tailieunhanh - ATOMISTIC BASIS OF ELASTICITY

Mô-đun đáp ứng đàn hồi giới thiệu hai tài sản vật chất rất quan trọng, căng sức mạnh cuối cùng σf và của Young modulus E. Để thiết kế cơ khí hiệu quả, đây không phải là chỉ số thông số được nhìn lên bảng và cắm vào phương trình. Bản chất của vật liệu được phản ánh trong các đặc tính này, và nhà thiết kế cố gắng để hoạt động mà không có một ý nghĩa như thế nào thực sự hoạt động rất apt chạy vào rắc rối. . | 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 properties the ultimate tensile strength ơf 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 trouble. 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 more complicated mechanical properties such as fracture which are controlled by a diverse combination 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 such as NaCl in which an .