Damage detecton n composte lamnates usng con-tap method S.J. Km Korea Aerospace Research Insttute, 45 Eoeun-Dong, Youseong-Gu, 35-333 Daejeon, Republc of Korea yaeln@kar.re.kr 45
The con-tap test has the ablty to ndcate damage n a structural element due to a localzed change of stffness. The change n vbraton sgnature may be detected by ear or more precsely by measurement of the dynamc contact force. In ths paper, a con-tap test method for dscrmnatng between measurements made on sound and delamnated structures s present. It has been shown that the characterstcs of contact force hstory durng a tap are changed by the presence of defect beneath the surface of the structure. For structurally radated nose, the sound feld s drectly coupled to the structural moton. Therefore, Impact response analyss should be performed. And the delamnaton model s used to analyze the mpact response analyss. Expermental results for delamnated composte structure are presented and correlatons between smulaton and measured data are shown. It s shown that con-tap test s useful and practcal dagnostc tool for detectng localzed delamnaton n composte lamnates. 1 Introducton The use of lamnated composte structures has many potental applcatons n a varety of engneerng felds. However, they are mostly used n a lamnated form wth relatvely weak nterlamnar nterfaces whch are vulnerable to transverse loads such as mpacts arsng from a fallng mass. The presence of delamnatons can cause sgnfcant degradaton of the structural response characterstcs [1]. The con-tap test s one of the oldest methods of nondestructve testng and t s regularly used for testng lamnated structures. The test requres an operator to tap each pont of the structure to be nspected wth a con, and lsten to the resultng sound radated by structure. When a structure s struck wth a hammer, the characterstcs of the mpact are dependent on the local mpedance of the structure and on the hammer used. Damage such as an adhesve dsband and fatgue damage results n a decrease n structural stffness, and hence a change n the nature of mpact []. Adams showed that t s possble to produce a verson of the con-tap test whch depends on the measurement of the force nput to the test structure durng the tap [3]. There are two knds of con-tap test method. One s based on the mpact force hstores that produced when the structure s struck. And the other s based on sound pressure hstores. The frst one requres the tme hstores of the force nput to the test structure to be captured, and ths can be done va transducer n tappng head so no transducer need be attached to the structure. The second requres the tme hstores of the sound that radated from the structure. The sound based con-tap test requres the Fourer transform of the sound pressure hstores to be computed and compared wth a standard from a sound structure. It s shown that the two knds of con-tap test are useful and practcal dagnostc tool for detectng localzed delamnaton n composte lamnates. In ths paper, an mpact force based non-destructve method s proposed. Ths method s usng the dfference between measured mpact force hstores of healthy structure and delamnated structure. For structurally radated nose, the sound feld s drectly coupled to the structural moton. Therefore, mpact response should be analyzed. Generally, the fnte element method on the mpact response of the lamnate has been known to requre long computaton tme. Shvakumar et al. dd not use the fnte element method to analyze the mpact response of the lamnate but used the sprng-mass model to effcently predct the mpact force hstory [4]. In ther study, the contact energy due to local ndentaton as well as transverse shear energy and bendng energy of plate are consdered, and they reported that the contact energy can be neglected n the mpact by relatvely low velocty foregn object on a flexble or thn plate. However, ther study was restrcted to crcular lamnates wth transversely sotropc materal propertes. Cho proposed the sprng element model usng lnearzed contact law. In ther study, they have shown that the lnearzed contact law approach could be appled to low-velocty mpact response analyss problem wth usng general purpose FEM software [5]. In ths study, to use the sprng-mass model, hammer shaped mpactor s modeled by concentrated mass. And to nvestgate the mpact response on delamnated lamnates, gap elements are used to avod the overlap and penetraton between the upper and lower sub-lamnates at delamnaton regon. The use of three-dmensonal elements to predct the mpact response of delamnated composte structures s nconvenent because of a qute number of elements necessary to obtan numercal solutons. In ths study, -D fnte elements are used n delamnated area. In ths study, from the mpact response analyss results, sound pressure s computed. Impact response analyss Fg.1 shows the FEM model for mpact response analyss usng general purpose FEM software. The mass of mpactor s lumped at the end of the sprng mass, and the other end of sprng element s attached to lamnate at mpacted locaton. After FEM analyss we can extract the compressve force hstory actng at the sprng. In present study, MSC/NASTRAN was used as general-purpose FEM software. In FEM modelng 4-node plate element was used and transent dynamc analyss was performed wth the ntal condton, whch s that ntal speed of the lumped mass was loaded as mpact velocty. Fg. 1 Sprng - mass model usng general-purpose FEM software.1 Smplfed sprng mass model Fg. shows the FEM model for mpact response analyss for hammer shaped mpactor. The hammer shaped mpactor 46
s modeled by sold elements and beam elements. The use of three-dmensonal elements to model the hammer shaped mpactor s nconvenent because of a qute number of elements necessary to obtan numercal solutons. And t s tme consumng work f the hammer shape s complcate. So the hammer shaped mpactor s smplfed by concentrated mass to use sprng mass model. The equvalent concentrated mass s determned as followed procedure. 1 Rc snθ tanθ = I & θ (1) v = & θ R Where R c s the center of mpactor, θ s rotated angle from neutral poston, I s the mass moment of nerta of mpactor wth respect to rotaton center, v s the mpact velocty of mpact poston and θ & s the angular velocty of mpactor. The equvalent mpactor mass s computed as followed. 1 & 1 θ mev () I o = Where m e s the equvalent mass of mpactor. The analyss model of the lamnate s 19 19 cm, and the boundary condton of the plate has four edges clamped. The lamnate has a lay-up [/9] s. And the materal propertes are shown n table 1. A comparson of mpact force hstores between detal FEM model and smplfed sprng - model when the mpact velocty s.954 m/sec s shown n Fg.4. As shown from the fgure, the mpact force hstory obtaned by smplfed sprng - model provded accurate result. Materal propertes of lamna Materal propertes of mpactor Table 1 Materal propertes E 1 = 13 GPa, E = 8. GPa G 1 =G 13 =G 3 = 3.74 Gpa ν 1 =.3 ρ =16 kg/m3 Thckness =.14 mm E = 7 Gpa ν =.3 7 6 Sprng-mass Detal model 5 4 3 Fg. Detal fnte element model of mpactor. Verfcaton of sprng mass model The confguraton of mpactor for type 1 and physcal propertes are shown n Fg.3. Equvalent mass s.9 kg that computed by Eq. (). 1. 1.. 3. 4. 5. 6. Fg. 4 Comparson of mpact force hstores between detal model and smplfed sprng mass model - Materal : Alumnum - Impactor mass :.137 kg - Center of mass (R c ) : 17 mm - Mass moment of nerta (I ) : 67 kg mm - Center of percusson (R ) : 17. mm.3 Comparson wth expermental result To compare the expermental result, a pendulum type tappng test system set up by the author s used. Fg.5 shows the schematc dagram of tappng system and test fxture. The test model of the lamnate s 19 19 cm, and the boundary condton of the plate has four edges clamped. The lamnate has a lay-up [/45//-45//-45//45/9] s. In Fg. 6, the mpact force hstory gven by smplfed sprngmass model and expermental one are shown. The type 1 mpactor s rotated 3 degrees upward from vertcal lne. and then released. In ths case, the mpact velocty s.954 m/sec. Fg.3 Confguraton of mpactor for type 1 and physcal propertes 47
The confguraton of mpactor for type and physcal propertes are shown n Fg.7. Equvalent mass s.956 kg that computed by Eq. (). In Fg. 8, the mpact force hstory gven by smplfed sprng-mass model and expermental one are shown. The type mpactor s rotated 15 degrees from vertcal lne. and then released. In ths case, the mpact velocty s.478 m/sec. 5 Analyss Test 15 1 5 16 Fg. 5 Test fxture and tappng system Analyss Test.. 4. 6. 8. Fg. 8 Comparson of mpact force hstores between expermental result and smplfed sprng mass model for type mpactor 1 8 4 3 Delamnaton model To prevent the overlap and penetraton, non-lnear fnte element analyss was performed usng MSC/NASTRAN to determne the mpact response of graphte/epoxy composte specmens...5 1. 1.5..5 Fg. 6 Comparson of mpact force hstores between expermental result and smplfed sprng mass model for type 1 mpactor W = 19 cm L= 19cm - Materal : Steel 3 x 3 mm - Impactor mass : 1.18 kg - Center of mass (R c ) : 133 mm - Mass moment of nerta (I ) : 764 kg mm - Center of percusson (R ) : 17. mm Fg.7 Confguraton of mpactor for type and physcal propertes Fg.9 Confguraton of lamnate wth a delamnaton and gap element stffness curve used to connect shell element 48
4-node shell elements and gap elements were used to model delamnaton. The gap elements are nserted along the delamnaton nterface surfaces for preventng penetraton of the upper and lower sub-lamnates durng mpact analyss process. Fg.9 shows the schematc of the delamnaton modelng. A comparson of mpact force hstores wth and wthout delamnated s shown n Fg.1 when the lamnate has a lay-up [/9] s. And type 1 mpactor s used. Impact response analyss has performed when the mpact velocty s.954 m/sec. From the results, we could know that maxmum mpact force s decreased by the presence of delamnaton. Ths result would be expected because plates wth delamnatons should be less stffness than undamaged plates. The larger delamnatons produced the greatest decrease n mpact force hstores and ncrease n contact tme duraton. 6. 5. 4. Sound Delamnated References [1] Km, S. J., and Hwang. I. H., "Predcton of fatgue damage for composte lamnate usng mpact response ", Internatonal Journal of fatgue 8, 1334-1339 (6) [] Cawley, P. and Adams, R. D., The mechancs of the con-tap method of non-destructve testng, Journal of Sound and Vbraton, Vol. 1, 1988, pp. 99-316 [3] Adams, R. D. and Cawley, P., 1985 n Research Technques n Non-destructve Testng (R. S. Sharp, edtor) 33-36. Vbraton technques n nondestructve testng. [4] Shvakumar, K. N., and Elber, W., Predcton of mpact force and duraton due to low-velocty mpact on crcular composte lamnates, ASME Journal of Appled Mechancs, Vol. 5, Sept. 1985, pp. 674-68. [5] Cho, I. H., and Lm, C. H., Low-velocty mpact analyss of composte lamnates usng lnearzed contact law, Composte Structures, Vol. 66, 4, pp. 15-13 3.. 1... 1.. 3. 4. 5. 6. 7. Fg. 1 Comparson of mpact force hstores between wthout and delamnaton 4 Concluson In ths paper, the physcal bass of the con-tap test has been demonstrated. And the smplfed sprng mass model has been proposed to model hammer shaped mpactor. From the test result, we could know that smplfed sprng - mass model provdes the approprate soluton. The numercal results show that the change of maxmum mpact force s related to stffness reducton. And a delamnaton model s used to numercally explore the effect of delamnaton. The numercal results show that there s strong correspondence between the extent of delamnaton (or local stffness reducton) and reducton n mpact force. The fnal goal of ths research s to provde useful tool for detectng the damage n composte structures. 49