Materials Evaluation
May 2013
NDCM-XIII
Le Mans
1
Variations in Crack Detectability Due to
Load History and State of Stress
John C. Duke, Jr. and Nicole Capella
Damage Science and Mechanics Laboratory
Virginia Tech
jcduke @vt.edu
May 2013
NDCM-XIII
Le Mans
2
Presentation Overview
• Presumptions and Assumptions
• The Ultimate Objective
• Comparison of Crack Detection and Sizing
•Different Load Conditions during Detection
•Ultrasonic and Eddy Current Scanning,
Fracture Surface examination
•Future Considerations
May 2013
NDCM-XIII
Le Mans
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Presumptions and Assumptions
• The structure built is the same as the
design.
– The loading of components is consistent with
the design model.
– All critical cracks/flaws (manufacturing) are
detected and sized correctly (90/95 POD)
• The properties of the materials used are
the same as the handbook values.
– Crack growth is predictable
– Service regime is the same as the design
regime
May 2013
NDCM-XIII
Le Mans
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What Designers Assume
• As technology improves smaller flaws can
be detected and sized properly.
• Computational tools can be used to verify
damage tolerant response… NASAGRO,
etc.
• Computational tools can be used to
determine POD --- model assisted POD
MAPOD
• The design assumptions are valid
May 2013
NDCM-XIII
Le Mans
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Damage development begins with the
1st cycle
Life: 1.1 million cycles
(N=0



failure)
50 μm
May 2013
NDCM-XIII
Le Mans
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Small cracks grow faster than
short cracks
May 2013
NDCM-XIII
Le Mans
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The Ultimate Objective
• Characterize the condition of the structure
• Assess how the condition of the structure
will affect performance under the expected
service regime
• Alter the regime if necessary or repair the
condition to allow for enhanced
performance
May 2013
NDCM-XIII
Le Mans
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Reliability
• Variability in actual material properties
• Variability if actual level and state of stress
experienced by flaws
• Variability of size of flaws
• Variability between assumed and actual
loads
• Variability of “ideal” PoD and actual PoD
May 2013
NDCM-XIII
Le Mans
9
The Study
• Two 7075 T651 COD specimens cycled to
initiate and grow cracks
• Subject cracks to different loading
conditions during examination by Eddy
Current and Ultrasonic Examination
• Load precracked specimens quasistatically to fracture; monitor AE measure
COD versus load.
May 2013
NDCM-XIII
Le Mans
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Specimen 1
May 2013
Specimen 2
NDCM-XIII
Le Mans
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Specimen 1 unwedged 15 MHz (focus 1.5 in)
Specimen 1 wedged 15 MHz (focus 1.5 in)
May 2013
NDCM-XIII
Le Mans
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Specimen 2 no load 15 MHz (focus 1.5 in)
May 2013
NDCM-XIII
Le Mans
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Specimen 2 different loading during
examination (7.3>6.6mm, 6.9 mm>6.2mm)
May 2013
NDCM-XIII
Le Mans
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Specimen 1 Wedged top 5.6mm bottom 7.1 mm
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NDCM-XIII
Le Mans
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Comparison of Loading to Failure
May 2013
NDCM-XIII
Le Mans
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Specimen 1
Specimen 2
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NDCM-XIII
Le Mans
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top
Specimen 1
top
Specimen 2
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NDCM-XIII
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Summary
• The size of the ultrasonic beam influences
crack sizing
• The size of eddy current probe influences
crack sizing
• The state of stress experienced by the
crack during examination influences sizing
by both UT and ECT
• Load spectrum during cycling affects crack
growth and AE response
• Much more NDMC is possible
May 2013
NDCM-XIII
Le Mans
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Questions/Reactions
jcduke@vt.edu
VT Damage Science and Mechanics Laboratory
May 2013
NDCM-XIII
Le Mans
20