Fatigue analysis method

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. 

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. 

Altstadt V, Loth W, Schlarb A (1996) Comparison of fatigue test methods for research and development of polymers and polymer composites. In Cardon AH, Fuduka H, Reifsnider KL (eds) Progress in Durability Analysis of Composite Systems. Balkema, Rotterdam, pp 75-80... 

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

This steady state stress analysis is identical to that for the EEUA method in section 13.3.3 (see also Table 13.1) This is followed by a fatigue analysis described in the next section. 

Several critical plane analysis methods have been independently proposed and studied in the last decade (Mars, 2002 Mars and Fatemi, 2006a,b Saintier et al., 2006a,b Harbour et al., 2008b Andriyana and Saintier, 2010 Zine, 2011 Ayoub et al., 2012). They are noteworthy for their ability to provide a detailed and accurate account of the effects of multiaxial loading on fatigue life, and for their ability to predict the plane in which cracks will develop. 

V Crupi, E Guglielmino, M Maestro, and A Marino. Fatigue analysis of butt welded AH36 steel joints Thermographic method and design S—N curve. Marine Structures, 22(3) 373—386, 2009. 

A fatigue analysis was made to determine if the tie rods will fail due to fatigue in the unlikely event that the rod vibrates in resonance with the von Karman vortex frequency. The modified Goodman diagram was used in this analysis. This method is described in Reference 6. 

Creep-fatigue design methods based on elastic analysis... 

Section Vin, Division 1 does not have either a screening method or a procedure for performing a fatigue analysis. However, Section Vin, Division 2 has both. However, it is stiU acceptable to build and stamp a vessel in cyclic service to Section VIII, Division 1 but to do the fatigue analysis per Section VIII, Division 2. There is no requirement for a vessel to be built to Section Vni, Division 2 simply because it is in cyclic service. On the other hand. Section VIII, Division 1 vessels are not exempt from fatigue analysis because they are not built to Section VIII, Division 2. 

Screening Method B Section VIII, Division 2, Paragraph 5.5.2.4 indicates that the fatigue screening Method B may be used for all materials. It is not a simple, single procedure as is outlined in method A. Six of the steps require stress calculations, and not satisfying any of the six calculation steps results in a requirement to perform a detailed fatigue analysis. 

Fatigue Assessment. Section VIII, Division 2, Part 5 contains methods for performing an actual fatigue analysis. They are as follows ... 

Fatigue evaluation using the simplified inelastic analysis method... 

There is the static analysis method that determines reaction forces at the attachment positions of resting mechanisms when a constant load is applied. As long as zero velocity is assumed, static analysis can be performed on mechanisms at different points of their range of motion. Static analysis allows the designer to determine the reaction forces on whole mechanical systems as well as interconnection forces transmitted to their individual joints. The data extracted from static analysis can be useful in determining compatibility with the various criteria set out in the problem definition. These criteria may include reliability, fatigue, and performance considerations to be analyzed through stress analysis methods. 

Peak stress intensity. If fatigue analysis is required for cyclic operating conditions, the maximum stress intensity S must be computed from the combined primary, secondary and peak stresses (operating conditions. The allowable value for this peak stress intensity S is obtained by the methods of analysis for cyclic operations with the use of the fatigue curves. 

Example 11.14. A cylindrical shell that is 36-in. ID by 2.5-in. thick contains a perpendicular nozzle th is 4-in. ID by 0.75-in. thick. The design pressure is 1900 psi at a design temperature of 450°F. The vessel is subjected to cyclic operation and a fatigue analysis is required. Peak stresses and stress concentration factors are not known for the specific geometry to be used. What method can be used to evaluate the peak stresses for a fatigue analysis ... 

It is to be noted that among the nondestructive methods for fatigue damage imitation assessments, x-ray diffraction method is considered to be one of the most suitable analysis methods [14]. Additional information on this method is available in Reference 14. 

Fatigue analysis in Division 3 can be done usingthe traditional SN method or the structural stress method (limited to the analysis of welds) only if leak-before-burst behavior can be demonstrated. Otherwise, the fracture mechanics method must be used. 

Division 2. With the advent of higher design pressures the ASME recognized the need for alternative rules permitting thinner walls with adequate safety factors. Division 2 provides for these alternative rules it is more restrictive in both materials and methods of analysis, but it makes use of higher allowable stresses than does Division 1. The maximum allowable stresses were increased from one-fourth to one-third of the ultimate tensile stress or two-thkds of the yield stress, whichever is least for materials at any temperature. Division 2 requkes an analysis of combined stress, stress concentration factors, fatigue stresses, and thermal stress. The same type of materials are covered as in Division 1. 

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