Showing papers on "Micromechanics published in 2023"

Micromechanics and strength of agglomerates produced by spray drying

Abstract: Colloidal suspension spray drying is often used as a route to produce micrometric agglomerates. The morphology and re-dispersion behavior of such agglomerates under fluid dynamic stress is expected to be strongly affected by the operative conditions at which drying is conducted. Motivated by the emerging research area of fluid-activated drug carriers, in this work we studied numerically both the generation mechanism of agglomerates during spray drying, and their mechanical and fragmentation behavior under fluid dynamics stresses. Capillary forces, Brownian motion and the contact mechanics of the particle-particle bonds were kept into account. Results showed that different spray drying conditions lead to substantially different morphologies and mechanical responses of the agglomerates: compact, uniformly dense clusters are obtained for low Péclet number conditions, hollow clusters, with a void core and a thick crust of contacting colloidal particles are instead formed at high Péclet conditions. We also observed that compact agglomerates are considerably stronger than hollow ones, and exhibit brittle rupture under fluid dynamic stresses. Hollow clusters were seen on the contrary to be looser and to undergo noticeable deformation prior to breakup. Finally, this work aims at providing the information needed for selecting spray drying conditions when aiming at a specific morphology and/or re-dispersability properties of colloidal agglomerates.

Multi-scale investigation of the mechanical properties of Loblolly pine wood at elevated temperature

Abstract: ABSTRACT The micromechanics of wood cell walls and the bending properties of wood were investigated by using nanoindentation and macromechanical test machine at elevated temperature, respectively. The microstructure of wood at the temperatures ranged from 20 to 160°C was also observed by optical microscope and scanning electron microscope. Results indicated that the increased temperature accompanying with the moisture release affected the mechanical behavior of wood on both cell-wall level and macroscale. The cell wall’s elastic modulus (E r) and hardness (H) decreased as temperature increased to 80°C due to the softening of wood polymers under hot and humid condition. On the contrary, both the E r and H increased when the temperature was above 100°C ascribed to the massive release of moisture. The increased mechanical properties of the thick-wall cells made a major contribution to the improved bending properties of wood at the macroscale. Furthermore, the cell walls at 80°C own the highest indentation creep ratio (C IT), that is, wood are easier to creep under a constant load. In hence, the mechanical stability of timber in constructions at about 80°C shall be paid more attention.

Mixed-modes (I/III) fracture of aluminum foam based on micromechanics of damage

Abstract: Nowadays, foam structures have several applications in industries. Basically, discontinuities such as pores cause complications in evaluating the mechanical and fracture behavior. In other words, understanding the multi scale of porous cells in cracked foams can play a significant role in fracture prediction. The present research proposes an analytical approach based on the experimental concept of damage in which the behavior of cracked metal foam was investigated under mixed-mode I/III (tensile and out-of-plane) fracture. In this method, pores in foam are modeled as circular and elliptical defects. In this context, mechanical properties and crack inclination angle are obtained based on loading and configuration of pores. Dispersion of circular and elliptical discontinuities determines the overall modulus and crack inclination angle trajectory based on the experimental results, pure mode I SIF were occurred on (0°) and pure mode III were obtained at (62°) loading angle. The aluminum foams were produced and prepared according to the edge notch disc bend (ENDB) test method to obtain the onset of fracture initiation (KIc and KIIIc) and trajectory of cracked foam body. The results demonstrated that analytical approach, based on circular pore distribution, had a good agreement with experimental results. Moreover, the results indicated that SIF in mode I and mode III had the most agreement with the experimental results at angles of 0° and 62°, respectively.

Additive manufacturing of fiber-reinforced polymer composites: A technical review and status of design methodologies

Abstract: Additive manufacturing (AM) of fiber-reinforced composites is gaining traction as an important manufacturing technology to produce complex, highly customized structures. AM processes enable the fabrication of complex 3D structures with control over fiber position and orientation, offering tremendous design freedom. Although the use of AM to fabricate fiber-reinforced composites is promising, various design difficulties remain. These include consideration of structural performance subject to manufacturing constraints on fiber placement, orientation, bend radius, and other relevant limitations. Design engineers will have to consider these limitations to be able to reap the full benefits of AM to fabricate fiber-reinforced polymer composites. This paper aims to provide a survey on the advances in multi-scale topology optimization, fiber orientation parameterization, micromechanics of fiber-matrix interactions and process planning specific for the AM of complex fiber reinforced composites. The status of recent research is summarized, and future directions outlined.

Unified analysis for tailorable multi-scale fiber reinforced cementitious composites in tension

Abstract: Fibers applied to reinforce the cementitious matrix exhibit a wide range of scales, from distributed carbon nanomaterials, chopped short fibers to continuous fibrous reinforcements. When a cementitious matrix is jointly toughened by reinforcing fibers at multiple scales, Multi-Scale Fiber Reinforced Cementitious Composite (MSFRC) tailored built on the micromechanics-based approach and bond-slip mechanism is proposed in this study. The composite actions of MSFRC, namely, tension stiffening, ductility enhancing and synergetic effects, are explained within a universal perspective. In addition, a 1-D numerical model using spring elements is developed to simulate the tensile behavior of MSFRC based on the crack band theory, fiber-bridging model and Monte Carlo simulation. The scales, types and contents of reinforcing fibers, interface behavior and stochastic nature can be considered in the model. Finally, it is found that the predicted mechanical response and crack evolution process match well with the experimental results obtained from literatures.

Micromechanical predictions on elastic moduli of a short fiber composite with arbitrary geometric combination

Abstract: The microstructure of a short fiber reinforced composite (SFRC) is different from point to point, particularly for an injection molded SFRC. In general, micromechanical models decompose such an SFRC into a set of groups, each of which contains short fibers with identical shape and orientation. We begin with examining the predictive ability of typical models, i.e., modified rule of mixture, Mori–Tanaka method, Bridging Model and Fu–Lauke scheme, for each group. Next, the effective properties of the decomposed SFRC are predicted by assembling all the groups via two-step homogenization and laminate analogous approach, respectively. The predictions are compared with FE results on representative volume elements which are constructed through a random sequential algorithm. Finally, an overall probability function that associates an LDF (length distribution function) and an ODF (orientation distribution function) with the fiber volume fraction is presented. The predictive accuracy of above decomposition methods incorporated with this function is improved.

A 3-D micromechanical framework to study damage propagation and failure of printed aligned discontinuous fiber-reinforced composites

Abstract: This paper presents a novel and efficient micromechanical framework for finite element simulation of damage and failure in 3-D printed aligned discontinuous fiber-reinforced composites. The framework can predict the initiation and propagation of different types of damage in the composite under tensile loading along the fibers’ axis. The micromechanical framework includes the microstructural representation of the composite with explicit fibers and matrix in addition to linear expansions at the ends. Accurate constitutive equations are utilized for fibers, matrix, and fiber/matrix interfaces in the microstructural representation. Fibers’ locations and lengths are generated randomly; based on a target distribution for the fibers’ aspect ratios measured experimentally; within the microstructural representation. Optimal microstructural representative dimensions for reliable investigation of the mechanical response are computed by conducting sensitivity analyses. The accuracy of the micromechanical framework is validated versus the experimental results of a 3-D printed aligned discontinuous fiber-reinforced composite as the composite of interest. It is shown that the proposed framework can simulate various aspects of the mechanical response, including the failure pattern and stress-strain behavior. Subsequently, the sensitivity of the mechanical response of the composite to a few constitutive equation-related parameters, including the strength and fracture toughness of the fiber/matrix interfaces and the matrix strength, is investigated. The correlation between the studied parameters and the composite’s strength and failure pattern is also analyzed and discussed. This paper delivers valuable guidance on the mechanical response of printed aligned discontinuous fiber-reinforced composites, eventually resulting in the paradigm-shifting design, manufacturing, and analysis of such advanced composites.

Yield Surfaces and Plastic Potentials for Metals, with Analysis of Plastic Dilatation and Strength Asymmetry in BCC Crystals

Abstract: Since the 1980s, constitutive modeling has steadily migrated from phenomenological descriptions toward approaches that are based on micromechanics considerations. Despite significant efforts, crystal plasticity remains an open field of research. Among the unresolved issues are the anomalous behavior of metals at low temperatures and the stress upturn at extreme dynamics. This work is focused on the low-temperature responses of body-centered-cubic (bcc) metals, among them, molybdenum (Mo). At these conditions, the plastic flow strength is governed by the motion of screw dislocations. The resultant non-planarity of core structures and slip causes the following: the shear stress includes non-glide components, the Schmid law is violated, there is a tension-compression asymmetry, and the yield surface and plastic potential are clearly decoupled. We find that the behavioral complexities can be explained by atomistically resolved friction coefficients in macroscopic yield and flow. The plastic flow mechanisms establish the departure point into the follow-up analysis of yield surfaces. For example, we know that while the von Mises stress is explained based on energy considerations, we will also show that the stress has a clear geometric interpretation. Moreover, the von Mises stress is just one case within a much broader class of equivalent stresses. Possible correlations among non-Schmid effects (as represented macroscopically by friction coefficients), volume change (i.e., residual elastic dilatation) from dislocation lines, and elastic anisotropy are investigated. Extensions to the shock regime are also established.

Vibration and buckling analysis of agglomerated CNT composite plates via isogeometric analysis using non-polynomial shear deformation theory

Abstract: The present study employs the non-polynomial shear deformation theory within the framework of isogeometric analysis to study the vibration and buckling behavior of carbon nanotubes reinforced composite plates. Owing to the higher aspect ratio of carbon nanotubes, they are prone to agglomeration in matrix. Therefore, to analyze the effect of carbon nanotubes agglomeration in composite plates, a two-parameter micromechanics model is used to determine the effective mechanical properties of the composite plate. Current study opts isogeometric analysis as numerical method, as it can represents the complex shape geometries, and provides higher-order element continuity. Higher-order element continuity is desirable for implementation of non-polynomial shear deformation theory. By the virtue of non-uniform rational B-splines, isogeometric analysis turns out to be better analysis numerical method for complex and shell structures. Governing isogeometric element equations are derived using Hamilton's principle. A detailed parametric study is also performed to investigate the effect of degree of agglomeration, thickness to length/radius ratio, different boundary conditions, and complex shape cutouts in a composite plate. The presented formulation is validated across analytical and finite element method solutions with available literature.

Mechanical Homogenization of Transversely Isotropic CNT/GNP Reinforced Biocomposite for Wind Turbine Blades: Numerical and Analytical Study

Abstract: One of the biggest problems facing the use of carbon nanotubes in reinforced composites is agglomeration within the matrix phase. This phenomenon—caused by Van der Waals forces—leads to dispersion problems and weakens the properties of the composites. This research presents a multi-stage homogenization approach used to investigate the influence of the aspect ratio, volume fraction, and agglomeration of the nanofillers on the effective mechanical properties of a polymer biocomposite containing randomly dispersed carbon nanotubes and graphene nanoplatelets. The first stage consisted in evaluating the properties of the reinforced polymers by the CNT/GNP. The second step consisted in combining the reinforced polymers with different natural and synthetic unidirectionally oriented fibers. It was found that agglomeration has a huge influence on the mechanical properties of the composite. The novelty of this work consisted of the consideration of the parameters influencing the elastic properties using different micromechanics approaches and numerical techniques.

Towards a numerical modeling of the coupling between RTM process and induced mechanical properties for rigid particle-filled composites

Abstract: In this work, a method is proposed for modeling RTM process and the associated mechanical behavior of composites filled with mono-sized spherical Alumina particles. This method combines (i) a numerical model (RTM model) that allows the simulation of the RTM process during the injection of particle filled resins and (ii) a computational strategy of mechanical properties based on the homogenization methods. These proposed models have already been validated with experimental results. The RTM model is based on 3 sub-models: the first one to describe the suspension flow, the second one to simulate the advance of the flow front, and the last one to model the particles filtration by the fibrous medium. The distribution result of the concentration of particles in the fibrous medium obtained at the end of the simulation of the injection is used as input data for mechanical models of homogenization. The homogenization numerical model was constructed from a representative volume element of the microstructures using the Poisson process. The idea here is to couple these two steps (RTM simulation + mechanical property computation) in a complete model which allows at the same time and in a single operation: to simulate the process of the manufactured composites loaded with particles and to deduce their induced mechanical properties. The pertinence of the proposed method is confirmed by the simulation of nine elastic properties of composites with the finite element method. The influence of post-filling on the induced mechanical properties has been studied.