Materials, Properties and Testing
•DEADLINES:
•Assignment 1 Submission : 22nd
December 2020
•Assignment 2 Submission: 19th January
2021
•Contact details: Mr. Omal Mumuni-
Timtey (o.mumuni-
timtey@eastcoast.ac.uk)
Failure Investigation
Objectives of Failure Investigation:
Failure investigation and subsequent analysis
should determine the primary cause of failure,
based on the determination, corrective action
should be initiated that will prevent similar
failure.
Important contributory causes of the failure must
be assessed, new experimental techniques may
have to be developed or an unfamiliar field of
engineering or science explored.
IMPORTANCE OF FAILURE
INVESTIGATION
Failure analysis reveals one or more the following:
Deficiencies in design
Material imperfection
Fabrication defects
Improper processing
Errors in assembly
Service abnormalities
Inadequate or improper maintenance
Unintended or inadvertent factors
ENGINEERING FAILURES
Engineering Failures may be categorised
as Technical, Operational and or
Unpredictable
Technical failures are most commonly due
to insufficient information about the nature
of the structure, its loading, its material, or
its operating conditions;
V90 3MW turbine
Operational failures are most commonly
due to improper operating practices or
conditions;
Unpredictable failures are most commonly the result of special
circumstances or acts of God
https://www.youtube.com/watch?v=qfPxLu46nLA
Stages involved in Failure Investigation:
1. Collection of background data.
2. Preliminary examination of the failed
part.
3. Non-destructive testing / Examination.
4. Mechanical Testing. 9. Analysis of Fracture
Mechanics.
5. Macroscopic Observation.
10. Testing under simulated
6. Microscopic studies.
conditions.
7. Determination of Failure
11. Analysis and Synthesis of
Mechanism.
all the evidences, formulation
8. Chemical analysis of the failed of conclusions.
portion.
12 Writing of report.
NEW TECHNIQUES FOR FAILURE ANALYSIS
ADVANCED NDE
MINIATURE SPECIMEN TESTING
X-RAY DIFFRACTION MAGNETIC BARKHAUSEN
NOISE
- RESIDUAL STRESSES
- MICROSTRUCTURE
IN-SITU METALLOGRAPHY
TRACE ELEMENTAL ANALYSIS
- SECONDARY ION MASS SPECTROSCOPY (SIMS)
- AUGER ELECTRON SPECTROSCOPY (AES)
- ELECTRON PROBE MICRO ANALYSIS (EPMA)
- ENERGY DISPERSIVE X-RAY ANALYSIS (EDAX)
MODELING
SIMULATION STUDIES
FINITE ELEMENT ANALYSIS
TYPES OF FAILURES
MECHANICAL FAILURES ENVIRONMENTAL MECHANICAL-ENVIRONMENTAL
FAILURES FAILURES
Ductile and brittle failures Stress-corrosion cracking
Fatigue failures Hydrogen embrittlement
Distortion failures Liquid metal embrittlement
Wear failures Corrosion failures Corrosion fatigue
Creep failures Corrosion-erosion failures Fretting fatigue
Cause of Mechanical Failure
• Misuse or abuse
• Assembly errors
• Manufacturing defects
• Improper or in adequate maintenance
• Design errors of design deficiencies
• Improper materials or poor selection of materials
• Improper heat treatments
• Unforeseen operating conditions
• Inadequate quality assurance
• Inadequate environmental protection/control
• Casting discontinuities
General types of Mechanical Failure include:
• Buckling in columns due to • Failure due to impact loading
compressive overloading. or thermal shock.
• Creep failure due to low strain • Failure due to fatigue fracture
rate at high temperature
• Failure due to combined effect of stress and
corrosion
• Failure by fracture due to compressive overload, the
fracture being either brittle or ductile.
• Failure due to excessive wear.
DUCTILE AND BRITTLE FRACTURES
Ductile and Brittle failures are terms that describe the amount
of macroscopic plastic deformation that precede fracture
Fracture mechanisms
• Ductile fracture
– Accompanied by significant plastic
deformation
• Brittle fracture
– Little or no plastic deformation
– Catastrophic
12
Ductile vs Brittle Failure
• Classification:
Fracture Very Moderately
Brittle
behavior: Ductile Ductile
•Here is a bit of statistics :
•4700 ships were built by 1946.
•1250 of these had suffered brittle fractures
by 1953. Adapted from Fig. 8.1,
Callister & Rethwisch 8e.
•230 of these fractures were classed as
serious.
•12 of the ships broke in two.
Large Moderate Small
• Ductile fracture is Ductile: Brittle:
usually more desirable Warning before No
than brittle fracture! fracture warning
13
Example: Pipe Failures
• Ductile failure:
-- one piece
-- large deformation
• Brittle failure:
-- many pieces
-- small deformations
Figures from V.J. Colangelo and F.A.
Heiser, Analysis of Metallurgical Failures
(2nd ed.), Fig. 4.1(a) and (b), p. 66 John
Wiley and Sons, Inc., 1987. Used with
permission.
14
Moderately Ductile Failure
• Failure Stages: void growth shearing
necking void fracture
and coalescence at surface
nucleation
s
• Resulting 50
50mm
mm
fracture
surfaces
(steel)
100 mm
particles From V.J. Colangelo and F.A. Heiser, Fracture surface of tire cord wire
serve as void Analysis of Metallurgical Failures (2nd loaded in tension. Courtesy of F.
ed.), Fig. 11.28, p. 294, John Wiley and Roehrig, CC Technologies, Dublin,
nucleation Sons, Inc., 1987. (Orig. source: P. OH. Used with permission.
sites. Thornton, J. Mater. Sci., Vol. 6, 1971, pp. 15
347-56.)
DUCTILE FRACTURE
Tearing of metal accompanied by appreciable gross
plastic deformation
Gray or fibrous appearance on fracture surface
Exhibit necking – Cup and cone formation
Microvoid formation and its coalescence – Dimpled structure
Dimples on a ductile fractured surface
BRITTLE FRACTURE
Rapid crack propagation with less expenditure of energy
Without gross plastic deformation
Bright and granular appearance on fracture surface
Little or No necking – Plane strain condition
Intergranular / Transgranular mode
Intergranular mode Transgranular mode
FATIGUE FAILURES
Fatigue fracture is caused by:
Repeated application of cyclic loads
Fatigue cracking results from:
Repeated application of cyclic stresses that are below the static yield
strength of the material – High cycle fatigue [HCF]
Repeated application of plastic strain – Low cycle fatigue [LCF]
Crack origin
High Cycle Fatigue Failure of a transmission shaft
Fatigue crack initiation
On planes oriented 45˚ to the applied stress axis
In persistent slip bands on specimen surface
At elevated temperature grain boundary may also act
as initiating sites
Crack propagates:
In planes perpendicular to the applied stress axis
Characterized by striations (beach marks) on the fracture surface
Clarity of fatigue striations depends on:
Ductility of the material
Stress levels (High stress levels – Widely spaced;
Low stress level – Small spacing)
Photograph of the failed aircraft wheel axle and its fracture surface
STRIATIONS INDICATING SLOW FATIGUE CRACK
GROWTH BEACH MARKS
OVERLOAD FAILURE MULTIPLE CRACK ORIGIN
Failure of a steam turbine blade from a nuclear power plant due to fatigue
To avoid fatigue failures:
Improvement in Design to:
Eliminate or minimize the stress raisers
Eliminate surface defects during manufacture
Relieve tensile residual stress
Ensure good surface finish
Creep Failure
Thermally assisted plastic deformation which is time dependent
at constant load or stress
At temp. > 0.3 Tm to 0.4 Tm; [Tm ] = Melting point in Kelvin
For metals as a general rule
For mild steel T= 1500
0C which is 1773 k and
At temp. > 0.4 Tm to 0.5 Tm; [Tm ]
= Melting point in Kelvin so there should be very
For ceramics as a general rule little creep below 0.4 x
1773 = 709 k
For Polymers – many of them creeps at room temperature. For
polymer the important thing is temperature is the glass
temperature, TG which is the temperature at which the bonds
solidify, above this temperature polymer is rubbery and creeps
rapidly
Creep - curve
MECHANISM OF CREEP
Primary creep : Work hardening dominates - Decrease in creep rate
Secondary creep : Balance between work hardening and recovery
Tertiary creep : Loss of cross section, formation and growth of cavities
Particle coarsening, Recovery in dislocation substructure
Failed Turbine wheel assembly from a
combustion turbine
Photomicrostructure of the failed turbine blade showing
creep cavities and grain boundary carbide precipitation
Design consideration for high temperature application
- Minimum of these three stresses is taken
Stress to cause 1% strain in 105 hours – Long term application
80% stress to cause onset of tertiary in 105 hours
67% stress to cause rupture in 105 hours
ENVIRONMENTAL FAILURE
Corrosion failure
Corrosion is the unintended destructive
chemical or electrochemical reaction of a
material with its environment
FACTORS THAT INFLUENCE CORROSION
Temperature and temperature gradients at metal
environment interface
Relative motion between the environment and the
metal parts
Presence of dissimilar metals in electrically conductive
environment
Processing and fabrication operations
Storage condition
TYPES OF CORROSION
Uniform corrosion
Pitting corrosion
Selective leaching
Intergranular corrosion
Concentration cell corrosion
Crevice corrosion
Galvanic corrosion
CORROSION RATE EXPRESSION
Corrosion rate in metals are expressed as mils per years (mpy)
or millimeters per year (mmpy) [1mpy = 0.0254mmpy]
Safe = < 5mpy
Moderate = 5 to 50 mpy
Severe = > 50 mpy
CORROSION CONTROL METHODS
Modification of metals
Modification of environments
Change of metal/environmental potential
Use of nonmetallic materials
Trepanned portion
Failure of a Monel-400 Boiler heatexchanger
from a nuclear power plant
SEM PHOTOGRAPH SHOWING THROUGH - THROUGH OPENING
AND INTERGRANULAR CORROSION AT THE DEFECTIVE LOCATION
IN MONEL–400 HEAT EXCHANGER TUBE DUE TO SLUDGE DEPOSIT
https://www.youtube.com/watc
h?v=KmnPaK_TaNI
Grain Boundary
Cu rich second phase
Cu depleted zone
Delamination
Intergranular corrosion on high strength aluminium
alloy due to exfoliation
Pitting corrosion observed on cut cross section of a
cupro-nickel tube from a turbine lub-oil cooler
MECHANICAL – ENVIRONMENTAL FAILURES
STRESS CORROSION CRACKING
HYDROGEN EMBRITTLEMENT
STRESS CORROSION CRACKING
Synergistic action of tensile stress and corrosive environment
General Characteristics:
Only specific environment cause failure
- Season Cracking (copper + ammonia)
Pure metals are less susceptible
Transgranular mode Intergranular mode
Stress corrosion cracking in Stainless steel
Hydrogen Embrittlement
Causes a reduction in ductility of the metal due to
absorption of hydrogen
Pickup of hydrogen from:
Processing - Melting
Fabrication - Welding/Electropolishing
Service in a hydrogen environment - Sour gas /
refineries
General characteristics of Hydrogen
embrittlement:
More susceptible in high strength steels > 1240 MPa
Failure does not occur below a critical stress
Sensitive to strain rate and temperature
Delayed failure – Static fatigue
Hydrogen embrittlement is reversible
MAIN CHARACTERISTIC OF HYDROGEN EMBRITTLEMENT
PHOTOMICROGRAPHS SHOWING BLISTERS AROUND INCLUSION AND
DECOHESION OF THE INCLUSION FROM THE MATRIX IN A FAILED pipe .
CONCLUSION
Engineering failure investigation is a detective process of
determining why and how things went wrong.
Failure investigation helps us to improve the reliability and
safety of machinery / plant and also contributes to the
enhanced productivity in addition to preventing many
industrial accidents.
Systematic investigations carried out on many failed
components has also generated a wealth of useful
information.
https://www.youtube.com/watch?v=eKMPojS10DY
https://www.youtube.com/watch?v=85Hxm
nLEkwM
