Crack initiation and propagation in a nanostructured nickel film were studied by molecular dynamic simulation as well as an interatomic-potential-based continuum approach. In the molecular dynamic simulation, the interatomic potential was described by using Embedded Atom Method (EAM), and a reduced 2D plane model was employed to simulate the mechanical behavior of nanofilms. Atomistic simulation shows that the reduced plane model in this paper can not only reveal the physical nature of crack initiation clearly but also give the critical time of crack initiation accurately as the continuum fracture analysis does. The normal stress and average atom energy at the crack tip which resulted from atomistic simulation at the time of crack initiation agree well with the analytical results. On the other hand, the crack propagation in nanofilms was studied by interatomic-potential-based continuum fracture mechanics analysis based on Griffith criterion. The coupled continuum-atomic analysis can predict the crack initiation and atomic stress accurately. Continuum analysis with material property parameters determined by interatomic potential is proved to be another promising way of solving failure problem on nanoscale.

Nanofilms have been more and more widely used in various industries up to date. An open and active challenge in the mechanical behavior of nanofilms is the mechanism of crack initiation and propagation. Recently, a wide variety of methods have been employed to measure the crack initiation, propagation, or healing in nanofilms [

Gao et al. [

In the present study, a joint continuum and atomistic investigation of intersonic crack propagation in nanofilms is proposed, and a study of propagation of mode

Figure

2D nanocrystalline nickel slab with 60 atoms along the horizontal direction and 100 atoms along the vertical direction. A horizontal slit of 11-atom distance is cut midway from the left boundary.

The motion of each atom is predicted through Newton’s law:

Values throughout this paper are expressed in terms of reduced units as follows: time is scaled by the selective time step, 3 fs, which is proved to be the optimum time step size through stability and efficiency tests, energies are scaled by the minimum value of EAM potential, 4.227 × 10^{−12} erg, mass is scaled by the atomic mass, and lengths are scaled by two-dimensional triangular lattice parameter

The initial temperature of the model was 0 K, which kept invariant during the simulation to avoid atomic heat activation and did not allow for the appearance of plastic zone and brittle-to-ductile transition during fracture simulation. A static tensile strain rate of 4 × 10^{9}/s along

Gao [

Figure

Atomistic simulation results: atom arrangement near the crack surfaces every 2000 time steps. The atoms in a triangular field around the crack surfaces depart from their perfect lattice location gradually and the crack starts to propagate when

With the stress intensity factor

Figure

Variation of atomic stress and atom total energy in the crack tip until the initiation of crack propagation. The stress was tensile and normal to the crack, and the total energy was the sum of interatomic potential and kinetic energy. Note that in the atomistic simulation the stress and energy were average data of the five selected atoms in the crack tip, which were on the crack extending line and nearest to the crack, as shown in Figure

Variation of crack tip atomic stress

Variation of atom total energy

From continuum analysis and atomistic simulation and the comparison of their results, we can conclude that the two-dimensional molecular dynamics model of nanonickel can predict the time of crack initiation accurately and close to the analysis result based on continuum fracture mechanics theories. And the results of tensile stress normal to the crack and the atom total energy of the crack tip atoms at the time of crack initiation by two different methods are also in good agreement with each other.

After a period of crack propagation, a symmetrical bifurcation ahead of the crack tip appears at

Bifurcation ahead of the crack tip and daughter crack growth during crack propagation.

With molecular dynamics method and continuum analysis, we have studied the mode

The authors do not have any direct financial relation with the commercial identity that might lead to a conflict of interests for any of them.

This work was partially supported by the National Natural Science Foundation of China through Grants no. 11132003 and no. 51209079.

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