NonDestructive Testing Ultrasonic & Thermography By: Romeo Zitha MSc Aeronautical Engineering

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  • Non-Destructive Testing

    Ultrasonic & Thermography

    By: Romeo Zitha

    MSc Aeronautical Engineering

  • 1


    In this study, Olymus OMNI Ultrasonic Scanner is used to detect and characterise

    defects in a 12 layers of Carbon resin vacuumed and infused with epoxy resin

    (Laminated Composite Material) measuring 220mm x 190mm and 3mm thick. The

    article also discusses the limitations; advantages and disadvantages of ultrasonic NDT

    on composites. As well as thermography techniques used for tracing defects in

    laminated composite materials, limitations, advantages and disadvantages.

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    Table of Contents


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    Composite structures are becoming increasingly popular within the aerospace

    industry, with Boeing stating, The 787 Dreamliner is primarily made of carbon fibre

    composite material, manufacturing processes produce less scrap material and waste.

    Airbus states, More than 50 per cent of the next-generation A350 XWB is made of

    composites, marking a significant milestone for aircraft production. Composites

    allow for greater strength to weight ratio, corrosion resistant and the opportunity to

    manufacture more complicated shapes. In commercial aviation in order to ensure

    safety and compliance are meet composite fundamentals of an aircraft need to be

    tested periodically during the operation life. NDT methods were developed in order to

    allow for inspection to detect, localize and determine a size of damage without the

    need to disassemble the structure. NDT techniques allow for possibly early damage


    Composites used for manufacturing aircraft components and structures, due to their

    complex internal structure they are subjected to different types of damage at various

    stages of their operation life as compared to aluminium alloys. Damages that occur

    within composites are for example delamination, fibre wrinkling, waviness; they are

    susceptible to impact damage, foreign object inclusion and ply separations can occur.

    Such damages can decrease the residual strength and durability of the structure

    leading potentially to a failure and jeopardizing the safety of the aircraft operation.

    Several NDT techniques have been developed for composites diagnostic purposes.

    Katunin, A., Dragan, K. and Dziendzikowski, M. (2015) states that Ultrasonic Testing

    (UT) is one of the most universal NDT methods allowing detecting different types of

    damage. NDT application on aircraft structures is a well-covered research topic, other

    researchers such as Feuillet, V., Ibos, L., Fois, M., Dumoulin, J. and Candau, Y.

    (2012), used other NDT techniques which can be applied for damage identifications

    of composites structures.

    Additional NDT methods applied in the inspection of aircraft composite structures

    cover: shearography, digital image correlation (DIC), X-ray computed tomography,

    lighting protection sheet (LPS) sensing

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    2.0 Ultrasonic NDT

    National Composites Network (2015), ultrasonic inspection (UT) its the most widely

    used non-destructive inspection method for the examination of composites. On

    microscopically homogenous materials (non-composite) it is commonly used in the

    frequency range 20kHz to 20MHz With composite materials the testing range is

    significantly reduced because of the increased attenuation, so the operating frequency

    limit is usually 5MHz or less. This reduces the ability to resolve small flaws within

    the composites hence it should be taken into consideration when performing the


    In most techniques short pulses of ultrasound (typically a few microseconds) are

    passed into the composite material and detected after having interrogated the

    structure. The techniques include pulse-echo, through-transmission, back scattering,

    acoustic ultra Sonics and ultrasonic spectroscopy.

    Figure 1. Ultrasonic Methods

    Ultrasonic coupling of the probes to the test specimen can be achieved in different

    ways, including close contact using coupling fluids, jells or soft materials, Immersion

    in water, Water jets or water columns and Non contact approaches. The advantages of

    using coupling fluids methods are that it simple, inexpensive, Suitable for field

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    applications, normal or angled probes can be used, provides instantaneous indication

    of flaws and can be used on relatively complex parts or areas with limited access.

    However it does have limitations such as it offers small coverage area, its requires the

    working surface to be smooth and clean and the coupling thickness variations could

    affect the results.

    In a typical scenario, the existing single-element inspection is simply emulated with

    the array but with one or more axes of physical transducer movement (e.g. translation)

    now performed electronically by the array. For example, this can be achieved by

    simply scanning a fixed width aperture over the length of the array4.

    2.1 Approach to inspection

    The experiment was carried out using Olympus OMNI Ultrasonic Scanner, which is

    connected to a 64-element probe (5L64-NW1) as show in figure 2. The frequency was

    set at 5 MHz with normal incidence beam, it is important to note that a low frequency

    reduces the ability to resolve small flaws.

    Figure 2. Olympus OMNI Ultrasonic Scanner

    2.2 Set up and calibration procedures

    The manual ultrasonic testing (UT) is contact-tested by scanning a probe by hand

    preforming raster scanning; this is suitable for inspecting small areas but requires high

    level of operating skills to insure consistent results. A water jell based coupling was

    used manually. It is important to ensure complete contact of couplant with all

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    surfaces. The material and the density of the material influence the distance the sound

    has to travel. With the couplant, it is more important to ensure complete contact with

    all surfaces. The thickness of couplant and the pressure of the probe is not the main

    attribute for the strength of the signal. The distance the sound has to travel through the

    material and the density of the material influences this more. If you do not have

    sufficient couplant then this will affect the transmission and reception of the signal.

    The signal amplitude is dependent on the thickness of the coupling fluid layer, which

    itself is dependent on the pressure applied. A, B, S and corrected and uncorrected C

    scans were recorded for further analysis of the images. The data is analysed using the

    software of the instrument. The data were saved to a disk, and were subsequently

    analysed analytically and visually.

    Figure 3. Raster scanning

    In order to gain accurate results, the raster scan followed the path as outlined on the

    figure to the above. This route would allow a visual representation in the results for

    the whole test piece.

    When the process was completed the results will be sent to the Olympus OMNI

    Ultrasonic Scanner. The scanner will then give a representation of the test piece. This

    information will be represented on the screen where the discrepancies can be analysed

    more intricately.

    2.3 Results

    From the scan data we can identify defects in the laminated carbon composite

    material, as depicted in the figure below.

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    As illustrated in the figure above, The results obtained from the A- scan. The B-Scan

    shows an approximate depth of the defects. The three images below show the B-Scan

    results at three different locations.

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    Defect Depth (Approx.) mm

    1 1.8

    2 2.2

    3 2.5

    4 1.4

    5 1.0

    The density of the defect can also be approximated and when validated against a test

    piece the material can be guessed with some accuracy. The C-Scan shown above

    shows a distinct density difference between each of the defects. Therefore by

    analysing the scan it is clear that the density of defect 4 is the highest with 2 being the

    lowest. Possibly of typical of composite flaw could be present such as: Delamination;

    matrix cracking; fibre breakage and core dis-bonds.

    The results obtained from this test have outlined certain limitations and potential

    errors that may occur from the ultrasonic test conducted on the test piece in this

    experiment. Therefore future ultrasonic methods have been researched to determine

    whether these limitations and errors will one day addressed and potentially mitigated

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    against. One future technique that has been said to potentially be able to address the

    issues mentioned is called Phased Array Ultrasonic Testing.

    Figure above shows a linear scan on a metal block. In this test the transducer is kept

    in one place but the elements waves are sent in a way that scans along the length of

    the probe.

    The results can be displayed in multiple scan views as show in Figure shown above.

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    Another benefit of the phased array method is the ability to take angular readings,

    which can benefit the test conductor when looking at areas that are not easily

    accessible. An example of this can be seen in figure above.


    Ultrasonic inspection was carried out on a Carbon resin vacuumed and infused with

    epoxy resin (Laminated Composite Material) measuring 220mm x 190mm and 3mm

    thick; five defects were located as indicated on the results section with the material

    within great accuracy. For improving the method it has be suggested that the use of

    Phase Array UT will yield better results.

    3.0 Infrared Thermography techniques

    Infrared thermography is a non-destructive technique that determines defects or flaws

    by measuring the temperature variations after some external introduction of a

    temperature gradient. The presences of defects disrupt the normal pattern of the heat

    flow that would be expected in a structure. The techniques are more sensitive to

    defects near the surface. National Composites Network (NCN) (2015) states that

    Modern thermography systems commonly use infrared (IR) cameras to detect

    radiated heat and are controlled by TV video electronics which sample the field of

    view at a typical rate of 50Hz, allowing temperature variations on a 20ms time-scale

    to be resolved. The camera is sensitive to change in temperature of about 0.005o C

    according to NCN.

    Thermography methods fall broadly into two groups: active methods, and passive

    methods. Active methods are thermal gradient are produced and continuously

    maintained by the application of cyclic stress. Passive methods are the results of

    transient change from the thermal gradient. Passive methods are the most widely

    applied NDT technique in composites inspection.

    There are two main techniques employed in active thermography: Lock-in

    thermography utilises a periodic harmonic heat to stimulate the target surface. The

    reflected heat is then captured as a series of thermal images. The thermal images are

    used to extract the sinusoidal wave pattern at each point in the image. The extraction

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    of the thermal wave from the thermal images relies on various signal-processing

    algorithms, including: Fourier Transforms, Time constant image, Four point

    correlation and Digital lock-in correlation. Each method has its advantages and

    disadvantages and is applicable for different applications.

    Pulse thermography consists of subjecting a component or structure to a pulse of heat

    and monitoring the temperature distribution. The heat will permeate the structure. In

    the presence of a defect the heat conduction becomes non-uniform and a thermal

    camera can detect this.

    Emissivity is a material property with some materials that are good emitters and some

    are poor emitters. The materials that are good emitters provide the best results; this

    can be affected by the surface finish, surface shape, viewing angle, metal oxidation

    and temperature.


    The main limitation in applying thermography to composites inspection is the

    anisotropy that produces different thermal properties in different directions. The

    presence of lightning protection mesh in some aerospace structures can mask


    Figure 4. Experimental set-up for thermal pulse thermography. [4]

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    Appendix A

    Appendix 1. Olympus OMNI Ultrasonic Scanner

    Appendix 2. Composite and Probe set-up

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    [3]. Feuillet, V., Ibos, L., Fois, M., Dumoulin, J. and Candau, Y. (2012). Defect

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    [5]. Ncn-uk.co.uk, (2015). National Composites Network. [online] Available at:

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    20 Apr. 2015].

    [6]. Olympus-ims.com, (2015). OmniScan MX2 Phased Array Flaw Detector.

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    [7]. Ramadas, C., Padiyar, J., Balasubramaniam, K., Joshi, M. and Krishnamurthy, C.

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