1 Chapter 7 - Mechanical Properties Introduction Mechanical behavior reflects the relationship between an _____________________________. Important properties are _________________________, etc. Topics: • tensile, compressive, and shear forces • stress vs. strain • elastic and plastic deformation Chapter 7 (continued) Topics (cont.): • yield and tensile strength • ductility, resilience, toughness • engineering properties vs. true properties • mechanical behavior of ceramics • mechanical behavior of polymers • viscoelasticity • hardness
Transcript
1
Chapter 7 - Mechanical Properties
Introduction
Mechanical behavior reflects the relationship between an _____________________________.
Important properties are _________________________, etc.
Topics:
• tensile, compressive, and shear forces
• stress vs. strain
• elastic and plastic deformation
Chapter 7 (continued)
Topics (cont.):
• yield and tensile strength
• ductility, resilience, toughness
• engineering properties vs. true properties
• mechanical behavior of ceramics
• mechanical behavior of polymers
• viscoelasticity
• hardness
2
Chapter 7 (continued)
Closed-Book Questions
29 Tell what you know about the different kinds of stress affecting solid parts and the ways to define and measure stress and strain.
30 Define elastic deformation and the terms used to characterize it.
31 How is the onset of plastic deformation defined and observed on a stress-strain diagram?
32 Explain the difference between strength and toughness and how they are measured on a stress-strain diagram. When is strength important and when is toughness important?
Chapter 7 (continued)
Closed-Book Questions (cont.)
33 Describe two different energy measurements that can be made using a stress-strain diagram.
34 Explain the significance of a decrease in engineering stress as strain increases.
35 Explain the influence of porosity on the mechanical properties of ceramics. Consider variability and stress concentration.
36 Describe three different typical stress-strain behaviors of polymers and relate this to temperature, crystallinity, and crosslinking.
3
Chapter 7 (continued)
Closed-Book Questions (cont.)
37 Describe the possibilities for the strain response of a polymer subjected to a constant stress for a finite period of time, after which the stress is removed.
38 Describe the relationship between relaxation modulus and temperature and the effect of crystallinity and molecular weight.
39 Explain the difference between hardness and strength. Consider both the behavior that each term characterizes and the measurement technique.
Chapter 7 (continued)
Open-Book Questions
10 Be able to calculate engineering stress and strain, both tensile and shear.
11 Be able to apply the definitions of elastic modulus and shear modulus.
12 Know the definition of Poisson’s ratio and how it is used to relate the elastic and shear moduli.
13 Be able to read the yield strength and tensile strength from a stress-strain diagram.
4
Chapter 7 (continued)
Stress - Strain
Types of stress and strain
• tension - applied forces are __________________________________ from a center
• compression - applied forces are _______________________________________ a center
Figure 7.1
Chapter 7 (continued)
Stress - Strain (cont.)
sample units: Pa = N/m2 (MPa or GPa)
engineering stress =
5
Chapter 7 (continued)
Stress - Strain (cont.)
units: dimensionless
engineering strain =
Chapter 7 (continued)
Stress - Strain (cont.)
Types of stress and strain (cont.)
• shear - the forces are _________________________
a
h
6
Chapter 7 (continued)
Stress - Strain (cont.)
Note: A0 is defined differently
shear stress =
Chapter 7 (continued)
shear strain = shear displacementdistance over which shear force acts
Stress - Strain (cont.)
7
Chapter 7 (continued)
Stress - Strain (cont.)
Often in practice it is more complicated
• bending - both _______________________ present
• torsion - a type of shearing force
Chapter 7 (continued)
Stress - Strain (cont.)
Geometric considerations demonstrate that a part subjected to a tensile force actually has both __________________ when the crystallographic planes are at an arbitrary angle to the applied force.
Figure 7.4
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Chapter 7 (continued)
Elastic Deformation vs. Plastic Deformation
• When a material is stressed, deformation occurs
• If the material _______________________ when the ______________ it is said to be __________________.
• Atoms are __________, but do not _________________.
• If the atoms take up new positions relative to each other the original dimensions don’t return. This is termed ______ _______________.
Chapter 7 (continued)
Elastic Deformation
• In the elastic regime, stress is ____________________
where E = the proportionality constant termed Young’s modulus (modulus of elasticity)
• known as Hooke’s law
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Chapter 7 (continued)
Elastic Deformation
Table 7.1
Chapter 7 (continued)
Elastic Deformation (cont.)
On a stress-strain curve the elastic region is seen as the _____________ _______ of the graph.
Figure 7.5
10
Chapter 7 (continued)
Elastic Deformation (cont.)
There can be _________ ___________ behavior.
This is stillreversible
E changes asstrain occurs
Figure 7.6
Chapter 7 (continued)
Elastic Deformation (cont.)
• Higher values of ‘E’ correspond to _________________
From Tables 2.3 and 7.1
Elastic modulus E (GPa)
Bond Energy(kJ/mol)
Melt Temperature (/C)
aluminum 69 324 660
tungsten 407 849 3410
magnesium oxide(crystalline)
225 1000 2800
soda-lime glass(amorphous)
69 1400
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Chapter 7 (continued)
Elastic Deformation (cont.)
As temperature ________, Young’s modulus __________
Figure 7.8
Chapter 7 (continued)
Elastic Deformation (cont.)
There is also elastic shear strain
where G = the shear modulus
12
Chapter 7 (continued)
Elastic Deformation (cont.)
Any lengthening (or compression) in one direction produces a contraction (expansion) in the other dimensions.
Chapter 7 (continued)
Elastic Deformation (cont.)
There is a relationship between the strains in the twodirections.
Commonly between ______________ (see Table 7.1)
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Chapter 7 (continued)
Elastic Deformation (cont.)
Poisson’s ratio can be used to relate the _______________ ___________ moduli
• G is often approximately 0.4E
• The value of this is that values of G are sometimes unavailable
• ( see Example Problem 7.2 )
Chapter 7 (continued)
Stress-Strain Diagrams
• Determined by a ______________
Figure Figure7.3 7.2
14
Chapter 7 (continued)
Stress-Strain Diagrams (cont.)
yield strengthThe stress at which a metal shows_______________ _____________.
Also called the proportional limit
Defined as ______ _________
Figure 7.10
Chapter 7 (continued)
Stress-Strain Diagrams (cont.)
tensile strengthThe _________ ______ reached on the stress-strain diagram.
If this stress level is held ____________ __________.
Figure 7.11
15
Chapter 7 (continued)
Stress-Strain Diagrams (cont.)
Figure 7.12
Chapter 7 (continued)
Stress-Strain Diagrams (cont.)
Table 7.2
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Chapter 7 (continued)
Stress-Strain Diagrams (cont.)
Stress-strain behavior is ______________________
Iron
Figure 7.14
Chapter 7 (continued)
Ductility, Resilience, and Toughness
Ductility - The degree of ________________________ _______________
Measured as percent elongation or percent reduction in area
Important both in use and during manufacture.
% elongation = final length - initial lengthinitial length
x 100%
% red. in area = initial area - final areainitial area
x 100%
17
Chapter 7 (continued)
Ductility, Resilience, and Toughness (cont.)
ResilienceThe capacity of a materialto ____________ when it
is ___________ deformed.
Represented by the _________ __________ stress-strain curve.
energy per volume to completely elasticallystrain the material
Figure 7.15
Chapter 7 (continued)
Ductility, Resilience, and Toughness (cont.)
toughnessThe capacity of a material to _______ _______________ ___________.
Represented by the area under the ______________________________.
Figure 7.13
18
Chapter 7 (continued)
Ductility, Resilience, and Toughness (cont.)For a material to be _______, It needs to have both ________ and __________. A brittle material often has higher strength, but lower toughness.
Chapter 7 (continued)
Engineering Stress vs. True Stress
The peak in the stress-strain curve is not due to a ______________ in the material.
In fact ________________________.
definition:
strain hardening - An increase in __________________ as ametal is ______________________.
19
Chapter 7 (continued)
Engineering Stress vs. True Stress (cont.)
Figure 7.16
Chapter 7 (continued)
Engineering Stress vs. True Stress (cont.)
The drop in the graph is due to the definition of stress as the applied force over the _________ cross-sectional area.
As the __________ cross-sectional area __________ the true stress continues to __________ because of strain hardening.
The engineering stress drops because of its' definition.
