Mechanics of Materials最新文献

Triply periodic minimal surfaces (TPMSs) are extensively used as effective tools for the microarchitectural designing of cellular structures. CoCrNi medium-entropy alloy, known for its high strength and high ductility, is an ideal candidate for fabricating TPMS-based structures. However, few studies have focused on this topic. This study investigated the compressive mechanical and energy absorption properties of CoCrNi D-sheet TPMS cellular structures fabricated via selective laser melting (SLM). The surface morphology of the fabricated D-sheet TPMS structure was observed through scanning electron microscopy, and the deformation and failure mechanisms of the structure were analyzed by the quasi-static compression test. The results indicated that the SLM-ed CoCrNi D-sheet TPMS structure exhibited stable collapse mechanisms compared to other metal-based TPMS structures. Meanwhile, the energy absorption characteristics of the modified finite element (FE) model agreed well with the experimental results. Furthermore, the impact of the level constant on the energy absorption performance of the D-sheet TPMS structure was investigated using the FE model. Thus, an optimal D-sheet (OD-sheet) TPMS structure with lower density and higher energy absorption capacity was obtained. Additionally, the Gibson–Ashby prediction model was established to aid in selecting CoCrNi OD-sheet TPMS structures with controlled relative densities for energy absorption applications.

An accurate estimation of the critical stress intensity factor for crack tip dislocation nucleation under Mode I loading is of great importance to determine whether a material is intrinsically ductile or not. Here, shear displacements and energy change at crack tip in FCC nickel, copper and aluminum are investigated during Mode I fracture process using atomistic simulations. In light of our simulation results, a new shear resistance model is formulated by a general Fourier expansion with coefficients identified by the computed energy curve. The new model involving the step formation energy which can be regarded as a new parameter and unstable stable stacking fault energy, reduces to Rice theory if no step exists. The criterion for nucleation is developed based on the idea that crack tip behaviors are controlled by the shear resistance and the maximum point serves as an obstacle to conquer. The predictions of the critical shear displacement corresponding to maximum shear resistance position and the critical nucleation energy show good agreement with simulation results. In addition, the new model can be further utilized to study the effect of complex stress state on Mode I crack tip dislocation nucleation.

Concrete is the most common material in the construction industry and almost the only material used for building security and protective structures. Such structures are intended to withstand extremely high blast and impact loading. One of the key mechanical properties of materials subjected to such extreme loading is the relationship between the hydrostatic pressure and the volumetric strain, which is known as the equation of state (EOS). In porous materials, this relationship is substantially inelastic due to the closure and collapse of pores. Commonly the EOS is obtained experimentally. Implementation of the experimental EOS curves in analytical and numerical solutions for different applications requires approximate simplified expressions for these curves. One of the most common approximations for describing the nonlinear compression of porous media is the p-alpha EOS. The present paper reviews the p-alpha EOS approximations appearing in the literature and discusses controversial issues concerning the full compaction pressure. New p-alpha EOS approximations are presented for a variety of experimental hydrostatic test results conducted by the authors for different concrete compositions with equal uniaxial compressive strength. An extended investigation is dedicated to the formulation of the p-alpha approximation in the case of limited available data, and a “blind prediction” of the pi-alpha EOS is proposed.

To demonstrate the implementation of the derived p-alpha EOS, an extended example is presented, where the quasi-static cavity expansion problem in a concrete medium that is characterized by the p-alpha EOS is worked out.

We propose a novel method for solving the static response of a conical indenter on a transversely isotropic and layered elastic half-space. The newly developed Fourier-Bessel series (FBS) system of vector functions, along with the unconditionally stable dual-variable and position method, is employed to derive the Green's function in the transversely isotropic and layered elastic half-space under a vertical ring load on the surface. To calculate the response at different field points on the surface, we apply discrete love numbers within the FBS vector system. The load densities in the discretized rings within the contact radius of the conical indenter are determined using the integral least-square method, along with a self-adaptive algorithm developed in this study. Finally, the relationship between the indentation depth (vertical displacement) and the applied load is obtained through force balance between the external load and the summed contact traction. The developed scheme is validated using existing exact solutions for the reduced homogeneous half-space case. Selected numerical results clearly demonstrate the effect of anisotropic material and layering on the indentation response. It is observed that, regardless of whether the structure is a stratified half-space or a layered structure with a rigid substrate, the material properties in the top layer have the most significant influence on the indentation behavior. In the case of a layered structure with an underlying elastic half-space, the material properties in the interlayer and bottom layer could also affect the indentation behaviors.

The stacking fault energy (SFE) effect on nanocrystalline metal deformation mechanisms has been extensively studied from dislocation and grain boundary perspectives. Compared to homogeneous nanocrystalline structures, the gradient nanograined (GNG) structure exhibits varied grain sizes, resulting in different SFE effects across regions during the hetero-deformation process. In this work, molecular dynamics (MD) simulation is conducted on GNG Cu-Ni binary alloys with incremental Ni concentration, to investigate the SFE effect at different deformation stages of the GNG structure. It is revealed that the elastic deformation stage shows a close relationship with alloy intrinsic property (e.g. elastic modulus) from Ni concentration variation, with neglectable SFE effect. Under the hetero-deformation stage, different from the homogeneous polycrystalline counterpart, the effect under high SFE condition is demonstrated from the overall dislocation density decline and the competition between the perfect and partial dislocations in the gradually varied grains across the GNG structure, which finally results in lower strain gradient, geometry necessary dislocation density, and a less effective hetero-deformation process. The active GB sliding and migration are found to offset this dislocation density decline in the sample with the relatively high SFE, which brings in higher GB stress concentration and larger GB free volume of the GNG structure. Besides, the GB relaxation across the GNG structure under the high strain rate tensile loading is also illustrated and discussed. The results further support the essential deformation mechanism underlying heterostructure materials.

As an empirically established design criterion, Nelson curves that relate the service temperature and the allowable hydrogen partial pressure have been developed and utilized for more than sixty years in pressure vessels and piping (PVP) safety design. Despite a relatively clear thermodynamic understanding of the high-temperature-hydrogen-attack (HTHA) problem, the detailed fracture process on the microstructural length scales, however, remains elusive, and a quantitative assessment of the PVP lifetime under HTHA from the available creep fracture dataset is still not possible. This work develops a microstructure-informed and micromechanics-based model by incorporating a synergy between hydrogen transport and intergranular-cavity-based fracture process. Based on the available creep lifetime data of C-0.5Mo steels, we are able to calibrate material constitutive parameters, and then conduct nonlinear finite element simulations that reveal a real-time stress-induced hydrogen diffusional transport along grain boundaries, coupled with a microstructure-explicit failure process, from which Nelson curves can be computed. Such failure analyses allow us to delineate two distinct regimes on the Nelson curves, i.e., dislocation-creep-controlled or grain boundary diffusion-assisted cavity growth. More importantly, we found that a small change of the pipe thickness and applied stresses can significantly shift these lifetime curves. However, these two parameters are usually not provided in Nelson curves, thus limiting their usage in material selection and safety design. This discrepancy can clearly be mitigated by extensive parametric studies from our micromechanical modeling/simulation framework.

In this work, the influences of shot peening (SP) on ultra-high strength steel were investigated. For the characterization of the strengthened layer, an inclined specimen was proposed for X-ray diffraction and nanoindentation, the micro-mechanical properties in the strengthened layer were studied with both experimental and crystal plasticity finite element (CPFE) analysis. It's noted that SP introduced grain refinement and compressive residual stress were highly concentrated in the surface layer due to the high strength and hardness, and the maximum depth influenced was only 0.1 mm. Based on simulation results, it's found that the differences in nanoindentation loading curves were introduced by the work done by residual stresses. To estimate the residual stress in the strengthened layer, a modified model was proposed with nanoindentation. The newly proposed model gives improved estimation accuracy compared with Wang's model, and it could give more precise results in contrast to the X-ray diffraction method for strengthened layers.

The study extends the locally-exact homogenization theory (LEHT) to determine the effective parameters and localized multiphysics fields of unidirectional piezoelectric nanocomposites with energetic surfaces. The interface is simulated using the generalized Gurtin-Murdoch model for piezoelectric nanocomposites, taking into account the discontinuities between surface stresses and electrical displacement. By characterizing hexagonal and square repeating unit cells (RUC), the interactions across a fiber's or pore's interface in piezoelectric nanocomposites with energetic surfaces have been analyzed. The extended LEHT possesses the advantages of fast convergence, excellent stability, and computational efficiency due to its avoidance of mesh discretization and pointwise satisfaction of interfacial conditions. To verify the validity of the present theory, this paper also derives the Eshelby solution for piezoelectric nanocomposite with energetic surfaces. The reliability and accuracy of the present theory are demonstrated by comparing the present results from the Eshelby solution and the finite element method (FEM) with generally good agreement. On this basis, the influence of microstructural effects, such as phase volume fractions and geometrical arrangement, on the effective and local responses of piezoelectric nanocomposites with energetic surfaces are investigated. The results show that effective piezoelectric/dielectric moduli and electric displacement field for piezoelectric nanocomposites are significantly affected by the size effect, along with several other parameters such as unit cell arrays and fiber/pore volume fractions. Those results highlight the significance of neighboring pore or fiber interactions for piezoelectric nanocomposites, which are often overlooked in traditional micromechanics models and are challenging to be captured using numerical methods.

A micromechanics-based yield criterion is developed to model void coalescence of three-dimensional voids under combined tension and shear. The analysis treats a cylindrical cell containing a coaxial cylindrical cavity, both having an elliptic cross section. Limit analysis is employed to first derive a criterion for homothetic cells. The model is then generalized to incorporate independent void spacing ratios (non-homothetic cells) The model predictions are assessed against finite-element based limit analysis on similar geometries. The effects of relative void spacing and void shape on effective yielding are investigated. In tension, the results indicate an increase in the coalescence stress with increasing in-plane anisotropy for both homothetic and non-homothetic cells. The new criterion is chiefly motivated by modeling shear failure. The extent to which the shear limit load reduces when shearing perpendicular to the largest transverse void dimension, as compared with shearing parallel to it, is discussed.

Pd₂₀Pt₂₀Cu₂₀Ni₂₀P₂₀ metallic glass exhibits a prominent β relaxation process, which is conducive to plastic deformation and is an ideal model alloy for studying the correlation between deformation mechanism and microstructure. In this work, the high-temperature rheological and creep behavior of Pd₂₀Pt₂₀Cu₂₀Ni₂₀P₂₀ metallic glass were systematically studied. Within the framework of the free volume model, the high-temperature rheological behavior near the glass transition temperature can be effectively examined through strain-rate jump and uniaxial tensile experiments. The results indicate that the plastic deformation behavior strongly depend on temperature and strain rate. A high value of the activation volume value for plastic deformation can be ascribed to the β relaxation. To further explore the high-temperature deformation behavior, creep experiments were performed near the β relaxation temperature range. Taking microstructural heterogeneity into account, the evolution of strain can be characterized using the empirical Kohlrausch-Williams-Watts equation and the generalized Kelvin model. The results show that annealing below the glass transition temperature leads to the annihilation of defects, an increase in the characteristic relaxation time. This work provides valuable insights into the mechanical behaviors of metallic glass at high temperatures, which is the key to develop the materials with improved mechanical properties for high temperature applications.