The role of non destructive testing in the airworthiness certification of civil aircraft composite structures

Prévia do material em texto

The role of non-destructive 
testing in the airworthiness 
certification of civil aircraft 
composite structures 
A, MAHOON 
(British Aerospace, UK) 
Airworthiness requirements for civil aircraft structures, 
particularly those fabricated from carbon fibre-reinforced 
resins, are discussed and the use of non-destructive testing 
to monitor the quality of these structures at each stage of 
development is reviewed. Non-destructive testing 
techniques for series-production items are described with 
information on the type of defects detected by the various 
techniques. Non-destructive testing techniques under 
development and future trends in the use of the proposed 
testing techniques are also included. 
Key words: composite materials; carbon fibre 
reinforcements; non-destructive testing techniques; aircraft 
structures 
Components manufactured from glass fibre-reinforced 
plastics have been in use as decorative panels in aircraft 
structures for a significantly long time. However, the 
use of high strength carbon fibre-reinforced composites 
(CFC) in the aircraft industry has steadily grown over 
the past twenty years due to the higher strength-to- 
weight ratios achievable with these materials than with 
the traditional aluminium alloys. In British Aerospace, 
the use of CFC began in 1970 with the aim of gradually 
extending the use of these materials from secondary 
structures and flying control surfaces to primary 
structures. Within British Aerospace, the use of cvc 
has extended into primary structures for military 
aircraft, whereas in civil aircraft it is still confined to 
secondary structures and flying control surfaces. A 
more significant application of cvc in civil aircraft 
primary structure has appeared in the form of the 
tailplane on the Airbus Industrie A320 Aircraft. 
Traditionally, the design of composite aircraft 
components had been based on the philosophy used for 
designing metal aircraft structures. This approach 
proved successful in learning the use of new-technology 
composite materials and served its purpose in building 
small demonstrator components to gain experience and 
confidence in the use of CFC in civil aircraft structures. 
However, with expanding use of cFc for high load- 
bearing structurally-significant components, it became 
necessary to adopt procedures which would optimize 
the design and manufacture of CFC components with 
the aim of fulfilling the requirements for obtaining 
airworthiness certification. The airworthiness 
requirements for composite structures for civil aircraft 
are discussed, and non-destructive testing (NOT) 
techniques which provide the means of fulfilling these 
requirements are detailed. 
Airworthiness requirements for CFC structures 
Airworthiness requirements for civil aircraft metal 
structures are well defined by the certification 
authorities of Europe, UK and the USA. 1-3 However, 
for civil aircraft structures manufactured from 
composite materials, only advisory documents which 
provide guidelines for demonstrating compliance with 
the regulations regarding airworthiness certification are 
available. In the absence of clearly defined 
airworthiness requirements and nationally agreed 
acceptance standards and with only limited long term 
performance data available, it has been essential to 
employ extensive structural testing programmes and 
demonstrate stringent manufacturing quality control 
and in-service maintenance support to obtain 
airworthiness certification for civil aircraft composite 
structures. The procedures currently adopted to fulfil 
airworthiness requirements for c F c structures for civil 
aircraft are detailed below. 
0010-4361/88/050229-07 $3.00@1988 Butterworth & Co (Publishers) Ltd 
COMPOSITES. VOLUME 19. NUMBER 3. MAY 1988 229 
Design and development 
At the inception stage of a particular structure, it is 
important to involve engineers specializing in materials 
knowledge, production manufacturing methods and 
NDT techniques. Selection of the right material of 
construction, such as the particular resin/fibre system, 
is very important to ensure that the structure will be 
able to perform the functions for which it is being 
designed. A suitable method of manufacture has to be 
developed that is economical and efficient. The 
designed structural configuration has to be amenable to 
inspection by NOT techniques commonly employed for 
production quality control. To fulfil these 
requirements, a research and development programme 
is normally undertaken at the design stage for each and 
every component. Out of this programme emerges the 
appropriate material and design data, the optimum 
method of manufacture and the suitable NDT 
techniques, including the NDT calibration standards. As 
the basic material design data and experience in the 
design and manufacture build up, the extent of research 
and development normally decreases for subsequently 
designed aircraft components. 
Structural testing 
Structural testing is the most important step towards 
obtaining a certificate of airworthiness for composite 
structures. It involves the application of static and 
cyclic loads equivalent to the level and frequency of 
stresses expected during the service life of the 
structure. Full life-size structures of the most important 
elements of the structure are tested. Artificial 
manufacturing defects such as delaminations and 
foreign inclusions are introduced into the structure to 
establish the effects of such defects on the structural 
integrity and to help in selecting the acceptance criteria 
for the manufacturing quality control. The structures 
are also aged in a humid environment to acquire a 
certain steady-state level of moisture and then 
subjected to structural testing. Impact damage is 
induced in various parts of the structure to establish the 
damage tolerance of the structure. Deliberately 
induced areas of impact damage are repaired and the 
repaired structure is then subjected to structural testing 
to ensure the integrity of the repaired area under the 
expected loading conditions. During the structural 
testing stage, various NOT techniques are employed to 
detect the presence of defects and to select the most 
appropriate technique for monitoring the structural 
integrity. NOT technique development also takes place 
at this stage to devise appropriate methods for 
in-service monitoring. Structural testing not only 
confirms the load-bearing capacity of the structure and 
its resistance to the expected operating environment 
but also provides data for obtaining the necessary 
certificate of airworthiness for the structure. 
Manufacturing quality control 
During the manufacture of composite structures, two 
processes occur simultaneously. The required shape is 
produced along with the final stage of the manufacture 
of the material, i.e. consolidation and cure of the resin. 
Therefore, in addition to checking the shape and 
dimensional accuracy, it is also important to ensure that 
the material itself is of an acceptable quality and does 
not contain discrete defects of a harmful nature. To 
ensure material quality at the manufacturing stage, test 
coupons are cured simultaneously with the respective 
component and tested by a number of destructive 
techniques. Recently, production quality control has 
moved towards the use of instrumental analytical 
techniques based on differential scanning calorimetry 
(DSC) and dynamic mechanical analysis (DMA) to assess 
the degree of cure in composites. Although the 
destructive techniques play an important role in the 
control of the manufacture of composite structures, 
they cannot completely fill the quality control 
requirements because complete consistency of the 
material quality cannot be ensured in all areas of the 
cured items, and coupon testing cannot reveal the 
presence of discrete defects in the ready-to-use 
components. 
Due to these two limitations, there is a requirement to 
employ NDT techniques for quality controlof the 
manufacturing process. During assembly of composite 
components into an aircraft structure, it is possible that 
damage may be induced due to drilling, machining, or 
rivetting. Accidental damage can also occur, either 
during assembly or in subsequent transportation and 
storage. Components containing such defects, if 
amenable, may be recovered by making repairs. The 
integrity of the repaired component is then confirmed 
by NDT. 
In-service monitoring 
The structural testing stage is designed to ensure that 
any aircraft structure put into service is safe and can 
withstand the service loads and environments. The 
manufacturing quality control procedures also 
guarantee that no structure containing material of 
unacceptable quality and/or discrete defects is allowed 
to enter service. Despite the rigorous control of 
structural testing and manufacturing processes, 
degradation of composite structures does occur during 
their service lives. In the case of cFc materials, 
environmental degradation occurs due to increase of 
moisture, chemical attack, thermal spiking, fatigue, 
overloading, erosion, impact damage and lightning 
strike. Accidental damage can also occur during 
transportation, inspection and maintenance. Due to the 
uncertainties of the service environment, there is 
always a prevalent requirement to undertake regular 
non-destructive inspections to detect any material 
degradation and to ensure the continuing structural 
integrity in service. To fulfil these requirements and to 
satisfy the airworthiness authorities, NDT techniques 
are developed and are often included either in the 
aircraft maintenance manual or are compiled into a 
separate NDT manual. 
Repair and inspection 
Regular in-service NDT detects the presence of damage 
induced in a composite structure due to the 
environmental effects. The extent of induced damage 
could be within the design allowable limits; repair 
230 COMPOSITES . MAY 1988 
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Fig. 1 A schematic view of Airbus A320-- aileron made from composite material 
action would not then be required but recording of the 
position of the damage and its continued monitoring 
would be essential. If the extent of the damage is 
beyond the acceptable limit, then the structure could 
either be recovered by undertaking necessary repairs or 
could be withdrawn from service. In the case where the 
integrity of the damaged structure could be restored by 
repair, an appropriate scheme is devised. Although 
quality control aspects are normally included in a repair 
scheme, it is an additional requirement to employ NDT 
techniques to ensure that the repair is sound and that 
the structural integrity has been restored. The integrity 
of the repair is then monitored during the routine 
planned inspections of the aircraft structure. 
ND T TECHNIQ UES FOR SERIES-PRODUC TION 
ITEMS 
Development of civil aircraft structures at the 
Weybridge plant of British Aerospace began in the mid 
1970s. Simple demonstrator components included 
panels on VC-10 and BAC 1-11 aircraft. Between 1978 
and 1981 the first series production item was 
manufactured. Launch of the Airbus A300-600 aircraft 
in 1981 lead to the development of a lightweight fixed 
trailing edge structure followed by the manufacture of 
A310-300 trailing edge and A320 aileron. These 
structures are basically wedge-shaped, the largest being 
4.5 m long and 1.5 m wide. The skin panels for trailing 
edges are co-cured Nomex sandwich with two layers of 
five harness satin weave carbon fibre fabric on each 
face. The fabric is preimpregnated with a 120°C-curing 
epoxy resin. These panels are then adhesively bonded 
to premoulded CFC ribs and a pre-laminated cotton- 
reinforced phenolic strip is used to build up the 
required edge thickness. The aileron (shown 
schematically in Fig. 1) is very similar in construction to 
the trailing edges except that the skins are a mixture of 
solid laminate and honeycomb sandwich and are 
adhesively bonded to nine premoulded ribs, a leading 
edge channel section spar and a pultruded trailing edge 
strip. The resin system for aileron cures at 170°C. 
Development of NDT techniques for cFc began with the 
design of the demonstrator items. Many Nox 
techniques were investigated for application to the 
Airbus series production items. The techniques 
currently employed for the manufacturing quality 
control are based on ultrasonics, radiography and the 
use of commercial bond testers. Acoustic emission has 
been used in conjunction with these techniques to 
monitor structual testing. Use of ultrasonics, 
radiography and commercial bond testers has also been 
extended to in-service inspection and examination of 
repaired structures. A brief description of these 
techniques with their application to various cFc 
structures is given below. 
Ultrasonics 
Four techniques based on the principles of ultrasonic 
measurements are: 
• Water jet-probe through-transmissionscanning 
• Immersion scanning with glass reflector plate 
• Irrigated probes manual scanning 
• Pulse-echocontact testing 
A typical water jet-probe automatic scanning rig as 
used for through-transmission examination of all skin 
panels for A300, A310 and A320 structures is shown in 
Fig. 2 and details of this technique are described in 
Reference (5). The testing is carried out at 1 MHz 
ultrasonic frequency with index steps of 0.2 mm to 
2 mm. The test data are presented as C-scan traces of 
five grey tones on electrosensitive paper. The 
technique detects material quality variations, 
delaminations in the skin, disbonds at the skin/ 
honeycomb interface and the presence of pre-preg 
backing film. Discrete defects down to 3 mm × 3 mm 
have been detected. 
All solid laminated ribs and A320 aileron channel spars 
are subjected to ultrasonic immersion scanning with 
glass plate reflector. The glass plate is normally placed 
parallel to and underneath the component at 
25-50 mm. Special jigging is required to place the glass 
plate under the spar flange due to the channel 
Fig. 2 Water jet-probe ultrasonic scanning rig 
COMPOSITES. MAY 1988 231 
~ Rib section 
Ultrasonic 
, r a n s d u c o , 
N a r T \ 
Fig. 3 Plan-view of i rr igated-probe assembly for ultrasonic 
examination of rib root radii (scale, 1:3) 
configuration of spar. The examination is conducted on 
an automatic scanning frame mounted on a water tank 
with test data recorded on electrosensitive paper (as 
with the C-scan trace). Webs of ribs and webs/flanges 
of spars are inspected at 10 MHz ultrasonic frequency. 
This technique has much higher sensitivity of defect 
detection when compared with the water jet-probe 
technique. 
Roots of ribs and spars, flanges of ribs and trailing edge 
wedges are inspected by hand-held irrigated probes. 
The principle of irrigated probe is similar to that of a 
water jet-probe. Transducers are mounted in special 
housing devices with water just irrigating the area 
between the transducer and the area of examination 
(hence irrigated probe). A typical irrigated-probe 
designed to inspect root radii of ribs is shown in Fig. 3. 
The examination is based on through-transmission 
using transducers operating at 1 or 5 MHz. Use of 
irrigated probe has enabled the detection of porosity 
and delaminations. 
In the case of the A320 aileron, adhesive bonds 
between the ribs, spar and trailing edge wedge to solid 
laminate skins are examined by manual ultrasonic 
pulse-echo using a 15 MHz probewith a 5 mm acrylic 
stand-off shoe. Porosity and disbonds are detected by 
this technique. 
Radiography 
Ultrasonic inspection is normally supplemented with 
radiographic inspection for honeycomb sandwich 
panels of A300/310 trailing edges and adhesive bonds at 
the spar/skin joints of A320 aileron. Radiographs of 
adequate density (1.5-2.5) and contrast are produced 
at 15 kV and 20 mA min- using fine or medium grain 
X-ray film. Radiographic examination is undertaken to 
ensure that no water ingress has occurred during 
ultrasonic testing. Water ingress was found to be a 
common defect at the initial manufacturing stages of 
A300 trailing edge skin panels. Radiographic 
examination is capable of detecting core defects such as 
splitting, crushing, distortion and displacement. 
Presence of self-adhesive tape pieces (25 mm × 60 mm) 
has also been detected by radiography. In the case of 
adhesive bonds, radiographic examination reveals the 
distribution of porosity at the bond lines. In some 
cases, fine distributed porosity is difficult to detect by 
ultrasonic examination. 
Penetrant-enchanced radiography is employed for 
detecting impact damage and disbonds. (A 
comprehensive investigation on the use of radio- 
opaque penetrants for inspection of composites is given 
in Reference (6)). Bond testing technique, in some 
cases, is unable to detect the extent of disbonds 
between honeycomb skin and ribs in A300 and A310 
trailing edges. Therefore, penetrant-enhanced 
radiography is used, where possible, for determining 
the extent of disbonds in these components. However, 
it is essential that the disbond extends to the edge of the 
rib flange so that the penetrant can impregnate into the 
disbonded area. 
Adhesive bond testing 
Conventional ultrasonic techniques fail to detect 
disbonds in honeycomb sandwich adhesively bonded to 
solid laminates. Commercially available bond testers 
are employed for the examination of these 
constructions, in particular, and for the inspection of 
adhesive bonds at an advanced assembly stage, in 
general. Adhesive bonds between the honeycomb 
sandwich skins and ribs in A300 and A310 trailing edge 
structures are inspected by Shurtronic Harmonic Bond 
Tester (Shurlock Corporation Inc, USA). The 
equipment is capable of detecting disbonds equal to 
and greater than 10 mm × 10 mm in the trailing edges. 
Inspection of adhesive bonds between the solid 
laminate skin and ribs/spar of A320 aileron was initially 
undertaken with commercial bond testers but was 
immediately replaced with ultrasonic pulse-echo 
technique. This approach was adopted because the 
bond testers that had been used were found to have 
either poor sensitivity or to be unable to detect 
disbonds and porosity in A320 aileron. 
Acoustic emission testing 
Acoustic emission testing is employed at the structural 
testing stage and is complemented by ultrasonic, 
radiographic and adhesive bond testing techniques. It 
has been used as an on-line monitoring technique for 
detecting failure during static and fatigue load testing of 
A320 aileron sections." Disbonds generated during 
loading have been detected by this technique and their 
extent determined by conventional NOT. The presence 
of deliberately induced impact damage and its 
propagation is also detected by this technique. 
Location of damage is achieved by using linear or 
triangular arrays of acoustic emission sensors and 
measuring the arrival times of emissions at these 
sensors. It has been found essential to employ guard 
transducers to eliminate the background noise 
generated due to mechanical functions of loading jacks 
and rams to enable detection of failure under fatigue 
loading. 
AID T TECHNIQUES UNDER RESEARCH AND 
DEVELOPMENT 
NDT techniques currently employed for series- 
production items, although sensitive enough to detect 
the majority of structurally significant defects, are not 
capable of detecting all the defects in composites and, 
232 COMPOSITES. MAY 1988 
in some cases, do not provide detailed identification 
and characterization of defects. Future composite 
aircraft structures such as the cFc wing demonstrator 
for BAe 125 aircraft will contain hybrid composites and 
will be manufactured using automated lay-up, curing 
and assembly techniques. The introduction of these 
new manufacturing technologies will require a different 
approach to NDT. Many new NDT techniques which are 
being developed with the aim of coping with new 
manufacturing technology (and improving the quality 
and capabilities of NOT) are currently under research 
and development. 
Ultrasonic spectroscopy 
Ultrasonic spectroscopy is based on the use of Fourier 
transformations of the ultrasonic time domain signal to 
display the frequency and phase content of the 
ultrasonic energy. It offers the potential of detecting 
subtle ultrasonic energy changes in the time domain 
signal following interaction with materials and defects. 
The suitability of the technique for detecting and 
characterizing discrete defects and material property 
variations has been investigated and it was found that 
manufacturing variables and in-service defects such as 
thickness, surface finish, void content, degree of cure, 
preimpregnate backing film inclusions, delaminations, 
impact damage and moisture ingress could be detected 
and characterized. Ultrasonic spectroscopy offers 
advantages over conventional ultrasonic techniques in 
the detection of degree of cure variations and water 
ingress and the inspection of thin honeycomb skins. In 
the majority of cases, however, the conventional 
techniques are equally effective. 8 
Real-time radiography 
With the increasing throughput and complexity of 
composite structures, it is becoming important to adopt 
automatic inspection techniques. Despite the high 
operational costs of radiographic inspection, it is still 
the most common complementary technique to 
ultrasonic examination. Automation of radiographic 
inspection by real-time imaging offers cost reductions 
by eliminating the use of X-ray films and reducing the 
time of inspection. By video recording the X-ray image, 
data storage is possible. Many real-time imaging 
systems are now commercially available. Most of these 
consist of a basic X-ray tube, a fluorescent screen and a 
video camera. The systems vary quite widely in 
complexity, ranging from simple video screen 
presentation of the X-ray image to digitization and 
computer processing of the recorded data. 
Computerized X-ray tomography, which is extensively 
employed in medical examination, has also been 
investigated for the testing of composite materials. In 
this method, the X-ray image consists of a cross- 
sectional through-the-thickness view in the form of a 
slice. Digitization of the X-ray image and the 
enhancement of the image quality by computer data 
processing offer a.means for detecting certain defects 
which are not amenable to detection by conventional 
techniques. Evaluations of currently available real-time 
imaging systems have revealed that the resolution 
provided by these systems (10-20 lines mm -1) is not 
equal to that achieved by the use of X-ray films 
(100-200 lines mm-1). Research is in progress to 
improve the resolution of image display systems. 
Eddy-current testing 
Eddy-current testing involves the use of a varying 
magnetic field produced by a test coil to induce small 
circulating currents, called eddy currents, in electrically 
conducting materials. Certain material properties, e.g. 
conductivity, have an effect on the eddy currents thus 
induced. The eddy currents themselves set up a 
magnetic field which interacts with the magnetic field of 
the coil such that the impedance of the test coil is 
changed. Thus, any change in the eddy currents is 
reflected by a change in the impedance of the test coil. 
Eddy-current testing is commonly used on metallic 
materials for the detection of cracks and the monitoring 
of material properties such as conductivity. Sincecarbon fibres are electrically conductive, eddy-current 
testing can be used for the inspection of carbon fibre 
composites. Using the impedance plane display of the 
eddy-current response, defects which can be detected 
include delaminations, translaminar cracks, barely 
visible impact damage, the presence of film inclusions 
and fibre volume fraction changes. 9 
Most eddy-current test procedures are based on manual 
testing with the test results being interpreted by visual 
observation of the eddy-current response as indicated 
on a meter or similar device. This approach is time 
consuming and can present difficulties in the 
interpretation and quantification of the test data. 
Procedures for automatic scanning and quantification 
of the eddy-current response, which have the capability 
of producing a record of quantized eddy-current test 
data on electrosensitive paper in the form of a facsimile 
C-scan trace have been developed. (A typical eddy- 
current C-scan trace is shown in Fig. 4.) Investigations 
based on this approach have confirmed that the 
majority of defects detected by other conventional NOT 
techniques can also be detected by eddy-current 
testing. The resolution of defect detection is influenced 
by the eddy-current test parameters such as operating 
frequency, amount of probe lift-off from the surface 
under inspection and the speed of scanning. Eddy- 
current C-scan presentation can identify areas of 
variable resin/fibre ratio and detect translaminar cracks 
better than the conventional ultrasonic scanning 
techniques. Research is in progress to optimize 
eddy-current test probes and to digitize the impedance 
plane response so that the eddy-current test data can be 
analysed on a computer and presented on a graphics 
terminal such that defect detection is enhanced. 
Thermography 
The basic principle of thermography is to apply heat to 
the test piece and measure the resulting variations in 
heat dissipation. Temperature-indicating substances 
such as sharp melting point chemicals, heat-sensitive 
paints, heat-sensitive liquid crystals and infrared 
heat-detection cameras have been employed for this 
purpose. The use of heat-sensitive materials to detect 
temperature changes is cumbersome in application and, 
C O M P O S I T E S . M A Y 1988 233 
An edge 
delamination 
i 
Impact 
damage 
Variation in 
resin/fibre 
ratio 
Fig. 4 Eddy current C-scan trace from an impact-damaged composite panel (scale, 1:2.75) (Eddy current test frequency, 2 MHz; 2 mm 
thick material with unidirectional carbon fibres laid in (00/_+450/90 ° ) orientation) 
in many cases, is not as sensitive in detecting low 
temperature changes as are optical methods based on 
infrared detection. In recent years, the development of 
video compatible infrared imaging cameras has led to 
the development of thermography as an efficient 
method of detecting discrete defects in composite 
materials. The method of applying the heat to the 
composite material requires careful consideration to 
avoid damage to the material during examination by 
the thermographic method. Hot water bags, immersion 
in hot water, hot-air blowers and transient heat 
generated due to aircraft flying conditions are all 
possible means of heating composite structures. In one 
method, heat is applied to the test object using a 
high-intensity photographic flash gun. 10 This technique 
of thermal inspection using a flash gun as the heat 
source in combination with a videq-compatible 
infrared-imaging camera has been termed thermal- 
pulse video-thermography. Detailed investigations 
have shown that in composites, the technique can 
detect the presence of delaminations, disbonds, impact 
damage, lightning strike damage and elongated voids in 
solid laminates and water ingress in honeycomb 
sandwich constructions. 
Acousto-ultrasonic techniques 
Acousto-ultrasonic measurements are based on a 
combination of fundamental ultrasonic testing with 
fundamental acoustic emission testing. Artificial stress 
waves are induced in the material using ultrasonic type 
piezo-electric transducers. The stress waves, after 
travelling through the material, are detected by an 
acoustic emission recording transducer on the same 
side of the specimen. Any change in the material 
characteristics influences the nature of the transmitted 
waves and is detected by the receiving acoustic- 
emission transducer. The degree of attenuation of the 
transmitted stress waves can be converted into a 
numerical value, commonly known as the stress wave 
factor (swF). The swF provides a means of rating the 
efficiency of transfer of dynamic strain energy in a 
given material. Higher values of swF indicate efficient 
transmission of stress waves and are indicative of good 
material quality and freedom from defects. A low swF 
is an indication of high attenuation of stress waves in 
the material, which could be indicative of either poor 
material quality or the presence of defects. 
Acousto-ultrasonic techniques can detect the majority 
of the defects detectable by conventional ultrasonic 
techniques and offer potential for predicting strength 
variations in composite materials by a quick and 
efficient means. 
Optical fibres 
In composite materials, the use of optical fibres is being 
developed for detecting damage and internal strain. 
For surface crack detection, the optical fibres can be 
adhesively bonded to the surface. If the surface 
immediately below the optical fibres suffers from 
cracking, then the optical fibres will break and the light 
transmitted along them will be attenuated. By bonding 
fibres at predetermined distances, crack initiation and 
propagation can be monitored in real time. For 
detecting the onset of mechanical damage in 
composites, the optical fibres can be incorporated into 
the resin matrix along with the reinforcing fibres. The 
234 COMPOSITES. MAY 1988 
onset and presence of damage is indicated by 
attenuation in the transmitted light. This particular 
approach offers considerable potential for in-service 
monitoring to detect damage invisible to the naked eye 
and could eliminate periodic inspection by 
conventional non-destructive means. Internal strain 
measurements in composites have been demonstrated 1 
by embedding optical fibres in the composite matrix 
and illuminating the system with a H e - N e laser. Any 
strain in the component matrix is transferred to the 
optical fibres and since lasers are a sensitive means of 
measuring minor displacements, the level of strain in 
the composite matrix can be determined. A laser 
operating at 633 nm is capable of detecting one 
micro-strain. In the future, if reinforcing fibres could be 
made capable of light transmission, damage detection 
by fibre optics could be achieved along with fibre 
reinforcement. 
Commercial competition forces airline operators to 
minimize the downtime of civil aircraft. Aircraft 
structures, such as control surfaces, tailplanes and 
wings, are quite large and their sheer size will mean 
that any inspection is very time-consuming. In order to 
survive in the very competitive world of the passenger 
airline business, it is vital to economize on inspection 
times and reduce aircraft downtimes. A need is 
therefore emerging to devise N D T techniques capable of 
inspecting large areas rapidly and effectively. 
ACKNOWLEDGMENT 
The author is grateful to British Aerospace for 
permission to publish this article. It must be 
emphasized that any opinions expressed are those of 
the author and do not necessarily represent the view of 
the Company. 
CONCLUSIONS 
Some of the composite-based civil aircraft components 
manufactured by British Aerospace have now been in 
service for over 15 years. Their satisfactory in-service 
performance has led to the use of composites for 
high-loaded applications such as flying control 
surfaces. It is envisaged that the use of composites in 
large civil aircraft primary structures such as wings will 
follow as confidencein both manufacturing methods 
and performance is built up. The projected expanding 
use of composite civil aircraft structures will require full 
exploitation of better design and advanced 
manufacturing technologies. Although automated 
tape-laying machines and enhanced control of the 
curing process might reduce the requirements for NDX 
due to the decreasing occurrence of defects, assembly 
techniques such as adhesive bonding will place a 
greater demand on NDT to ensure that bonds of 
consistent quality and adequate strength are achieved. 
In-service experience has revealed that mechanical 
damage, moisture ingress, heat damage and lightning 
strike damage are the most common factors which 
cause deterioration of composite structures. State-of- 
the-art NOT techniques can detect discrete defects 
generated by mechanical damage and lightning strike 
but are unable to detect heat damage and moisture 
ingress. Recovery of damaged composite structures by 
repair is feasible. Again, state-of-the-art techniques can 
detect the presence of discrete flaws but will not 
provide sufficient information on the strength of the 
material produced during the repair. Either new NOT 
techniques or the further development of existing 
techniques will be needed to fulfil this requirement. 
REFERENCES 
1 British CivilAirworthiness Requirements (issued by the Civil 
Aviation Authority, UK) 
2 Joint Airworthiness Requirements (issued by the European 
Airworthiness Authorities Steering Committee) 
3 Federal Aviation Requirements (issued by the Federal 
Aviation Administration, USA) 
4 Federal Aviation Administration Advisory Circular A C No 
20-107A (April 1984) 
5 Mahoon, A., Brant, C. and Maher, A, 'Computer controlled 
ultrasonic testing of aircraft structures' Brit J ofND T 24 
(1982) p. 316 
6 Mahoon, A. 'The use of radio-opaque penetrants for the study 
of damage in composites' Proc 21st Annual British Conference 
on NDT, September 1986 
7 Mahoon, A., Brant, C.P. and Hayes, L. 'Acoustic emission 
monitoring for structural integrity assessment of aircraft 
composite structures' Proc 4th European Conference on NDT, 
London, September 1987 
8 Brant, C.P. and Mahoon, A. 'Development of ultrasonic 
spectroscopy for the characterisation of carbon fibre 
composites' British Aerospace, Aircraft Laboratories, 
Weybridge, Report AL/MA T/3825 (November 1984) 
9 Stewart, F. and Mahoon, A. 'Eddy current test techniques for 
CFC' British Aerospace, Aircraft Laboratories, Weybridge, 
Report AL/MA T/4002 (February 1987) 
10 Milne, J.M. and Reynolds, W.N. 'Application of thermal 
pulses and infrared thermal imagers for observing sub-surface 
structures in metals and composites' Proc Soc Photo-Optical 
Instrument Eng 590 (1985) pp. 293-302 
11 Cuishaw, B. 'Optical fibres in NDT: a brief review of 
applications' ND T International 18 No 5 (1985) pp. 265-268 
A U THOR 
Formerly with the Weybridge site of the Military 
Aircraft Division, Dr Mahoon is now with Test and 
Development at British Aerospace, Richmond Road, 
Kingston-upon-Thames, Surrey, KT2 5QS, UK. 
COMPOSITES . MAY 1988 235