Review and Perspectives: Shape Memory Alloy Composite Systems

Following their discovery in the early 60's, there has been a continuous quest for ways to take advantage of the extraordinary properties of shape memory alloys (SMAs). These intermetallic alloys can be extremely compliant while retaining the strength of metals and can convert thermal energy to mechanical work. The unique properties of SMAs result from a reversible difussionless solid-to-solid phase transformation from austenite to martensite. The integration of SMAs into composite structures has resulted in many benefits, which include actuation, vibration control, damping, sensing, and self-healing. However, despite substantial research in this area, a comparable adoption of SMA composites by industry has not yet been realized. This discrepancy between academic research and commercial interest is largely associated with the material complexity that includes strong thermomechanical coupling, large inelastic deformations, and variable thermoelastic properties. Nonetheless, as SMAs are becoming increasingly accepted in engineering applications, a similar trend for SMA composites is expected in aerospace, automotive, and energy conversion and storage related applications. In an effort to aid in this endeavor, a comprehensive overview of advances with regard to SMA composites and devices utilizing them is pursued in this paper. Emphasis is placed on identifying the characteristic responses and properties of these material systems as well as on comparing the various modeling methodologies for describing their response. Furthermore, the paper concludes with a discussion of future research efforts that may have the greatest impact on promoting the development of SMA composites and their implementation in multifunctional structures

Homogenization of composites with extended general interfaces: comprehensive review and unified modeling

Abstract Interphase regions that form in heterogeneous materials through various underlying mechanisms such as poor mechanical or chemical adherence, roughness, and coating, play a crucial role in the response of the medium. A well- established strategy to capture a finite-thickness interphase behavior is to replace it with a zero-thickness interface model characterized by its own displacement and/or traction jumps, resulting in different interface models. The contributions to date dealing with interfaces commonly assume that the interface is located in the middle of its corresponding interphase. We revisit this assumption and introduce a universal interface model, wherein a unifying approach to the homogenization of heterogeneous materials embedding interfaces between their constituents is developed. The proposed novel interface model is universal in the sense that it can recover any of the classical interface models. Next, via incorporating this universal interface model into homogenization, we develop bounds and estimates for the overall moduli of fiber-reinforced and particle-reinforced composites as functions of the interface position and properties. Furthermore, we elaborate on the computational implications of this interface model. Finally, we carry out a comprehensive numerical study to highlight the influence of interface position, stiffness ratio and interface parameters on the overall properties of composites, where an excellent agreement between the analytical and computational results is observed. The developed interface-enhanced homogenization framework also successfully captures size effects, which are immediately relevant to emerging applications of nano-composites due their pronounced interface effects at small scales

Investigating the evolution of microtextured region in Ti-6242 using FE-FFT multiscale modeling method

Titanium alloy Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) is frequently used in the high-pressure compressor of aero engines due to its excellent resistance to fatigue and creep failure at high temperature. While exhibiting high strength at elevated temperatures, it is susceptible to dwell fatigue at temperatures below 473 K due in part to the presence of microtextured regions (MTRs), also known as macrozones. MTRs are clusters of similarly orientated alpha particles, which form during alpha/beta processing and remain stable even after large deformation. The major objective of this dissertation is to quantify the evolution of MTRs under different thermomechanical processing parameters, and predict the optimal processing parameters to eliminate the MTRs.Idealized MTRs with pure initial orientation are first employed as the benchmark case to investigate the loading direction effect on its breakdown efficiency. Three high-temperature compression processes are simulated with different loading directions using crystal plasticity finite element method, and the results are validated against high-temperature compression experiments and EBSD measurement. The evolution of equivalent plastic strain, accumulated shear strain, and misorientation distribution is analyzed in detail to reveal the relationship between loading direction and MTR breakdown efficiency. Lastly, the reorientation velocity divergence of arbitrary loading direction is expressed in the Rodrigues\u27 space in order to predict the optimal processing parameters for MTR elimination. The MTR breakdown efficiency also depends on the morphology and its position within the specimen. Two different length scales have to be analyzed in order to consider both factors, which present great challenge to the numerical simulation. In this dissertation, a high-efficient FE-FFT multiscale modeling framework is derived and developed to overcome this challenge. The Fourier-Galerkin method is utilized to solve the microscale unit cell problem, while total Lagrangian nite element is used to solve the macroscopic boundary value problems. Several numerical improvements are derived and implemented to further improve its numerical efficiency, including consistent linearization, consistent homogenized tangent stiffness, and inexact Newton method. A series of numerical studies is conducted to investigate the accuracy, efficiency, and robustness of this algorithm

Modeling and investigation of thermal conductivity of GF nanocomposites by molecular dynamics simulation and micro-mechanics

L'abstract è presente nell'allegato / the abstract is in the attachmen

Modelling and design of hierarchical fibre-graphene nanoplatelets reinforced elasto-viscoplastic polymer matrix composites to improve crashworthiness and energy absorption

Today, light-weighting for energy efficiency without sacrificing safety and performance attributes has become a primary focus in the automotive industry. In the field of modelling graphene nanocomposites' structural applications under severe loading conditions, literature is limited. In addition, the existing work only employs the so-called one-site (OS) modelling. This study develops an approach to study 3-phases hierarchical fibres/graphene nanoplatelets (GNPs)-reinforced polymer matrix composites utilising OS modelling and what is known as multi-site (MS) modelling. The MS modelling accounts for material anisotropy considering the interaction between neighbouring inclusions. Applicability of both models is then assessed for automotive components' crashworthiness response under combined mechanical and rate-dependent plasticity or viscoplasticity behaviours. A coherent micromechanical design is employed with elastic platelets and elasto-viscoplastic matrix assumptions. The micromechanics modelling combines rate-dependent constitutive laws and thermomechanical properties for the nonlinear response of composite materials. The heterogeneous material problem is resolved in the first instance for a thermoelastic case. The thermomechanical kinematic integral equation is used to derive the strain concentration tensor. Using the generalised Mori–Tanaka (GMT) homogenisation scheme, effective thermomechanical properties are obtained. For the nonlinear behaviour, a linearisation of the classical J2 rate-dependent model is considered with an isotropic hardening. Based on an implicit integration scheme, a consistent tangent modulus is obtained and serves as a uniform modulus for homogenisation of the rate-dependent thermomechanical composite material. An application is therefore performed on a short glass -fibres/graphene nanoplatelet/ Polyamide-Nylon 6 (GNP/PA6) composite. The current study's archival value is to provide an auspicious approach for a consistent design and application of this category of materials for automotive structural components

Deep material networks for efficient scale-bridging in thermomechanical simulations of solids

We investigate deep material networks (DMN). We lay the mathematical foundation of DMNs and present a novel DMN formulation, which is characterized by a reduced number of degrees of freedom. We present a efficient solution technique for nonlinear DMNs to accelerate complex two-scale simulations with minimal computational effort. A new interpolation technique is presented enabling the consideration of fluctuating microstructure characteristics in macroscopic simulations

Variational formulation of generalized interfaces for finite deformation elasticity

The objective of this contribution is to formulate generalized interfaces in a variationally consistent manner within a finite deformation continuum mechanics setting. The general interface model is a zero-thickness model that represents the finite thickness “interphase” between different constituents in a heterogeneous material. The interphase may be the transition zone between inclusion and matrix in composites or the grain boundaries in polycrystalline solids. The term “general” indicates that the interface model here accounts for both jumps of the deformation as well as the traction across the interface. Both the cohesive zone model and elastic interface model can be understood as two limits of the current interface model. Furthermore, some aspects of material modeling of generalized interfaces are elaborated and a consistent model is proposed. Finally, the proposed theory is elucidated via a series of numerical examples. © 2017, The Author(s) 2017

A unified computational framework for process modeling and performance modeling of multi-constituent materials

This thesis presents new theoretical and computational developments and an integrated approach for interface and interphase mechanics in the process and performance modeling of fibrous composite materials. A new class of stabilized finite element methods is developed for the coupled-field problems that arise due to curing and chemical reactions at the bi-material interfaces at the time of the manufacturing of the fiber-matrix systems. An accurate modeling of the degree of curing, because of its effects on the evolving properties of the interphase material, is critical to determining the coupled chemo-mechanical interphase stresses that influence the structural integrity of the composite and its fatigue life. A thermodynamically consistent theory of mixtures for multi-constituent materials is adopted to model curing and interphase evolution during the processing of the composites. The mixture theory model combines the composite constituent behaviors in an effective medium, thereby reducing the computational cost of modeling chemically reacting multi-constituent mixtures, while retaining information involving the kinematic and kinetic responses of the individual constituents. The effective medium and individual constituent behaviors are each constrained to mutually satisfy the balance principles of mechanics. Even though each constituent is governed by its own balance laws and constitutive equations, interactive forces between constituents that emanate from maximization of entropy production inequality provide the coupling between constituent specific balance laws and constitutive models. The mixture model is cast in a finite strain finite element framework that finds roots in the Variational Multiscale (VMS) method. The deformation of multi-constituent mixtures at the Neumann boundaries requires imposing constraint conditions such that the constituents deform in a self-consistent fashion. A set of boundary conditions is presented that accounts for the non-zero applied tractions, and a variationally consistent method is developed to enforce inter constituent constraints at Neumann boundaries in the finite deformation context. The new method finds roots in a local multiscale decomposition of the deformation map at the Neumann boundary. Locally satisfying the Lagrange multiplier field and subsequent modeling of the fine scales via edge bubble functions results in closed-form expressions for a generalized penalty tensor and a weighted numerical flux that are free from tunable parameters. The key novelty is that the consistently derived constituent coupling parameters evolve with material and geometric nonlinearity, thereby resulting in optimal enforcement of inter-constituent constraints. A class of coupled field problems for process modeling and for performance molding of fibrous composites is presented that provides insight into the theoretical models and multiscale stabilized formulations for computational modeling of multi-constituent materials

Applications of Crystal Plasticity in Forming Technologies

In this Special Issue, we have gathered work on simulations of polycrystalline metals and alloys at various length scales to model multiscale localization phenomena such as slip bands, cracks, and twins. The series highlights innovative techniques that combine simulation and experiments to capture material production and guide the development of forming theories. The published work helps to understand the effect of microstructure characteristics on deformation and damage behavior under multiaxial load conditions. Furthermore, these models and the studies can be used with machine learning technologies to optimize microstructure functions for materials application and process paths