A Study To Develop An Electronics Chassis Compound Cylinder Pressure Vessel Using Finite Element Modeling
Straley, Gordon Randall
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This thesis develops a process to design and analyze a two-layer compound cylinder pressure vessel utilizing an inner insert to house electronic circuit card assemblies. The process begins with sizing and analysis of a compound cylinder based on analytical formulas and progresses in complexity to a 2-Dimensional plane stress finite element model of the chassis assembly and ends with a computationally expensive 3-Dimensional finite element analysis of the pressure vessel. Comparing the results from the compound cylinder to the electronics chassis, it is observed that the inner insert geometry has a strong influence on the interfacial pressure at the upper circuit card assembly slot location. The values for the maximum contact pressure are approximately 125% higher when compared to a compound cylinder without an insert. However, the average contact pressure between the outer shell and inner insert is only 2% to 6% higher than the baseline compound cylinder. This difference is captured in a term deemed the Pressure Intensity Factor (PIF). This high-pressure region affects the stress values in both components which is captured in a Stress Concentration Factor (SCF) based on the equivalent stress values. The shell SCF values range from 1.4 to 1.6 for the 3D compound cylinder and the 2D plane stress electronics chassis models. The insert SCF values range from 5.6 to 6.8 for the same models. Both component SCF values increase in the 3D electronics chassis models. The thesis demonstrates that thin-walled cylindrical pressure vessels primary failure mode is buckling and that the inclusion of the interference fit insert increases the depth rating of the assembly by a factor of 8.0. The thesis employs the concept of margin of safety as the design pass/fail criteria and illustrates that the margins decrease as the electronics chassis stress values increase with the fidelity of the finite element models. The thesis illustrates that ending the analysis with a computationally inexpensive 2-Dimensional plane stress finite element analysis model may result in a failing pressure vessel design.