Failure Analysis

Failure Analysis

Dr. B. AnilPrincipal, GEC Wayanad

(Chairman, ISTE Kerala Section)

• There will be no design without flaw and there will be no construction without defect.

• Failures sometimes occur. • In several cases the aftermath of failures have

a significant impact to the people safety and economic risk.

Liberty Ships

Liberty Ships• Between 1941-45, 2751 liberty ships were

manufactured. These are the first ships to be made by welding.

• 400 of them developed Hull and deck fractures.

• Some of them broke into two. • The low temperature of the North Atlantic

caused the steel to be brittle• The cracks formed at stress raisers.

Tacoma Narrows Bridge

Tacoma Narrows bridge

• The Tacoma Narrows Bridge opened to traffic on July 1, 1940.

• Its main span collapsed into the Tacoma Narrows four months later on November 7, 1940

• Usually cited as an example of elementary forced resonance with the wind providing an external periodic frequency that matched the natural structural frequency

• The failure attributed to aeroelastic flutter caused by a 42 mph (68 km/h) wind.

• Another reason was due to its solid sides, not allowing wind to pass through the bridge's deck.

Space shuttle Challenger

Space shuttle Challenger disaster

• Occurred on January 28, 1986, when Space Shuttle Challenger (mission STS-51-L) broke apart 73 seconds into its flight, leading to the deaths of its seven crew members.

• The O-ring failure caused a breach in the SRB joint it sealed

The Joint and O rings

Aloha Airlines Flight 243

Aloha Airlines Flight 243 • On April 28, 1988, a Boeing 737-297 serving Aloha Airlines

Flight 243 (AQ 243, AAH 243) between Hilo and Honolulu in Hawaii suffered extensive damage after an explosive decompression in flight

• It was able to land safely the accident was caused by metal fatigue exacerbated by crevice corrosion (the plane operated in a coastal environment, with exposure to salt and humidity).

• Theroot cause of the problem was failure of an epoxy adhesive used to bond the aluminum sheets of the fuselage together when the B737 was manufactured. Water was able to enter the gap where the epoxy failed to bond the two surfaces together properly, and started the corrosion process.

American Airlines Flight 587

• The vertical stabilizer of the plane was torn apart during take off

• The plane flew in the wake of another larger aircraft which caused excessive vertical stabilizer movements(flutter)

• The pilot gave additional compensating commands which resulted in excessive stresses far beyond the design values causing fatigue failure

WHY STUDY Failure?

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

modes—• fracture, fatigue, and creep

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

Stop Insanity

• Failures do occur• Investigations teach many lessons• Earlier failed parts were disposed of without

proper analysis• Forensic Analysis of failed parts is an emerging

field• “Insanity” in Chinese means ‘When we do the

same thing again and again and expect a different result’

Every Accident provides an opportunity

• Industry gain a valuable experience every time an accident occurs.

• There are always opportunities to improve operation procedures, value perceptions, design, technical code revisions, and regulatory improvements.

Failures are caused by human errors

• Errors of knowledge– Involve insufficient knowledge, education,

training, experience • Errors of performance• Errors of intent(greed)

Root Cause Analysis tools– You can’t act on issues– But You can act on root causes of the problem

• Failure Mode and Effects Analysis• Sequence of Events Analysis• Cause and Effect Analysis• Fault Tree Analysis

• 5 Whys Analysis (Why-Why Analysis)– Should – Actual (Compare What should have been

with the Actual and keep asking why till the root cause is isolated)

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;

Simple axial failures

Fracture• Based on the ability of a material to

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

• Ductile materials:– substantial plastic deformation with

high energy absorption before fracture.

• Brittle materials:– little or no plastic deformation with

low energy absorption accompanying a brittle fracture.

Aluminum brittle fracture

Ductile fracture

Fracture

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

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

Crack propagation modes

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 inwhich the specimen necks 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) Crackpropagation.

(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-and-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.

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

• 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

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

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 MECHANICS

• 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: 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.

PRINCIPLES OF FRACTURE MECHANICS: Stress Concentration

• 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

PRINCIPLES OF FRACTURE MECHANICS: Stress Concentration

• 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.

PRINCIPLES OF FRACTURE MECHANICS: Stress Concentration

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

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

PRINCIPLES OF FRACTURE MECHANICS: Stress Concentration

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

stress raiser – maximum stress concentration < theoretical value.

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

PRINCIPLES OF FRACTURE MECHANICS: Stress Concentration

• The critical stress σc required for crack propagation in a brittle material is described by the expression

PRINCIPLES OF FRACTURE MECHANICS: Stress Concentration

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

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

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

Fracture Toughness

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

• For relatively thin specimens, fracture toughness depend on 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, for which appreciable plastic deformation is not possible in front of an advancing crack, – have low KIc values and – are vulnerable to catastrophic failure.

• Ductile materials, – have relatively large KIc values.

• Usage of fracture mechanics:– predicting catastrophic failure of materials 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:– by solid solution or dispersion additions – or by strain hardening

generally decrease KIc.

• KIc normally increases with reduction in grain size – at constant composition and other micro

structural variables.

Fatigue

• a form of failure that occurs in structures subjected to dynamic and fluctuating stresses. – Under these circumstances it is possible for failure to occur

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

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

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

Fatigue

• Polymers and ceramics (except for glasses) are also susceptible to this type of failure. – Fatigue is catastrophic and insidious, occurring very

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

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

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

Fatigue: THE S–N CURVE

• A schematic diagram of a rotating-bending test

Fatigue: THE S–N CURVE

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

strength;

• The number of cycles to failure is counted. – This procedure is repeated on other specimens at

progressively decreasing maximum stress amplitudes.

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

Fatigue: THE S–N CURVE

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

Fatigue: THE S–N CURVE• Fatigue limit (the endurance

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

stress that will not cause failure for essentially an infinite number of cycles.

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

Fatigue: THE S–N CURVE

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

Fatigue: THE S–N CURVE

• Statistic representation of S-N curve

high-cycle fatigue

low-cycle fatigue

Fatigue: CRACK INITIATION AND PROPAGATION

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

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

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

advancing crack has reached a critical size.

Fatigue: CRACK INITIATION AND PROPAGATION

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

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

and the like. – cyclic loading can produce microscopic surface

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

Fatigue: CRACK INITIATION AND PROPAGATION

• Two types of markings of a fracture surface that formed during the crack propagation step:– beachmarks– striations.

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

Fatigue: CRACK INITIATION AND PROPAGATION

• 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.

Fatigue: CRACK INITIATION AND PROPAGATION

• Final comment: – Beachmarks will not appear on the 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

FACTORS THAT AFFECT FATIGUE LIFE

• Surface Effects– Design Factors

FACTORS THAT AFFECT FATIGUE LIFE

• Surface Effects– Surface Treatments• Surface markings can limit the fatigue

life. • Surface finish that will improves the

fatigue life:– polishing – Shot peening: Introducing residual

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

FACTORS THAT AFFECT FATIGUE LIFE

• Surface 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

FACTORS THAT AFFECT FATIGUE LIFE

• ENVIRONMENTAL EFFECTS– Thermal fatigue

• is normally induced at elevated temperatures by fluctuating thermal stresses; – mechanical stresses from an external source need not be present.

• The origin of thermal stresses is the restraint to the dimensional expansion and/or contraction that would normally occur in a structural member with variations in temperature. – The magnitude of a thermal stress developed by a temperature

change is dependent on the coefficient of thermal expansion and the modulus of elasticity E .

FACTORS THAT AFFECT FATIGUE LIFE

• 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.

Creep• Creep is a time-dependent and permanent deformation of

materials when subjected to a constant load or stress. • Creep often occur when materials are placed in service at

elevated temperatures and exposed to static mechanical stresses – (e.g., turbine rotors in jet engines and steam generators that

experience centrifugal stresses, and high-pressure steam lines). • Creep is normally an undesirable phenomenon and is often

the limiting factor in the lifetime of a part. – It is observed in all materials types; for metals it becomes important

only for temperatures > ±0.4Tm ( absolute melting temperature).– Amorphous polymers, which include plastics and rubbers, are

especially sensitive to creep deformation.

Creep• 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: STRESS AND TEMPERATURE EFFECTS

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

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

– With either increasing stress or temperature, the following will be noted: 1) the instantaneous strain increases, 2) the steady-state creep rate is increased,3) the rupture lifetime is diminished.

Creep: STRESS AND TEMPERATURE EFFECTS

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

Creep: ALLOYS FOR HIGH-TEMPERATURE USE

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

Creep: ALLOYS FOR HIGH-TEMPERATURE USE

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

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

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

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

Creep: ALLOYS FOR HIGH-TEMPERATURE USE

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

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

insoluble in the matrix.

Creep: ALLOYS FOR HIGH-TEMPERATURE USE

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

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

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