Download - FGS General Presentation June 2011
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The Fatigue Growth Sensor (FGS) System:Background, Improvements, Steps Forward
Prepared By
Non Invasive Technologies, Inc
Los Angeles, CA 90049310-384-2428
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FGS System Background The FGS system is a nondestructive fatigue crack
inspection system.
FGS System includes: twoelectrolyte-filled sensors, aPotentiostat Data Link (PDL), and special software.
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The FGS System - Background Developed in the early 1990s, in part through sponsorship by
the US Air Force.
Internal and external verification (Rockwell Scientific);Validation studies conducted in both laboratory and field(including the AAR FRA TTCI facility).
Can detect cracks as small as 0.001 in. in lab.
Part of SIPS program.
Detects crack growth activity and crack growth precursors.
Adds value to all inspection and predictive maintenanceprograms, as well as provides better information for models.
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Similar to an EKG
Like an EKG:
EKG (electrocardiogram) - a test that records theelectrical activity of the heart, shows abnormal rhythms.
FGS The Fatigue Growth Sensor
Constant voltage applied between structure andelectrode
Current response due to mechanical loading -> FGSsignal
A crack changes how the current flows
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Normal EKG
Abnormal EKG
Normal FGS
Abnormal FGS
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Fundamentals of FGS Uses passivation of the metallic surface to measure
fatigue damage:
Sensor polarizes area, creates passive film.
In growing cracks, cyclic loadingbreaks down passive layer.
Micro-plasticity at crack tip changes amount of exposedmetal.
As new metal is exposed,new, additional passive filmforms which changes FGSsignal.
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The Differential FGS Set-up
An FGS sensor array is applied to an area
of interest and crack detection occurs under
or around the sensor.
The Crack Measurement Sensor (CM) is
placed over the tip of the crack. TheReference Sensor (R) is placed in the same
stress field, but where there is no crack.
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The FGS System Hardware FGS Sensors
Measurement & Reference
Customizable size, shape, geometry
PDL (Potentiostat Data Link).
Precisely controls FGS sensor array voltage
Measures the current flow for the two sensors(indicator of crack condition)
Controls Data Acquisition Rate
Controls Data Collection Times
Stores Data Locally
Wireless and battery powered
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Surface Preparation Sensor Installation Hardware Installation
Wireless Access Point Data Acquisition
Typical Bridge Installation
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Example Installations
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Basics of Data Interpretation Differential FGS used.
Comparison of CM and R Sensors.
Ratio of the CM sensor and R sensor data used as firstdecision metric -> Energy Ratio.
Special software uses specially designed algorithm todetermine growth activity.
Possible Output
No Growth
Microplasticity (Future Initiation)
Active Growth
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The FGS System - Software
Frequency content and
magnitude differencesindicate crack condition.
Algorithm
preliminarilyindicates crackactivity status.
Results = Energy Ratio
No growth
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Predictive Capabilities:Threshold Lab Verifications
Controlled environment
Precise measurements
Dynamic Input
Theoretical comparisons
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Experimental Setup
Crack
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Fatigue Data
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Crack Growth Data
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Fracture Surface
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FGS and Threshold Bridge Loading
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FGS and Threshold
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Energy Ratio
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Energy Ratio = Ratio of Areas Under Crack Measurement and
Reference Measurement Curves
Energy Ratio = 1.16
DK = 2.54 ksi in
Energy Ratio = 2.4
DK = 17.25 ksi in
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Improving the Standard of Care Federal Highway Administration -
Reliability of Visual InspectionStudy of 49 State DOT bridge inspectors.
Multiple fatigue crack indications at multiple locations
All at obviously fatigue sensitive locations
Inspectors correctly identified the crack indications lessthan 10% of the time.
That means 90 out of every 100 fatigue cracks aremissed by conventional methods
Also repairs are being made 4 out of 5 times at areaswhich do not require it (80% false positives).
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Frequency Domain Output
Location 1a: No Significant Activity, ER=1.3
Typical Active Sample
Energy Ratio = 3.1
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Predictive information available for fatigue criticallocations.
Integrate with FASTRAN II.
Find hotspots in all structures
Bridges, aircraft, offshore platforms, ships, shipping cranes
Obtain additional, more useful, more accurateinformation for decisions.
Almost immediate repair verification.
Conclusions
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Next Steps for Product Improvement
Shift Focus to 7075 AL.
Polarizing voltage
Threshold correlation curves
Crack growth rate.
Localized excitation method.
Develop fracture mechanics methodology totransfer FGS into FASTRAN based code.
Long term sensor/electrolyte change.
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Approximate Inspection Costs
Costs are based on number of areas to be
inspected.
Inspection crew can cover up to ten cracklocations in one shift, depending on ease of
access.
For a simple two span bridge cost will beapproximately US$15,000.