Table 6: POC Matrix for the FEM and the 250 fps Oblique Impact Test Pseudo-Orthogonality Check 3D Point-Tracking 25.94 63.94 78.19 98.69 FEM 26.03 .9999 .0000 .0392 .0000 70.69 .0240 .9984 .0000 .0010 77.68 .0221 .0000 1.0008 .0000 108.8 .0339 .0669 .0000 1.0000 In all correlation studies, including accelerometer and laser Doppler vibrometer test results, there is at least a 5% difference between the finite element model and the experimental results for the in-plane modes. The stereophotogrammetry oblique impact data show even more deviation – the differences for modes 2 and 4 are 9.55% and 9.29%, respectively. To confirm that these natural frequencies are consistent with another reference, the 3D point-tracking frequency response functions and poles were compared to the results obtained from collocated tri-axial accelerometers that measured data simultaneously during the oblique testing. The out-of-plane motion measured at points 1 and 3 are compared in Figures 4 and 5, respectively. At the peaks, the two measurement types agree very well. The curves for the stereophotogrammetry FRFs are not as smooth as those from the accelerometers, indicating a lower signal to noise ratio. This is even more apparent for the in-plane measurements shown in Figures 6 and 7. In addition to visually inspecting the FRFs, the poles for the 3DPT and accelerometers were compared. Table 7 compares the poles from the average 3D point-tracking linear input spectra and those from the FRFs from the tri-axial accelerometers at points 1 and 3 (top corners). The maximum difference between any two corresponding poles is 1.45%, so the 3DPT and accelerometer results are consistent. Furthermore, the accelerometer and LDV results were obtained as part of a previous study over the course of 2008 and 2009, while the 3DPT experiments were run in March, 2010. Though there are differences, the results are acceptable when considering the amount of time between the tests; small variations such as these are reasonable considering the length of time between tests and the nature of the bolted configuration used for these tests. Table 7: Comparison of 3DPT and Accelerometer Poles for the Oblique Impact Test 3D Point-Tracking Poles (Hz) Tri-axial Accelerometer Poles (Hz) Diff. (%) 25.94 25.65 1.11 63.94 64.44 -0.78 78.19 77.78 0.52 98.69 97.26 1.45 248
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