Fatigue design

Shell design fatigue criteria (20) for asphaltic concrete and sul-furized asphalt paving material are identical. 

Sa = allowable stress for any given number of cycles from design fatigue curves. 

The fatigue damage is calculated based on ASME III, Division I, NG criteria for cumulative damage (Miner s linear damage rule). For this procedure, design fatigue curves up to 10 cycles are used except for austenitic... 

In addition Section VIII, Division 2 contains data for smooth bar design fatigue curves and welded joint fatigue curves. The curves represent testing conducted in air, and can be adjusted for the effect of corrosive... 

Et = Modulus of elasticity at temperature evaluated Epc = Modulus of elasticity used to establish the design fatigue curve. 

Figure 1-10. Design fatigue curves for carbon, iow aiioy, series 4XX, high aiioy steeis and high tensiie steeis for temperatures not exceeding 700°F and A/< 10 (Use Fig. 5-110.1.1 for A/> 10 ). Reprinted by permission, ASME).

The ASME design fatigue curves are based on strain controlled data in which the best fit curves are constructed by a factor of 2 on stress or a factor of 20 on cycles to account for environment, size effect, and data scatter. 

The specified number of pressure cycling does not exceed the number of cycles on the design fatigue curve, corresponding to the stress amplitude of 3Sm (typically twice the yield strength). 

The temperature difference between two adjacent points of the vessel does not change during normal operation by more than the quantity Sa/2E , where Sa is the value obtained from the applicable design fatigue curve for the total specified number of temperature fluctuations. Significant temperature range exceeds Sa/2E , where S is the value of Sa for 10 cycles. 

The specified full range of mechanical loads does not result in load stresses whose range exceeds Sa, a value obtained from the design fatigue curve for total specified number of significant load fluctuations. A load fluctuation is considered significant if the total excursion of load stress exceeds the value of Sa for 10 cycles. 

The design fatigue curves for carbon steel to account for the adjustment for maximum mean stress is shown in Figure 8.4. Similar curves for austenitic steels and nickel-chrome steels are shown in Figure 8.5. Both these curves have been obtained from the ASME Boiler and Pressure Vessel Code. For 2024-T4 aluminum, the fatigue curve is shown in Figure 8.6. ... 

Figure 8.4 Design fatigue curve for carbon steel (ASME).

The largest absolute magnitude of 3Si, SS2 and SS3 is designated as S,. Half of Sr is the stress amplitude which is used for entering into the design fatigue curve. It may be necessary to investigate many pairs of instants to find the maximum value of Sr. 

Here Sy is the yield strength of the material at the mean value of the temperature of the cycle and is the value obtained from the applicable design fatigue curve for the specified number of cycles of the condition being considered. The use of Eq. (11.1) is recommended for local thermal stresses only. Such an approach with a modified expression for Poisson s ratio involving the secant modulus, Eg has been employed in low cycle fatigue evaluation by Gonyea and Moulin and Roche. In their assessments the Poisson s ratio expression is represented as... 

It seems worthwhile to use both the factors fCg and K, in a typical application by multiplying the product of these two factors with the elastically calculated stresses before entering stresses in a design fatigue... 

The material for the thermal liner as well as the liner-to-nozzle weld is Inconel 600, for which the modulus of elasticity, E, and the yield strength, Sy, are 207 GPa and 240 MPa, respectively. The design stress intensity Sm for the material is 2/3 Sy or 160 MPa. The design fatigue curve for the material is obtained from Reference 2 and is shown in Figure 12.5. This figure is based on an elastic modulus of 180 GPa. The alternating stresses... 

Mitchell, M.R. Fundamentals of modem fatigue analysis for design. Fatigue and Micro-structure, ASM, Ohio Metals Park, p. 385. 

Chapter 18 in EN 13445 Section 3 [8] Detailed calculation of the cyclic life gives a guideline to calculate the cyclic design life of pressure vessels. But EN 13445 Section 3 is not applicable for extremely high-pressure vessels, because the design fatigue curves are limited by a material tensile strength of 1000 MPa. 

Figure 8.26 Demonstration of how design can reduce stress amplification, (a) Poor design sharp corner, (b) Good design fatigue lifetime is improved by incorporating a rounded fillet into a rotating shaft at the point where there is a change in diameter.

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