PAMM · Proc. Appl. Math. Mech. 17, 235 – 236 (2017) / DOI 10.1002/pamm.201710087 Molecular dynamics simulations of the cooling rate influence on the tensile strength of silica glass Firaz Ebrahem 1, ∗ and Bernd Markert 1 1 Institute of General Mechanics, RWTH Aachen University, Templergraben 64, 52056, Aachen, Germany Classical molecular dynamics (MD) simulations are performed to study the stress-strain behaviour of armorphous silica glass (a-SiO2) at small time and length scales. Various amorphous states are generated by quenching molten SiO2 using a two- body and a three-body interaction potential. This quenching process is carried out for different cooling rates. The structural properties of a-SiO2 are validated through a comparison with other numerical and experimental results. Finally, tensile tests are performed on a-SiO2 until fracture occurs, and the cooling rate influence on the tensile strength is analysed. c  2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Silica glass is one of the most used materials and finds application in many technological fields. Several researchers from different parts of the scientific community make efforts to extend the understanding of a-SiO 2 . Especially the understanding of its mechanical properties and brittle fracture mechanism is of high interest. It is known that cracks propagate rapidly through brittle materials as soon as a critical stress level is exceeded. Fracture happens due to the breakage of inter-atomic bonds. Therefore, atomistic studies are essential in order to get a deep insight into the physical mechanisms of fracture at small scales, and hence, interpret the macroscopic material behaviour. Over the last decades, MD simulations have shown their efficiency in modelling material properties and mechanical be- haviour at the nano-scale. Classical MD is based on the numerical integration of the Newtonian equations of motion for a set of interacting atoms, where the dynamics of electrons is not considered explicitly [1]. The interactions between atoms are described by empirical potential functions U (r) in terms of a sum of bonded and non-bonded interactions U (r)= U bonded (r)+ U non-bonded (r) , (1) where r is the distance between two atoms. 2 Computational details MD simulations are performed with the LAMMPS software [2]. An interaction potential consisting of two-body and three- body covalent interactions is used [3]. The two-body part contains effects of steric repulsion, Coulombic charge transfer and electronic polarisabilities. The three-body part includes angular bond bending and bond stretching. The potential is defined between neighbouring atoms that are within a cut-off distance of r c =5.5 ˚ A. The simulations are carried out for cuboid systems with edge lenghts of 15 × 11 × 4 nm 3 containing 47988 atoms, while periodic boundary conditions are applied in all spatial dimensions. Various amorphous states are generated by a heating-quenching process, where the quenching is carried out for different cooling rates [4]. Starting from a random spatial arrangement of the atoms, the system is heated at a temperature of 8000 K for 200 ps under isothermal and isochoric conditions (NVT ensemble). In the second step, the melt is quenched to room temperature of 300 K, again by use of a NVT ensemble. Finally, the silica glass is equilibrated at 300 K for 200 ps using a isothermal and isobaric (NPT) ensemble. After the equilibration uni-axial tensile loadings are performed by deforming the simulation box in loading direction at a strain rate of ˙ ε =0.001 ps −1 under NPT conditions [5]. The stresses are calculated according to the definition of the virial theorem in discrete systems, in which the first part describes the stresses due to inter-atomic forces intercepted by an area and the second part due to a momentum flux across the area [6]. The components of the stress tensor are defined as σ ij = - 1 V  α   1 2  β=α  r β i - r α i  f αβ j + m α v α i v α j   , (2) where i and j represent x, y and z directions, V is the system volume, β the neighbours of atom α, m the mass of the atom, f αβ the force acting on atom α due to atom β, r the position and v the velocity. ∗ Corresponding author: e-mail ebrahem@iam.rwth-aachen.de, phone +49 241 80 96238 c  2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim