all-weather strain gage protection test results...typically: strain gages and acoustic pressure...
TRANSCRIPT
All-weather Strain Gage Protection Test Results
23rd ITEA Test Instrumentation WorkshopMay 2019
Presented by:Sid Jones
AVMI NAWCADPatuxent River, MD
All-weather Strain Gage Protection Test Results
NAVAIR Public Release 2019-269 Distribution Statement A – Approved for public release; distribution is unlimited
Ray Meilunas, Jeremy Hill, & Matt BucchinoSealants and Adhesives TeamNaval Air Warfare Center – Aircraft Division (NAWCAD)
Research and Testing Contributions:
Lead Project Engineer:Brandon NorrisAVMI Structures LabNaval Air Warfare Center – Aircraft Division (NAWCAD)
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AbstractWhen strain gages are mounted externally to an aircraft, they can be damaged by condensing water vapor from a flight regime that sees changes in altitude (induced vacuum/pressure), temperature (freeze/thaw), and humidity (condensation). The gages can also be damaged through the direct application of water whether it's from rain at 250 KCAS or maintainers washing the aircraft with power hoses, brushes, and harsh cleaning agents.
The Navy investigated better moisture protection for strain gage installations last year. We presented preliminary findings at last year’s ITEA. Now that the project is complete, this presentation will recount its approach and test results.
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Outline• Problem Statement• Scope• Requirements• Approach• Initial Method• Testing• Conclusion
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Problem StatementTest aircraft is designated as ‘Dry’
and is not allowed to fly or sit in rain
• When is a 'Dry Aircraft' designation used? Assigned when external sensors susceptible to damage
from moisture are installed Typically: Strain gages and acoustic pressure
transducers
• Test Engineers do not like their aircraft to be ‘Dry’ Lack of available test days (it is raining) Lack of schedule flexibility (it might rain) Cost and time to project if instrumentation is damaged
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Test Scope• Baseline current sealant practices against moisture
intrusion • Determine if better approaches and/or materials
are available Sealant Effectiveness Adhesion Temperature Effects Vibration Effects Pressure Effects Solar (UV) Effects
• If so, how much better?
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Operational Requirements• Withstand direct contact with rain
Flying with drops impacting gage area Sitting on tarmac with water running down wire
• Withstand condensing/freezing water vapor Flight changes in altitude (pressure), temperature, and
humidity Gage/wires cold at altitude, condensing vapor at sea level,
freezing at altitude • Withstand direct contact with jet fuel and exhaust• These are test aircraft; Sailors are not always gentle
Abrasion resistance when grabbing, stepping, etc. Withstand power washing / cleaning solvents
• Need to apply sealant in other than lab environments E.g. hanger, flight line, etc.
• Return Jet to operational configuration
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Approach• Testing involved several types of materials initially, then
down-selected to a couple of materials for in-depth testing Test A: Evaluated adhesion to wire types
A-1: Evaluate flat sections of wire jacket using Dyne Pens A-2: Added Atmospheric Plasma treatment to wire A-3: Tested Adhesive bond strength to wire jacket
Test B: Moisture protection testing of the sealants Water immersion and positive/negative pressure
Test C: Initial Environmental testing C-1: Combined Alt, Temp, Hum; & Pressure Washing C-2: Vibration, Pressure Washing
Test D: Full Environmental testing D-1: Alt/Temp/Hum, Vibration, Pressure Washing D-2: Solar (equivalent to full year of UV exposure) D-3: Thermal Shock and Pressure Washing
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Initial Method of Sealing Gages
• Previous method of protecting gages M-Coat A, 8802 / 870 Polysulfide
• Previous failure modes Water penetrating sealant barrier Water wicking along wire jacket Possibly wicking inside wire jacket
• Problems caused Corrosion
Can lead to circuit shorts Gage glue bond failure
• Unreliable data
Bare gage and solder tabsRed Dashed Line shows Sealant Coverage
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Test A-1&2: Wire Jacket Adhesion
• Dyne Pens measure surface energy. Using a Dyne test pen gives quick and easy results
indicating surface wettability. The low surface energy of polymer-based substrates
often leads to poor adhesion of inks, glues and coatings.
• Concern of uneven surface of wire jacket giving false readings using the Dyne Pens
• Switched to using a Water Contact Angle Measurement If contact angle is greater than 90°, the surface is
said to be non-wetting with that liquid.
• Explored use of Atmospheric Plasma (AP) wand to help increase surface wetting
Use of Dyne Pen on wire jacket
Water droplet angle measurement
Atmospheric Plasma Etch Tool
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Test A-3: Adhesive Bond Strength• ASTM D3032 Testing - Tests potting bond to wire jacket
0 lbs 10 lbs 20 lbs 30 lbs 40 lbs 50 lbs
XLETFE
XLETFE w/ AP
PFTE
PFTE w/ AP
PTFE (870)
Sealant to Wire Pull TestAll using PR-1826 except where indicated
Sample Prep Sample Test
XLETFE / 1826 Pull TestNote 1826 still bonded to wire jacket
Sealant
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Test B: Moisture Protection• Vacuum/Pressure Testing Moisture/air be drawn/forced inside the wire jacket Simulate ~50k ft-13 psi delta
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Test C: Initial Environmental Testing
• Test combinations of PTFE & XLETFE wire P/S 8802 & PR-1826
Sealant• Combined cycling: Temp, Alt, Hum; 30 cycles @ 8 hours/cycle
• Pressure Wash 1250 PSI, 1.75 GPM 5 to 10 min exposure of full
plate Washed samples along
wire jacket
Test Samples in Chamber
Test Cycle
Pressure Wash
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Test D: Full Environmental
• Test XLETFE / PR-1826 XLETFE, 1:30 AP, PR-1826 PTFE, 1:30 AP, PR-1826 PTFE, 1:30 HeP, PR-1826
• Environmental Testing Altitude, Temperature,
and Humidity Testing (sim F-18 envelope)
Pressure Washing (sim rain at 250 KCAS)
Thermal Shock Cycling (sim rapid thermal
expansion) Solar Testing
(sim 1 yr of UV rad)
Pressure WashCollecting Data
Thermal Shock
Solar Test
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Test D: Vibration Testing• Vibration Testing
(Sim effects of material fatigue)Test Plan D Strain SamplesTest Plan D Vibration Test Setup
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Conclusion• Using the identified XLETFE wire in combination
with PR1826 is a much better solution for sealing strain gage installations against tested environmental effects Cross Linked Extruded Tetrafluoroethylene (XLETFE)
wire jacket PR-1826 Class B Rapid Curing Fuel Tank Sealant
Service temperature range -80°F (-62°C) to 320°F (160°C) Two-part, epoxy cured Permapol® P-3 polythioether compound. The uncured material is a low sag, thixotropic paste, suitable for
application by extrusion gun or spatula. Cures at low temperatures and is unaffected by changes in relative
humidity
• Use in actual flight tests will determine whether it’s just a better solution or actually waterproof
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Contact Info
Principal InvestigatorBrandon Norris
Program ManagerSid Jones