Size matters in mechanics

“Crystals are like people: it is the defects in them that make them interesting.”

(Sir Frederick Charles Frank)

Understanding mechanical properties on very small scales is important, not only for developing a fundamental understanding of plasticity, but also to provide design guidelines for reliable nano- and micro-electromechanical devices. When the dimensions are reduced below the sub-micrometer scale, the mechanical properties differ from those of bulk materials and do not depend solely on the material properties. For instance, the strength of nanoscale metallic specimens depends both on size and geometry and may be a few orders of magnitude higher than the strength of their bulk counterparts. It is not surprising then that gold nanoparticles are ten times stronger than sword blades made of steel. Another example for the difference between bulk and nanoscale materials is the nature of the deformation, whereas in the latter case, deformation manifests itself by strain bursts.

In order to understand and control mechanical properties at the sub-micrometer scale, we must identify the link between the mechanical and the microstructure properties. It is well established that nucleation and motion of dislocations, line defects in the crystal structure, play a significant role in the irreversible (plastic) deformation of crystalline materials. It is a great challenge to correlate quantitatively between dislocation properties and mechanical behavior. Despite, the large density of dislocations in bulk materials allows treating them effectively, an approximation which allows us to construct simple constitutive laws in finite element modelling. However, as the dimensions are pushed towards the nanoscale, this approximated approach is no longer valid, and finite element simulations are brought to the fore.

The aim of the Nanomechanics Simulations Laboratory, under the supervision of Dr. Dan Mordehai, is to find computational and theoretical solutions to the engineering enigmas of nanomechanics and mechanical properties. In the framework of the laboratory, atomistic and mesoscopic tools are being developed and employed. In combination with analytical and finite element analysis, we tackle the challenges of understanding mechanical properties at the nanoscale. For instance, with molecular dynamics simulations, in which the dynamic evolution of the atoms in the crystal is calculated, and with finite element analysis we study the strength of metallic nanoparticles, dislocation nucleation and how their nucleation affects mechanical properties. Recently, similar simulation tools allowed us to identify in the detail the jump to contact process between two surfaces of finite size, with the nanoscale dimensions.

These simulation tools are computationally demanding and are performed on computer clusters, which solve the problem on several compute cores simultaneously. The main computer cluster that was established in the laboratory is an TTN/SGI cluster which includes about 200 Intel® Xeon® compute cores. This adds to the computational resources at the Technion, which are employed by the laboratory as well. These high performance computers allow us to expand our calculation capabilities to bring us one step forward towards revealing the nature of the deformation at the nanoscale.