mme 323 materials science week 11-12 - failure

MME 323: MATERIALS SCIENCE WEEK 11-12:

FAILURE*

Adhi Primartomo, PhDEmail: [email protected]: Room 191 – JIC Academic Building* Source: Materials Science and Engineering; 9th Edition; W.D.Callister;

Wiley; 2011

https://sites.google.com/site/primartomo/file-cabinet

WHY STUDY FAILURE?(page 286)

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• The design of a component of structure often callsupon the engineer to minimize the possibility offailure.

• It is important to understand the mechanics of thevarious failure modes: fracture, fatigue and creep.

• Be familiar with appropriate design principles that aybe employed to prevent in-service failures

INTRODUCTION(page 286)

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• The failure of engineering materials is almost always anundesirable event for several reasons: putting human lives injeopardy, causing economic losses and interfering with theavailability of products and services.

• Prevention of failure is difficult to guarantee.

• Usual causes of failure: improper materials selection &processing, inadequate design of the component, its misuse.

• Regular inspection and repair/replacement are critical to safedesign.

• It is the responsibility of engineer to anticipate and plan forpossible failure.

• In the event of failure does occur, engineer needs to asses itscause and take appropriate preventive measures against futureincidents.

FUNDAMENTAL OF FRACTURE(page 287)

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• Simple fracture the separation of a body into two or more

pieces in response to an imposes stress that is static and at low temperature relative to its melting points.

• Fracture can also occur from fatigue and creep.

• Present discussion, fracture result from: uniaxial tensile load.

• Two fracture modes (classification based on an ability of material to experience plastic deformation):

1. Ductile Fracture

2. Brittle Fracture

• Any fracture process involves two steps: crack formation and crack propagation – in response to an imposed stress.

• The mode of fracture is highly dependent on the mechanism of crack propagation.

DUCTILE FRACTURE (page 287-288)

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• Ductile metal typically exhibit substantial plastic deformation withhigh energy absorption before fracture.

• Ductile fracture characterizes by extensive plastic deformation in thevicinity of advancing cracks.

• The process proceeds relatively slowly as the crack length extended.Such a crack stable: it resists any further extension unless there isan increase in the applied stress.

• Ductile fracture is always preferred to brittle fracture for two reasons:

1. Brittle fracture occurs suddenly and catastrophically without anywarning; this is a consequence of the spontaneous and rapidcrack propagation. For ductile fracture, the presence of plasticdeformation gives warning that failure in imminent, allowingpreventive measures to be taken.

2. More strain energy is required to induced ductile fractureinasmuch as there materials are generally tougher.

DUCTILE FRACTURE (page 287-288)

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(a). Highly ductile fracture;(b). Moderate ductile fracture.

BRITTLE FRACTURE(page 287, 289-291)

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• Normally little or no plastic deformation with low energyabsorption accompanying a brittle fracture.

• Cracks may be spread extremely rapid with very littleaccompanying plastic deformation. Such a crack unstable:

crack propagation, once started, continuous spontaneouslywithout an increase in value of applied stress.

• Direction of cracks motion in nearly perpendicular to directionof the applied tensile and result in a relatively flat fracturesurface.

BRITTLE FRACTURE(page 287, 289-291)

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(a). Cup & Cone Ductile Fracture; (b). Brittle fracture.

PRINCIPLE OF FRACTURE MECHANICS(page 291-297)

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Fracture Mechanics quantification of the relationships among:

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

Stress Concentrations:

• The measured fracture strengths for most materialssignificantly lower than predicted by theoretical calculationsdue to the presence of microscopic flaws/cracks exist undernormal condition at the surface or in interior of the body.

• These flaws/cracks are detriment to the fracture strengthbecause an applied stress may be amplified/concentrated atthe tip. The magnitude of this amplification depending on thecrack orientation and dimension.

• These flaws/cracks stress raisers.


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The magnitude of this localized stress decreases with distance awayfrom the crack tip. At positions far away, the stress just the nominal

stress σ0 or the applied load divided by specimen cross-section area.


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Computation of Maximum Stress (σm) at a crack tip:

For a relatively long micro-crack, stress concentration factor (Kt):

Stress concentration factor (Kt) measure the degree to which anexternal stress is amplified at the tip of a crack.

Stress amplification may occur at: microscopic defects, internaldiscontinuities (voids/inclusions), sharp corners, scratches and notches.


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• The effects of a stress raiser is more significant in brittle thanin ductile materials.

• In ductile materials, plastic deformation ensues whenmaximum stress exceeds the yield strength. This leads to amore uniform distribution of stress in the vicinity of the stressraiser and to the development of a maximum stressconcentration factor less than the theoretical value.

• Such yielding and stress redistribution do not occur to anyappreciable extent around flaws in brittle materials.

• The critical stress (σc) required for crack propagation in abrittle materials:


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• All brittle materials contain a population of small flaws thathave variety of sizes. When the magnitude of tensile stress atthe tip of one of these flaws exceeds the value this criticalstress, a crack forms propagates which results in fracture.

• Very small and virtually defect-free metallic and ceramicmaterials have been produced with fracture strength thatapproach their theoretical values.


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• Fracture toughness (Kc) an expression to relate critical

stress for crack propagation (σc) and crack length (a):

• Plain Strain Fracture toughness (KIc) specimen thickness is

much greater than the crack dimension:


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The three modes of crack surface displacement:

(a). Mode I: tensile mode;

(b). Mode II: Sliding mode;

(c). Mode III: Tearing mode.


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• Brittle materials have low KIc value and vulnerable tocatastrophic failure.

• Ductile materials have relatively high Kic value.

• Fracture mechanics is particularly useful in predictingcatastrophic failure in materials having intermediate ductility.

• Kic depends on: temperature, strain rate and microstructure.


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Design using Fracture Mechanics:

• Variable to be considered to the possibility for fracture of acomponent: Kic , σ and a.

• It is important to decide which of these variables areconstrained and which are subjected to design control.

• Computation of design stress:

• Computation of maximum allowable flaw length:

FRACTURE TOUGHNESS TESTING(page 299-303)

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Impact testing technique used to measure the impact energy

(notch toughness).


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Ductile-to-Brittle transition:

• Primary function of impact test is to determine whether amaterial experiences a ductile to brittle transition withdecreasing temperature and if so, the range of temperatureover which it occurs.

• Widely used steels can exhibit this ductile to brittle transitionwith disastrous consequences.


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• At higher temperatures, the impact energy is relatively large,corresponding to a ductile mode of fracture.

• As the temperature is lowered, the impact energy drops suddenlyover a relatively narrow temperature range which corresponding tothe mode of brittle fracture.


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FATIGUE(page 304 - 315 )

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• Fatigue a form of failure that occurs in structuressubjected to dynamic and fluctuating stresses.

• Failure occurs at a stress level considerably lower than yieldstrength of a static load.

• The term of fatigue is used because this type of failureoccurs after lengthy period of repeated stress.

• Estimated to be involved about 90% of all metallic, polymerand ceramics (except glass failure).

• Fatigue failure is brittle-like even for ductile metals. It iscatastrophic, occurs very sudden, with very little (if any)plastic deformation and typically fracture surface isperpendicular to direction of applied stress.


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Three different fluctuating stress-time modes:

1. Reversed Stress Cycle:

Cyclic Stresses: (page 304 - 305)

• Amplitude is symmetrical about a mean zero stress level.

• Alternating from σmax to σmin of equal magnitude.


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2. Repeated Stress Cycle:

Cyclic Stresses:(page 304 - 305)

• σmax and σmin asymmetrical relative to zero stress level.


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3. Random Stress Cycle:


• Stress level vary randomly in amplitude and frequency.


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Parameters used to characterize the fluctuating stress cycle:

1. Mean Stress (σm):


2. Range of Stress (σr) :

3. Stress Amplitude (σa) :

4. Stress Ratio (R) :


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S-N Curve:(page 306 - 310)

• Most common fatigue test Rotating beam bending:

alternating tension and compression stresses of equalmagnitude on specimen as it simultaneously bend and rotate.

• Simulate the Reverse Stress Cycle (R= -1).

• During rotation, lower surface: tensile; upper surface:compression.


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• A series of tests is commenced by subjecting a specimento stress cycling at relatively high σmax (two-third of staticσyield). Number of cycles to failure is counted andrecorded.

• This procedure is repeated on other specimens atprogressively decreasing maximum stress levels.

• Data are plotted as stress (S) vs. logarithm of number Nof cycles to failure for each of the specimens.

• S parameter is either σmax or σa.

• The higher the magnitude of the stress, the smallernumber of cycles the material capable of sustainingbefore fracture.


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Two distinct types of S-N curves:

1. A material that display a fatigue limit most ferrous alloy:

• The curve becomes horizontal at higher N values –> fatiguelimit (endurance limit); below this fatigue will not occur.


FATIGUE (page 304 – 315)

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Two distinct types of S-N curves:

2. A material that doesn’t display a fatigue limit most non ferrous

alloy:

• The curve continuous downward trend at increasingly N values –> NOfatigue limit.


Fatigue strength: stress level at which failure occur for specific N.

Fatigue life (Nf): number of cycles to cause failure at a specified stress level.


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Example Problem 10.2:(page 309)

For cylindrical bar with diameter d0 ,maximum stress for rotating bending tests:

Calculation of Maximum Load to Avoid Fatigue in Rotating Bending Test

s =16FL

pd0

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σ= maximum stress = fatigue limit;

F = maximum applied load;

L = distance between two loadbearingpoint


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Example Problem 10.3:(page 310)

Calculation of Minimum Specimen Diameter to Yield a SpecifiedFatigue Lifetime:


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Factors that Affect Fatigue Life:(page 312 - 313)

1. Mean Stress.

2. Surface Effects.


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Factors that Affect Fatigue Life:(page 312 - 313)

3. Design Factors.

4. Surface Treatments.


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Methods to Increase Fatigue Performance:(page 313 - 314)

1. Imposing residual compressive stress within a thin outersurface layer.

Shot peening: ?

2. Case hardening.


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Environmental Effects:(page 314 - 315)

1. Thermal Fatigue:

• Occur at elevated temperature by fluctuating thermal stress.

• Cause by the restraint to dimensional expansion/contraction.

• To prevent: eliminate or reduce the restrain source.

2. Corrosion Fatigue:

• Occur by simultaneously action of a cyclic stress and chemical

attack.

• To prevent: reduce the rate of corrosion, apply protective

surface coatings, select a more corrosion resistance material,

reduce the corrosiveness of the environment, reduce the

applied tensile stress level, impose residual compressive stress

on the surface.

Two types of Environment-assisted Fatigue Failure:

CREEP(page 315 - 319 )

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• Creep time-dependent and permanent deformation of

materials when subjected to a constant low load or stress.

• It becomes important only for temperatures higher than 0.4Tm.

Minimum/steady-state creep rate

(εs): slope of the 2nd portion of

creep curve. It’s considered for long life applications.

For shorter life applications time

to rupture or rupture lifetime (tr).

The knowledge of these two creep characteristics allows the engineer to ascertain its suitability for a specific application.

CREEP (page 315 - 319 )

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1. The instantaneous strain at the time of stress applicationincreases.

2. Steady-state creep rate increases.

3. Rupture lifetime decrease.

With increasing stress or temperature:

Stress and Temperature Effects

CREEP (page 315 - 319 )

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Stress vs. Rupture Lifetime:

CREEP (page 315 - 319 )

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Creep Rate vs. Stress:

Dependence of creep rate on stress:

Dependence of creep rate on stress & temperature:

CREEP (page 315 - 319 )

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Example Problem 10.4: (page 318)

CREEP (page 315 - 319 )

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Alloys for High Temperature Use

• Factors affects creep characteristics of metals: meltingtemperature, elastic modulus and grain size.

• Stainless steels and Superalloys: resilience to creep andused in high-temperature service applications.

• Process to enhance creep resistance of metal alloy:

1. Solid-solution alloying.

2. Formation of precipitate phases.

3. Advanced process technique: directional solidification.

QUESTIONS AND PROBLEMS(page 324-325)

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Principle of Fracture Mechanics:

Questions: 10.1 – 10.10

Fracture Toughness Testing:

Questions: 10.12 – 10.13

Cyclic Stresses – The S-N Curve:

Questions: 10.14 – 10.22

Stress and Temperature Effects:

Questions: 10.28 – 10.31, 10.33 – 10.35

mme 323 materials science week 11-12 - failure

Education

moderate ductile fracture

cup cone ductile fracture

flat fracture surface

measured fracture strengths

stress concentrations

stress level

value of applied stress

nominal stress