Page 39 - Mechanical Behavior of Materials

2

Structure and Deformation

in Materials

2.1 INTRODUCTION
2.2 BONDING IN SOLIDS
2.3 STRUCTURE IN CRYSTALLINE MATERIALS
2.4 ELASTIC DEFORMATION AND THEORETICAL STRENGTH
2.5 INELASTIC DEFORMATION
2.6 SUMMARY

OBJECTIVES

• Review chemical bonding and crystal structures in solid materials at a basic level, and relate
these to differences in mechanical behavior among various classes of material.
• Understand the physical basis of elastic deformation, and employ this to estimate the
theoretical strength of solids due to their chemical bonding.
• Understand the basic mechanisms of inelastic deformations due to plasticity and creep.
• Learn why actual strengths of materials fall far below the theoretical strength to break
chemical bonds.

2.1 INTRODUCTION

A wide variety of materials are used in applications where resistance to mechanical loading is
necessary. These are collectively called engineering materials and can be broadly classified as
metals and alloys, polymers, ceramics and glasses, and composites. Some typical members of each
class are given in Table 2.1.
Differences among the classes of materials as to chemical bonding and microstructure affect
mechanical behavior, giving rise to relative advantages and disadvantages among the classes. The
situation is summarized by Fig. 2.1. For example, the strong chemical bonding in ceramics and
glasses imparts mechanical strength and stiffness (high E), and also temperature and corrosion
resistance, but causes brittle behavior. In contrast, many polymers are relatively weakly bonded
between the chain molecules, in which case the material has low strength and stiffness and is
susceptible to creep deformation.

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