Structural Safety Under Conditions of Ultimate Load Failure and Fatigue

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Report Number: WADD TR 61-177
Author(s): Shinozuka, M., Freudenthal, A. M.
Corporate Author(s): Columbia University
Laboratory: Directorate of Materials and Processes
Date of Publication: 1961-10
Pages: 123
DoD Task:
Identifier: AD0272390

The purpose of this investigation is to analyze the concept of the safety of structures subject to operational loads that cause fatigue damage as well as to occasional excessive overloads that might produce ultimate load failure. In Part I the relation between probability of failure and the reliability or the safety factor is discussed. Diagrams have been computed under the assumptions that the statistical variations of load and carrying capacity are expressed either by log-normal or by extremal distributions. The safety of multiple load-path structures, the probability of failure of simple structures under combined (primary and secondary) loads are also considered and the use of separate load factors for dead and live load is related to the concept of a single safety factor. Part II deals mainly with the statistical properties of fatigue life distributions. Assuming a statistical-mechanical model for the fatigue mechanism, a new distribution of fatigue lives is derived. The concept of stress-interaction established in previous experimental research is used to reproduce the survivorship functions under random loading from the known survivorship functions associated with constant stress amplitude fatigue. In Part III the risks of ultimate load and fatigue failures are combined and the reliability of aluminum specimens (AA 2024 Al) under both operational loads and occasional excessive overloads is investigated considering the interrelation with the risk-functions. The procedure is illustrated by a numerical example in which the truncated part of an exponential load spectrum is applied as operational (fatigue) loading while the rest of the spectrum produces the overloads.

Provenance: Lockheed Martin Missiles & Fire Control

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