Module 5: Experimental Modal Analysis for SHM Lecture 37: Velocity and displacement measurement. The Lecture Contains: Measurement of Damping

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1 The Lecture Contains: Measurement of Damping Comparison of Results with Standard Finite Element Software ABAQUS for Healthy Laminates file:///d /neha%20backup%20courses% /structural_health/lecture37/37_1.html [4/4/2013 4:02:58 PM]

2 The measurement Laser beam and the reflected Laser beam are made to interfere on the photo detector creating a fringe pattern. An optical sensor system OFV505 detects the changes in the fringe pattern and generates optical interference signal which is fed to the decoders. Decoders generate an analog signal in relation to this input optical interference signal. Measurement of damping A quantitative measure of damping factor is obtained by using the halfpower bandwidth method shown graphically in Figure Figure 37.1 Damping measurement using 3dB method The damping factor, can be determined by (37.1) is determined from the half power points down from the resonant peak value, X max. On a decibel scale, this corresponds to 3dB down from the peak value. Hence, is also referred as 3 db method. file:///d /neha%20backup%20courses% /structural_health/lecture37/37_2.html [4/4/2013 4:02:58 PM]

3 Sample preparation Glassepoxy composite plates with dimensions 270 mm x 200 mm are fabricated for experimental modal analysis. Laminated plates with three different ply orientations: all zero plies [0/0/0/0], all 90 plies [90/90/90/90] and a cross ply laminate [0/90/0/90] are used. All the plates are prepared using unidirectional glassepoxy mats by hand layup technique in the laboratory. Material properties of the glassepoxy composite laminate used in the experimental modal analysis are listed in Table Table 37.1 Material properties of glassepoxy laminate Composite Elastic modulus, E 1 Elastic modulus, E 2 Glassepoxy GPa 2.14 GPa Shear modulus, G GPa Poisson's ratio, 0.24 Density, 1670 kg/m3 file:///d /neha%20backup%20courses% /structural_health/lecture37/37_3.html [4/4/2013 4:02:58 PM]

4 Specimen with induced delamination Dynamic responses for healthy and delaminated composite plates are experimentally recorded for damage detection. Delamination is introduced by placing a square Teflon sheet (50mm x 50mm) in the laminate. Two different locations have been chosen for delamination in the laminated plates. In the first case Teflon sheet is placed at a distance of 50 mm from the clamped end. In the second case, it is placed at a distance of 150 mm from the clamped end. Teflon sheets are placed at top edge of the plate in both the cases to simulate delamination patterns of Delam TypeI and TypeII [Figure 37.2]. Figure 37.2 Composite plates with induced delaminations (Delam TypeI and Delam TypeII) file:///d /neha%20backup%20courses% /structural_health/lecture37/37_4.html [4/4/2013 4:02:59 PM]

5 Sample preparation For both the cases, delaminations have been introduced in two different layers of the 4ply laminates. The laminates with delaminations in the middle layer (i.e. II interface) are designated as Type A and those with delamination in the offset layer (i.e. I interface as shown in Figure 37.2) are designated as Type B. The experiments are conducted on these composite plates in fixedfree boundary condition (more precisely CFFF). Dynamic excitation is given at the center of the plate by an electrodynamic shaker. Laminated composite plates used in experimental modal analysis are detailed in Table Table 37.2 Laminated composite plates used in experimental modal analysis S.No. Ply configuration Type 1 [0/0/0/0] Healthy Delam TypeI A & B Delam TypeII A & B 2 [90/90/90/90] Healthy Delam TypeI A & B Delam TypeII A & B 3 [0/90/0/90] Healthy Delam TypeI A & B The experiments are carried out on Polytec Scanning (PSV) Vibrometer in 3D scan mode. The excitation details are provided in Table 37.3 Table 37.3 Detalis of the excitation signal used in 3D scan mode Excitation signal pseudorandom Frequency Range Hz FFT lines 1600 Window Rectangle Averages 10 (complex) file:///d /neha%20backup%20courses% /structural_health/lecture37/37_5.html (1 of 2) [4/4/2013 4:02:59 PM]

6 Velocity Decoder VD08 Digital decoder (velocity range 010 mm/s) file:///d /neha%20backup%20courses% /structural_health/lecture37/37_5.html (2 of 2) [4/4/2013 4:02:59 PM]

7 Comparison of Results with Standard Finite Element Software ABAQUS for Healthy Laminates Finite element modeling of healthy composite plates is carried out using a standard finite element software package ABAQUS. Conventional solid element is extruded with composite layup to model the composite plate. A fine mesh size of mm is used for accurate prediction of modal frequencies. First four modal frequencies are obtained through experimental modal analysis carried out on healthy laminates of different orientations. The comparisons between the experimental and numerical analyses results for the representative modes are presented in Table Table 37.4: Comparison of EMA and ABAQUS results Laminate Mode type B1(Hz) Tl(Hz) B2(Hz) B22(Hz) B32 (Hz) Experiment Healthy 0 o plies ABAQUS B02 (Hz) Healthy 90 o plies Experiment ABAQUS Healthy 0 o /90 o plies Experiment ABAQUS A standard FEM Package ABAQUS' is used for modal analysis of healthy composite plates to verify and validate the results of experimental modal analysis. A fairly good agreement in natural frequencies for different modes is seen for various ply orientations. The modal frequencies predicted by ABAQUS for different orientations of composite laminates are on the lower side in comparison to the experimental results for all the modes. The agreement in results is very good in general for higher frequency modes. Prediction of first modal frequency is in close conformity with the experimental results for all 90 0 plies and cross ply laminates while for all 0 0 plies, the results are somewhat conservative in comparison to the experimental results. file:///d /neha%20backup%20courses% /structural_health/lecture37/37_6.html [4/4/2013 4:02:59 PM]