Mean Load Stresses - Experimental and Simulation Investigations Regarding Their Effects on the Fatigue of Welded Structures
Jörg Rörup, Hamburg
ABSTRACT It is widely accepted that due to high residual tensile stresses the fatigue strength of welded joints is essentially independent of the mean load stress. On the other hand, theoretical considerations of the stable crack growth as well as experience from ship operation indicate a positive effect of mean compression stresses on the fatigue life. In order to further investigate this effect, fatigue tests with "longitudinal stiffener"- type specimens have been carried out under constant amplitude load with compression mean stress. The level of the residual stresses had a great influence on the fatigue life under compression cyclic load. The determined fatigue life for a series of tests with a high residual stress level was around the factor 2 lower than for a series of tests with a residual stress level which under usual welding conditions had developed. The shift of the mean stress into the compression range caused an increase of the fatigue life. With a constant crack initiation life for different stress ratios, the increased total fatigue life is caused by the crack propagation phase. Measurements were evaluated for the welding induced residual stresses and for the residual stresses at the crack tip. Due to the tension residual stresses measured at the weld toe the crack initiation under compression cyclic loads can be explained. The tension residual stresses determined at the crack tip, give a reason for the fact that the crack progresses into areas of the sample in those compression residual stresses from the welding process are present. The crack moves thereby a field of tension residual stresses in front of itself, which loses with increasing crack length at size and intensity.With the method of the finite elements, under consideration of the residual stresses computed before, a nonlinear structural analysis took place for the assessment of the closing effects for a through thickness crack. The determination of the effective stress intensity factor for crack lengths from 30 to 90 mm allowed a concrete estimation of the fatigue life under constant amplitude loads with a stress ratio of R = - . Both differently high load levels and different residual stress conditions could be considered with the model using the plane strain condition. The crack deceleration determined in fatigue tests was estimated in all cases by the computed crack propagation curve in satisfactory way.
Publication Source: Materialprüfung, ISSN: 0025-5300
Issue: 2004-5, pp -
Publisher: Carl Hanser Verlag München
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