32 Modeling and Verifying the Dynamic Response of Layered Plate Damping Systems 253 Fig. 32.12 Numerical versus experimental frequency response functions for configuration five bolt pattern two 32.5.2 Experimental Analysis We conduct our experimental analysis by first determining the factors influential to our responses. Then, we display the data graphically based on the critical factors. We finish our analysis with a regression using Minitab. We determine the importance of each factor in predicting each of the eight measured responses with a full factorial design in Minitab. Figures 32.15 and 32.16 display the contribution of each of the factors as well as the 2- and 3-factor interactions in predicting the modes’ frequency and damping. Figure 32.15 details that only bolt spacing, total thickness, and the number of interfaces significantly affect the natural frequencies, while Fig. 32.16 shows that all factors except thickness significantly influence the modal damping. Both figures indicate that the bolt spacing is the most important factor in both natural frequency and modal damping models. The thickness’s lack of influence on the modal damping indicates that material damping is negligible and reinforces the assumption that the bolted connection provides most—if not all—the damping present in these systems. The bolt preload’s negligible effect on natural frequency disagrees with past findings that the natural frequency should asymptotically increase with preload. This discrepancy can be explained if the two preloads tested herein translated to frequencies already close to the asymptote to which the natural frequency would converge with large amounts of preload. The impact amplitude’s lack of contribution suggests the natural frequency is linear in this impact range for these systems. The figures also demonstrate the significance of factor interactions for both natural frequency and modal damping. It is to be noted that the modal damping factor interactions are much greater than those for natural frequency.
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