Detailed dynamic stress analysis of complex assemblies
NX I-deas and Nastran allow accelerated solutions to challenging analyses
With a critical design review of a highly complex seeker assembly approaching, an efficient method to validate the design under severe dynamic environments was required.
The Theater High Altitude Area Defense (THAAD) missile system is a transportable defensive weapon that uses precision targeting imagery to protect against hostile incoming threats such as tactical and theater ballistic missiles. BAE Systems is responsible for providing the infrared (IR) seeker subsystem containing an infrared detector, stabilized on a two-axis integrated gimbal assembly (IGA). During operation and transportation, the seeker is subjected to severe static, random vibration, shock and thermal loads. With no margin for failure in the qualification test program, BAE Systems asked ATA Engineering, Inc. (ATA) to analytically qualify the IGA structural design for all load cases and identify any required design modifications before manufacturing.
Solving challenging structural analysis problems with superelements
One of the greatest challenges in the analysis of complex assemblies is the trade-off between model fidelity and solution time. In the case of the IGA, the dynamic response is highly sensitive to the geometric features of the parts and how they connect to one another. This eliminated the possibility of less detailed models or part-by-part analyses providing accurate predictions of the dynamic loads and stresses. To balance the need for accurate stress predictions and reduced solution time, a superelement analysis approach was used. A system dynamic model of the entire assembly was created that included detailed finite element models of all the components. An efficient modal solution of the assembly was achieved by representing individual subassemblies as superelements in the solution. The system modal response was then used in a forced response analysis to predict the dynamic stresses on the components under the various random vibration and shock loads. The modularity and efficiency of the solution allowed design issues to be identified sufficiently early to implement improvements in a cost effective manner, and helped BAE Systems deliver the first unit one week ahead of schedule.
Increasing accuracy, decreasing schedule
The IGA components were approaching final detailing, and comprehensive stress analysis was required to validate each of the designs. Complex part geometries, and components whose dynamic response is very sensitive to how they connect to one another, meant that part-by-part analysis with assumed boundary conditions at connector locations would introduce too many unknowns to allow credible, representative results to be generated within the available time.
To reduce schedule and remove uncertainties associated with the connection boundary conditions, a detailed stress model of the entire assembly was built with all connections accurately represented. Detailed finite element models of each of the parts were created in NX™ I-deas™ MasterFEM software and assembled into a detailed system model. Superelement models of each of the subassemblies were created and exported to Nastran for modal analysis. System natural frequencies were predicted using the superelement results. By using a super-element approach, the effects of individual component design changes on the overall system response could also be rapidly evaluated without having to reperform a complete solution for the entire assembly.
Nastran modal results were imported into NX I-deas Response Analysis for forced response analysis. By using a forced response approach, a single set of system modal results could be used to predict all component responses under any random, shock or transient input, greatly reducing analysis time. NX I-deas Advanced Durability module was used to evaluate the life of each of the parts based on the dynamic stresses predicted by NX I-deas Response Analysis. Loads at critical interfaces were automatically extracted and exported to a spreadsheet where detailed connector margin of safety calculations could be performed.
Powerful tools lead to powerful solutions
By using a superelement approach and powerful pre- and postprocessing tools, it was possible to exceed all of BAE Systems’ technical and schedule requirements. Critical design modifications were identified and implemented quickly, while allowing sufficient schedule to analyze and verify the updated design. The final design was modeled and verified for more than 20 distinct load cases in less than eight weeks. The exacting analysis approach provided a level of credibility that led to unanimous acceptance of the structural design at the critical design review.