All-weather Strain Gage Protection Test Results...Typically: Strain gages and acoustic pressure transducers • Test Engineers do not like their aircraft to be ‘Dry’ Lack of available

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)

2NAVAIR Public Release 2019-269; Distribution Statement A – Approved for public release; distribution is unlimited

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.


Outline• Problem Statement• Scope• Requirements• Approach• Initial Method• Testing• Conclusion


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


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?


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


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


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


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


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


Test B: Moisture Protection• Vacuum/Pressure Testing Moisture/air be drawn/forced inside the wire jacket Simulate ~50k ft-13 psi delta


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


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


Test D: Vibration Testing• Vibration Testing

(Sim effects of material fatigue)Test Plan D Strain SamplesTest Plan D Vibration Test Setup


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


Contact Info

Principal InvestigatorBrandon Norris

[email protected]

Program ManagerSid Jones

[email protected]

Documents

All-weather Strain Gage Protection Test Results...Typically: Strain gages and acoustic pressure transducers • Test Engineers do not like their aircraft to be ‘Dry’ Lack of available