Evaluating quality of adhesive joints in glass fiber plastic piping by using active thermal NDT

M.Grossoa, C.A.Marinhob, D.A.Nesterukc, J.M.A.Rebellod, S.D.Soaresb, V.P.Vavilov*c

a LNCD/COPPE/UFRJ, CEP 21941-972, Rio de Janeiro, Brazil; bPetrobras Research Center

(CENPES); cTomsk Polytechnic University, Russia, 634030 Tomsk, Lenin Av.; dFederal University of Rio de Janeiro Department of Metallurgical and Materials Engineering - COPPE/UFRJ P.O. Box

68505 CEP 21941-972, Rio de Janeiro, Brazil

ABSTRACT

GRP-type composites (Glass-fibre Reinforced Plastics) have been continuously employed in the oil industry in recent years, often on platforms, especially in pipes for water or oil under moderate temperatures. In this case, the pipes are usually connected through adhesive joints and, consequently, the detection of defects in these joints, as areas without adhesive or adhesive failure (disbonding), gains great importance. One-sided inspection on the joint surface (front side) is a challenging task because the material thickness easily exceeds 10 mm that is far beyond the limits of the capacity of thermography applied to GRP inspection, as confirmed by the experience. Detection limits have been evaluated both theoretically and experimentally as a function of outer wall thickness and defect lateral size. The 3D modeling was accomplished by using the ThermoCalc-6L software. The experimental unit consisted of a FLIR SC640 and NEC TH-9100 IR imagers and some home-made heaters with the power from 1,5 to 30 kW. The results obtained by applying pulsed heating have demonstrated that the inspection efficiency is strongly dependent on the outer wall thickness with a value of about 8 mm being a detection limit.

Keywords: thermal nondestructive testing, composite, data processing

1. INTRODUCTION PETROBRAS, the Brazilian Energy Company, counts on a large number of pipes made of Glass-fibre Reinforced Plastics (GRP) installed at its facilities and has taken efforts to seek the best practices in service inspection of joints in GRP lines. There are three main types of mechanical joints available for GRP piping: concentric adhesive-bonded joints, laminated joints and threaded joints with the last design being only applied to onshore installations. Among them, concentric adhesive-bonded joints and laminated joints, both adhesive-based, represent the most used configuration in platforms and, consequently, the detection of defects in these joints acquires great importance. One of the nondestructive testing (NDT) methods implemented in a partnership with the Federal University of Rio de Janeiro is active thermography (Pulse Thermography and Pulsed Phased Thermography). Basically, Pulse Thermography (PT) involves the brief heating of samples under test and then the recording of temperature decay curves. Pulsed Phase Thermography was introduced to make a frequency analysis from measurements obtained by means of PT [1]. In the test set up, both the camera and heat source are pointing to the outer surface of the joint. If the outer thickness is under 8 mm – 9 mm, defects, such as a lack of adhesive (considering a rectangular area of 10x10 mm or bigger), can be detected, but, with a thicker material, the tests fail, that is expected according to literature [2-5]. In order to evaluate if an outer thickness about 9mm specifies a technical limit to active thermography, a 3D modeling was accomplished by using the ThermoCalc-6L software. This study was performed at Tomsk Polytechnic University.

* vavilov@tpu.ru; phone 7 913 821 9749; fax 7 3822 41; www.innovation.ru

Thermosense: Thermal Infrared Applications XXXV, edited by Gregory R. Stockton, Fred P. Colbert, Proc. of SPIE Vol. 8705, 87050T · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2016762

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2. TEST OBJECT DESCRIPTION Were prepared some GRP samples, joined through concentric adhesive-bonded joints, on which some defects (lack of adhesion and lack of adhesive) were introduced in the adhesive area. Figure 1a shows two pipes (5 mm thick) joined with a collar. An adhesive by the thickness of about 1 mm was applied both over the collar’s inner surface and over a prepared surface of the pipes, which were introduced into the collar (collar thickness 12 mm). The two pipes are assembled inside the collar similarly to a butt welded joint.

a) b)

Figure 1. Full-scale standard samples of GRP tubes joined with a CRP collar (a) and defect (lack of adhesive) scheme (b)

Two other fragments also used in experiments are shown in Fig. 2. These test pieces also contained defects in the adhesive area.

a) b) c)

Figure 2. Fragments of standard collar-tube samples with a disbond and a lack of adhesive

outer view (a), inner view (b) and defect scheme (c)

3. EXPERIMENTAL TESTS AND RESULTS Using a FLIR SC640 IR imager and an external heat sources (two 1.5 kW halogen lamps or a heat blower), each sample was positioned 500 mm away from the camera-heaters set. The trials were performed by applying different heating cycles, according to two configurations: 1) the camera and the heating set pointing onto the outer surface of the collar, and 2) pointing onto the inner surface of the pipes. IR image sequences were taken during the sample cooling and consisted of 1086 images further processed with the ThermoFit Pro software. Figures 3 and 4 show some results. These tests were performed at Federal University of Rio de Janeiro. On the inner surface, it was possible to detect the defects which represented a lack of adhesive (Fig. 1) in images obtained by applying the PT approach (Fig. 3). The original image (Fig. 3a), without any treatment, showed clearly the

100 mm 15 mm

15 mm

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s

defective area. The PCA image (Fig. 3b) has been used to produce a binary defect map (Fig. 3c) that is preferred by end-users instead of spotty color or grey-scale images.

a) b) c) Figure 3. One-sided thermal NDT inspection of the full-scale collar-tube sample on the inner GRP surface (see Fig. 2):

source image (a), PCA image (b), binary defect map (PCA-based) (c). When the inspection was done on the outer surface, the results obtained have been discouraging. The well-visible image texture was seen shortly after the end of heating (Fig. 4b) probably representing surface features (clutter) rather than defect 'footprints' which were expected at much longer times. However, at the end of the thermal event, the sample temperature decayed closely to the ambient temperature by revealing noise rather than expected defect signals (Fig. 3c).

a) b) c)

Figure 4. One-sided thermal NDT inspection of the full-scale collar-tube sample on the outer CRP surface (see Fig. 1): source image (a), shortly after heating (100 images processed, PCA image) (b), end of the thermal event (100 images processed, PCA image) (c).

3.1 Pulsed IR thermography

The tests have been conducted on a computerized IR thermographic system at Tomsk Polytechnic University. The heating was done by using a 30 kW heater including 6 halogen lamps, 5 kW each. At a 0.5 m distance from the heater, the absorbed heat power was about 14.7 kW/m2 to provide the excess temperature of about T=56oC at the end of a 10 s-long heat pulse. The maximum allowed acquisition rate was 60 Hz but in the experiments described it was only 1 Hz in the accordance with the recommendations presented in Table 1 (see below). The results have been processed by using the ThermoFit Pro software with the emphasis being made on the principal component analysis (PCA) as one of the most promising processing techniques in thermal NDT [6].

Defects

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.

r

In the inspection on the inner GRP surface, source images, even taken at an optimum observation time (Fig. 5a), exhibit mostly surface roughness of the composite, but, after having applied the PCA technique, some imperfections in the adhesive layer have become clearly seen. However, they can be hardly associated with the programmed defects in these samples. On the outer GRP surface, source images (Fig. 6a) showed only some regular marks made by a pencil. PCA images revealed some anomalies which slightly correlated with the defect 'footprints' in Fig. 6b by taking into account that images in Fig. 6 are mirror ones in regard to Fig. 5. However, the detection reliability seemed to be vague in this case.

a) b) Figure 5. One-sided thermal NDT inspection of the collar-tube fragments on the inner GRP surface (see Fig. 2b):

source images at a 'best' observation time (a), PCA images (b).

a) b)

Figure 6. One-sided thermal NDT inspection of the collar-tube fragments on the outer CRP surface (see Fig. 2a): source images at a 'best' observation time (a), PCA images (b).

3.2 X ray tomography

Samples #1 and #2 have been tested on the X ray computer tomograph TOLMI-150-10 at Tomsk Polytechnic University (X ray radiation energy 120 kEv, minimum in-depth step 100 μm). Some tomographic images shown in Fig. 7 prove the presence of programmed defects in the adhesive layer which are seen as dark areas, both localized and extended.

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a) b)

Figure 7. X ray tomographic images: sample #1 (a), sample #2 (b).

4. MODELING RESULTS Our experience in the numerical modeling of TNDT problems has shown that, in many practical cases, a cylindrical geometry can be safely substituted with Cartesian without a noticeable loss in temperature amplitudes. The samples shown in Figs. 1,2 have been analyzed in the planar (Cartesian) geometry by using the ThermoCalc-6L software [7]. A sample scheme is shown in Fig. 8. The analyzed model included two adhesively-linked plates made of GRP. The model input parameters used in calculations are as follows:

• outer GRP plate thicknesses: 5, 8 and 12 mm; inner GRP plate thickness: 5 mm; • adhesive layer thickness: 1 mm; • defect (air-filled) lateral size: 5x5 mm 15x15 mm and 30x30 mm;

Voids

Extended defects

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• defect (air-filled) thicknesses: 0.3; 0.6 and 1 mm (3 defects in each calculation session); • GRP thermal conductivity: K=0.33 W/(m.K); • product of GRP heat capacity by density: Cρ==2.54.106 J/(m3.K); • adhesive thermal conductivity: K=0.66 W/(m.K) (50% epoxy glue and 50% aluminum powder); • product of adhesive heat capacity by density: Cρ=4.89.106 J/(m3.K); • defect (air in thin gaps) thermal conductivity: K=0.07 W/(m.K); • product of air heat capacity by density: Cρ=1.21.103 J/(m3.K).

The sample is uniformly heated with a heat flux Q=10000 W/m2. Such value can be achieved with powerful air blowers (fans) or middle-power halogen lamps. It is important, that differential temperature signals over defects ΔT are linearly proportional to Q. Therefore, if we have ΔT1 with Q1, the ΔT2 with Q2 will be ΔT2= ΔT1 (Q2/Q1). This comment seems to be important because it allows avoiding calculations for many values of Q. The stimulation time was 10 s in all cases to provide the excess temperature T~37oC at the end of heating. It is important that, with longer heating, ΔT grows up but the so-called running contrast C=ΔT/T (see more about this parameter below) typically decreases. It will be shown below that, for better defect detection, it is desirable to ensure higher C rather than ΔT. Also, as it has been shown elsewhere, heating duration does not affect much inspection efficiency in the case of thicker composites. Computation parameters are: numerical grid size 90x160x180; computation time step 1 s; total process time 600 s including 10 s heating. Computation results represented sets of up to 600 images evolving in time. An example of the image at the optimum observation time is shown in Fig. 4.

Figure 8. Sample cross-section (left) and top view (right) Most simulation results have been obtained in the case of one-sided inspection on the outer collar surface, as it is expected to be done in practice. In order to supply comparative results, some data has been also obtained in the case of one-sided inspection on the inner tube surface.

Q

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Figure 9. Temperature distribution at optimum observation time 116 s for a 0.3 mm-thick defect (one-sided inspection on the outer GRP collar surface; collar thickness 12 mm; heating power and duration: 10 kW/m2 and 10 s; air-filled 15x15 mm defects in the

adhesive) Two temperature parameters introduced above have been chosen to identify the inspection efficiency: the differential temperature signal ΔT and the running temperature contrast C. As it is well known [6], these parameters evolve in time and reach their maximum values at ( )m mTτ Δ and ( )m mCτ respectively (see examples of the corresponding evolutions in Fig. 10). In total, 36 test cases have been modeled, of which 27 cases have been related to a practically interesting situation of one-sided inspection on the outer GRP collar surface (Table 1) and 9 cases illustrate a speculative case of one-sided inspection on the inner GRP tube surface (Table 2).

TΔ vs. τ /C T T= Δ vs. τ

Figure 10. Evolution of differential temperature signals and running contrasts in time (one-sided inspection on the outer GRP collar surface; collar thickness 12 mm; heating power and duration: 10 kW/m2 and 10 s; air-filled 15x15 mm defects in adhesive)

mTΔ

( )m mTτ Δ

mC

( )m mCτ

Defect thickness d=1 mm

d=0.6 mm

d=0.3 mm

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Table 1. Optimal detection parameters (one-sided inspection on the outer GRP surface; heating power and duration: 10 kW/m2 and 10 s; air-filled defects in adhesive; CL -GRP thickness, d -defect thickness; h -defect lateral size; mTΔ -maximum differential signal;

( )m mTτ Δ -optimum observation time for mTΔ ; mC -maximum running contrast; ( )m mCτ -optimum observation time for mC )

CL , mm d , mm h x h , mm mTΔ , oC ( )m mTτ Δ , s mC ( )m mCτ , s 5 0.3 5x5 0.10 88 0.020 122 15x15 0.43 111 0.1 141 30x30 0.51 126 0.13 167 0.6 5x5 0.15 91 0.032 117 15x15 0.71 118 0.17 151 30x30 0.88 139 0.24 183 1.0 5x5 0.20 95 0.042 122 15x15 0.96 125 0.24 160 30x30 1.24 151 0.36 200 8 0.3 5x5 0.016 197 0.0052 247 15x15 0.11 224 0.038 271 30x30 0.17 262 0.066 319 0.6 5x5 0.026 203 0.0086 254 15x15 0.18 235 0.066 286 30x30 0.31 279 0.12 340 1.0 5x5 0.035 210 0.012 265 15x15 0.25 245 0.095 299 30x30 0.44 295 0.19 361

12 0.3 5x5 0.0030 317 0.0015 474 15x15 0.024 405 0.012 468 30x30 0.053 469 0.029 546 0.6 5x5 0.0050 411 0.0026 505 15x15 0.043 430 0.023 507 30x30 0.097 499 0.056 589 1.0 5x5 0.0071 432 0.0038 538 15x15 0.062 448 0.033 531 30x30 0.15 525 0.087 623

Table 2. Optimal detection parameters (one-sided inspection on the inner GRP tube surface; heating power and duration: 10 kW/m2

and 10 s; air-filled defects in adhesive; other specifications same as in Table 1)

GL , mm d , mm h x h , mm mTΔ , oC ( )m mTτ Δ , s mC ( )m mCτ , s

5 0.3 5x5 0.10 88 0.020 113 15x15 0.43 111 0.10 144 30x30 0.52 128 0.14 176 0.6 5x5 0.15 91 0.032 118 15x15 0.71 119 0.18 154 30x30 0.89 140 0.25 195 1.0 5x5 0.20 95 0.042 123 15x15 0.96 125 0.25 164 30x30 1.25 153 0.38 215

Analysis of basic thermal NDT features presented by the data in Tables 1 and 2 is beyond of the scope of this paper, see [2-8]. In short, it can be stated that: 1) temperature signals caused by defects decay with growing defect depth and diminishing thickness and lateral size; 2) maximal temperature contrasts appear later than maximal differential signals; 3) optimum observation times, both ( )m mTτ Δ and ( )m mCτ , are more resistant against variations in defect thickness

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and lateral size to compare to amplitude parameters; therefore, they are mostly used for the characterization of defect depth; 4) in the case of Q=10 kW/m2, CL =12 mm, d=0.3 mm, hxh=5x5 mm (Table 1), the maximum temperature signal

mTΔ =18 mK that is under temperature sensitivity of most IR imagers; by five-fold increasing the heating power, i.e. up to 50 kW/m2, it is possible to ensure the very reasonable value of mTΔ =90 mK, however, the running contrast

nC =0.0037 will remain the same and much lower than a typical level of noise (see below).

5. THEORETICAL EVALUATION OF INSPECTION LIMITS 5.1 Detection conditions

Four defect detection conditions in active thermal NDT have been formulated elsewhere [6, 7]. Without going deeper into details which outline requirements to heating power and duration, temperature resolution and acquisition interval of a test equipment, we shall use below the most important detection condition which states that running temperature contrasts over defects must exceed the so-called noise running contrast nC that is a characteristic of a tested material. To determine nC , one should evaluate statistically experimental images by calculating temperature standard deviation

nσ and dividing it by the sample excess temperature: /nnC Tσ= . For example, it has been reported that the so-called

'black' coatings on metals and homogeneous non-metals are characterized by nC ~ 0.02, or 2%. Therefore, it might happen that TΔ over a particular defect may exceed a temperature resolution of a used IR camera (if heating power is high enough) but a value of C over the same defect could be lower than nC for a particular material, hence, such defect will not be detected. The data in Table 1 can be presented graphically, as shown in Fig. 6 for the case of d=1 mm (polynomial approximation has been used to smooth the curves). It will be shown in the next section that, in the inspection of GRP, a limiting value of nC is about 0.08, or 8% (while inspecting collar surface). By assuming nC =8%, as shown in Fig. 6, the detection limits by the collar thickness cL will be determined by the corresponding crossing points. For example, a 15x15x1 mm defect can be detected up to depths about 8.6 mm, while for smaller defects (5x5x1 mm) the detection limit is about 3.8 mm. It is worth noting that thinner defects, as it follows from Table 1, are characterized by lower cL values. The same methodology applied to the data in Table 2 shows that the inspection on the inner GRP surface allows the detection of all defects larger than 15x15x0.3 mm (the limiting value nC =6%, see the next section). In any case, the main conclusion from the analysis above is that a 8 mm-thick GRP collar specifies a critical case for the application of thermal NDT on the outer tube surface. The optimum delay times are within the 250-350 second range depending on defect size.

5.2 Noise estimates

In order to obtain a limiting value of nC , time evolutions of /n Tσ have been studied on the surfaces of standard samples. Since the GRP composites used to manufacture pipes and collars are different, that is visually seen in Figs. 1 and 2, the evolution of nC used to be slightly different on the outer and inner surfaces (see Fig. 7). On the outer GRP surface, nC decays from 0.08 to 0,05, while, in the case of the inner GRP surface, nC decays from 0.06 to 0.03. Generally, these results are in a good accordance with earlier reported data [6-8]. The revealed noise behavior can be explained by lateral heat diffusion which smoothes temperature surface patterns, thus decreasing temperature standard deviation nσ . Since, according to the results in Table 1, 2, the detection of defects should be performed at rather longer times after heating stopped, the estimates of nC should be about 4% and 3% for two types of GRP composite (collar and pipe). Finally, by assuming that for reliable detection one needs a temperature contrast at least twice higher than a noise level, the limiting value of nC is assumed to be 8% (collar) and 6% (pipe).

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0.08

0.05 0.03

7.5 10 12.5 15 17.5 20

0.1

0.2

0.3

0.4

Figure 10. Graphical presentation of the data in Table 1 (defect thickness d=1 mm)

Figure 11. Noise evolution in time (zero time is the heater turn-on time, heating duration 10 s )

6. CONCLUSION The potentials of thermal NDT for the detection of defects between a 12 mm-thick GRP repair collar and a 5 mm-thick GRP pipe have been studied, both theoretically and experimentally. It has been shown that detection limits are determined by a level of noise expressed in terms of running temperature contrast that is the temperature standard deviation normalized by the sample excess temperature. In the case of GRP composites, the limiting noise value is about 8 %. By comparing numerically-calculated temperature signals and experimentally-found noise estimates, it is stated that a collar thickness of 8 mm is the critical value characterizing thermal NDT possibilities in a practically-important one-sided inspection on the outer GRP tube surface. Such limitation of material thickness prevents the use of the thermal NDT technology in real situations.

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SPIE 5073, 363-373 (2003).

C

cL , mm

nC =8%

hxh=30x30 mm

15x15 mm

5x5 mm

Cn Cn GRP (outer surface)

30 200 τ, s 30 200 τ, s

GRP (inner surface)

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