ssc - 419 supplemental commercial design guidance for fatigue ship ...

Glossary of TermsClassification—The process of establishing and administering standards or Rules. Those seeking classificationmust adhere to these Rules to gain class.Fatigue—Failure of material from repeated cyclic application of loads. The number of load cycles in thelifetime of a ship is 10 8 or greater.Fatigue Loading Spectrum—A representation of the random amplitude loads applied to ship structure from shipoperations in a random seaway. A loading spectrum is generally characterized by an exceedance curve,which has the load on the ordinate, and the number of times a load of that magnitude is exceeded duringthe lifetime of the ship on the abscissa. In a semi-log curve, with the abscissa a logarithmic scale, theexceedance curve is close to a straight line (an exponential distribution), or has a slight curvature toapproximate a Weibull distribution.Linear Cumulative Fatigue Theory—A theory of fatigue failure for loading of variable amplitude. The amountof fatigue damage by all the stress cycles of a particular stress amplitude is the fraction of the number ofcycles at that amplitude to the number of cycles to cause failure under constant amplitude loading of thatstress amplitude.Response Amplitude Operator (RAO)—Also known as Frequency Response Functions (FRF) or transferfunctions. The amount of response to a unit wave height of some hull response parameter, such asbending moment. The value of the response is determined over the range of all anticipated waveencounter frequencies.Sea Spectrum—A characterization of the randomly occuring waves at a particular loacation at a particular time.Paramaters used include the low-frequency and high frequency of the significant wave height, modalfrequency, and a shape parameter.Sea State—A characterization of the amplitude of waves at a particular location at a particular time. Sea statesare generally characterized by the average of the one-third highest waves. There are several standardtables of probability that the wave height will be within a given range at a particular location in theocean.S-N Curve—A graphic representation of the results of a number of fatigue tests on identical specimens testedunder repeated loading at the same amplitude of stress. The number of loading cycles (N) is plotted onthe abscissa, and the amplitude of the applied stress range (S) is plotted on the ordinate. When the S-Ncurve is plotted on a logarithmic scale, the shape of the curve through the data points tends toapproximate a straight line.Weibull Distribution—A random distribution with a cumulative distribution function of:F S (s) = P(S ≤ s) = 1 – exp[-(s/S m ) ξ ] ln (N T )Where:S is the stressS m is the design stressN T is the number of times in the lifetime that the stress S m is exceededξ is the Weibull shape parameter, generally ranging between 0.7 and 1.3 for hull girder waveloading.6