Effects of Load Sequence on Fatigue Crack Growth in Pressure Vessels

Abstract

Low alloy steels such as ASTM A508 and A533 and their equivalent materials have been extensively applied in fabricating pressure vessels due to their relatively excellent mechanical properties and moderately good weldability. The integrity of such materials governs the safety of the power plants. These vessels mainly are subjected to random loading in service and the load cycle interactions can have a significant effect in fatigue crack growth. Studying of fatigue crack growth rate and fatigue life calculation under spectrum loading is important for the reliable life prediction of vessels. Many models have been proposed, but as yet no universal model exists. In this paper, a fatigue life predicted under various load spectra, using three different fatigue crack growth models namely the Austen, modified Forman and NASGRO models. These models are validated with fatigue crack growth test data under various variable amplitude loadings. This application is performed with aids of three-point bend specimens. The results show clearly the load sequences effect and the predicted results agree with some discrepancies between the different models as well as with the test data. Neglecting, the cycle interaction effects in fatigue calculation under variable amplitude loading lead to invalid life prediction.

Development of fatigue prediction model for steel bridges based on characteristics of variable stress spectra

“…in which f denotes the crack opening function; R is the stress ratio; C, n, p, and q are material parameters; ÁK is the stress intensity factor range; ÁK th is the threshold stress intensity factor range; and K C is the critical stress intensity factor. For evaluation of stress sequence effects (stress interaction) in variable stress spectra, the NASGRO equation is the most reliable method among other fatigue equations (Abdullah et al, 2011;Beden et al, 2010Beden et al, , 2011Beretta and Villa, 2010;Rui and Xiang, 2010).…”

Fatigue life prediction methodology using entropy index of stress interaction and crack severity index of effective stress

“…When we analyzed stress sequence effects (stress interaction) in stress spectra, the reason to use NASGRO fatigue equations is the best reliable method among other fatigue equations (Abdullah et al., 2011; Beden et al., 2010, 2011; Beretta and Villa, 2010; Rui and Xiang, 2010).…”

Crack Propagation Behaviour under Corrosion and Thermomechanical Loads

“…The effect of load ratio on fracture propagation. [18] The propagation of a fatigue crack measuring 5 mm in length as a function of the number of cycles for load ratios of 0.1, 0.3, 0.4, 0.5, and 0.7 under a maximum stress of 118MPa demonstrates that the crack spreads more slowly under similar circumstances. (Figure 2).…”

“…The number of cycles and the load ratio are directly related to one another in a meaningful way. Therefore, as compared with the same level of maximum stress, a larger load ratio results in a stronger resistance to cracking [18].…”