Physical Mesomechanics最新文献

This paper discusses a subcritical crack nucleation mechanism in a brittle solid within a real range of applied stress. A medium deformed by uniaxial tension is considered as an open nonequilibrium system of nuclei and electrons. Structural relaxation of the medium begins with the excitation of dynamic displacements during nonadiabatic Landau–Zener transitions. Dynamic displacements induce the instability of the medium to the longitudinal displacement wave. The kinetics of structural relaxation is described by two nonlinear parabolic kinetic equations for dynamic order parameters. Conditions are derived for the existence of localized solutions (autosolitons). The excitation of autosolitons leads to local elongation and cross-sectional reduction of the specimen. The resulting neck is a subcritical crack.

Notched 30CrMnSiA steel specimens were exposed to rupture load (mode I) at an angle of 90° between their fracture surface and load direction and to shear load (mode II) at an angle of 45° and 15°. For shear loading, Richard’s grips were used allowing one to vary the load from pure tension to pure shear by varying the notch orientation angle to the tensile load direction. Assessed under loading were the parameters of acoustic emission (AE) and strain fields (by the digital image correlation (DIC) method), and after failure, the damage parameters and microhardness on the polished lateral surface of the specimens, and the macro- and microreliefs of fracture surfaces. It is shown that increasing the shear component under tension changes the mechanical and the acoustic parameters of the specimens (total number of AE signals, their activity, b_AE-value), and the critical temperature of brittleness, changing the fracture surface morphology from ductile to brittle at a load orientation of 45°. Simultaneously, a nonlinear dependence of the damage parameters (relative area of microcracks S^*, their average length l_av, orientation to the loading axis) on the load angle is observed, showing a correlation with principal strains estimated by the DIC method.

Selective laser melting (SLM) is one of the most promising metal additive manufacturing technologies. SLMed Al-Si alloys have been widely used in the rail transport, aerospace, and automotive industries. Recently, the fatigue and fracture properties of SLMed Al-Si alloys have attracted considerable attention due to their application in critical load-bearing structures. This review aims to better understand the recent progress on the fatigue and fracture investigations of SLMed Al-Si alloys, especially AlSi10Mg, with emphasis on the effect of defects, heterogeneous microstructure, residual stress, and post-treatment methods. In addition, fatigue and fracture modeling methods are discussed. Finally, the challenges and future research opportunities are prospected.

A porous structure is widely used in additive manufacturing of orthopedic implants to reduce the stiffness mismatch between the implant and the bone. The development and improvement of porous structures for orthopedic implants is still a major challenge. It is essential to study mechanical properties of different porous structures and their relation to the deformation mechanism. In this paper, the relation between the deformation mechanism and the mechanical properties of Ti6Al4V triply periodic minimal surface (TPMS) structures, such as stretching-dominated IWP and bending-dominated gyroid structures, are investigated using the finite element analysis for uniform and density gradient scaffolds. The method for designing network-based and sheet-based TPMS structures is presented. The numerical results show that failure in the stretching-dominated structure (IWP) starts with buckling of the vertical struts, whereas failure in the bending-dominated structure (gyroid) occurs with the formation of the 45° shear band. The gyroid structure shows a higher shear modulus than the IWP structure. The numerical results exhibit good agreement with the previous experimental data for uniform and density gradient structures. Finally, the effect of the void defect on the elastic and shear moduli is evaluated. The results indicate that the elastic modulus of the bending-dominated structure shows a greater reduction in the presence of void defects than that of the stretching-dominated structure, and the shear modulus of the stretching-dominated structure is more sensitive to void defects than that of the bending-dominated structure.

A method is proposed for analyzing the relative energy distributions of grain boundaries in ultrafine-grained materials measured by grain boundary grooving using a scanning tunneling microscope. The grain boundary energy distribution in a grain boundary ensemble is considered as a superposition of individual distributions or populations, which can be identified by cluster analysis based on statistical criteria and each of which has its own average energy, variance, and share in the total distribution. The analysis is performed for 12Cr15Mn9NiCu steel with a coarse-grained structure in the as-received state and with an ultrafine-grained structure produced by hot helical rolling and subsequent cold rolling. It is shown that the number of boundary populations and their main characteristics revealed by clustering depend on the steel structure. The results of cluster analysis of experimental distributions are compared with the EBSD measurement data on grain boundary misorientation distributions. Discrepancy between the clustering results for the energy and misorientation distributions of grain boundaries is discussed taking into account the difference in the type of information obtained.

The paper reports on a dynamic finite element simulation of the propagation of longitudinal elastic waves in cylindrical plates and Pochhammer waves in rods. The simulation shows that in cylindrical specimens of compact shape, a single longitudinal wave first propagates till unloading waves from their lateral surfaces start to interact, and then, two types of longitudinal elastic waves propagate at a time. In rods, first comes a longitudinal elastic wave, and then, two concurrent waves—a longitudinal elastic wave and a Pochhammer wave—due to the effect of unloading from their lateral surfaces. With the example of aluminum, it is demonstrated in which range of geometric parameters the elastic properties of near-compact specimens cannot be analyzed by ultrasonic methods.

The paper proposes a mechanical simulation model based on continuum damage mechanics and physical mesomechanics to describe the accumulation of dispersed damages in polycrystalline materials, considering that the main damaging factors are dispersed microcracks and internal stresses produced primarily by linear structural defects. From the proposed model follows a statistical limit state criterion consistent with failure conditions for brittle and ductile structural materials. The limit state criterion is applied to several typical cases of failure and elastic-to-elastoplastic strain transition in polycrystalline structural materials. Based on the model, an acoustic approach to damage assessments of structural materials is also proposed. With the approach, several acoustic effects are identified from the propagation of elastic pulses in a damaged material. Such effects can be useful for instrumental damage assessment of materials (specimens, structural elements) at any time of loading or operation. The acoustic approach can provide a basis for a method of measuring the damage parameters included in the model. The experimental data available to us suggest that the proposed approach to damage assessment is correct for structural materials and is promising for further experimental research to develop instrumental express methods of monitoring dispersed damages in metal structures exposed to thermomechanical loads.

This paper presents a general constitutive model to describe the coupling among bulk solid stress, pore fluid pressure, and adsorption in a fluid-saturated multiple-porosity elastic solid. The general case of N coexisting porosities is considered. Each porosity system may contain fluid in the free or adsorbed phase, as well as in both phases. Thus, the utility of the developed constitutive relations extends to materials with complex pore structures, which may involve several scales of pore size. These relations are obtained from the thermodynamic free energy density function for deformation of the porous solid frame while incorporating the surface energy stored at the interface between the pore fluid and solid phase. A number of previously published models for coupled adsorption and poroelasticity in microporous and mesoporous materials are examined as special cases. Possible means of estimating the constitutive model parameters are discussed.

In the present paper, the effect of adding graphene on the fracture strength of the Araldite adhesive was studied. Experimental specimens were made of PMMA and then were bonded using a thin adhesive layer. Different loading modes were created by using the modified Arcan fixture. The effect of adding graphene to the adhesive layer was studied at four different weight ratios of graphene, including 0.00, 0.25, 0.50, and 1.00%. The results derived suggest that the experimental specimens with 0.5 wt % graphene have the highest fracture force. For specimens with the same amount of graphene, the highest fracture force was obtained under the mode II loading condition. The experimental results were compared with the results of the finite element model. The fracture behavior of an adhesive layer was modeled using the cohesive zone model. The maximum nominal stress criterion and the quadratic power law criterion were used for the crack initiation and propagation in the adhesive layer, respectively. The comparison between the numerical and experimental results shows overall good agreement.

The present work is aimed at analyzing free longitudinal vibrations of nanorods composed of a functionally graded (FG) material with deformable boundaries within a hardening nonlocal elasticity approach. For this purpose, a FG nanorod composed of the ceramic and metal constituents is considered to be elastically supported by means of axial springs at both ends. Then the analytical method based on the association of the Fourier sine series and the Stokes transformation is developed to solve the free axial vibration problem of a FG nanorod with both deformable and nondeformable boundaries. Free axial vibration of a restrained FG nanorod is first studied within hardening nonlocal elasticity. To show the validity and profitability of the proposed analytical method, the presented Fourier series method with the Stokes transformation is used for the analysis of axial vibration of a rigidly supported homogeneous nanorod by setting the appropriate spring stiffness values. The main superiority of this new approach is in its power of dealing with numerous boundary conditions to determine longitudinal vibration frequencies of FG nanorods. Using the present solution method, various numerical applications are given for different small-scale parameters, gradient index, and nanorod length.