ssc-367 - Ship Structure Committee

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life of a structure,cannot be defined bya closed-formmathematical
function. Most often a numerical long-termdensity function of the
stress range is used to determine the fatigue damage and the method
is identifiedas the “long-termnumerical method”. If the long-term
stress range density function is idealized, an approximate density
function can be used. “Weibull” distribution is one commonly
accepted shape parameter used to describe the long-term stress
density function. The fatigue damage computed is closed
form. Incorporating the Weibull shape parameter is generally
referred to as the “Long-TermClosed-FormMethod”.
6.2.3 Uncertainties and Gaps in Stress SDectrum Develo~ment
There are several important variables contribut
uncertainties in the development of the spectrum.
ng to the
Analysis assumptions substantially influence the calculated
results. The most important of these is the selection of scatter
diagram blocks. While atypical scatterdiagram has40t060 blocks
(each representingthe joint probabilityof Hs and T), these blocks
are often arbitrarily grouped into 10 to 15 super blocks to
facilitate analyses. In addition to the uncertainties introduced
dueto lumpingof these blocks,validityof Rayleighdistributionis
also jeopardizeddue to limitednumber of blocksdefining the entire
environment.
Other analyses uncertainties result from the use or omission of
various parameters (rainflow counting, Weibull d stribution) and
their validity for the problem at hand.
Work carried out by various investigatorshave he” ped enhance the
reliability of spectral fatigue analysis. Chen and Maurakis
(Reference6.10) offer a close form spectralfatigue analysesmethod
that eliminates some of the uncertainties due to analyses
assumptions and computational procedures. The computer program
developed, incorporating the self-contained algorithm, appears to
minimize the uncertainties due to analytical assumptions (i.e.,
6-12