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Transcript of IE 337: Materials & Manufacturing Processes Lecture 2: Nature and Mechanical Properties of Metals...
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IE 337: Materials & Manufacturing Processes
Lecture 2:
Nature and Mechanical Properties of Metals
Chapters 2 and 3
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Last Time
Material-Geometry-Process Relationships Manufacturing Materials Manufacturing Processes How do we characterize processes?
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This Time
Metals Origin of metallic properties (Chapter 2) Mechanical properties (Chapter 3)
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Ionic Bonding
Figure 2.4 Three forms of primary bonding: (a) Ionic
Atoms of one element give up their outer electron(s), which are in turn attracted to atoms of some other element to increase electron count in the outermost shell to eight
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Covalent Bonding
Figure 2.4 Three forms of primary bonding: (b) covalent
Electrons are shared (as opposed to transferred) between atoms in their outermost shells to achieve a stable set of eight
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Metallic Bonding
Figure 2.4 Three forms of primary bonding: (c) metallic
Sharing of outer shell electrons by all atoms to form a general electron cloud that permeates the entire block
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Figure 2.1 Periodic Table of Elements. Atomic number and symbol are listed for the 103 elements.
Periodic Table
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Macroscopic Structures of Matter
Atoms and molecules are the building blocks of more macroscopic structure of matter
When materials solidify from the molten state, they tend to close ranks and pack tightly, arranging themselves into one of two structures: Crystalline Noncrystalline
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Crystalline Structure
Structure in which atoms are located at regular and recurring positions in three dimensions
Unit cell - basic geometric grouping of atoms that is repeated
The pattern may be replicated millions of times within a given crystal
Characteristic structure of virtually all metals, as well as many ceramics and some polymers
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Three Crystal Structures in Metals
1. Body-centered cubic
2. Face centered cubic
3. Hexagonal close-packed
Figure 2.8 Three types of crystal structure in metals.
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Crystal Structures for Common Metals
Room temperature crystal structures for some of the common metals:
Body‑centered cubic (BCC) Chromium, Iron, Molybdenum, Tungsten
Face‑centered cubic (FCC) Aluminum, Copper, Gold, Lead, Silver,
Nickel Hexagonal close‑packed (HCP)
Magnesium, Titanium, Zinc
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Imperfections (Defects) in Crystals
Imperfections often arise due to inability of solidifying material to continue replication of unit cell, e.g., grain boundaries in metals
Imperfections can also be introduced purposely; e.g., addition of alloying ingredient in metal
Types of defects:
1. Point defects
2. Line defects
3. Surface defects
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Point Defects
Imperfections in crystal structure involving either a single atom or a few number of atoms
Figure 2.9 Point defects: (a) vacancy, (b) ion‑pair vacancy, (c) interstitialcy, (d) displaced ion (Frenkel Defect).
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Line Defects
Connected group of point defects that forms a line in the lattice structure
Most important line defect is a dislocation, which can take two forms: Edge dislocation Screw dislocation
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Edge Dislocation
Figure 2.10 Line defects: (a) edge dislocation
Edge of an extra plane of atoms that exists in the lattice
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Surface Defects
Imperfections that extend in two directions to form a boundary
Examples: External: the surface of a crystalline object
is an interruption in the lattice structure Internal: grain boundaries are internal
surface interruptions
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Elastic Strain
When a crystal experiences a gradually increasing stress, it first deforms elastically
If force is removed lattice structure returns to its original shape
Figure 2.11 Deformation of a crystal structure: (a) original lattice: (b) elastic deformation, with no permanent change in positions of atoms.
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Plastic Strain
If stress is higher than forces holding atoms in their lattice positions, a permanent shape change occurs
Figure 2.11 Deformation of a crystal structure: (c) plastic deformation (slip), in which atoms in the lattice are forced to move to new "homes“.
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Effect of Dislocations on Strain
In the series of diagrams, the movement of the dislocation allows deformation to occur under a lower stress than in a perfect lattice
Figure 2.12 Effect of dislocations in the lattice structure under stress
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Mechanical Properties in Design and Manufacturing
Mechanical properties determine a material’s behavior when subjected to mechanical stresses Properties include elastic modulus, ductility,
hardness, and various measures of strength Dilemma: mechanical properties desirable to the
designer, such as high strength, usually make manufacturing more difficult The manufacturing engineer should appreciate the
design viewpoint And the designer should be aware of the
manufacturing viewpoint
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Stress‑Strain Relationships
Three types of static stresses to which materials can be subjected: 1. Tensile - tend to stretch the material
2. Compressive - tend to squeeze it
3. Shear - tend to cause adjacent portions of material to slide against each other
Stress‑strain curve - basic relationship that describes mechanical properties for all three types
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Tensile Test
Most common test for studying stress‑strain relationship, especially metals In the test, a force pulls the material, elongating it and reducing its diameter
Figure 3.1 Tensile test: (a) tensile force applied in (1) and (2) resulting elongation of material
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Tensile Test Specimen
ASTM (American Society for Testing and Materials) specifies preparation of test specimen
Figure 3.1 Tensile test: (b) typical test specimen
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Tensile Test Setup
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Tensile Test Sequence Figure 3.2 Typical progress of a tensile test: (1) beginning
of test, no load; (2) uniform elongation and reduction of cross‑sectional area; (3) continued elongation, maximum load reached; (4) necking begins, load begins to decrease; and (5) fracture. If pieces are put back together as in (6), final length can be measured.
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Engineering Stress
Defined as force divided by original area:
oe A
F
where e = engineering stress, F = applied force, and Ao = original area of test specimen
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Engineering Strain
Defined at any point in the test as
where e = engineering strain; L = length at any point during elongation; and Lo = original gage length
o
o
LLL
e
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Stress-Strain Relationships
Figure 3.3 Typical engineering stress‑strain plot in a tensile test of a metal.
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Two Regions of Stress‑Strain Curve
The two regions indicate two distinct forms of behavior: 1. Elastic region – prior to yielding of the material
2. Plastic region – after yielding of the material
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Elastic Region in Stress‑Strain Curve
Relationship between stress and strain is linear Material returns to its original length when stress
is removed
Hooke's Law: e = E e
where E = modulus of elasticity
E is a measure of the inherent stiffness of a material
Its value differs for different materials
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Yield Point in Stress‑Strain Curve
As stress increases, a point in the linear relationship is finally reached when the material begins to yield Yield point Y can be identified by the change in
slope at the upper end of the linear region Y = a strength property Other names for yield point = yield strength,
yield stress, and elastic limit
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Plastic Region in Stress‑Strain Curve
Yield point marks the beginning of plastic deformation
The stress-strain relationship is no longer guided by Hooke's Law
As load is increased beyond Y, elongation proceeds at a much faster rate than before, causing the slope of the curve to change dramatically
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Tensile Strength in Stress‑Strain Curve
Elongation is accompanied by a uniform reduction in cross‑sectional area, consistent with maintaining constant volume
Finally, the applied load F reaches a maximum value, and engineering stress at this point is called the tensile strength TS (a.k.a. ultimate tensile strength)
TS =
oAFmax
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Ductility in Tensile Test
Ability of a material to plastically strain without fracture
Ductility measure = elongation EL
where EL = elongation; Lf = specimen length at fracture; and Lo = original specimen length
Lf is measured as the distance between gage marks after two pieces of specimen are put back together
o
of
LLL
EL
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True Stress
Stress value obtained by dividing the instantaneous area into applied load
where = true stress; F = force; and A = actual (instantaneous) area resisting the load
AF
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True Strain
Provides a more realistic assessment of "instantaneous" elongation per unit length
o
L
L LL
LdL
o
ln
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True Stress-Strain Curve
Figure 3.4 ‑ True stress‑strain curve for the previous engineering stress‑strain plot in Figure 3.3.
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Strain Hardening in Stress-Strain Curve
Note that true stress increases continuously in the plastic region until necking In the engineering stress‑strain curve, the
significance of this was lost because stress was based on an incorrect area value
It means that the metal is becoming stronger as strain increases This is the property called strain hardening
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True Stress-Strain in Log-Log Plot
Figure 3.5 True stress‑strain curve plotted on log‑log scale.
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Flow Curve
Because it is a straight line in a log-log plot, the relationship between true stress and true strain in the plastic region is
where K = strength coefficient; and n = strain hardening exponent
nK
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Compression Test
Applies a load that squeezes the ends of a cylindrical specimen between two platens
Figure 3.7 Compression test:
(a) compression force applied to test piece in (1) and (2) resulting change in height.
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Compression Test Setup
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Shear Properties
Application of stresses in opposite directions on either side of a thin element
Figure 3.11 Shear (a) stress and (b) strain.
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Hardness
Resistance to permanent indentation Good hardness generally means material is
resistant to scratching and wear Most tooling used in manufacturing must be
hard for scratch and wear resistance
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Hardness Tests
Commonly used for assessing material properties because they are quick and convenient
Variety of testing methods are appropriate due to differences in hardness among different materials
Most well‑known hardness tests are Brinell and Rockwell
Other test methods are also available, such as Vickers, Knoop, Scleroscope, and durometer
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Widely used for testing metals and nonmetals of low to medium hardness
A hard ball is pressed into specimen surface with a load of 500, 1500, or 3000 kg
Figure 3.14 Hardness testing methods: (a) Brinell
Brinell Hardness Test
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Brinell Hardness Number
Load divided into indentation area = Brinell Hardness Number (BHN)
)( 22
2
ibbb DDDD
FHB
where HB = Brinell Hardness Number (BHN), F = indentation load, kg; Db = diameter of ball, mm, and Di = diameter of indentation, mm
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Rockwell Hardness Test
Another widely used test A cone shaped indenter is pressed into
specimen using a minor load of 10 kg, thus seating indenter in material
Then, a major load of 150 kg is applied, causing indenter to penetrate beyond its initial position
Additional penetration distance d is converted into a Rockwell hardness reading by the testing machine
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Rockwell Hardness Test
Figure 3.14 Hardness testing methods: (b) Rockwell: (1) initial minor load and (2) major load.
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Effect of Temperature on Properties
Figure 3.15 General effect of temperature on strength and ductility.
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Hot Hardness
Ability of a material to retain hardness at elevated temperatures
Figure 3.16 Hot hardness ‑ typical hardness as a function of temperature for several materials.
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You should have learned:
The nature of metals Different crystalline structures Different crystalline defects that affect properties
The properties of metals Mechanical properties What they are and what they mean
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Next Class
Manufacturing Materials (Chapter 6) How can we modify mechanical properties in
metals? (Chapter 6 and 27) How are metal alloys classified and how are
they used? (Chapter 6)
Assignment #1