Fatigue Strength Of Tubular T- And X-Joints

Abstract

ABSTRACT The results of fatigue tests on 40 tubular T and X-joints are presented. The chord diameters of the joints are 168, 457 and 914 mm. The most important conclusions are that the lifetime decreases with increasing joint size and that in the high cycle rance (N ? 107) some test results of the large joints fall below the AWS-X curve. INTRODUCTION The weather conditions which induce a fatigue crack are more severe in the North Sea than in the Gu1f of Mexico, Because of these severe conditions, a research project, sponsored by the European Coal and Steel Community (ECSC) on the fatigue behaviour of steel in offshore structures, entitled "Fatigue and corrosion fatigue behaviour of offshore steel structures", has been carried out. The research has been carried out in the U.K., France, Germany, Italy and the Netherlands. There is a special cooperation between Norway and the participating ECSC countries. In the programme, flat plate specimens and tubular joints have been tested. In addition, theoretical work (e.g. finite element analysis on tubular joints) has also been carried out. An interim report of the program was presented at a siminar in November 1978 in Cambridge. In this paper, the results of tests on tubular joints, carried out in the Netherlands under the sponsorship of the ECSC, are discussed. Also, the results of additional work (strain measurements, crack measurements and calculations) sponsored by MaTS (Netherlands Marine Technological Research) are discussed. TEST PROGRAMME Forty tests on tubular joints were carried out by the Stevin Laboratory of Delft University of Technology and the Institute TNO* for Building Materials and Building Structures (IBBC-TNO). T- and X-joints were tested with chord diameters of 168, 457 and 914 mm. Table I gives a survey of the test programme. Three small T-joints were loaded with in plan8 bending. All other joints were axially loaded. One X- joint was tested with both chord and brace axially loaded. Some joints have one (Tl and Xl) or two (T2) additional unloaded braces. Two joints are tested with a random load and all others with a constant amplitude load (R = Fmin/Fmax = 0 or -1). Four joints have been tested in artificial seawater, one of them with cathodic protection. TEST SPECIMENS AND EQUIPMENT The test specimens were made by a manufacturer with experience in offshore structures. The specimens were welded in a fixed position with both chord and brace horizontal. Fig. 1 shows the weld shape of a large joint. The steel quality of the joints is in accordance with the standards mentioned in table II. Fig. 2 shows the test rig for the large joints. To avoid secondary effects, all supports in the test rigs are hinges. STRAIN MEASUREMENTS General Before starting the fatigue tests, the strain distribution and the maximum (hot spot) strains in the joint were measured under a static load smaller than the fatigue load. The results are related to the nominal strain in the loaded brace (SNCF = Elocal/Enominal).