68 L.G. Horta et al. 7.2 Composite Subfloor Test Article To take advantage of sections of a composite helicopter fuselage previously tested by Sikosrky, the front frame section was removed from the fuselage and used in our study. Figure 7.1 shows a side view of the entire section as tested by Sikorsky that included the crew access opening with subfloor, front and rear bulkhead frames, and roof with overhead mass that represented the engine. Because parts of the fuselage section were damaged during a prior impact test, this study discusses results using only the front frame section of the fuselage, see Fig. 7.1. An LS-DYNA simulation model, initially developed by Sikorsky, was revised and updated at NASA Langley. It now contains 37,111 nodes, 17,216 Belytschko-Tsay shell elements, 8,300 solid elements; and 2,200 concentrated masses. The model is depicted in Fig. 7.2. Results included in this chapter were obtained using the LS-DYNA Mat 58 material property card to represent the composite layups with Type 2 shell elements. Type 16 fully-integrated shells were used for vibration analysis only because Type 16 shells typically require up to four times the execution time of the Type 2 shells. Prior to switching the simulation to Type 2, simulation runs were completed to verify that the results did not change by using Type 2 shell elements. For Mat 58, some key material properties used in the simulation are listed in Table 7.1. The impact simulation lasted 20 ms, which required about 12 min of CPU to execute on a Windows-based computer with eight processors. 7.3 Time Domain Calibration 7.3.1 Metric 1: Displacement Norm Calibration metrics quantitatively assess the accuracy of an analytical model. The metrics used in this work were initially discussed in [3] and are summarized here for completeness. To contrast these metrics with other approaches, the readers are referred to work by Oberkampf and Barone [4] and Schwer [5], which set forth scalar statistical metrics for use with time history data. Metrics in terms of mean, variance, and confidence intervals facilitate assessment of experimental data. For the current problem, instead of using response predictions at a particular point as a metric, a vector 2-norm (magnitude of vector) of the system response is used instead. An important benefit of using this metric is that it provides for a direct measure of multi-dimensional closeness of two models. In addition, when tracked as a function of time, closeness is quantified at each time step. Model parameters not well known are assumed to be uncertain to evaluate the effect of parameter variations on the system response. By assuming that parameter values are uncertain, statistical measures of the metric can be computed and used to conduct model assessments. To facilitate the sampling process, parameter variations followed a uniform distribution Fig. 7.1 Initial configuration of Sikorsky Composite Helicopter
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