Structural rejuvenation and relaxation of a metallic glass under the periodically thermal-mechanical loading

The current work focuses on effect of the thermomechanical protocol which can lead either to aging or rejuvenation of glass through decoupling of the thermal and mechanical processes. We demonstrated that the Labased metallic glass shows a kind of material hypomnesia because a clearer rejuvenation trend is observed just after imposing strain oscillations of increasing amplitude than after a long period from that, as it is proved by the results of stress relaxation. There is a threshold value of the oscillation amplitude that separates the effect of the protocol between acceleration of aging and rejuvenation. The activated energy spectrums imply plasticity and mechanical heterogeneity of the metallic glass promoted by simple mechanical stimulation. Mechanical perturbations applied not long ago in time are vital for hypomnesia metallic glasses, in which different histories lead to various degrees of aging or rejuvenation. The correlation between the thermomechanical properties of metallic glasses and the previous application of strain oscillations of various amplitude is unveiled in this work, revealing an effective tool for regulating the structural state of metallic glasses through an easy-operated method

Unsteady interaction and dynamic stability analysis of a two-stage-to-orbit vehicle during transverse stage separation

Hypersonic stage separation is a significant process for the future two-stage-to-orbit (TSTO) vehicle. Strong and complex interstage aerodynamic interference may result in drastic aerodynamic forces and moments, potentially conflicting with the safe separation. Therefore, the dynamic stability of the vehicle during separation is critical to aerospace safety. In this study, numerical analysis of a hypersonic flow with Ma = 6.7 past a parallel-staged TSTO vehicle during stage separation is performed by laminar flow simulations. The TSTO vehicle consists of a wave-rider and a spaceplane as booster and orbiter, respectively. Considering the different centers of gravity for the orbiter during separation, the longitudinal dynamic stability of the orbiter is analyzed based on the dynamic characteristic of the center of pressure (CoP) in different cases. The dynamic separation behavior, aerodynamic characteristics, and typical flowfield patterns are clarified. Moreover, the derivative of CoP to time is proposed and analyzed in detail, which serves as an indicator to determine the dynamic stability and the safe stage separation. A safety separation judgment criterion is also proposed based on the CoP dynamic characteristics for the TSTO vehicle, and the mechanism of CoP variations associated with the unsteady aerodynamic interference during stage separation is revealed.</p

Microstructure features induced by fatigue crack initiation up to very-high-cycle regime for an additively manufactured aluminium alloy

Fatigue failure can still occur beyond 10(7) cycles, i.e. very-high-cycle fatigue (VHCF), in many metallic materials, such as aluminium alloys and high-strength steels. For VHCF of high-strength steels, a fine granular area (FGA) surrounding an inclusion is commonly identified as the characteristic region of crack initiation on the fracture surface. However, no such FGA feature and related crack initiation behaviour were observed in VHCF of conventionally cast or wrought aluminium alloys. Here, we first reported the distinct mechanisms of crack initiation and early growth, namely the microstructure feature and the role of FGA in VHCF performance for an additively manufactured (AM) AlSi10Mg alloy. The AM pores play a key role in fatigue crack initiation similar to that of the inclusions in high-strength steels, resulting in almost identical FGA behaviour for different materials under a range of mean stress with a stress ratio at R 0. The profile microstructure of FGA is identified as a nanograin layer with Si rearrangement and grain boundary transition. This process consumes a large amount of cyclic plastic energy making FGA undertake a vast majority of VHCF life. These results will deepen the understanding of VHCF nature and shed light on crack initiation mechanism of other aluminium and AM alloys. (c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology

Experimental study on particle deposition of Fe3O4 in supercritical heat exchange tube

Particle deposition poses a significant challenge to the economics and safety of supercritical boilers. Under-standing the deposition behavior of particles on the steam-water side wall is essential. A supercritical particle deposition system was designed and build according to the actual conditions. The effects of fluid thermal state, flow rate and exposure time on particle morphology, deposition layer morphology and deposition distribution were investigated. The results showed that FeCl2 was oxidized to micron sized Fe3O4 particle, and the salt crystals had a redissolution behavior in supercritical water. The particle deposition layer was a three-layer structure, possibly related to the gas-like fluid clusters and turbulence. The deposition distribution was related to the flow state, and there was a forward shift of the peak point with exposure time. Our results help to un-derstand the particle deposition behavior in supercritical heat exchangers

Numerical investigation for subsonic performance of the high-pressure capturing wing configuration with wing dihedral

High-pressure capturing wing (HCW) aerodynamic configuration demonstrates favorable aerodynamic perfor-mance under hypersonic conditions, and its novel additional lifting wing (also known as HCW) has the potential to enhance lift characteristics under subsonic conditions. Therefore, this configuration presents a promising option for wide-speed-range vehicles. However, the stability characteristics of this novel configuration under subsonic conditions have not yet been investigated. In this paper, the effects of wing dihedral angles on the subsonic aerodynamic characteristics of a parametric conceptual HCW configuration with two lifting wings were investigated. Specifically, the design variables for this study were the dihedral angles of the upper HCW and the lower delta wing. To obtain the distributions of various aerodynamic parameters over the design space, a combination of the uniform experimental design method, computational fluid dynamics numerical simulation techniques, and kriging surrogate model algorithm was employed. The findings suggest that wing dihedral angles have a greater impact on the lift-drag ratio (L/D) at low angles of attack compared to high angles of attack. L/D can be enhanced by incorporating a positive dihedral angle in HCW, and as the delta wing's negative dihedral angle rises, L/D tends to increase earlier and decrease later at low angles of attack. Furthermore, for the lon-gitudinal, lateral, and directional stability characteristics of this configuration, the positive dihedral angles of the delta wing offer greater overall advantages than negative ones in improving them, and the positive dihedral angles of HCW yield more significant enhancements in stability compared to negative ones

Hyperelastic constitutive relations for soft elastomers with thermally-induced residual stress

Residual stress widely exists in soft materials. Besides growth, inhomogeneous thermal expansion is also a primary cause of residual stress. However, establishing a proper hyperelastic constitutive relation is a great challenge since the existing theories cannot capture the change of underlying mechanical responses triggered by temperature variations. In this paper, a general hyperelastic constitutive relation for soft elastomers with thermally-induced residual stress is developed. We first reveal the initial temperature dependence of conventional thermoelastic models. This property attributes the alteration of the underlying thermoelastic response to free thermal expansions. Then, a compatibility-broken curvature compensation (CBCC) framework is established based on finite thermoelasticity. It generates a free thermal expansion to eliminate the Riemannian curvatures of the virtual stress-free configuration derived from the isothermal stress release. Such a mechanism indicates the non-local effect of the residual stress, which fundamentally modifies the traditional view that invariant formulations cover all the possible functional dependence of residual stress. Also, the obtained governing equations are similar to Einstein field equations of the general theory of relativity. This similarity may deeply imply a standard mechanism concerning the curvature compensation leading to residual stress genesis. We finally conduct comparative analyses of the spherically symmetric and axisymmetric problems between the current constitutive relation and the existing models. The influences of adopting distinct residual stresses, the performance of the non-local effect, and the availability of the new constitutive relation are investigated in detail. This framework can shed some light on the constitutive modeling of soft materials

Multi-scale fatigue failure features of titanium alloys with equiaxed or bimodal microstructures from low-cycle to very-high-cycle loading numbers

Fatigue failure types and their characteristics of titanium alloys with equiaxed or bimodal microstructures were systematically studied in low-cycle, high-cycle and very-high-cycle regimes. Based on the fractography, there are multi-scale features closely related to the behavior of crack initiation and early growth in specific microstructure under different loading cycles. At macro-scale, the presence and location of crack initiation with a rough area (RA) are dominated by the external loads and the number of equiaxed alpha grains in local microstructure domain. At micro-scale, facets are the most prominent features as the mean stress and the failure life increase. There is a trade-off between facets and the granular RA surface in very-high-cycle fatigue (VHCF) under stress ratio R from a positive to a negative value. At nano-scale, due to numerous cyclic pressing, the microstructure underneath the fracture surface is refined to form nanograins and shaping the granules within RA region, which keeps a relatively high VHCF strength at R =-1. As mean stress increases, the fatigue resistance dramatically degrades in VHCF under R > 0, because the RA morphology changes from granules to facets

Plastic deformation capacity obtained by the process of strain delocalization in Hf_0.5Nb_0.5Ta_0.5Ti_1.5Zr multi-principal-element alloy

Multi-principal element alloys (MPEAs) with body-centered-cubic (BCC) structures composed of elements of IVB, VB, and VIB usually exhibit high compressive strength and superior high-temperature performance. However, premature necking under tensile loading at ambient temperature limits their applications. Herein, we report the Hf0.5Nb0.5Ta0.5Ti1.5Zr MPEA with a single BCC phase, which performs considerable tensile plasticity by the process of strain delocalization. The formation of dispersed slip bands and two major strain localized regions suppress premature necking. The strain-localized region with a larger strain gradient realized strain delocalization during non-uniform deformation, resulting in considerable tensile plasticity (similar to 20%) with a yield strength of 922 MPa. Two dominated work hardening mechanisms were revealed. One is the geometrically necessary dislocations (GNDs) produced by non-uniform deformation which can coordinate deformation incompatibility, thus enhancing plastic deformation capability. The other is the lattice distortion which can provide an easy path for the cross slip of dislocations and realize strain delocalization. These two kinds of work-hardening mechanisms jointly contribute to the significant plastic deformation capacity of the Hf0.5Nb0.5Ta0.5Ti1.5Zr MPEA

Induction of deformation twinning by dynamic strain aging effect: Rate-temperature coupling effect and constitutive modeling

This study focuses on the rate-temperature dependence analysis and constitutive modeling of the DSA-induced deformation twinning, which is a new plastic deformation mechanism discovered in several low-stacking-fault-energy metallic materials. Two types of deformation twinning, namely primary deformation twinning and DSA-induced deformation twinning, were noticed during the plastic deformation of the commercially pure titanium (CP-Ti). The strain, temperature, and strain rate ranges of the DSA-induced deformation twinning, as well as its rate-temperature coupling dependence, were systematically analyzed. The DSA-induced deformation twinning becomes more pronounced with the increasing strain rate and thereby plays a more significant role in enhancing the strain hardening rate than the primary deformation twinning. According to the dynamic Hall-Petch effect of the deformation twinning, a thermo-viscoplastic constitutive model based on the microstruc-tural evolution and the thermal activation theory was developed. The volume fraction of the deformation twins during plastic deformation was expressed as a function of strain, temperature, and strain rate. The developed model was shown to be able to describe the S-shaped true stress vs. true strain curves over the wide ranges of temperature (77-998 K) and strain rate (0.001-8000/s). Finally, considering the interaction between the DSA and deformation twinning, the authors proposed the synergistic strengthening & toughening effect of DSA and deformation twinning, which was believed as a promising mechanism to achieve the simultaneous strengthening and toughing of the metallic materials

Development of BB model and investigation of P-wave propagation across jointed rock masses using CDEM

This paper describes an advanced coupling method of finite element and discrete element called the ContinuumDiscontinuum Element Method (CDEM). In this study, the Barton-Bandis model(BB model) for joint non-linear deformation is introduced into the CDEM to investigate the propagation of stress waves in jointed rock masses. The paper provides a detailed account of the development process of the BB model. Then, the developed model is utilized to simulate several representative cases of stress wave propagation in jointed rock masses, revealing the principles of stress wave propagation in jointed rock masses. The findings of this study demonstrate that the simulation results of the CDEM exhibit favorable consistency with experimental and theoretical results. This confirms the capability of the CDEM to accurately and effectively simulate stress wave propagation in jointed rock masses, thereby expanding the applicability of the CDEM. This research provides powerful support for further understanding and analysis of stress wave propagation problems in jointed rock masses