<p>Material mechanical properties such as yield strength and fatigue strength have some uncertainties due to material manufacturing processes.&nbsp; The qualified components have their dimensions inside their dimension tolerances.&nbsp; Load on a component could be varied due to different service conditions.&nbsp; Therefore, design parameters in mechanical component design have typically some uncertainties and should be described by random variables.&nbsp; Thus, reliability instead of a factor of safety is a better way to describe the status of safety of a component.&nbsp; The number one design criterion for mechanical design is that the component must be safe.&nbsp; Since “Reliability of Military Electronic Equipment” was published in 1957 by the Advisory Group on Reliability of Electronic Equipment (AGREE), we all know and accept that to make a component safe, the component must be designed with required reliability.&nbsp; But the implementation of mechanical reliability design theory in mechanical components design is very slow.&nbsp; There are maybe two main reasons: (1) there is a lack of statistical descriptions of material mechanical properties such as mechanical yield strength and fatigue strength &amp; fatigue life; and (2) there is a lack of appropriate mechanical reliability design theory for mechanical component reliability design such as theory for a component under some typical cyclic fatigue load spectrum.</p><p>A thin-wall vessel is a typical mechanical component under multi-axis stress. This paper presents how to implement mechanical reliability design theory on a thin-wall vessel structure.&nbsp; Mechanical reliability design theory typically consists of three key parts: (1) Statistical descriptions of the design variables such as mechanical properties, dimensions, and external loads; (2) A limit state function, which links the design parameters with reliability through a failure mode; and (3) Computational methods for reliability or dimension with required reliability.&nbsp; The critical part is to create a limit state function. &nbsp;It is not difficult to build a limit state function for a thin-wall vessel structure under static loading by the Maximum Shear Stress theory or the Distortion Energy theory. &nbsp;However, for a fatigue design problem, if P-S-N curve fatigue design theory is used, the limit state functions, for some typical cyclic loading such as multiple distributed loads or multiple distributed number of cycles, is hardly established due to a lack of corresponding failure theories.&nbsp; But if the K-D probabilistic fatigue damage theory is used, which was proposed by the author, the limit state function can be established under any type of cyclic fatigue load spectrum.&nbsp; This paper will show how to get approximate estimated statistical descriptions of material’s mechanical properties when they are not available.&nbsp; Then, three general limit state functions for a thin-wall vessel structure under static loads and cyclic fatigue loads with infinite life or finite fatigue life will be established and explained in detail.&nbsp; Finally, four case design examples of a thin-wall vessel structure will be provided.&nbsp; These four case design examples are reliability calculation and dimension with required reliability under static loads or cyclic fatigue loads. &nbsp;The contribution of this paper is to present a complete set of mechanical reliability design theory for a thin-wall vessel structure under static load or under cyclic fatigue loads.&nbsp;&nbsp; This theory can be implemented on other typical mechanical components such as beams, bars, and shafts.</p>

Implementation of Mechanical Reliability Design Theory on a Thin-Wall Vessel Structure

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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