Diversity of used materials in automobiles · Ferrous Amorphous Nonferrous Plastics Thermop lastics...
Transcript of Diversity of used materials in automobiles · Ferrous Amorphous Nonferrous Plastics Thermop lastics...
Production techniques_material properties
Mohsen Badrossamay 1
Dep. of Mech. Eng.
DEPARTMENT OF MECHANICAL ENGINEERINGISFAHAN UNIVERSITY OF TECHNOLOGY
روشهاي تولید و كارگاه PRODUCTION TECHNIQUES
FUNDAMENTALS OF MATERIALS: BEHAVIOR AND MANUFACTURING PROPERTIES
PART ONE
Dep. of Mech. Eng.
Diversity of used materials in automobiles
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Engineering Materials
Engineering materials
Metals
Ferrous
Amorphous
NonferrousPlastics
Thermoplastics
Thermosets Elastomers
ceramics and others
Composites
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Selecting Materials
Considerations:
Properties of materials
Mechanical properties
Physical properties
Chemical properties
Manufacturing properties
Cost and availability
Appearance, service life, and recycling
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Behavior & Manufacturing Properties
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The Structure of Metals
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Crystal Structure of Metals
Body-centered cubic (BCC) - alpha iron, chromium, molybdenum, tantalum, tungsten, and vanadium.
Face-centered cubic (FCC) - gamma iron, aluminum, copper, nickel, lead, silver, gold and platinum.
Hexagonal close-packed - beryllium, cadmium, cobalt, magnesium, alpha titanium, zinc and zirconium
Common crystal structures for metals:
Why different crystal structure?To minimize the energy required to fit together in a regular pattern
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Body-centered Cubic Crystal Structure
The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells
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Face-centered Cubic Crystal Structure
The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells
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Hexagonal Close-packed Crystal Structure
The hexagonal close-packed (hcp) crystal structure: (a) unit cell; and (b) single crystal with many unit cells.
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Deformation and strength of single crystal
Elastic deformation Plastic deformation Mechanisms of plastic deformation:
Slipping of one plane of atoms over an adjacent plane (called slip plane) under a shear stress The shear stress required to cause slip in a single crystal is directly
proportional to the b/a ratio, where a is the spacing of the atomic planes and b is inversely proportional to the atomic density in the atomic plane.
As b/a decreases, the shear stress required to cause slip decreases Slip in a single crystal takes place along planes of maximum atomic density,
i.e. slip takes place in closely packed planes and in closely packed directions Because b/a ratio properties varies for different directions within crystal, a
single crystal has different properties that is called anisotropic Twinning: a portion of the crystal forms a mirror image of itself across
the plane of twinning Twins form abruptly and are the cause of the creaking sound (tin cry) that occurs when
a tin or zinc rod is bent at room temperature 11
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Permanent Deformation
Permanent deformation (also called plastic deformation) of a single crystal subjected to a shear stress: (a) structure before deformation; (b) permanent deformation by slip.
The shear stress required to cause slip in a single crystal is directly proportional to the b/a ratio, where a is the spacing of the atomic planes and b is inversely proportional to the atomic density in the atomic plane.
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Permanent Deformation and Twinning in Crystal
(a) Permanent deformation of asingle crystal under a tensileload. Note that the slip planestend to align themselves in thedirection of the pulling force.This behavior can be simulatedusing a deck of cards with arubber band around them.
(b) Twinning in a single crystal intension.
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Slip systems Slip system: the combination of a slip plane and its
direction of slip Generally metals with 5 or more slip systems are ductile In bcc crystals
There are 48 possible slip systems i.e. high probability to slip Because of relatively high b/a ratio, the required shear stress is high Generally have good strength and moderate ductility
In fcc crystals There are 12 possible slip systems i.e. moderate probability to slip The required shear is low because of the relatively low b/a ratio Generally have moderate strength and good ductility
In hcp crystals There are 3 possible slip systems i.e. low probability to slip More slip systems become active at elevated temperatures Brittle at room temperature 14
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Slip Lines and Slip Bands in Crystal
Schematic illustration of slip lines and slip bands in a single crystal (grain) subjected to a shear stress. A slip band consists of a number of slip planes. The crystal at the center of the upper illustration is an individual grain surrounded by several other grains
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Imperfections in the crystal structure
The actual strength of metals is approximately one to two orders of magnitude lower than the strength levels obtained from theoretical calculations
the reason is addressed by imperfections and defects Structure-sensitive and structure-insensitive properties
Types of defects in crystals:1. Point defects, such as a vacancy (missing atom), an interstitial
atom (extra atom in the lattice), or an impurity (foreign atom that has replaced the atom of the pure metal)
2. Linear, or one-dimensional defects called dislocations3. Planar, or two-dimensional imperfections such as grain boundaries
and phase boundaries4. Volume, or bulk imperfections such as voids, inclusions, other
phases, or cracks16
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Defects in a Single-Crystal Lattice
Schematic illustration of types of defects in a single-crystal lattice: self-interstitial, vacancy, interstitial, and substitutional
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Dislocations in Crystals
Dislocations are defects in the orderly arrangement of a metal’s atomic structure and help explain the discrepancy between the actual and theoretical strengths of metals
Presence of dislocation lowers the shear stress required to cause slip
Types of dislocations in a single crystal: (a) edge dislocation; and (b) screw dislocation18
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Edge Dislocation Movement
Movement of an edge dislocation across the crystal lattice under a shear stress. Dislocations help explain why the actual strength of metals is much lower than that predicted by theory.
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Work hardening Dislocations can:
1. Become entangled and interfere with each other; and2. Be impeded by barriers, such as grain boundaries and impurities
and inclusions in the material
Entanglements and impediments increase the shear stress required for slip
The effect of an increase in shear stress, thus causing an increase in overall strength and hardness of the metal, is known as work hardening or strain hardening
Work hardening is used for strengthening metals in metalworking processes at ambient temperature
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Solidification of Molten Metal
Schematic illustration of the stages during solidification of molten metal; each small square represents a unit cell.
(a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different.
(b) and (c) Growth of crystals as solidification continues.
(d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other.
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Grain Size
where
N = Grains per square inch at 100x magnification
n = ASTM grain size number
N = 2n-1
ASTM Grain Size:
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Mechanical properties of metals are affected from grain size
Large grain size generally is associated with low strength, low hardness, and low ductility
Grain size is measured by counting the number of grains in a given area, or by counting the number of grains that intersect a length of a line randomly drawn on an enlarged photograph of the grains
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Plastic Deformation of Idealized Grains
Plastic deformation of idealized (equiaxed) grains in a specimen subjected to compression (such as occurs in the forging or rolling of metals): (a) before deformation; and (b) after deformation. Note the alignment of grain boundaries along a horizontal direction; this effect is known as preferred orientation.
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Recovery, recrystallization, and grain growth
Plastic deformation at room temperature causes: 1. The deformation of grains and grain boundaries2. A general increase in strength3. A decrease in ductility4. Anisotropic behavior
Heating the metal to a specific temperature range for a period of time (annealing) can reverse those effects
The events take place during heating1. Recovery (stress relief)
Occurs at a certain temperature range below the recrystalization temperature
2. Recrystallizationnew equiaxed and strain-free grains are formedTemperature range approximately between 0.3 and 0.5 melting point of the metalRecrystallization decreases the density of dislocations, lowers the strength, and
rises the ductility
3. Grain growth 24
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Recovery, Recrystallization, and Grain Growth Effects
Schematic illustration of the effects of recovery, recrystallization, and grain growth on mechanical properties and on the shape and size of grains. Note the formation of small new grains during recrystallization.
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Temperature Ranges for Cold, Warm and Hot Working
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DEPARTMENT OF MECHANICAL ENGINEERINGISFAHAN UNIVERSITY OF TECHNOLOGY
PRODUCTION TECHNIQUES
ACADEMIC YEAR 90-91, SEMESTER TWO
FUNDAMENTALS OF MATERIALS BEHAVIOR AND MANUFACTURING
PART TWO
Dep. of Mech. Eng.
Mechanical Behavior, Testing, and Manufacturing Properties of Materials
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Mechanical Properties
Strength, toughness, hardness, elasticity, fatigue and creep
Mechanical test methods:
Tension, compression, torsion, bending, hardness, fatigue, creep, impact
The properties that a material reveals under loading
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Relative mechanical Properties of Materials
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Tension Test The most common test for determining such mechanical
properties of materials as strength, toughness, elastic modulus, and strain-hardening capability
Three types of strain: (a) tensile, (b) compressive and (c) shear
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Tensile-test Specimen and Machine
(a) A standard tensile-test specimen before and after pulling, showing original and final gage lengths.
(b) A tensile-test sequence showing different stages in the elongation of the specimen. 32
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Tension Test Stress-strain Curve
A typical stress-strain curve obtained from a tension test, showing various features 33
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Tension test stresses and strains
Engineering Stess,
Engineering Strain,
Modulus of Elasticity,
True stress, =
True strain, =ln
o
o
o
o
PA
l le l
E ePAll
Engineering stress: the ratio of applied load, P, to the original cross-sectional area A0, specimen
Young’s modulus: the ratio of stress to strain in the elastic region
True stress: the ratio of the load, P, to the instantaneous cross-sectional area, A, of the specimen
Ultimate tensile strength (UTS): the maximum engineering stress
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Tension test stresses and strains Hooke’s law: the linear relationship between stress and
strain E: a measure of the slope of the elastic portion and,
hence, the stiffness of the material The higher the E value, the higher the load required to
stretch the specimen to the same extent and, thus the stiffer the material
Poisson’s ratio: the absolute value of the ratio of the lateral strain to the longitudinal strain
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Mechanical Properties of Materials
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Loading and Unloading of Tensile-test Specimen
Schematic illustration of the loading and the unloading of a tensile-test specimen. Note that, during unloading, the curve follows a path parallel to the original elastic slope.
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Ductility
There are two common measures of ductility Elongation:
Reduction of area:
The extent of plastic deformation that the materialundergoes before fracture
100 Elongation0
0
l
ll f
100 area ofReduction 0
0
A
AA f
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Elongation vs. reduction of area
Approximate relationship between elongation and tensile reduction of area for various groups of metals 39
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Tension and Stress Curves(a) Load elongation curve in tension
testing of a stainless steel specimen.
(b) Engineering stress-engineering strain curve, drawn from the data in Fig. a.
(c) True stress-true strain curve, drawn from the data in Fig. b. Note that this curve has a positive slope, indicating that the material is becoming stronger as it is strained.
(d) True stress-true strain curve plotted on the log-log paper and based on the corrected curve in Fig. c. The correction is due to the tri-axial state of stress that exists in the necked region of the specimen.
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Power Law Constitutive Model
Kn
where
K = strength coefficient
n = strain hardening exponent
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True Stress-strain CurvesTrue stress-strain curves in tension at room temperature for various metals. The curves start at a finite level of stress: The elastic regions have too steep a slope to be shown in this figure, and thus each curve starts at the yield stress, Y, of the material
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Resilience and Toughness• Area under stress–strain curve up to the yield point, Y, of the
material is known as the modulus of resilience,
• Area has the units of energy per unit volume.E
YYe22
resilience of Modulus2
0
• The area under the true stress–true strain curve is known as toughness,
where εf is the true strain at fracture.
• Toughness is the energy per unit volume (specific energy) dissipated up to the point of fracture.
df
0
Toughness
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Temperature effects on stress-strain curves
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Typical effects of temperature on stress-strain curves. Note that increasing the temperature generally 1) raises the ductility and toughness (area under the curve) ; 2) lowers the yield stress, the ultimate tensile strength, and the modulus of elasticity of materials.
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Strain and deformation rate in manufacturing
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Deformation rate: the speed at which a tension test is being carried out strain rate is a function of the specimen lengthIncreasing strain rate increases the strength of material (strain-rate hardening)
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Effect of Strain Rate on Tensile Strength of Al
The effect of strain rate on the ultimate tensile strength for aluminum. Note that, as the temperature increases, the slopes of the curves increase; thus, strength becomes more and more sensitive to strain rate as temperature increases.
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mC where
C = strength coefficient
= true strain rate
m = strain-rate sensitivity exponent
Cold working: up to 0.05
Hot working: 0.05 to 0.4
Superplastic materials: 0.3 to 0.85
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Compression - Disk Test
Disk test on a brittle material, showing the direction of loading and the fracture path.
Tensile stress, 2Pdt
where
P = load at fracture
d = diameter of disk
t = thickness of disk
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Torsion-Test Specimen
A typical torsion-test specimen; it is mounted between the two heads of a testing machine and twisted. Note the shear deformation of an element in the reduced section of the specimen.
2Shear stress, =2
Shear strain, =
Tr t
rl
where
T = torque
r = average tube radius
t = thickness of tube at narrow section
l = length of tube subjected to torsion
= angle of twist
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Bend-test Methods
Two bend-test methods for brittle materials: (a) three-point bending; (b) four-point bending. The areas on the beams represent the bending-movement diagrams, described in texts on mechanics of solids. Note the region of constant maximum bending movement in (b); by contrast, the maximum bending moment occurs only at the center of the specimen in (a).
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Hardness-testing methods and formulas
General characteristics of hardness-testing methods and formulas for calculating hardness.
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Indentation Geometry for Brinell Testing
Indentation geometry in Brinellhardness testing:
(a) annealed metal;
(b) work-hardened metal;
(c) deformation of mild steel under a spherical indenter. Note that the depth of the permanently deformed zone is about one order of magnitude larger than the depth of indentation. For a hardness test to be valid, this zone should be developed fully in the material
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Hardness Scale
Conversions
Chart for converting various hardness scales. Note the limited range of most scales. Because of the many factors involved, these conversions are approximate.
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FatigueS-N Curves
(a) Typical S-N curves for two metals. Note that, unlike steel, aluminum does not have an endurance limit. (b) S-N curves for common polymers
S: stress amplitudes, N: the number of cycles
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Endurance Limit vs. Tensile Strength
Ratio of endurance limit to tensile strength for various metals, as a function of tensile strength. Because aluminum does not have an endurance limit, the correlations for aluminum are based on a specific number of cycles, as is seen in previous figure.
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Creep Curve
Schematic illustration of a typical creep curve. The linear segment of the curve (secondary) is used in designing components for a specific creep life.
Creep: the permanent elongation of a component under a static load maintained for a period of time 55
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Impact Test Specimens
Impact test specimens: (a) Charpy; (b) Izod.
Impact tests are useful particularly in determining the ductile-brittle transition temperature of materials
Materials that have high impact resistance generally are those that also have high strength and high ductility, and hence high toughness 56
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Material Failures
Schematic illustrations of types of failures in materials:
(a) necking and fracture of ductile materials;
(b) buckling of ductile materials under a compressive load;
(c) fracture of brittle materials in compression;
(d) cracking on the barreled surface of ductile materials in compression57
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Fracture Types in Tension
Schematic illustration of the types of fracture in tension:
(a) brittle fracture in polycrystalline metals;
(b) shear fracture in ductile single crystals;
(c) ductile cup-and-cone fracture in polycrystalline metals;
(d) complete ductile fracture in polycrystalline metals, with 100% reduction of area. 58
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Ductile Fracture in Low-carbon Steel
Surface of ductile fracture in low-carbon steel, showing dimples. Fracture usually is initiated at impurities, inclusions, or preexisting voids (microporosity) in the metal. Source: Courtesy of K. H. Habig and D. Klaffke
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Progression of a Fracture
Sequence of events in the necking and fracture of a tensile-test specimen:
(a) early stage of necking;
(b) small voids begin to form within the necked region;
(c) voids coalesce, producing an internal crack;
(d) the rest of the cross-section begins to fail at the periphery, by shearing;
(e) the final fracture surfaces, known as cup- (top fracture surface) and cone-(bottom surface) fracture.
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Temperature Transition in Metals
Schematic illustration of transition temperature in metals.61
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Fracture Surface of Steel
Fracture surface of steel that has failed in a brittle manner. The fracture path is transgranular (through the grains). Magnification: 200x. Source: Courtesy of B. J. Schulze and S.L. Meinley and Packer Engineering Associates, Inc.
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Intergranular Fracture
Intergranular fracture, at two different magnifications. Grains and grain boundaries are clearly visible in this micrograph. The fracture path is along the grain boundaries. Magnification: left, 100x; right, 500x. Source: Courtesy of B.J. Schulze and S.L. Meiley and Packer Engineering Associates, Inc. 63
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Fatigue-Fracture Surface
Typical fatigue-fracture surface on metals, showing beach marks. Magnification: left, 500x; right, 1000x. Source: Courtesy of B.J. Schulze and S.L. Meiley and Packer Engineering Associates, Inc.
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Reduction in Fatigue Strength vs. Ultimate Tensile Strength
Reductions in fatigue strength of cast steels subjected to various surface-finishing operations. Note that the reduction becomes greater as the surface roughness and the strength of steel increase. Source: Courtesy of M. R. Mitchell
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Residual Stresses in Bending a Beam
Residual stresses developed in bending a beam having a rectangular cross-section. Note that the horizontal forces and moments caused by residual stresses in the beam must be balanced internally. Because of nonuniform deformation and especially during cold-metalworking operations, most parts develop residual stresses.
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