CHAPTER6 Mechanical Behavior
Mechanical testing machines can be automated
to simplify the analysis of the mechanical per-
formance of materials in a variety of product
applications.(Courtesy of MTS Systems Cor-
poration.)
Figure6-1Tensile test.
100
50
012345
L o a d (103 N )
Elongation, mm
Figure 6-2Load-versus-elongation curve ob-tained in a tensile test.The specimen was alu-minum 2024-T81.
5000
00.020.040.060.080.10
400300200100Figure 6-3Stress-versus-strain curve obtained by normalizing the data of Figure 6–2for specimen geometry.
5000
00.0020.0040.0060.0080.010
400300200100Figure 6-4The yield strength is de?ned relative to the intersection of the stress–strain curve with a “0.2%offset.”This is a convenient indication of the onset of plastic deformation.
50000
0.02
0.01
400300200100
Figure 6-5Elastic recovery occurs when stress is removed from a speci-men that has already undergone plastic deformation.
Stress
Figure6-6The key mechanical properties obtained from a ten-sile test:1,modulus of elasticity,E;2,yield strength,Y.S.; 3,tensile strength,T.S.;4,ductility,100× failure(note that elastic recovery occurs after fracture);and5,toughness=
σd (measured under load;hence,the dashed line is verti-cal).
Figure6-7Neck down of a tensile test specimen within its gage length after extension beyond the tensile strength.(Courtesy of R.S.Wort-
man)
Engineering or true strain (in./in. or m/m) × 10–2
E n g i n e e r i n g o r t r u e s t r e s s (p s i ) × 103
Figure 6-8True stress (=load divided by actual area in the necked-down region)continues to rise to the point of fracture,in con-trast to the behavior of engineering stress.(After R.A.Flinn/P .K.Trojan:Engineering Materials and Their Applications,
2nd Ed.,Copyright c
1981,Houghton Mif?in Company,used by permission.)
Low strength,
high ductility,
low toughness Figure6-9The toughness of an alloy depends on a combination of strength and ductil-
ity.
Stress
Strain Figure6-10For a low-carbon steel,the stress-versus-strain curve includes both an upper and lower yield point.
(a) Unloaded
(b) Loaded
εx
εz ν = –
Figure6-11The Poisson’s ratio(ν)characterizes the contraction per-pendicular to the extension caused by a tensile stress.
500
400
410
480300
200
100
b
= 3FL/(2bh2)
Figure6-14The bending test that generates a modulus of rupture.This strength parameter is similar in magnitude to a tensile strength.Fracture occurs along the outermost sample edge,which is under a tensile load.
? ?Figure6-15Stress(σm)at
the tip of a Grif?th crack.
032,00028,000
24,00020,000
16,000
12,0008,000
4,000
0240
200
160
120
80
40
2
46
1
357Strain (%)
T e n s i l e s t r e s s (M P a )
T e n s i l e s t r e s s (p s i )
Figure 6-16Stress-versus-strain curves for a polyester engineer-ing polymer.(From Design Handbook for Du Pont Engi-neering Plastics,used by permission.)
12015,00010,0005,000
05,00010,00015,000
100806040
20
020406080100120
106284
0Strain (%)
S t r e s s (M P a )
S t r e s s (p s i )
482610
Figure 6-17Stress-versus-strain curves for a nylon 66at 23?C showing the effect of relative humidity.(From Design Handbook for Du Pont Engi-neering Plastics,used by permission.)
Tensile test specimen
B o n d i n g f o r c e
B o n d i n g e n e r g y
S t r e s s
Figure 6-18Relationship of elastic deformation to the stretching of atomic bonds.