Fatigue Behavior of Welded Steel Joints in Air and Seawater

Abstract

Summary Constant-amplitude fatigue tests were carried out in air and artificial seawater on T-shaped welded steel joints. The main objective of the test program was to compare the behavior of welded steel joints in air and seawater. In addition, the effects of weld-profile/weld-finishing, stress relieving, and stress ratio were investigated. The influence of cathodic protection and overprotection on corrosion fatigue behavior also was studied. Introduction The recent use of offshore structures in deeper and rougher areas (e.g., the northern part of the North Sea), where there are lower air and sea temperatures, high winds, and severe marine conditions, has increased the probability of fatigue failure. For safe design of probability of fatigue failure. For safe design of offshore structures in such areas, knowledge of the corrosion fatigue behavior of steels and welded joints under representative conditions is of vital importance. The vast majority of fatigue data on which current fatigue design rules are based have been derived from tests with laboratory-size specimens in air.The corrosion effect normally is catered for, in the case of offshore structures, by extrapolation of the design S-N curves beyond the fatigue limit. Otherwise, the same basic concepts as for "in-air" structures are applied"stress range philosophy" and Miner's rule. The justification for this approach is a limited number of laboratory tests in simulated seawater, very limited experience from laboratory test results that corrosion protection is effective in delaying corrosion fatigue protection is effective in delaying corrosion fatigue failures, and service experience (which at present does not include the North Sea).To obtain fatigue data appropriate to steel offshore structures, it is desirable to obtain more data about the corrosion fatigue behavior of tubular joints under simulated North Sea conditions. However, these tests are expensive and time consuming; therefore, it is possible to perform only a very limited number of such tests. The vast majority of corrosion fatigue tests has to be performed with laboratory-size specimens under conditions performed with laboratory-size specimens under conditions simulating the conditions for offshore structures in the North Sea.This, paper describes the results of fatigue tests with welded joints in air and artificial seawater. The tests were carried out on nonload-carrying welded joints. The parameters varied in this investigation are environment parameters varied in this investigation are environment (air and artificial seawater), stress ratio R (R = 0.1 and - 1), stress relief treatment [postweld heat treatment (PWHT)], weld profile, weld finishing [as-welded, grinding, and tungsten inert gas (TIG) and plasma dressings], and cathodic protection and overprotection. The work forms pan of a large research program on the corrosion fatigue behavior of welded steel joints. Some preliminary tests results are given. preliminary tests results are given. Material The material used for the fabrication of the test specimens was in accordance with Euronorm Fe 510(BS 4360 Grade D steel). The plate steel thicknesses were 40 and 70 mm. The chemical composition and mechanical properties of the plates are described in Tables 1 and 2. properties of the plates are described in Tables 1 and 2. The microstructure of the steel contained about 20 to 25 % pearlite (grain size of 10.5 to 11 mu m according to ASTM specification). The impurity content of the steel was found to be low. The analysis and properties of the steel are within specification values (Euronom Fe 510). Experimental Work Test Specimens (Design and Fabrication) Fig. 1 illustrates the specimen configurations and weld profiles. Each specimen was fabricated such that the profiles. Each specimen was fabricated such that the longitudinal direction of the specimen was aligned parallel to the rolling direction of the plate. JPT P. 440