Basic Elasticity and viscoelasticity（基本的弹性和粘弹性）.pdf

© Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher • c h a p t e r o n e • Basic Elasticity and viscoelasticity In the physically stressful environment there are three ways in which a material can respond to external forces. It can add the load directly onto the forces that hold the constituent atoms or molecules together, as occurs in simple crystalline (includ- ing polymeric crystalline) and ceramic materials—such materials are typically very rigid; or it can feed the energy into large changes in shape (the main mechanism in noncrystalline polymers) and flow away from the force to deform either semiper- manently (as with viscoelastic materials) or permanently (as with plastic materials). 1.1 hookean Materials and Short- range forces The first class of materials is exemplified among biological materials by bone and shell (chapter 6), by the cellulose of plant cell walls (chapter 3), by the cell walls of diatoms, by the crystalline parts of a silk thread (chapter 2), and by the chitin of arthropod skeletons (chapter 5). All these materials have a well-ordered and tightly bonded structure and so broadly fall into the same class of material as metals and glasses. What happens when such materials are loaded, as when a muscle pulls on a bone, or when a shark crunches its way through its victim’s leg? In a material at equilibrium, in the unloaded state, the distance between adjacent atoms is 0.1 to 0.2 nm. At this interatomic distance the forces of repulsion between two adjacent atoms balance the forces of attraction. When the material is stretched or compressed the atoms are forced out of their equilibrium positions and are either parted or brought together