Service de Physique de l'Etat Condensé

Molecular Dynamics
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Over the last century, a number of developments have occurred to significantly advance our understanding of fracture at the macroscopic scale. For several decades, a vast analytical field known as fracture mechanics addressed the need for reliable safety criteria for engineering design. In recent years, the focus has shifted towards microscopic processes that occur near the crack front.  From the simulation viewpoint, molecular dynamics (MD) has become the method of choice to study fracture at the atomic scale and can easily provide atomic-level stresses and strains as well as the dynamics features of crack propagation.

The recent emergence of parallel computers and highly-efficient simulation algorithms has had a significant impact on fracture simulations.   Over the last decade, massively-parallel computers delivering hundreds of teraflop (1014 floating point operations per second, see www.top500.org for the current list of the fastest computers in the world) performance have become available and when combined with highly-efficient, linearly scaling algorithms for MD simulations, parallel architectures have made it possible to simulate materials with realistic interactions and system sizes approaching micrometer dimensions (108-109 atoms). Furthermore, data compression algorithms and advanced visualization tools for three-dimensional, immersive, and interactive visualization environments provide the means to analyze and to obtain new information from massively parallel MD simulations [1,2].

Previous massively parallel MD simulations probed the atomistic aspects of dynamic fracture in silica glass (i.e. amorphous SiO2) These simulations reveal nanometer scale cavities nucleating and coalescing with one another up to 20 nm ahead of the crack tip [3-5]. After which these cavities were seen to merge with the advancing crack to cause mechanical failure. This scenario was also observed experimentally during stress corrosion ultra-slow fracture of glass using Atomic Force Microscopy (AFM) [6, 7].

In order to characterize the irreversible changes in structure taking place within the process zone (i.e. the zone ahead of the crack tip where pores are opening), a variety of simulations have been carried out using 1) cyclic loading and unloading in hydrostatic pressure and 2) cyclic loading and unloading in shear. Structural changes revealed by these simulations have been analyzed in various ways (static structure factor, analysis of the ring structure, evolution of the fabric tensor…).

 

Computing resources

 As computational scientist within CEA/DSM/DRECAM/SPCSI we have access to several clusters. Scientists at Saclay have access to a Vectorial/Parallel NEC SX6 rated at 352 GFlops and two Scalar/Parallel HP SC rated at 2 TFlops and 2.38TFlops. In addition, we will have exclusive access to a 16 processor and 10 processor PC cluster located at SPCSI.
 

Maj : 15/04/2016 (763)

 

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