Study on crack propagation in tubular joints under compressive fatigue loadings
Large scale tubular truss beams, approximately of 9 m long and 2 m high, were tested under constant amplitude fatigue loading. The beams were made out of circular hollow sections of steel S355, welded to form a uni-planar truss with K-joints, in a shape common to bridge construction. The main goal of these tests was to focus on the fatigue behavior of the joints loaded in compression that is with chord in compression, one diagonal in compression and the remaining diagonal in tension. The tests showed, as in other studies, that fatigue cracks may develop in compressive stress field zones from applied loads due to the presence of large welding tensile residual stresses. An alternate current potential drop system was used to follow during the tests the crack development from the weld toes. This paper contains the results, presented in terms of number of cycles versus crack depth and crack growth rate, from the cracks that developed. In order to evaluate the level of residual stresses near the weld toes, both measurements by hole-drilling and neutron diffraction were carried out. This paper includes the first results of the residual stress field, more measurements being in progress. Among the interesting results, it was observed that crack initiation and growth occurred first from hot spot 1c (weld toe in-between the braces, compression brace side) until the crack reached about 2 mm deep. Then, the crack growth decreased and, simultaneously, a crack started to develop from hot spot 1 (tension brace side). Both crack continued to grow, the failure being triggered by a fatigue crack in a joint on the tension chord. Characteristics of the different cracks (shape, angle of propagation, etc.) are given in the paper. The S-N results are compared with previously obtained results on beams with different tube dimensions.
Record created on 2009-11-16, modified on 2016-08-08