Issam Doghri - Academia.edu (original) (raw)

Papers by Issam Doghri

Mechanics of Advanced Materials and Structures

Structure and Properties of Additive Manufactured Polymer Components, 2020

Abstract Plastic part complexity has historically been strongly driving part cost. Recent develop... more Abstract Plastic part complexity has historically been strongly driving part cost. Recent developments of additive technologies have led to a shift from rapid prototyping to production of actual end-use parts, thus widening the range of design possibilities. By opposition with the so-called now subtractive methods, additive manufacturing (AM) regroups all manufacturing processes allowing to process parts from 3D numerical CAD model by building them layer by layer. The now irrelevant need for specific mold tooling design allows to produce low volume of highly complex parts in less time. Among additive processes of polymers and polymer-based composites, the most mature and industrially widespread are selective laser sintering (SLS) and fused filament fabrication (FFF), sometimes referred to as fused deposition modeling (FDM). In FFF, fused material filaments are deposited through a moving head while in SLS, powdered material is sintered using a laser heat source.

Mechanics of Advanced Materials and Structures, 2021

This work is concerned with the micromechanical modeling of porous short fiber-reinforced composi... more This work is concerned with the micromechanical modeling of porous short fiber-reinforced composites considered as three-phase microstructures: cavities and short fibers embedded in a matrix phase....

International Journal for Multiscale Computational Engineering, 2021

Computational Materials Science, 2021

Abstract An enhanced phase field model recently proposed by the authors enables to model polymer ... more Abstract An enhanced phase field model recently proposed by the authors enables to model polymer crystallization and to predict spherulite growth and representative semi-crystalline micro-structures in 2D or 3D. After solidification, the value of the phase field variable in each subcell (2D pixel or 3D voxel) indicates whether the material is amorphous or crystalline. In the present paper, full-field micromechanical simulations are conducted on the microstructures generated by the enhanced phase field model in order to predict the effective mechanical properties. A Fast Fourier Transform (FFT) method with periodic boundary conditions is used. Care is taken to obtain representative volume elements (RVEs) by computing the number of subcells in each spherulite and the number of spherulite nucleations in each RVE. Numerical FFT predictions of elastic properties for a range of crystallinity ratios are validated against experimental data and compared to simpler composite inclusion models. Finite element simulations of thermal shrinkage are also shown. The approach was numerically implemented in a research version of the Digimat software.

POLYMER CRYSTALLIZATION, 2020

An enhanced phase field model is proposed for the numerical simulation of crystallization in semi... more An enhanced phase field model is proposed for the numerical simulation of crystallization in semicrystalline polymers. As with other models, it is based on coupling the heat equation with the Alle-Cahn equation, which is derived from the Gibbs-Thomson solid-liquid interface equation. Starting from spherulite nucleation,

Composites Part A: Applied Science and Manufacturing, 2015

This paper presents a new micromechanical damage model, called "First Pseudo-Grain Damage" (FPGD)... more This paper presents a new micromechanical damage model, called "First Pseudo-Grain Damage" (FPGD) model, to predict the overall elasto-plastic behavior and damage evolution in short fiber reinforced thermoplastic materials typically produced by injection molding. The model combines mean-field homogenization theory with

International Journal of Plasticity, 2015

A two-scale time homogenization formulation and the corresponding algorithms are proposed for cou... more A two-scale time homogenization formulation and the corresponding algorithms are proposed for coupled viscoelasticeviscoplastic (VEeVP) homogeneous solids subjected to large numbers of cycles. The main aim is to predict the long time response while reducing the computational cost considerably. The method is based on the definition of macro and micro-chronological time scales, and on asymptotic expansions of the unknown variables. First, the VEeVP constitutive model is formulated based on a thermodynamical framework. Next, the original VEeVP initial-boundary value problem is decomposed into coupled micro-chronological (fast time scale) and macro-chronological (slow timescale) problems. The former is purely VE, and solved once for each macro time step, whereas the latter problem is nonlinear and solved iteratively using fully implicit time integration. For micro-scale time averaging, one-point and multi-point integration algorithms are developed. Several numerical simulations on uniaxial and multiaxial cyclic loadings illustrate the computational efficiency and the accuracy of the proposed methods.

Presentation of an original homogenization scheme for the simulation of the off-axis loading of b... more Presentation of an original homogenization scheme for the simulation of the off-axis loading of biaxial woven carbon-epoxy composites.

International Journal of Solids and Structures, 2013

This study proposes a micromechanical modeling of inclusion-reinforced viscoelastic-viscoplastic ... more This study proposes a micromechanical modeling of inclusion-reinforced viscoelastic-viscoplastic composites, based on mean-field approaches. For this, we have generalized the so-called incrementally affine linearization method which was proposed by Doghri et al. (2010a) for elasto-viscoplastic materials. The proposal provides an affine relation between stress and strain increments via an algorithmic tangent operator. In order to find the incrementally affine expression, we start by the linearization of evolution equations at the beginning of a time step around the end time of the step. Next, a numerical integration of the linearized equations is required using a fully implicit backward Euler scheme. The obtained algebraic equations lead to an incrementally affine formulation which is form-similar to linear thermoelasticity, therefore known homogenization models for linear thermoelastic composites can be applied. The proposed method can deal with general viscoelastic-viscoplastic constitutive models with an arbitrary number of internal variables. The semi-analytical predictions are validated against finite element simulations and experimental results.

Communications in Applied Numerical Methods, 1988

This paper deals with the implementation in finite element calculations of complex elastoplastic ... more This paper deals with the implementation in finite element calculations of complex elastoplastic constitutive equations exhibiting non-linear kinematic and isotropic hardening, and fully coupled to a continuous ductile damage evolution model. The Newton method is used to solve the non-linear global equilibrium equations as well as the non-linear local equations obtained by fully implicit integration of the constitutive equations. The consistent local tangent modulus is obtained by exact linearization of the algorithm. The procedure described has been implemented in the general purpose code ABAQUS.

International Journal of Material Forming, 2009

This work presents a new mean-field approach for the modeling of two-phase elasto-plastic composi... more This work presents a new mean-field approach for the modeling of two-phase elasto-plastic composites. The multisite model was initially designed for texture prediction in polycrystalline aggregates [L. Delannay et al., Comput. Mater. Sci. (2008), doi:10.1016/j.commatsci.2008.06.013]. Here, it is used to simulate short range interactions between the matrix and the inclusions. Stacks of two subregions lying on either sides of a planar interface are treated as pseudo- grains. A self-consistent scheme governs the strain partitioning among the pseudo-grains and the remaining matrix. Predictions of the model are compared to finite element results.

International Journal of Solids and Structures, 2007

ABSTRACT A micromechanical model is proposed for multiphase metals consisting of a ductile matrix... more ABSTRACT A micromechanical model is proposed for multiphase metals consisting of a ductile matrix reinforced by hard, equiaxed inclusions. The model belongs to a class of incremental mean-field theories of the first order and is suitable for general, non-monotonic loading paths. This paper specifically addresses the definition of the effective, instantaneous shear modulus of the isotropic comparison materials which intervene in the solution of the linearized, homogenisation problem. Time integration of the composite response is calculated using a Newton–Raphson scheme and a consistent tangent operator is derived. Predictions of the phase stresses developed under various loading modes and for a wide range of volume fractions of inclusions are assessed by a comparison with finite element simulations of periodic unit cells. It is argued that the latter predictions depend dramatically on the topological arrangement of the inclusions. Considering a cubic ordering of inclusions, it is possible to reproduce FE predictions with the mean-field model on the condition that the comparison materials are stiffer than the real phases during the first few percent of plastic strain. The mean-field model provides an accurate prediction of the average stress developed within individual phases.

International Journal of Solids and Structures, 2003

We develop homogenization schemes and numerical algorithms for two-phase elasto-plastic composite... more We develop homogenization schemes and numerical algorithms for two-phase elasto-plastic composite materials and structures. A Hill-type incremental formulation enables the simulation of unloading and cyclic loadings. It also allows to handle any rate-independent ...

AIP Conference Proceedings, 2011

ABSTRACT A micromechanical damage modeling approach is presented to predict the overall elasto-pl... more ABSTRACT A micromechanical damage modeling approach is presented to predict the overall elasto-plastic behavior and damage evolution in short fiber reinforced composite materials. The practical use of the approach is for injection molded thermoplastic parts reinforced with short glass fibers. The modeling is proceeded as follows. The representative volume element is decomposed into a set of pseudograins, the damage of which affects progressively the overall stiffness and strength up to total failure. Each pseudograin is a two-phase composite with aligned inclusions having same aspect ratio. A two-step mean-field homogenization procedure is adopted. In the first step, the pseudograins are homogenized individually according to the Mori-Tanaka scheme. The second step consists in a self-consistent homogenization of homogenized pseudograins. An isotropic damage model is applied at the pseudograin level. The model is implemented as a UMAT in the finite element code ABAQUS. Model is shown to reproduce the strength and the anisotropy (Lankford coefficient) during uniaxial tensile tests on samples cut under different directions relative to the injection flow direction.

III European Conference on Computational Mechanics

We develop mean-field homogenization schemes for the prediction of the effective mechanical and a... more We develop mean-field homogenization schemes for the prediction of the effective mechanical and acoustical properties of viscoelastic inclusion-reinforced materials. For mechanical applications, the overall behavior (eg stiffness tensor) of composite materials is estimated using Eshelby-based homogenization models. The effective acoustical properties (eg attenuation factor) are obtained in a similar way, based on the one particle scattering problem. Viscoelastic materials are known to be mechanical dampers characterised by ...

Mechanics of Deformable Solids, 2000

In the previous chapter, we presented the local or “strong” formulation, which allows to solve va... more In the previous chapter, we presented the local or “strong” formulation, which allows to solve various (but rather simple) problems in statics. Often, as we shall see in subsequent chapters, the formulation is used under approximate forms (e.g., theories of beams, plates and shells). However, the most powerful numerical methods (e.g., finite elements) which are used to find approximate solutions to problems that cannot be solved in closed form (i.e., most industrial problems) are not based on the local formulation but on the global formulations which will be developed in Secs. 2.2 to 2.6; this constitutes one of the major interests of this chapter.