Fatigue analysis

Some items that must be considered are wind, earthquake, piping and support loads internal and external pressure and thermal loads. Stresses developed by these loads must be calculated separately and then combined with shell or head stresses caused by internal pressure at their point of application. These standards must be met whether or not a fatigue analysis is required. 

In addition, tiie vessel engineer must decide, in accordance with Paragraph AD-160, whether or not fatigue is the critical factor. The vessel designer must determine the necessity of a fatigue analysis, which in turn tells whether a detailed stress analysis of the vessel must be made. 

The Division 2 User s Specification states that a fatigue analysis may or may not be required in accordance with Paragraph AD-160. In general, Paragraph AD-160 deals with the cyclic conditions of the vessel and is divided into two parts, Condition A and Condition B with subparts. Conditions AP and BP, which cover nonintegral-type nozzles. 

Condition A is an evaluation based strictly on pressure and temperature cycles. Pressure cycles within 20% of the design pressure are unlimited those over 20% of the design pressure are to be included with the cycles of differential temperature between adjacent points, as described by the code, with a limit of 1,000 cycles for the life of the vessel. 

Condition B is evaluated if conditions in A are not met. Condition B compares cycles, determined in Condition A, with fatigue allowables, Appendix 5. A fatigue analysis is not required for pressure, temperature or materials joined with different coefficients of expansion if either Conditions A or B are met. 

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Part AD This part contains requirements for the design of vessels. The rules of Division 2 are based on the maximum-shear theoiy of failure for stress failure and yielding. Higher stresses are permitted when wind or earthquake loads are considered. Any rules for determining the need for fatigue analysis are given here. 

In order to accurately model the fatigue behavior of rubber, fatigue analysis methods must account for various effects observed for rubber during constant amplitude testing. Effects associated with load level, 7 -ratio (ratio of minimum to maximum loading level), and crack closure are presented in this section. 

A detailed fatigue analysis is required if any of these conditions is likely to occur to any significant extent. Fatigue failure will occur during the service life of the vessel if the endurance limit (number of cycles for failure) at the particular value of the cyclic stress is exceeded. The codes and standards should be consulted to determine when a detailed fatigue analysis must be undertaken. 

This example of using a hierarchical multiscale modeling methodology for redesign of the Corvette cradle shows that not only can the multiscale modeling be used for monotonic considerations like the control arm, but can be used for fatigue analysis as well. 

Pipe stress and pressure vessel analysis Pressure, dead-weight, thermal expansion, vibration modal analysis of fatigue. Analysis to ensure that the piping and pressure vessels conform to the codes of ASME, API or WRC as the case may be... 

Although the Finite Element Method is still not very common for calculating the static strength of a vessel, it became a standard tool for computation of peak stresses (Figure 1) for fatigue analysis. 

The most common pressure vessel codes that include the evaluation of fatigue are the North-American "ASME Section VIII Division 2" [1] and the German "AD-Merkblatt S2" [2], the latter being the most detailed fatigue analysis currently available in pressure vessel codes. 

If NDI techniques can detect initial cracks as small as 10 in., then using Ui = 10 in., the estimated resulting fatigue life would be about 5 x 10 cycles. If, on the other hand, the initial crack size must be estimated from proof testing, say at 150 ksi, the assumed initial crack size for use in the fatigue analysis must be equal to the critical size for the proof stress (cr r), Eqn. (7.14). 

The code includes an easily applied rule for determining whether or not a fatigue analysis must be made. Special rules are included for the design... 

Conditions AP and BP evaluate nonintegral type nozzles only. Pad-type nozzles are considered nonintegral. Condition AP is related to Condition A except that pressure cycles in the nozzle cannot exceed 15% of the design pressure and still be unlimited. Condition BP is related to Condition B but has some criteria changes. If either AP or BP satisfies the requirements, a fatigue analysis is not mandatory. 

If a fatigue analysis is required, the code provides design methods (Article 5-1 for vessels... 

Fatigue analysis Rohmert 1973a, b Laurig 1973 Joint torques, strength equations Suitable... 

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