Types of Material

Why Materials ??? Ashby,: Material Selection in Mechanical Design

Engineering Materials

MaterialsNanomaterials, shape-memory alloys, superconductors, Ferrous metals: carbon-, alloy-, stainless-, tool-and-die steelsNon-ferrous metals: aluminum, magnesium, copper, nickel, titanium, superalloys, refractory metals, beryllium, zirconium, low-melting alloys, gold, silver, platinum, Plastics: thermoplastics (acrylic, nylon, polyethylene, ABS,) thermosets (epoxies, Polymides, Phenolics, ) elastomers (rubbers, silicones, polyurethanes, )Ceramics, Glasses, Graphite, Diamond, Cubic Boron NitrideComposites: reinforced plastics, metal-, ceramic matrix composites

Properties of materials

Properties of materials Mechanical properties of materialsStrength, Toughness, Hardness, Ductility,Elasticity, Fatigue and CreepChemical propertiesOxidation, Corrosion, Flammability, Toxicity, Physical propertiesDensity, Specific heat, Melting and boiling point,Thermal expansion and conductivity,Electrical and magnetic properties

Physical propertiescolour light wave length

specific heat the heat required to raise the temperature of one gram of a substance by one degreecentigrade (J/kg K)density mass per unit volume expressed in such units as kg/cm 3

thermal conductivity rate at which heat flows through a given material (W/m K)melting point a temperature at which a solid begins to liquify

electrical conductivity a measure of how strongly a material opposes the flow of electric current (m)

*Densities of structural materialsComparison: density of water is 1000 kg/m3

Density (kg/m3)

Engineering materials

Steel

7800

Concrete

2300

Rubber

1100

Biological materials

Bone

2000

Cartilage

1100

Tendon

1300

Locust cuticle

1200

Physical propertiescoefficient of thermal expansion degree of expansion divided by the change in temperature (m/C)permeabilityis the measure of the ability of a material to support the formation of amagnetic fieldwithin itself. Magnetic permeability is typically represented by the Greek letterInSIunits, permeability is measured inhenriesper meter (Hm1), orNewtonperamperesquared (NA2). The permeability constant (0), also known as themagnetic constantor the permeability of free space, 0= 4107Hm1ferromagnets(f),paramagnets(p), free space(0) anddiamagnets(d)Simplified comparison of permeabilities

Chemical propertiescorrosion resistance - is the property of a metal, or in general a material, to resist to corrosion attack in a particular environment at defined operating conditions, pressure, temperature and fluid velocity. Usually the resistance to corrosion is expressed in terms ofCorrosion rate, mm/y or mils per year (mpy). 1 mpy = 0.0254 mm/y = 25.4 microm/y

To calculate the corrosion rate from metal loss: mm /y = 87.6 x (W / DAT) where: W = weight loss in milligrams, D = metal density in g /cm3 A = area of sample in cm2, T = time of exposure of the metal sample in hours

Mechanical propertiestensile strength measures the force required to pull something such as rope,wire or a structural beam to the point where it breaks

ductility a measure of how much strain a material can take before rupturingmalleability the property of a material that can be worked or hammered or shaped without breaking

brittleness breaking or shattering of a material when subjected to stress (when force is applied to it)

Mechanical propertieselasticity the property of a material that returns to its original shape after stress (e.g. external forces) that made it deform or distort is removed

plasticity - the deformation of a material undergoing non-reversible changes of shape in response to applied forcestoughness the ability of a material to absorb energy and plastically deform without fracturing

hardness the property of being rigid and resistant to pressure; not easily scratchedmachinability the property of a material that can be shaped by hammering, pressing, rolling

*types of stressesTensionCompressionTorsionShear

*Tensile Test

*Stress-Strain Testspecimenmachine

*Important Mechanical Properties from a Tensile Test Young's Modulus: This is the slope of the linear portion of the stress-strain curve, it is usually specific to each material; a constant, known value. Yield Strength: This is the value of stress at the yield point, calculated by plotting young's modulus at a specified percent of offset (usually offset = 0.2%). Ultimate Tensile Strength: This is the highest value of stress on the stress-strain curve. Percent Elongation: This is the change in gauge length divided by the original gauge length.

TerminologyLoad - The force applied to a material during testing.Strain gage or Extensometer - A device used for measuring change in length (strain).Engineering stress - The applied load, or force, divided by the original cross-sectional area of the material.Engineering strain - The amount that a material deforms per unit length in a tensile test.

*1. Initial2. Small load3. UnloadElastic means reversible.Elastic Deformation

*1. Initial2. Small load3. UnloadPlastic means permanent.Plastic Deformation (Metals)

*c07f10abTypical stress-strain behavior for a metal showing elastic and plastic deformations, the proportional limit P and the yield strength y, as determined using the 0.002 strain offset method (where there is noticeable plastic deformation). P is the gradual elastic to plastic transition.

*Plastic Deformation (permanent)From an atomic perspective, plastic deformation corresponds to the breaking of bonds with original atom neighbors and then reforming bonds with new neighbors. After removal of the stress, the large number of atoms that have relocated, do not return to original position.Yield strength is a measure of resistance to plastic deformation.

*c07f25

Localized deformation of a ductile material during a tensile test produces a necked region. The image shows necked region in a fractured sample

*Permanent DeformationPermanent deformation for metals is accomplished by means of a process called slip, which involves the motion of dislocations.Most structures are designed to ensure that only elastic deformation results when stress is applied. A structure that has plastically deformed, or experienced a permanent change in shape, may not be capable of functioning as intended.

*Yield Strength, sy

Stress-Strain Diagram Strain ( ) (DL/Lo)41235Stress (F/A)Elastic RegionPlasticRegionStrainHardeningFractureultimatetensile strength

Slope=EElastic region slope =Youngs (elastic) modulus yield strengthPlastic region ultimate tensile strength strain hardening fractureneckingyieldstrength

Stress-Strain Diagram (cont) Elastic Region (Point 1 2) - The material will return to its original shape after the material is unloaded( like a rubber band). - The stress is linearly proportional to the strain in this region. : Stress(psi)E : Elastic modulus (Youngs Modulus) (psi) : Strain (in/in) Point 2 : Yield Strength : a point where permanent deformation occurs. ( If it is passed, the material will no longer return to its original length.) or

Strain Hardening - If the material is loaded again from Point 4, the curve will follow back to Point 3 with the same Elastic Modulus (slope). - The material now has a higher yield strength of Point 4. - Raising the yield strength by permanently straining the material is called Strain Hardening.Stress-Strain Diagram (cont)

Tensile Strength (Point 3) - The largest value of stress on the diagram is called Tensile Strength(TS) or Ultimate Tensile Strength (UTS) - It is the maximum stress which the material can support without breaking. Fracture (Point 5) - If the material is stretched beyond Point 3, the stress decreases as necking and non-uniform deformation occur. - Fracture will finally occur at Point 5.Stress-Strain Diagram (cont)

The stress-strain curve for an aluminum alloy.

*Stress-strain behavior found for some steels with yield point phenomenon.

*c07tf02TENSILE

PROPERTIES

*Room T valuesa = annealedhr = hot rolledag = agedcd = cold drawncw = cold workedqt = quenched & temperedYield Strength: Comparison

* After yielding, the stress necessary to continue plastic deformation in metals increases to a maximum point (M) and then decreases to the eventual fracture point (F). All deformation up to the maximum stress is uniform throughout the tensile sample. However, at max stress, a small constriction or neck begins to form. Subsequent deformation will be confined to this neck area. Fracture strength corresponds to the stress at fracture. Region between M and F: Metals: occurs when noticeable necking starts. Ceramics: occurs when crack propagation starts. Polymers: occurs when polymer backbones are aligned and about to break.Tensile Strength, TS

*In an undeformed thermoplastic polymer tensile sample, the polymer chains are randomly oriented. When a stress is applied, a neck develops as chains become aligned locally. The neck continues to grow until the chains in the entire gage length have aligned. The strength of the polymer is increased

*Room T valuesBased on data in Table B4, Callister 6e.a = annealedhr = hot rolledag = agedcd = cold drawncw = cold workedqt = quenched & temperedAFRE, GFRE, & CFRE =aramid, glass, & carbonfiber-reinforced epoxycomposites, with 60 vol%fibers.Tensile Strength: Comparison

* Another ductility measure: Ductility may be expressed as either percent elongation (% plastic strain at fracture) or percent reduction in area. %AR > %EL is possible if internal voids form in neck. Ductility, %ELDuctility is a measure of the plastic deformation that has been sustained at fracture:A material that suffers very little plastic deformation is brittle.

*c07f13ToughnessLower toughness: ceramicsHigher toughness: metalsToughness is the ability to absorb energy up to fracture (energy per unit volume of material).

A tough material has strength and ductility.

Approximated by the area under the stress-straincurve.

Energy to break a unit volume of material Approximate by the area under the stress-strain curve.21Toughness

*c07f05Linear Elastic PropertiesModulus of Elasticity, E: (Young's modulus) Hooke's Law:s = E e Poisson's ratio: metals: n ~ 0.33 ceramics: n ~0.25 polymers: n ~0.40Units:E: [GPa] or [psi]n: dimensionlessn = ex/ey

*MetalsAlloysGraphiteCeramicsSemicondPolymersComposites/fibersE(GPa)Composite data based onreinforced epoxy with 60 vol%of aligned carbon (CFRE),aramid (AFRE), or glass (GFRE)fibers.Youngs Moduli: Comparison

Material SpecificationChemical compositionMechanical properties Strength, hardness (under various conditions: temperature, humidity, pressure)Physical properties density, optical, electrical, magneticEnvironmental green, recycling

MetalsFerrous MetalsCast ironsSteelsSuper alloysIron-basedNickel-basedCobalt-basedNon-ferrous metalsAluminum and its alloysCopper and its alloysMagnesium and its alloysNickel and its alloysTitanium and its alloysZinc and its alloysLead & TinRefractory metalsPrecious metals

General Properties and Applications of Ferrous AlloysFerrous alloys are useful metals in terms of mechanical, physical and chemical properties.Alloys contain iron as their base metal.Carbon steels are least expensive of all metals while stainless steels is costly.

Carbon and alloy steelsCarbon steelsClassified as low, medium and high:Low-carbon steel or mild steel, < 0.3%C, bolts, nuts and sheet plates.Medium-carbon steel, 0.3% ~ 0.6%C, machinery, automotive and agricultural equipment.High-carbon steel, > 0.60% C, springs, cutlery, cable.

Carbon and alloy steelsAlloy steelsSteels containing significant amounts of alloying elements.Structural-grade alloy steels used for construction industries due to high strength.Other alloy steels are used for its strength, hardness, resistance to creep and fatigue, and toughness.It may heat treated to obtain the desired properties.

Carbon and alloy steelsHigh-strength low-alloy steelsImproved strength-to-weight ratio.Used in automobile bodies to reduce weight and in agricultural equipment.Some examples are:Dual-phase steelsMicro alloyed steelsNano-alloyed steels

Stainless steelsCharacterized by their corrosion resistance, high strength and ductility, and high chromium content.Stainless as a film of chromium oxide protects the metal from corrosion.

Stainless steelsFive types of stainless steels:Austenitic steelsFerritic steelsMartensitic steelsPrecipitation-hardening (PH) steelsDuplex-structure steels

Typical Selection of Carbon and Alloy Steels for Various Applications

TABLE 5.1

Product

Steel

Product

Steel

Aircraft forgings,

tubing, fittings

Automobile bodies

Axles

Ball bearings and races

Bolts

Camshafts

Chains (transmission)

Coil springs

Connecting rods

Crankshafts (forged)

4140, 8740

1010

1040, 4140

52100

1035, 4042, 4815

1020, 1040

3135, 3140

4063

1040, 3141, 4340

1045, 1145, 3135, 3140

Differential gears

Gears (car and truck)

Landing gear

Lock washers

Nuts

Railroad rails and wheels

Springs (coil)

Springs (leaf)

Tubing

Wire

Wire (music)

4023

4027, 4032

4140, 4340, 8740

1060

3130

1080

1095, 4063, 6150

1085, 4063, 9260, 6150

1040

1045, 1055

1085

Mechanical Properties of Selected Carbon and Alloy Steels in Various Conditions

TABLE 5.2 Typical Mechanical Properties of Selected Carbon and Alloy Steels in the Hot-Rolled, Normalized, and Annealed Condition

AISI

Condition

Ultimate tensile strength (MPa)

Yield Strength (MPa)

Elongation in 50 mm (%)

Reduction of area (%)

Hardness (HB)

1020

1080

3140

4340

8620

As-rolled

Normalized

Annealed

As-rolled

Normalized

Annealed

Normalized

Annealed

Normalized

Annealed

Normalized

Annealed

448

441

393

1010

965

615

891

689

1279

744

632

536

346

330

294

586

524

375

599

422

861

472

385

357

36

35

36

12

11

24

19

24

12

22

26

31

59

67

66

17

20

45

57

50

36

49

59

62

143

131

111

293

293

174

262

197

363

217

183

149

Room-Temperature Mechanical Properties and Applications of Annealed Stainless Steels

TABLE 5.4 Room-Temperature Mechanical Properties and Typical Applications of Selected Annealed Stainless Steels

AISI

(UNS)

Ultimate tensile strength (MPa)

Yield strength (MPa)

Elongation in 50 mm

(%)

Characteristics and typical applications

303

(S30300)

550620

240260

5350

Screw machine products, shafts, valves, bolts, bushings, and nuts; aircraft fittings; bolts; nuts; rivets; screws; studs.

304

(S30400)

565620

240290

6055

Chemical and food processing equipment, brewing equipment, cryogenic vessels, gutters, downspouts, and flashings.

316

(S31600)

550590

210290

6055

High corrosion resistance and high creep strength. Chemical and pulp handling equipment, photographic equipment, brandy vats, fertilizer parts, ketchup cooking kettles, and yeast tubs.

410

(S41000)

480520

240310

3525

Machine parts, pump shafts, bolts, bushings, coal chutes, cutlery, tackle, hardware, jet engine parts, mining machinery, rifle barrels, screws, and valves.

416

(S41600)

480520

275

3020

Aircraft fittings, bolts, nuts, fire extinguisher inserts, rivets, and screws.

TABLE 5.5 Basic Types of Tool and Die Steels

Type

AISI

High speed

Hot work

Cold work

Shock resisting

Mold steels

Special purpose

Water hardening

M (molybdenum base)

T (tungsten base)

H1 to H19 (chromium base)

H20 to H39 (tungsten base)

H40 to H59 (molybdenum base)

D (high carbon, high chromium)

A (medium alloy, air hardening)

O (oil hardening)

S

P1 to P19 (low carbon)

P20 to P39 (others)

L (low alloy)

F (carbon-tungsten)

W

TABLE 5.6 Processing and Service Characteristics of Common Tool and Die Steels

AISI designation

Resistance to decarburization

Resistance to cracking

Approximate hardness (HRC)

Machinability

Toughness

Resistance to softening

Resistance to wear

M2

Medium

Medium

6065

Medium

Low

Very high

Very high

T1

High

High

6065

Medium

Low

Very high

Very high

T5

Low

Medium

6065

Medium

Low

Highest

Very high

H11, 12, 13

Medium

Highest

3855

Medium to high

Very high

High

Medium

A2

Medium

Highest

5762

Medium

Medium

High

High

A9

Medium

Highest

3556

Medium

High

High

Medium to high

D2

Medium

Highest

5461

Low

Low

High

High to very high

D3

Medium

High

5461

Low

Low

High

Very high

H21

Medium

High

3654

Medium

High

High

Medium to high

H26

Medium

High

4358

Medium

Medium

Very high

High

P20

High

High

2837

Medium to high

High

Low

Low to medium

P21

High

Highest

3040

Medium

Medium

Medium

Medium

W1, W2

Highest

Medium

5064

Highest

High

Low

Low to medium

Source: Adapted from Tool Steels, American Iron and Steel Institute, 1978.

Tool and die steelsDesigned for high strength, impact toughness, and wear resistance at a range of temperatures.

Processing and Service Characteristics of Common Tool and Die Steels

TABLE 5.6 Processing and Service Characteristics of Common Tool and Die Steels

AISI designation

Resistance to decarburization

Resistance to cracking

Approximate hardness (HRC)

Machinability

Toughness

Resistance to softening

Resistance to wear

M2

Medium

Medium

6065

Medium

Low

Very high

Very high

T1

High

High

6065

Medium

Low

Very high

Very high

T5

Low

Medium

6065

Medium

Low

Highest

Very high

H11, 12, 13

Medium

Highest

3855

Medium to high

Very high

High

Medium

A2

Medium

Highest

5762

Medium

Medium

High

High

A9

Medium

Highest

3556

Medium

High

High

Medium to high

D2

Medium

Highest

5461

Low

Low

High

High to very high

D3

Medium

High

5461

Low

Low

High

Very high

H21

Medium

High

3654

Medium

High

High

Medium to high

H26

Medium

High

4358

Medium

Medium

Very high

High

P20

High

High

2837

Medium to high

High

Low

Low to medium

P21

High

Highest

3040

Medium

Medium

Medium

Medium

W1, W2

Highest

Medium

5064

Highest

High

Low

Low to medium

Source: Adapted from Tool Steels, American Iron and Steel Institute, 1978.

Aluminium and aluminium alloysFactors for selecting are:High strength to weight ratioResistance to corrosionHigh thermal and electrical conductivityEase of machinabilityNon-magnetic

Aluminium and aluminium alloys

Magnesium and magnesium alloysMagnesium (Mg) is the lightest metal.Alloys are used in structural and non-structural applications.Typical uses of magnesium alloys are aircraft and missile components.Also has good vibration-damping characteristics.

Copper and copper alloysCopper alloys have electrical and mechanical properties, corrosion resistance, thermal conductivity and wear resistance.Applications are electronic components, springs and heat exchangers.Brass is an alloy of copper and zinc.Bronze is an alloy of copper and tin.

Nickel and nickel alloysNickel (Ni) has strength, toughness, and corrosion resistance to metals. Used in stainless steels and nickel-base alloys.Alloys are used for high temperature applications, such as jet-engine components and rockets.

Nickel and nickel alloys

SuperalloysSuperalloys are high-temperature alloys use in jet engines, gas turbines and reciprocating engines.

Titanium and titanium alloysTitanium (Ti) is expensive, has high strength-to-weight ratio and corrosion resistance.Used as components for aircrafts, jet-engines, racing-cars and marine crafts.

Refractory metalsRefractory metals have a high melting point and retain their strength at elevated temperatures.

Applications are electronics, nuclear power and chemical industries.

Molybdenum, columbium, tungsten, and tantalum are referred to as refractory metal.

Other nonferrous metalsBerylliumZirconiumLow-melting-point metals: - Lead - Zinc - TinPrecious metals: - Gold - Silver - Platinum

Special metals and alloysShape-memory alloys (i.e. eyeglass frame, helical spring)Amorphous alloys (Metallic Glass)NanomaterialsMetal foams

Mechanical Failure & Failure Analysis

ISSUES TO ADDRESS... How do flaws in a material initiate failure? How is fracture resistance quantified; how do different material classes compare? How do we estimate the stress to fracture? How do loading rate, loading history, and temperature affect the failure stress?Ship-cyclic loadingfrom waves.Computer chip-cyclicthermal loading.Hip implant-cyclicloading from walking.Adapted from Fig. 22.30(b), Callister 7e. (Fig. 22.30(b) is courtesy of National Semiconductor Corporation.)Adapted from Fig. 22.26(b), Callister 7e.Mechanical Failure & Failure AnalysisAdapted from chapter-opening photograph, Chapter 8, Callister 7e. (by Neil Boenzi, The New York Times.)

Fracture mechanismsDuctile fractureOccurs with plastic deformation

Brittle fractureOccurs with Little or no plastic deformationThus they are Catastrophic meaning they occur without warning!

Ductile vs Brittle Failure Ductile fracture is nearly always desirable!Ductile: warning before fractureBrittle: No warning

Ductile failure: --one piece --large deformationFigures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., 1987. Used with permission.Example: Failure of a Pipe

Evolution to failure:Moderately Ductile Failure

Ductile vs. Brittle FailureAdapted from Fig. 8.3, Callister 7e.cup-and-cone fracturebrittle fracture

Brittle FailureArrows indicate point at which failure originatedAdapted from Fig. 8.5(a), Callister 7e.

Intergranular(between grains) Intragranular (within grains)Al Oxide(ceramic)Reprinted w/ permission from "Failure Analysis of Brittle Materials", p. 78. Copyright 1990, The American Ceramic Society, Westerville, OH. (Micrograph by R.M. Gruver and H. Kirchner.)316 S. Steel (metal)Reprinted w/ permission from "Metals Handbook", 9th ed, Fig. 650, p. 357. Copyright 1985, ASM International, Materials Park, OH. (Micrograph by D.R. Diercks, Argonne National Lab.)304 S. Steel (metal)Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p. 650. Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge National Lab.)Polypropylene(polymer)Reprinted w/ permission from R.W. Hertzberg, "Deformation and Fracture Mechanics of Engineering Materials", (4th ed.) Fig. 7.35(d), p. 303, John Wiley and Sons, Inc., 1996.3 mm4 mm160 mm1 mm(Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p. 1119.)Brittle Fracture Surfaces: Useful to examine to determine causes of failure

Failure Analysis Failure AvoidanceMost failure occur due to the presence of defects in materialsCracks or Flaws (stress concentrators) Voids or inclusionsPresence of defects is best found before hand and they should be determined non-destructivelyX-Ray analysisUltra-Sonic InspectionSurface inspectionMagna-fluxDye Penetrant

Impact (high strain rate) Testing Impact loading (see ASTM E23 std.): -- severe testing case -- makes material act more brittle -- decreases toughness Useful to compare alternative materials for severe applicationsAdapted from Fig. 8.12(b), Callister 7e. (Fig. 8.12(b) is adapted from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc. (1965) p. 13.)

Flaws are Stress Concentrators!Results from crack propagationGriffith Crack Model:

where t = radius of curvature of crack tipso = applied stresssm = stress at crack tip

tAdapted from Fig. 8.8(a), Callister 7e.

Concentration of Stress at Crack TipAdapted from Fig. 8.8(b), Callister 7e.

Engineering Fracture Design Avoid sharp corners!sAdapted from G.H. Neugebauer, Prod. Eng. (NY), Vol. 14, pp. 82-87 1943.)max is the concentrated stress in the narrowed region

Crack PropagationCracks propagate due to sharpness of crack tip A plastic material deforms at the tip, blunting the crack. brittle

Energy balance on the crackElastic strain energy- energy is stored in material as it is elastically deformedthis energy is released when the crack propagatescreation of new surfaces requires (this) energyPlastic deformed region

When Does a Crack Propagate?Crack propagates if applied stress is above critical stress

whereE = modulus of elasticitys = specific surface energya = one half length of internal crackKc = sc/s0

For ductile materials replace gs by gs + gp where gp is plastic deformation energyi.e., sm > sc or Kt > Kc

Fatigue behavior: Fatigue = failure under cyclic stress Stress varies with time. -- key parameters are S (stress amplitude), sm, and frequency Key points when designing in Fatigue inducing situations: -- fatigue can cause part failure, even though smax < sc. -- fatigue causes ~ 90% of mechanical engineering failures. Because of its importance, ASTM and ISO have developed many special standards to assess Fatigue Strength of materials (Fig. 8.18 is from Materials Science in Engineering, 4/E by Carl. A. Keyser, Pearson Education, Inc., Upper Saddle River, NJ.)

Some important Calculations in Fatigue Testing

Figure 8.8 Fatigue corresponds to the brittle fracture of an alloy after a total of N cycles to a stress below the tensile strength.

Fatigue limit, Sfat: --no fatigue failure if S < Sfat Fatigue Limit is defined in: ASTM D671

Adapted from Fig. 8.19(a), Callister 7e. Fatigue Design Parameters

Lets look at an Example

For metals other than Ferrous alloys, F.S. is taken as the stress that will cause failure after 108 cycles

Figure 8.21 Fatigue behavior for an acetal polymer at various temperatures. (From Design Handbook for Du Pont Engineering Plastics, used by permission.)For polymers, we consider fatigue life to be (only) 106 cycles to failure thus fatigue strength is the stress that will lead to failure after 106 cycles

Cracks in Material grows incrementallytyp. 1 to 6increase in crack length per loading cycle Failed rotating shaft --crack grew even though Kmax < Kc --crack grows faster as Ds increases crack gets longer loading freq. increases.Adapted fromfrom D.J. Wulpi, Understanding How Components Fail, American Society for Metals, Materials Park, OH, 1985.Fatigue Mechanism

Figure 8.12 Illustration of crack growth with number of stress cycles, N, at two different stress levels. Note that, at a given stress level, the crack growth rate, da/dN, increases with increasing crack length, and, for a given crack length such as a1, the rate of crack growth is significantly increased with increasing magnitude of stress.

Improving Fatigue Life1. Impose a compressive surface stresses (to suppress surface crack growth)Adapted fromFig. 8.24, Callister 7e.

Figure 8.17 Fatigue strength is increased by prior mechanical deformation or reduction of structural discontinuities.

Other Issues in Failure Stress Corrosion CrackingWater can greatly accelerate crack growth and shorten life performance in metals, ceramics and glasses Other chemicals that can generate (or provide H+ or O2-) ions also effectively reduce fatigue life as these ions react with the metal or oxide in the material

Figure 8.18 The drop in strength of glasses with duration of load (and without cyclic-load applications) is termed static fatigue. (From W. D. Kingery, Introduction to Ceramics, John Wiley & Sons, Inc., New York, 1960.)

Figure 8.19 The role of H2O in static fatigue depends on its reaction with the silicate network. One H2O molecule and one Si OSi segment generate two SiOH units, which is equivalent to a break in the network.

Figure 8.20 Comparison of (a) cyclic fatigue in metals and (b) static fatigue in ceramics.

SUMMARY

Why Failure?All unanticipated mechanical failures must have a cause:Designed incorrectlyManufactured incorrectlyMis-maintainedMis-operated

Engineering materials don't reach theoretical strength. Flaws produce stress concentrations that cause premature failure. Sharp corners produce large stress concentrations and premature failure. Failure type depends on T and stress:- for noncyclic s and T < 0.4Tm, failure stress decreases with: - increased maximum flaw size, - decreased T, - increased rate of loading.- for cyclic s: - cycles to fail decreases as Ds increases.- for higher T (T > 0.4Tm): - time to fail decreases as s or T increases.SUMMARY

Case StudyConventional tensile failure mode that we are all familiar with.

Brittle FractureDuctile Fracture

Failure in CompressionFailure in Torsion

Failure in Bending

Failure in little bits!

Sneaky failure

Fatigue Failure

CORROSION AND DEGRADATION

ISSUES TO ADDRESS... Why does corrosion occur?1 What metals are most likely to corrode? How do temperature and environment affect corrosion rate? How do we suppress corrosion?

CORROSION AND DEGRADATION

What is Corrosion?Corrosion is the oxidation of a metal due to an ELECTROCHEMICAL reaction. The oxidizing agent is most often O2 (atmospheric corrosion) or H+ (chemical corrosion) or both.Why is it a problem? Financial - $350 Billion Dollar Annual Problem in U.S. (4.25% of GNP) Department of Defense spends $6 8 Billion

3 Two reactions are necessary: -- oxidation reaction: -- reduction reaction: Other reduction reactions:-- in an acid solution-- in a neutral or base solutionCORROSION OF ZINC IN ACID

4 Two outcomes:--Metal sample mass--Metal sample mass--Metal is the anode (-)--Metal is the cathode (+)(relative to Pt)(relative to Pt)Standard Electrode PotentialSTANDARD HYDROGEN (EMF) TEST

5 EMF series Metal with smaller V corrodes. Ex: Cd-Ni cellmetaloAuCuPbSnNiCoCdFeCrZnAlMgNaK+1.420 V+0.340- 0.126- 0.136- 0.250- 0.277- 0.403- 0.440- 0.744- 0.763- 1.662- 2.262- 2.714- 2.924metalVmetaloDV = 0.153VoSTANDARD EMF SERIES

6CORROSION IN A GRAPEFRUIT

7 Ex: Cd-Ni cell with standard 1M solutions Ex: Cd-Ni cell with non-standard solutionsn = #e-per unitoxid/redreaction(=2 here)F = Faraday'sconstant=96,500C/mol. Reduce VNi - VCd by --increasing X --decreasing YEFFECT OF SOLUTION CONCENTRATION

Ranks the reactivity of metals/alloys in seawaterPlatinumGoldGraphiteTitaniumSilver316 Stainless SteelNickel (passive)CopperNickel (active)TinLead316 Stainless SteelIron/SteelAluminum AlloysCadmiumZincMagnesium8GALVANIC SERIES

9 Uniform AttackOxidation & reductionoccur uniformly oversurface. Selective LeachingPreferred corrosion ofone element/constituent(e.g., Zn from brass (Cu-Zn)). IntergranularCorrosion alonggrain boundaries,often where specialphases exist. Stress corrosionStress & corrosionwork togetherat crack tips. GalvanicDissimilar metals arephysically joined. Themore anodic onecorrodes.(see Table17.2) Zn & Mgvery anodic. Erosion-corrosionBreak down of passivatinglayer by erosion (pipeelbows). PittingDownward propagationof small pits & holes. Crevice Between twopieces of the same metal.FORMS OF CORROSION

Stress & Saltwater... --causes cracks! Heat treatment: slows crack speed in salt water! 4mm--material: 7150-T651 Al "alloy" (Zn,Cu,Mg,Zr)10DETERIORATIVE

Uniform Corrosion: Rust!Prevention:PaintPlateSacrificial anode

Galvanic CorrosionCauses:Dissimilar metals Electrolyte Current PathDescribed by Galvanic SeriesSolutions:Choose metals close in galvanic seriesHave large anode/cathode ratiosInsulate dissimilar metalsUse Cathodic protection

Pitting and Creviced CorrosionPrevention:Weld dont rivetUse non-absorbing gasketsPolish surfacesAdd drains avoid stagnant waterAdjust composition; e.g., add Mo to SSCauses: concentration gradients in electrolyte cause some areas high in ion concentrations that accelerate oxidation

Intergranular CorrosionOccurs in specific alloys precipitation of corrosive specimens along grain boundaries and in particular environmentse.g. : Chromium carbide forming in SS, leaving adjacent areas depleted in CrSolutions: High temp heat treat to redissolve carbides Lower carbon content (in SS) to minimize carbide formation Alloy with a material that has stronger carbide formation (e.g., Ti or Nb)

Erosion CorrosionCauses: abrasive fluids impinging on surfacesCommonly found in piping, propellers, turbine blades, valves and pumpsSolutions: Change design to minimize or eliminate fluid turbulence and impingement effects.Use other materials that resist erosionRemove particulates from fluids

Selective LeachingOccurs in alloys in which one element is preferentially removed e.g., in Brass, Zinc is electrically active and is removed, leaving behind porous CopperOccurs in other metals, such as Al, Fe, Co, CrSolutions:Use protective coating to protect surfacesUse alternative materials

Stress CorrosionAka: stress corrosion crackingCracks grow along grain boundaries as a result of residual or applied stress or trapped gas or solid corrosion productse.g., brasses are sensitive to ammoniaStress levels may be very lowSolutions: Reduce stress levelsHeat treatmentAtmosphere control

Hydrogen EmbrittlementMetals loose strength when Hydrogen is absorbed through surface, especially along grain boundaries and dislocationsOften occurs as a result of decorative platingHigh strength steels particularly susceptibleCan be removed by baking the alloy

11 Self-protecting metals! --Metal ions combine with O to form a thin, adhering oxide layer that slows corrosion. Reduce T (slows kinetics of oxidation and reduction) Add inhibitors --Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). --Slow oxidation reaction by attaching species to the surface (e.g., paint it!). Cathodic (or sacrificial) protection --Attach a more anodic material to the one to be protected.Adapted from Fig. 17.14, Callister 6e.CONTROLLING CORROSION

Corrosion preventionSacrificial AnodeApplied Voltage

Surface coatings & PassivationSome materials, such as Aluminum or Stainless Steel, form oxide barrier coatings that prevent oxidation at active surface this is called passivationSurface can be coated with protective layers: painted, anodized, plated (Caution!!! Cracks in plating or paint can lead to crevice corrosion!)

12 Corrosion occurs due to: --the natural tendency of metals to give up electrons. --electrons are given up by an oxidation reaction. --these electrons then are part of a reduction reaction. Metals with a more negative Standard Electrode Potential are more likely to corrode relative to other metals. The Galvanic Series ranks the reactivity of metals in seawater. Increasing T speeds up oxidation/reduction reactions. Corrosion may be controlled by: -- using metals which form a protective oxide layer -- reducing T-- adding inhibitors-- painting--using cathodic protection.SUMMARY

*********************Stress concentrated at crack tip***********************But they can be distracting if used too much. Poor contrast here. I changed font sizes and size of picture***********************