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TCU Structural Analysis. Brent R. Lee Southwest Research Institute (210) 522-6093 brent.lee@swri.org. Agenda. Requirements Compliance Table Analysis Approach and Assumptions Natural Frequencies Results Summary. Requirements Compliance Table. Structural Analysis Approach and Assumptions.

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## TCU Structural Analysis

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**TCU Structural Analysis**Brent R. Lee Southwest Research Institute (210) 522-6093 brent.lee@swri.org**Agenda**• Requirements Compliance Table • Analysis Approach and Assumptions • Natural Frequencies • Results • Summary**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**Chassis Qualification**DI PIA OE SCA WIPER TCU**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**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**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**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**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**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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