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P1: KVK Journal of Nondestructive Evaluation [jone]

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C 2004) Journal of Nondestructive Evaluation, Vol. 23, No. 4, December 2004 ( DOI: 10.1007/s10921-004-0819-z

Geometrical Limitations to Detection of Defects in Composites by Means of Infrared Thermography Carosena Meola,1,3 Giovanni Maria Carlomagno,1 and Luca Giorleo2 Received March 3, 2003; revised September 10, 2004

The aim of the present experimental study is to gain information about limits in detection of defects in composites by infrared thermography. Specimens are manufactured of either carbon/epoxy, or glass/epoxy, and with inclusions of foreign materials to simulate defects of different size and positioned at different depths. Tests are carried out by using both pulse and lockin techniques. Results are presented in terms of difference of temperature (pulse), or difference of phase angle (lockin), between damaged zones and sound material. It seems that, apart from diameter and depth, the thickness is very crucial for defects visibility.

KEY WORDS: Composites; glass fibres; carbon fibres; nondestructive control; infrared thermography; pulse thermography; lockin thermography.

1. INTRODUCTION

In this context, infrared thermography, thanks to its two-dimensional and non contact character, can be usefully exploited for nondestructive evaluation of composites.(2,3) The inspection may be performed with either pulse thermography (PT), or modulated lockin thermography (MT), or a combination of the two as pulse phase thermography (PPT). Pulse thermography visualizes defects through surface temperature gradients in the transient heating/cooling phase while the object is thermally stimulated. Two approaches are possible: transmission and reflection modes. In the transmission mode the specimen is heated from one side while the infrared camera views the rear part (opposite to heating). In the reflection mode instead both the heating source and the camera are positioned on the same side. The size of defects can be measured directly from the thermal image by exactly known the spatial resolution of the employed optics. Instead, quantitative information about depth and nature of defects can be acquired by processing the thermal image.(4,5) The PT technique is affected by local variation of the emissivity coefficient and nonuniform heating that can mask the defect visibility.

Composites offer significative benefits over metallic materials in terms of lighter weight, fatigue and corrosion resistance and aptitude to take complex shapes. Adversely, composites, throughout their life cycle, are susceptible to the formation of many possible defects such as delaminations, cracking, fibres fracture, fibres pullout, matrix cracking, inclusions, voids, and impact damage and can arise either during the multiple step production process, or in service. Among others, impact damage, which is invisible to naked eyes, can have deleterious effects on the capability of composite structures to operate in service.(1) So that, the requirements of high performance involve accurate quality controls of laminates as well as periodic inspection of parts in service. Thus, urged the development of nondestructive techniques able to discover defects of every kind and at an incipient stage. 1 2 3

DETEC – Universita` di Napoli Federico II, P.le Tecchio, 80 80125 Napoli, Italy. DIMP – Universita` di Napoli Federico II, P.le Tecchio, 80 80125 Napoli, Italy. Corresponding author: E-mail:

125 C 2004 Springer Science+Business Media, Inc. 0195-9298/04/1200-0125/0


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The modulated thermography was first described by Carlomagno and Berardi(6) and later further investigated by many researchers.(7−13) The object is stimulated by a modulated heat flux; the thermal wave propagates inside the object thickness and becomes reflected when it reaches zones where the heat propagation parameters change (local inhomogeneities). The interference between reflected wave and surface wave can be measured on the surface and transformed into phase, or magnitude images. These images allow for a direct evaluation of depth and nature of defects without troublesome post-processing procedures. In fact, from the relationship: µ=

α πf

(1)

it is possible to calculate the thermal diffusion length µ, by considering the frequency f at which defects become visible and by knowing the thermal diffusivity α of the material under test, and so the depth p since p = 1.8 µ as indicated by Busse.(7) Details on the material characteristics can be related to the variation of the phase angle which can be evaluated directly on the phase image. The PPT technique, developed by Maldague and Marinetti,(14) combines some advantages of both PT and MT. The specimen is pulse heated as in PT and the mix of frequencies of the thermal waves launched into the specimen is unscrambled by performing the Fourier transform of the temperature evolution over the field of view; the phase, or magnitude, image can be presented as in MT. A comparison between the different techniques was made by Carlomagno and Meola.(15) The performance of a nondestructive technique is assessed through its ability to individuate defects of small dimensions and positioned quite in depth. The success of a control, performed with infrared thermography, depends on the material (both sound and defects) thermal properties mainly thermal conductivity and thermal diffusivity. Generally, infrared thermography is thought to be able to detect defects of diameter proportional to depth. One question occurs to us: “How thick must be a defect to be detectable?” The main scope of the present investigation is an attempt to search for an answer. Therefore, the influence of: diameter, depth and thickness on the visibility of defects in carbon/epoxy and glass/epoxy is analysed.

Fig. 1. Sketch of Carb-A specimens.

2. DESCRIPTION OF SPECIMENS Two composites: carbon/epoxy and glass/epoxy are considered. 2.1. Carbon/Epoxy • Several specimens are manufactured by superimposing resin pre-impregnated weave substrates (0.90)f to have an overall thickness st variable from 1 mm up to 6 mm. Each specimen, as sketched in Fig. 1, includes three inserts of Teflon of thickness s = 200 µm, equivalent diameter d equal to 6.35, 12.7 and 16.9 mm and positioned at depth p variable from 0.5 up to 3 mm. The size of each specimen is 50 × 185 mm2 . These specimens are simply called Carb-A. • Two specimens, manufactured of the same weaves as Carb-A, are step wedged, as sketched in Fig. 2, with an insert of Teflon at half depth in the middle of each step; each step is about 50 mm long and 70 mm wide. Thus, these specimens, which are called CarbB1 (Fig. 2a) and Carb-B2 (Fig. 2b), include defects of d = 12.7 mm and positioned at depth p variable from 0.5 mm up to 1.75 mm (Fig. 2a) and p from 1.9 mm up to 3.5 mm (Fig. 2b). It is important to note that all these specimens are manufactured in a firm by following precise standards. In particular, defects are composed of two superimposed thin foils of Teflon (which is generally used for sealing) and are positioned between two contiguous weaves. The thickness of defects is found to be s = 200 µm, as measured, after nondestructive tests, by a scanning electron microscope SEM; an image is shown in Fig. 3. 2.2. Glass/Epoxy The material under test is a glass/epoxy laminate obtained by a staking sequence of (M10/50%759120CM) resin preimpregnated substrates (0.1 mm


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Fig. 2. Sketch of Carb-B specimens, all dimension are in mm; a) Carb-B1; b) Carb-B2.

thick) positioned at 0◦ and 90◦ to have a total thickness st = 5 mm. To simulate delaminations and/or non-homogeneities, diskettes of aluminium, cork and Teflon of five different diameters, d, ranging from 2 up to 8 mm and two thicknesses, s, equal to 1 and 2 mm are manufactured, from plates of given thickness, by means of punch knives and positioned at different depths, p, from 0.125 up to 4 mm (for more details see Ref. 5). The final laminate is obtained by superposition of three thinner prepreg laminates. Holes are drilled over one prepreg and filled with pieces of one of the three different materials: Teflon, cork, and aluminium; then, this prepreg (with a thin layer of resin in both sides) is inserted between the two

sound prepregs, put in a press and let curing at ambient temperature (about 20◦ C). Each specimen is 150 × 150 mm2 ; a sketch is shown in Fig. 4.

3. TEST PROCEDURE Most of the tests are carried out by the Agema 900 LW Thermovision, equipped with the Lockin option and by using the experimental setup of previous works(16,17) with heating source and camera on the same side. The field of view (which depends on the optics focal length and on the viewing distance) is scanned by the Hg-Cd-Te detector in the

Fig. 3. SEM image.


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Meola, Carlomagno, and Giorleo Table I. Material Aluminium Carbon/epoxy–(Zalameda, 1999) Carbon/epoxy // (Infrared Obs.) Carbon/epoxy ⊥ (Infrared Obs.) Carbon/epoxy-A present paper Cork Glass/epoxy // (Infrared Obs.) Glass/epoxy present paper Epoxy resin Teflon

Fig. 4. Sketch of glass/epoxy specimens.

8–12 µm infrared window. Normal sensitivity, expressed in terms of noise equivalent temperature difference, is 0.07◦ C when the scanned object is at ambient temperature. The scanner spatial resolution is 235 instantaneous fields of view per line at 50% slit response function. Each image is digitized in a frame of 136 × 272 pixels at 12 bit. Both pulse and lockin methods are used; PT is performed either in transmission, or in reflection, sequences of images are acquired during the transient heating phase. The surface of the specimen is thermally stimulated by a circular quartz lamp (1 kW) positioned at a distance variable from 0.5 m up to 1.5 m depending on the specimen dimension and on the testing technique. The PT technique suffers from nonuniform heating and reflections and thus requires accurate positioning of the heating source; with the MT technique the lamp position is not so important and a few degrees (down to 1◦ C) temperature rise are sufficient for defects detection.(15) The infrared camera is positioned at a distance so as to view the entire surface of each specimen. For the MT technique, the thermal diffusivity α, as stated by Eq. (1), represents a key parameter for flaws detection. The α values, of the materials involved in the present study, are compiled in Table I. These values are either found in literature, or measured by the MT technique itself by following the procedure described by Meola et al.(17) Generally, tests start at a value of the wave frequency quite high at which only surface (or low depth) defects are visible; after, the frequency is decreased until the minimum value available in the Agema Lockin option ( f = 0.0037 Hz) is set, or the entire thickness

α[cm2 /s]– literature

α[cm2 /s]– measured

0.9 0.0035–0.0062 0.02 0.0044 0.0045 0.0016 0.00169 0.00116 0.0004 0.0007

is investigated. The images are stored for successive analysis. Some tests, on specimens of type Carb-B, are performed by the Cedip JADE III focal plane array camera with spectral response in the 3–5 µm infrared band, normal sensitivity 20 mK at 25◦ C, spatial resolution of about 65000 pixels. In this case a different setup is employed with the lamp positioned on the rear of the specimen, i.e. on the opposite side with respect to the camera (transmission mode). 4. DATA ANALYSIS Thermal and phase images of the specimen of type Carb-A with inserts of Teflon at p = 0.5 mm are shown in Fig. 5. At first sight, since the three defects appear visible over both thermal and phase images, both techniques could be assumed to work well. However, by deepen, it can be noted that the phase image gives more details with respect to the thermal one; in fact: • the defect countour is better resolved and this helps for the evaluation of the defect real size; • it is possible to see also the material weave apart from the label present on the surface top left side. Thermal and phase images are analysed to acquire information about the capabilities of the two techniques. In particular, it has to be reminded that the main aim of the present work is to individuate the smallest detectable imperfection. In general, manuals of infrared systems state that a defect to be visible should have diameter (equivalent diameter) at least equal to the depth, but another question arises: what role plays the defect thickness? To the authors’ knowledge, this aspect, had not received enough attention.


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Fig. 5. Images relative to the carbon/epoxy specimen with inserts at p = 1 mm; a) thermal image; b) phase image for f = 0.47 Hz.

To try to answer this question, the role played by three parameters: diameter (equivalent diameter), thickness and depth of defects is investigated. Both thermal and phase images are analysed in terms of temperature (phase angle) difference between damaged zones and sound material. Sometimes, such difference is called contrast even if is more appropriate to define the contrast in a normalised way.(18)

129

Fig. 6. |T | against p for varying the diameter of Teflon defects in glass/epoxy.

probably due to the higher thermal conductivity of carbon fibres with respect to glass fibres. In fact, the thermal contrast enhances for increasing the relative conductivity between basic material and defects. It is also worth noting that glass/epoxy has been tested in transmission mode while carbon/epoxy in reflection mode. At least for the considered testing conditions, transmission seems to favour contrast. 4.2. Phase Images

4.1. Thermal Images The difference of temperature T between sound and damaged zones is evaluated. The absolute temperature difference |T| measured over images, acquired in transmission mode, and which refer to glass/epoxy with Teflon inserts is plotted against the depth p in Fig. 6 and against the diameter d in Fig. 7. More specifically, Fig. 6 shows the variation of |T| with p and d for s = 1 mm; Fig. 7 instead shows the variation of |T| for varying s with p = 1 mm. As expected, |T| increases as d and s increase and p decreases. In an attempt to find a limiting value in defects thickness and depth, |T| is plotted against the ratio s/p in Fig. 8 for carbon/epoxy and in Fig. 9 for glass/epoxy. As can be seen |T| increases with s/p until s/ p ∼ = 2 and after it remains practically constant and depending only on the diameter. The influence of the basic material on defects visibility is shown in Fig. 10; as can be seen, points relative to carbon/epoxy display better contrast. This effect is

The absolute value of the difference between the phase angle over the centre of the damaged zone and the phase angle of the sound material |φ| is

Fig. 7. |T | against d for varying the thickness of Teflon defects in glass/epoxy.


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Fig. 8. |T | against s/p for varying the diameter of Teflon defects in carbon/epoxy.

plotted against p in Fig. 11 for carbon/epoxy and in Fig. 12 for glass/epoxy. In particular, only values for defects of aluminium and cork are reported in Fig. 12. Defects, made of Teflon, are masked by the presence of nonuniform distribution of adhesive, which has thermal diffusivity close to that of Teflon. By comparing the two figures (11 and 12) it seems that inclusions in glass/epoxy are better visualized; this can be explained by considering the geometry of inserts and the manufacturing process. In fact, glass/epoxy specimens include inserts of thickness 1 mm (or 2 mm), while inserts in carbon/epoxy are only 200 µm thick.

Fig. 9. |T | against s/p for varying the diameter of Teflon defects in glass/epoxy.

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Fig. 10. Comparison between carbon/epoxy and glass/epoxy.

To quantify the influence of the defect thickness on detection by MT, values of |φ| are plotted against s/p in Fig. 13 for carbon/epoxy. It seems that |φ| increases sharply with s/p for s/p ≤ 0.2 regardless of the diameter; while for greater s/p values the influence of the diameter becomes important.

4.3. Comparison Between Thermal and Phase Images In general, by comparing phase and thermal images, the main thing to be observed is that the thermal image is strongly influenced by nonuniform heating and this can lead to misleading interpretation of results. In fact, the PT technique cannot distinguish

Fig. 11. |φ| against p for carbon/epoxy.


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Fig. 14. Comparison between |φ| and |T | for carbon/epoxy. Fig. 12. |φ| against p for glass/epoxy.

whether an effect is real or due to the potential error sources linked to the object, the environment and the acquisition system, which may entail modulation of the temperature distribution. For a comparison between the two techniques, |T| and |φ| for carbon/epoxy, are plotted together against s/p in Fig. 14. It can be noted that, in the log-log plane, all |φ| points lie over a line of equation: |φ| = 13.88

s 1.393 p

(2)

with coefficient of determination R2 = 0.99; instead, all |T| points lie on a line of equation: |T | = 3.42

s 1.478 p

(3)

with coefficient of determination R2 = 0.97. The two lines are quasi-parallel; each |φ| value is shifted of a quantity which is about four times the correspondent |T | value. The main feature is that the MT technique is capable of detecting thin defects deeper (lower s/p values) than PT does. However, for both techniques, as p increases the contrast worsens because of noise effects and thermal diffusion within the material; systems of high sensitivity and low noise are necessary. 4.4. Results by Focal Plane Array (FPA) Camera As anticipated in section 3, some tests are performed by considering the Cedip JADE MWIR focal plane array camera. A phase image of the specimen Carb B2 is shown in Fig. 15 together with a profile along its length; note that the thickness increases going from right to left (Fig. 2). Two observations can be made: • the phase angle increases with increasing the thickness; • local discontinuities in the phase angle distribution indicate the presence of material inhomogeneities (defects).

Fig. 13. |φ| against s/p for carbon/epoxy.

As an important feature, it has to be pointed out that, as s/p decreases below the value of 0.1, the contrast strongly decreases becoming of the same order of the instrument noise; thus, defects in specimens like the Carb B2 cannot be evidenced by a single detector as the Agema 900. Instead, the focal plane array system is able to distinguish between sound material and very small anomalies.


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Meola, Carlomagno, and Giorleo phy. However, more tests are necessary including a larger variation of basic material and types of defects. REFERENCES

Fig. 15. Results from Cedip camera on specimen Carb-B2; a) phase image; b) phase angle profile.

5. CONCLUSIONS In general, infrared thermography, in the two modes PT and MT, is suitable for nondestructive evaluation of composites, which include either carbon or glass fibers. From the analysis of results the following observations can be made: • carbon fibers, due to their higher thermal properties (conductivity and diffusivity), permit the inspection of deep material layers; • phase images allow for a better deftects detection; • generally, the contrast decreases with decreasing the defect size and with increasing the depth; • a crucial point in defect detectability is the thickness; at a fixed depth, a large but very thin defect (like a disbonding) may be more difficult to discover than a small but thick one. However, the possibility to discern very thin and deep defects depends on the sensitivity of the thermographic system. The old interlaced systems suffer from noise dampening; the focal plane array technology of modern systems solved this problem allowing for images of high definition and therefore for detection of very small imperfections. The purpose of this investigation was mainly to focus attention on geometrical parameters which limit defects detection with infrared thermogra-

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