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.