Non-Destructive Testing

Ultrasonic & Thermography

By: Romeo Zitha

MSc Aeronautical Engineering

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Abstract

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

1.INTRODUCTION 3 2.0 ULTRASONIC NDT 4 2.1 APPROACH TO INSPECTION 5 2.2 SET UP AND CALIBRATION PROCEDURES 5 2.3 RESULTS 6 CONCLUSION 10 3.0 INFRARED THERMOGRAPHY TECHNIQUES 10 APPENDIX A 12 REFERENCES 13

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1.Introduction

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

detection.

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

inspection.

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.

Conclusion

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.

Limitations

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

indications.

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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