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

Suggested Modifications to the SafeHull ApproachWhere:N ABS = number of cycles for ABS = 20 years x 0.80 x 5 x 10 7N S = number of cycles for actual service = years x percent operability x 5 x 10 7S ABS = Permissible stress range by ABSS 0 = Permissible stress range for actual serviceThe following result is shown in Appendix J for Ship J from the SafeHull Phase Acomputations:SafeHull stiffener #11, Side Longitudinal #17,Computed stress range — 3,013 kg/cm 2 (295 MPa)Weibull long-term distribution factor — 0.928Class F2 detail permissible stress range — 2,443 kg/cm 2 (239 MPa)Ratio of stress range to permissible stress — 1.23 (MPa/MPa)If the ship is to operate for 40 years at 35 percent operability, equation (11.2) gives theallowable stress range to be 2,572 kg/cm 2 (252 MPa) and the ratio of stress range to permissiblestress as 3,013/2,572 = 1.17. In this calculation, the slope, m, for a Class F2 detail is 3.0.Another method to account for years service and percent operability comes fromintegration of the Weibull function. A procedure given by Hughes (1995) divides a stress blockof 5 x 10 7 cycles into 25 blocks, with the stress range for each block, SR i , containing N i cycles,with a Weibull parameter, γ, as⎛ ⎛ ⎞ ⎞⎜ ⎜Niln ⎟ ⎟⎜ ⎜ 7 ⎟ ⎟⎜⎝ 5×10SR⎠⎟i= S0(11.3)7⎜ − ln ( 5×10 ) ⎟⎜⎟⎝⎠where S 0 is the maximum stress range. This procedure is shown in Table 11.1, which is anEXCEL spreadsheet for the computations. The following describes the computations performedin each column:Cycles Exceeded — The exceedance curve for 5 x 10 7 cyclesNumber of Cycles — The number of cycles for each blockStress Range Factor — The fraction of the maximum stress range allocated for the block,computed using equation (11.3)Stress Range — The average stress range for each block using the maximum stress rangemultiplied by the average of the factors from same row and the row above from theprevious columnCycles to Failure Upper — The number of cycles for a linear S-N curve based on thecoefficients A 1 and B 1Cycles to Failure Lower — The lower limit of a bilinear S-N curve based on thecoefficients A 2 and B 2Cycles to Failure (Bilinear) — The maximum of the previous two columns⎛ 1⎞⎜ ⎟⎝ ? ⎠11-3 3