TITLE THE PRINCIPLES OF COIN-TAP METHOD OF NON-DESTRUCTIVE TESTING

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1 TITLE THE PRINCIPLES OF COIN-TAP METHOD OF NON-DESTRUCTIVE TESTING Sung Joon Kim*, Dong-Chul Che Kore Aerospce Reserch Institute, 45 Eoeun-Dong, Youseong-Gu, Dejeon, , Kore Phone : FAX : yelin@kri.re.kr Abstrct The coin-tp test hs the bility to indicte dmge in structurl element due to loclized chnge of stiffness. The chnge in vibrtion signture my be detected by er or more precisely by mesurement of the dynmic contct force [1]. In this pper, coin-tp test method for discriminting between mesurements mde on sound nd delminted structures is present. It hs been shown tht the chrcteristics of rdited sound from structure during tp re chnged by the presence of defect beneth the surfce of the structure. For structurlly rdited noise, the sound field is directly coupled to the structurl motion. Therefore, Impct response nlysis should be computed. In this study, the rdited sound induced by impct is obtined by solving the Ryleigh integrl eqution. And the 2-D equivlent delmintion model is used to nlyze the impct response nd coustic nlysis. Predicted impct responses using 2-D equivlent delmintion model re compred with the numericl ones from the 3-D non-liner finite element model. From the results, it is shown tht the 2-D simplified model ws found to be relible for predicting the impct force histories nd sound pressure. INTRODUCTION The use of lminted composite structures hs mny potentil pplictions in vriety of engineering fields. However, they re mostly used in lminted form with reltively wek interlminr interfces which re vulnerble to trnsverse lods such s impcts rising from flling mss. The presence of delmintions cn cuse significnt degrdtion of the structurl response chrcteristics. The coin-tp test is one of the oldest methods of non-destructive testing nd it is regulrly used for testing lminted structures. The test requires n opertor to tp ech point of the structure to Eds.: Sung Joon Kim, Dong-Chul Che

2 Sung Joon Kim, Dong-Chul Che be inspected with coin, nd listen to the resulting sound rdited by structure. When structure is struck with hmmer, the chrcteristics of the impct re dependent on the locl impednce of the structure nd on the hmmer used. Dmge such s n dhesive disbnd nd ftigue dmge results in decrese in structurl stiffness, nd hence chnge in the nture of impct [2]. Adms showed tht it is possible to produce version of the coin-tp test which depends on the mesurement of the force input to the test structure during the tp [3]. In this pper, sound bsed non-destructive method is proposed. This method is using the difference between rdited sound of helthy structure nd delminted structure. For structurlly rdited noise, the sound field is directly coupled to the structurl motion. Therefore, impct response should be nlyzed. Generlly, the finite element method on the impct response of the lminte hs been known to require long computtion time. Shivkumr et l. did not use the finite element method to nlyze the impct response of the lminte but used the spring-mss model to efficiently predict the impct force history [4]. In their study, the contct energy due to locl indenttion s well s trnsverse sher energy nd bending energy of plte re considered, nd they reported tht the contct energy cn be neglected in the impct by reltively low velocity foreign object on flexible or thin plte. Choi proposed the spring element model using linerized contct lw. In their study, they hve shown tht the linerized contct lw pproch could be pplied to low-velocity impct response nlysis problem with using generl purpose FEM softwre [5]. To investigte the impct response on delminted lmintes, 3-D finite element model is used using gp element to void the overlp nd penetrtion between the upper nd lower sub-lmintes t delmintion region. But the use of three-dimensionl elements to predict the impct response of delminted composite structures is inconvenient becuse of quite number of elements necessry to obtin numericl solutions. Impct response nlysis using generl-purpose FEM softwre Figure 1 shows FEM model for impct response nlysis using 1-D spring element supported in generl purpose FEM softwre. The mss of impctor is lumped t the end of the spring mss, nd the other end of spring element is ttched to lminte t impcted loction. After FEM nlysis we cn extrct the compressive force history cting t the spring. In present study, MSC/NASTRAN ws used s generl-purpose FEM softwre. In FEM modeling 3-node plte element ws used nd trnsient dynmic nlysis ws performed with the initil condition, which is tht initil speed of the lumped mss ws loded s impct velocity. Delmintion Model To prevent the overlp nd penetrtion, 3-D non-liner finite element nlysis ws performed using MSC/NASTRAN to determine the impct response of grphite/epoxy composite specimens.

3 ICSV13, July 2-6, 26, Vienn, Austri Mss of plte,m P Lumped Mss of Impctor, M I Spring Element Figure 1 Spring element model using generl-purpose FEM softwre Eight-node solid elements nd gp elements were used to model delmintion. The gp elements re inserted long the delmintion interfce surfces for preventing penetrtion of the upper nd lower sub-lmintes during impct nlysis process. In this study, 2-D simplified delmintion model ws used for comprison with 3-D non-liner finite element model. Figure 2 shows the schemtic of the delmintion modeling. The delminted re elements re copied, nd ech hlf lminte ws ssigned the pproprite properties: lminte A: [] 8, thickness = 1. mm; lminte B: [] 4, thickness =.5 mm; lminte C: [] 4, thickness =.5 mm. Finlly the outlining nodes of both groups were tied together by constrining ll of their degree of freedom. Seth hs proposed the simplified delmintion model nd hs shown tht the frequency response results re very greed with tests [6]. Delmintion model verifiction Delmintions were locted t the midplne of the pltes. The nlysis model of the lminte is 15 X 15 X.1 cm, nd the boundry condition of the plte hs four edges clmped. The delminted re of cse1 is 1.5 X 1.5 cm, nd cse2 is 4.5 X 4.5cm. And the mteril properties re shown in tble 1. A comprison of impct force histories between 3-D non-liner model nd 2-D simplified model is shown in Figure 3. As shown from the figures, the numericl results obtined by simplified model provided pproprite result. A comprison of impct force histories with nd without delminted is shown in Figure 4 when the mss rtio (mss of impctor/mss of plte) is 35. nd impct velocity is 5 m/sec.

4 Sung Joon Kim, Dong-Chul Che Exploded view for delminted region C A B Element Node Figure 2 Schemtic of delmintion modeling nd configurtion of delminted re Tble 1 Mteril properties [5] E 1 = 12 GP, E 2 = 7.9 Gp Mteril properties of lmin Mteril properties of impctor G 12 =G 13 =G 23 = 5.5 Gp ν 12 =.3 ρ =1582 kg/m 3 Thickness =.125 mm E = 27 Gp ν =.3 Impct force histories re decresed by the presence of delmintion. This result would be expected becuse pltes with delmintions should be less stiffness thn undmged pltes. The lrger delmintions produced the gretest decrese in impct force histories nd increse in contct time durtion. The difference in sound produced by the two tps my be explined by studying the frequency content of the two force pulses. This is chieved by crrying out Fourier trnsform of the force-time records to produce the power spectrum density spectr shown in Figure 5. The mplitude of power spectrum density input to the delminted structure flls off rpidly with incresing frequency, while the impct on the sound structure hs much lower rte of decrese of PSD with frequency. This mens tht the impct on delminted re will not excite the higher structurl modes s strongly s the impct on the sound zone. Therefore the sound produced does not contin the higher frequencies nd the structure sounds duller.

5 ICSV13, July 2-6, 26, Vienn, Austri Cse 2 : Simplified Cse 2 : Non-liner Contct Force (N) Time (msec) Figure 3 Impct force histories nlyzed using the simplified model nd the 3-D non-liner model Sound Cse 2 Impct Force (N) Time (msec) Figure 4 Impct force histories for cse2

6 Sung Joon Kim, Dong-Chul Che 25 2 Power Spectrum Density Sound Cse 1 PSD 15 1 Cse Frequency (Hz) Figure 5 PSD spectr of time histories shown in Figure 4 Clcultion of coustic pressure The coustic pressure rdited from vibrtion plte cn be obtined by evluting the Ryleigh surfce integrl where ech elementl re on the plte surfce is regrded s simple point source of n outgoing wve nd its contribution is dded with n pproprite time dely. The coustic pressure p(r,t) t the observtion point P with Crtesin coordinte(x,y,z ) t time t cused by the vibrtion of the plte is clculted by using the Ryleigh integrl. ρ p( r, t) = 2π ( S ) 1 r r 2 r r W ( r, t 2 t c ) ds (1) Where ρ nd c re, respectively, the mss density nd wve velocity of the coustic 2 w medium, is the ccelertion time history of the plte obtined from impct 2 t response nlysis, R is the distnce between the observtion point P nd infinitesiml element t (x,y) on the plte surfce. The properties of ir s the coustic medium re density ρ =1.21 kg/m 3 nd speed of sound c =34m/s. Computtionl Results

7 ICSV13, July 2-6, 26, Vienn, Austri To investigte the effect of delmintion on rdited sound, smple cse ws nlyzed when the mss rtio (M I /M P ) is 1.92 nd impct velocity is.5 m/sec. The difference in the coustic sound pressure produced by impct is shown in Figure 6. The sound pressure is clculted t the observtion point. The observtion point is 15 cm bove the center of the plte. Figure 7 shows tht the lrger delmintions produced the decrese in the center frequency of sound pressure. Sound Pressure (P) Sound Cse 1 Cse Time (msec) Figure 6 Sound Pressure Histories 25 2 Sound Cse 1 Cse 2 15 PSD Frequency (Hz) Figure 7 PSD spectr of time histories shown in Figure 6

8 Sung Joon Kim, Dong-Chul Che CONCLUSION The physicl bsis of the coin-tp test hs been demonstrted, the chnge in the sound produced when delminted re is tpped being due to chnge in the force trnsmitted to the structure by the tp. It hs been shown tht this process my be modeled by representing the delmintion s 2-D simplified delmintion. In this pper, comprisons between 3-D non-liner delmintion model nd 2-D simplified model hve performed to investigte the impct response behvior. From the results, the 2-D simplified model ws found to be relible for predicting the impct force histories nd sound pressure. The chief dvntge of the coin-tp method over higher ultrsonic testing is tht no couplnt is needed between the structure nd tpping hed. The technique is therefore prticulrly ttrctive for use in the field. There re two kinds of coin-tp test method. One is bsed on the impct force histories tht produced when the structure is struck. And the other is bsed on sound pressure histories. The first one requires the time histories of the force input to the test structure to be cptured, nd this cn be done vi trnsducer in tpping hed so no trnsducer need be ttched to the structure. The second requires the time histories of the sound tht rdited from the structure. The sound bsed coin-tp test requires the Fourier trnsform of the sound pressure histories to be computed nd compred with stndrd from sound structure. It is shown tht the two kinds of coin-tp test re useful nd prcticl dignostic tool for detecting loclized delmintion in composite lmintes REFERENCES [1] Willims, S. M., nd Smith, J. W., An intelligent tp test s n inspection tool for corrosion in chequer plte floors, Journl of Sound nd Vibrtion, Vol. 257, pp (22). [2] Cwley, P. nd Adms, R. D., The mechnics of the coin-tp method of non-destructive testing, Journl of Sound nd Vibrtion, Vol. 122, pp (1988). [3] Adms, R. D. nd Cwley, P., 1985 in Reserch Techniques in Non-destructive Testing (R. S. Shrp, editor) Vibrtion techniques in non-destructive testing. [4] Shivkumr, K. N., nd Elber, W., Prediction of impct force nd durtion due to low-velocity impct on circulr composite lmintes, ASME Journl of Applied Mechnics, Vol. 52, Sept., pp (1985). [5] Choi, I. H., nd Lim, C. H., Low-velocity impct nlysis of composite lmintes using linerized contct lw, Composite Structures, Vol. 66, pp (24). [6] Kesseler, S.S et l., Dmge detection in composite mterils using frequency response methods, Composites, Prt B Vol.33, pp (22).