18 Operating Mode Shapes of Electronic Assemblies Under Shock Input 183 Fig. 18.7 Sum of effective modal mass in the out of plane direction (w.r.t. the PCB) for an extended number of mode shapes. This chart is a summary of six cases each with a geometry identical the PCB studied in this paper, but using slightly different material properties Fig. 18.8 (a) Comparison of base excitation measured underneath PCB on fixture. (b) Frequency domain representation of time history shown in (a) 18.3.1 Experimental Validation of Model Model validation using explicit dynamic simulations and implicit modal analysis was performed. First the acceleration measured underneath the PCB on the aluminum fixture was studied. A thorough understanding of the friction between the plates of the fixture was required to accurately model the acceleration of the fixture. Figure 18.8 illustrates the similarity between experiment and simulation in both the time and frequency domain. For the purposes of model validation the results are deemed acceptable for the intended use of the model. Quantitative mode shape correlation proved difficult due to the limited sensing ability of the single point velocity measurement. The frequency content of the time history shown in Fig. 18.3a has dominant peaks that match with predicted mode shapes, but the predicted modes seem unlikely to physically be observable using our current measurement
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