#Calculate fatigue strength crack

Neuber proposed a microstructural support concept which considers the effect of a stress gradient on fatigue behaviour. The underlying assumption is that, in order for fatigue failure to occur, the average stress must exceed the fatigue limit of the material without stress concentration, О”S o, over some critical volume surrounding the notch, rather than the maximum stress range. A number of researchers have attempted to produce methods of calculating a local stress that takes account of this behaviour by assuming that the apparent increase in strength for small notches is due to the fact that the stress concentration occurs over only a small volume of material. However, for a hole with a sufficient size (for example, in mm), K f approaches K t. For a very small hole (for example, in Ојm), K f is found to be 1.0. A simple example is a plate with a hole in it, where K t = 3.0. In other words, the SCF is higher than the fatigue strength reduction factor, K f. However, even then the approach can prove to be too conservative, with the actual life being significantly higher than that determined by this method. This is a simple task if the assessment concerns a well-defined geometry for which a published SCF is available. In this context, one way to determine the local stress range is to multiply the applied nominal stress range by the maximum stress concentration factor (SCF), K t. This might be an S-N curve for the same material without a stress-concentrating feature, typically that for smooth specimens when assessing machined notches.

#Calculate fatigue strength crack

Local stress-based fatigue assessment approaches involve use of the stress range at the point of potential fatigue crack initiation under consideration in conjunction with a suitable S-N curve. To overcome these inadequacies, there is increasing interest in the development of local stress approaches that include allowance for the notch-effect of the weld itself, for the fatigue design of welded joints. Thus, although the above design approach is convenient for designers, weaknesses are that the long-endurance fatigue life and the constant amplitude fatigue limit (CAFL) are not well defined and that it is not possible for the designer to make allowance for known characteristics related to the factors that influence the local stress concentration and hence fatigue life. Variations in these factors lead to scatter in fatigue test data, scatter that is particularly wide in the long-endurance regime. That stress concentration depends on the local weld geometry which in turn depends on such factors as the welding process, the welding position and the type and extent of any welding flaws. Instead, this is built into the S-N curve by basing it on the results of fatigue tests on actual welded specimens. Neither of these stresses includes any allowance for the notch effect of the stress concentration at the fatigue crack initiation site. IntroductionДЇatigue design of welded joints is generally based on S-N curves expressed in terms of either the nominal or structural hot-spot stress range. This paper presents the results of the fatigue strength estimates and draws conclusions about the applicability of the local stress approaches to girth welds. To use the critical distance approach, the relevant material properties, such as threshold stress intensity factor range and fatigue limit for flush ground welds, were determined experimentally. Two local stress approaches, critical distance and reference radius, were used to estimate the fatigue strength of girth welds at 5x10 6 and 10 7 cycles, which were then compared with available full-scale fatigue test results. Finite element modelling was carried out to determine the dependence of the stress concentration and through-wall stress distribution on weld root bead profile, with hi-lo values ranging from 0.25 up to 1.0mm. In this paper, use of local stress approaches was attempted to estimate the long endurance fatigue strength of girth welded pipes. Nominal and hot spot stresses are conventionally used for fatigue design of welded joints. Paper presented at Proceedings 33rd International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2014) June 8-13, 2014, San Francisco, USA Abstract TWI Ltd, Granta Park, Great Abington, Cambridge CB21 6AL, UK National Structural Integrity Research Centre.Structural Integrity Research Foundation.