文档介绍：ATOMISTIC BASIS OF ELASTICITY
David Roylance
Department of Materials Science and Engineering
Massachusetts Institute of Technology
Cambridge, MA 02139
January 27, 2000
Introduction
The Introduction to Elastic Response Module introduced two very important material proper-
ties, the ultimate tensile strength σf and the Young’s modulus E. To the effective mechanical
designer, these aren’t just numerical parameters that are looked up in tables and plugged into
equations. The very nature of the material is reflected in these properties, and designers who
try to function without a sense of how the material really works are very apt to run into trou-
ble. Whenever practical in these modules, we’ll make an effort to put the material’s mechanical
properties in context with its processing and microstructure. This module will describe how for
most engineering materials the modulus is controlled by the atomic bond energy function.
For most materials, the amount of stretching experienced by a tensile specimen under a
small fixed load is controlled in a relatively simple way by the tightness of the chemical bonds
at the atomic level, and this makes it possible to relate stiffness to the chemical architecture of
the material. This is in contrast to plicated mechanical properties such as fracture,
which are controlled by a bination of microscopic as well as molecular aspects of
the material’s internal structure and surface. Further, the stiffness of some materials — notably
rubber — arises not from bond stiffness but from disordering or entropic factors. Some principal
aspects of these atomistic views of elastic response are outlined in the sections to follow.
Energetic effects
Chemical bonding between atoms can be viewed as arising from the electrostatic attraction
between regions of positive and negative electronic charge. Materials can be classified based on
the nature of these electrostatic forces, the three principal classes being
1. Ionic materials, s