 mm3/N∙m, friction coefficient - 0.65. In testings for tension, the material failure occurred by the mechanism of intragrain cleavage. The formation of brittle cracks along boundaries and in grain boundary junctions, i.e. in sites containing the inclusions of second phases, is revealed. The HEA's irradiation by a pulsed electron beam with the energy density of 10–30 J/cm2 (pulse duration 200 μs, number of pulses 3) results in material homogenization, decreased crystal lattice microdistortions, and an increase in sizes of coherent scattering regions. High-velocity crystallization of the molten surface layer of HEA samples is accompanied by columnar structure formation having a submicro-nanocrystalline structure.

Deformation behavior of high-entropy alloy system Al – Co – Cr – Fe – Ni achieved by wire-arc additive manufacturing

A non-equiatomic high-entropy alloy (HEA) of the Al – Co – Cr – Fe – Ni system was obtained using wire-arc additive manufacturing technology in the atmosphere of pure argon. The initial wire had 3 conductors with different chemical composition: pure aluminum wire (Al ≈ 99.95 %), chromium-nickel wire (Cr ≈ 20 %, Ni ≈ 80 %), and cobalt alloy wire (Co ≈ 17 %, Fe ≈ 54 %, Ni ≈ 29 %). The resulting sample of high-entropy alloy was a parallelepiped consisting of 20 deposited layers in height and 4 layers in thickness. The alloy had the following elemental composition, detected by energy-dispersive X-ray spectroscopy: aluminum (35.67 ± 1.34 at. %), nickel (33.79 ± 0.46 at. %), iron (17.28 ± 1.83 at. %), chromium (8.28 ± 0.15 at. %) and cobalt (4.99 ± 0.09 at. %). Scanning electron microscopy revealed that the source material has a dendritic structure and contains particles of the second phase at grain boundaries. Element distribution maps obtained by mapping methods have shown that grain volumes are enriched in aluminum and nickel, while grain boundaries contain chromium and iron. Cobalt is distributed in the crystal lattice of the resulting HEA quasi-uniformly. It is shown that during tensile tests, the material was destroyed by the mechanism of intra-grain cleavage. The formation of brittle cracks along the boundaries and at the junctions of grain boundaries, i.e., in places containing inclusions of the second phases, is revealed. It was suggested that one of the reasons for the increased fragility of HEA, produced by wire-arc additive manufacturing, is revealed uneven distribution of elements in microstructure of the alloy and also the presence in material volume of discontinuities of various shapes and sizes.

Деформация и разрушение высокоэнтропийного сплава AlCoCrFeNi

Using wire-arc additive manufacturing (WAAM)technology in an atmosphere of argon gas a non - equatomic high entropy alloy (HEA) of AlCoCrFeNi system is obtained: Al (35.67±1.34 at%), Ni (33.79±0.46 at%), Fe (17.28±1.83 at%), Cr (8.28±0.15 at%), Co (4.99±0.09 at%). Scanning electron microscopy method revealed that HEA is a polycrystal material having the grain size (4-15) µm with the particles of second phase located along the grain boundaries. Mapping methods showed that grain volumes are enriched in aluminum and nickel, while grain boundaries contain chromium and iron. Cobalt is distributed in the crystal lattice of the resulting HEA quasi-uniformly. It is shown that during tensile tests, the material was destroyed by the mechanism of intra-grain cleavage. The formation of brittle cracks along the boundaries and at the junctions of grain boundaries, i.e., in places containing inclusions of the second phases, is revealed. It was suggested that one of the reasons for the increased brittleness of HEA, is revealed uneven distribution of elements in the microstructure of the alloy and also the presence in the volume of material discontinuities of various shapes and sizes.

Features of weld metal brittle fracture in Charpy tests

The behavior of the metal of welded joints and low-alloy low-carbon steel in the process of impact bending is studied from the point of view of energy partitioning due to crack growth process. It is shown that for metal with different microstructures there are significant differences in the degree of deformation before the formation of a brittle crack. The relationship between the work spent before the formation of a brittle crack and the overall work of fracture is shown. Statistical data confirming the increased susceptibility of the metal of welded joints to early formation of brittle cracks are presented. It is shown that the destruction of the metal of welded joints takes place with a large spent of energy on the final fracture of the specimen relative to the base metal.

Crevasse Propagation on Brittle Ice: Application to Cycloids on Europa

AbstractExisting lineaments on the surface of the Jovian moon Europa are thought to be the result of ongoing brittle crack formation in the elastic regime. Arcuate features are called cycloids and can be modeled using linear elastic fracture mechanics. Here, we build on existing terrestrial models of rift propagation and extend them to cycloids on the moon. We propose that these cracks tend to grow as a series of nearly instantaneous events, spaced by periods of inactivity. The behavior is similar to what is observed on Antarctic ice shelves, where rifts can remain dormant for years. We argue that dormant periods between growth events could explain the presence of cycloids on Europa even without invoking secular motion of the crust. Furthermore, being able to model propagation events and their timing should help future missions exploring the moon.

Acoustic emission from finite two-dimensional cracks: Directivity functions and frequency spectra

In this paper, the acoustic emission accompanying the formation of brittle cracks of finite length is investigated theoretically using the approach based on the application of Huygens' principle for elastic solids. In the framework of this approach, the main input information required for calculations of acoustic emission spectra is the normal displacements of the crack edges as a function of frequency and wavenumber. Two simple approximate models defining this function are used in this paper for calculations of the acoustic emission spectra and directivity functions of a crack of finite length. The simplest model considers a crack that opens monotonously to its static value. The more refined model accounts for oscillations during crack opening and considers a crack of finite size as a resonator for symmetric modes of Rayleigh waves propagating along the crack edges and partly reflecting from the crack tips. Analytical solutions for generated acoustic emission spectra are obtained for both models and compared with each other. It is shown that resonant properties of a crack are responsible for the appearance of noticeable peaks in the frequency spectra of generated acoustic emission signals that can be used for evaluation of crack sizes. The obtained analytical results are illustrated by numerical calculations.

Nonlinear elastic finite fracture mechanics: Modeling mixed-mode crack nucleation in structural glazing silicone sealants

Requiring both stress and energy conditions to be met simultaneously proved key to modeling brittle crack formation at singular and nonsingular stress concentrations in linear elastic materials. The present work extends this so-called coupled stress and energy criterion to brittle crack nucleation in hyperelastic media using the example of silicone adhesives. For this purpose, we provide a comprehensive constitutive as well as fracture mechanical characterization of the structural silicone adhesive DOWSIL™ 993 using a large set of experiments and propose a mixed-mode failure model for crack initiation in nonlinear elastic materials. Characterized in independent experiments, the model is used to determine critical loads of hyperelastic adhesive bonds in both shear and tension dominated configurations. For any of the examined adhesive joint configurations the model predicts and explains size effects and agrees well with experimental findings. We study stable and unstable crack propagation observed in video recordings of our experiments. It is shown that crack initiation, crack growth and crack arrest are caused by nonmonotonic energy release rates and can be predicted. Effects of excess energy available after crack nucleation and initial unstable crack growth are discussed.

Tribological behavior of an amorphous Zr20Ti20Cu20Ni20Be20 high-entropy alloy studied using a nanoscratch technique

In this study, friction and wear behaviors of an amorphous Zr20Ti20Cu20Ni20Be20 high-entropy alloy (a-HEA) were characterized using a nanoscratch method. Scratch tests were carried out under both ramping and constant loading conditions to measure the coefficient of friction (COF) and wear resistance of the amorphous alloy. The morphology of scratched surface and subsurface was further examined using scanning and transmission electron microscopes. It was found that friction behavior of the a-HEA can be divided into two regions: elastic and plastic. In the elastic region, the COF dramatically decreases with increasing normal force, however, it monotonically increases in the plastic region. Morphological observation indicated that brittle fracture dominated in the plastic region, which was consistent with the result that a series of load serrations were accompanied by the formation of brittle cracks along the scratched track. Wear performance of the a-HEA was also evaluated and compared with “conventional” metallic glasses and crystalline high-entropy alloys. As a result of high hardness and large elastic recovery, the current a-HEA exhibited good wear resistance and low COF, behaved more like brittle amorphous alloys rather than ductile high-entropy alloys.

Microstructural Characterization and Mechanical Properties of Diffusion Bonded Ti–Ni In Situ Metal Intermetallic Laminates

In this work, Ti–Ni based in situ metal intermetallic laminates (MILs) were prepared and the various intermetallic phases formed were analyzed. Solid state diffusion bonding technique was adopted to produce transition joints between commercially pure Ti and Ni sheets of thickness 0.5 and 0.2 mm respectively at three different temperatures such as 770, 800 and 850 °C for various bonding duration. Microstructural characterization and phase analysis along the cross section of the bonded samples revealed the presence of three different intermetallic phases namely Ti2Ni, TiNi and TiNi3. Quasi static compression tests carried out on the MILs along the parallel and perpendicular direction revealed that the strength of the MILs increased with increase in total intermetallic layer thickness. The fractographs taken on fracture surface revealed that the failure of the MILs was mainly due to the formation of brittle cracks and de-bonding in the intermetallic layers. However, the metallic layers added strength to the MILs.

Compliant Yet Brittle Mechanical Behavior of Li2S–P2S5 Lithium‐Ion‐Conducting Solid Electrolyte

Young's modulus, hardness, and fracture toughness are measured by instrumented nanoindentation for an amorphous Li2S–P2S5 Li-ion solid electrolyte. Although low elastic modulus suggests accommodation of significant chemomechanical strain, low fracture toughness can facilitate brittle crack formation in such materials.

Compilation of a thermodynamics based process signature for the formation of residual surface stresses in metal cutting

The abrasive wear behaviour and fatigue life of many components in mechanical engineering depend on their surface stress states. Compressive surface stresses hinder ductile or brittle crack formation as well as crack propagation into the bulk material and therefore increase the lifetime of dynamically or abrasively loaded parts. The thermo-mechanical loads acting on the workpiece surface during metal cutting determine the stress state after processing. Under the high strain rates of cutting, the underlying mechanisms include the accumulation of stress fields around dislocations resulting from plastic deformation as well as thermal expansion and shrinking phenomena associated with the dissipation of mechanical energy. Until now, the underlying thermodynamics of residual stress formation in metal cutting are hardly understood quantitatively, which explains the current dominance of empirical-iterative design procedures of cutting processes regarding residual stresses.In this work, the derivation and experimental validation of a thermodynamics based finite element model for the energy transformations during residual stress formation are presented. For the first time, the residual surface stress state is correlated with the mechanical and dissipative thermal energies, which are transformed during processing. It is shown how each residual stress component relates to these energy transformations. These findings are applied to formulate characteristic process signatures, which may be used to describe the formation of residual stresses in other manufacturing technologies as well.The proposed work includes orthogonal cutting tests on quenched and tempered AISI 4140, subsequent determination of the residual stress states using a diffractometric measurement technique, the analytical description of the energy transformations during residual stress formation as well as its implementation into a finite element process model.

Tracking stress and hydrothermal activity along the Eastern Lau Spreading Center using seismic anisotropy

Using P-wave refraction data from the L-SCAN controlled-source seismic experiment, the three-dimensional anisotropy structure of the upper crust is determined along the Eastern Lau Spreading Center (ELSC). The tomographic image of anisotropy magnitude and orientation is constructed from ∼197 000 travel time measurements of P-wave arrivals recorded on 83 ocean bottom seismometers, providing the most detailed and extensive view of upper-crustal spreading center anisotropy to date. P-wave speeds vary with the azimuth of the seismic ray path, a type of anisotropy produced by stress-aligned lithospheric cracks and microcracks. Across the study area, the fast axes of anisotropy are generally oriented parallel to the trend of the spreading center, as expected for ridge-parallel cracks that form in association with seafloor spreading. The seismic study encompassed four individual spreading segments of the ELSC separated by three overlapping spreading centers (OSC). The two larger OSCs exhibit high anisotropy that penetrates deep into the upper crust. Surrounding these OSCs, and in the wake of the largest, are regions where the fast axes of anisotropy are misaligned relative to the general trend. In these areas, the observations agree with numerical models and seafloor morphology that suggest tensile stress concentrations and a high degree of brittle crack formation. The tomographic images indicate that the location and dynamics of persistent ridge offsets can be tracked back through time via their anomalous anisotropy. Along the spreading centers, and away from ridge tips, the anisotropy is greater where the melt supply is inferred to be greater, and smaller where the melt supply is inferred to be smaller. This is opposite to the trend expected if simple tectonic stress models govern anisotropy. Alternatively, hydrothermal activity is expected to increase with magma supply and can explain higher anisotropy via hydrofracturing. This study provides the first evidence that seismic anisotropy tracks variations in hydrologic activity along the crests of oceanic spreading centers.

Modelling brittle crack formation at geometrical and material discontinuities using a finite fracture mechanics approach

In this study, finite fracture mechanics procedures are employed to predict crack formation at geometrical and material discontinuities in brittle elastic structures. A hybrid failure model is utilised taking into account the stress field in the undamaged structure and the energy balance for the formation of cracks. Asymptotic formulations are compared to a direct numerical implementation. Experiments carried out on notched brittle specimens exhibiting various geometries and loading-modes are analysed by means of both approaches. Additionally, free-edge effects in composite laminates are analysed. It is found that the predictions from the model agree well with experimental results.

Mechanical properties of a 316L/T91 weld joint tested in lead–bismuth liquid

The mechanical strength of T91/316L weld joint assembled by electron beam process is investigated in air and in a liquid lead bismuth bath at 300 and 380 °C using the small punch test. It is shown that the mechanical response in air of the weld joint is similar to that of the T91 base material. The plastic deformation is mainly concentrated in the T91 part of the weld joint which promotes cracking in this material. Testing in liquid lead bismuth bath results in a reduction in ductility and the formation of brittle cracks. The T91/weld interface is found to be rather resistant as it cracks late in the test and after a large crack propagated in the T91 steel.

Orientation and the extent of exfoliation of clay on scratch damage in polyamide 6nanocomposites

The major objectives of this work are to understand the effects of organoclay, its extent ofexfoliation and orientation, and indenter geometry on the scratch characteristics ofpolyamide 6/organoclay nanocomposites. Two different organically treated clays are usedfor this purpose and their structural parameters in a polyamide 6 matrix quantified. It isshown that, while the material properties are important for scratching resistance, they arenot the only determinants of the scratch performance of materials. Further, despite provingbeneficial to scratch resistance, in terms of residual depth, the presence (and exfoliation) oforganoclay promotes the formation of brittle cracks during scratching. But with noorganoclay layers, plastic flow controls the scratch damage in neat polyamide 6 with largeresidual depths. Factors such as orientation of clay layers and variations of indentertip geometry also exert dominant effects on scratch penetration resistance anddamage. Additionally, significant plastic flow and rotation of organoclay layersfrom the original configuration are observed underneath the sliding indenter.

Behavior of unirradiated Zr based uranium metal fuel under reactivity initiated accident conditions

Reactivity initiated accident (RIA) tests with 4 unirradiated Zr based uranium metal fuel rods were performed to establish a criterion which should be observed under RIA conditions. Of the four tests, fuel failures were observed in the two tests that experienced the maximum energy depositions of 188 and 212 cal/g, respectively. However, the fuel failures were not observed at the place of a maximum energy deposition but at the position where the thermocouples were installed; one failed at the position whose local energy deposition was 150 cal/g, and the other one at the place with energy deposition of 170 cal/g. The fuel failures seem to have occurred because excessive pressure, which was caused by the partial melting of the fuel meat, was applied to the cladding with a reduced thickness. However, other parts of the fuel rods including the place of a maximum energy deposition maintained their integrity and a big change in the temperature and pressure in the internal capsule, which would be an indication of the fragmentation and dispersion of the fuel meat into the internal capsule, was not observed. Visual inspection also showed that, except for the thermocouple positions, there was no trace of clad failure such as the formation of brittle cracks in the cladding or melting of the cladding. Therefore, for the Zr based uranium metal fuel rods, it can be concluded that the threshold energy deposition above which fragmentation and dispersion of fuel meat into the primary coolant system is expected to occur could be higher than 212 cal/g.

Analysis of the causes of the appearance of brittle cracks in large stamped preforms from steel 30KhGSNMA-VD

1. Intermediate annealing after stamping large preforms of steel 30KhGSNMA-VD can be accompanied by temper brittleness of kind II, which causes the formation of brittle cracks in subsequent grinding and quenching. 2. In order to prevent the formation of brittle cracks in stamped preforms, they should be subjected to sandblasting after grinding and decelerated heating for quenching in the final treatment.

An Inadequacy in a Common Micromechanics Approach to Analysis of Damage Evolution in Composites

This article examines a commonly used micromechanics approach to evolution of damage in composites. In this approach an initial crack pattern is assumed and on the basis of the local stress fields and a (brittle) fracture criterion, further evolution of the same crack pattern is determined. We question this assumed self-similarity in the damage mode by considering two alternative crack modes, one of which would result in deviation from the assumed crack pattern. Using a simple maximum stress criterion for formation of brittle cracks, it is found that the self-similarity breaks down before saturation of cracking in the assumed crack pattern. This seemingly inherent inadequacy in the damage evolution analysis is discussed.