Auxetic metamaterials have attracted much attention due to their outstanding advantages over traditional materials in terms of shear capacity, fracture resistance, and energy absorption. However, there are lack of design inspirations for novel auxetic structures. According to the materials databases of atomic lattice, some natural crystals possess negative Poisson’s ratio (NPR). In this paper, the mechanism of auxeticity in microscale Ti crystal is investigated through density functional theory simulation. Then we propose a macroscopic auxetic metamaterial by mimicking the microscopic atomic lattice structure of the bodycentered cubic Ti crystal. The NPR property of the macroscopic metamaterial is verified by theoretical, numerical and experimental methods. The auxeticity keeps effective when scaling up to macroscopic Ti crystal-mimic structure, with the similar deformation mechanism. Furthermore, from the geometric parameter investigation, the geometric parameters have great influence on the Poisson’s ratio and Young’s modulus of the macroscopic metamaterial. Importantly, an optimized structure is obtained, which exhibits 2 times enhancement in auxeticity and 25 times enhancement in normalized Young’s modulus, compared to the original architecture. This work establishes a link between the physical properties at micro-nanoscale and macroscale structures, which provides inspirations for high load-bearing auxetic metamaterials.

A multiscale nonlocal continuum model is proposed to describe the superelastic deformation of gradient nano-grained NiTi shape memory alloys (SMAs). At the mesoscopic scale, the polycrystalline aggregate is regarded as a composite, i.e., the grain-interior (GI) phase is assumed to be a cuboidal inclusion embedded in a matrix of grain-boundary (GB) phase. An intrinsic energetic length and a gradient energy are introduced into the Helmholtz free energy of the GI phase. The criterion of martensite transformation (MT) is derived based on the principle of virtual power and second law of thermodynamics. The hindering effect of GB on MT in GI phase is addressed. By deriving the analytical solution of the proposed model and introducing a scale transition rule, the overall and local stress-strain responses of the specimen at the macroscopic scale are obtained. The prediction capability of the proposed model is verified by comparing the analytical solution with the experiment. The influences of the distribution form for the grain size (GS) on the superelastic deformation of gradient nano-grained NiTi SMAs are further predicted and discussed. The analytical form and low computational cost of the proposed model make it an appropriate theoretical tool to design the gradient nano-grained SMAs with desired mechanical property.

The stability of gaseous detonation waves is crucial for the operation of detonation-based propulsion systems and the assessment of industrial explosion hazards. However, research on the stability of detonation waves in complex reactive systems that are composed of actual fuels and oxidants and can be described by numerous elementary chemical reactions, has not been fully carried out. To investigate the relationship between linear and nonlinear stabilities in gaseous detonation wave propagation for complex reactive systems, the linear stability analysis and the one-dimensionally nonlinear numerical simulations of H₂/O₂/Ar (argon) detonations based on the reactive Euler equations and detailed reaction mechanisms are carried out. The results show that in complex reactive systems characterized by elementary chemical reactions, the results of linear stability computation of detonation are consistent with those from one-dimensionally nonlinear oscillations of detonation wave. Utilizing these linear stability results, a neutral stability curve and a perturbation frequency transition curve in the phase plane of initial pressure versus inert gas (Ar) dilution ratio are derived, especially the new frequency transition curve clearly describes the transition of perturbations from low-frequency to high-frequency mode. One-dimensional nonlinear simulations show that near the perturbation frequency transition curve, the oscillations of the detonation wave can also transform between the low-frequency, high-amplitude oscillation mode and the high-frequency, low-amplitude oscillation mode, with the oscillation frequency corresponding to the mode that exhibits the maximum growth rate identified in the linear stability analysis. This investigation into detonation stability in complex reactive gases offers guidance for selecting appropriate initial conditions and gas compositions in practical applications of detonation.

In order to solve the vertical vibration negative effect problem caused by the increase of the unsprung mass in the hub motor driven vehicle (HMDV), a novel mechatronic suspension using the bridge electrical network is proposed. Firstly, the bridge electrical networks composed of two capacitors, two inductors, and one resistor are summarized and their impedance functions are analyzed forward through the structural method. Then a quarter HMDV model is constructed, and the optimal element parameters in the electrical networks are selected through the Pattern Search algorithm. The influence of element parameters perturbation of the optimal structure on the output response of HMDV suspension is further analyzed. Results show that the proposed bridge electrical network can be realized as a biquartic impedance. It can be equivalent to a mechanical impedance of the suspension through a linear motor. Compared with the conventional suspension, the root-mean-square values of the dynamic tire load and the suspension working space are reduced by 10.76% and 18.10%, respectively. The vibration at low and high frequencies of the unsprung mass is suppressed, effectively improving the grounding and handling stability of the vehicle.

Equiatomic NiTi shape memory alloys (SMAs) can exhibit multiple martensitic transformations from a parent phase, significantly influencing the advanced macroscopic properties of SMAs, such as the large deformation/strain ability. A comprehensive atomic-scale understanding of the selection rule of the martensite phase/variant and its impact on the macroscopic mechanical behavior of SMA could be helpful for the development of high-performance SMAs. This work studies the transformation pathway, preferred martensite variant and corresponding macroscopic behavior of single crystal and bicrystal NiTi SMAs based on molecular dynamics and theoretical analysis. It is found that the transformation strain of single crystal NiTi is significantly influenced by the crystal orientation-dependent transformation pathway and martensite variant. The selection rule is that the transformation pathway and preferred martensite variant, leading to maximum transformation strains for each orientation, are energetically preferred. It can be predicted theoretically and agrees well with the molecular dynamic simulations. In addition, the stress-strain response of bicrystal NiTi can be modulated by changing its transformation pathway based on the orientation effect. This work provides atomic insights into the orientation-dependent deformation ability of NiTi and could be helpful for the development of high-performance SMAs through orientation modulation.

Creep is an important mechanical property of refractory high-entropy alloys (RHEAs) at high temperatures. The existence of short-range order (SRO) and its ability to improve the strength or plasticity of high-entropy alloys (HEAs) have been experimentally proven. However, there is still little research on the correlation between SRO and creep behavior. The mechanism of SRO influencing creep behavior is not yet clear. In this work, the creep behaviors of TiVTaNb RHEA with and without SRO were simulated at various temperatures and stresses using molecular dynamics methods, and the effects of SRO on creep behavior were analyzed. The results show that the SRO is energetically favorable for occurrence in this RHEA. For polycrystalline RHEAs, grain boundary energy is an important driving force for the formation of SRO. Significantly, under the same conditions, the SRO can reduce the steady-state creep rate and change the creep mechanism of the RHEA. Specifically, the models with SRO will exhibit lower stress exponent and grain-size exponent. A mechanism by which SRO reduces the effects of grain boundaries on creep has been discovered. These phenomena can be well explained by the effects of SRO on atomic diffusion. In addition, by analyzing the diffusion ability of different elements, SRO can induce localization of atomic diffusion, resulting in strain localization under high stresses. This work highlights the importance of SRO on the creep of RHEAs and provides a reference for establishing a reasonable creep model of RHEAs.

Non-Schmid (NS) effects in body-centered cubic (BCC) single-phase metals have received special attention in recent years. However, a deep understanding of these effects in the BCC phase of dual-phase (DP) steels has not yet been reached. This study explores the NS effects in ferrite-martensite DP steels, where the ferrite phase has a BCC crystallographic structure and exhibits NS effects. The influences of NS stress components on the mechanical response of DP steels are studied, including stress/strain partitioning, plastic flow, and yield surface. To this end, the mechanical behavior of the two phases is described by dislocation density-based crystal plasticity constitutive models, with the NS effect only incorporated into the ferrite phase modeling. The NS stress contribution is revealed for two types of microstructures commonly observed in DP steels: equiaxed phases with random grain orientations, and elongated phases with preferred grain orientations. Our results show that, in the case of a microstructure with equiaxed phases, the normal NS stress components play significant roles in tension-compression asymmetry. By contrast, in microstructures with elongated phases, a combined influence of crystallographic texture and NS effect is evident. These findings advance our knowledge of the intricate interplay between microstructural features and NS effects and help to elucidate the mechanisms underlying anisotropic-asymmetric plastic behavior of DP steels.

The present study proposes a modified random sequential absorption (RSA) algorithm to generate a representative volume element (RVE) model for predicting the elastic properties of discontinuous curved fiber reinforced composites (DCFRCs) with varying fiber waviness functions and orientations. A small-move method was proposed to modify the traditional RSA algorithm. In comparison with the original RSA algorithm, the generation efficiency of the proposed modified RSA algorithm increased by over 40%, and the achievable maximum fiber volume fraction could reach up to 15% with a fiber aspect ratio of 15. The generated RVE model was utilized in conducting finite element analysis to investigate the effect of fiber waviness and wavy functions on the elastic properties of DCFRCs. Finally, a modified rule-of-mixture was proposed to predict the elastic properties of DCFRCs with various fiber orientations. The results indicated that the elastic properties predicted by the modified rule-of-mixture were in good agreement with those obtained from the RVE model, thereby demonstrating its effectiveness.

We examine the electromechanical field and charge redistribution within a flexoelectric semiconductor (FS) nanobeam, accounting for bending, fundamental thickness-shear, and antisymmetric thickness-stretch deformations. The coupled governing equations include microstructure, flexoelectric, and semiconductor effects, highlighting the interplay between mechanical displacement, electric potential, and charge carriers. For applications in flexoelectronic devices, the static bending of a simply supported FS beam induced by uniform pressure and wave propagation in an unbounded FS beam are analytically addressed using the derived framework. The effects of antisymmetric thickness-stretch on mechanical displacements and electron concentration perturbation, as well as size dependence of microstructure and flexoelectric effects, are identified. An interesting finding reveals that wave frequencies of the antisymmetric thickness-stretch mode, as anticipated by the proposed model, are larger compared to those of the model neglecting flexoelectric and semiconductor effects. For the first time, the cutoff frequency of antisymmetric thickness-stretch impacted by the two features is explained mathematically. These findings are beneficial for enhancing the performance of flexoelectronic sensors and electroacoustic devices.

Ceramic matrix composites have broad application prospects in the aerospace field due to their high temperature resistance and oxidation resistance. The effect of temperature and environment atmosphere on the fracture toughness and failure mechanisms of two-dimensional plain-woven SiC_f/SiC composites was investigated. The results show that they exhibit pseudo-plastic deformation behavior at different temperatures. The fracture toughness is as high as 48 MPa m^1/2 at room temperature, and gradually decreases with rising temperature. The difference in fracture toughness between argon and air initially increases and then decreases with rising temperature. Furthermore, the high-temperature failure mechanisms of these composites were analyzed through macro and micro analysis. Based on this, a physic-based temperature-dependent fracture toughness model considering matrix toughness, plastic power, fiber pull-out, and residual thermal stress was developed for fiber-reinforced ceramic matrix composites. The model has been well validated by experimental results. An analysis of influencing factors regarding the evolution of fracture toughness was conducted by the proposed model. This work contributes to a better understanding of the mechanical performance evolution and failure mechanisms of ceramic matrix composites under multifield coupling conditions, thereby promoting their applications.