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by: Herman Saputroby: Herman Saputroby: Herman Saputroby: Herman Saputro
Selendang Ayu suffered a catastrophic fracture in number 4 hold in December 2004.
The 1980s and 1990s were a very unsafe time for bulk carriers. Many bulkerssank during this time, 99 were lost between 1990 and 1997. The latest case isSelendang Ayu in Desember 2004.
KASUSKASUSKASUSKASUS KASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALAN
The leading causes:
1. Stability problems2. Structural problems
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On March 27th, 1980, the semi-submersible platform Alexander Kielland suddenlycapsized during a storm in the North Sea, because one of its five vertical columnssupporting the platform was broken off. 123 workers among the 212 people on
board were killed in the accident.
The investigation showed that a fatigue crack had propagated
from the double fillet weld.
KASUSKASUSKASUSKASUS KASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALAN
http://www.weldreality.com/navy%20weld%20problems.htm
An oil tanker that fractured in a brittle manner by crack propagation
around its girth. (Photography by Neal Boenzi. Reprinted with permissionfrom The New York Times.)
Each year from 1995 to 2001, an average ofEach year from 1995 to 2001, an average ofEach year from 1995 to 2001, an average ofEach year from 1995 to 2001, an average of 408 tankers break apart at sea408 tankers break apart at sea408 tankers break apart at sea408 tankers break apart at sea or barelyor barelyor barelyor barelyescaped that fate, according to the International Association of Independent Tankerescaped that fate, according to the International Association of Independent Tankerescaped that fate, according to the International Association of Independent Tankerescaped that fate, according to the International Association of Independent TankerOwners, known asOwners, known asOwners, known asOwners, known as IntertankoIntertankoIntertankoIntertanko. The leading cause was collision, but nearly as many. The leading cause was collision, but nearly as many. The leading cause was collision, but nearly as many. The leading cause was collision, but nearly as manysuffered suffered suffered suffered unknown structural failuresunknown structural failuresunknown structural failuresunknown structural failures or technical problems. or technical problems. or technical problems. or technical problems.
KASUSKASUSKASUSKASUS KASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALAN
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Aloha Airlines On April 28, 1988, a Boeing 737-200 (line number 152)
The investigation determined that the failure mechanism was a result of multiple sitefatigue cracking of the skin adjacent to rivet holes along the lap joint upper rivet row and tearstrap disbond which negated the fail-safe characteristics of the fuselage. Finally, the fatiguecracking initiated from the knife edge associated with the countersunk lap joint rivet holes;the knife edge concentrated stresses that were transferred through the rivets because of lapjoint disbanding.
http://lessonslearned.faa.gov/ll_main.cfm?TabID=4&LLID=20&LLTypeID=2
KASUSKASUSKASUSKASUS KASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALAN
KASUSKASUSKASUSKASUS KASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALANKASUS KEGAGALAN
Southwest Airlines planeSouthwest Airlines planeSouthwest Airlines planeSouthwest Airlines plane
The plane's nose gear collapsed as theaircraft landed on Runway 4
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ANALISISANALISISANALISISANALISIS KEGAGALANKEGAGALANKEGAGALANKEGAGALAN
The cause of most structural failures generally falls into one of the followingThe cause of most structural failures generally falls into one of the followingThe cause of most structural failures generally falls into one of the followingThe cause of most structural failures generally falls into one of the following
categories:categories:categories:categories:1. Negligence during design, construction or operation of the structure.2. Application of a new design or material, which produces an unexpected (and
undesirable) result.
Why structure fail ???Why structure fail ???Why structure fail ???Why structure fail ???
FailureFailureFailureFailure in structures leads to lostlostlostlost of propertiesof propertiesof propertiesof properties and sometimes lost ofhuman lives.
Failure analysisFailure analysis is an engineering approach to determining how and why equipment or acomponent has failed.
The goal of a failure analysis is to understand the root cause of thefailure so as to prevent similar failures in the future.
ANALISISANALISISANALISISANALISIS KEGAGALANKEGAGALANKEGAGALANKEGAGALAN
The cause of most structural failures generally falls into one of the followingThe cause of most structural failures generally falls into one of the followingThe cause of most structural failures generally falls into one of the followingThe cause of most structural failures generally falls into one of the followingcategories:categories:categories:categories:1. Negligence during design, construction or operation of the structure.
2. Application of a new design or material, which produces an unexpected (andundesirable) result.
Why structure fail ???Why structure fail ???Why structure fail ???Why structure fail ???
FailureFailureFailureFailure in structures leads to lostlostlostlost of propertiesof propertiesof propertiesof properties and sometimes lost ofhuman lives.
Failure analysis
Failure analysis is an engineering approach to determining how and why equipment or a
component has failed.
The goal of a failure analysis is to understand the root cause of thefailure so as to prevent similar failures in the future.
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Failure TheoriesFailure TheoriesFailure TheoriesFailure Theories
Failure under load can occur due to excessive elastic deflections or due toexcessive stresses.
Failure prediction theories due to excessive stresses fall into two classes:
1. Failure when the loading is static or the number of load cycles is one orquite small, and
2. Failure due to cyclic loading when the number of cycles is large often inthousands of cycles.
FailureFailureFailureFailure underunderunderunder staticstaticstaticstatic loadloadloadloadParts under static loading may fail due to:a)a)a)a) DuctileDuctileDuctileDuctile behaviorbehaviorbehaviorbehavior:::: Failure is due to bulk yielding causing permanent
deformations that are objectionable. These failures may cause noise, lossof accuracy, excessive vibrations, and eventual fracture. In machinery,
bulk yielding is the criteria for failure. Tiny areas of yielding are OK inductile behavior in static loading.
b)b)b)b) BrittleBrittleBrittleBrittle behaviorbehaviorbehaviorbehavior:::: Failure is due to fracture. This occurs when thematerials (or conditions) do not allow much yielding such as ceramics,grey cast iron, or heavily cold-worked parts
Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
Adapted from Fig. 8.3, Callister 7e.
ductile fracture in aluminum
brittle fracture in a mild steel
Ductile materials - extensive plastic deformation and energy absorption (toughness)
before fracture
Brittle materials - little plastic deformation and low energy absorption before fracture
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Ductile fractureDuctile fractureDuctile fractureDuctile fracture
Cup-and-cone fracture inaluminum.
Final shearfracture.
Initial necking.
Small cavity
formationCoalescence ofcavities to form acrack
Crackpropagation
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Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
Microvoid Formation, Growth And Coalescence
MicrovoidsMicrovoidsMicrovoidsMicrovoids are easily formed at inclusions,intermetallic or second-phase particles and
grain boundaries.GrowthGrowthGrowthGrowth andandandand coalescencecoalescencecoalescencecoalescence of microvoids progress
as the local applied load increases.
Random planar arrayof particles acting asvoid initiators
Growth of voidsto joineach other asthe appliedstress increases.
Linkage orcoalescenceof these voids toform free fracturesurface.
Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
Ductile fractureDuctile fractureDuctile fractureDuctile fracture
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Ductile Fracture of Alloys
If materials is stretched, itfirstly deforms uniformly.
Inclusions stress concentrators
Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
Ductile fractureDuctile fractureDuctile fractureDuctile fracture
1)1)1)1) DecohesionDecohesionDecohesionDecohesion at particle -matrix interface.2) Fracture of brittle particle
3) Decohesion of an interface associatedwith shear deformation or grainboundary sliding.
Decohesion of carbide particlesfrom Ti matrix.
Fractured carbides aidingmicrovoid formation.
Microvoids are from by:
Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
Ductile fractureDuctile fractureDuctile fractureDuctile fracture
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Brittle FractureBrittle FractureBrittle FractureBrittle Fracture
Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
Fractografic Studies
Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
Transgranular fracture: Crackspass through grains. Fracturesurface: faceted texturebecause of different orientationof cleavage planes in grains.
Intergranular fracture: Crack propagation is along grainboundaries (grain boundaries areweakened/ embrittled byimpurity segregation etc.)
Brittle FractureBrittle FractureBrittle FractureBrittle Fracture
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Intergranular Fracture
Intergranular failureIntergranular failureIntergranular failureIntergranular failure isa moderate to low energybrittle fracture mode resultingfrom grain boundaryseparation or segregation ofembrittling particles orprecipitates.
Embrittling grainboundary particles are
weakly bonded with thematrix, high free energyand unstable, which leads topreferential crackpropagation path. Intergranular fracture with and without
microvoid coalescence
Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
Brittle FractureBrittle FractureBrittle FractureBrittle Fracture
Characteristic for ceramics and glassesCharacteristic for ceramics and glassesCharacteristic for ceramics and glassesCharacteristic for ceramics and glasses
Distinct characteristics of brittle fracturesurfaces:
1) The absence of gross plastic deformation.2) Grainy or Faceted texture.3) River marking or stress lines.
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Brittle FractureBrittle FractureBrittle FractureBrittle Fracture
Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
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Strength and Toughness
Strength
Resistance of a materialto plastic flow
ToughnessResistance of a material
to the propagationof a crack
How concerned should you be if you read in thepaper that cracks have been detected in thepressure vessel of the nuclear reactor of thepower station a few miles away?
Ductile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle FailureDuctile vs. Brittle Failure
TESTING FOR TOUGHNESS
This type of test provides a comparison of the toughness ofmaterials
however, it does not provide a way to express toughness asa material property (no true material property that isindependent on size and shape of the test sample)
Tear test Impact testMeasuringthe energy
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INTRODUCTION TOINTRODUCTION TOINTRODUCTION TOINTRODUCTION TOFRACTURE MECHANICSFRACTURE MECHANICSFRACTURE MECHANICSFRACTURE MECHANICS
The fracture strength of a solid material is a function of thecohesive forcescohesive forcescohesive forcescohesive forces that exist between atoms.
On this basis, the theoretical cohesive strength of a brittle elastic solid
has been estimated to be approximately E/10, where E is the modulusof elasticity.
The experimental fracture strengths of most engineering materialsnormally lie between 10 and 1000 times below this theoretical value.
Why?
surface energy
unstrained interatomic spacing
STRESS CONCENTRATIONSTRESS CONCENTRATIONSTRESS CONCENTRATIONSTRESS CONCENTRATION
Crack reduces the cross section =>increase in stress
What will happenwith tough material?
Cracks concentrate stress
Flaws are detriment to the fracture strength because an appliedstress may be amplified or concentrated at the tip, the magnitude ofthis amplification depends on crack orientation and geometry.
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What Force Is Required To Break The Samples?
THEORIES OF BRITTLE FRACTURETHEORIES OF BRITTLE FRACTURETHEORIES OF BRITTLE FRACTURETHEORIES OF BRITTLE FRACTURE
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STRESS CONCENTRATORS
Schematic stress profile along the lineXX
the magnitude of this localized stressdiminishes with distance away from the crack tip
The maximum stressat the crack tip
stress
concentration
factor
A measure of the degree to
which an external stress is
amplified at the tip of a crack
PROBLEMPROBLEMPROBLEMPROBLEM
1. Consider a circular hole in a plate loaded in tension. When will material nearthe hole yield?
2. A plate with a rectangular section 500 mm by 15mm carries a tensile load of 50kN. It is made of a
ductile metal with a yield strength of 50 MPa. Theplate contains an elliptical hole of length 100 mmand a minimum radius of 1 mm, oriented as shownin the diagram.
What is
(a) the nominal stress(b) The maximum stress in the plate?(c) Will the plate start to yield?
(d) Will it collapse completely?
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THEORETICAL STRESSCONCENTRATION FACTOR
CURVES
GRIFFITH THEORY OF BRITTLEGRIFFITH THEORY OF BRITTLEGRIFFITH THEORY OF BRITTLEGRIFFITH THEORY OF BRITTLEFRACTUREFRACTUREFRACTUREFRACTURE
Inherent defects in brittlematerials lead to stress
concentration.
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If stress exceeds thecohesive strength ofbonds, crackextension is possible.
Thermodynamic criterion:There are two energies to be taken into
account when a crack propagates:(1) New surfaces should be created and
a certain amount of energy must beprovided to create them;
(2) Elastic strain energy stored in thestressed material is released during
crack propagation.
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THEORY OF BRITTLE FRACTURETHEORY OF BRITTLE FRACTURETHEORY OF BRITTLE FRACTURETHEORY OF BRITTLE FRACTURE
The stress required to
create the new crack
surface
Critical stress for crack propagation
G 2G 2G 2G 2
PROBLEMPROBLEMPROBLEMPROBLEM
A relatively large plate of a glass is subjected to a tensile stress of 40 MPa. Ifthe specific surface energy and modulus of elasticity for this glass are 0.3J/m2 and 69 GPa, respectively, determine the maximum length of a surface flaw that is possible withoutfracture.
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PROBLEM: PROPERTIES OF SIALON CERAMICS
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Assume that an advanced ceramic, SiAlON (silicon aluminum oxynitride),has a tensile strength of 414 MPa.
Let us assume that this value is for a flaw-free ceramic. (In practice, it isalmost impossible to produce flaw-free ceramics.)A crack 0.025 cm deep is observed before a SiAlON part is tested.The part unexpectedly fails at a stress of 3.5 MPa by propagation of the crack. Estimate the radius of the crack tip.
BASIC MODES OF CRACK TIP DEFORMATION
KIC the critical stress intensity inmode I fracture (plain strain)
critical stress forcrack propagation
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K = (EG) 1/2
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FRACTURE TOUGHNESS
FT is a material propertymaterial propertymaterial propertymaterial property;Value is independent of the way it is measured;Can be used for designCan be used for designCan be used for designCan be used for design
Fracture toughness of a material is obtained by determiningthe ability of a material to withstand the load in the presence
of a sharp crack before failure.
Y is a dimensionless parameter or function
that depends on both crack and specimen
sizes and geometries, as well as the manner
of load application33
Crack propagates when the stressintensity factor exceeds a critical value.
ENERGY RELEASE RATE
Irwin later modified the Griffith theory by replacing the
term 2 with the potential strain energy release rate G
When a samples fractures, a new surface is created =>
necessary conditions for fracture sufficient energy
release
G 2The critical condition to which the crackpropagates to cause global failure is when this
G value exceeds the critical value
Irwin showed that G is measurable and can
be related to the stress intensity factor, K
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Y VALUES OF VARIOUS CRACK GEOMETRIESY VALUES OF VARIOUS CRACK GEOMETRIESY VALUES OF VARIOUS CRACK GEOMETRIESY VALUES OF VARIOUS CRACK GEOMETRIES
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PROCESS ZONE
A plastic zone forms at thecrack tip where the stress
would otherwise exceedthe yield strength
Size of process zone:
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BRITTLE CLEAVAGE FRACTURE
Materials of high yield strength
Near tip stress are very high =>tear the atomic bonds apart =>increase in the crack length results in increase in K,causing crack to accelerate
FRACTURE TOUGHNESS AND DESIGNFRACTURE TOUGHNESS AND DESIGNFRACTURE TOUGHNESS AND DESIGNFRACTURE TOUGHNESS AND DESIGN
If the KIC value of material is known and the presence of a crack isallowed, we can then monitor the crack propagation during service priorto failure =>How long we can use the component before it fails.
Brittle materials, for which appreciableplastic deformation is not possible in front ofan advancing crack, have low KIc values and
are vulnerable to catastrophic failure.
Crack length necessaryfor fracture at a materials
yield strength
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Tough metals are able to containlarge cracks but still yield in apredictable, ductile, manner
Critical crack lengths are a measure of thedamage tolerance of a material
DAMAGE TOLERANCE
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FRACTURE RESISTANCE
The ability of a material to resist the growth of a crackdepends on a large number of factors:
Larger flaws reduce the permitted stress. The ability of a material to deform is critical.
Increasing the raterateraterate of application of the load, such asthat encountered in an impact test, typically reducesthe fracture toughness of the material.
Increasing the temperature normally increases thefracture toughness.
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VARIABLES AFFECTINGVARIABLES AFFECTINGVARIABLES AFFECTINGVARIABLES AFFECTINGFRACTURE TOUGHNESSFRACTURE TOUGHNESSFRACTURE TOUGHNESSFRACTURE TOUGHNESS
Metallurgical factorsMetallurgical factorsMetallurgical factorsMetallurgical factors
Microstructure, inclusions,impurities Composition Heat treatment Thermo-mechanical processing
FRACTURE TOUGHNESS MODULUS CHART
Values range from 0.01 100 MPam
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Transition crack length plotted on chart values can range from near-atomic dimensions for ceramics to almost a meter for ductile metals
FAIL-SAFE DESIGN
Yield-before-break
Requires that the crack will
not propagate even if thestress causes the part to yield
Leak-before-break
Requires that a crackjust large enough to
penetrate both the innerand outer surface of the
vessel is still stable
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DESIGN USING FRACTURE MECHANICSDESIGN USING FRACTURE MECHANICSDESIGN USING FRACTURE MECHANICSDESIGN USING FRACTURE MECHANICS
Consider the thin-walled spherical tankof radius r and thickness tthat may beused as a pressure vessel.
One design of such a tank calls for yielding of the wall material prioryielding of the wall material prioryielding of the wall material prioryielding of the wall material priorto failureto failureto failureto failure as a result of the formation of a crack of critical size and itssubsequent rapid propagation.Thus, plastic distortion of the wall may be observed and the pressure
within the tank released before the occurrence of catastrophic failure.Consequently, materials having large critical crack lengths aredesired.
On the basis of this criterion, rank the metal alloys listedin Table, as to critical crack size, from longest to shortest.
wall stress
DESIGN PROCESSDESIGN PROCESSDESIGN PROCESSDESIGN PROCESS YIELDYIELDYIELDYIELD----BEFOREBEFOREBEFOREBEFORE----FRACTUREFRACTUREFRACTUREFRACTURE
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Requirement:The stresses are everywhere lessthat required to make a crack ofcritical length to propagate.BUT!!! It is not safe
Requirement:Crack should not propagateeven if the stress is sufficient tocause general yield for then thevessel will deform stably in a waythat can be detected.
Tolerable crack size
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DESIGN PROBLEM - LEAK-BEFORE-BREAK
An alternative design that is also often utilized with pressure vessels is termed leak-
before-break. Using principles of fracture mechanics, allowance is made for the growth
of a crack through the thickness of the vessel wall prior to the occurrence of rapid crack
propagation. Thus, the crack will completely penetrate the wall without catastrophic
failure, allowing for its detection by the leaking of pressurized fluid.
With this criterion the critical crack lengthac (i.e., one-half of the total internal crack
length) is taken to be equal to the pressure vessel thicknesst.
Using this criterion, rank
the metal alloys in Table as
to the maximum allowable
pressure.47
2a = t
FORENSIC FRACTURE CASE
48
K1c
of the tank material measured to be
45 MPam
10 mm crack found in longitudinal weld
Stress based on
maximum design
pressure
Stress at which a plate with the given K1cwill fail with a 10 mm crack
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DUCTILE-TO-BRITTLE TRANSITION
At low temperatures some metals and all polymers become
brittle
As temperatures decrease, yield strengths of most materials
increase leading to a reduction in the plastic zone size
Only metals with an FCC structure remain ductile at the lowest
temperatures
The ductile to brittle transition temperature is the temperature atwhich the failure mode of a material changes from ductile to brittle
fracture.
DUCTILE TO BRITTLE TRANSITIONBEHAVIOUR
Some metals and polymers experience ductile-to-brittle transition behaviour
when subjected to decreasing temperature, resulting from a strong yield stress
dependence on temperature.
Metals possess limitedslip systems available at low
temperature, minimising the
plastic deformation during the
fracture process.
Increasing temperature
allows more slip systems to
operate, yielding general
plastic deformation to occur
prior to failure.
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WHEN DUCTILE TURN TO BRITTLE
The criterion for a material to change its fracture behaviour from ductile
to brittle mode is when the yield stress at the observed temperature is larger
than the stress necessary for the growth of the micro-crack indicated in the
Griffith theory
The criterion for ductile to brittle
transition is when the term on the
left hand side is greater than the right
hand side.
is the lattice resistance to dislocation
movement
k is a parameter related to the release of
dislocation into a pile-up
D is the grain diameter (associated with slip
length).
G is the shear modulus
is a constant depending on the stress system
WHY DONT SOME MATERIALSUNDERGO TRANSITION?
Unlike steel, aluminium does not undergo a ductile-brittle transition. The reason can
be explained in terms of their crystal structure.
The yield stress of steel is temperature sensitive because of its BCC structure. At low
temperatures it is more difficult for the dislocations to move (they require a degree of
diffusion to move due to the non-close packed nature of the slip planes) and thereforeplastic deformation becomes more difficult. The effect of this is to increase the yield
stress at low temperatures.
Aluminium has a FCC structure, this means
that it has lots of easily operated close-
packed slip systems operating at low
temperatures. As a result its yield strength is
not temperature sensitive and aluminium
remains ductile to low temperatures.
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BAD LUCK OF TITANIC
The sinking of the Titanic was caused primarily by the brittleness of thesteel used to construct the hull of the ship.
In the icy water of the Atlantic, the steel was below the ductile to brittle
transition temperature.
FACTORS AFFECTING MODES OFFRACTURE
The yield stress of steel is temperature
sensitive. The fracture stress remains relatively
constant with temperature.
At room temperature steel is a ductile material, this
means that it will undergo plastic deformation before
fracture i.e. the yield strength of the material is less
than the fracture stress.
At low temperatures the yield stress of steel
increases, when the yield stress increases above
the fracture stress the material will undergo a
ductile-to-brittle transition.
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THE STRENGTH-TOUGHNESS TRADE-OFF
Increasing the yieldstrength of a metal
decreasing the size
of the plastic zone
surrounding a crack
this leads to decreased
toughness
INSPECTIONS
1. Visual2. Liquid Penetrant
3. Magnetic
4. Ultrasonic5. Eddy Current
6. X-ray
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Most basic and common
inspection method.
Tools include fiberscopes,
borescopes, magnifying
glasses and mirrors.
Robotic crawlers permit
observation in hazardous or tight
areas, such as air ducts, reactors,
pipelines.
Portable video inspection unit
with zoom allows inspection
of large tanks and vessels,
railroad tank cars, sewer lines.
1. VISUAL INSPECTION
INSPECTIONS
A liquid with high surface wetting characteristics is applied to
the surface of the part and allowed time to seep into surface
breaking defects.
The excess liquid is removed from the surface of the
part.
A developer (powder) is applied to pull the trapped
penetrant out the defect and spread it on the surface where it
can be seen.
Visual inspection is the final step in the process. The
penetrant used is often loaded with a fluorescent dye and the
inspection is done under UV light to increase test sensitivity.
2. LIQUID PENETRANT INSPECTION
INSPECTIONS
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3. MAGNETIC PARTICLE INSPECTION
The part is magnetized. Finely milled iron particles coated with a dye pigment
are then applied to the specimen. These particles are attracted to magnetic fluxleakage fields and will cluster to form an indication directly over thediscontinuity. This indication can be visually detected under proper lightingconditions.
INSPECTIONS
Magnetic Particle Crack Indications
INSPECTIONS
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4. RADIOGRAPHY
The radiation used in radiography testing is ahigher energy (shorter wavelength) version of
the electromagnetic waves that wesee as visible light. The radiation can comefrom an X-ray generator or a radioactivesource.
High Electrical Potential
Electrons-+
X-rayGenerator orRadioactive
Source CreatesRadiation
Exposure Recording Device
Radiation
Penetratethe Sample
INSPECTIONS
5. EDDY CURRENT TESTING
Eddy current testing is particularly well suited for detecting surface cracks but can also be
used to make electrical conductivity and coating thickness measurements. Here a small
surface probe is scanned over the part surface in an attempt to detect a crack
INSPECTIONS
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High frequency sound waves are introduced into a material and they are reflected backfrom surfaces or flaws.
Reflected sound energy is displayed versus time, and inspector can visualize a crosssection of the specimen showing the depth of features that reflect sound. f
plate
crack
0 2 4 6 8 10
initialpulse
crackecho
back surfaceecho
Oscilloscope, or
flaw detector screen
6. ULTRASONIC INSPECTION (PULSE-ECHO)
INSPECTIONS
ULTRASONIC IMAGING
Gray scale image produced using the
sound reflected from the front surface
of the coin
Gray scale image produced using the
sound reflected from the back surface of
the coin (inspected from heads side)
High resolution images can be produced by plotting signal
strength or time-of-flight using a computer-controlled scanning
system.
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COMMON APPLICATION OF NDT
Inspection of Raw Products
Inspection Following Secondary Processing
In-Services Damage Inspection
INSPECTION OF RAW PRODUCTS
Forgings, Castings,
Extrusions, etc.
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Machining
Welding Grinding Heat treating Plating etc.
INSPECTION FOLLOWINGSECONDARY PROCESSING
Cracking
Corrosion
Erosion/Wear
Heat Damage
etc.
INSPECTION FORIN-SERVICE DAMAGE
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POWER PLANT INSPECTION
Probe
Signals produced by
various amounts of
corrosion thinning.
Periodically, power plants are
shutdown for inspection.
Inspectors feed eddy current probes
into heat exchanger tubes to checkfor corrosion damage.
Pipe with damage
WIRE ROPE INSPECTION
Electromagnetic devices andvisual inspections are used to findbroken wires and other damage tothe wire rope that is used inchairlifts, cranes and other lifting
devices.
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STORAGE TANK INSPECTION
Robotic crawlers useultrasound to inspect thewalls of large above
ground tanks for signsof thinning due tocorrosion.
Cameras on longarticulating armsare used to inspectunderground
storage tanks fordamage.
AIRCRAFT INSPECTION Nondestructive testing is used
extensively during the manufacturingof aircraft.
NDT is also used to find cracks andcorrosion damage during operation
of the aircraft. A fatigue crack that started at the site
of a lightning strike is shown below.
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JET ENGINE INSPECTION
Aircraft engines are overhauled afterbeing in service for a period of time.
They are completely disassembled,cleaned, inspected and then reassembled.
Fluorescent penetrant inspection is usedto check many of the parts for cracking.
Sioux City, Iowa, July 19, 1989
A defect that went
undetected in an engine
disk was responsible for the
crash of United Flight 232.
CRASH OF UNITED FLIGHT 232
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PRESSURE VESSEL INSPECTION
The failure of a pressure vessel can
result in the rapid release of a large
amount of energy. To protect against
this dangerous event, the tanks areinspected using radiography and
ultrasonic testing.
RAILINSPECTION
Special cars are used to inspectthousands of miles of rail tofind cracks that could lead to aderailment.
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BRIDGE INSPECTION
The US has 578,000 highwaybridges.
Corrosion, cracking and otherdamage can all affect a bridgesperformance.
The collapse of the SilverBridge in 1967 resulted in lossof 47 lives.
Bridges get a visual inspectionabout every 2 years.
Some bridges are fitted with
acoustic emission sensors thatlisten for sounds of cracksgrowing.
NDT is used to inspect pipelines toprevent leaks that could damage theenvironment. Visual inspection,radiography and electromagnetic testingare some of the NDT methods used.
Remote visual inspection using arobotic crawler.
Radiography of weld joints.
Magnetic flux leakage inspection. This
device, known as a pig, is placed in thepipeline and collects data on thecondition of the pipe as it is pushedalong by whatever is being transported.
PIPELINE INSPECTION
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SPECIAL MEASUREMENTS
Boeing employees in Philadelphia were given the privilege of evaluatingthe Liberty Bell for damage using NDT techniques. Eddy current methodswere used to measure the electrical conductivity of the Bell's bronzecasing at various points to evaluate its uniformity.
REFERENCES
1. Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition,McGraw-Hill, ISBN 0-07-100406-8.
2. Sanford, R.J., Principles of fracture mechanics, 2003, Prentice Hall,New Jersey, ISBN 0-13-092992-1.
3. Callister WD, Material Science anda Engineering an Introduction,six edition, 2003, John Wiley & Sons, Singapure.