Lecture 1 Why Study Fracture

FailureFailure

WHY STUDY WHY STUDY FailureFailure??

The engineer has to minimize the The engineer has to minimize the possibility of failure since the design possibility of failure since the design step.step.– Understand the mechanics of the Understand the mechanics of the

various failure modes—various failure modes— fracture, fatigue, and creepfracture, fatigue, and creep

– Be familiar with appropriate design Be familiar with appropriate design principles to prevent in-service failures. principles to prevent in-service failures.

Fracture

The separation of a body into two or more pieces in response to a static stress and at temperatures far below the MP of the material.

The applied stress may be – tensile, – compressive, – shear, – or torsional;

Fracture Based on the ability of a Based on the ability of a

material to experience material to experience plastic deformation, two plastic deformation, two fracture modes are possible: fracture modes are possible: ductile, and brittle fracture.ductile, and brittle fracture.

Ductile materials:Ductile materials:– substantial plastic deformation substantial plastic deformation

with high energy absorption with high energy absorption before fracture. before fracture.

Brittle materials:Brittle materials:– little or no plastic deformation little or no plastic deformation

with low energy absorption with low energy absorption accompanying a brittle accompanying a brittle fracture. fracture.

Fracture

In response to an imposed stress, In response to an imposed stress, any fracture process involves two any fracture process involves two steps:steps:– crack formation crack formation – and propagation. and propagation.

The mechanism of crack propagation The mechanism of crack propagation determine the mode of fracture. determine the mode of fracture.

Ductile Fracture

Ductile fracture:– extensive plastic deformation in the vicinity of

an advancing crack. – proceeds relatively slowly as the crack length

is extended. – often said as stable crack.

it resists any further extension unless there is an increase in the applied stress.

Normally there will be evidence of appreciable gross deformation at the fracture surfaces (e.g., twisting and tearing).

Ductile Fracture

Highly ductile fracture inHighly ductile fracture inwhich the specimen necks which the specimen necks

down to a point.down to a point.

Moderately ductile fracture after some

necking.

Brittle fracture without any

plastic deformation.

Ductile Fracture Normal fracture process stages:

– necking – formation of small cavities (microvoids) in the interior of

the cross section, – as deformation continues, these microvoids enlarge,

come together, and coalesce to form an elliptical crack, which has its long axis perpendicular to the stress

direction.– The crack continues to grow in a direction parallel to its

major axis by this microvoid coalescence process.– Finally, fracture occurs by the rapid propagation of a

crack around the outer perimeter of the neck, by shear deformation at an angle of about 45° with the tensile axis—

this is the angle at which the shear stress is a maximum.

(a) Initial necking.

(b) Small cavity formation.

(c) Coalescence of cavities to form a crack. (d) Crack

propagation.

(e) Final shear fracture at a 45° angle relative to the tensile direction.

Ductile Fracture fracture having this

characteristic surface contour is called a cup-cup-and-cone fractureand-cone fracture – because one of the mating

surfaces is in the form of a cup, the other like a cone.

– In this type of fractured specimen, the central interior region of the surface has an irregular and fibrous appearance, which is indicative of plastic deformation.

Ductile Fracture

Brittle Fracture

Brittle fracture:– cracks may spread

extremely rapidly, – very little plastic

deformation.– said to be unstable crack,

once it started, crack propagation will continue spontaneously without an increase in magnitude of the applied stress.

Brittle Fracture

(a) Photograph showing V-shaped “chevron” markings characteristic of brittlefracture. Arrows indicate origin of crack. Approximately actual size.

Brittle Fracture

(b) Photograph of a brittle fracture surface showing radial fan-shaped ridges. Arrow indicates origin of crack. Approximately 2.

Brittle Fracture

For most brittle crystalline materials, – crack propagation

corresponds to the successive and repeated breaking of atomic bonds along specific crystallographic planes (cleavage).

– Type of fracture: transgranular (or transcrystalline), the fracture cracks pass through

the grains.

Brittle Fracture

Macroscopically, the fracture surface may have a grainy or faceted texture,

as a result of changes in orientation of the cleavage planes from grain to grain.

Brittle Fracture

In some alloys, crack propagation is along grain In some alloys, crack propagation is along grain boundaries (type of fracture: boundaries (type of fracture: intergranularintergranular).).

Fracture Ductile fracture is almost always preferred for

two reasons. First,

– brittle fracture occurs suddenly and catastrophically without any warning; this is a consequence of the spontaneous and rapid crack propagation.

– ductile fracture, the presence of plastic deformation gives warning that fracture is imminent, allowing preventive measures to be taken.

Second, – more strain energy is required to induce ductile

fracture inasmuch as ductile materials are generally tougher.

Fracture

Under the action of an applied tensile stress, – Most metal alloys are ductile, – Ceramics are notably brittle, – Polymers may exhibit both types of

fracture.

PRINCIPLES OF FRACTURE PRINCIPLES OF FRACTURE MECHANICSMECHANICS

Brittle fracture of normally ductile materials, can be explained through the mechanisms of fracture (i.e. the field of fracture mechanics).– quantification of the relationships between

material properties, stress level, the presence of crack-producing flaws, and crack propagation mechanisms.

Design engineers are now better equipped to anticipate, and thus prevent, structural failures.

PRINCIPLES OF FRACTURE MECHANICS:PRINCIPLES OF FRACTURE MECHANICS: Stress Concentration

The measured fracture strengths for most brittle materials are significantly lower than those predicted by theoretical calculations based on atomic bonding energies.– This discrepancy is explained by the presence of

very small, microscopic flaws or cracks that always exist under normal conditions at the surface and within the interior of a body of material.

These flaws are a detriment to the fracture strength because an applied stress may be amplified or concentrated at the tip, the magnitude of this amplification depending on crack orientation and geometry.

The magnitude of the localized stress diminishes with distance away from the crack tip.

At positions far removed, the stress is just the nominal stress.

Due to their ability to amplify an applied stress in their locale, these flaws are sometimes called stress raisers.


Assume that a crack is – an elliptical hole through a plate, – oriented perpendicular to the applied

stress,

the maximum stress, σm , occurs at the crack tip and may be approximated by


Sometimes the ratio σm/σ0 is denoted as the stress concentration factor, Kt

A measure of the degree to which an external stress is amplified at the tip of a crack.


Stress amplification is not restricted Stress amplification is not restricted to microscopic defects; it may occur to microscopic defects; it may occur at macroscopic internal at macroscopic internal discontinuities (e.g., voids), at sharp discontinuities (e.g., voids), at sharp corners, and at notches in large corners, and at notches in large structures.structures.

The effect of a stress raiser is more The effect of a stress raiser is more significant in brittle than in ductile significant in brittle than in ductile materials. materials.


For a ductile material, plastic deformation For a ductile material, plastic deformation ensues when the maximum stress exceeds the ensues when the maximum stress exceeds the yield strength. yield strength. – more uniform distribution of stress in the vicinity of more uniform distribution of stress in the vicinity of

the stress raiser the stress raiser – maximum stress concentration < theoretical value. maximum stress concentration < theoretical value.

Yielding and stress redistribution Yielding and stress redistribution do not occurdo not occur to any appreciable extent around flaws and to any appreciable extent around flaws and discontinuities in brittle materials; discontinuities in brittle materials; – maximum stress concentration = theoretical value.maximum stress concentration = theoretical value.


The critical stress The critical stress σσcc required for required for crack propagation in a brittle crack propagation in a brittle material is described by the material is described by the expressionexpression


All brittle materials contain a population All brittle materials contain a population of small cracks and flaws that have a of small cracks and flaws that have a variety of sizes, geometries, and variety of sizes, geometries, and orientations.orientations.– When the magnitude of a tensile stress at When the magnitude of a tensile stress at

the tip of one of these flaws exceeds the the tip of one of these flaws exceeds the value of this critical stress, a crack forms value of this critical stress, a crack forms and then propagates, which results in and then propagates, which results in fracture. fracture. Very small and virtually defect-free metallic and Very small and virtually defect-free metallic and

ceramic whiskers have been grown with fracture ceramic whiskers have been grown with fracture strengths that approach their theoretical values.strengths that approach their theoretical values.


Fracture Toughness

A measure of a material’s resistance A measure of a material’s resistance to brittle fracture when a crack is to brittle fracture when a crack is presentpresent

For relatively thin specimens, For relatively thin specimens, fracture toughness depend on fracture toughness depend on specimen thickness.specimen thickness.

Fracture Toughness For specimen with thickness >> the crack

dimensions, fracture toughness independent of thickness; – a condition of plane strain exists. – when a load operates on a crack in the mode I,

there is no strain component perpendicular to the front and back faces.

– The value is known as the plane strain fracture toughness, KIc

Fracture Toughness

Brittle materials, Brittle materials, for which appreciable for which appreciable plastic deformation is not possible in front plastic deformation is not possible in front of an advancing crackof an advancing crack, , – have low have low KKIcIc values and values and – are vulnerable to catastrophic failure. are vulnerable to catastrophic failure.

Ductile materials, Ductile materials, – have relatively large have relatively large KKIcIc values. values.

Usage of fracture mechanics:Usage of fracture mechanics:– predicting catastrophic failure of materials predicting catastrophic failure of materials

having intermediate ductility's. having intermediate ductility's.

Fracture Toughness

KIc depends on many factors, the most influential of are:– temperature, – strain rate, – microstructure.

KIc diminishes with increasing strain rate and decreasing temperature.

Fracture Toughness

Yield strength enhancement:Yield strength enhancement:– by solid solution or dispersion additions by solid solution or dispersion additions – or by strain hardening or by strain hardening

generally decrease generally decrease KKIcIc. .

KKIcIc normally normally increasesincreases with with reductionreduction in grain size in grain size – at constant composition and other micro at constant composition and other micro

structural variables.structural variables.

Flaw test Flaw test

There is a number of nondestructive There is a number of nondestructive test (NDT) techniques to detect and test (NDT) techniques to detect and measure both internal and surface measure both internal and surface flaws.flaws.– These techniques does not destroy the These techniques does not destroy the

material/structure being examined. material/structure being examined. – Some testing methods must be Some testing methods must be

conducted in a laboratory setting; others conducted in a laboratory setting; others may be adapted for use in the field. may be adapted for use in the field.

Flaw testFlaw test

IMPACT FRACTURE TESTINGIMPACT FRACTURE TESTING

Impact test conditions were chosen Impact test conditions were chosen to represent those most severe to represent those most severe relative to the potential for fracture: relative to the potential for fracture: – (1) deformation at a relatively low (1) deformation at a relatively low

temperature, temperature, – (2) a high strain rate (i.e., rate of (2) a high strain rate (i.e., rate of

deformation), and deformation), and – (3) a triaxial stress state (may be (3) a triaxial stress state (may be

introduced by the presence of a notch).introduced by the presence of a notch).


Two standardized tests were designed to Two standardized tests were designed to measure the measure the impact energy impact energy ((notch notch toughness):toughness): – CharpyCharpy ( (commonly used in the USA)commonly used in the USA)– IzodIzod. .

One of the primary functions is to One of the primary functions is to determine whether or not a material determine whether or not a material experiences a experiences a ductile-to-brittle ductile-to-brittle transitiontransition with decreasing temperature.with decreasing temperature.– If so, what is the range of temperatures over If so, what is the range of temperatures over

which it occurs.which it occurs.


The ductile-to-brittle transition is related to The ductile-to-brittle transition is related to – the temperature dependence of the measured the temperature dependence of the measured

impact energy absorption. impact energy absorption. – curve curve A A in Figure 8.13 for a steel.in Figure 8.13 for a steel.

At higher temperatures the CVN energy is At higher temperatures the CVN energy is relatively large, in correlation with a relatively large, in correlation with a ductile mode of fracture. ductile mode of fracture. – As the temperature is lowered, the impact As the temperature is lowered, the impact

energy drops suddenly over a relatively narrow energy drops suddenly over a relatively narrow temperature range, below which the energy temperature range, below which the energy has a constant but small value; that is, the has a constant but small value; that is, the mode of fracture is brittle.mode of fracture is brittle.

Temperature dependence of the Charpy V-notch impact Temperature dependence of the Charpy V-notch impact energy (curve A) and percent shear fracture (curve B) for an energy (curve A) and percent shear fracture (curve B) for an A283 steel.A283 steel.



Schematic curves for the three general types of impactSchematic curves for the three general types of impactenergy-versus-temperature behavior.energy-versus-temperature behavior.

Influence of carbon content on the Charpy V-notch Influence of carbon content on the Charpy V-notch energy versus temperature behavior for steel.energy versus temperature behavior for steel.

FatigueFatigue a form of failure that occurs in structures a form of failure that occurs in structures

subjected to dynamic and fluctuating stresses. subjected to dynamic and fluctuating stresses. – Under these circumstances it is possible for failure Under these circumstances it is possible for failure

to occur at a stress level considerably lower than to occur at a stress level considerably lower than the tensile or yield strength for a static load. the tensile or yield strength for a static load.

The term “fatigue” is used because this type The term “fatigue” is used because this type of failure normally occurs after a lengthy of failure normally occurs after a lengthy period of repeated stress or strain cycling. period of repeated stress or strain cycling.

Fatigue is largest cause of failure in metals, Fatigue is largest cause of failure in metals, estimated approximately 90% of all metallic estimated approximately 90% of all metallic failures. failures.

FatigueFatigue Polymers and ceramics (except for glasses) Polymers and ceramics (except for glasses)

are also susceptible to this type of failure. are also susceptible to this type of failure. – Fatigue is catastrophic and insidious, occurring Fatigue is catastrophic and insidious, occurring

very suddenly and without warning.very suddenly and without warning.– Fatigue failure is brittle-like in nature even in Fatigue failure is brittle-like in nature even in

normally ductile metals, in that there is very normally ductile metals, in that there is very little, if any, gross plastic deformation little, if any, gross plastic deformation associated with failure. associated with failure.

The process occurs by the initiation and The process occurs by the initiation and propagation of cracks, and ordinarily the propagation of cracks, and ordinarily the fracture surface is perpendicular to the fracture surface is perpendicular to the direction of an applied tensile stress.direction of an applied tensile stress.

Fatigue: THE Fatigue: THE S–N S–N CURVECURVE

A schematic diagram of a rotating-bending test


The specimen is subjected to a relatively The specimen is subjected to a relatively large maximum stress cycling amplitude, large maximum stress cycling amplitude, – usually on the order of two thirds of the static usually on the order of two thirds of the static

tensile strength; tensile strength; The number of cycles to failure is counted. The number of cycles to failure is counted.

– This procedure is repeated on other specimens This procedure is repeated on other specimens at progressively decreasing maximum stress at progressively decreasing maximum stress amplitudes. amplitudes.

Stress Stress S S vs. log(vs. log(N N to failure) for each of the to failure) for each of the specimens are plotted. specimens are plotted.


Two distinct types of Two distinct types of S–N S–N behavior are:behavior are:– Materials with fatigue limitMaterials with fatigue limit– Materials with no fatigue limit.Materials with no fatigue limit.

Fatigue: THE Fatigue: THE S–N S–N CURVECURVE Fatigue limitFatigue limit (the (the

endurance limitendurance limit): ): below which fatigue below which fatigue failure will not occur. failure will not occur. – The largest value of The largest value of

fluctuating stress that fluctuating stress that will will notnot cause failure for cause failure for essentially an infinite essentially an infinite number of cycles. number of cycles.

– For many steels, fatigue For many steels, fatigue limits range between limits range between 35% and 60% of the 35% and 60% of the tensile strength.tensile strength.


The higher the The higher the magnitude of the magnitude of the stress, the smaller stress, the smaller the number of the number of cycles sustained cycles sustained before failure.before failure.


Statistic representation of Statistic representation of S-NS-N curve curve

high-cycle fatiguehigh-cycle fatigue

low-cycle fatiguelow-cycle fatigue

Fatigue: CRACK INITIATION Fatigue: CRACK INITIATION AND PROPAGATIONAND PROPAGATION

The process of fatigue failure is The process of fatigue failure is characterized by three distinct steps: characterized by three distinct steps: – (1) crack initiation: a small crack forms at (1) crack initiation: a small crack forms at

some point of high stress concentration;some point of high stress concentration;– (2) crack propagation: crack advances (2) crack propagation: crack advances

incrementally with each stress cycle; incrementally with each stress cycle; – (3) final failure: occurs very rapidly once (3) final failure: occurs very rapidly once

the advancing crack has reached a the advancing crack has reached a critical size. critical size.


Cracks associated with fatigue failure Cracks associated with fatigue failure almost always initiate (or nucleate) on the almost always initiate (or nucleate) on the surface of a component at some point of surface of a component at some point of stress concentration.stress concentration.

Crack nucleation sites include:Crack nucleation sites include:– surface scratches, sharp fillets, keyways, surface scratches, sharp fillets, keyways,

threads, dents, and the like. threads, dents, and the like. – cyclic loading can produce microscopic surface cyclic loading can produce microscopic surface

discontinuities resulting from dislocation slip discontinuities resulting from dislocation slip steps that may act as stress raisers, and steps that may act as stress raisers, and therefore as crack initiation sites.therefore as crack initiation sites.


Two types of markings of a fracture surface Two types of markings of a fracture surface that formed during the crack propagation that formed during the crack propagation step:step:– beachmarksbeachmarks– striationsstriations..

Both of these features indicate the position of Both of these features indicate the position of the crack tip at some point in time and the crack tip at some point in time and appear as concentric ridges that expand appear as concentric ridges that expand away from the crack initiation site(s), away from the crack initiation site(s), frequently in a circular or semicircular frequently in a circular or semicircular pattern. pattern.


Beachmarks (sometimes also called “clamshell marks”) are of macroscopic dimensions, – may be observed with the

unaided eye. – found on components

that experienced interruptions during the crack propagation stage

for example: a machine that operated only during normal work-shift hours. Each beachmark band represents a period of time over which crack growth occurred.




Final comment: – Beachmarks and striations will not

appear on thet region over which the rapid failure occurs.

– Rather, the rapid failure may be either ductile or brittle; evidence of plastic deformation will be

present for ductile, and absent for brittle, failure.

FACTORS THAT AFFECT FATIGUE LIFE

Mean Stress


Surface Effects– Design Factors

FACTORS THAT AFFECT FATIGUE LIFE Surface EffectsSurface Effects

– Surface TreatmentsSurface Treatments Surface markings can limit the Surface markings can limit the

fatigue life. fatigue life. Surface finish that will Surface finish that will

improves the fatigue life:improves the fatigue life:– polishing polishing – Shot peeningShot peening: Introducing : Introducing

residual compressive stresses residual compressive stresses into the ductile metals into the ductile metals mechanically by localized plastic mechanically by localized plastic deformation within the outer deformation within the outer surface region. surface region.


Surface EffectsSurface Effects – Case hardening: a

technique by which both surface hardness and fatigue life are enhanced for steel alloys. a component is exposed to a

carbonaceous (carburizing) or nitrogenous (nitriding) atmosphere at an elevated temperature.

A carbon- or nitrogen-rich outer surface layer (or “case”) is introduced by atomic diffusion from the gaseous phase, normally on the order of 1 mm deep.

Case

Core


ENVIRONMENTAL EFFECTSENVIRONMENTAL EFFECTS– Thermal fatigueThermal fatigue

is normally induced at elevated temperatures by is normally induced at elevated temperatures by fluctuating thermal stresses; fluctuating thermal stresses;

– mechanical stresses from an external source need not mechanical stresses from an external source need not be present.be present.

The origin of thermal stresses is the The origin of thermal stresses is the restraintrestraint to the to the dimensional expansion and/or contraction that would dimensional expansion and/or contraction that would normally occur in a structural member with variations normally occur in a structural member with variations in temperature. in temperature.

– The magnitude of a thermal stress developed by a The magnitude of a thermal stress developed by a temperature change is dependent on the coefficient of temperature change is dependent on the coefficient of thermal expansion and the modulus of elasticity thermal expansion and the modulus of elasticity E E ..


ENVIRONMENTAL EFFECTS– Corrosion fatigue:

Failure that occurs by the simultaneous action of a cyclic stress and chemical attack.

– Corrosive environments have a deleterious influence and produce shorter fatigue lives. Even the normal ambient atmosphere will affect the fatigue behavior of some materials.

– Small pits may form as a result of chemical reactions between the environment and material,

which serve as points of stress concentration and therefore as crack nucleation sites.

– Crack propagation rate is enhanced in the corrosive environment.– The nature of the stress cycles will influence the fatigue behavior;

for example, lowering the load application frequency leads to longer periods during which the opened crack is in contact with the environment and to a reduction in the fatigue life.

CreepCreep CreepCreep is a time-dependent and permanent is a time-dependent and permanent

deformation of materials when subjected to a deformation of materials when subjected to a constant load or stress.constant load or stress.

Creep often occur when materials are placed in Creep often occur when materials are placed in service at elevated temperatures and exposed to service at elevated temperatures and exposed to static mechanical stresses static mechanical stresses – (e.g., turbine rotors in jet engines and steam generators (e.g., turbine rotors in jet engines and steam generators

that experience centrifugal stresses, and high-pressure that experience centrifugal stresses, and high-pressure steam lines). steam lines).

Creep is normally an undesirable phenomenon and Creep is normally an undesirable phenomenon and is often the limiting factor in the lifetime of a part. is often the limiting factor in the lifetime of a part. – It is observed in all materials types; for metals it becomes It is observed in all materials types; for metals it becomes

important only for temperatures > important only for temperatures > ±±0.40.4TmTm ( absolute ( absolute melting temperature).melting temperature).

– Amorphous polymers, which include plastics and rubbers, Amorphous polymers, which include plastics and rubbers, are especially sensitive to creep deformation.are especially sensitive to creep deformation.

CreepCreep GENERALIZED

CREEP BEHAVIOR– A typical creep test

consists of subjecting a specimen to a constant load or stress while maintaining the temperature constant; deformation or strain is measured and plotted as a function of elapsed time.

Creep: Creep: STRESS AND TEMPERATURE EFFECTS

Both temperature and the Both temperature and the level of the applied stress level of the applied stress influence the creep influence the creep characteristics. characteristics. – At a temperature < 0.4At a temperature < 0.4TmTm, and , and

after the initial deformation, the after the initial deformation, the strain is virtually independent of strain is virtually independent of time.time.

– With either increasing stress or With either increasing stress or temperature, the following will temperature, the following will be noted: be noted: 1)1) the instantaneous strain increases, the instantaneous strain increases, 2)2) the steady-state creep rate is the steady-state creep rate is

increased,increased,3)3) the rupture lifetime is diminished.the rupture lifetime is diminished.

Creep: STRESS AND Creep: STRESS AND TEMPERATURE EFFECTSTEMPERATURE EFFECTS

Stress (logarithmic scale) versus rupture Stress (logarithmic scale) versus rupture lifetime (logarithmic scale) for a low lifetime (logarithmic scale) for a low carbon–nickel alloy at three temperatures.carbon–nickel alloy at three temperatures.

Logarithm stress Logarithm stress versus the Larson–versus the Larson–Miller parameter for Miller parameter for

an S-590 iron.an S-590 iron.

Creep: STRESS AND Creep: STRESS AND TEMPERATURE EFFECTSTEMPERATURE EFFECTS

Creep: ALLOYS FOR HIGH-Creep: ALLOYS FOR HIGH-TEMPERATURE USETEMPERATURE USE

There are several factors that affect There are several factors that affect the creep characteristics of metals, the creep characteristics of metals, e.g.: e.g.: – melting temperature, melting temperature, – elastic modulus, elastic modulus, – grain size. grain size.


In general, In general, – the higher the melting temperature, the higher the melting temperature, – the greater the elastic modulus, the greater the elastic modulus, – and the larger the grain size, and the larger the grain size,

the better is a material’s resistance to creep. the better is a material’s resistance to creep. Relative to grain size, smaller grains permit Relative to grain size, smaller grains permit

more grain-boundary sliding, which results in more grain-boundary sliding, which results in higher creep rates. higher creep rates. – This effect is in opposite to the influence of grain This effect is in opposite to the influence of grain

size on the mechanical behavior at low size on the mechanical behavior at low temperatures [i.e., increase in both strength and temperatures [i.e., increase in both strength and toughness.toughness.


Materials especially resilient to creep in Materials especially resilient to creep in high temperature service applications:high temperature service applications:– Stainless steels, Stainless steels, – the refractory metals, the refractory metals, – the superalloys. the superalloys.

The creep resistance of the cobalt and The creep resistance of the cobalt and nickel superalloys is enhanced nickel superalloys is enhanced – by solid-solution alloying, by solid-solution alloying, – and by the addition of a dispersed phase that and by the addition of a dispersed phase that

is virtually insoluble in the matrix. is virtually insoluble in the matrix.


Advanced processing techniques have Advanced processing techniques have been utilized; been utilized; – one such technique is directional one such technique is directional

solidification, which produces either highly solidification, which produces either highly elongated grains or single-crystal elongated grains or single-crystal components.components.

Another is the controlled unidirectional Another is the controlled unidirectional solidification of alloys having specially solidification of alloys having specially designed compositions wherein two-designed compositions wherein two-phase composites result.phase composites result.


(a)(a) Polycrystalline turbine blade that was produced by a conventional casting Polycrystalline turbine blade that was produced by a conventional casting technique. technique.

(b)(b) High-temperature creep resistance is improved as a result of an oriented High-temperature creep resistance is improved as a result of an oriented columnar grain structure produced by a sophisticated directional columnar grain structure produced by a sophisticated directional solidification technique. solidification technique.

(c)(c) Creep resistance is further enhanced when single-crystal blades are used. Creep resistance is further enhanced when single-crystal blades are used. (Courtesy of Pratt & Whitney.)(Courtesy of Pratt & Whitney.)

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Lecture 1 Why Study Fracture

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