A model for calculating the fatigue crack growth in welded tubular joints, based on fracture mechanics is established. Various aspects of fatigue crack growth calculations, which can be found in literature, are incorporated in this calculation model. The size of a fatigue crack can be calculated by this method for any time in the fatigue life. The crack growth calculation model is verified by comparing the results with the actual fatigue crack growth in a large scale welded tubular joint specimen. The crack growth data of this specimen were obtained from crack marks introduced during testing. The fatigue results of other specimens are also compared with calculated results.
Fatigue test results can be presented as S-N lines (Wohler-lines). For designing against fatigue failure these S-N lines can be used. This presentation and analysis is completely based on experiments and gives only information about the fatigue life for the specific detail and material. By using fracture mechanics the fatigue crack growth in an arbitrary structural detail can be calculated. Furthermore the remaining fatigue life of a cracked structure can be calculated. The fatigue crack propagation rate of the material can be derived by standard fatigue tests. This means that basically fracture mechanics is a more powerful tool to handle fatigue problems. The fatigue crack growth can be calculated using relations giving the fatigue crack propagation rate as a function of the stress intensity factor range. A variety of these relations are given in literature. The relation given by Paris is the most elementary one and can in most cases be applied satisfactorily.(Mathematical equation available in full paper)
In this relation the crack propagation rate, expressed by the amount of crack extension per load cycle, is related to the stress intensity factor range (?K) with C and m being material constants. This stress intensity factor range is the difference between the maximum and minimum stress intensity factor (K) during the load cycle. The stress intensity factor describes the stress field near the tip of the crack. In a tubular joint where the crack initiates at the weld toe (figure 1), the value of K is rather difficult to calculate due to the complex geometry and stress situation. Therefore, the crack .growth calculation method is established for a simplified geometry and verified with crack growth data of actual fatigue tests on large scale welded tubular specimens. To calculate the stress intensity factors the formulae for a semi-elliptical crack in an infinite plate are basically used. The influence of the weld geometry is taken into account using a magnification factor. For the crack propagation of the semielliptical crack, Paris? law (equation 1) is applied in two directions with a possible reduction factor for the surface crack growth. The fact that smaller cracks grow faster is simulated by an additional crack length.
In this section the different aspects of the crack growth calculation for the simplified geometry are discussed. The calculation procedure is treated at the end of this section.
