Formation and Anisotropic Mechanical Behavior of Stacking Fault Tetrahedron in Ni and CoCrFeNiMn High-Entropy Alloy

Stacking fault tetrahedron (SFT) is a kind of detrimental three-dimensional defect in conventional face-centered cubic (FCC) structural metals; however, its formation and anisotropic mechanical behavior in a CoCrFeNiMn high-entropy alloy (HEA) remain unclear. In this work, we first performed molecular dynamics simulations to verify the applicability of the Silcox-Hirsch mechanism in the CoCrFeNiMn HEA. The mechanical responses of the SFT to shear stress in different directions and that of the pure Ni counterpart were simulated, and the evolutions of the atomic structures of the SFTs during shear were analyzed in detail. Our results revealed that the evolution of the SFT has different patterns, including the annihilation of stacking faults, the formation and expansion of new stacking faults, and insignificant changes in stacking faults. It was found that the effects of SFT on the elastic properties of Ni and HEA are negligible. However, the introduction of SFT would reduce the critical stress, while the critical stress of the CoCrFeNiMn HEA is much less sensitive to SFT than that of Ni.

Study of the destruction of the chassis main cross member made of VT22 alloy

The reasons for the destruction of the chassis main cross member made of alloy VT22 are considered and analyzed in bench test conditions. The chemical composition, mechanical properties, as well as macro- and microstructure of the material were studied. The tests of the cross-arm material for crack resistance and low-cycle fatigue (LCF) with the determination of the durability were carried out. The results of analysis proved that material meets the declared performance characteristics. A fractographic study of the traverse fracture showed that the fracture occurred from several foci according to the fatigue mechanism. The length of the longest fatigue crack was 1.7 mm and the critical stress intensity factor KIc was thus attained. Proceeding from the dimensions of the part at the site of fracture, the maximum crack length and the value of the critical stress intensity factor obtained experimentally KIc = 56.5 MPa • m1/2, we have calculated the nominal tensile stress at the moment of fracture. The calculated value of the nominal stresses is 1022 MPa, which is comparable to the yield strength of the material (1100 MPa). A high level of tensile stresses in the loading cycle is considered the most probable reason for the destruction of the chassis main cross member in the conditions of bench tests.

Characterization of Critically Stressed Fractures Using Fluid-Flow Models for Naturally Fractured Reservoirs

Fracture characterization, including permeability and deformation due to fluid flow, plays an essential role in hydrocarbon production during the development of naturally fractured reservoirs. The conventional way of characterization of the fracture is experimental, and modeling approaches. In this study, a conceptual model will be developed based on the structural style to study the fracture distributions, the influence of the fluid flow and geomechanics in the fracture conductivity, investigate the stress regime in the study area. Understanding the fracture properties will be conducted by studying the fracture properties from the core sample, image log interpretation. 3D geomechanical models will be constructed to evaluate the fluid flow properties; the models consider the crossflow coefficient and the compression coefficient. According to the model results, the fracture permeability decreases with increasing effective stress. The degree of decline is related to the crossflow coefficient and the compression coefficient. Most of these reservoirs are mainly composed of two porosity systems for fluid flow: the matrix component and fractures. Therefore, fluid flow path distribution within a naturally fractured reservoir depends on several features related to the rock matrix and fracture systems' properties. The main element that could help us identify the fluid flow paths is the critical stress analysis, which considers the in-situ stress regime model (in terms of magnitude and direction) and the spatial distributions of natural fractures fluid flow path. The critical stress requires calculating the normal and shear stress in each fracture plane to evaluate the conditions for critical and non-critical fractures. Based on this classification, some fractures can dominate the fluid-flow paths. To perform the critical stress analysis, fracture characterization and stress analysis were described using a 3D stress tensor model capturing the in-situ stress direction and magnitude applied to a discrete fracture model, identifying the fluid flow paths along the fractured reservoir. The results show that in-situ stress rotation observed in the breakouts or drilling induce tensile fractures (DITFs) interpreted from borehole images. The stress regime changes are probably attributed to some influence of deeply seated faults under the studied sequence. the flow of water-oil ratio through intact rock and fractures with/without imbibition was modeled based on the material balance based on preset conceptual reservoir parameters to investigate the water-oil ratio flow gradients

Enhanced assessment of technological factors for Ti-6Al-4V and Al-Cu-Mg strength properties

Introduction. The strength of construction materials when used under cyclic loads is of great importance in design engineering. A significant number of factors that affect the fatigue resistance have predetermined the creation of numerous methods that consider such influence. Nondestructive methods that are based on the connection of the physical degradation of material with strain properties enable evaluating experimentally the fatigue properties of materials. Purpose of study: the analysis of the processes of energy dissipation and strain accumulation during the inelastic cyclic strain of samples, using the VT6 (Ti-6Al-4V) titanium alloy and the D16 (Al-Cu-Mg) aluminum alloy before and after the technological impact. The work experimentally investigates the physical processes of degradation of the VT6 and D16 alloy samples that accompany the process of fatigue failure in materials with homogeneous and inhomogeneous stress-strain states in the concentrator (in the form of a hole and a weld). Typical modes are used to reach the fatigue testing that determine the critical stress in a material sample – the stress at which physical properties (temperature, strain) change without reaching the fatigue failure of samples. Critical stress amplitudes in the cycle, based on the data obtained during the experiment and the results of mathematical simulation, are compared. The effect of stress concentrators on critical loads that a detail can withstand after a unit operation is estimated by the finite-element method (FEM). As a result, the effect of the operational and technological factors on critical stress determined by strain and temperature is estimated. Comparative tests of the VT6 and D16 alloy samples with and without stress concentrators showed that the amplitudes of critical stress decrease by more than 30% in comparison with the ones that are without stress concentrators. The low-cycle fatigue tests of the D16 alloy samples are carried out. Mathematical simulation of the cyclic strain of the samples is carried out using MSC.Marc package. The results of the cyclic loading tests, which show that the characteristics of the technological process reduce the amplitudes of the critical stress of the VT6 and D16 alloys and affect the fatigue properties of the D16 aluminum alloy, are discussed. Mathematical simulation corresponded positively to the experimental data. Such correspondence indicates the possibility of conducting qualitative numerical assessments of the beginning of the inelastic strain accumulation process in structures with stress concentrators under the cyclic stress and the increasing stress amplitude, using the typical sample made of hardening elastoplastic material.

Randomized approach to determine dynamic strength of ice

The randomized method of Sign-Perturbed Sums (SPS) is applied within the framework of the incubation time approach to evaluate the dynamic strength of ice. The experimental data of [Carney et al., 2006; Wu and Prakash, 2015; Saletti et al., 2019] is analysed in order to estimate strength parameters of ice and describe the observed strain-rate sensitivity curves. The independence of incubation time value on the ice temperature is established in contrast with the significant dependency of the critical stress parameter. The obtained confidence interval of the spalled ice is in good correspondence with the scatter observed experimentally.

Stability of nanobeams and nanoplates with defects

The sensitivity of critical buckling load and critical stress concerning different geometrical and physical parameters of Euler-Bernoulli nanobeams with defects is studied. Eringen’s nonlocal theory of elasticity is used for the determination of critical buckling load for stepped nanobeams subjected to axial loads for different support conditions. An analytical approach to study the impact of discontinuities and boundary conditions on the critical buckling load and critical stress of nanobeams has been developed. Critical buckling loads of stepped nanobeams are defined under the condition that the nanoelements are weakened with stable crack-like defects. Simply supported, clamped and cantilever nanobeams with steps and cracks are investigated in this article. The presented results are compared with the other available results and are found to be in a close agreement.

The Ageing and Rejuvenation of Soft, Glassy Complex Fluids

<p>Suspensions of multiarm star polymers are studied as models for soft colloidal interactionsin colloidal glasses. Establishing a pre-shearing protocol which ensures a reproducible initial state (the "rejuvenation" of the system), we report here the stress evolution from startup for two different concentrations for a range of shear rates using conventional rheological techniques. We show the existence of critical shear rateswhich are functions of the concentration. When the suspensions are sheared at rates below the critical rates, the stress rises to a common value which is also a function of the concentration. The system thus evolves into a yield stress-like fluid. This behavior manifests itself as an evolution from a monotonic, slightly shear-thinning flow curve to a flow curve dominated by a stress plateau. Complementary to the controlled-rate measurements, stress-controlled measurementsshow that for a stress below the critical stress, the rate at which strain is acquired drops several orders of magnitude, providing evidence of a lower branch of the flow curve. In stress-controlled ageing experiments, the material recovers an increasing fraction of the strain acquired under stress with waiting time upon cessation of the (less than critical) stress. The freshly rejuvenated suspension recovers a mere 2%of the acquired strain, while for a waiting time of 104 s the material recovers 97% of the acquired strain. The material thus appears to evolve from a nearly ideal fluid to a nearly ideal solid. We relate this bulk evolution to spatially and temporally resolved Rheo-NMR velocity profiles which clearly show an evolution to a strongly shear-banded state. The velocity of the suspension in the lower shear band is below the uncertainty of the experiment. The growth of the (assumed) zero-shear band is well described by a Gompertz relation. The effects of shear-rate, temperature and waiting time on the Gompertzparameters are investigated. Phenomenological understanding is provided through a scalar model that describes the stress-dependent free-energy landscape. Using a dual-minimum free-energy landscape, the model is able to replicate the behaviour of the stress after startup in shearratecontrolled experiments, the flow curve and the velocity profiles across the gap of a Couette geometry. The Large-Amplitude-Oscillatory-Shear (LAOS) response is reported along with discussions of current LAOS analysis techniques. The stress response to LAOS of the star suspensions is well described in a Cox-Merz manner by a modified Cross model. The modified Cross model highlights an asymmetry in the LAOS response. This constitutes the first ever report of asymmetric LAOS responses. The asymmetry is followed as a function of time using two complementary scalar variables. A speculativeinterpretation is given to account for the evolution of the asymmetry.</p>