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Structural Analysis

Scope and Description

This topic covers structural analysis of spacecraft hardware – from full spacecraft to subsystems and components. This includes processes used to simulate or otherwise estimate response to vibration, shock, static acceleration, and acoustic environments. The responses of structural elements and the components that they enclose and support (e.g. electronics, optics, sensors) are analyzed to ensure satisfactory margins against mechanical failure. Failure can be caused by material yield, fracture, or displacement beyond operational limits.

Resources under this topic area are primarily references that provide analysis formulas and methods, software tools for finite element analysis (FEA), and standards that provide engineering factors and environmental parameters.

Best Practices and Lessons Learned

  • When developing an FEA workflow, make sure to experiment with simplified models to understand the influence of various modeling options/parameters and to verify that the tool is behaving as expected. CAD embedded FEA software tools like SolidWorks Simulation provide streamlined workflows and enable quick design iteration. However, their “black box” interfaces can lead to significant modeling errors. Use documentation, vendor engineering support, and the testing mentioned above to ensure that your process produces accurate results.
  • FEA shock simulation is notoriously lacking in accuracy. Conservative modeling based on non-FEA analyses (using a spreadsheet or custom software tool) is a good alternative. Steinberg’s book provides the formulas for this type of analysis on electronics. However, consider the assumptions used in these non-FEA analyses that preclude their use on circuit boards with complex geometries and/or mechanical interfaces.
  • Exhaustive, traditional structural analysis is expensive and time consuming. In many cases, limited analyses can be completed before flight hardware design and used to impose design guidelines. Using these guidelines will allow for significantly reduced structural analysis on the completed design.
  • Torque-tension factors for the smaller fasteners used widely in small satellites do not match standard values. This can have a significant effect on fastener stress margins and optimal preload torques. Refer to the article from Hemminger Et al. for empirically derived torque-tension factors for your combinations of fastener/mating thread material and lubrication (if applicable).

Resources

This conference article presents the methods and results of testing to determine the torque-tension relationship ... for various fasteners. Results are presented for various fastener materials, female thread materials, and lubricants. The results for small fasteners are significantly different than standard values and can be valuable to accurate stress/gapping analysis and preload torque assignment for smallsat hardware.

Standard
NASA

Section A.3.2 provides guidance on the attenuation of the shock response spectrum (SRS) with distance ... from the source (i.e. system-level interface) and with the interfaces between the source and the subassembly or component being analyzed.

Section 4 provides the formulas and process for estimating fatigue damage in solder joints for surface ... mount electronic components. This can be used to produce a spreadsheet or software tool to determine fatigue damage ratio (i.e. percent of the way to 50% probability of failure) for solder joints due to thermal cycling.

Section 6 provides formulas and logic for the analysis of threaded fasteners to ensure satisfactory tension, ... shear, and bending margins. This information can be used to produce a spreadsheet or software tool to determine design margins, size fasteners, and set preload torques.

Website
Tom Irvine

This website provides in-depth tutorials and software tools for structural analysis. This includes sections ... on vibration response spectrum, damping, shock and shock response spectrum, random vibration, and many other topics relevant to the structural analysis of space hardware. This is a good place to go to improve your understanding of the fundamental principles and assumptions behind structural analysis methods. Some content can be accessed without a subscription, but a subscription is required for full access.

Software Tool
Dassault Systemes

SolidWorks Simulation is a suite of FEA tools integrated with the SolidWorks computer aided design (CAD) ... tools. This software can be used to complete all the most common structural analyses for smallsats and their components (e.g. modal, random vibration, shock response spectrum, sine, static, fatigue). SolidWorks Simulation can also be used to carry out static and transient thermal analyses. Its integration with a powerful CAD tool streamlines design iteration. However, it is not appropriate for solution of some complex (e.g. high strain-rate) models and its simplified interfaces and workflows mean that some effort is required to fully understand the settings and avoid FEA modeling errors.

Software Tool
Siemens

This is a stand-alone FEA tool that can be used to complete all the most common structural analyses for ... smallsats and their components (e.g. modal, random vibration, shock response spectrum, sine, static, fatigue) along with less common, more complex analyses like acoustic analysis. Nastran can also be used to carry out static and transient thermal analyses. A stand-alone program like Nastran generally requires more experience to understand and use than a CAD-enabled FEA software tool (e.g. SolidWorks Simulation); however, it has more capabilities and provides a more detailed level of direct control and reporting of modeling parameters.

Book
Dave Steinberg

This book provides practical methods for the design and analysis of electronics to survive severe shock ... and vibration environments. The formulas and processes presented here can be used to develop spreadsheets or simple software tools to analyze fatigue damage in component solder joints due to shock and random vibration. Steinberg’s vibration and shock analysis methods are typically restricted to rectangular circuit boards supported at the edges. Non-FEA analysis techniques in this text can often be combined with board-level results from FEA tools to provide EEE component-level reliability metrics.

Article
Julie Fagerudd

This masters thesis covers the structural analysis of a smallsat. It includes background on random vibration ... and shock and how to model a structure's response to these environments. An example structural analysis for a university 3U cubesat is presented in detail. It is a useful case-study and example report for smallsat structural analysis engineers.

White Paper
NASA

This page in the NASA Public Lessons Learned System describes the importance of establishing a "mandatory ... closed-loop system for detailed, independent, and timely technical reviews of all analyses performed in support of the reliability/design process." These reviews are important for detecting design defects.

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