nondestructive evaluation of aromatic polyimide spacecraft and aircraft wiring john e. white,...
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Nondestructive Evaluation of Aromatic Polyimide Spacecraft
and Aircraft Wiring
John E. White, Materials Science Dept., Johns Hopkins UniversityEdward J. Tucholski, Physics Dept. US Naval Academy
Robert E. Green, Jr., Materials Science Dept., Johns Hopkins University
Sponsor: NASA Langley Research Center
Motivation
• Significant Electrical & Fire Hazards Associated With Aging and Deteriorating Wire Insulation
• 200 Miles of Electrical Wiring on Typical Commercial Jet• Currently Wires Are Inspected Visually and Only When There is an
Existent Fault in the Wiring or When Modifications Are Made• Several Aircraft Crashes and Emergency Landings Attributed to
Electrical Wiring:– 24 Nov 1993 – MD-87 – Smoke in cabin– 17 July 1996 – Boeing 747 – In-flight breakup– 28 Nov 1998 – Boeing 747 – Several arced wires– 22 Dec 1998 – L-1011 – Electrical wires arcing– 29 Mar 1999 – MD-11 – Evidence of insulation fire– 29 Dec 2000 – L-1011- Electrical fire caused by arcing– 10 Jan 2001 – Boeing 767 – Small cabin fire on landing
Common Insulation Flaws and Factors Contributing to Insulation Deterioration
• Chafes• Cracks• Cuts• Delaminations• Embrittlement• Burns
• Vibrations• Mechanical Stresses• Moisture• Elevated Temperatures• Repeated Temperature
Variations• Exposure to Chemicals
Arcing in Polymeric Insulated Wires
• Arcing major contributor in electrical fires involving degraded aviation wiring
• Electrical discharge of an arc can have a temperature several thousand degrees Celsius
• Scintillations and flashing (400 mJ to 8.25 J) caused substantial charring and erosion of wire insulation and conductors but rarely did circuit breakers trip
• Non-contact arcing between conductors separated by insulation may require several kilovolts to initiate arcing, but if separated by carbonized insulation, arcing can occur at normal operating voltages
Swissair Flight 111/TWA Flight 800
• Major TSBC Findings on SA 111– Arcing in cockpit area quickly spread
to bulkhead insulation– Many examples of previously heat
damaged wiring found – Chafed and cut wire bundles found
near cockpit overhead circuit breaker panel
• NTSB Conclusions on TWA 800– Fuel tank indicator wiring found to
have significant damage prior to crash– 10 separate instances of recorded
repairs near or at indicator wiring in preceding 10 year span
– Ignition source traced to possible arced wire incident
– Fire spread along indicator wiring to fuel tank assembly causing explosion
Nondestructive Evaluation Techniquesto Characterize the Insulation
• Infrared Thermography
• Time Domain Reflectometry
• Pulse X-ray
Kapton® Insulation
• Samples from Boeing 707 Aircraft Provided by NASA Langley Research Center
• Samples included braided 16, 18, and 20 AWG (nominal bare wire diameters were: 0.0508 in., 0.0403 in., and 0.0320 in., respectively)
• Max. OD with insulation approx. twice nominal diameter
• Insulation existed in two or three distinct layers
Infrared Thermography
Specimen Test Stand
08:04:25 REC
IR CameraIR ControllerInframetrics 600
..IR Monitor
VHS Recorder
Computer
Video Converter
Heat Gun
Infrared Thermography System
Thermography – Single Wire Animation
Thermography – Single Wire Animation
Wire Photo
Bundle Animation
Thermography – Wire Bundles
Visible image Infrared image
Six Wire Animation
Best Image Showing Damage
TDR Method• Time Domain
Reflectometer• Measures impedance of
the wire (vertical axis) as a function of time
• Time corresponds to position along the wire (horizontal axis)
• Impedance of the wire is affected by insulation
TDR Schematic
Sample TDR Results
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2155
160
165
170
175
180
Imp
ed
an
ce (
Oh
ms)
Time (ns)
Control Wire Damaged Test Wire
Burned Insulation
Frayed Insulation
Metal Shard in Insulation
Pulse X-ray
Pulser-Specimen-CCD Array
EXR200
Pulser
CCD Array
Sample Holder
Laptop Computer
WireSample
. Trigger
Film
Film vs CCD Array Detector
X-Radiographs of Single Wires
Burnt Wire
Crimped Wire
Undamaged Metallic Braided Wires
Burned wire used for TDR
Conclusions• IR thermography is a simple, portable method for a
moderately trained technician to find defects in wire insulation.– Quick, non-contact
– Can’t see into the center of a wire bundle
• TDR is an efficient, very sensitive method of detecting and locating the position of insulation flaws.– Compare periodic surveys to baseline measurements
• Battery operated pulse x-ray with CCD array detector is a portable means of investigating and recording wire defects in-situ.
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