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The study of the dissipation heat flow and the acoustic emission during the fatigue crack propagation in the metal A N Vshivkov, A Yu Iziumova, I A Panteleev et al. noAbstract. Fatigue is one of the main causes of failures in mechanical and structural systems. Offshore installations, in particular, are susceptible to fatigue failure due to their exposure to the combination of wind loads, wave loads and currents. In order to assess the safety of the components of these installations, the expected lifetime of the component needs to be estimated. The fatigue life is the sum of the number of loading cycles required for a fatigue crack to initiate, and the number of cycles required for the crack to propagate before sudden fracture occurs. Since analytical determination of the fatigue crack propagation life in real geometries is rarely viable, crack propagation problems are normally solved using some computational method. In this review the use of the finite element method (FEM) and the extended finite element method (XFEM) to model fatigue crack propagation is discussed. The basic techniques are presented, together with some of the recent developments. IntroductionComponents which are subjected to fluctuating loads are found virtually everywhere: Vehicles and other machinery contain rotating axles and gears, pressure vessels and piping may be subjected to pressure fluctuations (e.g. water hammer) or repeated temperature changes, structural members in bridges are subjected to traffic loads and wind loads, while those in ships and offshore structures are subjected to the combination of wind loads, wave loads and currents. If the components are subjected to a fluctuating load of a certain magnitude for a sufficient amount of time, small cracks will nucleate in the material. Over time, the cracks will propagate, up to the point where the remaining cross-section of the component is not able to carry the load, at which the component will be subjected to sudden fracture [1]. This process is called fatigue, and is one of the main causes of failures in structural and mechanical components [2]. In order to assess the safety of the component, engineers need to estimate its expected lifetime. The fatigue life is the sum of the number of loading cycles required for a fatigue crack to nucleate/initiate, and the number of cycles required for the crack to propagate until its critical size has been reached [2,3]. In this paper, computational methods to estimate the lifespan of a propagating crack whose initial geometry is known will be considered.Estimations of the fatigue crack propagation rate, da/dN, are normally based on a relation with the range of the stress intensity factor, ΔK, which is a linear elastic fracture mechanics (LEFM) parameter for quantifying the load and geometry of the crack. Paris, Gomez and Anderson [4] first proposed the existence of such a relation in 1961, and its simplest form is the Paris law [5]:

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In order to assess the structural integrity of tubular members or pipes containing circumferential through‐wall cracks, their stress intensity factor solutions are required. While stress intensity factors for tension and bending are available, few solutions exist for the case of torsion, even though these components may also be subjected to torque. In this paper, the finite element method is used to compute the stress intensity factors for this geometry under tension and torsion. Shell elements are employed to compute the results for thin shells by the means of the displacement extrapolation technique. The computed results indicate that the available analytical solution for torsional loading, which is based on shallow shell theory, is nonconservative for long cracks in thin shells. Shallow shell theory is in general not applicable to long cracks, and the present work is therefore able to provide solutions for a wider range of crack lengths than what is currently available.

As offshore structures are reaching their original design life, effective repair and life-extension techniques are required, in order to ensure continued safe operation. A stop hole may be drilled at the end of a crack, in order to delay further fatigue crack propagation. To compare the stop hole technique with other relevant repair techniques, its effect needs to be modelled. Here, a procedure for modelling the stop hole-induced fatigue crack growth delay for a crack propagating under mixed-mode I + II conditions is presented. The procedure combines the S–N curve for fatigue crack initiation and the Paris law for fatigue crack propagation, with models for mixed-mode crack propagation and finite-element analysis of the cracked geometry. The failure criterion inherent in the S–N curves is used to define the transition between initiation and propagation. The procedure is implemented on an example, and the importance of considering the mixed-mode conditions is indicated.

The total fatigue life of a brace in an offshore jacket structure is conventionally considered in four parts. N 1 is the number of cycles to initiate the first discernible surface cracking as noted by any available method. N 2 is the number of cycles to detect surface cracking by visual examination without the use of crack enhancement or optical aids. N 3 is the number of cycles until the first through wall cracking and N 4 is the total number of cycles to the end of test or final separation of the member. The majority of fatigue tests conducted on tubular connections or on girth welds in brace members obtained only N 3 results, it being common practice to stop testing when a through wall crack was present. In the HSE Guidance the S-N curves for tubular connections and girth welds in braces are therefore based on N 3 data. (In fact it should be noted here that there were very few test results for single sided girth welds available at the time of drafting the HSE guidance; the choice of Class F2 for these joints was therefore based largely on judgement rather than data). In UK waters, flooded member detection (FMD) by ultrasonic inspection with a remotely operated vehicle is used to check whether through cracks are present; however, in practice, fatigue cracks are likely to continue to grow around the brace circumference after breaking through-wall. A review by Sharp (Ref.1) concluded that detailed knowledge of the crack shape development after breakthrough together with a value for the ratio N 4 /N 3 are required. From the structural safety viewpoint, there is clearly a need to quantify the rate of fatigue crack growth after development of a through wall crack, but prior to the point at which final separation becomes a possibility. The present study was designed to examine these factors for circumferential welds in tubular members, and hence allow the efficacy of the FMD strategy to be assessed. This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy. HSE BOOKS

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