Showing papers on "Micromechanics published in 2022"

Impact response of inclined self-weighted functionally graded porous beams reinforced by graphene platelets

Abstract: This study presents a theoretical analysis on the low-velocity impact response of inclined porous nanocomposite beams under various impulsive loads. The laminated beam model consists of multiple layers modelled as closed-cell cellular solids with identical thickness, where each layer contains uniformly distributed internal pores and is reinforced by dispersing graphene platelets into the matrix. The layer-wise continuous variations in both internal pore size/density and graphene fraction result in functionally graded lightweight beams with controllable density distributions and varying elastic moduli across the thickness direction. The material properties of each layer are determined according to Halpin–Tsai micromechanics model and the extended rule of mixture. The governing equations of the inclined beam are derived based on Timoshenko beam theory then solved by employing Ritz method for space domain and Newmark method for time domain. The static bending due to the self-weight of the beam is examined first, and then imported into the dynamic analysis as the initial stress state for the beam under impulsive impacts with six different pulse shapes. A comprehensive parametric study is conducted with a special focus on the combined effects of graded material distributions and inclined angle on the beam behaviour. Results show that a larger inclined angle reduces the beam deflection, and the rectangular impulsive load can lead to the largest mid-span deflection of fully clamped graded beams that may reach over 70% more than those under some of other impact load types. This study should provide insights into the design of lighter and stiffer inclined structural components subjected to various impulsive loading conditions.

Review on study of internal load transfer in metal matrix composites using diffraction techniques

Abstract: A systematic review of the various aspects of internal load transfer using different diffraction-based methods has been carried out in this work to strengthen the understanding of metal matrix composites (MMCs) and their micromechanics. The load transfer from the softer and more compliant metallic matrix to the harder reinforcement plays a major role in conventional MMCs with a relatively high volume fraction of reinforcement particles. The mechanism of load transfer is dependent upon several factors, and a thorough understanding of them is important to design MMCs with optimum properties. Advanced diffraction-based techniques, such as neutron diffraction and synchrotron X-ray diffraction, have been applied successfully to study internal load transfer in MMCs by several authors. These techniques have allowed the measurement of elastic lattice strains in all crystallographic phases of a bulk MMC. The phase stress can then be calculated from the measured lattice strain using standard elasticity relations. Evolution of both the lattice microstrain and stress in each crystalline phase as a function of the applied stress yields information about the deformation and damage within the composite material. This review gives an overview of the studies carried out so far on internal load transfer in MMCs and the insights obtained this way.

A micromechanics-based variational phase-field model for fracture in geomaterials with brittle-tensile and compressive-ductile behavior

Abstract: This paper presents a framework for modeling failure in quasi-brittle geomaterials under different loading conditions. A micromechanics-based model is proposed in which the field variables are linked to physical mechanisms at the microcrack level: damage is related to the growth of microcracks, while plasticity is related to the frictional sliding of closed microcracks. Consequently, the hardening/softening functions and parameters entering the free energy follow from the definition of a single degradation function and the elastic material properties. The evolution of opening microcracks in tension leads to brittle behavior and mode I fracture, while the evolution of closed microcracks under frictional sliding in compression/shear leads to ductile behavior and mode II fracture. Frictional sliding is endowed with a non-associative law, a crucial aspect of the model that considers the effect of dilation and allows for realistic material responses with non-vanishing frictional energy dissipation. Despite the non-associative law, a variationally consistent formulation is presented using notions of energy balance and stability, following the energetic formulation for rate-independent systems. The material response of the model is first described, followed by the numerical implementation procedure and several benchmark finite element simulations. The results highlight the ability of the model to describe tensile, shear, and mixed-mode fracture, as well as responses with brittle-to-ductile transition. A key result is that, by virtue of the micromechanical arguments, realistic failure modes can be captured, without resorting to the usual heuristic modifications considered in the phase-field literature. The numerical results are thoroughly discussed with reference to previous numerical studies, experimental evidence, and analytical fracture criteria.

A micromechanics-based variational phase-field model for fracture in geomaterials with brittle-tensile and compressive-ductile behavior

Abstract: This paper presents a framework for modeling failure in quasi-brittle geomaterials under different loading conditions. A micromechanics-based model is proposed in which the field variables are linked to physical mechanisms at the microcrack level: damage is related to the growth of microcracks, while plasticity is related to the frictional sliding of closed microcracks. Consequently, the hardening/softening functions and parameters entering the free energy follow from the definition of a single degradation function and the elastic material properties. The evolution of opening microcracks in tension leads to brittle behavior and mode I fracture, while the evolution of closed microcracks under frictional sliding in compression/shear leads to ductile behavior and mode II fracture. Frictional sliding is endowed with a non-associative law, a crucial aspect of the model that considers the effect of dilation and allows for realistic material responses with non-vanishing frictional energy dissipation. Despite the non-associative law, a variationally consistent formulation is presented using notions of energy balance and stability, following the energetic formulation for rate-independent systems. The material response of the model is first described, followed by the numerical implementation procedure and several benchmark finite element simulations. The results highlight the ability of the model to describe tensile, shear, and mixed-mode fracture, as well as responses with brittle-to-ductile transition. A key result is that, by virtue of the micromechanical arguments, realistic failure modes can be captured, without resorting to the usual heuristic modifications considered in the phase-field literature. The numerical results are thoroughly discussed with reference to previous numerical studies, experimental evidence, and analytical fracture criteria.

Coupled dynamics of axially functionally graded graphene nanoplatelets-reinforced viscoelastic shear deformable beams with material and geometric imperfections

Abstract: This paper is the first to explore the coupled dynamics of geometrically and material-wise imperfect axially functionally graded (AFG) graphene nanoplatelets-reinforced viscoelastic third-order shear deformable beams. Four AFG graphene nanoplatelets distribution patterns are considered. Porosity, as the material imperfection, is modelled using a Gaussian Random Field model. Four thickness-wise functionally graded porosity distribution patterns are modelled. Effects of geometric imperfection are included by assigning an initial curvature to the beam. To consider the influences associated with energy dissipation caused by internal friction, the Kelvin-Voigt model for viscosity is used. External dissipative energy is modelled using a transverse damper. The effective material properties of the AFG beams are calculated using a modified Halpin-Tsai micromechanics model, together with a rule of mixture. Coupled axial, transverse, and rotational motion equations are obtained by employing a Hamiltonian approach and third-order shear deformation. The natural frequencies are obtained using a modal decomposition method. A simplified version of the graphene nanoplatelets-reinforced AFG structure is verified by a computer code-based finite element method. This novel study on the effects of geometrical imperfection on the sensitivity of beams towards porosity imperfections demonstrates the significance of scrutinising the effects of one imperfection on another.

The Effect of Agglomeration on the Electrical and Mechanical Properties of Polymer Matrix Nanocomposites Reinforced with Carbon Nanotubes

Abstract: In this work, we investigated the effect of carbon nanotubes addition and agglomeration formation on the mechanical and electrical properties of CNT–polymer-based nanocomposites. Six specimens with carbon nanotubes (CNTs) fractions of 0%, 0.5%, 1%, 2%, 4% and 5% were manufactured and characterized by dynamic mechanical analysis (DMA) and four-probe method. The stress–strain curves and electrical conductivity properties were obtained. Scanning electron microscopy (SEM) was used to characterize both agglomeration and porosity formation. By employing micromechanics, through representative volume element (RVE), finite element analysis (FEA) and resistor network model (RNM), the Young’s modulus and electrical conductivity values were calculated. The samples’ elastic moduli showed an increment, reaching the maximum value at a CNTs fraction of 2%, thereafter an adverse effect was caused in the high CNT percentage samples. The final electrical conductivity seemed greatly altered with the addition of CNTs, reaching the percolation threshold at 2%. The unavoidable formation of CNT agglomerates appeared to influence the final physical properties. The CNT agglomerates adversely affect the mechanical performance of high-CNT-percentage samples. Conversely, an exponential increment in the electrical conductivity was presented as the agglomerates formed networks allowing the transport of electrons through the tunnelling effect. These phenomena were experimentally and numerically confirmed, showing a good correlation.

Graphene-reinforced nanocomposites: synthesis, micromechanics models, analysis and applications – a review

Abstract: Graphene is playing an important role in the enhancement of new composite materials and is capable of enhancing the performance and functionality of many applications. Graphene-reinforced structures are used in nano-sized systems, energy storage devices, automotive and combustible devices, defence system, aerospace systems, medical instruments, biomedical system, and electronics devices. Due to the crucial mechanical behaviour of nano-structures design, the complete understanding of the flexural, buckling, and vibration analysis of various nanocomposite (NC) structures such as beams, plates, and shells have received great attention in recent years. The present article reviews the synthesis techniques of graphene-reinforced nanocomposites (Gr-NCs). The article also discusses the existing micromechanics models to determine the mechanical properties of Gr-NCs. The comprehensive review also presents a detailed assessment on mechanical analysis and applications of Gr-NCs structures. The article further discusses the future scope and technical challenges that may help in the appropriate design and analysis of graphene nanosystems, applicable in the various field of nanotechnology and nanocomposite materials. The purpose of the present review is to critically review the existing development in Gr-NCs and provide the comprehensive overview of Gr-NCs. By reinforcing graphene and its derivatives into the matrix, the resulting composite may enhance the functionality and explore new NCs for of various nanostructures.

Micromechanics-based phase field fracture modelling of CNT composites

Abstract: We present a novel micromechanics-based phase field approach to model crack initiation and propagation in carbon nanotube (CNT) based composites. The constitutive mechanical and fracture properties of the nanocomposites are first estimated by a mean-field homogenisation approach. Inhomogeneous dispersion of CNTs is accounted for by means of equivalent inclusions representing agglomerated CNTs. Detailed parametric analyses are presented to assess the effect of the main micromechanical properties upon the fracture behaviour of CNT-based composites. The second step of the proposed approach incorporates the previously estimated constitutive properties into a phase field fracture model to simulate crack initiation and growth in CNT-based composites. The modelling capabilities of the framework presented is demonstrated through three paradigmatic case studies involving mode I and mixed mode fracture conditions.

Micromechanics-based multiscale progressive failure simulation of 3D woven composites under compressive loading with minimal material parameters

Abstract: A novel micromechanics-based multiscale progressive damage model, employing minimal material parameters, is proposed in this paper to simulate the compressive failure behaviours of 3D woven composites (3DWC). The highly realistic constructions of microscopic and mesoscopic representative volume cells are accomplished, and a set of strain amplification factor is employed to bridge the meso-scale and micro-scale numerical calculations. Considering that the multiple failure mechanisms of 3DWC under compression are all caused by the matrix failure from the microscopic perspective, a new method incorporating the micromechanics of failure (MMF) theory and 3D kinking model is developed to identify the micro matrix failure associated with the kinking of yarns, inter-fiber fracture and pure matrix failure. As a result, only the matrix parameters are required for the failure simulation of 3DWC, eliminating the necessity of using other material parameters such as the fracture toughness and failure strengths of fiber yarns, which are generally difficult to accurately obtain through experiments. The newly proposed damage model is numerically integrated into ABAQUS with a user-defined subroutine UMAT. The numerical predictions and the experimental results exhibit good agreement, verifying the feasibility and accuracy of the novel damage model.

Effective thermo-viscoelastic behavior of short fiber reinforced thermo-rheologically simple polymers: An application to high temperature fiber reinforced additive manufacturing

Abstract: This paper presents a procedure for the estimation of the effective thermo-viscoelastic behavior in fiber-reinforced polymer filaments used in high temperature fiber-reinforced additive manufacturing (HT-FRAM). The filament is an amorphous polymer matrix (PEI) reinforced with elastic short glass fibers treated as a single polymer composite (SPC) holding the assumption of thermo-rheologically simple matrix. Effective thermo-viscoelastic behavior is obtained by implementing mean-field homogenization schemes through the extension of the correspondence principle to continuous variations of temperature by using the time–temperature superposition principle and the internal time technique. The state of the fibers in the composite is described through the use of probability distribution functions. Explicit forms of the effective properties are obtained from an identification step, ensuring the same mathematical structure as the matrix behavior. The benchmark simulations are predictions of residual stress resulting from the cooling of the representative elementary volumes (REVs) characterizing the composite filament. The computation of the averaged stress in the benchmarking examples is achieved by solving numerically the stress–strain problem via the internal variables’ framework. Reference solutions are obtained from Fast Fourier Transform based full-field homogenization simulations. A comparative analysis is performed, showing the reliability of the proposed homogenization procedure to predict residual stress against extensive computations of the macroscopic behavior of a given microstructure.

Micromechanics of soft materials using microfluidics

Abstract: Micron-scale soft materials are finding a wide range of applications in bioengineering and molecular medicine, while also increasingly emerging as useful components for consumer products. The mechanical characterization of such microscale soft objects is conventionally performed with techniques such as atomic force microscopy or micropipette aspiration that measure the local properties of micron scale objects in a serial manner. To permit scalable characterization of the global mechanical properties of soft microscale objects, we developed and describe here a microfluidic platform that can be used for performing parallelized integrated measurements of the shear modulus of individual microscale particles. We demonstrate the effectiveness of this approach by characterizing the mechanical properties of multiple protein microgels in parallel, and show that the obtained values are in good agreement with conventional serial measurements. This platform allows parallelized in situ measurements of the mechanical properties of soft deformable micron-scale particles, and builds on scalable single-layer soft-photolithography fabrication, making the measurement system readily adaptable for a range of potential applications.

Micromechanics-based material networks revisited from the interaction viewpoint; robust and efficient implementation for multi-phase composites

Abstract: A material network consists of discrete material nodes, which, when interacting, can represent complex microstructure responses. In this work, we investigate this concept of material networks under the viewpoint of the hierarchical network interactions. Within this viewpoint, the response of the material network is governed by a well-defined system of equations and an arbitrary number of phases can be considered, independently of the network architecture. The predictive capability is achieved by, on the one hand, sufficiently deep and rich network interactions to tie the discrete material nodes together, and, on the other hand, an efficient offline training procedure. For this purpose, a unified and efficient framework for an arbitrary network architecture is developed, not only for the offline training, but also for the online evaluation. The efficiency and prediction accuracy of the material network as a surrogate of a homogenization-based multiscale model in predicting the stress–strain response in both contexts of a virtual test and of FE 2 multiscale simulations are demonstrated through numerical examples with two-phase and three-phase fiber-reinforced composites.