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Dr. Sanjib Chowdhury
Center for Composite Materials, University of Delaware, Newark, 19716 DE, USA

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0 Machine Learning
0 Multiscale Modeling
0 Nanostructured materials
0 Composite mechanics and manufacturing
0 Composite interphase

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Journal article
Published: 15 December 2020 in Applied Surface Science
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In this study, the interaction of epoxy resin with the glass fiber in the presence of monolayer glycidoxypropyltrimethoxy silane is studied using molecular dynamics simulations. To quantify the fiber–matrix adhesion, the interphase traction-separation response is developed by loading the interphase in mode-I and mode-II. The overall composite model is also loaded in tension and shear to predict the stress–strain responses and failure loci. Interphase consisting of different silane-epon-amine connectivity patterns has thickness in the range of 1.3–1.7 nm as determined by the root mean squared fluctuation method. In the absence of silane, fiber-epoxy non-bonded interaction is very weak and failure is at the fiber surface. Simulations indicate that higher fiber surface reactivity (i.e., SiOH number density) does not improve adhesion unless there is silane in the interphase. Presence of silane introduces covalent bonding interactions in the fiber-epoxy interphase improving the interphase properties. As a result, composite strength and energy absorption capability improves significantly with the number of bond sites at the fiber surface and promotes progressive failure through multiple damage modes. Simulation results suggest that silane number density of 1–2 nm−2 should be the optimum to achieve high strength and energy absorption for the composite system investigated.

ACS Style

Sanjib C. Chowdhury; Riley Prosser; Timothy W. Sirk; Robert M. Elder; John W. Gillespie. Glass fiber-epoxy interactions in the presence of silane: A molecular dynamics study. Applied Surface Science 2020, 542, 148738 .

AMA Style

Sanjib C. Chowdhury, Riley Prosser, Timothy W. Sirk, Robert M. Elder, John W. Gillespie. Glass fiber-epoxy interactions in the presence of silane: A molecular dynamics study. Applied Surface Science. 2020; 542 ():148738.

Chicago/Turabian Style

Sanjib C. Chowdhury; Riley Prosser; Timothy W. Sirk; Robert M. Elder; John W. Gillespie. 2020. "Glass fiber-epoxy interactions in the presence of silane: A molecular dynamics study." Applied Surface Science 542, no. : 148738.

Journal article
Published: 06 November 2020 in Computational Materials Science
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Molecular dynamics (MD) simulation requires an accurate potential energy function to describe atomic interactions of interest. Optimization of the function’s numerous parameters is often time-consuming and labor-intensive. In this study, a machine learning inspired evolutionary parametrization technique using the genetic algorithm is developed to decrease the time required to optimize the parameters of the ReaxFF interatomic potential. An artificial neural network is used as a surrogate for the ReaxFF potential to reduce computational time. Changes to the genetic algorithm are incrementally benchmarked for accuracy and time cost with respect to a moderately complex zinc-oxide model to find superior operators for ReaxFF parametrization. It is found that utilizing an artificial neural network significantly boosted performance, as measured by the final total error and the rate of decrease of total error with respect to time. The double-Pareto probability density based crossover operator and a multiple standard deviation based Gaussian mutation scheme outperform their counterparts. The computational time cost to achieve the same level of accuracy relative to manual training is decreased from months to days.

ACS Style

Chaitanya M. Daksha; Jejoon Yeon; Sanjib C. Chowdhury; John W. Gillespie Jr.. Automated ReaxFF parametrization using machine learning. Computational Materials Science 2020, 187, 110107 .

AMA Style

Chaitanya M. Daksha, Jejoon Yeon, Sanjib C. Chowdhury, John W. Gillespie Jr.. Automated ReaxFF parametrization using machine learning. Computational Materials Science. 2020; 187 ():110107.

Chicago/Turabian Style

Chaitanya M. Daksha; Jejoon Yeon; Sanjib C. Chowdhury; John W. Gillespie Jr.. 2020. "Automated ReaxFF parametrization using machine learning." Computational Materials Science 187, no. : 110107.

Journal article
Published: 27 January 2020 in Composites Part B: Engineering
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The effects of molecular weight (MW) of cross-linker and degree of cure on the structure and thermo-mechanical properties of the Bisphenol A diglycidyl ether epoxy resin have been studied using MD simulations with reactive force field ReaxFF and non-reactive General AMBER Force Field (GAFF). Cross-linked structures are created from stoichiometric mixtures of Epon and Jeffamine® using a multi-step cross-linking algorithm. The glass transition temperature (Tg) is determined by annealing where the cross-linked epoxy is cooled from the rubbery state to below room temperature. Deformation mechanisms of the cross-linked epoxy including bond breakage are studied under tensile and shear loadings. The effects of cross-linkers of increasing MW (Jeffamine® D-230, Jeffamine® D-400 and Jeffamine® D-600) are studied for highly cured (98.5% degree of cure) systems. MD predicted Tg is in good agreement with experiments after cooling rate correction using the WLF relationship. The highest Tg is obtained for the lower MW cross-linker that exhibits a denser network structure. In addition, the effects of varying degrees of cure on properties are studied for Epoxy/Jeffamine® D-230. In this case, the MD results shows that Tg increases linearly with degree of cure and that the DiBenedetto relationship can be applied using the MD fitted parameters. Lower MW cross-linker yields higher modulus and yield stress and reduced strain to failure and energy absorption than the higher MW cross-linkers. Results from GAFF, which is about 100 times more computationally efficient, agree well with ReaxFF predictions up to the strain limit at which bond breakage becomes significant.

ACS Style

Sanjib C. Chowdhury; Robert M. Elder; Timothy W. Sirk; John W. Gillespie. Epoxy resin thermo-mechanics and failure modes: Effects of cure and cross-linker length. Composites Part B: Engineering 2020, 186, 107814 .

AMA Style

Sanjib C. Chowdhury, Robert M. Elder, Timothy W. Sirk, John W. Gillespie. Epoxy resin thermo-mechanics and failure modes: Effects of cure and cross-linker length. Composites Part B: Engineering. 2020; 186 ():107814.

Chicago/Turabian Style

Sanjib C. Chowdhury; Robert M. Elder; Timothy W. Sirk; John W. Gillespie. 2020. "Epoxy resin thermo-mechanics and failure modes: Effects of cure and cross-linker length." Composites Part B: Engineering 186, no. : 107814.

Journal article
Published: 26 October 2019 in Computational Materials Science
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This paper investigates the inter-molecular interactions during tensile loading in ultrahigh molecular weight polyethylene (UHWMPE) single crystals at the atomistic scale. Molecular dynamics (MD) simulations of velocity controlled chain pullout are employed to study inter-molecular load transfer mechanisms. The transfer of tensile load is governed by van der Waals forces that dominate the inter-molecular shear interactions. The tensile stress build up occurs over a length of approximately 40c, where c is the lattice constant along the chain axis. Atomistic MD models incorporate the influence of surrounding neighboring atoms. Therefore, a nonlocal shear lag continuum model is developed for the first time to bridge length scales by extending the classical shear lag model of stress transfer in composites. The nonlocal model predictions correlate better with the MD results compared to the classical shear lag formulation. Combining the MD and shear lag model results, a bilinear mode II cohesive traction-separation behavior is identified to describe the inter-molecular interactions of the continuum with interface stiffness (2.38 GPa/nm), peak traction (0.14 GPa) and mode II fracture toughness (17 mJ/m2).

ACS Style

Sanjib C. Chowdhury; Subramani Sockalingam; John W. Gillespie Jr.. Inter-molecular interactions in ultrahigh molecular weight polyethylene single crystals. Computational Materials Science 2019, 172, 109360 .

AMA Style

Sanjib C. Chowdhury, Subramani Sockalingam, John W. Gillespie Jr.. Inter-molecular interactions in ultrahigh molecular weight polyethylene single crystals. Computational Materials Science. 2019; 172 ():109360.

Chicago/Turabian Style

Sanjib C. Chowdhury; Subramani Sockalingam; John W. Gillespie Jr.. 2019. "Inter-molecular interactions in ultrahigh molecular weight polyethylene single crystals." Computational Materials Science 172, no. : 109360.

Journal article
Published: 21 December 2018 in Engineering Fracture Mechanics
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In this paper, fracture mechanism and the effects of surface crack on the mechanical properties (modulus and strength) of silica glass are studied through reactive all-atom molecular dynamics (MD) simulations. Tapered surface cracks of different length ranging from 0.25 to 10.0 nm are created by deleting atoms to study the crack length effects. Interatomic interactions are modeled with state-of-the-art reactive force field ReaxFF and fracture energy release rate is determined from discretized atomistic J-integral approach. Simulation results indicate that surface cracks have no effect on fiber modulus, however, fiber strength is significantly affected by surface crack. With the increase in crack length, strength decreases and MD predicted strength-crack length response is in good agreement with theoretical prediction. MD simulations project that about 35 nm size crack could reduce glass fiber strength to 3.5 GPa, which is experimentally observed fiber mean strength. MD simulations show that there is no inelastic process zone with cavities in front of the crack tip. Fracture mode is brittle type where crack growth initiates through Si-O bond breakage and it propagates through sequential bond ruptures.

ACS Style

Sanjib C. Chowdhury; Ethan A. Wise; Raja Ganesh; John W. Gillespie. Effects of surface crack on the mechanical properties of Silica: A molecular dynamics simulation study. Engineering Fracture Mechanics 2018, 207, 99 -108.

AMA Style

Sanjib C. Chowdhury, Ethan A. Wise, Raja Ganesh, John W. Gillespie. Effects of surface crack on the mechanical properties of Silica: A molecular dynamics simulation study. Engineering Fracture Mechanics. 2018; 207 ():99-108.

Chicago/Turabian Style

Sanjib C. Chowdhury; Ethan A. Wise; Raja Ganesh; John W. Gillespie. 2018. "Effects of surface crack on the mechanical properties of Silica: A molecular dynamics simulation study." Engineering Fracture Mechanics 207, no. : 99-108.

Proceedings article
Published: 15 November 2017 in American Society for Composites 2017
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Theoretical strength of glass fiber is much higher than the experimental values. It is believed that nano-meter size surface cracks, which are developed on the fiber surface during manufacturing and later on handling, deteriorate fiber strength. In the paper, effects of surface crack on the mechanical properties (modulus and strength) of glass fiber are studied through reactive all-atom molecular dynamics (MD) simulations. Surface cracks of different length are created by deleting atoms. Two types of reactive force fields – ReaxFF and Tersoff, are considered to assess their accuracy and computational expense. Simulation results indicate that surface cracks have no effect on glass fiber modulus, however, fiber strength is significantly affected by surface crack. Presence of surface cracks reduces fiber strength. With the increase in crack length, strength decreases and MD derived strength-crack length response is in good agreement with theoretical prediction.

ACS Style

Sanjib C. Chowdhury; Ethan A. Wise; John W. Gillespie Jr.. Modeling of Glass Fiber with Surface Cracks—A Molecular Dynamics Simulation Study. American Society for Composites 2017 2017, 1 .

AMA Style

Sanjib C. Chowdhury, Ethan A. Wise, John W. Gillespie Jr.. Modeling of Glass Fiber with Surface Cracks—A Molecular Dynamics Simulation Study. American Society for Composites 2017. 2017; ():1.

Chicago/Turabian Style

Sanjib C. Chowdhury; Ethan A. Wise; John W. Gillespie Jr.. 2017. "Modeling of Glass Fiber with Surface Cracks—A Molecular Dynamics Simulation Study." American Society for Composites 2017 , no. : 1.

Ceramics
Published: 27 July 2017 in Journal of Materials Science
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In this paper, strength of the interphase between silica and glycidoxypropyltrimethoxy silane (GPS) coupling agent has been studied using molecular dynamics (MD) simulations. Silica–GPS interphase model is created by coupling the hydroxylated silica surface with monolayer-hydroxylated GPS molecules. The interphase model is subjected to mode-I (normal), mode-II (shear) and mixed-mode (normal–shear) mechanical loading to determine the interphase cohesive traction–separation (T–S) response (i.e., cohesive traction law). In MD simulations, atomic interactions are modeled with the reactive force field ReaxFF. Effects of interphase thickness and GPS bond density on the T–S response are studied. Simulation results indicate that interphase strength decreases with increase in the interphase thickness before attaining a plateau level at higher thickness. For a particular thickness, strength improves significantly with increase in the GPS bond density with the silica surface. Damage mode is adhesive at the silica interface at lower thickness and transitions to mixed mode and cohesive failure within the silane interphase at higher thickness. Mixed-mode T–S responses are bounded by the mode-I and mode-II responses. Characteristic parameters of the continuum-level potential-based cohesive zone model (PPR–CZM) are determined by fitting the MD-based mode-I and mode-II T–S responses with PPR–CZM functional. Development of the PPR–CZM parameters enables bridging length scales from the MD to the continuum scale for fracture modeling of the fiber–matrix interphase in composites subjected to mixed-mode loading. Results on mode-I and mode-II unloading are also presented.

ACS Style

Sanjib C. Chowdhury; John W. Gillespie. Silica–silane coupling agent interphase properties using molecular dynamics simulations. Journal of Materials Science 2017, 52, 12981 -12998.

AMA Style

Sanjib C. Chowdhury, John W. Gillespie. Silica–silane coupling agent interphase properties using molecular dynamics simulations. Journal of Materials Science. 2017; 52 (22):12981-12998.

Chicago/Turabian Style

Sanjib C. Chowdhury; John W. Gillespie. 2017. "Silica–silane coupling agent interphase properties using molecular dynamics simulations." Journal of Materials Science 52, no. 22: 12981-12998.

Journal article
Published: 14 February 2017 in Fibers
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Ballistic impact induces multiaxial loading on Kevlar® and polyethylene fibers used in protective armor systems. The influence of multiaxial loading on fiber failure is not well understood. Experiments show reduction in the tensile strength of these fibers after axial and transverse compression. In this paper, we use molecular dynamics (MD) simulations to explain and develop a fundamental understanding of this experimental observation since the property reduction mechanism evolves from the atomistic level. An all-atom MD method is used where bonded and non-bonded atomic interactions are described through a state-of-the-art reactive force field. Monotonic tension simulations in three principal directions of the models are conducted to determine the anisotropic elastic and strength properties. Then the models are subjected to multi-axial loads—axial compression, followed by axial tension and transverse compression, followed by axial tension. MD simulation results indicate that pre-compression distorts the crystal structure, inducing preloading of the covalent bonds and resulting in lower tensile properties.

ACS Style

Sanjib C. Chowdhury; Subramani Sockalingam; John W. Gillespie. Molecular Dynamics Modeling of the Effect of Axial and Transverse Compression on the Residual Tensile Properties of Ballistic Fiber. Fibers 2017, 5, 7 .

AMA Style

Sanjib C. Chowdhury, Subramani Sockalingam, John W. Gillespie. Molecular Dynamics Modeling of the Effect of Axial and Transverse Compression on the Residual Tensile Properties of Ballistic Fiber. Fibers. 2017; 5 (1):7.

Chicago/Turabian Style

Sanjib C. Chowdhury; Subramani Sockalingam; John W. Gillespie. 2017. "Molecular Dynamics Modeling of the Effect of Axial and Transverse Compression on the Residual Tensile Properties of Ballistic Fiber." Fibers 5, no. 1: 7.

Original paper
Published: 27 July 2016 in Journal of Materials Science
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Assessment of the empirical reactive force field ReaxFF to predict the formation of amorphous silica from its crystalline structure and the determination of mechanical properties under tension using molecular dynamics simulations is presented. Detailed procedures for preparing amorphous silica from crystalline silica are presented and the atomic structure is in good agreement with experimental results. Tensile properties of silica are predicted over a wide range of strain rates (2.3 × 108 s−1–1.0 × 1015 s−1) allowing comparison with results reported in the literature for other force fields. Quasi-static modulus obtained from power-law fitting of the low-stain rate modulus predicted by ReaxFF is in good agreement with experimental results. A transition strain rate of approximately \( 2.5 \times 10^{11} {\text{s}}^{ - 1} \) is identified where modulus increases rapidly to a plateau level. Tensile strength also increases significantly in this range of strain rate and plateaus at the theoretical upper bound for silica. A detailed study is presented to understand the mechanisms associated with strain rate effects on the overall stress–strain response of silica. Bond breakage which evolves into void growth leading to failure is predicted to occur at approximately 27 % strain for all strain rates. Stress relaxation simulations indicates that the transition strain rate occurs when the characteristic time for high-strain rate loading and stress relaxation times are the same order. The effects of cooling rate and temperature on the structure and the stress–strain response of the silica glass are also investigated. Low-cooling rate and low-cooling temperature enhance the properties of silica.

ACS Style

Sanjib C. Chowdhury; Bazle Z. (Gama) Haque; John W. Gillespie. Molecular dynamics simulations of the structure and mechanical properties of silica glass using ReaxFF. Journal of Materials Science 2016, 51, 10139 -10159.

AMA Style

Sanjib C. Chowdhury, Bazle Z. (Gama) Haque, John W. Gillespie. Molecular dynamics simulations of the structure and mechanical properties of silica glass using ReaxFF. Journal of Materials Science. 2016; 51 (22):10139-10159.

Chicago/Turabian Style

Sanjib C. Chowdhury; Bazle Z. (Gama) Haque; John W. Gillespie. 2016. "Molecular dynamics simulations of the structure and mechanical properties of silica glass using ReaxFF." Journal of Materials Science 51, no. 22: 10139-10159.

Journal article
Published: 01 June 2016 in Carbon
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ACS Style

Bazle Z. (Gama) Haque; Sanjib C. Chowdhury; John W. Gillespie. Molecular simulations of stress wave propagation and perforation of graphene sheets under transverse impact. Carbon 2016, 102, 126 -140.

AMA Style

Bazle Z. (Gama) Haque, Sanjib C. Chowdhury, John W. Gillespie. Molecular simulations of stress wave propagation and perforation of graphene sheets under transverse impact. Carbon. 2016; 102 ():126-140.

Chicago/Turabian Style

Bazle Z. (Gama) Haque; Sanjib C. Chowdhury; John W. Gillespie. 2016. "Molecular simulations of stress wave propagation and perforation of graphene sheets under transverse impact." Carbon 102, no. : 126-140.

Journal article
Published: 12 November 2013 in Computational Materials Science
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In this paper, mechanical responses of the carbon nanotubes (CNTs) intramolecular junctions (IMJs) under three generic modes of mechanical loadings – tension, compression, and torsion have been studied using molecular dynamics simulations. (5,5)-(10,0), (7,7)-(14,0), (10,10)-(20,0) armchair–zigzag and (8,0)-(6,0) zigzag–zigzag IMJs have been simulated by connecting two constituent CNTs with pentagon and heptagon rings. Classical molecular dynamics based on the velocity-Verlet algorithm has been used to solve the Newtonian equation of motion and carbon–carbon interaction in the CNT has been modeled by the Brenner potential. Mechanical properties, particularly stiffness and maximum force/torque and failure modes for different loading conditions are studied. Simulation results show that stiffness of the IMJ falls between those of the constituent CNTs. Compressive failure load of the IMJ is lower than either of the constituent CNTs. However, failure loads and damage modes of the IMJs under tensile and torsional loadings depend on the transition region in the IMJs.

ACS Style

Sanjib C. Chowdhury; Bazle Z. (Gama) Haque; John W. Gillespie. Molecular simulations of the carbon nanotubes intramolecular junctions under mechanical loading. Computational Materials Science 2013, 82, 503 -509.

AMA Style

Sanjib C. Chowdhury, Bazle Z. (Gama) Haque, John W. Gillespie. Molecular simulations of the carbon nanotubes intramolecular junctions under mechanical loading. Computational Materials Science. 2013; 82 ():503-509.

Chicago/Turabian Style

Sanjib C. Chowdhury; Bazle Z. (Gama) Haque; John W. Gillespie. 2013. "Molecular simulations of the carbon nanotubes intramolecular junctions under mechanical loading." Computational Materials Science 82, no. : 503-509.