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TCU Structural Analysis

This document outlines the structural analysis conducted on the TCU chassis, focusing on vibration compliance and fatigue lifetime for components. The analysis adheres to safety factors and emphasizes the fatigue life of leads and solder joints, as well as the shock sensitivity of components. Using ANSYS for finite element analysis, natural frequencies were determined to ensure mission requirements are met. Despite challenges with the CPU circuit board under vibration loads, several design modifications are proposed to enhance performance and mitigate risks. Further analysis and testing will verify findings.

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TCU Structural Analysis

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  1. TCU Structural Analysis Brent R. Lee Southwest Research Institute (210) 522-6093 brent.lee@swri.org

  2. Agenda • Requirements Compliance Table • Analysis Approach and Assumptions • Natural Frequencies • Results • Summary

  3. Requirements Compliance Table

  4. Structural AnalysisApproach and Assumptions • Analysis addresses three areas • Fatigue lifetime of component leads and solder joints • Shock sensitivity of components • Safety critical structures • Chassis structurally similar to previously qualified hardware (DI PIA, SWIFT XEP, OE SCA) • Fasteners are preloaded to 65% of material yield strength • Factors of safety: 1.25 on yield and 1.4 on ultimate • ANSYS (finite element analysis) used to determine natural frequencies of circuit board assemblies • Use board natural frequencies in component fatigue lifetime calculations based on Steinberg approach

  5. Chassis Qualification DI PIA OE SCA WIPER TCU

  6. Chassis Qualification (cont.) • No qualification problems anticipated • WIPER environment more severe, but lower enclosure height gives improved ruggedness • Chassis to be modeled mathematically and vibration tested OE SCA Finite Element Model

  7. Circuit Board Models • Typical WIPER VME module details • First natural frequency: 187 Hz • Ecircuit board = 3 x 106 psi, Estiffener = 10 x 106 psi • Simple and fixed supports at connectors and card retainers • Mass evenly distributed across entire module

  8. Circuit Board Natural Frequencies • Summary of WIPER module 1st mode frequencies • Desired frequency indicates minimum natural frequency needed for worst case component to meet mission fatigue requirements

  9. Circuit Board Natural Frequencies (cont) • CPU circuit board fails to meet WIPER random vibration requirements • Planned to reuse previously qualified board design with minimal changes (low-risk, low-cost) • Current configuration does not meet WIPER load environment • High peak PSD levels in random vibration spectrum (1.4 g2/Hz compared to 0.2 g2/Hz on previous program where board was used) • TCU currently located on spacecraft panel with most severe shock and random vibration environment

  10. Possible Solutions • Three suggested options to overcome this problem • Add cross-shaped stiffener to CPU circuit board • Requires modifying layout of CPU and other modules • Increases natural frequency of CPU board to 283 Hz (249 Hz required) • Modify the current chassis design in order to reorient modules (to stand on their ends) • Relocate the entire TCU to a different spacecraft panel

  11. Summary • TCU chassis meets all design requirements • Further analysis and testing will be used for verification • TCU circuit boards not able to meet current random vibration loads • Not confident that components on CPU board will meet fatigue lifetime requirements due to high peak PSD levels • Several alternatives are available to mitigate this problem

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