Page 21 - Mechanical Behavior of Materials

20 Chapter 1 Introduction

The most basic concern in design to avoid structural failure is that the stress in a component
must not exceed the strength of the material, where the strength is simply the stress that causes a
deformation or fracture failure. Additional complexities or particular causes of failure often require
further analysis, such as the following:

1. Stresses are often present that act in more than one direction; that is, the state of stress is
biaxial or triaxial.
2. Real components may contain flaws or even cracks that must be specifically considered.
3. Stresses may be applied for long periods of time.
4. Stresses may be repeatedly applied and removed, or the direction of stress repeatedly
reversed.

In the remainder of this introductory chapter, we will define and briefly discuss various types
of material failure, and we will consider the relationships of mechanical behavior of materials to
engineering design, to new technology, and to the economy.

1.2 TYPES OF MATERIAL FAILURE

A deformation failure is a change in the physical dimensions or shape of a component that is
sufficient for its function to be lost or impaired. Cracking to the extent that a component is separated
into two or more pieces is termed fracture. Corrosion is the loss of material due to chemical
action, and wear is surface removal due to abrasion or sticking between solid surfaces that touch.
If wear is caused by a fluid (gas or liquid), it is called erosion, which is especially likely if the
fluid contains hard particles. Although corrosion and wear are also of great importance, this book
primarily considers deformation and fracture.
The basic types of material failure that are classified as either deformation or fracture are
indicated in Fig. 1.1. Since several different causes exist, it is important to correctly identify the
ones that may apply to a given design, so that the appropriate analysis methods can be chosen to
predict the behavior. With such a need for classification in mind, the various types of deformation
and fracture are defined and briefly described next.

Figure 1.1 Basic types of deformation and fracture.