Hashim Naji Azooz Al Mahmud

Significant research effort has been dedicated for decades to improve the mechanical properties of aerospace polymer-based composite materials. Lightweight epoxy-based composite materials have increasingly replaced the comparatively heavy and expensive metal alloys used in aeronautical and aerospace structural components. In particular, carbon fibers (CF)/graphene nanoplatelets (GNP)/epoxy hybrid composites can be used for this purpose owing to their high specific stiffness and strength. Therefore, this work has been completed to design, predict, and optimize the effective mechanical properties of CF/GNP/epoxy composite materials at different length scales using a multiscale modeling approach. The work-flow of modeling involves a first step of using molecular dynamics (MD) with a reactive force field (ReaxFF) to predict the structure and mechanical behavior of the GNP/epoxy materials at the molecular level. A micromechanics approach is then used to model and predict the mechanical properties of the CF/GNP/epoxy hybrid composite at the bulk level. One of the major findings of this study refers to an alignment behavior of phenyl rings in epoxy with the planar GNP surface at the interphase region. This alignment plays an important role to drive the molecular density of epoxy at the interphase and promote the GNP-epoxy interfacial adhesion. The results also validate the use of ReaxFF in MD modeling of such nanocomposites as the predicted properties compare well with experiment. The impact on the mechanical properties of aerospace epoxy materials reinforced with pristine GNP, highly concentrated Graphene Oxide (GO), and Functionalized Graphene Oxide (FGO) has also been investigated in this study. A systematic computational approach to simulate the reinforcing nanoplatelets and probe their influence on the mechanical response of the epoxy matrix at both nanoscale and bulk levels. The nanoscale outcomes indicate a significant degradation in the in-plane elastic and shear moduli of the nanocomposite when introducing large amounts of oxygen and functional groups to the robust sp2 structure of the GNP. However, the wrinkled and rough topology of GO and FGO promotes the nanoplatelet-matrix interlocking mechanism which produces a significant improvement in the out-of-plane shear modulus. In addition, surface functionalization of GNP promotes the nanoplatelet-epoxy interfacial interaction/adhesion significantly which is important for the material toughness. Using micromechanics analysis, the influence of the nanoplatelets content and aspect ratio on the mechanical response of the proposed nanocomposites has also been predicted and validated with experimental data available from the literature. Generally, there is an improvement in the predicted mechanical response of the bulk nanocomposite materials with increasing nanoplatelets content and aspect ratio. The predicted mechanical properties of the nanoplatelet/epoxy nanocomposites are then used to generate hybrid composite models reinforced with unidirectional CF. The micromechanics predictions are used to analyze the reinforcing effect of the proposed nanoplatelets on the unidirectional CF/nanoplatelet/epoxy hybrid composites. Three laminated hybrid composite panels are also modeled and analyzed to address the reinforcing effect of the proposed nanoplatelets on the laminated hybrid composite panels. The predicted mechanical properties of the laminated hybrid composite panels are important in assessing the mechanical performance of in-service structural components.

New fabrication methods for carbon nanotube (CNT) networks have been reported, producing self-assembled stacks of flattened CNTs (flCNTs), which could potentially be infiltrated with polymers for enhanced load transfer. In this work, Molecular Dynamics simulation is used to model flCNT/polyimide nano-composites to characterize the interfacial characteristics. Two polyimides, fluorinated and non-fluorinated, are simulated and compared on the basis of interfacial interaction energy, friction force, and transverse strength. The results indicate that the fluorinated polyimide exhibits a lower interaction energy, higher frictional force, and higher peak strength in transverse tension relative to the non-fluorinated polyimide, which is likely due to the steric hindrance and interlocking of the fluorine atoms. The presence of polyimide significantly enhances the interfacial friction over bare incommensurate flCNTs. This study is intended to drive the design of new light-weight, high-strength materials involving flCNTs for aerospace structural applications.

A novel multi-scale Integrated Computational Material Engineering (ICME) approach to predict the evolution of residual stress during curing of thermoset composites is presented. Molecular Dynamics (MD) simulations and Finite Element Analysis (FEA) are used at their respective length scales to determine the cure induced effect on thermoset fiber-reinforced composites. First, curing is simulated at the molecular level to predict the density (shrinkage), Young’s modulus, and yield strength as a function of the degree of cure. Second, the correlation between polymer properties and tensile transverse strength is obtained by modeling the residual stress build-up at the micro-scale. Preliminary results are presented for EPON 862 and EPON 828 epoxy systems at the nano-scale. A theoretical framework for future work is outlined.

The impact on the mechanical properties of aerospace epoxy materials reinforced with pristine Graphene Nanoplatelets (GNP), highly concentrated Graphene Oxide (GO), and Functionalized Graphene Oxide (FGO) has been investigated in this study. Molecular Dynamics (MD) using a reactive force field (ReaxFF) has been employed in predicting the effective mechanical properties of the interphase region of the three proposed nanocomposite materials at the nanoscale level. A systematic computational approach to simulate the reinforcing nanoplatelets and probe their influence on the mechanical properties of the epoxy matrix are included in this study. The nanoscale outcome indicates a significant degradation in the in-plane elastic Young’s (decreased by ~89%) and shear (decreased by ~72.5%) moduli of the nanocomposite when introducing large amounts of oxygen and functional groups to the robust sp2 structure of the GNP. However, the wrinkled morphology of GO and FGO improves the nanoplatelet-matrix interlocking mechanism which produces a significant improvement in the out-of-plane shear modulus (increased by ~95%). The influence of the nanoplatelet content and aspect ratio on the mechanical response of the nanocomposites has also been determined in this study. Generally, the predicted mechanical response of the bulk nanocomposite materials demonstrates an improvement with increasing nanoplatelet content and aspect ratio. The results show good agreement with experimental data available from the literature.

Numerous research efforts have been focused on developing lightweight epoxy-based composite materials that rival expensive metal alloys in aerospace structural components. Due to their high specific stiffness and strength, carbon fiber (CF)/graphene nanoplatelet (GNP)/epoxy hybrid composites are excellent candidates for this purpose. The objective of this study is to develop a multiscale modeling approach to predict the effective mechanical properties of a CF/GNP/epoxy composite material. The work-flow of this study involves molecular dynamics (MD) simulation with a reactive force field to predict the structure and behavior of the GNP/epoxy material at the molecular level and micromechanics to predict the mechanical properties of the CF/GNP/epoxy hybrid composite at the bulk level. The study provides evidence of an alignment behavior of phenyl rings in epoxy with the planar GNP surface at the interphase region. The results also indicate the validity of using a reactive force field as they compare well with experiment.

Numerous research efforts have been focused on developing lightweight epoxy-based composite materials that can rival expensive metal alloys in specific aerospace structural components. Due to their high-specific stiffness and strength and corrosion resistance, epoxy-graphene nanoplatelets (GNP) composite materials can be used as the matrix of carbon fiber-reinforced hybrid composites. This work provides a multiscale computational analysis to simulate the hybrid composite and predict its effective mechanical properties. The work-flow of this study involves two main sequential stages that can be specified towards acquiring the effective mechanical properties of the hybrid composite. Molecular dynamics (MD) simulations using large-scale atomic/molecular massively parallel simulator (LAMMPS) to create a nanoscale unit cell from EPON 862-DETDA epoxy and GNP was the first stage. The reactive force field (ReaxFF) was chosen to represent the atomic/molecular level interactions in these simulations. The interaction between aromatic rings in epoxy molecules and GNP was investigated in this study. It was found that such interactions can affect the spatial density distribution of the epoxy molecules at the GNP/epoxy interphase. The second stage involved a micromechanical analysis of the carbon fiber-reinforced hybrid composite using NASA's micromechanics analysis code based on the generalized method of cells (MAC/GMC). The effective mechanical properties of the hybrid composite were predicted at different volume fractions of GNP in the composite matrix.

This paper presents an efficient meshless method in the formulation of the weak form of local Petrov-Galerkin method MLPG. The formulation is carried out by using an elliptic domain rather than conventional isotropic domain of influence. Therefore, the method involves an MLPG formulation in conjunction with an anisotropic weight function. In the elliptic weight function, each node has three characteristic indicated that were major radius, inner radius, and the direction of the local domain. Furthermore, the space that will be covered by the elliptical domain will be less than the area of the circle (isotropic) at the same main diameter. This means leaving many points of integration are not necessary. Therefore, the computational cost will be decreased. MLPG method with the elliptical domain is used in solving problems of linear elastic fracture mechanism LEFM. MATLAB and Fortran codes are used for obtaining the results of this research .The results were compared with those presented in the literature which shows a reduction in the computational cost up to 15%, and an error criteria enhancement up to 25%.

A Stress Intensity Factor (SIF) Mode I is calculated for longitudinal composite plate with dimensions (104 mm × 40 mm) and fiber volume fraction (37. 5%). The finite element method (FEM) (ANSYS) and MFree method are used for calculating the SIF of quarter model (i. e. (52 mm × 20 mm)) with constant displacement in y- direction that is applied at the ends of the plate. The crack length, material properties of fiber and material properties of matrix affect the SIF and these parameters are studied to understand the crack growth and the fracture mechanism in composite material. The value of SIF depends on the position of crack tip in additional to the material properties of fiber and matrix. A comparison between the FEM and MFree method is made to study the advantages and disadvantages of each method and to choose the suitable method for calculating SIF for composite plate. Generally, there is a good agreement between the FEM and MFree method but there is a slightly difference between them specially when the crack length increases. This slight difference happens due to the calculation technique of SIF used in each method. This difference makes the FEM accurate than the MFree method unless increasing the number of nodes around the crack tip in MFree method.

This work presents an efficient numerical Meshless method in the formulation of the local weak form by Meshless Local Petrov-Galerkin "MLPG" method whose shape function is derived using the Moving Least Squares "MLS" approximation. In the formulation of the local weak form, a comparison study has been presented based on the efficiency, time cost, and accuracy of a variety of meshless trial functions without transgressing the general concepts of the MLPG method. Unconventional local support domains such as elliptic, hexagonal, and rhombic domains are investigated to check the possibility of using these local domains. The formulation has been stabilized by using elliptic local support domains rather than conventional isotropic domains of influence, therefore, the method involves an MLPG formulation in conjunction with an anisotropic weight function. In the elliptic weight function, each node has three characteristic indications that are major radius, minor radius, and the direction of the local domain. Furthermore, the space that will be covered by the ellipse will be less than that when using a circular isotropic domain at the same main diameter which means leaving many points that are not necessary. Thus, the computational time cost will drop about 20-25%, as well as, improving the accuracy by decreasing the error in the obtained results. The modified MLPG formulation is employed for problems in Linear Elastic Fracture Mechanics "LEFM", wherein an efficient method for evaluating the stress intensity factors for the mode-I and mode-II depending on the potentiality of the MLPG has been employed. The basic objective of the present work has been accomplished by developing an efficient "elastostatic" and "elastodynamic" fracture mechanics FORTRAN codes, based upon the MLPG method. Several case studies have been considered in the present work, in which, the results are verified with those presented in the literature.

Significant research effort has been dedicated for decades to improve the mechanical properties of aerospace polymer-based composite materials. Lightweight epoxy-based composite materials have increasingly replaced the comparatively heavy and expensive metal alloys used in aeronautical and aerospace structural components. In particular, carbon fibers (CF)/graphene nanoplatelets (GNP)/epoxy hybrid composites can be used for this purpose owing to their high specific stiffness and strength. Therefore, this work has been completed to design, predict, and optimize the effective mechanical properties of CF/GNP/epoxy composite materials at different length scales using a multiscale modeling approach. The work-flow of modeling involves a first step of using molecular dynamics (MD) with a reactive force field (ReaxFF) to predict the structure and mechanical behavior of the GNP/epoxy materials at the molecular level. A micromechanics approach is then used to model and predict the mechanical properties of the CF/GNP/epoxy hybrid composite at the bulk level. One of the major findings of this study refers to an alignment behavior of phenyl rings in epoxy with the planar GNP surface at the interphase region. This alignment plays an important role to drive the molecular density of epoxy at the interphase and promote the GNP-epoxy interfacial adhesion. The results also validate the use of ReaxFF in MD modeling of such nanocomposites as the predicted properties compare well with experiment. The impact on the mechanical properties of aerospace epoxy materials reinforced with pristine GNP, highly concentrated Graphene Oxide (GO), and Functionalized Graphene Oxide (FGO) has also been investigated in this study. A systematic computational approach to simulate the reinforcing nanoplatelets and probe their influence on the mechanical response of the epoxy matrix at both nanoscale and bulk levels. The nanoscale outcomes indicate a significant degradation in the in-plane elastic and shear moduli of the nanocomposite when introducing large amounts of oxygen and functional groups to the robust sp2 structure of the GNP. However, the wrinkled and rough topology of GO and FGO promotes the nanoplatelet-matrix interlocking mechanism which produces a significant improvement in the out-of-plane shear modulus. In addition, surface functionalization of GNP promotes the nanoplatelet-epoxy interfacial interaction/adhesion significantly which is important for the material toughness. Using micromechanics analysis, the influence of the nanoplatelets content and aspect ratio on the mechanical response of the proposed nanocomposites has also been predicted and validated with experimental data available from the literature. Generally, there is an improvement in the predicted mechanical response of the bulk nanocomposite materials with increasing nanoplatelets content and aspect ratio. The predicted mechanical properties of the nanoplatelet/epoxy nanocomposites are then used to generate hybrid composite models reinforced with unidirectional CF. The micromechanics predictions are used to analyze the reinforcing effect of the proposed nanoplatelets on the unidirectional CF/nanoplatelet/epoxy hybrid composites. Three laminated hybrid composite panels are also modeled and analyzed to address the reinforcing effect of the proposed nanoplatelets on the laminated hybrid composite panels. The predicted mechanical properties of the laminated hybrid composite panels are important in assessing the mechanical performance of in-service structural components.

The remarkable properties of Thermoplastic Matrix Composites (TMCs) including their ability to tolerate damage, low weight, high resistance to impact, and mechanical toughness, make them highly desirable for use in aerospace structures. TMCs undergo various cooling and heating cycles at elevated temperatures during the processing, which triggers the formation of crystalline regions resulting in matrix shrinkage and the generation of residual stresses in the final composite. To minimize the development cycle time and cost of future applications of TMCs, it is crucial to optimize materials manufacturing parameters, and component design. Manufacturing process of TMCs has a significant impact on their microstructure, which plays a critical role in controlling their in-service performance. Given that the thermo-mechanical properties of composite materials are influenced by phenomena at various length scales, a multiscale computational approach is necessary to predict manufacturing parameter-property relationships accurately and efficiently. Also, thermomechanical properties in semi-crystalline thermoplastics are closely linked to their crystallinity content, which is precisely regulated by both material properties such as melting and crystallization temperatures, and processing parameters like crystallization temperature and temperature holding time. Therefore, molecular-level material properties should be determined across a broad range of crystallinity levels to facilitate the multi-scale process modeling of thermoplastic composites. This study implemented molecular dynamics (MD) modeling to predict the thermo-mechanical properties and volumetric shrinkage of PolyEther Ether Ketone (PEEK) resin, a thermoplastic material, as a function of temperature and crystallinity content. The thermo-mechanical properties are linked to processing time and temperature via crystallization-kinetics models. © 2023 by DEStech Publications, Inc. and American Society for Composites. All rights reserved.

Graphene oxide (GO) nanoplatelets can be used to reinforce neat resin or the matrix phase of fiber composites for improved mechanical properties. Although the oxygen content levels in GO and reduced graphene oxide (rGO) strongly influence the interface with the polymer, experimental-based optimization of the oxygen content for enhanced composite properties is difficult and time-consuming. Fortunately, molecular dynamics (MD) simulation can be used to efficiently predict the interfacial properties of composite on the molecular level and provide physical insight into the effect of the oxygen content of rGO. In this study, MD is used to predict the elastic properties of rGO/epoxy interfaces and the corresponding interfacial shear strength (IFSS) for different oxygen content levels. The results indicate that increasing levels of oxygen in rGO results in interfaces with a reduced in-plane elastic modulus but a substantially higher IFSS. These results are important for the design of rGO/epoxy composites for specific engineering applications.

Al Mahmud participated in the American Institute of Aeronautics and Astronautics (AIAA SciTech Forum 2020) . The participation involved publishing a research work entitled: Multiscale Modeling for Virtual Manufacturing of Thermoset Composites which has been accepted and published in the proceeding of the conference. AIAA 2020-0882 Session: ICME: Integrating Composite Manufacturing Modeling with Design Published Online:5 Jan 2020 https://doi.org/10.2514/6.2020-0882

Al Mahmud participated in the Earth & Space Virtuale Technical Conference in the United States of America in April 19-23, 2021. The participation involved presenting his research work entitled: Molecular Dynamics Modeling to Probe the Effect of Surface Functionalization on the Interfacial Adhesion and Shear Strength of Graphene/Epoxy Nanocomposites which has been accepted and published in the proceeding of the conference. https://doi.org/10.1061/9780784483381.001

Al Mahmud participated in the ASC 36th Virtuale Technical Conference in Texas A&M University in the United States of America in September 19-23, 2021. The participation involved presenting his research work entitled: COMPUTATIONAL MODELING OF EPOXY-BASED HYBRID COMPOSITES REINFORCED WITH CARBON FIBERS AND FUNCTIONALIZED GRAPHENE NANOPLATELETS which has been accepted and published in the proceeding of the conference. http://www.dpi-proceedings.com/index.php/asc36/article/view/35846

Al Mahmud attended and participated in the the ASC 34th Technical Conference at Georgia Tech University, Georgia Institute of Technology, in Atlanta, GA, USA in September 23-25, 2019 The participation involved presenting his research work entitled: MD MODELING OF EPOXY-BASE NANOCOMPOSITES REINFORCED WITH FUNCTIONALIZED GRAPHENE NANOPLATELETS which has been accepted and published in the proceeding of the conference http://dpi-proceedings.com/index.php/asc34/article/view/31377