Interfacial Evaluation of Wind Blade Carbon Spar-Cap Depending on Elimination Method of Intermediate Medium

Abstract

An Ultrasonic Test (UT), a type of non-destructive test, is used to inspect the manufacturing integrity of carbon spar-caps of wind blades. When performing a UT, an intermediate medium is used to improve the signal detection ability between the inspection target and the probe. However, if the intermediate-medium residue is not removed, it acts as a contaminant in the interface between the spar-cap and the blade skin. This has a negative effect on the adhesion characteristics. A quantitative method is required for removing the intermediate medium and peel ply after the UT. After the UT, the interfacial characteristics of the spar-cap surface are examined according to the method of removing the intermediate medium in this study. The static contact angle and the work of adhesion (W_a) were measured according to various surface treatment conditions. In addition, shear strength of the Carbon Fiber-Reinforced Plastic (CFRP) spar-cap was evaluated by the lap shear test. An optimized method of peel-ply removal combined with an intermediate medium was found in this study. An optimal guideline for intermediate-medium treatment could be proposed when evaluating the manufacturing integrity of real wind blade spar-caps using UT.

1. Introduction

In order to achieve net zero, many countries are making great efforts to expand renewable energy. In particular, offshore wind power, which can be used for large-scale power plants, is receiving attention. Wind turbines are gradually becoming larger, and the trend is to develop the length of wind blades to increase annual energy production [1,2]. To solve the issue of increasing weight and load due to the enlargement of wind blades, composite materials with excellent mechanical properties such as the strength-to-weight ratio and elastic modulus are being used. The spar-cap is an important structure that directly withstands the load applied to the wind blade. As wind blades become larger, the spar-cap material, Glass Fiber-Reinforced Plastic (GFRP), is gradually being replaced with Carbon Fiber-Reinforced Plastic (CFRP) [3,4]. Since the manufacturing method of large wind blades is complex, the structural integrity of the spar-cap and the interfacial adhesion between the components are very important to acquire excellent quality.

Micro-defects such as non-impregnation of resin, debonding, and wrinkles can occur during the spar-cap manufacturing process. This can cause damage and fracture, reduce the strength of the spar-cap, and finally induce catastrophic failure of the blade [5,6,7]. Complex bonding methods are used in the wind blade manufacturing process [8,9]. In order to secure the structural integrity of the spar-cap, non-destructive evaluation techniques such as UT are widely used for quality control of the blade by detecting microscopic defects [10,11]. The UT is performed after applying the intermediate medium on the peel ply. In the case of UT, intermediate media such as silicone, glycerol, and liquid soap water are used at the interface to improve the signal sensitivity between the probe sensor and the specimen surface [12]. After the UT is completed, residues of the intermediate medium remaining on the composite surface can act as contaminants at the interface between the adhesive and the surface, potentially reducing the interfacial strength between the blade skin and the spar-cap. This ultimately has a negative effect on the quality of the wind blade. Kinloch verified that the third material existing at the interface has a negative effect on the bonding properties using the thermodynamic W_a [13]. Kraft et al. experimentally demonstrated that various types of surface contamination on CFRP surfaces lead to a pronounced reduction in adhesive strength [14]. Ames and Chelli reported that surface contamination alters the interfacial energy and thereby decreases adhesion [15]. In addition, Hoffmann et al. quantified the changes in adhesive strength caused by different contaminants and concluded that prebond contamination can significantly degrade the adhesive strength [16]. The correlation between contaminants on polymeric composite surfaces and interfacial strength has been investigated in various studies. Borges et al. comprehensively reviewed how moisture ingress and various contaminants alter the adhesive interface and reported that surface contaminants remaining at the interface can locally reduce adhesion, potentially leading to joint failure [17].

Because contaminants such as intermediate media at composite interfaces significantly reduce adhesive strength, numerous studies have been conducted to develop methods for their removal. Gaviolli et al. demonstrated that different contaminants and cleansing agents influence both the adhesive strength and conversion at the composite–adhesive interface, highlighting that inadequate removal of contaminants leads to a significant deterioration in interfacial properties [18]. Mandolfino et al. showed that nanosecond-pulsed Ytterbium-doped fiber laser treatment of CFRP surfaces can be optimized to remove surface contaminants and enhance the shear strength of adhesive-bonded joints, whereas excessive ablation thickness degrades joint performance [19]. In addition, surface treatment of CFRP composites using a picosecond ultraviolet laser has been investigated as an effective means of eliminating adhesion-impairing contaminants while avoiding damage to the fibers [20]. However, for the intermediate-medium residues remaining after the UT process, mechanical removal is difficult; although organic solvents can be used to remove these residues, solvent-based cleaning generally exhibits low efficiency and may alter the surface properties of the material, making it difficult to implement in actual blade manufacturing processes.

As an alternative, peel plies are widely used in industry to protect composite surfaces from contamination and to enable their long-term storage, and various studies on peel plies are currently being carried out. Kanerva and Saarela analyzed the interfacial properties produced by different peel-ply surface treatments and demonstrated their potential for adhesive bonding [21]. Kuppusamy et al. evaluated the effectiveness of polyester and Diatex peel plies in controlling mold-release-agent contamination on carbon fiber laminates and showed that Diatex peel ply more effectively suppresses the transfer of release residues to the laminate surface than the other conditions [22]. Buchmann et al. quantitatively analyzed the removal method of peel plies from CFRP laminates and showed that peel angle, peel-ply orientation, and peel-ply material strongly influence the required peel force and the occurrence of fiber breakouts on the composite surface [23]. However, the peel-ply removal process and its effects when contaminants such as intermediate-medium residues are present on peel-ply-treated composite surfaces have not yet been quantitatively studied.

In this study, the method of removing intermediate-medium residues of peel ply on CFRP was quantitatively studied. The static contact angle and single-lap shear tests were performed according to the method of removing the intermediate medium applied to CFRP to evaluate the interfacial characteristics and adhesive strength. In addition, the penetrability of the intermediate medium was verified through surface observation and chemical composition analysis of the peel ply after removing the intermediate medium. Finally, the intermediate-medium and peel-ply removal method applicable to the quality inspection of real large-scale wind blade spar-caps using ultrasonic inspection was provided.

2. Experimental Section

2.1. Material and Sample Preparation

The CFRP and peel ply used in this study are materials used in wind blade spar-caps and were provided by a wind blade manufacturer (Human Composite Co., Ltd., Gunsan, Republic of Korea). The 1.6 mm thick CFRP plate used an adhesive mixed with bisphenol A-based epoxy resin (KFR-730FL, Kukdo, Busan, Republic of Korea) and amine-based hardener (KFH-740FL, Kukdo, Republic of Korea) at a ratio of 100:45, and the CFRP surface was protected with poly (amide)-based peel ply (PA66 Red Stripe, ACG, TongXiang, China). The intermediate medium was sulfate-based liquid soap (Trio, Aekyung, Seoul, Republic of Korea), which was diluted with water at a ratio of 1:10.

In order to confirm the adhesion characteristics according to the peel-ply removal method after applying the intermediate medium, static contact angle and single-lap shear tests were performed, and the schematic diagram for producing CFRP specimens for this is shown in Figure 1. Case 1 was produced to confirm the normal status of static contact angle and single-lap shear strength with CFRP with peel ply separated without applying the intermediate medium. Case 2 and Case 3 were produced to confirm the change in adhesive properties depending on the method of removing the peel ply according to the condition of the intermediate medium used in the UT. Case 2 was CFRP with peel ply attached; the intermediate medium was applied to the surface of the peel ply and dried, and the peel ply was removed. In Case 3, the intermediate medium was applied to CFRP with peel ply attached, and then the peel ply was removed in a wet state and dried. To evaluate the effect of the intermediate medium on contact angle and adhesive strength, the peel ply was removed, the medium was applied to the CFRP surface and dried, and the Case 4 specimen was fabricated. The intermediate medium was applied with a sprayer for 20 min and dried at 25 °C for 24 h.

Figure 2 shows the type of specimen for checking intermediate-medium permeability. For the primary peel ply (Case 5) that was not used in the curing process, the peel ply (Case 6) attached to CFRP after the curing process was completed, and the peel ply (Case 7) separated from CFRP after the curing process was completed, the shape of the peel ply was observed, and chemical composition and intermediate-medium penetration experiments were performed.

2.2. Methodologies

Methods for evaluating interfacial and adhesive properties include static contact angle measurement and a lap shear test, which are mechanical evaluations. Static contact angle measurements are widely used to investigate the surface properties of materials. Contact angle and wettability are key factors in evaluating the interfacial properties of composite materials. W_a, calculated from the contact angle, is used as a quantitative indicator to evaluate the interface properties [24,25]. The lap shear test is a method of measuring the shear strength between an adhesive and a composite. Forces and displacements of composites can be measured directly using a single-lap shear test [26]. This is widely used for the representative evaluation of adhesive properties in real industrial fields [27].

2.2.1. Surface Energy Measurement of CFRP

A static contact angle test was performed on a CFRP specimen produced by the peel-ply removal method (Figure 1) after application of the intermediate medium, cut into 10 mm × 10 mm sizes, and the surface energy of CFRP was measured using water, formamide, ethylene glycol, and diiodomethane solvents [28,29,30]. The CFRP specimen was placed on a horizontal stage, and the four solvent droplets were dropped on the CFRP surface (Case 1, Case 2, Case 3, Case 4). The droplet diameter was set at 1 mm. If, for each of the four liquid solvents, the droplet was positioned on the CFRP, adhesion between the liquid solvent and solid CFRP surface occurred and its static contact angle depended on the W_a. Four specimens were tested for each case. The relationship between W_a and surface energy can be expressed by the Young–Dupre equation [31]:

$$W_a={\gamma }_l-{\gamma }_{sl}+{\gamma }_s$$

(1)

where ${\gamma }_l$ is the surface tension of the liquid, ${\gamma }_{sl}$ is the interfacial energy between the liquid and the solid, and ${\gamma }_s$ is the surface energy of the solid.

The surface energy of polar and nonpolar elements can be expressed using the Owens–Wendt equation [32,33,34]:

$$W_a={\gamma }_l(1+cos\theta )={2({\gamma }_s^d{\gamma }_l^d)}^{1/2}+{2({\gamma }_s^p{\gamma }_l^p)}^{1/2}$$

(2)

where ${\gamma }^d$ represents the dispersive component of surface energy, ${\gamma }^p$ represents the polar component of surface energy, and $\theta$ represents the static contact angle.

To evaluate the influence of the intermediate medium, which has a basic component, the Oss–Chaudhury–Good (vOCG) equation expressed in terms of acidic and basic components is given as follows [35]:

$$\gamma ={\gamma }^{LW}+{\gamma }^{AB},{\gamma }^{AB}=2{({\gamma }^{+}{\gamma }^{-})}^{1/2}$$

(3)

$$W_a={\gamma }_l(1+cos\theta )={2({\gamma }_s^{LW}{\gamma }_l^{LW})}^{1/2}+{2({\gamma }_s^{+}{\gamma }_l^{-})}^{1/2}+{2({\gamma }_s^{-}{\gamma }_l^{+})}^{1/2}$$

(4)

where $\gamma$ is the total surface energy, and ${\gamma }^{LW}$ and ${\gamma }^{AB}$ are the Lifshitz–van der waals and acid–base components, respectively. ${\gamma }^{+}$ and ${\gamma }^{-}$ are the Lewis acidic and Lewis basic components, repectively.

2.2.2. Single-Lap Shear Test for CFRP

To check the adhesive strength according to the peel-ply removal method (Figure 1) after applying the intermediate medium, a single-lap shear test was performed based on the ASTM D3164 standard. A single-lap shear specimen was manufactured in compliance with the shape and dimensions specified in ASTM D3164 [36] and ASTM D1002 [37]. Seven specimens were used for the single-lap shear test under each condition. The adhesive part of the single-lap shear specimen was coated with an adhesive area of 25.4 mm × 12.7 mm and a thickness of 0.5 mm of epoxy adhesive and cured for 10 h at 80 °C. For the single-lap shear test, a universal testing machine (3382, Instron, Norwood, MA, USA) was used, and the test speed was set at 1.3 mm/min. Shear strength was calculated using the following equation [38]:

$${\tau }_a={\displaystyle \frac{F}{w\times l_{lap}}}$$

(5)

where $F$ is the shear force, $w$ is the adhesive width, and $l_{lap}$ is the adhesive length.

2.2.3. Observation of Peel Ply and Spreading Test Using Intermediate Medium

To verify the permeability of the intermediate medium, a drop of the intermediate medium was placed on the peel ply, as shown in Figure 2. The initial diameter of the intermediate medium drop was set to 1 mm using a syringe with a syringe pump (KDS100, KD Scientific Inc., Holliston, MA, USA), and this procedure was repeated five times. Thereafter, the spread and degree of impregnation of the intermediate medium on the peel ply were confirmed using a digital microscope (AM4815TL, Dino-Lite, New Taipei City, Taiwan). To analyze the cause of penetration of the intermediate medium, the peel-ply surfaces of Case 5, Case 6, and Case 7 were examined at 40× magnification using a metallurgical microscope (BX53M, Olympus, Tokyo, Japan). The chemical composition of the peel ply removed after CFRP molding was confirmed using a Fourier-transform infrared spectroscope (FT-IR; 6300FV, Jasco, Easton, MD, USA) equipped with an Attenuated Total Reflection (ATR) cell.

3. Results and Discussion

3.1. The Analysis of the Static Contact Angle and Surface Energy for the CFRP

The static contact angle was measured to obtain the polar surface energy and nonpolar surface energy of CFRP. Figure 3 shows photographs of water, formamide, ethylene glycol, and diiodomethane droplets on a CFRP surface, in which the static contact angles are recorded. The static contact angle of hydrophilic distilled water was 66° and 61° and the static contact angle of hydrophobic diiodomethane was 14° and 13° in Case 1 and Case 2. CFRP specimens protected from the intermediate medium exhibited a higher contact angle with water and a lower contact angle with diiodomethane. The similar static contact angles measured for Cases 1 and 2 support the validity of the peel-ply removal method used in Case 2 and indicate that the intermediate medium does not significantly affect the CFRP surface. The static contact angle of hydrophilic distilled water was 23° and 26° and the static contact angle of hydrophobic diiodomethane was 73° and 81° in Case 3 and Case 4. CFRP exposed to the intermediate medium has a low contact angle with water and a high contact angle with diiodomethane.

The surface energies of the polar and dispersive components obtained using the Owens–Wendt model are shown in

Table 1. Surface energy components and work of adhesion using Owens–Wendt model (mJ/m²).

Condition	${\mathit{\gamma }}_{\mathit{d}}$	${\mathit{\gamma }}_{\mathit{p}}$	${\mathit{\gamma }}_{\mathit{s}}$	${\mathit{W}}_{\mathit{a}}$
Case 1	41.3	8.4	49.6	85.6
Case 2	38.2	11.3	49.5	86.3
Case 3	8.9	60.1	69.0	84.0
Case 4	5.0	63.4	68.3	77.6
Epoxy adhesive [39]	26.1	11.9	38.0

and Figure 4. CFRP exposed to an intermediate medium has a higher polar surface energy, and CFRP protected from an intermediate medium has a higher dispersive surface energy. In Case 1 (the intermediate medium was not applied), the dispersive surface energy was 41.3 mJ/m² and the polar surface energy was 8.4 mJ/m². In Case 4 (the intermediate medium was applied), the dispersive surface energy was 5.0 mJ/m², which was lower than Case 1 CFRP, and the polar surface energy was 63.4 mJ/m², which was higher than Case 1 CFRP. In Case 2, the dispersive surface energy was 38.2 mJ/m² and the polar surface energy was 11.3 mJ/m². In Case 3, the dispersive surface energy was 8.9 mJ/m² and the polar surface energy was 60.1 mJ/m².

The surface energies of the Lifshitz–van der Waals and acid–base components obtained using the vOCG model are shown in

Table 2. Surface energy components and work of adhesion using vOCG model (mJ/m²).

Condition	${\mathit{\gamma }}_{\mathit{S}}^{\mathit{L}\mathit{W}}$	${\mathit{\gamma }}_{\mathit{S}}^{\mathit{A}\mathit{B}}$	${\mathit{\gamma }}_{\mathit{S}}^{+}$	${\mathit{\gamma }}_{\mathit{S}}^{-}$	${\mathit{\gamma }}_{\mathit{s}}$	${\mathit{W}}_{\mathit{a}}$
Case 1	49.3	3.9	0.4	9.1	53.2	90.7
Case 2	49.5	3.4	0.2	14.1	52.9	91.2
Case 3	21.2	30.5	4.0	57.8	51.7	81.4
Case 4	17.0	26.3	2.5	69.8	43.3	74.9
Epoxy adhesive [39]	32.6	6.8	1.1	10.3	39.4

and Figure 5. Based on the vOCG analysis, the Lifshitz–van der Waals and acid–base components of the surface energy for Case 1 were 49.3 and 3.9 mJ/m², respectively, while those for Case 2 were 49.5 and 3.4 mJ/m². These values are identical to the results obtained from the Owens–Wendt model for Cases 1 and 2. The Lewis acid components for Cases 1 and 2 were 0.4 and 0.2 mJ/m², and the corresponding Lewis base components were 9.1 and 14.1 mJ/m², indicating that both surfaces are dominated by the Lifshitz–van der Waals component and exhibit only small acid–base contributions. For Case 3, the Lifshitz–van der Waals and acid–base components of the surface energy were 21.2 and 30.5 mJ/m², respectively, whereas for Case 4, they were 17.0 and 26.3 mJ/m². Consistent with the Owens–Wendt model, the acid–base component was slightly higher than the Lifshitz–van der Waals component in both cases. The Lewis acid components for Cases 3 and 4 were 4.0 and 2.5 mJ/m², and the corresponding Lewis base components were 57.8 and 69.8 mJ/m², indicating that the CFRP surfaces are strongly dominated by basic characteristics. These results suggest that residues of the intermediate medium remain on the CFRP surface in Case 3. The overall trends in total surface energy obtained from the Owens–Wendt and vOCG models were found to differ. This discrepancy arises from differences in the model parameters [40] and, in particular, from the fact that the vOCG approach explicitly accounts for the acidic and basic components of the polar contribution. Therefore, the vOCG model could be more suitable for research on surface energy caused by surface contamination.

The intermediate medium is the surfactant composed of both polar elements in the form of hydroxyl groups and dispersive elements in the form of hydrocarbon chains. CFRP contains relatively hydrophobic carbon fiber and epoxy resin. When neat CFRP is treated with soapy water, the CFRP surface comes into contact with the hydrophobic segments of the surfactant, and the hydrophilic groups are oriented toward the outermost region so that the surface ultimately becomes hydrophilic [41]. The dispersive and polar surface energy components of the Case 2 CFRP were similar to those of the Case 1 CFRP. This suggests that, when the peel ply is removed using the same procedure as in Case 2, the intermediate medium does not reach the CFRP surface. The dispersive and polar surface energy components of the Case 3 CFRP were similar to those of the Case 4 CFRP. This indicates that, when the peel ply is removed using the same procedure as in Case 3, residues of the intermediate medium remain on the CFRP surface.

3.2. Work of Adhesion and Lap Shear Test for the CFRP

Figure 6 shows the lap shear strength and W_a used to evaluate the interfacial adhesion between the epoxy adhesive and the CFRP. The lap shear strength and W_a exhibited similar trends for all four cases. The shear strengths of Case 1 and Case 2 were 25.5 MPa and 26.2 MPa, respectively, whereas those of Case 3 and Case 4 were 23.6 MPa and 15.9 MPa, corresponding to decreases of approximately 7% and 38% relative to Case 1.

The W_a values obtained from the Owens–Wendt model were 85.6, 86.3, 84.0, and 77.6 mJ/m² for Cases 1–4, respectively, whereas those calculated using the vOCG model were 90.7, 91.2, 81.6, and 74.9 mJ/m². For Cases 1 and 2, both models yielded similar W_a values. In contrast, compared with Case 1, the W_a of Case 3 decreased by approximately 2% (Owens–Wendt) and 10% (vOCG), and that of Case 4 decreased by about 9% (Owens–Wendt) and 17% (vOCG). These results suggest that the presence of the intermediate medium is the primary factor responsible for the reduction in both shear strength and work of adhesion.

3.3. Observation of Surface and Analysis of Chemical Composition for Peel Ply

Figure 7 shows enlarged photos of the surface of the peel ply. In Case 5, the shape of the weave was observed, whereas the shape of the weave for Case 6 peel ply was not observed. In Case 7, the micro-cracks were observed at regular intervals. It is thought that the weak parts of the peel ply after the curing process were damaged upon detaching it.

Figure 8 shows the FT-IR spectra of the peel ply and the polymer resin. In the case of the untreated peel ply, a -NH- peak at 3298 cm⁻¹, -CH₂- peak at 2931 cm⁻¹, and C=O peak at 1633 cm⁻¹ were observed. Based on these peaks, the chemical composition of the untreated peel ply was identified as being similar to that of nylon 66 [42]. In the case of the polymer resin collected from the cured carbon prepreg, an -OH peak at 3390 cm⁻¹, -CH₂- peak at 2924 cm⁻¹, and C-O peak at 1235 cm⁻¹ were observed. As a result of analyzing the peak, the chemical composition of the polymer resin was similar to that of bisphenol A-based epoxy resin [43]. In addition, the peak of the peel ply after the curing process was analyzed and the peak position of the existing untreated peel ply was completely changed to the peak position of the polymer resin. When the epoxy resin contained in the carbon prepreg is heated, it will temporarily become low-viscosity fluid. The low-viscosity epoxy fluid will penetrate the peel ply, and then it will form an epoxy layer with peel ply. The peak of the existing untreated peel ply could not appear since the upper layer of the peel ply was covered with an epoxy resin layer of a certain thickness.

3.4. The Intermediate-Medium Droplet Penetration Test for the Peel Ply

Figure 9 shows photographs of penetration of the intermediate-medium droplet through the peel ply. Upon dropping the intermediate-medium droplet on the peel ply, the intermediate-medium droplet on the peel ply was first observed, and then a trace of spreading of the intermediate medium in the peel ply was observed.

The change in droplet diameter on the peel ply and spreading diameter into the peel ply of the intermediate medium over time is illustrated in Figure 10. In Case 5, the droplet diameter increased dramatically from 1.5 mm to 2.6 mm in 1 s, and the droplet disappeared after about 1 s. The spreading diameter increased dramatically from 1.5 mm to 4.6 mm in 2 s. The spreading diameter increased slowly and was maintained after 10 s at 5.1 mm. In Case 6, the droplet diameter increased slowly from 1.5 mm to 2.9 mm at 30 s, whereas the phenomenon of penetration of the intermediate medium in the peel ply could not be observed. The spreading pattern of the Case 7 peel ply was similar to Case 5 peel ply. The spreading rate of the intermediate medium into Case 7 peel ply was lower at 0.7 mm/s after 1 s than the rate for Case 5 peel ply at 1.4 mm/s after 1 s. It is considered that the intermediate medium applied to the peel ply could easily penetrate the empty weaves of the Case 5 peel ply and the micro-cracks of the Case 7 peel ply. However, the intermediate medium could not penetrate the Case 6 peel ply due to the non-gap polymer resin layer on the peel ply.

Figure 11a shows the schematic plot of protection of the CFRP surface from the intermediate medium by using peel ply. When the peel ply was applied to CFRP manufacturing, the polymer resin in carbon prepreg penetrated the peel ply and filled the empty space [44]. Although the intermediate medium was applied on the CFRP surface, the intermediate medium could not penetrate deep enough to reach the CFRP surface due to the barrier effect of peel ply with polymer resin. Figure 11b shows a schematic illustration of the behavior of the intermediate medium in the peel ply during its detachment from the CFRP. Micro-cracks in the peel ply can form during its detachment from the CFRP. In the case of detaching in the dry state of the intermediate medium, solid soap elements remained on the peel ply without penetration. In contrast, if the peel ply wetted by the intermediate medium was detached immediately, the intermediate medium partially penetrated through the peel ply due to the micro-cracks and the intermediate medium reached the CFRP surface.

4. Conclusions

After UT, the intermediate medium acts as a contaminant during the bonding process between the spar-cap and the blade skin part, thereby ultimately degrading mechanical properties of the wind turbine blade.

The effect of the intermediate medium remaining after UT on the interfacial adhesion of wind blade CFRP was investigated in this study.

Also, the degree of permeability of the intermediate medium was evaluated by surface observation and chemical composition analysis of the peel ply.

The highest work of adhesion and shear strength were found on the CFRP surface from which the peel ply was removed through complete drying.

It could be assumed that the intermediate medium could not pass through the polymer resin layer inside the peel ply if it was completely dried.

A post-UT cleaning method was proposed through various experiments in this study. It is expected to be utilized as a means of preventing residual contamination of composite materials used in industrial fields such as wind energy, aerospace, shipbuilding, and defense.

Further studies are required to identify appropriate cleaning solutions and procedures when contaminants remain after UT. Such studies would help prevent the degradation of interfacial properties following UT and thereby enhance the structural integrity of composite structures.