Advanced T-Ray Nondestructive Evaluation of Defects ... - Springer Link

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6, pp. 1093-1098

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DOI: 10.1007/s12541-013-0147-2

Advanced T-Ray Nondestructive Evaluation of Defects in FRP Solid Composites Kwang-Hee Im1,#, Kil-Sung Lee2, In-Young Yang3, Yong-Jun Yang3, Young-Hwan Seo4,

and

David Kuei Hsu5

1 Department of Automotive Eng., Woosuk University, 490 Hujung-ri, Samrae-up, Wanju-kun, Jeonbuk, Korea, 565-701 2 Wind Energy Division, Human Composites Co. Ltd., 1588 Soryong-dong, Gunsan-si, Jeonbuk, Korea, 573-879 3 Dept. of Mechanical Design Eng., Chosun University, Gwangju, Korea, 501-759 4 Dept. of Physical Education, Chosun University, Gwangju, Korea, 501-759 5 Center for Nondestructive Evaluation, Iowa State University, Ames, Iowa 50011, USA # Corresponding Author / E-mail: [email protected], TEL: +82-63-290-1473, FAX: +82-63-291-9312 KEYWORDS: NDE, T-ray, FRP, Composites

Nondestructive testing technique using T-ray (terahertz ray) receives attention these days as a new field of test engineering. In this study, T-ray time-domain spectroscopy was used for inspecting and evaluating physical property and defect characteristics in FRP composite material. Also, general refraction and transmission mode of this T-ray spectrography was used for finding refraction coefficient (n) and for obtaining T-ray image. First, in order to obtain the terahertz ray refraction index, refractive and transmission mode technique was induced, with which refraction index of GFRP composites, balsa and epoxy of wind turbine blade could be obtained. In GFRP and CFRP composites, T-ray propagation faces hindrance by carbon fiber. Accordingly, the authors have analyzed directional dependence between E-field of T-ray and the carbon fiber. Also, evaluating method of wind turbine blade composites having two (2) saw-blade delaminations integrally inserted was presented in this study. It was found that the time of flight of T-ray using T-ray transmission mode technique coincides well with that using refractive index in the wind turbine blade. Manuscript received: September 28, 2012 / Accepted: March 18, 2013

1. Introduction Recently, utilization of Terahertz wave (T-ray) is in rapid progress today with interests, which is applied in the field of inspection engineering thanks to the development of T-ray technique and its peripheral system. T-ray gives compared to micro waves.1 Terahertz Time Domain Spectroscopy (THz-TDS) plays leading role in precision detection of internal defects and damages in composite materials nondestructively. Use of THz-TDS here is based on optical conductivity, which relies on generating low cycle terahertz.2 Since THz wave of less than picosecond could be generated, it was consequently possible to detect defects using high S/N ratio, high signal over noise, The energy generated at this time is distributed over a wide range of terahertz and affects very wide bandwidths. Transient change in terahertz emitter is made because of the resistance of optical conductivity switch on the terahertz time scale.3 In addition to this, there is another method: optical anisotropic conversion or optical mixing. This can be obtained by use of two continuous wave (CW) lasers. When two lasers are mixed together, a beating is generated, which can be modulated by conductance of optical conductivity switch

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using differential terahertz waves, and thus continuous wave of terahertz wave (CW-THz) can be obtained. Not only limited to the application in wide ranges, THz-TDS has, due to its excellent utility of application technique, potential of becoming the first THz image system, small, portable and reliable enough to be used actually in the field. Also, wind energy is being considered as hot issues on evaluating the quality of parts in blades; so a method of inspection was needed.4 It is thought that such system get bring the manufacturing system. First of all, the authors try to use it for fiber reinforced plastics (FRP) composite laminates, which is used widely in the field where specific strength and specific rigidity are required in view of the utility and applicability the T-ray has. First, the authors handled refractive index (n) and the effect of electrical conductivity of FRP composites, also made analysis of THz scan image for carbon fiber reinforced plastics (CFRP) and evaluated. While the carbon fiber is electrically conductive, the epoxy matrix is not conductive. However, the carbon fiber of CFRP composites has electrical conductivity.5 The effect of electrical conductivity of fiber was analyzed through test at this time. It was decided to use T-ray for nondestructive inspection and evaluation of wind turbine blade. Wind turbine blade was composed of

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INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6

various composites including GFRP, balsa and epoxy.6,7 Since general ultrasonic wave has certain limit in transmission, highly transmissive Tray became to use. Terahertz pulse having frequency of 0.1 to 1.0 THz proved to be transmissive in the tail part of wind turbine blade 100 mm (4 inches) thick that is composed of GFRP, balsa and epoxy. Time-offlight (TOF) of the emitted terahertz pulse was calculated by respective thickness and wave speed (refractive index) at each layer of GFRP, balsa and epoxy of the tail part of the wind turbine. Samples taken from the tail part of the wind turbine blade were of wedge shape having 360 mm width and 85-95 mm at thick end and 30 mm at thin end respectively. Time-of-flight (TOF) of the emitted terahertz pulse was measured along the center line of the width. Thickness of the middle part of GFRP, balsa and epoxy layers was determined as average of thicknesses measured at the two (2) exposed ends of the fabricated sample. Even though these samples have voids inside of epoxy, the size of the void could not be measured. In comparative analysis of time of flight actually measured against that of calculated, various sizes of the void were used.8,9 Two (2) saw cuts delaminations were fabricated and inserted into the wind turbine blade for inspection and evaluation using T-ray system. Therefore, the authors have also worked out measuring technique of refractive index (n) as physical property of various materials using terahertz wave, in addition to performing application of terahertz wave with consideration of interrelation between fiber direction and E-field of the electrically conductive CFRP composites and non-conductive GFRP composites respectively. The T-ray images were analyzed in order to detect the two saw cuts in the wind turbine blades.

2. Theory 2.1 Measuring refractive index Terahertz wave is also used in security monitoring system in airports, medical imaging, polar liquid, various industrial systems and composite materials. This is because of the shorter wave length and relatively high definition of images This method is inducing the refractive index by transmission mode at time domain of terahertz wave through media of test specimen and air. Fig. 1 shows Diagram showing the geometry of the reflection mode and the propagation direction of terahertz wave signal. In this setup, by obtaining respective TOF through measurement of the time required for T-ray to reach pulse

receiver from the pulse emitter without test specimen and the time when refracted by test specimen of certain thickness, the refractive index can be obtained.

2.1.1 Reflection mode This method was to determine the index of refraction used to calculate the optical path length different between the front and back reflections in the time domain. A diagram8 showing the geometry of the two THz signals is shown as in Fig. 1. 4

2

2

∴n – An – Asin θp1 = 0 2

2

(1)

2

Here, A = ( T Vair ) ⁄ ( 4d2 ) where T is the transmission time of the sample, d2 is the sample thickness, Vair is the light speed in air and θp1 is the incident angle of the sample.

2.1.2 Through-transmission mode In through-transmission mode,8 the index of refraction (n) could be calculated using the following equation as shown in Fig. 2. Δtvair ∴n = 1 + -----------d

(2)

Where Δt is the difference time between with sample and without sample, d is the sample thickness, Vair is the light speed in air and L is the distance between pulsed emitter and pulsed receiver. A method to determine the absorption coefficient (α) was also developed as a conventional way. In through-transmission mode, the pulses were captured from two samples with different thickness and the absorption coefficient (α) could be developed by a sample proportional equation from Fig. 3(a) and (b). The absorption coefficient (α) could then be calculated using following formula as shown in Fig. 3.9 I ln ---2I1 α = –-------------d1 – d2

(3)

Fig. 2 Diagram showing the geometry of the through-transmission mode

Fig. 1 Diagram showing the geometry of the reflection mode

Fig. 3 Diagram showing the geometry of the through-transmission mode with two samples in (a) thinner and (b) thicker sample


where d2 is the thicker sample, d1 is the thinner sample, I2 is the transmitted E-field in the thicker sample and I1 is the transmitted Efield in the thinner sample. A goal of this research was to determine if the refractive index of various materials could be calculated with a THz TDS system in reflection and through-transmission modes. In addition to such samples, a CFRP composite laminate was prepared to assess the utility of a THz TDS system for NDE of orientation in carbon fibers. Another sample consisting of one ply laminated piece was prepared to try to detect a flaw at the back side in order to evaluate the effect of E-field and to investigate the backside flaw with THz TDS imaging.

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4 THz and maximum 300 ps delay time. T-ray beam is concentrated at focus length of 50 mm and 150 mm respectively and full width at half maximum (FWHM) is 0.8 mm and 2.5 mm respectively. The TDS System can be set for measurement of transmission or reflection (small angle pitch catch). Frequency range of CW System is 50 GHz 1.5 THz and maximum definition can be obtained at 100 MHz. Focus length of CW System is 50 mm and 150 mm respectively. TDS and CW Systems are connected by optical fiber each other. Fig. 5 shows schematic drawing of Transmission Mode in T-ray System.

4. Discussion and results 3. Experimental setup and measurement 3.1 Measuring System Fig. 4 shows photograph of terahertz time-domain spectroscopy system (THz-TDS), the nondestructive test system. This system is intended for collecting material property of the test specimen and their scan images. Terahertz System used in this study is the one manufactured by TeraView, U.K. The system is composed of TDS Pulse System and Frequency-domain Continuous Wave (CW) System, and has TDS technology of generating and controlling terahertz pulses for detecting defects. THz-TDS System can make images with enhanced data collection, and has unique structural property of optical instrument for controlling terahertz wave directly and making images. The TDS System used in this study has frequency range of 50 GHz -

Fig. 4 THz TDS system for imaging and material parameter measurement

4.1 Measuring Terahertz Refraction Index In order to measure parameters of T-ray that shows the material's physical property, THz pulse was obtained from GFRP in reflection mode. In Fig. 6, time difference is apparently shown when there is test specimen, where GFRP test specimen of approximately 6.0 mm thick. For obtaining refractive index, method of using reflection mode. one of the measuring techniques, was employed to calculate optical time difference by equation (1). Table 1 shows refractive indices of GFRP composites, Balsa and Epoxy samples measured in the reflection mode. Standard deviation of data did not spread away from 1 to 2%. Also, physical phenomena were classified on several things between T-ray and ultrasonic waves as shown in Table 2. This is thought because terahertz measuring technique in reflection mode is relatively easy in experiment compared to other methods

Fig. 6 THz TDS pulses from transmitted GFRP sample (t = 85.6 ps for GFRP 6.0 mm thick) Table 1 Average THz refractive indices of the material studied Materials GFRP Epoxy Balsa

Throughtransmission mode 2.18 ± 0.06 1.77 ± 0.02 1.19 ± 0.09

Reflection mode 2.19 ± 0.06 1.76 ± 0.04 1.19 ± 0.06

*Known data by References10,11 Table 2 Comparison table between T-ray and ultrasonic waves

Fig. 5 THz measurement method with through-transmission mode

Physical phenomena Snell’s law and velocity In vacuum Shadow effect Attenuation

T-ray Same Going through Not hampered Same

Ultrasonic Same Not going through Hampered Same

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even though there are many parameters to consider in actual measuring. In case of test specimen of composite materials, however, there was difficulty in comparing the result to the existing data because the method of fabrication and properties were different among them.9-13

4.2 Evaluation of E-Field Terahertz wave has certain limitation in its transmissivity in electrically conductive materials unlike non-conductive materials. Therefore, we had to study comparatively whether it may be applicable for carbon fiber and glass fiber as well. Particularly, the carbon fiber reinforced plastics (CFRP) is composed of electrically conductive carbon fiber and non-conductive matrix. Microscopic observation of CFRP composite laminates shows diverse composition of fiber and matrix, which may cause serious effect in the electrical conductivity. Therefore, it is required to make property evaluation of terahertz waves in CFRP quantitatively. According to the existing references, it is said that the electrical conductivity in radial direction of the carbon fiber is approximately 3 times bigger than that in axial direction. CFRP composites is composed of uni-directional fibers and its lamination is made in many different methods, which affects the electrical conductivity. Particularly, the lateral (vertical to fiber axis) generation mechanism of the conductivity depends on the fiber contacts between contiguous fibers. It is thought that the lateral value of conductivity in laminates using uni-directional pre-preg sheet depends seriously on the fabrication process and the quality of the laminates.10 Therefore, the experiment was conducted in terahertz transmission mode as shown in Fig. 7. In Fig. 7(a) and (b), it was studied about the transmission in E-fields 90o and 0o to the fiber direction of CFRP composite laminates. Where the directions of E-field and fiber coincides (case of (b) 0o), the receiving signal of terahertz wave could not be seen since the fiber hinders the terahertz wave travel. Namely the direction of unidirectional carbon fibers lies parallel to the electric field terahertzes which are conductors due to blocking the terahertz waves. However, where it makes 90o crossing (case a), the terahertz wave travels between fibers and the receiving signal of terahertz wave

could be observed. Particularly, since the terahertz signal was biggest when the fiber direction makes 90o to E-field in CFRP composite laminates, it is thought that the high transmissivity of terahertz wave at this mode makes the nondestructive test signal most optimum. In other words, the terahertz waves could penetrate the carbon fibers to some degree because the fibers lie in a direction perpendicular to the electric field of terahertz waves. Consequently the terahertz waves go into the resin of CFRP composites which are consisting of non-conducting resin and fibers. Also, as in Fig. 7(c) and (d), GFRP laminate test specimen was used, where the receiver could sense the terahertz signal irrespective of T-ray and E-field directions. Since GFRP laminates are electrically non-conductive, they could receive terahertz signal irrespective of directions of the fiber and E-field.11-14 Therefore, it is impossible to detect small cracks because of larger cracks. Contrary to this, terahertz wave does not have such limitation. In order to clarify this 'shadow effect", two (2) saw cuts defects of 101.6 mm and 127 mm diameters respectively were inserted into the turbine blade simulating delaminations. Blade thickness at this time was 30 mm to 85 mm, and the small delamination (101.6 mm dia.) was made to locate right below the larger delamination (127 mm dia.). These two (2) delaminations were made parallel to the blade surface as shown in Fig. 8, but there was some difficulty in doing this because of different thickness of the blade. In T-ray experiment, THz-TDS made T-ray scan images for test specimen having two saw cut defects inside using transmission mode. As shown in Fig. 9(a) and (b), the image of amplitude of transmitted terahertz pulse is the two (2) saw cuts delaminations. Fig. 9(a) and (b) shows T-ray image of saw cuts defects of 127 mm 101.6 mm respectively at this time. Even though the images are not sufficiently clear due to the diverse composite composition of the wind turbine blade, it shows clearly in the right bottom parts (point A and B). This means that one T-ray scan can detect two (2) saw cuts delamination at the same time. Fig. 9(c) and (d) show T-ray B-scan images of A-A' and B-B' in Fig. 9(a). The position of A-A‘ is of no delamination while that of B-B' is of two (2) delaminations. Through-transmission mode was utilized. The scan image of B-B’ at this time shows the difference of TOF. This means that the

Fig. 7 Angular dependence of transmitted power of THz terahertz waves through (a) and (b) of a 10-ply unidirectional carbon composite laminate and (c) and (d) of GFRP laminate with 12 mm in thickness

Fig. 8 Wind turbine blade with two simulated delaminations in the same location of sample by using a saw cutter


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delamination has caused change of TOF to make it short, which made bigger difference when there were two (2) delaminations. Particularly, in Fig. 9(d), distinct difference of TOF is shown at the position where two saw cuts delaminations are located (point C and D). Therefore, it was found that the terahertz wave does not receive any hindrance from “shadow effect” in inspecting the thick wind turbine blade.

4.3 Transmission Force in Wind Turbine Blade Wind turbine blade is made typically of three (3) different materials. They are GFRP composites, balsa and epoxy. Since these three materials are electrically non-conductive, terahertz wave can be used in inspecting the wind turbine. In an attempt to evaluate the transmission of terahertz wave in the wind turbine blade, samples were taken from the tail part of the blade for use in experiment of reflection and transmission mode using terahertz system. These samples were of wedge shape having balsa core and glass fiber composite skin. Two (2) adjoining skins were glued together by epoxy adhesives as shown in Fig. 10. Thickness was approximately 95 mm at the thick end of the wedge while 45 mm at the thin end. Before entering into the experiment for these samples taken from the tail part of the blade, refractive indices for each component parts were measured using 6 mm thick plate cut off from other part of the blade. Refractive indices thus measured were 1.18 in balsa, 1.76 in epoxy and 2.17 in glass fiber respectively. Measuring TOF in transmission mode was done at 7 locations in the tail part. (See Fig. 10). T-ray delayed TOF (t) at the tail part of wind turbine blade was calculated and also found the experimental values. When measuring respective test specimen, each specimen was cut off from the blade, machine and then measured the refractive index (n). Value of TOF at respective positions was calculated using actually measured refractive index and measured thickness of the part. Thickness in use was the average of thickness at the exposed end of both sides. Even though the actually measured TOF and calculated value meets well each other as shown in Fig. 11, some difference were found at positions 1 and 2 because of different thicknesses along the longitudinal direction. This is thought that partial voids in thick epoxy layer or matrix shortage for glass fiber may have caused the effect also.

Fig. 9 Detection of double saw cuts in wind blade spar cap. Scan size is 160 × 144 mm, step size is 1.0 mm and focal length is 150 mm

Fig. 10 Complex structure with GFRP, epoxy and balsa

Fig. 11 Transmission of T-ray pulse through the trailing edge of a wind turbine blade

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5. Conclusions As a study for application of Terahertz wave (THz) in nondestructive inspection of wind turbine blade, measuring technique of refractive index, one of the material properties, was used and also studied the property of E-field for terahertz wave. Particularly, for wind turbine blade composites, TOF of terahertz wave in transmission mode was measured for evaluating the wind turbine blade. Following results could be obtained. 1) For wind turbine blade composites, refractive index in transmission mode could be measured by use of terahertz wave. 2) Even though terahertz wave had certain limited transmission power depending upon carbon fiber direction, it had no effect in the electrically non-conductive GFRP composites. 3) Unlike ultrasonic nondestructive inspection for wind turbine blade having two (2) saw cuts delamination, terahertz wave was found free from “shadow effect.” 4) When terahertz wave system has been used in the tail part of wind turbine blade, the actually measured transmission TOF meets well with the calculated value, but some part has shown slight difference. This is thought because of the complexity of blade formation.

ACKNOWLEDGEMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 20110008391).

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