18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS TAPPING TEST AND ANALYSIS FOR DAMAGE DETECTION S. J. Kim 1 , S. M. Ahn 1 , I. H. Hwang 1 , C. H. Hong 2 1 Korea Aerospace Research Institute, Daejeon, Korea, 2 Chungnam National University, Daejeon, Korea * S. J. Kim (yaelin@kari.re.kr) Keywords : Damage, Detection, Tapping, Composite, Impact, Sound 1 Introduction dimensional elements to model the hammer shaped The tapping test has the ability indicating damage in impactor is inconvenient because of a quite number a structural element due to a localized change of of elements necessary to obtain numerical solutions. stiffness [1]. The change in vibration signature may And it is time consuming work if the hammer shape be detected by ear or more precisely by is complicated. So the hammer shaped impactor is measurement instrumentation. In this paper, a simplified by concentrated mass to use spring – tapping test method for discriminating between mass model. Fig.2 shows the FEM model for impact measurements made on undamaged and delaminated response analysis using general – purpose FEM structures is presented. It has been shown that the software [2]. The mass of impactor is lumped at the characteristics of radiated sound from a structure end of the spring mass, and the other end of spring during a tapping are changed by the presence of element is attached to laminate at impacted location. damage in composite laminate. For structurally The equivalent concentrated mass is determined as radiated sound, the sound field is directly coupled to followed procedure. Where R c is the mass center of the structural motion. Therefore, Impact response impactor, θ is rotated angle from neutral position, I 0 analysis should be performed. In this study, the is the mass moment of inertia of impactor with radiated sound induced by tapping is computed by respect to rotation center, v i is the impact velocity of � is the angular velocity of solving the Rayleigh integral equation. And the impact position and θ delamination model is used to analyze the impact impactor. The equivalent impactor mass is computed response analysis of delaminated composite laminate. as followed. Predicted impact force histories and sound pressure 1 � 2 × − θ = θ histories are compared with tapping test data. The m gR ( 1 cos ) I (1) e c 0 2 results of tests and analyses are presented and it is � v = θ × R concluded that the impact force and acoustic i i pressure data can be used to identify the presence of delamination. And it is shown that the presented Where the m e is the equivalent concentrated mass of impactor. analysis model was found to be reliable for predicting the tapping phenomena. 2 Impact response analyses 2.1 Spring-mass model Spring-mass models are simple and provide accurate solutions. The most complete model consists of one spring representing the linear stiffness of contact law and plate element representing the composite laminate. Fig.1 shows the FEM model for impact response analysis for hammer shaped impactor. The hammer shaped impactor is modeled by solid elements and beam elements. The use of three- Fig.1 3-D finite element model for impact analysis
Table 1 Material properties �������������� �������������� ������������� ������������� �������� �������� E 1 = 132.0 GPa, E 2 = 8.0 GPa G 12 =G 13 =G 23 = 3.74 GPa Material properties of ν = 0.3 12 lamina ρ = 16 kg/m 3 Thickness = 0.14 mm E = 207 GPa Material properties of impactor ν = 0.3 70.0 Spring-mass Fig.2 Spring-mass model using general-purpose 3-D FEM 60.0 FEM software 50.0 2.2 Verification of spring – mass model Force (N) 40.0 The configuration of impactor and physical 30.0 properties are shown in Fig.3. Equivalent mass of impactor is 0.092 kg. 20.0 10.0 - Material : Aluminum - Material : Aluminum 0.0 - Impactor mass : 0.1237 kg - Impactor mass : 0.1237 kg 0.0 1.0 2.0 3.0 4.0 5.0 6.0 - Center of mass ( R c ) : 127 mm - Center of mass ( R c ) : 127 mm Time (msec) - Mass moment of inertia ( I 0 ) - Mass moment of inertia ( I 0 ) Fig.4 Comparisons of impact force histories between : 2670 kg mm 2 : 2670 kg mm 2 3D FEM model and simplified spring - model when - Center of percussion ( R i ) - Center of percussion ( R i ) the impact velocity is 0.954 m/sec. : 170.0 mm : 170.0 mm The configuration of type 2 impactor and physical properties are shown in Fig.5. Fig.3 Configuration of type 2 impactor and physical properties - Material : Steel - Material : Steel - Impactor mass : 1.218 kg - Impactor mass : 1.218 kg The analysis model of the laminate is 19.0 × 19.0 - Center of mass ( R c ) : 133 mm - Center of mass ( R c ) : 133 mm cm 2 , and the boundary condition of the plate is four - Mass moment of inertia ( I 0 ) - Mass moment of inertia ( I 0 ) edges clamped. The laminate has a lay-up [0/90] 2s . : 27642 kg-mm 2 : 27642 kg-mm 2 And the material properties are shown in table 1. A - Center of percussion ( R i ) - Center of percussion ( R i ) comparison of impact force histories between detail : 170.0 mm : 170.0 mm FEM model and simplified spring - model is shown in Fig.4. In this case, the impact velocity is 0.954 m/sec. As shown from the figure, the impact force history computed by simplified spring - model Fig.5 Configuration of type 2 impactor and physical provided accurate result. properties
The equivalent mass is 0.956 kg. The type 2 impact force is decreased by the presence of impactor is rotated 15 degrees from vertical line and delamination. The size of laminate is 19.0 × 19.0 cm 2 , and the boundary condition of the plate has four then released. In this case, the impact velocity is 0.478 m/sec. In Fig.6, the impact force history given edges clamped. by simplified spring-mass model and experimental one are shown. 250.0 5 cm 19 cm Analysis Test 200.0 5 cm Force (N) 150.0 19 cm 100.0 50.0 Gap element Shell element 0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Fig.7 Configuration of laminate with a delamination Time (msec) Fig.6 Comparison of impact force histories between test and analysis for Type B impactor for [0/45/0/- 80.0 45/0/-45/0/45/90] s laminate Test 70.0 Analysis 2.3 Delamination model 60.0 To prevent the overlap and penetration, non-linear 50.0 Force (N) finite element analysis has been performed to compute the impact response of graphite/epoxy 40.0 composite specimens subjected to be struck by 30.0 hammer. 4-node shell elements and gap elements were used to model delamination. The gap elements 20.0 are inserted along the delamination interface 10.0 surfaces for preventing penetration of the upper and lower sub-laminates during impact analysis process 0.0 [3]. Fig.7 shows the schematic diagram of the 0.0 4.0 8.0 12.0 16.0 20.0 delamination modeling. Time (msec) Fig.8 Comparison of impact force histories for delaminated laminate between test and analysis 2.4 Comparison of impact force histories between test and analysis A comparison of impact force histories with and 3 Tapping sound analysis without delaminated is shown in Fig.8 when the The acoustic pressure radiated from a vibration plate laminate has a lay-up [0/90] 2s . Type 2 impactor is can be obtained by evaluating the Rayleigh surface used as impactor. Impact response analysis has integral where each elemental area on the plate performed when the impact velocity is 0.337 m/sec. surface is regarded as a simple point source of an From the results, we could know that maximum 3
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