Ansatz zur Berücksichtigung von Schweißeigenspannungen bei der Ermüdungsfestigkeitsbewertung mittels direkter numerischer Simulationen
Zusammenfassung der Projektergebnisse
Although it is known that residual stresses may influence the fatigue strength of welded structures, they are usually not included explicitly in fatigue assessments. In this project the influence of welding residual stresses on fatigue was investigated using numerical simulations and experimental tests to develop an approach to consider welding residual stresses in fatigue analysis. A numerical welding simulation approach using a prescribed temperature heat source is used to investigate the influence of various geometry parameters on the resulting residual stresses and to compare residual stresses in small-scale specimens and large structures. The simulations were used to design small-scale specimens containing a multilayer K-butt weld and a longitudinal stiffener with tensile transversal residual stresses at the weld toe. Residual stresses were measured by X-ray diffraction and hole drilling on both specimen geometries and the simulation results were verified. Fatigue tests at different load ratios were conducted on both specimen geometries. The initiation of macroscopic cracks was detected using digital image correlation. The influence on crack initiation and propagation showed differences for both investigated weld geometries. A correlation between the fatigue test results and the calculated stresses was found. Based on these findings an approach to predict S-N curves depending on the residual stress condition and stress ratio was developed. The approach is based on numerical welding simulations and allows to consider the influence of welding residual stresses in fatigue analysis. From the results of the numerical simulations and experimental tests it was concluded the following: Small-scale specimens without structural constraints may present considerable tensile welding residual stresses. These are preserved also after cutting orthogonal to the weld to produce test specimens of reduced width. Depending on the weld pass sequence, tensile transversal residual stresses can form at the weld toe of cruciform joints or K-butt welds and thus have a relevant influence on fatigue. This might be used for experimental investigations, but have undesired effects for other applications. - The simulations showed how welding residual stresses differ for the same weld in a small-scale specimen or a larger structure. However, depending on the weld geometry and the welding sequence they are not necessarily higher (more tensile) in the large structure. Numerical simulations can be a practical tool to assess welding residual stresses case by case. - The proposed procedure using DIC allows to detect the initiation of macroscopic cracks and monitor crack propagation in fatigue tests. It is applicable on resonance testing machines and welded specimens. - The influence of welding residual stresses on fatigue depends on the weld geometry. Residual stresses affected crack initiation and propagation differently for both investigated geometries. On the K-butt weld specimens residual stresses affected mainly crack initiation. On the longitudinal stiffener specimens residual stresses influenced both crack initiation and propagation. Overall, this differentiated effect on crack initiation and propagation distinguishes the influence of residual stresses on fatigue from that of mean stresses. - Despite this different influence on crack initiation and propagation, it was possible to assess the residual stress influence based on the stresses at the weld toe. For the investigated K-butt weld the tensile residual stresses from the weld toe resulted in an underestimation of the crack propagation phase. On the longitudinal stiffener, according to the simulations residual stress some millimetres below the weld toe are in the same magnitude as on the surface and thus affect also crack propagation. For both geometries, assuming the stresses from the weld toe as effective through crack initiation and propagation resulted in only a small error in total fatigue life. - The presented approach allows to predict S-N curves for different welding residual stress conditions and load ratios R based on numerical simulations without residual stress measurements. In this way it allows to consider the influence of welding residual stresses in fatigue analysis. In research involving fatigue tests the approach could be used to assess if results are affected by welding residual stresses and stress-relieve treatments or costly residual stress measurements might be required. In fatigue design it could help to possibly take advantage of low stress ratios in welded structures.
Projektbezogene Publikationen (Auswahl)
-
(2019). A simplified welding simulation approach used to design a fatigue test specimen containing residual stresses. Ship Technology Research 66 (1), pp. 22–37
N. Friedrich, S. Ehlers
-
(2019). Crack Monitoring in Resonance Fatigue Testing of Welded Specimens Using Digital Image Correlation. Journal of visualized experiments: JoVE (151)
N. Friedrich, S. Ehlers
-
(2019). Experimental validation of a simplified welding simulation approach for fatigue assessments. In C. Sommitsch, N. Enzinger, P. Mayr (Eds.): Mathematical Modelling of Weld Phenomena 12. Graz: Verlag d. Technischen Universität Graz, pp. 875–891
N. Friedrich, S. Ehlers
