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Fatigue Limit-State Design in .NET Creator QR Code 2d barcode in .NET Fatigue Limit-State Design




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Fatigue Limit-State Design use vs .net denso qr bar code generation toprint qr code iso/iec18004 with .net Microsoft .NET Micro Framework 5 years for extensive i qr barcode for .NET nspection and repairs. It will be appreciated that in the case of a tanker conversion to a ship-shaped offshore unit, fatigue safety factors closer to those for the trading tanker may usually be economically necessary; in such cases, it is also common that an extensive structural integrity monitoring program will be employed in service, at least in harsh environments.

Tanker conversions, however, are typically targeted for shorter times of onsite service than their new-build counterparts. The FLS assessment and design should, in principle, be undertaken for every suspect location of fatigue cracking that includes welded joints and local areas of stress concentrations and for all relevant loads. Although wave-induced actions are primary sources of fatigue, the effects due to the following loads may need to be considered depending on the design and circumstances: r Functional loads including those related to loading and offtake of cargo r Wind loads; for example, the effect of vortex-induced vibrations and vortex shedding r Slamming loads r Sloshing loads r Local load effects arising from mooring and riser systems Procedures and criteria related to slamming and sloshing effects for FLS purposes are not generally well de ned, and neither are these two load effects amenable to a closed form spectral fatigue method.

In any event, it appears in practice that these conditions are primarily evaluated for strength, even if fatigue is sometimes said to be suspected in related failures. The dry-docking condition, any docking condition a oat, and any damage condition, although relevant for strength assessment, normally do not need to be considered for FLS. The calculations should address all transit conditions, for example, tow to the eld or to a shipyard for repair and all onsite operating conditions.

The structural design criteria for FLS are usually based on cumulative fatigue damage under repeated uctuation of loading, as measured by the Palmgren Miner cumulative damage accumulation rule for purposes of using the S N curve approach (S = uctuating stress, N = number of stress cycle). A particular value of the Miner sum (in principle, unity) is taken to be synonymous with the formation or initiation of a crack and is calculated as follows: n1 n2 ni nk + + + + + = 1, N1 N2 Ni Nk (7.1).

where ni = number of cy cles at the ith stress range level; N i = fatigue life at the ith stress range level; and k = number of stress range levels. The structure is designed so that when analyzed for fatigue, a reduced target damage sum results, implying that cracks will not form with a given degree of certainty or safety factor. In applying the S N curve approach, the following three steps are required: (1) de ne the histogram of cyclic stress ranges; (2) select the relevant S N curve; and (3) calculate the cumulative fatigue damage.

. 7.2 Design Principles and Criteria max = Stress range a mean a time = 2 a Figure 7.1. Cyclic stress range versus time. min 7.2.1 Cyclic Stress Ran ges In the fatigue damage assessment of welded structural details, our primary concern is the ranges of cyclic maximum and minimum stresses rather than mean stresses, as shown in Figure 7.

1. When the maximum and minimum stresses are denoted by max and min , respectively, the mean stress is given by mean = ( max + min ) /2 and the stress (or loading) ratio is de ned by R = min / max . The stress range is de ned by = max min = 2 a , where a is called the stress amplitude.

In principle, the fatigue characteristics are affected by the R value as well as , but the effect of mean stress is typically neglected for welded structures for two interrelated reasons: (a) the likely presence of near yield magnitude tensile residual stresses in vicinity of a weld, which tends to make the entire stress range damaging for fatigue purposes; and (b) the typical use of S N curves based on the R = 0 data. Note that in nonwelded cases, R effects may be signi cant, but in ship-shaped offshore structures, it is typically the welded details that usually govern for FLS purposes except possibly cutouts. In the context of spectral-analysis-based FLS design of ship-shaped offshore units for wave-induced effects, the vessel motion and load response amplitude operators (RAOs) are obtained by the seakeeping analysis for a range of wave frequencies and headings as well as for representative cases of each of the vessel loading cases identi ed.

Then, the global and local structural analyses with relevant nite-element models are used to obtain the wave-induced cyclic stress range transfer function for each wave heading angle and frequency. Note that the load transfer functions are complex numbers; hence, there actually are two stress analysis cases that must be gone through before the stress transfer function value can be established for a given heading, speed, frequency, and load case. This can be quite an intensive process of computation.

There are variations of this process in which stress in uence functions for loads are used. Using scatter diagrams of waves and swell that represent a number of sea states describing short-crested seas in terms of joint probability of occurrence of wave height and period, the spectral density of stress range is calculated by integrating the product of the wave energy spectrum and the modulus-squared of the stress range spectrum for each combination of signi cant wave height and period; therefore, the variance of stress range is obtained for the short-term sea state. A series of such.

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