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NOTICE

This document is disseminated under the sponsorship of the U.S.
Department of Transportation in the interest of information exchange.
The United States Government assumes no liability for the contents or
use thereof. The United States Government does not endorse products or
manufacturers. Trade or manufacturer\'s names appear herein solely
because they are considered essential to the objective of this report.

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PREFACE

The authors are grateful to the Transportation Development Centre (TDC)
of Transport Canada and the Federal Aviation Administration (FAA) for
their support in funding this project. The authors also wish to
acknowledge the funding assistance of the National Research Council
Canada, Institute for Aerospace Research (NRC/IAR) under a collaborative
research agreement.

Special thanks go to Ms. Danielle Favreau, Science Contracting Officer,
for her support and assistance, and Mr. Jean‑Louis Rene, Senior
Development Officer at TDC, who assumed the efficient coordination of
this project. Further thanks go to Mr. Bill Miller of Transport Canada
(Airthworthiness) and Mr. David G. Galella of the FAA for their input
and guidance during the project.

We are especially thankful to Jerzy Komorowski and his research team
from NRC/IAR for their technical skill, enthusiasm, and timely execution
of critical aspects of this project involving the accelerated corrosion
experiments and the corrosion modelling effort.

Research Team:

Diffracto Ad.

Dr. Frank Karpala, Project Manager, Researcher

Dr. Omer Hageniers, Researcher

William James, Researcher

Marc Noel, Researcher

Eldon Cooper, Researcher

Rodger Reynolds, Researcher

Don Clarke, Senior Technician

NRC/IAR

Jerzy Komorowski, Project Leader, Senior Research Officer

Ronald Gould, Technical Officer

Anton Marincak, Technical Officer

Nick Bellinger, Associate Research Officer

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EXECUTIVE SUMMARY

In 1992, an agreement was reached between Transport Canada Aviation
(TCA),Transportation Development Centre (TDC), and the Federal Aviation
Administration (FM) providing for research funds for Phase I of this
project. In 1994, after a very successful Phase I program, additional
funding for Phase II was agreed to by these agencies.

This report summarizes the results of the second phase of development
for a nondestructive inspection (NDI) system based on *D SIGHT.* It is a
natural follow‑on program that had four primary objectives:

develop specifications and software and design and build a field use
prototype

develop a further understanding of the corrosion process and the
sensitivity of *D SIGHT* by artificially accelerating corrosion in lap
splices and monitoring the results for additional specimens

develop a computer model of the corrosion process in a lap splice in
order to be able to predict the *D SlGHTsignatures* for a variety of lap
splice geometries that would otherwise be difficult to acquire

develop experience using the new hardware under field conditions and
record feedback from NDI managers and technicians to improve system
features, reliability, and user satisfaction

For Phase II, both hardware and software were completely changed to
reflect the knowledge gained from Phase I experience. Primarily, the
user interface was changed both in hardware and software because it is
so crucial to a successful data collection, retrieval, and analysis
system. A graphical Windows environment was introduced and a large VGA
monitor was provided to the pendant operator. Image acquisition and
retrieval becomes very easy with the introduction of a graphical turtle
diagram representation of the aircraft surface.

The accelerated corrosion testing continued from the first phase and
provided additional support for the high sensitivity of *D SlGHTto*
pillowing in corroded lap splices down to the 2 to 3 percent level. The
computer modelling and D SlGHTsimulation also provides a useful way to
study the potential of *D SIGHT* on new joint configurations as well as
draw attention to configurations that may be unacceptable or visual
inspection due to the low levels of pillowing that develop. The fact
that *D SIGHT is* measuring the extent of pillow deflections rather than
material loss also draws attention to techniques that may be reporting
false material loss simply because of manufacturing tolerances
especially at low corrosion levels. The realistic images that come from
the surfaces generated by simulation of corroded lap splices followed by
simulated *D SIGHT* inspection reinforces the need to build an image
library that can be used as a visual stand in making evaluations during
image analysis. This activity will be carried out in the follow‑on work
in Phase lII.

During Phase II, field trips were made to Northwest Airlines, in
Minneapolis‑St.Paul, Air Canada,

in Montreal, and United Airlines, in San Francisco. A field trip to the
RAF in England is also

ix

summarized to include the experience of corrosion detection with the
original prototype developed in Phase 1. The new hardware performed well
in the field trials with the exception of Connectors that were difficult
to manufacture and broke down easily. These connectors were eventually
discarded in favour of a newer style that provided better strain relief.
The major modifications to the optical train were particularly
successful in eliminating the contamination from dust and highlighter.
The new software philosophy using a turtle diagram (planar surface
drawing) graphical user interface was particularly well received by NDI
personnel but the software itself was not entirely error free during the
field trials. However, the process of planning, acquiring, and analyzing
images did force many of the features to be exercised in the field.
Improvements to the software and hardware were introduced based on these
findings.

x

1 INTRODUCTION

The first phase of this project was to develop a breadboard system for
the nondestructive inspection (NDI)ofaircraft lap joints for corrosion
and to develop an understanding of the detection capability and
sensitivity of *D SIGHT* relative to other technologies. The breadboard
system developed, based on *D SIGHT,* was extremely successful and the
reasons for its high detection sensitivity to corrosion became known.
The results of this first phase are summarized in the Transportation
Development Centre (TDC) report, TP 11 983E, or the equivalent Federal
Aviation Administration (FM) report DOT/FM/CT‑94/56, entitled,
\"Characterization of Corrosion and Development of a Breadboard Model of
a *D SlGHTAircraft* Inspection System\", published in Feb. and Aug.
1994, respectively. The system became known as DAIS which stands for D
SlGHT Aircraft Inspection System.

The second phase of this project extended the results of the first phase
and included the following major tasks. The primary organization
responsible for completing each task is indicated in parentheses.

develop specifications, design, and build the prototype hardware
(Diffracto)

design, code, and debug system software (Diffracto)

perform accelerated corrosion testing and NDI on library corrosion
specimens to document detection sensitivity (NRC/IAR)

develop a computer model (finite element) of the corrosion process in
lap splices and use the result to generate simulated *D SlGHTimages*
from the output surface (NRC/IAR and Diffracto for image generation)

obtain hands‑on experience and performance evaluation from field trials
with the new prototype system at airline overhaul facilities (Diffracto,
NRC/IAR)

These tasks were divided into essentially two areas: hardware and
software development of the technology and determination of sensitivity
and corrosion detection ability from a practical and theoretical level.
The two came together during field trials that attempted to make use of
both the developed hardware and the knowledge of detection capability
derived from the modelling and laboratory work.

1

2 HARDWARE DEVELOPMENT

2.1 Summary of Major Revisions

At the end of Phase I, a number of important recommendations were made
to improve the breadboard sensor and host hardware. The main problems
were as follows:

sensor weight too great for ease of use

sensor optics were susceptible to highlighter contamination

small mirror could not be used for illumination and viewing due to the
constant danger of dust contamination

camera depth of field needed improvement

host computer weight was excessive

power supply was bulky along with the host computer

pendant monitor image was too small and of poor quality

loose connector mating

All of these problems were addressed in Phase II. In fact, a major
redesign of the sensor, host, and pendant was initiated. The sensor was
redesigned completely to take advantage of a new board‑level camera with
greater sensitivity than the previous model. The optics were packaged to
be as free of contamination as possible. Also, the enclosure was
redesigned with sections removed and replaced with cloth to produce a
lighter sensor. The weight was reduced by 10 percent while eliminating
many of the contamination issues experienced with the first prototype.

Because of a major skim in software philosophy, major hardware changes
were made to move away from a reliance on RS‑170 video to VGA video. The
VGA video mode would permit the display of both images as well as
graphics. The VGA mode also required a new custom cable to permit VGA
signals to the pendant. In addition, a new compact power supply
providing a 48 VDC power bus was mounted directly in the host. The host
computer was also changed with a smaller and lighter model having an LCD
display for the VGA. The result was fewer components and a lighter
system overall. The VGA graphical display mode also required a new
display for the pendant along with new packaging. All the improvements
and specifications of the new hardware are outlined in the sections that
follow.

2.2 Hardware Description

The new DAISsystem consists of the following components:

*D SIGHT* image acquisition sensor

host controller

pendant controller

printer

These components are depicted in figure 1 in the typical configuration.
The purpose of the system is to acquire *D SIGHT* image data from
aircraft surfaces for visual interpretation by an inspector. The system
also provides the ability to display and print the image data for

2

interpretation, store, and retrieve the image data for record keeping,
record analysis results and repair areas, and permit user input of
identifying information.

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2.2.1 Host Controller

The host controller consists of an industrial PC computer with an LCD
VGA display monitor, keyboard, and disk drives as well as hardware
necessary for image capture, communication with the pendant and sensor,
and printing functions. Image video display capability is provided at
the host on the 10‑inch VGA display. User input is made through keyboard
entry and a built‑in mouse. The power and control modules for the remote
pendant and sensor reside in the host controller enclosure in an
extended chassis. A custom cable connect panel is supplied to interface
to the special boards and power modules. The system can be powered with
switchable 1 20/240 VAC.

3

The minimum computer specifications are as follows:

.486‑66 MHz processor

16 MB RAM

1.44 MB, 3.5\" disk drive

540 MB hard drive

parallel port

TFT LCD 10‑inch VGA monitor (256 simultaneous colors) 101‑key keyboard
built‑in mouse

Additional hardware include:

8 bit, 640x480 image capture board

D/A l/O board for light control

power supplies for pendant and sensor

2.2.2 Pendant Controller

A portable pendant device is an integral part of the system to provide
control of the sensor functions at a remote location from the host. The
pendant is connected to the host by a cable approximately 30.5 m (100
ft.) to minimize the movement of the host controller during inspection
tasks. A cable, about 4.5 m (15 ft) in length, is provided to connect
the remote pendant to the sensor. Each cable is equipped with industrial
connectors and mating connectors are provided on the sensor, pendant,
and host controller. An industrial custom cable is provided for RS‑170,
VGA signals, RS‑232 signals, power, and sense lines.

The pendant consists of a TFT LCD 10‑inch VGA video display monitor with
an integral touch screen for user input of control commands. The monitor
and cable connectors are packaged in an industrial enclosure with
convenient handles. The pendant will provide full control of image
acquisition and the ability to display video and system menu items. The
new pendant and host computer are shown in figure 2.

2.2.3 DAIS‑250C Sensor

The DAIS‑250C corrosion sensor consists of an enclosure with integral
handles, a COD camera and lens, a white light source, a retroreflective
screen, and glass mirrors to reduce package size. The optical components
for the sensor have been optimized to maximize detection sensitivity for
pillow signatures caused by corrosion. The camera, lens, light source,
and electronics are enclosed in a self‑contained module equipped with a
filtered fan system in order to protect the components from dust and
contamination. A thermal switch is included to protect the components
from excessive heat in case of component failure or blocked air
circulation.

Each corrosion sensor is equipped with an identification (ID) number
chip to identify it uniquely to the DAIS system. In addition, a
solid‑state COD camera is provided with a industrial camera lens. A
low‑power white light source is provided with a bulb life exceeding 2000
hours. For ease of cleaning, a commercial retroreflector is used with a
protective coating.

4

The sensor enclosure for the DAIS‑250C sensor is a hybrid design having
both rigid end caps to protect critical components and a central
cloth/frame section to reduce weight. The enclosure serves two purposes:
to support the internal components and geometry and to block ambient
light that would reduce image contrast and sensitivity from entering the
inspection area. Manipulation of the sensor into any orientation is made
possible by rigid handles at approriate locations. The enclosure
footprint and field of view are rectangular in shape and the inspection
area varies in she depending on the sensor type. No‑mar feet and a
light‑blocking rubber skirt are provided at the base of the sensor to
reduce possible damage to the aircraft surface and to prevent ambient
light from entering the sensor. The completed sensor is shown in figure
3 and a drawing showing internal structure is shown in figure 4.

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3 SOFTWARE DEVELOPMENT

3.1 General

The problem of coordinating and finding inspection data from specific
locations on the aircraft has led to the development of a graphical user
interface for the collection, retrieval, and analysis of image data
based on the concept of a turtle diagram (a planar drawing of the
surface of an aircraft). Figure 5 shows a turtle diagram with the basic
characteristics required: reference labels having meaning to the
aircraft and NDI technician and critical placement positions for the
inspection along lap splices representing areas requiring inspection.
Reference to the image data can be made by selecting the box associated
with the position on the aircraft drawing.

DAIS software uses this Windows graphical user interface during the
inspection process both at the host and pendant. In addition, a modular
structure is used to simplify and isolate major functions that need not
require all system hardware. The Windows software environment is ideal
for this modularity and has the added capability of simultaneously
displaying live video in a window and graphics on a VGA monitor.

The software structure for DAIS is based on five broad operational
requirements that have been identified for the inspection system:
inspection plan creation, sensor installation and calibration, image
acquisition, image analysis and reporting, and repair planning. Only the
sensor install and calibrate module and the image acquisition module
require connection of the entire system hardware; otherwise, the
remaining modules could be used at the host in the NDI shop.

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3.2

DAIS File Structure

Three main types of files in the DAIS system will record the inspection
activities. The inspection plan files (.PLN) are designed once for a
specific aircraft type and can be reused or modified depending on
maintenance requirements and schedules. The aircraft work files
(.AIR)embody a particular inspection plan file (i.e., contain the plan
within it) along with the inspection status, analysis results, repair
results, and image filenames for a particular aircraft tail number. The
aircraft work file will incorporate information from the acquisition
module, the image analysis module, and the repair module as it is
updated. All image data will be stored in individual files in either
Bit‑MaPped image file format (.BMP) or in Tagged Image File format
(.TIF) depending on the user\'s preference. The names of the image files
will be created automatically from information related to the position
of the placement in the plan, the sequential group number, and the
sequential number of the placement within the group. Users will
interface to these image files graphically through the turtle diagram.
The aircraft work file could be archived for comparison with future
inspections even if all the images associated with it have been deleted,
since it records the defects found and any repair actions on the turtle
diagram and not the images themselves. Figure 6 shows the five modules
schematically and the corresponding input and output files.

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3.3 Description of Program Modules

3.3.1 Inspection Plan Module

The inspection plan module is basically a graphical editor that creates
an inspection plan from an existing planar drawing of the aircraft
(turtle diagram) and the footprint of sensors needed to accomplish the
desired inspection. The goal of this module is to allow an experienced
inspector to design a plan of inspections for a set of DAIS sensors from
the inspection requirements of a regular maintenance schedule or service
bulletin. The inspection plan can then be executed by NDI technicians
who will be guided by the plan each time the inspection is needed. This
will result in a consistent set of inspections that can be compared over
time. The aircraft drawing is presumed to already exist and must satisfy
a limited set of constraints during its creation. Once an inspection
plan is created it can be used over and over again. More than one
inspection plan may be created due to different inspection schedules or
needs.

Table 1 summarizes the functions and the menu system for inspection
planning.

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corrosion once it is initiated without specimen disassembly. This points
to the need for any studies attempting to compare various corrosion
NDItechniques to be carried out in a short period of time to prevent
significant changes in corrosion levels.

Corrosion library panel 56T19 was the source for one accelerated
corrosion specimen representing the lap splice construction of a Douglas
DC‑9 aircraft. This specimen is from the intersection of a lap and
circumferential splice.

Three DC‑9 lap splice specimens from Phase I were included in the last
corrosion exposure period. These specimens (25, 26, and 27) were part of
an investigation into the influence of rivet spacing on *D SIGHT*
inspection signatures.

The specimens are each nominally of 0.19 m2 to 0.14 m2 (1.5 to 2 sq.ft.)
in surface area. Each of the specimens was inspected by *D SIGHT* and
shadow moire prior to inclusion in the corrosion test and at subsequent
inspection intervals and with eddy current during the final inspection.

4.2 Specimen Disassembly and Reassembly

Selected specimens were opened to prepare one or more surfaces of the
joint for corrosion activity. The fasteners were removed, and if
present, the adhesive layer or sealant was also removed. The formed tail
of the aluminum rivet was first drilled with a bit having a smaller
diameter than the rivet . This reduced the rivet strength and made it
easy to chisel off the formed tail. The remaining rivet was then punched
out with the specimen placed face down on a padded surface.

Those specimens which incorporated an adhesive layer were heated to 50°C
to facilitate separation of the skins. The selected surface was abraded
with a medium or ultrafine ScotchBrite disc to remove the protective
primer and/or cladding. The speed of the tool was regulated to avoid
overheating the skin.

The specimen components were relocated with and held initially by Kleeko
fasteners. New rivets were installed in the original holes. An air
driven riveting gun was mounted in an arbour press and configured to
allow a single operator to carry out the riveting procedure. This
procedure also reduced the introduction of deformations and marks from
hand‑held riveting. The original countersinks were adjusted when
required to provide a flush fit for the head of the new rivet.

The outboard surfaces, which were to be monitored during the test, were
brought to a uniform condition. Where the original paint/primer system
was damaged and degraded, it was chemically striped and repainted in a
gloss white finish. Intact original paint systems were retained but were
spray painted gloss white to cover multiple colors. The inboard surface
and new fasteners were spray painted with a zinc chromate primer.

21

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4.3 Corrosion Test Setup and Specifications

The prepared specimens were suspended by plastic straps inside a
Singleton Salt Fog Test Cabinet (SCCHModel #22) and arranged so that
they did not contact or overlap each other.

The cabinet is made of plain steel with an interior lining of inert,
oven‑cast, seamless PVC. It is uniformly heated by water jackets on all
four sides and the bottom. The top is covered with a Plexiglas dome. The
cabinet is equipped with a humidifying (bubble) tower with a micro
bubble aerator for generating humidified air. Distilled water inside the
bubble tower, which is fed from a 10‑gallon supply reservoir, is
maintained at a constant level by a Level‑matic (patented) control unit.
The bubble tower is instrumented with a pressure gauge and an airflow
meter. The output flow rate is controlled by regulating the pressure of
the air supply to the tower. The humidified air from the bubble chamber
is fed into a fog tower reservoir inside the cabinet where it is mixed
with the salt solution before being dispelled through an atomizer into
the dispersion tower. The latter is fitted with internal baffles to
produce a homogenous mist of free‑falling spray without directional
corrosion effects. The salt solution is brought into the fog tower from
another reservoir (36 gallon capacity) by gravity feed. The temperatures
inside the bubble tower and the cabinet are monitored by automatic
electronic controls with digital displays. The corrosion cabinet was
fitted with an exhaust condenser assembly, wherein the salt fog exhaust
from the cabinet was condensed by mixing it with a continuous supply of
recirculated water and collected for disposal into a third reservoir.

The cabinet was operated in the manual mode under a continuous spray
cycle with daily system function checks by the operator.

For the application of accelerated corrosion to the lap and
circumferential splice specimens, the humidifying (bubble) tower
temperature was maintained at 35°C (95°F) and the temperature inside the
salt spray cabinet at 48°C (118°F).

A bubble tower pressure of 90 KPa (13 psi) is used in the operation of
this cabinet to prevent contamination of the room environment as was
experienced in Phase I when the chamber was operated at the recommended
pressure of 125 KPa (18 psi). This results in a fog collection rate of
0.8 ml per hour.

As per ASTM standard B638 for Copper Accelerated Acetic Acid Salt Spray
Testing (CASS Test), the salt solution was prepared with 5 parts by
weight of sodium chloride dissolved in 95 parts of distilled or reagent
grade water and reagent grade copper chloride (CuCI2\~2H2O) added at a
rate of 0.25 g/litre. The pH of the salt solution is lowered to a value
of 2.8‑2.9 by the addition of glacial acetic acid, which provided the
atomized salt spray with a pH value between 3.1 and 3.3.

4.4

Pre‑ and Postexposure NDI

At intervals, the specimens were removed from the salt spray cabinet and
inspected for corrosion activity. Prior to inspection, the specimens
were washed with water to remove the accumulated salt deposits. The test
matrix includes a total of five inspection periods: at O hr.
(precorrosion), and at 695, 1145, 1455, and 1647 furs.

23

4.4.1 *D SIGHT*

*D SlGHTand* shadow moire image records were obtained of the outer
surface condition of each specimen at each inspection. *D SIGHT*
inspections were carried out with a DiffractoLimited DAIS‑250C unit
prior to testing and at each inspection interval. As the test progressed
the surface condition of the paint system degraded sufficiently to
require the use of a temporary highlighter fluid. The paint system was
chemically stripped from all specimen outer surfaces for the final
inspection. *D SlGHTimages* of the specimens prior to testing and at the
final inspection are on file with NRC/IAR.

4.4.2 Shadow Moire

Shadow moire image records were obtained for all specimens prior to
testing and at each inspection interval. Initially a single 8 x 10 inch
200 line per inch screen was used to obtain a sequence of images
(typically 3) which spanned the length of the specimen and required
splicing of the separate images into one record of the entire lap
splice. From the second inspection interval onwards, a system was
available which allowed the imaging of the entire splice at one time.

Shadow moire images of the specimens at the second inspection and at the
final inspection are on file with NRC/IAR.

4.4.3 Eddy Current

Reference standards or calibration specimens were constructed for each
particular type of joint included in the accelerated corrosion program.
Each was made from the actual aircraft specimens so as to duplicate the
material properties, conductivity, thickness, joint construction, and
paint system. Slots were machined into the calibration specimens to
simulate loss of material at increments of 5, 7, 10, and 15 percent as
shown in figure 9. The eddy current equipment was set up according to
the appropriate calibration specimen each time a corrosion specimen was
checked for material loss. Prior to the specimens being introduced into
the salt­fog corrosion tank a section of the joint was removed and
retained as a sample of noncorroded structure.

The following is a list of equipment used to carry out the eddy current
inspections:

Zetec MIZ‑40 Eddy Current Instrument

Dupont Portascan 2‑axis Scan Hardware and Software for MIZ‑40

486‑66MHZ Notebook PC with Docking Station

Probes Optimized for First Layer Corrosion Detection

Experimental Probes for Second Layer Corrosion Detection

Previous eddy current inspections using the Portascan (Phase I) were
setup and gated to detect corrosion in the first layer. The effect of
sealant and gaps between the first and second layer was thought to be
minimal and the Dating of the eddy current signal was set to remove
lift‑off only. With the arrival of a more powerful computer, new gates
became available in the software to collect the MIZ‑40 signal. The new
gates provided better data collection and the ability to remove the
effect of gaps between the first and second layers. A more reliable map
of the

24

corrosion specimens could now be generated. At present five percent
thickness loss or greater can be reliably detected in the first layer.

The selection of the appropriate eddy current probe was not a
straightforward. Initially the probe was a standard off‑the‑shelf single
coil unit selected to match the first layer skin thickness. Optimum
setup with the MIZ 40 gave results of 10 percent or greater thickness
loss with a \"go\" or \"no‑go\" type of mapping on the Portascan. To
improve upon this sensitivity, a reflection type probe was obtained. In
this type of probe the pickup coil is separate from the drive coil. The
new probe increased the sensitivity of the system to material loss and
gave fewer false calls. Through numerous setups and different suppliers,
an optimum probe and configuration were found which detected a loss of
thickness to 5 percent or greater with the ability to map various levels
of corrosion. One probe, which had the greatest sensitivity, was sent to
the manufacturer along with our calibration specimen and MIZ‑40 settings
for improvement. This advanced probe is currently in use. Other sources
of corrosion probes are being investigated.

The DC‑10 specimens have introduced problems for eddy current
inspection. Until now the corrosion specimens were inspectable because
the lap joint was in the first layer. In the DC‑10 specimens, the joints
are covered with a \"beauty strip\" so the lap is in the second layer. A
calibration specimen was constructed from a DC‑10 structure. Material
was removed to represent a 5 percent and 10 percent thickness loss in
the back face of the beauty strip, the lap joint front face, and the lap
joint back face. The current probes and setup can reliably detect
material loss in the first layer only. Corrosion detection in the second
layer is difficult.. A lower frequency must be used to penetrate to the
second layerwhich results in a diminished sensitivity to material loss.
The effect of gaps and sealant between the first and second layer
results in an increase in the number of false calls due to the inability
of the single frequency technique to distinguish between an air gap and
the presence of corrosion.

In an effort to solve this problem a set of probes manufactured by Zetec
is currently being evaluated. The manufacturer has made these probes
specifically for the detection of corrosion in the second layer. As
well, a new technique utilizing \"signal mixing\" is now under
development. The eddy current probe is now excited at two separate
frequencies. Corrosion detection in the second layer is enhanced by
minimizing the undesirable signals by virtue of the mixing process.

The test matrix includes a total of five inspection periods: at 0 hr.
(precorrosion) and at 695, 1145, 1455, and 1647 furs. There were some
departures from this inspection regime and these are all shown in tables
7 to 12.

The calibration test pieces are made from the same material as the
corrosion test specimens. The material is machined from the skins as a
ratio of thickness loss.

25

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height="8.981944444444444in"}

4.5 Results and Discussion

Periodic inspections of the exposed specimens were limited to *D SIGHT*
and shadow moire since most of these did not indicate any corrosion
activity. Only during the last inspection interval were positive
corrosion indications recorded. This would suggest that corrosion
requires a gestation or initiation period even on the surfaces on which
the alloys were exposed following the removal of the protective
coatings.

The considerable time of total exposure resulted in a degradation of the
exterior specimen surfaces. Both optical inspection methods require a
suitable surface condition. For *D SIGHT,* a temporary application of
solid film highlighter was necessary in some cases. Shadow moire
requires a flat uniformly coloured surface, hence a white powder coating
was applied prior to inspection.

Specimen number 21, which was a backdrilled lower window belt lap joint
and doubler, skin/stringer specimens 14 and 16, and lap splice specimen
18 (all typical of DC‑10 constructions) did not develop detectable
levels of corrosion.

The rivet spacing ratio in the L1011 joints is approximately 0.5 (i.e.,
the distance between fasteners along the splice is twice the distance
between the rows). As could be expected from the model of pillowing
developed under Task 1.6, the *D SIGHT* signature is dominated by a wave
occurring between fasteners rather than the depressed fasteners which
are typical of a rivet spacing ratio 1.0. The rivet spacing ratio of 0.5
results in a configuration amenable to *D SIGHT* corrosion detection.

Specimen number 24 contains both a circumferential butt splice and a
longitudinal multi‑layered splice, both typical DC‑9 lap constructions.
The modelling of corrosion pillowing for the DC‑9 longitudinal joint
indicated that corrosion detection will be more difficult for this type
of joint. As predicted by the pillowing model, the bulging along the lap
splice was not readily detectable in the longitudinal view but quite
pronounced in the perpendicular view. This is directly related to the
rivet spacing ratio for this splice being close to 2.0. The eddy current
inspection of this lap splice is difficult as well due to the rivet
spacing and probe size requirements.

As in the Phase I accelerated corrosion testing, the *D SIGHT* and
shadow moire methods produced similar corrosion indications. Eddy
current indications were absent in specimens 9, 12, 15, and 24. The
corrosion levels in these specimens seem to be below the 5 percent
thickness loss threshold of the eddy current corrosion detectability as
used in this study. All other eddy current findings correlated with *D
SIGHT* and shadow moire indications. The level of corrosion found by
eddy current varied from 5 to 7 percent thickness loss.

All specimens will be subjected to teardown and microfocus X‑ray
inspections during Phase lil of the project.

A significant corrosion initiation period was observed. Once corrosion
had formed, the accumulation seemed to be relatively rapid. Significant
differences in *D SIGHT* corrosion signatures were recorded. These can
be related to the different rivet spacing ratio as predicted by
modelling.

31

D SlGHTcan detect corrosion in a lap splice below 5 percent corrosion
thinning level. For rivet spacing ratios greater than 1.0, DAIS head
placement along the greater rivet spacing direction might facilitate the
inspection.

Future studies of NDImethod corrosion detection capabilities will have
to take place in a short period of time prior to test specimen teardown
as it is very difficult to arrest the corrosion process.

Temporary solid film application to corroded external specimen surfaces
for D SlGHTinspections was shown to be an effective highlighting
altemative.

32

5 CORROSION MODELLING AND D *SIGHT* SIMULATION

5.1 Motivation

The application of *D SIGHT* to detect corrosion in lap joints is based
on the observation that corrosion produces surface defommations in the
aircraft skin commonly known as \"pillowing\" in between the fasteners
of the joints. These deformations are caused by the accumulation of
corrosion products between the faying (surfaces) elements of the joint.

Preliminary work carried out in Phase I of this agreement indicated that
the level of bulging was influenced by the corrosion product volumetric
increase. This increase was found to be over 6 times greater than the
thickness loss which explained the high sensitivity of *D SlGHTto* lap
joint corrosion. Phase II of this project proposed that finite element
techniques be used to determine the surface defommation of corroded lap
joints. These deformations would then be used to simulate *D SIGHT
images* of the surfaces so that other joint structures could be
simulated in the future without relying on actual samples. In most cases
it is difficult, if not impossible, to obtain corroded specimens of a
wide variety of joint configurations at various levels of corrosion.

The chemical composition of the corrosion product detemmined in Phase I
was used to develop a mathematical model for correlating the amplitude
of the pillowing deformation of the outer skin of a lap joint to the
degree of corrosion inside the joint. The model was first used to
detemmine the degree of corrosion in a plate that was fixed at four
comers. Finite element techniques were then used in conjunction with the
mathematical model to determine the amplitude of the pillowing in actual
lap joints. These results were used to generate simulated *D
SlGHTimages* of corroded lap joints for calibration of the *D* SlGHT
Aircraft Inspection System (DAIS).

Corrosion tests will be carried out on a limited number of specimens to
verify the modelling in Phase lilt Given the large variety of lap joint
designs, modelling should be a more cost effective way to calibrate the
*D SIGHT* lap splice images.

The following sections summarize technical report, LTR‑ST‑2005, written
by N.C. Bellinger, J.P. Komorowski, and S. Krishnakumar from NRC/IAR,
that describes the model derivation and computer simulation results.

5.2 Mathematical Model and Assumptions

An analytical model of the pillowing of fuselage lap joints was
developed on the premise that after the lap joint disbands, assuming the
surfaces were initially bonded, the aircraft skin between the rivets
deforms perpendicularly to the joint surface. This creates a space
within the joint due to the additional volume required by the corrosion
product. Chemical analysis carried out at NRC on corrosion samples
indicated that the insoluble product consisted of aluminum oxide
trIhydrate (aluminum hydroxide) mixed with a small proportion of
aluminum oxide monohydrate. The molecular volume ratios for the
different oxides are given in table 13. As can be seen from this table,
the molecular volume of aluminum hydroxide is approximately 6.5 times
that of pure aluminum.

33

![](media/image30.emf){width="6.708333333333333in"
height="3.2895833333333333in"}

The model assumes that the product of corrosion is distributed within
the joint such that a uniform lateral pressure is exerted on the
fuselage skins. It was also assumed that the lap joint was symmetrical
about its mid‑plane, thus only the outer skin was modeled. The
closed‑form classical plate theory of Timoshenko and Kriger was used to
calculate the deformation of the outer skin supported by equidistant
rivets and subjected to a uniform lateral pressure. As a first
approximation, the dimensions of the rivets were taken to be small, that
is, they offered only point supports. The plate segment bounded by four
rivets was considered to be pinned at its four comers and subjected to a
uniform pressure. Symmetrical boundary conditions were applied to all
four edges of the plate.

5.3 Finite Element Model

The mathematical model was derived for a section of an infinite flat
plate with rivets at each comer simulated by point supports. However, an
actual lap joint, can contain free edges and stiffeners, which cannot be
modeled using a closed‑form solution. Therefore, finite element
techniques were required to develop a model of an actual aircraft lap
joint. The commercially available NISA finite element code, which
operates on the Silicon Graphics Series L Challenge at NRC, was used for
all numerical analysis. Initially, a convergence study was carried out
on the flat plate model to determine the effect that element type and
size had on the accuracy of the results. Two finite element models were
generated for the convergence study; one used 3‑D second‑order general
shell elements with six degrees of freedom (three translation and three
rotational), while the other used 3‑D second‑order brick elements with
three degrees of freedom (three translational). Both models were fixed
at the four comers, and symmetrical boundary conditions were placed
along the edges. A constant pressure of 6.89 kPa (1 psi) was applied
nommal to the surface of the plate. As a criterion of convergence, when
the finite element simulation results were within 3 percent of the
closed‑form solution, the model was assumed to have converged.

34

Using the results from the convergence study, two fuselage lap joint
configurations were modeled; one with a rivet spacing ratio, b/a,of 1
(equally spaced rivets) and the other with a ratio of 2. Since lap
joints contain stiffeners, the joints were not symmetrical about their
mid­planes and thus both skins had to be modeled. Initially, a constant
pressure of 6.89 kPa (1 psi) was applied to both the outer and inner
skins of the joint.

5.4 Results and Discussion

5.4.1 Mathematical Model

A FORTRAN program was written to calculate the central deflection and
volume under the deflected plate as well as the deformed shape for
various rivet spacings. The central deflection height and volume under
the deflected plate were used to normalize the data for analysis
purposes. The maximum deflection occurs when the rivet spacing is equal
(i.e., fomms a square) and tends to level off as the rivet spacing ratio
increases. The deformed shapes of plates with rivet spacing ratios of
1.0, 1.5, and 2.0 are shown in figure 10. The plots are normalized with
respect to the central deflections. For equally spaced rivets, the
deflection at the mid‑points of the edges is 0.75 times the central
deflection. However, as the rivet spacing ratio increases, the relative
deflections at the shorter edges decrease while those at the longer
edges increase. For example, the deflections at the mid‑points of the
short and long edges for a rivet spacing ratio of 2 are 0.18 and 0.995
times the central deflection, respectively. This indicates that if
visual inspection was used to determine the extent of corrosion within a
joint with a high rivet spacing ratio, it may be difficult to detect the
deformation. Also, since eddy current inspection techniques require a
relatively large probe due to the low frequencies required to penetrate
the skin thickness, it could be difficult to detect corrosion between
the closely spaced rivets. On the other hand, *D SlGHTwould* have a
clear advantage since it is adaptable to the detection of any
out‑of­plane phenomena leading to a change in surface topography greater
than 4 micrometers.

5.4.2 Finite Element

As mentioned earlier, two finite models were generated to determine the
accuracy of the 3‑D general shell and 3‑D brick elements to model the
out‑of‑plane displacements. The mesh size was refined to determine the
effect that element size has on the accuracy of the results. From this
study it was concluded that the 3‑D general shell elements could not
accurately model the pillowing effect. Upon refining the mesh, the 3‑D
general shell elements produced results that were, at best, 7 percent
higher than the closed‑form solution while the 3‑D brick elements were
within 2 percent. Having met the convergence criterion, the 3‑D brick
elements were used to model all lap joints.

A finite element model was generated using first‑order brick elements
for a lap joint with three rows of equally spaced rivets. First‑order
elements were used in order to increase the number of data points which,
in tum, would increase the accuracy of the simulated *D SIGHT* image.
Symmetrical boundary conditions were applied along the long edges of the
joint, while clamped boundary conditions were applied along the short
edges. The z‑displacements were fixed along the centerline of the
rivets. A constant pressure of 6.89 kPa (1 psi) was applied to both the
outer and inner skins.

35

The out‑of‑plane displacements obtained from the initial run were used
to determine the volume under the deformed shape. The required volume
for a number of percentage thickness losses ranging from 2 to 20 percent
were then determined. The applied pressure was modified and the finite
element analysis was re‑runfor each case. The displacement results for
the outer skin were then used in a program, written by Diffracto Ltd.
which simulates the D *SIGHT* inspection technique. This program uses
the algorithm referred to as \"ray tracing\" which is a three
dimensional rendering technique that produces realistic signatures.
Since symmetrical boundary conditions were used in the finite element
analysis, the length of the lap joint was increased by mirroring the
out‑of‑plane displacements. The resulting images for a joint of
approximately 30 cm (12 inches) in length, are shown in figure 1 1 for a
percentage thickness loss of 2, 6, 10 and 15 percent. These four
signatures are presented to show the clear difference which occurs due
to the increase in corrosion products. Note that as the corrosion
products increase, the rivets are more pronounced.

Further analysis was carried out on the simulated images using the image
processing system (IPS) developed by the Structures, Materials, and
Propulsion Laboratory of IAR/NRC. Light intensity profiles were obtained
along five lines in each image, one along each rivet row and one in
between the rows. All the profiles had a sinusoidal waveform with the
maximum peak occurring in the vicinity of the rivets. To compare the
different profiles, the average light amplitude was calculated for the
row of rivets near the edge of the joint for each image. The average
light amplitude increased linearly as the thickness loss increased up to
8 percent. When these results are normalized with respect to the average
overall light intensity (obtained by averaging the line profiles) for
each image the values continuously increase, nonlinearly, until the 10
percent thickness loss after which they appear to remain constant.
However, since figure 11 shows a distinct difference in the simulated D
*SIGHT* signatures between the 10 and 15 percent thickness loss, then
the profiles should also be distinctive. The difference between the 10
and 15 percent thickness loss profiles is the presence of what can be
temmed a speak secondary waveform\" at the location of the rivets.

36

![](media/image31.emf){width="6.746527777777778in" height="9.375in"}

![](media/image32.emf){width="6.746527777777778in" height="9.375in"}

The line intensity profiles also indicate that the amplitude of this
secondary waveform increases as the corrosion products increase above 10
percent. These image processing results indicate that it may be possible
to attempt an automated *D SIGHT* image analysis. It is important to
note that the most common NDI techniques (i.e., eddy current) measure
the thickness of the skins and estimate corrosion by comparing the
measured thickness to the original (or manufactured) thickness
specification. The measured thickness may be affected by prior
maintenance activity, manufacturing tolerances, and corrosion. Therefore
the amount of material loss in a thin skin for a small amount of
corrosion (less than 5 percent) could be well within the manufacturers
tolerance for the aluminum sheet. On the other hand, *D SIGHT* measures
the surface deformation or deflection caused by the formation of the
corrosion product and not the actual thickness loss. The magnitude of
the defommation is significantly larger than the magnitude of the
material loss. As a result, *D SIGHT is* capable of distinguishing a
very small change in percentage thickness loss by measuring the surface
deflection, as shown by the ray tracing results.

The finite element model of a multilayer lap joint with a rivet spacing
ratio of 2 was also generated. The applied boundary conditions are
identical to those used in the previous model. Since this was a
multi‑layer joint, it was assumed that corrosion would occur in the
first layer where the outer and inner skin make contact and in the
second layer between the finger doubler and the inner skin. The
configuration of the joint is such that a gap is present between the
inner and outer skins caused by the placement of the finger doubler.
This gap is significant in that it can contain corrosion products
without causing out‑of‑plane displacements. To take this into account,
the gap volume was determined and compared to the volume required by a
specified thickness loss and the pressure applied to the gap was
modified accordingly.

The out‑of‑plane displacements for a 5 and 10 percent thickness loss
were used to generate the simulated D *SlGHT signatures* in figure 12.
The signatures are not as pronounced as in the case for the rivet
spacing ratio of 1 due to the fact that the out‑of‑plane displacements
are a magnitude smaller. However, of significance is the fact that there
was a *D SIGHT* indication of the corrosion in the second layer,
although subtle.

The results can be summarized as follows:

1\. A mathematical model has been derived that correlates the pillowing
displacements and

the thickness loss due to corrosion.

2\. This model is independent of the thickness of the skin.

3\. The rivet spacing affects the magnitude of the corrosion induced
pillowing in lap joints

and may significantly reduce the ability to detect corrosion visually.

4\. It has been shown that *D SIGHT is* capable of detecting small
changes in percentage

thickness loss, thus demonstrating important improvement over currently
deployed

methods.

It has been shown D *SIGHT* images are a measure of the volume of the
corrosion product which is present in a joint, and using the simulated
signatures determined in this report, the thickness loss that has
occurred in the skin can be estimated.

39

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6 FIELD TRIALS

6.1 British Royal Air Force

6.1.1 Background

The field trip to England was not an official under the contract.
However, the results are reported here because they reinforce some of
the concerns that have been found with the original prototype in field
use. Several of the criticisms and suggestions have already been
acknowledged and have been implemented in the new equipment developed in
Phase II. At the time of this field trial, the new equipment was not yet
completed so the original prototype developed in Phase I was sent
instead. The field trial was performed in November, 1994, at various Air
Force bases in England. Diffracto\'s representative at the field trial
was David Willie.

The RAF has initiated a 3‑year investigation into exploring practical
and functional nondestructive inspection equipment. The RAF began its
inquiries into the DAIS‑250C at the ASNT show in Atlanta, GA, in the
fall of 1994. Flight Lieutenant Noel Murray brought the D
SlGHTtechnology to the attention of Sgt. Bill Bolton of the NOT group of
the Central Servicing Development Establishment. Sgt. Bolton expressed
interest in the speed and ease of the DAIS‑250C and asked for the
Diffracto equipment to be tried at different military locations in
England.

6.1.2 Inspections

6\. 1.2.1 RAF Brize Norton Airforce Base

The first task of the DAIS‑250C was to inspect the belly skin of a
VC‑10. While it was made expressly clear that the greatest advantage of
the DAIS‑250C was unmistakably along lap joints, the RAF team wished to
document rivet rows just forward of the wing fairings on both sides of
the VC‑10. The VC‑10 has a long history with the RAF and is mostly
employed as a \"workhorse,\" including considerable duty as a fuel
tanker. The tail number of the aircraft inspected was actually the last
VC‑10 in the fleet to not have its belly replaced: it was scheduled to
do so within the next sixty days.

The lap joint below the window belt seemed to indicate light to moderate
corrosion. There were definite areas of severe anomalies where the lap
joint butted together. The \"lifting\" could very well be an indication
of heavy machining or structural stress.

The rivet rows were extremely difficult to position in the field of
view. Basically, heavy paint with no markings made for slower
acquisition. In several areas a wavy type of signal prevailed, but few
if any typical corrosion signatures were found. The VC‑10 uses a double
skin and the RAF was looking for indications of corrosion between the
layers of the skin, not necessarily along lap joints.

The DAIS‑250C was also used around the front access portal. It was
relatively easy to see the indications of corrosion around this access
area. However, the area also showed signs of previous repairs and we
very well may have been looking at considerable amounts of bonding
adhesive.

41

6.1.2.2 RAF Boscoome Downs AirforceBase

The Boscoome Downs facility provided us with two aircraft for corrosion
inspection, a BAe Andover C.Mk and a BAe One‑Eleven. Both aircraft were
first manufactured in 1963. The Andover proved to be a difficult subject
for *D SIGHT.* Most of the Andover was refurbished with rather tall
rivet heads, not broad flat mushroom rivets, but the raised flat head
type of rivet. The images showed some pillowing; however, the typical
haloing signature surrounding the rivet head was not apparent in the
images.

The BAe One‑Eleven had been inspected prior to Diffracto\'s arrival on
base. The RAF NDT team had apparently acquired some images along a lap
joint, but again most of the data acquisition was targeted along rivet
rows in the belly area of the aircraft. A great deal of these images
proved to be unusable because highlighter fluid got onto the optics. But
this was overcome in subsequent inspections with a cleaning of the
optics.

6.1.2.3 DRA ‑ Defence Research Agency, Ministry of Defence

The originally stated purpose of this visit was to look only at various
composite materials. The agenda quickly changed when DRA brought out
completely characterized specimens of VC‑10. The better part of a
morning was spent capturing images for Robert Smith, one of the
developers of the Andscan System which was purchased along with DAIS by
ARINC last year. The RAF team handled the entire acquisition and
captured some very good images. The group of scientists that gathered to
witness the demonstration were impressed with the immediacy of DAIS.
Smith seemed to know every inch of his samples and was soon able to read
the signatures very easily. The images were saved under direction of
Smith. He planned on correlating the *D SIGHT* images with his own
scans.

When all of the VC‑10 images had been taken, the demonstration moved to
another part of the complex to look at a Jaguar wing. The composite
material was extremely used with considerable impact damage present. The
RAF took several images of obvious impact sites and presumably clear
areas of the wing.

6.1.2.4 Unattended Inspections

The RAF NDT team retained the DAIS‑250C for an additional week. The only
other aircraft openly discussed was a Boeing 707, located at RAF Swanton
Morley. The intention with the 707 inspection was to concentrate on lap
joint inspection. Sgt. Bolton indicated that they would like
Diffracto\'s interpretation of the images upon return of the DAIS‑250C.

6.1.2.5 Difficulties with the Equipment and Procedure

\(a\) The most serious problem of the entire trip originated not from
the equipment, but from

the power source. There were continual \"brown outs\" while working at
RAF Brize Norton.

Once the NDT team was convinced the problem was not with the DAIS‑250C,
but with

power demands, things proceeded quickly.

\(b\) The RAF was not pleased with the demands of setting up log files.

42

\(c\) The original concentration on acquiring images from heavily
painted rivet rows, not lap joints, quickly turned the two‑man
requirement into a four‑man effort.

\(d\) Highlighter drips and dust.

6.1.3 RAF Initial Recommendations and Summary

\(a\) The RAF would like to see a simpler system for cataloguingimages.

Due to the alignment difficulties encountered with rivet rows, they
suggested a small LCD monitor be affixed to the sensor head.

The Royal Air Force was extremely impressed with the speed and
simplicity of the system. When it was suggested that images could be
acquired by technicians at any location for further investigation by a
small group of experts, it was made clear that they favour putting
together a \"squad\" of DAIS inspectors that would travel to various
bases. Further, the original purpose of buying a DAIS‑250C would be to
document pieces of various aircraft for long temm condition studies. The
speed of the DAIS would allow them to record several images from several
different tail numbers, quickly building a Surface Condition Archive.

The RAF NDT team will be looking at their own correlations between DAIS,
eddy current and other NDI methods, and actual tear down to evaluate the
DAIS‑250C for \"viability in current maintenance programs\". The
highlighting procedure was generally sloppily done. They had little
patience with thinning out streaks, stopping runs, or re‑applying
highlighter when necessary. Speed was definitely the objective, not the
quality of the images. When we really took our time at the DRA facility
in RAF Famborough, the RAF NDT team seemed detemmined to match the
quality of the specimen images at the following week RAF Swanton Morley
707 inspection.

The RAF NDT team is extremely interested in the next generation of the
DAIS‑250C. They very much want to observe a demonstration at a North
American air carrier. The turtle diagram mapping is much like the
\"Kipper\' diagram they currently use and they feel there might be a
good fit. They were very critical of the first generation (menu‑driven)
software and did not like the nomenclature of printer functions in
particular. They definitely want to see and try the new generation
DAIS‑250C.

6.2 Northwest Airlines

6.2. 1 Background

Field Trial ‑ Northwest Airlines, St. Paul, MN. ‑ December 20, 21, 1994

Attended by:

Frank Karpala Diffracto Limited

Marc Noel Diffracto Limited

David Willie Diffracto Limited

43

Jerzy Komorowski NRC/IAR

Ron Gould NRC/IAR

Dave Galella FAA

The purpose of this field trip was to observe and test the new DAIS
hardware and software in the field and to capture D *SIGHT* images with
the newly configured DAIS‑250C sensor head, operator pendant, and
graphical user interface software.

6.2.2 Inspections

12/20/94 DAY ONE *747‑200*

The initial attempts to bring up the acquisition module software and
implement it resulted in a fallback to a \"manual\" image acquisition
and archiving routine. This laborious system of keystroking placement
identification, image acquisition and archiving, reiterated the problems
recognized with the first DAIS prototypes. Lap joints were imaged along
stringers 39L and 44L between body stations 460 and 720. A total of *29
D SIGHT* images were acquired in 26 minutes.

Diffracto staff worked to correct software problems via modem through
the day and evening of the 20th.

12/21/94 DAY TWO *727‑200*

With replaced and reworked files, the image acquisition software
performed well within its design expectations. The first inspection task
on the 727 was to acquire images from crown stringers 4L and 4R. The
second task was to acquire images from the aft belly section of the 727.
The locations of the belly inspection, released to us the day before,
were included in the inspection plan prepared on the evening of December
20th.

The crown was completely unobstructed and totally accessible. The
inspection plan for the crown, created on the morning of the 21 st,
allowed for \"production\" type image acquisition. The acquisition along
stringers 4R and 4L was demarcated by body stations 360 and 1183.
Beginning aft and working counterclockwise to the nose and aft again, a
total of 116 *D SIGHT* images were archived in 45 minutes of
*acquisition time.* The average placemenVcapture time was 24 seconds per
*D SIGHT* image with a travel rate of 3 fVminute. Acquisition time was
calculated by noting the stamped time of the first and last image in an
array.

The aft belly inspection area of the 727 was also readily accessible;
however, the belly was thoroughly coated with grease and dirt. It was
successfully wiped clean with Electron highlighter fluid and countless
sheets of shop towelling. Body stations 1183 and 950 marked the
boundaries of lap joint inspection along stringers, 26L, 26R, 27L, 27R,
28L, 28R, 29L, 29R, and 30. A total of 152 *D SIGHT* images were
acquired and archived in 1 hour and 16 minutes for an average
placemenVcapture time of 30 seconds per *D SIGHT* image. The travel rate
for this particular inspection was 2.3 feet per minute.

44

The acquisition rates may seem slower than first anticipated simply
because there were a lot of stops and starts during acquisition to
change operator positions, to explain the inspection procedure to
Northwest personnel, and to explain what the images meant.

6.2.3 Observations by the DAIS Inspection Crew

6.2.3. 1 Hardware

Sensor

The sensor‑head seemed to be unbalanced when the operator used the
handles for sensor placement to vertical positions on the aircraft. It
was recommended that a large handle be mounted at the balance point of
the sensor. The handle would \"wrap\" around the sensor, facilitating an
operator\'s comfort preference and need for placement control. And the
sensor\'s feet (pegs) seemed to slide far too easily on highlighted
surfaces.

Pendant

The support neck strap was not overly favored as the sole method of
controlling the pendant\'s weight. The monopod was very effective in
transferring all the pendant\'s weight to the floor. The combination of
monopod (as weight bearer) and the loosened neck strap (as an equipment
safety precaution) was favored by all. During long acquisition nuns, the
monopod is a necessity.

Pendant‑to‑Sensor Cable

The length of cable between the pendant and sensor seemed limiting. It
was noted by Noel that under the belly of the aircraft the sensor
operator often \"tugged the pendant operator along\". The sensor
operator generally began to move immediately to the next location after
hearing \"OK\" from the pendant operator. Meantime, the pendant operator
was watching the screen and not where the sensor was going. A longer
cable should allow for enough slack between the two at all times
alleviating cable and connector wear and tear.

PEP Power Portable

The overall performance of the PEP was good. The physically fixed mouse
requires some getting used to, but ultimately performs well. Positioning
the LCD screen so more than two people can see a clear screen image at
one given time was a bit taxing. This was overcome by splitting the
audience between the computer screen and the pendant screen.

6.2.3.2 Software

The acquisition and analysis modules required a severe shakedown to
indicate the proper direction to finish coding.

45

The lamp within the sensor head was set to turn off after each
placement. We thought it might be slowing down the acquisition process.

There was a problem with \"double‑clicking\" on the pendant\'s touch
screen. The touch screen\'s setup preferences were adjusted.

The pendant\'s interface is now being optimized for \"large controls
only\". Small controls are going to be avoided at the pendant.

6.2.4 Northwest Airlines ‑ Reaction to the DAIS

At the inspector level there was some concern the need to record
inspection results on the computer. The idea that entire lap joints, at
every panel location, could be inspected with relative ease and speed;
however, created interest and excitement. The interpretation of the
actual *D SIGHT* images seemed to raise the greatest number of
questions. Inspectors were interested in reaming how to read corrosion
signatures.

Inspector comments on the software, even in its Windows environment,
gravitated to a basic \"fear of computers\"; however, after watching the
inspection crew for a while, observers began to understand the
simplicity of the acquisition module. Because there was writing, saving,
printing, and jumping to other applications (Mosaic, Program Manager,
File Manager), the process looked more complicated than it is to people
familiar with computers. The simplicity of picking a footprint off the
.AIRcraft plan (after acquisition) and recalling that exact *D SIGHT*
image proved to be the most exciting aspect of the software.

Northwest management seemed more interested in what was found rather
than how it was found or how fast the inspection proceeded. Both Jeff
Register and John Yuen (NOT Managers) questioned what image indications
were being looked for and how to put a \"value\" on the signatures. The
NOT managers were first and foremost interested in the final analysis on
the corrosion findings. Only light corrosion indications were found in
some areas including the aft belly area.

6.3 Air Canada

Air Canada ‑ Dorval Airport, Montreal, February 2, 1995

Plane Inspected:

Boeing 747‑200

Attended by:

Frank Karpala

Dave Willie

Omer Hageniers

Jerzy Komorowski

46

![](media/image34.emf){width="0.5854166666666667in"
height="0.15694444444444444in"}![](media/image35.emf){width="0.8527777777777777in"
height="0.15694444444444444in"}

![](media/image36.emf){width="6.715972222222222in"
height="9.331944444444444in"}

before the platform to be moved. However, it did become clear that the
creation of the inspection plan has to account for the staging as well
as the normal requirements of the inspection. Different forms of the
inspection plan could have reduced the amount of movement needed while
covering the required areas. At the time of the field trial, not all the
software features were in place to increase the flexibility of image
acquisition. Either there must be enough software flexibility for
acquisition, or the inspection plan must take the movement around the
aircraft into consideration during the initial plan creation.

6.4 United Airlines

United Airlines ‑ San Francisco, CA, February 15, 1995

Plane Inspected:

Boeing 737‑291A: Flight Hours 44797 hours 49 minutes at time of
inspection. Cycles: 34801 at time of inspection.

Attended by:

David J. Willie

Dr. Frank Karpala

6.4.1 Inspections

The DAIS 250C was used on Febnuary 15th in Dock 4 of United Airlines\'
San Francisco Maintenance Facility.

The crown of the B‑737 was chosen as the first acquisition site for two
reasons: the area was not being worked on, and we were expecting a group
of interested people from United to observe a demonstration. The space
on the crown would have afforded everyone a good vantage point.

Work began at Body Station 1016 and proceeded towards the radome along
Stringer 4 Right to Body Station 259.5. Moving across to Stringer 4
Left, DAIS acquisition started at BS 259.5 and carried on to Body
station 540. A total of 65 *D SIGHT* images were acquired from the crown
in 45 minutes of placemenVcapture time. PlacemenVcapture time was
calculated by noting the beginning and ending time stamps from the
actual .BMP image files. Further calculations reveal a travel rate of
1.7 feet per minute.

DAIS operators encountered some positioning problems due to an extremely
heavy coat of paint: the body station indicators (circumferentials and
ribs) were particularly hard to see until they were actually viewed live
by DAIS. Acquisition time‑outs were extended to allow for extra
positioning time, which consequently added to overall acquisition times.

48

While gathering data along the crown stringers, the system encountered
its first (and only) hardware problem of the visit. The
pendant‑to‑sensor cable began to short at the sensor‑head connector. As
more and more time was monopolized correcting for the uncooperative
coupling device, the acquisition rate finally slowed to a stop. The
inspection was terminated on Stringer 10 between BS540 and 360
rightside.

6.4.2 Performance and Results

The DAIS 250C equipment performed very well overall with the exception
of the failed connector. These connectors (eight connectors in all) are
absolutely critical to the operation of DAIS in acquisition mode. After
this field trip, alternative connection products were researched and
ultimately replaced the original coupling devices.

Due to the heavy paint on the fuselage, the sensor operator was very
much at the command of the pendant operator. Although the live DAIS
image guided the proper placement and alignment of the sensor‑head, it
became clear that an additional visual guide for this situation would be
a great help, for example, by indicating the location of the ribs, at
some point above the sensor\'s footprint of the lap joint, with a small
grease pencil mark *before* acquisition in the particular area. While
this additional step has not been necessary in other field trials (on
commercial aircraft), it is apparent that heavily applied paint will
slow placement location and acquisition rates.

The pendant touch‑screen performed very well under the \"tap mode\",
which was set up under user preferences. Both inspectors found the tap
mode to be a more accommodating technique than the \"double click\" mode
based on time. As more time is spent operating the pendant, the finger
placement and screen manipulation tasks become more effortless to
execute.

While the majority of this field test was spent on the crown of the 737,
the operators did gather some images along Stringer 10 above the window
belt on the right side. The only problem experienced here was that the
sensor head remains unbalanced with considerably more weight to the
camera end. An additional handle location towards the balance point
might alleviate a great deal of this balance problem.

The areas investigated on the crown of the B‑737 were not expected to be
areas with a high probability of finding obvious corrosion signatures,
and the heavy paint presented a more difficult analysis task. Paint sag
was evident at all areas on the aircraft. Inspection rate was lower than
expected partly due to the problem with the failed connector.

6.5 On‑Aircraft Inspections

The following figures (figures 13 to 21) show the new DAIS equipment
being used at various facilities during the field trials. A few typical
D *SIGHT* images of the aircraft surfaces are also shown at different
corrosion levels (figure 15, figure 20, and figure 21).

49

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![](media/image38.emf){width="6.769444444444445in"
height="9.406944444444445in"}

7 CONCLUSIONS

Many important advances resulted from the Phase 11 effort. The design of
a new sensor and host helped reduce the overall weight of the system and
provide more reliable equipment. The software developments were also
received with great enthusiasm by NDI technicians because they made the
user interface to the location of the image data easier to manage.

The result of thousands of hours of accelerated corrosion testing have
also helped to improve the confidence in D *SIGHT* technology as a means
of detecting low levels of corrosion. The computer modelling is
particularly significant in that additional work can begin to predict
the stress levels in all types of joint configurations and to address
issues of damage tolerance. For *D SIGHT,* a method is now available to
derive signatures of corroded lap splices at different levels of
pillowing. This will provide a visual standard to judge the corrosion
level.

The results of the field trials remain an important basis for
improvement of the equipment and techniques for inspection. A great deal
is reamed by performing inspections on actual aircraft to determine the
problems with the design of the equipment. These field trials also
indicate areas of improvement because each situation is slightly
different from the last.

52

8 PUBLICATIONS FROM PHASE I AND PHASE 11

Phase I

1\. J. P. Komorowski, R. W. Gould, *\"D SlGHT for* Aircraft Corrosion
Inspection\", Report of Task 5.1, 5.2, and 5.3, LTR‑ST‑1943, NRC/IAR,
Aug. 1993

2\. S. Krishnakumar, J. P. Komorowski, l. Sproule, \"Chemical
Characterization of Corrosion Products in

Fuselage Lap Joints\", LTR‑ST‑1952, NRC/IAR, Nov. 1993

3\. J. P. Komorowski, R. W. Gould, A. Marincak, and S. Krishnakumar,
\"Application of *D SIGHT* for

Corrosion Detection in Fuselage Lap and Butt Joints\", LTR‑ST‑1960,
NRC/IAR, Feb. 1994

4\. F. Karpala and O. L. Hageniers, \"Characterization of Corrosion and
Development of a Breadboard

Model of a *D SlGHTAircraft* Inspection System: Phase I\", Final Report,
Transport Canada Publication

No. TP11983E, Feb. 1994, and FM Report No. DOT/FAlVCT‑94/56, Aug. 1994

Phase 11

1\. F. Karpala, O. L. Hageniers, J. P. Komorowski, \"Theory and
Application of *D SlGHT for* Corrosion

Detection\", Proceedings of ASNT Spring Conference, March, 1994

2\. O. L. Hageniers and F. Karpala, \"Aging Aircraft Inspection Using *D
SIGHT\",* 1994 ASNT Fall Conference and Quality Testing Show, Atlanta,
Georgia, Sept 19‑23, 1994

3\. N. C. Bellinger, S. Krishnakumar, J. P. Komorowski, \"Modelling of
Pillowing Due to Corrosion in

Fuselage Lap Joints\", 7th CASI Symposium on Aerospace Structures and
Materials, Winnipeg,

Manitoba, May, 1994 (Proceedings), also published in Canadian
Aeronautics and Space Joumal, Vol.

40, No. 3, Sept. 1994

4\. J. P. Komorowski, S. Krishnakumar, R. W. Gould, N. C. Bellinger, F.
Karpala, O. L. Hageniers,

\"Double Pass Retroreflection for Corrosion Detection in Aircraft
Structures\", AGARD 79th meeting

of the Structures and Materials Panel, Specialist Meeting on:
\"Corrosion Detection in Aircraft

Structures\", AGARD CP‑565, Oct. 1994, Seville, Spain

Note: this paper has been accepted for publication in \"Materials
Evaluation\"

5\. D. J. Willie, \"Applying *D* SlGHT Technology to Aging Aircraft
Corrosion NDI\", Airframe Finishing,

Maintenance and Repair Conference and Exposition, Bellevue, Washington,
Mar 13‑16, 1994, SAE

Technical Paper Series #951140

6\. N. C. Bellinger, J. P. Komorowski, and S. Krishnakumar, \"Numerical
Modelling of Pillowing Due to Corrosion in Fuselage Lap Joints\",
LTR‑ST‑2005, NRC/IAR, Apr. 1995

7\. R. W. Gould, A. Marincak, and J. P. Komorowski, \"Accelerated
Corrosion Testing and NDI ‑ Report on Task 1.5\", LTR‑ST‑2006, NRC/IAR,
Apr. 1995

53