Engineering Research and Development Center

Vicksburg, MS, United States

Engineering Research and Development Center

Vicksburg, MS, United States
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Padilla Espinosa I.M.,North Carolina A&T State University | Hodo W.,Engineering Research and Development Center | Rivas Murillo J.S.,North Carolina A&T State University | Rajendran A.M.,University of Mississippi | Mohan R.V.,North Carolina A&T State University
Journal of Engineering Materials and Technology, Transactions of the ASME | Year: 2017

Cement paste is a material with heterogeneous composite structure consisting of hydrated and unhydrated phases at all length scales that varies depending upon the degree of hydration. In this paper, a method to model cement paste as a multiphase system at molecular level for predicting constitutive properties and for understanding the constitutive mechanical behavior characteristics using molecular dynamics is presented. The proposed method creates a framework for molecular level models suitable for predicting constitutive properties of heterogeneous cement paste that could provide potential for comparisons with low length scale experimental characterization techniques. The molecular modeling method followed two approaches: one involving admixed molecular phases and the second involving clusters of the individual phases. In particular, in the present study, cement paste is represented as two-phase composite systems consisting of the calcium silicate hydrate (CSH) phase combined with unhydrated phases tricalcium silicate (C3S) or dicalcium silicate (C2S). Predicted elastic stiffness constants based on molecular model representations employed for the two phases showed that, although the individual phases have anisotropic characteristics, the composite system behaves as an isotropic material. The isotropic characteristics seen from two-phase molecular models mimic the isotropic material nature of heterogeneous cement paste at engineering scale. Further, predicted bulk modulus of the composite system based on molecular modeling is found to be high compared to the elastic modulus, which concurs with the high compression strength of cement paste seen at engineering length scales. © 2017 by ASME.

Mohan R.V.,North Carolina A&T State University | Hodo W.D.,Engineering Research and Development Center
COMPDYN 2015 - 5th ECCOMAS Thematic Conference on Computational Methods in Structural Dynamics and Earthquake Engineering | Year: 2015

Computational modeling of complex, heterogeneous, multi-scale features of cement paste requires starting from their fundamental building blocks that includes material chemistry, microstructural morphology. This would enable capturing scale relevant features that influence the properties and behavior of materials through associated computational, material and mechanistic models. Such modeling starting from nanoscale material features through material chemistry modeling via molecular dynamics (MD); modeling of complete three-dimensional virtual microstructure including the evolution of microstructure due to hydration of cementitious materials are briefly highlighted. Material chemistry modeling discussions from our recent work on nanoscale shear deformation to obtain the stress-strain behavior solely based on the material chemistry structure of hydrated cementitious material constituent CSH Jennite is summarized. Micro-scale modeling involving finite element based repeated volume element (RVE) modeling applied to the virtual three-dimensional complex microstructures at different degrees of hydration of the cement paste is also summarized. Complete details are presented in our other current and future publications in the literature. Multi-scale modeling that links across various length scales and material features in complex heterogeneous material systems provides an effective way of coupling material science and engineering features for their better understanding and tailored material design. These approaches present a new future direction for integrated material science and engineering of materials and structures.

Eller P.,Engineering Research and Development Center | Cheng J.-R.C.,Engineering Research and Development Center | Albert D.,Cold Regions Research and Engineering Laboratory ERDC CRREL
Proceedings - 2010 DoD High Performance Computing Modernization Program Users Group Conference, HPCMP UGC 2010 | Year: 2011

A Two-Dimensional Finite Difference Time Domain (2D-FDTD) simulation is used to find the source location of an acoustic wave in an urban area using a time-reversal technique. This method potentially allows soldiers on the battlefield to locate the source of an acoustic wave produced by gunfire or other sources. The simulation has been demonstrated to accurately find the location of the acoustic waves, but required hours to compute the solution. For practical use in the future, the simulation must run quickly to allow soldiers to find the location of their attacker before the attacker can leave the area, requiring us to accelerate the code to produce a solution in a reasonable amount of time. The simulation code requires many independent computations for each element of a large 2D grid. Graphics Processing Units (GPUs) perform best for highly-parallel and computationally-intense problems, making this an ideal simulation to compute using GPUs to significantly reduce the running time. GPUs also allow the solution to be obtained locally (with the soldiers) rather than at a centralized high performance computing center. This work develops a GPU version of the 2D-FDTD code and experiments with a variety of optimizations to produce an accurate solution as quickly as possible. GPU-only and CPU-GPU versions are developed, with the CPU-GPU version showing slightly better performance. Careful selection of thread block parameters is needed to load data from memory as quickly as possible. Over 11 times speedups are produced, providing progress towards a solution that can allow people on the battlefield to locate the source of gunfire and other projectiles in close to real-time. © 2011 IEEE.

Rivas Murillo J.S.,North Carolina A&T State University | Mohamed A.,North Carolina A&T State University | Hodo W.,Engineering Research and Development Center | Mohan R.V.,North Carolina A&T State University | And 2 more authors.
International Journal of Damage Mechanics | Year: 2016

Calcium silicate hydrate Jennite is a molecular structure commonly accepted as a representation of the complex calcium silicate hydrate gel formed during the hydration of typical Portland cement. In this paper, the behavior of nanoscale calcium silicate hydrate Jennite under shear deformation was investigated using molecular dynamics simulations. Computational samples representing the nanoscale structure of calcium silicate hydrate Jennite were subjected to shear deformation in order to investigate not only their mechanical properties but also their deformation behavior. The simulation results indicated that the nanoscale calcium silicate hydrate Jennite under shear deformation displays a linear elastic behavior up to shear stress of approximately 1.0 GPa, and shear deformation of about 0.08 radians, after which point yielding and plastic deformation occurs. The shear modulus determined from the simulations was 11.2 ± 0.7 GPa. The deformation-induced displacements in molecular structures were analyzed dividing the system in regions representing calcium oxide layers. The displacement/deformation of the layers of calcium oxide forming the structure of nanoscale calcium silicate hydrate Jennite was analyzed. The non-linear stress-strain behavior in the molecular structure was attributed to a non-linear increase in the displacement due to sliding of the calcium oxide layers on top of each other with higher shearing. These results support the idea that by controlling the chemical reactions, the tailored morphologies can be used to increase the interlinking between the calcium oxide layers, thus minimizing the shearing of the layers and leading to molecular structures that can withstand larger deformation and have improved failure behavior. © SAGE Publications.

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