AIR-COUPLED ULTRASONIC INSPECTION TECHNIQUE FOR FRP STRUCTURE

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1 1 th A-PCNDT 6 Asia-Paifi Conferene on NDT, 5 th 1 th Nov 6, Aukland, New Zealand AIR-COUPLED ULTRASONIC INSPECTION TECHNIQUE FOR FRP STRUCTURE Yoshikazu Yokono 1, Shigeyuki Matsubara, Shigeki Matsui, Makoto Kitajima, Hideki Yabushita, Satoru Utoyama 3, Noboru Wakita 3 1 Pony Industry Co.,Ltd. Osaka, JAPAN Non-Destrutive Inspetion Co.,Ltd. Osaka, JAPAN 3 All Nippon Airways Co., Ltd. Tokyo, JAPAN Abstrat An investigation of the air-oupled ultrasoni tehnique, whih ould be done from one side of a testing material, was arried out in order to detet raks in Fiber Reinfored Plastis (FRP) by non-ontating. The aessibility of probe should be improved to apply this tehnique for examination of raks in FRP at maintenane fields of airplane, beause in general some obstales are loated on its test surfae. In this study, the probe layout was arranged to examine whole test area and some experimental investigation of this tehnique was arried out using testing speimens. As a result, all raks loated in the testing speimen an be deteted learly. Also it was onfirmed that small raks of 5mm in length ould be deteted and that the air-oupled ultrasoni tehnique an be used to detet invisible raks. 1. Introdution Fiber reinfored plastis (FRP) that are the most popular omposite materials help to lighten omponents, beause of their superior strength-toweight ratio. Therefore, they have ome to be utilized as parts of aerospae et. Composite materials have already been used for seondary strutures of ommerial airrafts, suh as vertial and horizontal stabilizer as well as skin plate of wing. Moreover, some ommerial airrafts in whih omposite materials are used for major strutures are developed. Under this situation, reliable and effiient examination tehniques by whih omposite materials are examined need to be developed. Though tapping testing, ultrasoni testing, soft X- ray radiography and infrared thermography testing have been applied to omposite material as nondestrutive examination tehniques, their reording ability, effiieny and auray are required to be improved. Ultrasoni testing is applied to omposite material of airplane during both onstrution and maintenane, beause it is the advantageous inspetion tehnique of a planer defet like as rak or delamination. However, onventional ultrasoni testing has a disadvantage that liquid oupling medium suh as water and oil are neessary and leaning must be done before and after testing. Moreover stable oupling between probe and material to be tested must be kept, beause it affets the examination result. For the purpose immersion tehnique an be applied at the manufaturing stage, but it is diffiult to apply it at the maintenane stage, and large-sale system is neessary for large omponent. Under these bakgrounds it is meaningful to establish the speedy and quantitative inspetion tehnique that exeuted without any oupling medium of a liquid. The purpose of this study is to develop the pratial air oupled ultrasoni inspetion tehnique. Moreover, we deided that the tehnique exeuted only from one side of an internal or external surfae (it is hereafter alled the One Side Aess Inspetion Method) would be developed in order to make inspetion at the maintenane stage easier.. Air-oupled ultrasoni inspetion method In order to transmit ultrasound effetively, liquid is usually used to ouple a probe with a material to be tested in the onventional ultrasoni testing. Equation 1 indiates eho transmittane that is a ratio of the sound energy bak to the transduer after transmitting through a interfae (P T ) to the original energy (P I ) as shown in Fig. 1. T P P = = 4Z Z T T 1 1 (1) P P ( Z ) I T 1 + Z Where Z 1 and Z show aoustial impedanes of medium 1 and medium respetively.

2 P I P T tehnique or through transmission tehnique. Lamb wave has been used for the ase of steel plate. P T P T medium medium1 medium1 medium Figure 1: Shemati drawing of ultrasound s transmission at an interfae. Table 1: Aoustial properties Density ρ (1 3 kg/m 3 ) Sound veloity of longitudinal wave C (m/s) Aoustial impedane Z = ρ C (1 6 kg/m s) Water Air Aryli resin Epoxy resin 1.15~1.3 5~8.8~3.7 Table : Eho transmittane Medium1 Medium Water Air Aryli resin Epoxy resin.8~ ~ Table 1 shows the aoustial properties of water, air, aryli resin and epoxy resin, and Table summarizes eho transmittanes. It is lear that the sound energy transmitting in the air is thousands times smaller than that in the water. As desribed above, transmission oeffiient of airoupled ultrasoni tehnique is quite low, it is of great signifiane to onsider frequeny of sound, probe property and apability of equipment. Air-oupled ultrasoni tehnique desribed below was investigated by using suitable system. 3. Propagation of Lamb wave Though omposite material onstitutes omplex struture suh as honeyomb, hannel and stringer, it is based on plate struture, whih is often inspeted by ultrasoni testing using refletion Lamb waves are a omplex vibrational wave that travels through the entire thikness of a material. Propagation of Lamb waves depends on density, elasti, and material properties of a omponent, and they are influened remarkably by seleted frequeny and material thikness. With Lamb waves, there are the two most ommon modes, symmetrial and asymmetrial, and many higher harmonis of them are produed. The equation and 3 are requirements for existing symmetrial and asymmetrial Lamb wave, respetively. πfd ( p 1) tan = πfd 4 pq tan q + p ( 1) πfd πfd p tan q + 4 pq tan p = Where, f and d mean phase veloity of Lamb wave, frequeny of ultrasound and thikness of material, respetively. Equation 4 and 5 show parameters p and q, and D and S mean sound veloity of longitudinal and transverse wave, respetively. () (3) p = 1 (4) S q = 1 (5) D As an example, alulation results of phase veloity of Lamb wave in CFRP, in whih sound veloity of longitudinal wave is 367m/s and that of transverse wave is 144m/s, is shown in Fig.. Phase 位相速度 Veloity (m/s) (m/s) S S1 S S3 S4 S5 S6 A1 A A3 A4 A5 1 A CFRP CFRP 縦波速度 VL=367m/s D 5 横波速度 VS=144m/s S Frequeny 周波数 thikness 板厚 (MHz (MHz mm) mm) Figure : Calulation results of phase veloity of Lamb wave in CFRP The dotted vertial line in the figure, whih represents the ondition of 5mm in thikness and

3 43kHz in frequeny, predits that there are symmetrial mode S and asymmetrial mode A having similar sound veloity as well as higher harmonis. 4. Propagation of Lamb wave In order to onfirm the propagation behavior of this Lamb wave experimentally, Lamb wave was tried to emit and propagate into the CFRP with the layout shown in Fig. 3. Probe, whih transmits or reeives longitudinal wave in the air, has a omposite piezo-eletriity element with 5mm in diameter and a nominal frequeny of 4kHz. Transmitter 5mm Burst purser/reeiver Reeiver 1mm Filter Osillosope Pre amp. Figure 3: Blok diagram for experiment of Lamb wave propagation. Examples of olleted waves are shown in Fig.4, and experimental results and theoretial solution are ompared in Table 3. Although some differenes between the alulation result and the experiment result were shown, waves of the mode of S1, A1, and mixed mode of S and A, whih were predited by alulation, were learly observed. Moreover, The distane amplitude harateristi urve, whih is obtained by sanning the transmitter, is shown in Fig. 5. The figure shows that attenuation of mode of S+A is greater than that of the other modes. Then, it was presumed that this mode had muh leakage of ultrasound to the air, and it was not suitable for long-distane propagation beause of high attenuation. However, it was easy to detet propagation of the Lamb wave in the air. S+A A1 S1 Amplitude(dB) Table 3: Comparison between experiment and theoretial solution experiment Theoretial solution Estimated Phase Phase Mode mode veloity veloity S+A 11m/s S m/s A 134.3m/s A1 196m/s A m/s S1 46m/s S1 86.m/s S+A A1 S Probe Position (mm) Figure 5: Distane amplitude urves of Lamb wave. The One Side Aess Inspetion Method was developed to detet the lamb wave in this mode propagating a testing objet. When disbonding et. exists in the path of propagating Lamb wave, the boundary onditions about propagation of the wave hange, therefore the Lamb wave is attenuated and the amplitude of a deteted wave dereases as shown in Fig.6. That is why disbonding, impat damage, rak et. an be deteted by this inspetion tehnique from the amplitude hange. Probes Sound part Sound part Probes Lamb wave A B C µs/div mv/div µs/div 1mV/div µs/div 1mV/div Figure 4 Examples of olleted wave Derease in the amplitude Disbonding of Lamb wave Disbonding part Figure 6: Example of olleted waves at the sound part and disbonding part.

4 5. Crak detetion of omposite material using Lamb wave The photographs of flap of B747 airraft are shown in Fig.7. Visual inspetion is exeuted beause there is possibility that rak ours at the fold of skin plate shown by red arrow in the piture. To improve detetability of rak, hereinafter, an investigation of air-oupled ultrasoni inspetion was arried out. The testing speimen that simulated the fold in mid flap had artifiial rak of 5.4mm and 1.7mm as shown in figure 8. To onfirm the detetability of rak by Lamb wave tehnique, the examination of the testing speimen with setting probes shown in Fig.9 was arried out. The typial example of a reeived wave in a sound part and raking part are shown in Fig.1. It was onfirmed that the Lamb wave that propagates the sound part is deteted by reeiving probe learly, and the amplitude dereases in the raking part. It means that propagation of Lamb wave was interrupted by rak. Transmitter Reeiver (a) Triple-slotted flaps Figure 9: Layout of probes for Lamb wave tehnique. 1 1 Amplitude 振幅 (mv) (mv) Amplitude 振幅 (mv) (mv) 時間 Time (μs) (µs) Time 時間 (μs) (µs) (b) Mid flap Figure 7: Photograph of flap of B747 (a) Sound part (b) Crak part Figure 1: Example of waves olleted by Lamb wave tehnique. Inaessible region 5.4mm 1.7mm 5.4mm Length of artifiial raks Crak length 5.4mm Amplitude 振幅 (mv) (mv) Crak length 1.7mm 1 Position 距離 (mm) (mm) Figure 8: Photograph of testing speimen simulated the flap of B747. Figure 11: Amplitude profile by Lamb wave tehnique.

5 Fig.11 shows the amplitude profile of deteted wave in the enter part of testing speimen shown in Fig.8. The amplitude remarkably dereased at the point of raks. Therefore it was lear the hange in amplitude of Lamb wave ould indiate the existene of rak. To investigate the detetability of rak by using this tehnique, testing speimen shown in Fig.1 with smaller raks was examined. The Amplitude profile was obtained as shown in Fig.13 and the derease in the amplitude at the rak of 5mm in length was learly observed. It is onfirmed that the rak of 5mm in length an be deteted by this tehnique. The region near the support is so diffiult to be aessed that it annot be examined, therefore the transmitter and reeiver have to be plaed on eah side of fold in flap by this tehnique. The dotted arrow in Fig.8 shows the inaessible region of the testing speimen. 6. Improved Lamb wave tehnique The aessibility of probe should be improved to apply this tehnique for examination of raks in FRP at maintenane fields of airplane, beause in general some obstales suh as supports are loated on its test surfae. In this study, the probe layout shown in Fig.14 was arranged to examine whole test area. Fig.15 shows the shemati drawing of prinipal of this tehnique and example of olleted waves. This figure indiates that refleted Lamb wave an be deteted at the raking part by this tehnique and the amplitude profile is expeted to be the reverse. As rak might our near a support due to stress ondition, it is meaningful to derease the inaessible region. Crak Reeiver Transmitter Figure 14: Probe layout for improved Lamb wave tehnique. mm 15mm 1mm 5mm Transmitter Reeiver Transmitter Reeiver 送信 受信 送信 受信 Crak 割れ Amplitude(mV) (mv) 1 測定配置振幅小 (μ s) Time (ms) FRP Amplitude(mV) (mv) 1 測定配置振幅大 (μ s) Time (ms) Figure 1: Smaller raks. mm 15mm 1mm 5mm Figure 13: Amplitude profile of smaller raks by Lamb wave tehnique. Figure 15: Prinipal of improved Lamb wave tehnique and example of olleted waves. The testing speimen shown in Fig.16 is the same as the sample shown in Fig.8 exept the invisible raks that are the artifiial raks under the paint. Profile of signal amplitude of it by improved Lamb wave tehnique is shown in Fig.17. As a result of this examination, it is lear that the rak lose to the support an be deteted as well as the raks deteted by the previous experiment, and raks under the paint an also be deteted. Therefore it an be said from the above-mentioned results that the whole test area of flap of B747 inluding fold lose to supports an be inspeted by this tehnique.

6 Detetion of raks and disbondings in the omposite material are so easy and speedy by using this tehnique that it will be applied in the many fields. 8. Aknowledge This researh was entrusted in part from Safety and Mission Assurane Department of Japan Aerospae Exploration Ageny, and I would like to aknowledge here the generosity of this organization. Figure 16: prinipal of improved Lamb wave tehnique and example of olleted waves. Table 4: Artifiial visible and invisible raks in the testing speimen. 9. Referenes [1] S. Matsubara, T. Nagai, T. Yoshiara, M. Shirai and H. Miyamoto, Air-Coupled Ultrasoni Inspetion Method for CFRP, Key Engineering Materials Vols (4), pp188 Crak No. Clak Length #1 (visible) 5.4mm # (visible) 1.7mm #3 (visible) 5.4mm #4 (invisible) 1.7mm #5 (invisible) 5.4mm Signal Amplitude 振幅 (% (%) Invisible Craks Detetion Level #4 #5 # # 距離 (m m) Position (mm) Visible Craks #3 Figure 17: Amplitude profile of visible and invisible raks by improved Lamb wave tehnique. 7. Conlusions An investigation of the air-oupled ultrasoni tehnique, whih ould be done from one side of a testing material, was arried out in order to detet raks in Fiber Reinfored Plastis (FRP) by nonontating. As a result, it was onfirmed that small raks of 5mm in length an be deteted and that the air-oupled ultrasoni tehnique an be used to detet invisible raks. Moreover it was lear that inaessible area an be dereased by using improved Lamb wave tehnique and whole test area an be examined.