Random load fatigue tests have been carried out on T and Y joints. The crack depth has been measured in all cases and the results of a Y joint subjected to in-plane bend loading are reported here.
The results from these tests have been used to propose an "Inherent Flaw" model which uses Fracture Mechanics in a simple way to predict the fatigue life of tubular welded joints and has been developed to incorporate thickness effects.
Detailed analysis of the crack growth data has also shown that the mode of loading should influence the fatigue life. Thus it might be necessary to modify the hot spot stress concept if accurate fatigue life calculations are needed, and one such modification is proposed.
Tubular welded joints are used extensively in the fabrication of fixed steel platforms. These joints, however, experience fatigue damage, mainly due to the presence of small defects caused by welding, high stress concentrations at the intersection and the load variations - wave and other environmental loads. The problem has not been simplified by the extremely complex geometry near the intersection which makes a theoretical stress analysisal most impossible.
This uncertainty in predicting the stresses at the intersection, and the fatigue life, has prompted extensive research programmed to be carried out in many countries. From these tests, fatigue life curves have been drawn based on the hot spot stress. It is important to realize however, that these S-N curves are not 'true' S-N curves, for the steel used but are 'fictitious' S-N curves relevant to the geometry of the tubular joints. This does not mean that one cannot use these curves to predict the fatigue life of a tubular joint, it merely means that part of the full analysis of the failure mechanism can be omitted because the data has come from the full scale test. Identification of a characteristic stress, in this case the hot spot stress, and relating this stress to a failure curve is an acceptable engineering approach when detailed analysis is impracticable.
The work at University College London has concentrated on formulating a fracture mechanics based fatigue life calculation. In doing this work it has become evident that the growth of a crack through the thickness' can also be simply related to a characteristic field stress [1] and recently this observation has been used as the basis for an 'Inherent Flaw' model of fatigue [2] which can be used to predict total fatigue life or the remaining life of a flaw discovered during inspection. These observations may prove to be sufficient for the production of an elementary fracture mechanics based method for estimating fatigue life, but it should be noted that the complexity of the real behavior is contained within the assumption of a constant crack depth growth rate. However, as in the case of the 'fictitious' S-N curve this assumption could prove adequate in many situations.
Since completing our studies on tubular welded T joints some work has commenced on Y joints and this will be reported here.
