JERE YLIANUNTI, MADAN PATNAMSETTY, WENXIN ZHANG,YASHWANTH GOWDA

DIFFERENT NDT METHODS FOR POLYMER AND COMPOSITEMATERIALS

Seminar group work, MOL-32246 Introduction to NDT-techniques

iTABLE OF CONTENTS

1. INTRODUCTION ................................................................................................ 12. COMMONLY USED NDT METHODS ............................................................... 2

2.1 Visual NDT methods .................................................................................. 22.2 Ultrasound and acoustic emission ............................................................... 42.3 Radiographic methods ................................................................................ 42.4 Magnetic and electrical methods ................................................................. 4

3. ACOUSTIC EMISSION ....................................................................................... 63.1 Background ................................................................................................ 63.2 Acoustic emission for composites ............................................................... 63.3 Case study on Acoustic emission technique applied in real-time evaluation ofenergy attenuation for damage monitoring of ceramic matrix composites ............ 73.4 Case study on Acoustic emission technique applied in real-time frequencydetermination of different fracture mechanisms for carbon/epoxy composites...... 8

4. ULTRASOUND TECHNIQUE ............................................................................ 94.1 Background ................................................................................................ 94.2 Ultrasonic to composites ............................................................................ 94.3 Ultrasonic testing of Polymer composites ..................................................104.4 Comparative study of Pulse echo and TTU in glass/epoxy composites [19] .11

5. RADIOGRAPHIC METHODS............................................................................135.1 Background ...............................................................................................135.2 Radiography to Composites .......................................................................135.3 Radiography testing of Polymer Composites .............................................135.4 Case Study for inspection of possible defects in the composite structure ofhelicopter rotor blades [20] ...................................................................................15

REFERENCES ............................................................................................................17

11. INTRODUCTION

This paper focuses on the matter of using different NDT methods for inspecting polymer andcomposite materials. Metallic materials are usually the most common examples used whendiscussing the use of NDT methods, but they are also largely used on polymers and compo-sites. There are certain distinctions in the methods to be considered, though, which raise fromthe differences in the chemical and physical composition of these material groups. The elec-trical insulating nature of polymers is probably the most distinct quality that differentiates itfrom metals. Composite materials, however, are often mixtures of polymer, metallic and/orceramic materials, which makes matters more complicated.

This paper will first outline some of the most used NDT methods, and later on continue toelaborate their use and applicability in polymer and composite testing. Although widely usedmethods for metallic materials, magnetic and electrical methods are used less often on plasticmaterials, and will be only briefly considered in this paper. The focus will be on more widelyapplicable methods regarding plastic materials. Case studies are included with some of theNDT techniques, where considered important in depicting the use of these methods.

22. COMMONLY USED NDT METHODS

This chapter will outline the common NDT methods in general. Although some limitationsarise because of the basic chemical composition of polymeric materials, many common NDTmethods are applicable to polymers just as well as other materials. Most important techniquesregarding polymers and composites will be treated more specifically in later chapters.

While some qualities of polymers limit the use of certain NDT techniques, in most cases theysimply require specific knowledge on how to correctly apply the techniques to these kinds ofmaterials. For example, speed of sound is greatly lower in lighter polymer materials com-pared to ceramics and metals. This may cause confusion in studying ultrasound inspectionresults from a polymer component, if this factor is not known and accounted for. The greatestdifference in polymer-based materials that separates them in this sense from testing of metal-lic materials, is electrical conductivity. Most polymers are electrical insulators, which limitsthe usability of some testing methods for them. They are usually also poor thermal conduc-tors, which has to be taken into account when using thermal inspection methods.

It has to be noted that while generally, polymeric materials are both poor electrical and ther-mal conductors, development in material sciences is narrowing the gap between metals, ce-ramics and polymers in terms of qualities and possible applications. In the last few decades,research and development in the fields of electric engineering has generated conducting pol-ymers specifically for electrical applications [1]. One example is research in polymer-basedflexible displays, in which the mobile phone market has a keen interest. This widens the pos-sibilities of using NDT methods which are based on electrical conductivity, to inspect poly-mers.

2.1 Visual NDT methods

These are by far the simplest and most used techniques. These include various photographicand video camera applications, as well as microscopic methods. In the most primal sense,visual inspection consists of using just basic human senses, such as touch and eyesight to de-termine the possible flaws in a component. Nowadays machines are used more and more toconduct visual testing of components. Machine vision has several obvious advantages over aperson conducting the inspection, such as speed and consistent accuracy.

Dying and penetrating liquids are also a sort of visual inspection methods. They work by us-ing specific dyes or paints, which enhance the contrast of surface flaws on a component. Visi-

3ble light, sometimes assisted by fluorescent substances, and ultraviolet are all used. Somevery porous types of polymers may prevent the use of these methods.

Microscopic inspection methods are sometimes categorized under visual methods, along rep-lica techniques. These two techniques are tightly connected, as they are often used together.Replica formation means using a compound to create a 3D-mirror image of the componentunder inspection. This replica contains the components surface topology along with possiblesurface cracks, porosity and other anomalies. In some cases residue from the surface is alsopicked up by the replica compound, which in some cases is a desired occurrence. This replicacan then be taken for off-site inspection. This eliminates the use of portable equipment andallows the use of more powerful magnifying tools than would be practical to use on the field.Picture 1 shows the microscopic differences between the original component and a replica.

Figure 1: Microscopic images of a crack in a boiler tube (top), and of the replica of the samecrack (bottom)

The resolution of replicas is largely dependent on the quality of replica compound. In theabove picture it can be seen that some of the finest cracks seem to have disappeared. The

4main crack, however, is well visible, and resolution of this scale is adequate for many inspec-tion cases.

2.2 Ultrasound and acoustic emission

The basis of these methods lie on the sound waves traveling inside the component, which canbe detected by appropriate sensors. The sound waves carry information about the internalstructures and stresses of the material. Ultrasound methods usually first produce the waves,and then try to capture the returning, reflected waves from the component. Acoustic emissionmethod works a bit differently, since it relies on the sound waves generated by the componentitself. Both methods are useful tools when inspecting polymer and composite materials [3].This method will be reviewed more thoroughly in chapter 4.

2.3 Radiographic methods

The name refers to different kinds of x-ray, gamma ray and other short wavelength radiationmethods of inspection. Small particle methods, such as different electron scattering methodsand neutron radiography, can also be included in the same category. The most known method,x-ray radiography, is most suitable for revealing porosity inside a component. Different kindsof fractures can also be examined. Polymer materials are typically less dense than e.g. metalsor ceramics, and partially because of this, attenuation of electromagnetic radiation is generallylower. This can make it more difficult to achieve high contrast in radiographic methods. Alt-hough, it can also be beneficial in the way of allowing the inspection of thicker components[4]. Chapter 5 will elaborate on the use of this non-destructive inspection method.

2.4 Magnetic and electrical methods

These include several methods including magnetic field testing, magnetic particle testing, andtechniques using induction phenomenon. Barkhausen effect is a less known technique fornon-destructive component evaluation. Use of these methods is often partially restricted dueto the electrical insulating nature of polymers. Some methods generate their own magneticfield, and some use one generated in the component under inspection. This is relevant whenconsidering methods usable for polymers and composites. For example, eddy current methoduses alternating current in a coil to induce an alternating magnetic field into a component. Forthis phenomenon to work at all, certain requirements have to be fulfilled for the componentmaterial [4], [5]. Since magnetic methods have somewhat less usability with plastics and com-posites, they will not be reviewed furthermore in this paper, but instead focus will be on morewidely used methods.

Composites are a different case in many aspects, since they can include metallic, ceramic andfor example, carbon fiber materials. This can result in very different qualities regarding e.g.thermal and electrical conductivity. The following chapters will give an in-depth view on

5some of the mentioned NDT methods, and consider their usability and possible restrictions ininspecting polymers and composites.

63. ACOUSTIC EMISSION

3.1 Background

The physical nature of acoustic emission is very simple compared to other NDT methods. Thetransient elastic waves that generated by the rapid stress redistribution inside materials aredetected and analyzed for material characterization. From this point of view, the differencebetween AE and ultrasound technique is the source of generated wave. The other difference isthat for AE technique, an external stimulation should be applied to the sample while in ultra-sound technique no external stimulation required. During AE testing, the specimen is appliedwith an external stimulus such as load, pressure or temperature. Then the sensor records thereleased stress wave propagated to the surface [6]. Different wave signal reveals different fail-ure mechanism [7]. There are mainly two types of AE signals conducted: burst emission andcontinuous emission. One records a sudden change and the other records the change over aperiod of time. The emitted wave ranges from around cm in laboratory sample to several mfor real structural [8]. To achieve a good acoustic coupling effect between sensor and specimensurface, the interface is usually applied with a gel material or adhesive.

Due to the principle of the AE technique, it is widely used for homogeneous materials suchas metals and ceramics, as well as inhomogeneous materials such as composites etc. Besides,it is also greatly applied on different structures and processes. As a result, AE is an essentialtechnique in both laboratory and industry. Several important applications of AE are: the struc-tural health monitoring (SHM), material research, leakage, corrosion, in-service failure [9].

3.2 Acoustic emission for composites

Different from metals from which phase transformation and twining generates the acousticwave, matrix cracking and fiber breakage and matrix-fiber interface debonding conduct as thethree main sources [10]. The properties such as dynamic response, flaw density, threshold foronset of crack growth can be obtained [11]. Due to the complex damage mechanisms of com-posites, enormous amounts of noises are collected together with acoustic emission waves.Three dominant types of noises originate from mechanic, thermal and electrical magnetic in-terference. To eliminate the noises to an acceptable level, threshold criterion is selected andfilters are incorporated [12]. The acoustic emission wave which falls out at ultrasonic range

7follows the same physical rule when it propagates in the composites as in the metals but someproperties such as wave velocity and attenuation differs from each other.

Compared to other NDT techniques, AE technique which mainly detects various micro-structural failures complies to composites with some particular advantages, such as real-timecontrol, remote inspection and on-going monitoring. Besides, due to AE signal directly corre-lates to the failure mechanism, this technique gains a great advantage for inspecting real sizestructures [8]. Meanwhile, AE technique is not only widely applied but also quite efficient dur-ing inspection. Moreover, AE technique is especially sensitive than other NDT methods suchas ultrasound so that it is a suitable technique for detecting micro damage, crack initiation andincipient failure. However, there are also some limitations when applying AE technique.Firstly, the spatial flaw is difficult to locate. Secondly, the damage severity it difficult to iden-tify. Thirdly, as unique to composites, the specimen anisotropy provides trouble in locatingsource. Fourthly, increased number of sensors is required owing to large signal attenuation[13]. Fifthly, noise can be a crucial factor in data analyzing. Lastly, data analyzing requirestrained expertise to interpret [14].

3.3 Case study on Acoustic emission technique applied in real-time evaluation of energy attenuation for damage monitoringof ceramic matrix composites

E. Maillet et al.[15] proposed an approach to evaluate the real-time energy attenuation due todamage accumulation by analyzing the acoustic emission signal energy data collected. Theceramic matrix composites were applied with intermediate temperatures so that the acousticemission data during static fatigue tests are studied. According to figure 1, the results showedthat there was a significant increase of energy attenuation during beginning which indicatesthe transverse matrix crack opening. Besides, this method also can also be used as lifetimeprediction due to reproducible and characteristic properties of attenuation coefficient evolu-tion. The results also proved that this method can be applied to many other materials besidesceramic matrix composites as long as the acoustic emission activity is high enough.

8Fig 2. Attenuation coefficient B vs. time a/ during the first hour and b/ throughout the fatigue test.

3.4 Case study on Acoustic emission technique applied in real-time frequency determination of different fracture mecha-nisms for carbon/epoxy composites

As illustrated before, fracture mechanism for composites can be achieved by applying acous-tic emission technique. Peter J. de Groot et al. [7] research work showed a convincing resultsabout the above concepts. They applied specimen with load from zero to failure and measuredthe real-time frequency acoustic wave emitted. From figure 2, it can be obviously obtainedthat different fracture mechanism generated certain range of acoustic wave. In detail, matrixcracking released frequencies between 90 and 180 kHz, fiber failure frequencies above 300kHz, debonding frequencies between 240 and 310 kHz and pull-out frequencies between 180and 240 kHz.

Fig 3. Overview of frequency failure relationships as found in the literature as well as our results.

94. ULTRASOUND TECHNIQUE

4.1 Background

The testing is underwent with high frequency sound waves introducing into materials for sur-face or subsurface detections. These waves are reflected after some attenuation and thus theanalysis of reflected beams locates the flaws or discontinuities. The degree of reflection de-pends on largely physical state of materials forming interfaces and specific physical proper-ties. Normally the UT is done at the frequencies between 0.1 and 25 MHz, not only the inter-nal flaws the technique may also be used to detect surface flaws, to define bond characteris-tics, corrosion, and determine physical properties, structural, grain size and elastic constants[2]. Technically in ultrasonic inspection a transducer transforms a voltage pulse into an ultra-sonic wave and when the transducer is placed on to the specimen then signal is either trans-mitted to another transducer (Pitch Catch method) or reflected back to same transducer(Pulse echo method). Either way the signal is transformed into an electrical pulse, which isobserved on an oscilloscope. And the signal can determine the ultrasonic wave velocitythrough the specimen, the presence of flaw, defect, or delamination and its size, shape, posi-tion and compositions may be known and material properties such as density, elastic constantscan be determined. [16] On contrast there also exists UT technique by non-contacting the sam-ple, the technique is currently exploited which is known as Laser Ultrasonic Inspection Sys-tem (LUIS) developed by Ultra Spec [18]

4.2 Ultrasonic to composites

For composites the testing range is simply reduced because of increased attenuation, so theoperating frequency is normally ranged between 1 and 5 MHz [17] In most techniques shortpulses are of ultrasonic are passed into composite and detected after having interrogated thestructure [18] The most important techniques include Pulse-echo, through transmission ultrasound (TTU), Pitch catch and guided wave techniques.

Fig 4 (a) TTU; (b) PE; (c) pitch catch; and (d) guided wave[17]

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Various categories of polymer composites can be tested such as Laminated (Hand layup, Tapelayup, Fibre placement, Resin transfer moulding), Sandwich (Honeycomb core, Foam core),Three-dimensional (3D) performs and other forms such as braided, stitched, or chopped fibreor tape, Bonds (Film or paste, Secondary or co-bond). The major flaws that could be detectedare porosity, delamination and disbond, Skin to skin care disbond. [17]. In this method it isimportant to avoid frequencies at which resonance occurs between ply interfaces, for examplefor unidirectional plies spaced at 8 plies/mm this frequency is usually about 12 MHz . Formanual ultrasonic testing the area is contact tested by ultrasonic probe and skilled operatormay be required, for some composites that are water sensitive or absorbent, the roller probeswith water retentive rubber tyres are used as they leave surface dry. [18]

4.3 Ultrasonic testing of Polymer composites

A typical velocity of ultrasound across the laminate is close to 2.8mm/s, the common plythickness for aerospace application would be in a range of 0.1-0.35 mm. In composites theultrasound attenuation is a measure of material consolidation. The UT attenuation is meas-ured as

Where = attenuation coefficient

A0 = initial pressure amplitude

A = Pressure amplitude after transmission

X = transmitted distance

The values of frequencies and ultrasonic attenuations used are subjected to specifics of thefibre, resin, fibre-to-resin ration, ply type, and layup. They are also subjective to specifics ofultrasonic equipment, because consolidation is comprised by porosity, thus attenuation mayincrease. So every calibration curves must be developed for specific equipment. Only 2% po-rosity is acceptable for manufacturing process. And if greater than 4% then the performancewould be knocked down. When the ultrasonic sound is transmitted then the measurement ofimpedance of individual material will give the transmission co-efficient and reflection co-efficient.

The acoustic impedance Z can be defined as

Where v = acoustic velocity and = density

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And thus the transmission co-efficient is

And reflection co-efficient is

Where Z1 and Z2 are impedance of two difference materials, considering acoustic impedanceof polymer composite is 470,000 g /cm2-s and for air it is 40 g/cm2-s thus we get transmissionco-efficient as 0.00017 and refection co-efficient as 0.9998 thus the small value of Transmis-sion co-efficient and large value of Reflection co-efficient gives us clear idea of delaminationof composites. [17]

4.4 Comparative study of Pulse echo and TTU in glass/epoxycomposites [19]

A study of two UT techniques (Pulse echo and Through Transmission Ultrasonic testing)is used for a comparative study of the materials of the following properties.

Table 1: Properties of constituent materials. [19]

Parameter Unit E-glass Epoxy resin

Density g/cm3 2.58 1.13

Tensile strength MPa 3500 65.4

Elastic modulus GPa 75 3.1

After the analysis of different weight percent glass content specimens the following resultsare obtained

Table 2: Determined properties of investigated glass/epoxy specimens [19]

no:Glass content

[Wt.%]

Average wave velocity [m/s]

Pulse-echoThrough-transmission

1 31 2461 26562 37.2 2580 26763 56.8 2949 28084 57.3 2963 28665 65.2 3045 2920

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From the above comparison a considerable dispersion of data in the wave velocity can be at-tributed to the local void content in the specimens indicating that the accuracy of presentmethods could be influenced by this factor. However the extent to which void content affectsthe wave velocity is yet to be established. The graph below shows the dispersion of wave ve-locity with respect to Glass weight percent with different techniques.

[19]

Figure 5: Ultrasonic wave velocity Vs Fibre content

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5. RADIOGRAPHIC METHODS

5.1 Background

The radiography includes three basic elements as radiation source, test piece, and a sensingmaterials. The radiation can be of many types as X-ray radiation, gamma radiation, Beta radi-ation and neutron radiation which are generated from different materials at high voltage. Theproblem with this process is that it is very much hazardous to human or any living beings,among these above radiation types X-ray radiation is the most famous and is largely used invarious applications. This technique majorly is applicable with industrial applications as in-spection of welding defects, Casting defects and defects in electronic application materials [3].

5.2 Radiography to Composites

For composites inspection two types of radiation are relevant they are X-ray and neutronsradiations. So the basic principle here is that the parts of the specimen that have different ra-diation absorption properties can be easily discriminated by an image formed on a film withbeam transmitted through the specimen. And x-rays techniques used here are classified into 2categories as Conventional X-ray Radiography and enhanced X-ray radiography (speciallyformulated liquids are used to enhance the contrast of the radiographic images). This en-hancement is to improve contrast of some polymer matrix and carbon fibers which have lowerabsorption co-efficient. The conventional radiography can be applied to detection of voids incomposites (when absorption is 2%or more). Penetration enhanced radiography also detectsdelamination and cracks. Enhanced radiography can also determine fiber volume fraction andfiber alignment on composites with radiation absorptive additives such as boron or glass(GFRP). The advantage over ultrasonic testing is that more thicknesses can be inspected andbetter resolution images can be produced. [18]

5.3 Radiography testing of Polymer Composites

The mechanism here is that the transmitted beam intensity is recorded by an image detectorsystem as film or digital imaging system.

The general equation of attenuation of radiation beam is

Where I (E) = transmitted beam intensity as a function of energy E

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I0 (E) = Initial X-ray beam spectrum intensity,

(E) = material linear attenuation co-efficient

x = thickness

Figure 6: Transmission through a sample for internal detection

The below image show a plot between linear attenuation co-efficient up to 50KeV for CRFPand aluminum which infers minor change in thickness gives the respective change in intensity

After differentiating the attenuation of the beam equation we get

Thus when comparing the equation to plot it means the intensity % change is related to plythickness for composites in this condition. That is for example an approximate one-ply(~0.19mm) change in thickness represents about 1.2% change in intensity.

Figure 7: Plot between Energy and Linear attenuation co-efficient [17]

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The radiography can be sensitive to volumetric features and it is not sensitive to planar defectlike Delamination. Although radiography is not usually selected for laminate damage detec-tion, it is useful for volumetric features for sandwich structures. [17]

5.4 Case Study for inspection of possible defects in the compo-site structure of helicopter rotor blades [20]

For a rotor blade of MI 24-type helicopter which are made of composite structure (18 sec-tions of honeycomb constrictions and many bonded surfaces). The dimensions of the bladeare 10m and 0.7m and weighs 110Kg, a biological shield had also been made for the inspec-tion of larger blades. A portable X-ray generator was used for X-ray radiography adjusted to150kV and 3 mA. There are 3 measurements done

In first step, each blade was radiographed by neutrons in its original state (Neutron ra-diography)

And next step was the blade is watered on surface and measured to simulate the rainenvironment and radiographed by the neutron (wet NR-neutron radiography)

And then X-ray radiography was made.

Then radiographic images are developed individually

Figure 8: Water percolation into the honeycomb structure near the stiffener: (a) dry NR; (b) wet NR;

C) Resin rich area in the honeycomb structure [20]

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The most important defects were cavities, holes, and cracks in the sealing element of the in-terface of section borders. But were not visible to X-ray or NR, but for wet condition the wa-ter over flows into the defects and thus due to high neutron hydrogen attenuation they becomevisible as in figure 6(b). But conditionally if water is present prior to wetting thus no changecan be visible some times. The resin rich and poor areas are also more problematic. This typeof defect is identified because the X-ray attenuation is very small (thus not visible in contrastto the water content), while dry and wet NR gives similar images. The most important pointsof this study have been the visualization of the possible imperfections in the honeycombstructure, the defects at adhesive filling and water percolation at sealing interfaces. [20]

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[20] M. Balask, I. Veres, G. Molnr, Z. Balask, and E. Svb, Composite structure of heli-copter rotor blades studied by neutron- and X-ray radiography, Phys. B Condens. Mat-ter, vol. 350, no. 13, pp. 107109, Jul. 2004.