2 Fundamental Mechanical Properties of Materials The goal of the following pages is to characterize materials in terms of some of the fundamental mechanical properties that were introduced in Chapter 1. A qualitative distinction between ductile,brittle,and elastic materials can be achieved in a relatively simple experiment us- ing the bend test,as shown in Figure 2.1.A long and compara- tively thin piece of the material to be tested is placed near its ends on two supports and loaded at the center.It is intuitively obvious that an elastic material such as wood can be bent to a much higher degree before breakage occurs than can a brittle material such as stone or glass.Moreover,elastic materials re- turn upon elastic deformation to their original configuration once the stress has been removed.On the other hand,ductile materi- als undergo a permanent change in shape above a certain thresh- old load.But even ductile materials eventually break once a large enough force has been applied. To quantitatively evaluate these properties,a more sophisti- cated device is routinely used by virtually all industrial and sci- entific labs.In the tensile tester,a rod-shaped or flat piece of the material under investigation is held between a fixed and a mov- able arm as shown in Figure 2.2.A force upon the test piece is exerted by slowly driving the movable cross-head away from the fixed arm.This causes a stress,o,on the sample,which is de- fined to be the force,F,per unit area,Ao,that is, (2.1) Ao Since the cross section changes during the tensile test,the ini-2 The goal of the following pages is to characterize materials in terms of some of the fundamental mechanical properties that were introduced in Chapter 1. A qualitative distinction between ductile, brittle, and elastic materials can be achieved in a relatively simple experiment us￾ing the bend test, as shown in Figure 2.1. A long and compara￾tively thin piece of the material to be tested is placed near its ends on two supports and loaded at the center. It is intuitively obvious that an elastic material such as wood can be bent to a much higher degree before breakage occurs than can a brittle material such as stone or glass. Moreover, elastic materials re￾turn upon elastic deformation to their original configuration once the stress has been removed. On the other hand, ductile materi￾als undergo a permanent change in shape above a certain thresh￾old load. But even ductile materials eventually break once a large enough force has been applied. To quantitatively evaluate these properties, a more sophisti￾cated device is routinely used by virtually all industrial and sci￾entific labs. In the tensile tester, a rod-shaped or flat piece of the material under investigation is held between a fixed and a mov￾able arm as shown in Figure 2.2. A force upon the test piece is exerted by slowly driving the movable cross-head away from the fixed arm. This causes a stress, , on the sample, which is de￾fined to be the force, F, per unit area, A0, that is,
A F 0 . (2.1) Since the cross section changes during the tensile test, the ini￾Fundamental Mechanical Properties of Materials