ASM Practical Fractography.pdf
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Transcript of ASM Practical Fractography.pdf
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FRACTOGRAPHY
David M. ChristieSenior Failure AnalystIMR Test Labs
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Focus of this Presentation: Examples of fracture:
cast, wrought, and powder metals Overload
ductile and brittle Fatigue Stress corrosion cracking Hydrogen embrittlement Liquid metal embrittlement Cleaning techniques
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CAST METALS Fractures tend to be more difficult to
interpret than in wrought material Non-uniform microstructure and
chemistry Section-size dependent properties Fracture along second phase
particles Generally rougher fracture surfaces
than wrought material
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Type 383 Aluminum die cast
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Aluminum die cast Cast-in letter
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Aluminum die cast fracture surface
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Fatigue striations are often difficult to find in castings, compared with wrought material
Fatigue origins are also difficult to determine, because the fracture surfaces tend to be rougher than in wrought material
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Fatigue Striations die cast aluminum
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Ductile overload in some casting alloys will occur primarily through or at the boundaries of secondary phases
Only small ligatures of matrix will show ductile dimples
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Aluminum die cast overload fracture
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Fatigue features and origins are difficult to determine, compared to wrought material
Ductile fracture will often occur primarily at boundaries of secondary phases
From this Example:
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Cast aluminum impact wrench housing
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What are the small planar regions at the fracture edge?
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Note sharp boundary
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Fracture ridges and beachmarks = multiple origin fatigue.... Low nominal load or high nominal load?
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Overload zone Where are the dimples?
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Overload zone fracture primarily through second
phase particles
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Ductile Iron but is this a ductile fracture?
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Microstructure shows carbides
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Ductile iron - overload
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-Nodules loose in matrix- Many sites of lost nodules
- Ductile dimples in matrix ligatures
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Ductile dimples
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Nodules loose in matrix, dimples evident
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Ductile Iron Differential Case
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Ductile Iron Fatigue fracture
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Ductile iron Fatigue - Nodules are tight in matrix, fatigue goes through nodules
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Ductile iron fatigue fracture
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Ductile iron Fatigue striations
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Ductile Iron fatigue striations in matrix
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Fatigue of cast alloys with multiple phases
Fatigue will usually go through secondary phases, rather than around them: Graphite nodules of ductile iron Eutectic silicon of aluminum alloys
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GRAY CAST IRON OVERLOAD
Fracture mostly along graphite flake boundaries
Matrix ligatures show ductile dimples and/ or evidence of microstructure (pearlite).
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GRAY CAST IRON OVERLOAD
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Gray Iron Backscatter SEM image, shows graphite flakes well
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GRAY CAST IRON OVERLOAD
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GRAY CAST IRON OVERLOAD
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White Cast Iron Brittle Fracture
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White Cast Iron Brittle Fracture
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Brittle Fracture - White Cast Iron. Note cleavage facets and fracture at carbide boundaries
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Cast Magnesium AZ 91C Mg
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Cast Magnesium Fracture
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Cast Magnesium - AZ 91C Mg
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Alloy 319 Cast Aluminum Commercial Juicer
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Cast Aluminum 319 Al
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Cast Aluminum 319 Al
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Stainless Steel Pump Impeller (CG-8M)
Stainless steel pump impeller failed after 14 months in service
One of six vanes fractured off
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Pump Impeller
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Vane Radius is highest stress location
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Fracture surface generally rough
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Fatigue beachmarks present
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Fatigue features at high magnification
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Dendritic region close to origin
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Manufacturing Related CG-8M
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Pump Impeller Conclusions Fatigue began at a small weld repair
crack Weld repair to fill in casting void was in
a critical location Recommended revising procedure to
prohibit welding at the leading vane radius, and to include dye penetrant check of welds
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Observations of Fatiguein cast metals
Due to rougher surfaces and poorly developed fracture ridges, the locations of fatigue surfaces and origins can be difficult.
The locations of fatigue origins can sometimes be determined from striation direction and curvature.
Frequently fatigue direction will change from grain to grain in cast material, as the crack follows the weakest crystallographic plane. This complicates things!
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Mn Bronze Alloy C863 Automotive Transmission Fork
Failed very early in vehicle life Low stress part Mature part, no significant history of
failures
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Planar Fracture Regions
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Discolored Regions = Shrinkage porosity
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Overload Region not discolored
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Cross section of Shift Fork
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Shift Fork Conclusions Gas and shrinkage porosity occupied
50% of cross section Resulting reduction in load-bearing
cross section increased stress intensity Result was fatigue initiation at inside
radius of fork
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Mn Bronze (C670) adjusting nut
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Adjusting Nut Conclusions Mn Bronze (C670) adjusting nut from
offshore oil rig was exposed to salt water, mud, hydrogen sulfide, diesel fuel environment
Nut is held in constant tension and exposed to radial vibration, mated to 316 stainless
Nut failed by intergranular stress corrosion cracking (IGSCC)
Recommended alloy change to (SCC resistant) cast nickel (K-500)
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Chrome-plated Leaded Brass Flush Valve (C857)
Chrome-plated leaded yellow brass flush valves developed leaks after six months of service
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Small Cracks resulted in leaking
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Fracture Surface was discolored
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Tip: When cleaning fracture surfaces of
leaded material, avoid the use of Alconox detergent, as it can remove the lead!
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TIP: When the main fracture surface is
heavily corroded or damaged, look near the edges of the fracture (at the crack tip)
There is often less damage in this location, and the fracture mode is probably consistent with the rest of the fracture surface
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Fracture at Crack Tip
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Tip: Fractures produced in the laboratory
can aid in your interpretation of the field fractures To confirm fracture mode and compare to
the field fracture. To determine if the material has been
embrittled. To test response of freshly exposed
material to different environments, cleaning techniques.
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Material was not embrittled
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Branched transgranular cracking -indicative of SCC
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Brass Flush Valve Conclusions Failure was due to transgranular stress
corrosion cracking No specific corrosive agent was
determined Alloy contained 35% zinc, which makes
it a susceptible alloy Not a highly stressed part, suspected
residual casting stresses Recommended stress relief of castings,
or material change to aluminum bronze
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Cast Nickel Pump Impeller (Cast Super-Duplex Stainless Steel -
Jessup 700)
After nine months of pumping a low pH (1.5 2.0) slurry of 50% wet phosphoric acid, one of four vanes fractured from an impeller
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Pump Impeller
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One Vane Fractured
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Tip: Fatigue fractures in cast material will
often change direction with each grain, depending on crystallographic planes
Faceted fracture surfaces are often fatigue fractures
SEM can aid in determining fracture mode
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Damage near fracture origins dont panic.
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Remember look at crack tip!
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Tip: When fracture surfaces are corroded or
damaged, look for secondary cracks Opening secondary cracks will reveal
fresher fractures, with more detail
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Near Secondary Crack Origin
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Cast JS700 Pump Conclusions
Corrosion fatigue had occurred, with all four vanes showing cracks
Alloy was appropriate, met specification Recommended checking chemistry of
pumpage, checking residual stress of cast impellers, checking balance of impellers
Corrosion Fatigue depends on environment and stress intensity
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Cast Bismuth Bronze Wear Rings
Cast Bismuth Bronze (lead-free C89320) wear rings failed prematurely in a pump
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Bronze Wear Ring Conclusions
Metallographic section showed intergranular cracking
Microstructure indicated the parts had run dry and overheated
Molten bismuth had embrittled the part, resulting in fracture by Liquid Metal Embrittlement (LME)
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Powder Metals Green crack vs. Sinter Bond fracture Ductile overload Fatigue
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Green Cracks PM parts are pressed and ejected
Green at this point The stresses of pressing and/or ejection
can result in cracks at this stage Sintering to produce diffusion bonding
between particles will not bridge the gap created by a crack
The result: a green crack
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Powder Metals Green crack
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Powder Metals Green crack
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Powder Metals Green crack
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Green crack of a steam-treated part
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Powder metal - Overload
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Powder metal - Overload
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Powder Metal Overload The percentage of sinterbond fracture is
directly related to the part density The higher the density, the greater the
percentage of sinterbond fracture
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Powder metal - Fatigue
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Powder metal - Fatigue
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Powder metal - Fatigue
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Powder Metal Fatigue Fatigue does not seek the weakest
path, as overload fracture does The result is that generally a larger
percentage of the fracture surface is actual fracture, as opposed to void area
The patches of fatigue fracture are generally larger than overload sinterbond fracture
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Powder metal Ferrite core fractureby thermal shock
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Powder metal Ferrite core fracture by thermal shock
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WROUGHT METALS OVERLOAD
Ductile Brittle
FATIGUE CORROSION CRACKING
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OVERLOAD AXIAL TENSION
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OVERLOAD - SHEAR
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LONGITUDINAL OVERLOAD FRACTURE DIMPLE NUCLEATION AT MnS INCLUSIONS
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Brittle Fracture Cleavage Fracture below DBTT
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Same Steel Ductile Fracture above DBTT
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Brittle Intergranular fracture in hardened case of carburized steel this is the expected overload morphology
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Type 303 Stainless steel Fatigue
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Use of topographic backscatter mode in the SEM can show steps or ridges in the fracture,
indicative of multiple fatigue origins.
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Type 303 Stainless steel Fatigue MnS inclusions evident on fracture surface
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Type 303 Stainless steel Overload
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Wrought Aluminum - Fatigue
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Wrought Aluminum - Fatigue
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Wrought Aluminum - Fatigue
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Wrought Aluminum - well-developed striations
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Wrought Aluminum - Overload
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Steel Shaft Rotating Bending Fatigue
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Steel Shaft Rotating Bending Fatigue
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Steel Shaft Rotating Bending Fatigue
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Steel Shaft Rotating Bending Fatigue
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Steel Shaft Rotating Bending Fatigue
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Steel Shaft Rotating Bending Fatigue
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Stainless Steel Bellows Fatigue Fracture
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Stainless Steel Bellows Fatigue Fracture
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Tip: Fatigue striation curvature indicates direction of crack propagation
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Brass Intergranular Stress Corrosion Cracking
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Crack tip Field crack to left, overload at right
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Brass Laboratory overload fracture
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Hydrogen Embrittlement
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Hydrogen embrittlement
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Hydrogen Embrittlement ductile ornamentation of grain boundaries
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Hydrogen Embrittlement ductile ornamentation of grains, gaping grain boundaries
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Hydrogen Embrittlement patches of ductile fracture
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Use of the SEM for Fractography Always examine fracture surface optically
before SEM examination Scan first at high refresh rate, high probe
current Once the critical areas are established, take
photographs, adjusting SEM conditions Consider the use of Backscatter and
Secondary modes Consider the use of topographic modes in
both Backscatter and Secondary
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Optimizing the SEM for Fractography Photographs
Working distance (WD) should be minimized (e.g. 10 15 mm)
Accelerating voltage should be 10 Kevor less
Probe current (spot size) should be low (~100 picoamps)
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SEM not optimized: 31 mm WD, 30 Kev, 1.8 nanoamps
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SEM not optimized: 31 mm WD, 30 Kev, 100 picoamps
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SEM optimized: 15 mm WD, 10 Kev, 100 picoamps
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SEM Low Magnification Techniques
BEI composition mode BEI topographic mode SEI mode SEI mode with reverse voltage bias
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Backscatter composition mode
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Backscatter topographic mode
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Secondary mode
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Secondary mode, negative bias
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Cleaning Fracture Surfaces Photodocument As-received condition Clean starting with least aggressive method Use step-wise approach and examine at each
step It is often not necessary to remove all oxides
or contamination from the fracture and attempting to do so may damage the surface
If in doubt, submit a polished metallographic mount of your material to the proposed cleaning method, examine for etching or other damage
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Cleaning Fracture Surfaces The use of an alkaline detergent
(Alconox) has proven most useful. Mix 160 g to one gallon of DI water Can be used at room temperature or
heated to 100 degrees F Ultrasonic for up to 15 30 minutes in
five minute increments, with examinations after each five minutes.
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Cleaning Fracture Surfaces Before Alconox
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Cleaning Fracture Surfaces After Alconox
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SUMMARY Examples of cast, wrought, and powder
metals were reviewed Overload ductile and brittle Fatigue Stress corrosion cracking Hydrogen embrittlement Liquid metal embrittlement Cleaning techniques were presented