Results - The University of Virginia · Discrete Pits IGC Corroded LS surfaces ... specimen exposed...
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Discrete Pits IGC Corroded LS surfaces(top) and their corresponding fracture surfaces (bottom) T S L S L T The Effect of Galvanically Induced Corrosion Damage on the Fatigue Crack Formation Behavior of AA 7050-T7451 Noelle Easter Co and Prof. James Burns Center for Electrochemical Science and Engineering Motivation Knowledge Gaps Objectives The use of stainless steel fasteners in aircraft with aluminum substructure creates a galvanic couple when exposed to atmospheric conditions, leading to the formation of galvanic corrosion damage. Collaborative research program is in place to quantify, understand and model this behavior. 1. Characterize the corrosion damage induced under electrochemical condition representative of a galvanic crevice in atmospheric conditions 2. Quantify the crack formation and small crack growth behavior for different galvanically induced corrosion morphologies 3. Identify the salient features of the corrosion damage that govern the crack formation behavior for each morphology 1. How do corrosion morphologies typical of galvanic couples influence overall fatigue life behavior in AA 7050-T7451? 2. What features of the corrosion morphology influence the fatigue crack formation and small crack growth behavior of AA 7050-T7451? Water layer Stainless steel fastener AA 7050 - T7451 substructure Primer/Top Coating Pit depth does not dictate the location of the initiation site, this points toward a strong effect of micro-geometry or microstructure. Experimental Approach Step 1: Geometry dependent modeling to determine the the chemistry, pH and potential distribution for a AA 7050- CRES304 galvanic couple (C Liu/RG Kelly) Step 2: Study the microstructure interactions and establish the corrosion morphology associated with these conditions. (V Rafla/JR Scully) Surface Recession SAMPLE PREPARATION AA7050-T7451 fatigue specimens polished to 600 grit CORROSION GENERATION 2mm x 2mm area in the reduced-gage section (LS surface) of the fatigue specimen exposed to different electrochemical conditions IMAGE ANALYSIS 3D profile and top view of generated pits obtained using interferometer and optical microscope FATIGUE TEST Specimens with pits in the reduced-gage section subjected to fatigue test with a pre- determined loading protocol at 90% relative humidity FRACTOGRAPHY Fracture surfaces investigated using the scanning electron microscope 1.5-hour potential hold at -700 mV with 0.5 M NaCl + 8x10 -4 M NaAlO 2 (pH 8) generates discrete pits. 72-hour potential hold at -700 mV with 0.5 M NaCl + 8x10 -4 M NaAlO 2 (pH 8) generates surface recession. 7-day hold inside the RH chamber at 96% RH and 30 o C with droplet of 1 M NaCl + 0.022 M AlCl 3 + 0.05 M K 2 S 2 O 8 on top of the exposed area generates IGC. Results Element Al Zn Cu Mg Zr Fe Si Ti Wt % Balance 6.1 2.2 2.2 0.11 0.08 0.04 0.02 AA 7050- T7451 Composition: DATA ANALYSIS da/dN vs crack length (a) plot determined using marker band spacing 3D profile obtained using white light interferometer Top view obtained using optical microscope LOADING PROTOCOL: Constant maximum stress: 200 MPa Baseline cycle: R=0.5, f=20 Hz Marker cycle: R=0.1, f=10 Hz T L S Load induced fracture marks (marker bands) are produced on the fracture surface. T S L Conclusions References Future Work 1. Successfully developed and characterized corrosion damage typical of the galvanic couples 2. Crack formation life and feature size dominate the total fatigue life; morphology influence on small scale crack propagation diminishes after 50-100 μm beyond corrosion damage. 3. Macro-scale corrosion features do not fully capture the crack formation behavior; 2D-3D techniques have been successfully utilized to characterize micro-geometry features of surface recession/pits. 4. XCT and EBSD techniques have been initially employed to characterize IGC morphology and to identify microstructure features pertinent to crack formation. Acknowledgement 1. Use x-ray computed tomography (XCT) to locate secondary cracks and constituent particles with respect to the corrosion damage (particularly for IGC) Grain width: L: 22-1230 μm S: 12-112 μm T: 14-264 μm Fatigue specimen loaded in hydraulic frame with flexi-glass chamber to control humidity; loading direction is along L. Fatigue specimen XCT image (top), fracture surface (bottom left), EBSD image (bottom right) This work is funded by the US Office of Naval Research (B. Nickerson). 1. Burns, J.T., J.M. Larsen, and R.P. Gangloff, Effect of initiation feature on microstructure-scale fatigue crack propagation in Al–Zn–Mg–Cu. International Journal of Fatigue, 2012. 42(0): p. 104-121. 2. Spear, A.D., Li, Shiu Fai, Lind, J.F., Suter, R.M. and Ingraffea, A.R., Three-dimensional characterization of microstructurally small fatigue-crack evolution using quantitative fractography combined with post-mortem X-ray tomography and high-energy X-ray diffraction microscopy. Acta Materialia Inc. Discrete Pits Top view images of the corroded LS surfaces using the white light interferometer (top) and optical microscope (bottom) The white light interferometer is able to capture the 3D features as well as the true depths of discrete pits and surface recession. However it is not capable of determining the true depth of the IGC fissures. L S T Histogram of pit depths and crack initiation sites for discrete pits (left) and for IGC (right) Taking all the pit depth measurements, cracks do not initiate at the deepest pit for both discrete pits and IGC. However, among all initiation sites (primary and secondary), the initiation site of the primary crack is the deepest. Most secondary cracks initiated at shallow discrete pits (lower tail end of the histogram). Step 3: Determine the influence of varying morphology on the fatigue behavior and structural integrity of AA 7050-T7451 Fracture surface with marker bands is used to quantify the microstructurally small scale fatigue crack growth (left) Plot of total fatigue life and initiation life to create a 10 um crack size (right) Samples with discrete pits have the highest total fatigue life, whereas samples with surface recession have the shortest total fatigue life. Samples with longer initiation life have higher total fatigue life. Microstructurally small fatigue crack growth behavior becomes independent of the micro-feature when the crack extends away from the initiation point. 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 0 500 1000 1500 2000 da/dN, (um/cycle) Crack length a (μm) Crack growth rate vs crack length A1 A2 A3 B1 B2 B3 C1 C2 C3 Surface Recession Discrete Pits IGC The micro-feature of the crack initiation site for discrete pits and surface recession corrosion damage can be characterized by the combination of 2D and 3D imaging techniques. XCT will be used for IGC. Even for surface recession, cracks do not initiate at the deepest portion of the damage pit. Combination of 2D and 3D imaging techniques is necessary to identify the micro-features where crack initiates. Initiation site for primary crack Initiation site for secondary crack CORROSION DAMAGE CHARACTERIZATION QUANTIFICATION OF CRACK FORMATION AND SMALL CRACK GROWTH BEHAVIOR IDENTIFICATION OF CORROSION DAMAGE FEATURE Plot of crack growth rate (da/dN) versus crack length (a) for all fatigue samples with crack lengths obtained from marker band spacing 2. Use EBSD to determine the influence of crystallographic orientation on the crack growth behavior (1) SEM image of corroded surface (2) optical image of corroded surface (3) SEM image of the fracture surface (4) white light interferometer image of the corroded surface (5) 3D image of the corrosion damage 1 mm 1 mm 1 mm 1 mm (1) (2) (3) (4) (5) 1mm Once crack extends 50-100 μm beyond the corrosion damage, the growth rates merge and are consistent with each condition, thus supporting the conclusion that crack formation life and corrosion feature depth dominate any secondary effect of crack propagation behavior. *Average pit depth where primary crack initiates *52 μm *633 μm *165 μm 1 mm μm 0 50 100 150 200 1 mm μm 0 100 200 300 400 500 600 700 800 900 1000 NM 1 mm μm 0 50 100 150 200 250 300 350 400 NM IGC Surface Recession μm 0 50 100 150 200 250 300 350 400 450 500 550 600
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Transcript of Results - The University of Virginia · Discrete Pits IGC Corroded LS surfaces ... specimen exposed...
Discrete Pits IGC
Corroded LS surfaces(top) and their corresponding fracture surfaces (bottom)
T
SL
S
L
T
The Effect of Galvanically Induced Corrosion Damage on the Fatigue Crack Formation Behavior of AA 7050-T7451
Noelle Easter Co and Prof. James BurnsCenter for Electrochemical Science and Engineering
Motivation
Knowledge Gaps
Objectives
The use of stainless steel fasteners in aircraft with aluminum substructurecreates a galvanic couple when exposed to atmospheric conditions,leading to the formation of galvanic corrosion damage.
Collaborative research program is in place to quantify, understand andmodel this behavior.
1. Characterize the corrosion damage induced under electrochemicalcondition representative of a galvanic crevice in atmosphericconditions
2. Quantify the crack formation and small crack growth behavior fordifferent galvanically induced corrosion morphologies
3. Identify the salient features of the corrosion damage that govern thecrack formation behavior for each morphology
1. How do corrosion morphologies typical of galvanic couples influenceoverall fatigue life behavior in AA 7050-T7451?
2. What features of the corrosion morphology influence the fatigue crackformation and small crack growth behavior of AA 7050-T7451?
Water layer
Stainless steelfastener
AA 7050 - T7451substructure
Primer/Top Coating
Pit depth does not dictate the location of the initiation site, this points toward a strong effect of micro-geometry or
microstructure.
Experimental Approach
Step 1: Geometry dependent modeling todetermine the the chemistry, pH andpotential distribution for a AA 7050-CRES304 galvanic couple (C Liu/RG Kelly)
Step 2: Study the microstructure interactionsand establish the corrosion morphologyassociated with these conditions. (V Rafla/JRScully) Surface Recession
SAMPLE PREPARATIONAA7050-T7451 fatigue
specimens polished to 600 grit
CORROSION GENERATION2mm x 2mm area in the
reduced-gage section (LS surface) of the fatigue
specimen exposed to different electrochemical conditions
IMAGE ANALYSIS3D profile and top view of
generated pits obtained using interferometer and optical
microscope
FATIGUE TESTSpecimens with pits in the
reduced-gage section subjected to fatigue test with a pre-
determined loading protocol at 90% relative humidity
FRACTOGRAPHYFracture surfaces investigated
using the scanning electron microscope
1.5-hour potentialhold at -700 mVwith 0.5 M NaCl +8x10-4 M NaAlO2
(pH 8) generatesdiscrete pits.
72-hour potentialhold at -700 mVwith 0.5 M NaCl +8x10-4 M NaAlO2
(pH 8) generatessurface recession.
7-day hold inside theRH chamber at 96%RH and 30oC withdroplet of 1 M NaCl +0.022 M AlCl3 + 0.05M K2S2O8 on top ofthe exposed areagenerates IGC.
Results
Element Al Zn Cu Mg Zr Fe Si Ti
Wt % Balance 6.1 2.2 2.2 0.11 0.08 0.04 0.02
AA 7050- T7451 Composition:
DATA ANALYSISda/dN vs crack length (a) plot
determined using marker band spacing
3D profile obtained using whitelight interferometer
Top view obtained usingoptical microscope
LOADING PROTOCOL:
Constant maximum stress: 200 MPaBaseline cycle: R=0.5, f=20 HzMarker cycle: R=0.1, f=10 Hz
T
L
S
Load induced fracture marks (marker bands) are produced onthe fracture surface.
T
SL
Conclusions
References
Future Work1. Successfully developed and characterized corrosion damage typical
of the galvanic couples
2. Crack formation life and feature size dominate the total fatigue life;morphology influence on small scale crack propagation diminishesafter 50-100 μm beyond corrosion damage.
3. Macro-scale corrosion features do not fully capture the crackformation behavior; 2D-3D techniques have been successfullyutilized to characterize micro-geometry features of surfacerecession/pits.
4. XCT and EBSD techniques have been initially employed tocharacterize IGC morphology and to identify microstructurefeatures pertinent to crack formation.
Acknowledgement
1. Use x-ray computedtomography (XCT) to locatesecondary cracks andconstituent particles withrespect to the corrosiondamage (particularly for IGC)
Grain width:L: 22-1230 μmS: 12-112 μmT: 14-264 μm
Fatigue specimen loaded inhydraulic frame with flexi-glasschamber to control humidity;loading direction is along L.
Fatigue specimen
XCT image(top), fracturesurface(bottom left),EBSD image
(bottom right)
This work is funded by the US Office of Naval Research (B. Nickerson).1. Burns, J.T., J.M. Larsen, and R.P. Gangloff, Effect of initiation feature on microstructure-scale fatigue crack propagation in Al–Zn–Mg–Cu. International Journal of
Fatigue, 2012. 42(0): p. 104-121.2. Spear, A.D., Li, Shiu Fai, Lind, J.F., Suter, R.M. and Ingraffea, A.R., Three-dimensional characterization of microstructurally small fatigue-crack evolution using
quantitative fractography combined with post-mortem X-ray tomography and high-energy X-ray diffraction microscopy. Acta Materialia Inc.
Discrete Pits
Top view images of the corroded LS surfaces using the white light interferometer (top) and opticalmicroscope (bottom)
The white light interferometer is able to capture the 3D features as wellas the true depths of discrete pits and surface recession. However it isnot capable of determining the true depth of the IGC fissures.
L
S
T
Histogram of pit depths and crack initiation sites for discrete pits (left) and for IGC (right)
Taking all the pit depth measurements, cracks do not initiate at thedeepest pit for both discrete pits and IGC. However, among allinitiation sites (primary and secondary), the initiation site of theprimary crack is the deepest. Most secondary cracks initiated atshallow discrete pits (lower tail end of the histogram).
Step 3: Determine the influence of varying morphology on the fatigue behavior and structural integrity of AA 7050-T7451
Fracture surface with marker bands is used to quantify themicrostructurally small scale fatigue crack growth (left)
Plot of total fatigue life and initiation life to create a 10 umcrack size (right)
Samples with discrete pits have the highest total fatigue life, whereassamples with surface recession have the shortest total fatigue life.Samples with longer initiation life have higher total fatigue life.
Microstructurally small fatigue crack growth behavior becomes independent of the micro-feature when the crack
extends away from the initiation point.
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
0 500 1000 1500 2000
da/
dN
, (u
m/c
ycle
)
Crack length a (μm)
Crack growth rate vs crack length
A1 A2 A3
B1 B2 B3
C1 C2 C3
Surface Recession
Discrete Pits
IGC
The micro-feature of the crack initiation site for discrete pits andsurface recession corrosion damage can be characterized by thecombination of 2D and 3D imaging techniques. XCT will be used forIGC. Even for surface recession, cracks do not initiate at the deepestportion of the damage pit.
Combination of 2D and 3D imaging techniques is necessary to identify the micro-features where crack initiates.
Initiation site for primary crack
Initiation site for secondary crack
CORROSION DAMAGE CHARACTERIZATION
QUANTIFICATION OF CRACK FORMATION AND SMALL CRACK GROWTH BEHAVIOR
IDENTIFICATION OF CORROSION DAMAGE FEATURE
Plot of crack growth rate (da/dN) versus crack length (a) for all fatigue samples with crack lengthsobtained from marker band spacing
2. Use EBSD todetermine theinfluence ofcrystallographicorientation onthe crack growthbehavior
(1) SEM image of corroded surface (2) opticalimage of corroded surface (3) SEM image of thefracture surface (4) white light interferometerimage of the corroded surface (5) 3D image ofthe corrosion damage
1 mm
1 mm
1 mm
1 mm
(1) (2)
(3)
(4)
(5)
1mm
Once crack extends 50-100 μm beyond the corrosion damage, thegrowth rates merge and are consistent with each condition, thussupporting the conclusion that crack formation life and corrosionfeature depth dominate any secondary effect of crack propagationbehavior.
*Average pit depth where primarycrack initiates
*52 μm
*633 μm
*165 μm
1 mm
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IGCSurface Recession
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