Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

286 Chapter 10 Force and Force-Related Parameters

Based on their mechanical behavior, solid materials are commonly classified as either

ductileorbrittle. A ductile material, when subjected to a tensile load, will go through significant

permanent deformation before it breaks. Steel and aluminum are good examples of ductile

materials. On the other hand, a brittle material shows little or no permanent deformation

before it ruptures. Glass and concrete are examples of brittle materials.

The tensile and compressive strength are other important properties of materials. To pre-

dict failure, engineers perform stress calculations that are compared to the tensile and com-

pressive strength of materials. The tensile strength of a piece of material is determined by

measuring the maximum tensile load a material specimen in a shape of a rectangular bar or

cylinder can carry without failure. The tensile strengthor ultimate strength of a material is

expressed as the maximum tensile force per unit of the original cross-sectional area of the spec-

imen. As we mentioned earlier, when a material specimen is tested for its strength, the applied

tensile load is increased slowly so that one can determine the modulus of elasticity and the elas-

tic region. In the very beginning of the test, the material will deform elastically, meaning that

if the load is removed, the material will return to its original size and shape without any per-

manent deformation. The point to which the material exhibits this elastic behavior is the elas-

tic point. The elastic strength represents the maximum load that the material can carry without

any permanent deformation. In many engineering design applications, the elastic strength or

yield strength (the yield value is very close to the elastic strength value) is used as the tensile

strength.

Some materials are stronger in compression than they are in tension; concrete is a good

example. The compression strength of a piece of material is determined by measuring the

maximum compressive load a material specimen in a shape of a cube or cylinder can carry with-

out failure. The ultimatecompressive strengthof a material is expressed as the maximum com-

pressive force per unit of the cross-sectional area of the specimen. Concrete has a compressive

strength in the range of 10 to 70 MPa. The strength of some selected materials is given in

Table 10.6.

Example 10.13 A structural member with a rectangular cross section, as shown in Figure 10.31, is used to sup-

port a load of 4000 N distributed uniformly over the cross-sectional area of the member. What

type of material should be used to carry the load safely?

Material selection for structural members depends on a number of factors, including the

density of the material, its strength, its toughness, its reaction to the surrounding environment,

and its appearance. In this example, we will only consider the strength of the material as the

design factor. The average normal stress in the member is given by

Aluminum alloy or structural steel material with the yield strength of 50 MPa and 200 MPa,

respectively, could carry the load safely.

s

4000 N

1 0.05 m 21 0.005 m 2

16 MPa

10 cm

5 cm

5 mm

■Figure 10.31

The structural member of

Example 10.13.

62080_10_ch10_p251-302.qxd 5/22/10 12:32 AM Page 286

Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀