An experimental and theoretical stress analysis has been conducted on large scale tubular welded T-joints subjected to out-of-plane bending. This work has shown that it is possible to predict the local stress at the weld toe.
Random load fatigue tests have also been conducted on these T-joints using the same out-of-plane bend loading. The results have been analysled successfully using fracture mechanics and shown the inadequacy of life estimates based on the stress-life approach.
It is expected that offshore steel structures, subjected to environments of the type found in the Northern North Sea, will suffer from fatigue damage and that this damage will be in the form of cracks throughout most of the fatigue life.
Two approaches are possible for life estimates involving either the stress-life method or fatigue fracture mechanics. The latter, models the physical situation more closely and would be preferable if possible. Previous work has shown that the fracture mechanics approach is possible for in-plane bend loading and that the stress-life approach has some important drawbacks. In the present paper this work has been extended to include out-of-plane bend loading.
Fatigue tests and an experimental stress analysis have been conducted on tubular welded 'T' joints constructed from BS 4360 grade 50 C steel. The joints were fabricated using manual metal arc welding, stress relieved and finally inspected radiographically. The dimensions of the joints are given in Table 1.
The static and dynamic loads were applied to the joint using the out-of-plane bend rig shown in Fig. 1. In this rig both ends of the chord are clamped and the bending load is applied to the brace by a servohydraulic actuator.
The experimental stress analysis was carried out on two joints, using a combination of rosette and linear strain concentration gauges. Both types are needed because the stress field close to the intersection is biaxial. Use of linear gauges alone could give misleading results.
Stress concentration factors are required for the weld toe as this is the crack initiation site. The weld toe geometry makes it impossible to measure the strain at this point. Instead stresses have to be measured close to the weld and then extrapolated to give the toe stress. In this work the rosette gauges were mounted at about 7 mm from the toe, with strain concentration gauges alongside. The gradients indicated by the latter were then used to extrapolate the rosette gauge results.
Three fatigue tests have been conducted using the out-of-plane bend loading configuration. In all cases the fatigue loading was a stationary random signal with an approximate gaussian probability density function (Clipping Ratio = 3.9). The frequency spectrum (P.S.D.) had two equal peaks at 0.6 and 1.8 HZ.
