Experimentelle und numerische Untersuchung des Ermüdungsverhaltens von verfestigten Kerben und Schweißverbindungen nach dem Hochfrequenzhämmern

Die Ermüdungsfestigkeitsteigerung von Schweißverbindungen durch das High Frequency Mechanical Impact (HFMI)-Verfahren ist hinreichend statistisch belegt. Die Wirkweise des Verfahrens beruht auf der Änderung der Kerbgeometrie am Nahtübergang, sowie auf der Verfestigung der Randschicht und die Entstehung oberflächennaher Druckeigenspannung. Der Einfluss dieser Effekte wird im Rahmen dieser Arbeit sowohl experimentell als auch numerisch an gekerbten und geschweißten Proben aus niederfesten Stahl (S355J2+N) und hochfesten Stahl (S960QL) untersucht. Durch gezielte Seperationsexperiemente an gekerbten Grundwerkstoffproben erfolgte experimentell eine teilweise Trennung der Effekte der Randschichtverfestigung sowie der Induzierung von Druckeigenspannung im Schwingfestigkeitsversuch. Durch Prozesssimulation des Schweißprozesses, der HFMI-Nachbehandlung sowie der Schwingbelastung mit der Finiten Elemente Methode (FEM) wurde der Eigenspannungszustand nach der Behandlung errechnet und mit Röntgen- und Neutronenbeugungsmessungen abgeglichen. Bei der numerischen Analyse des Ermüdungsverhaltens lag der Fokus auf der Phase der Rissbildung, die durch dehnungsbasierte Schädigungsparameter abgebildet wurde. Dabei wurde die zyklisch-stabile Eigenspannung als zusätzliche Mittelspannungskomponente berücksichtigt. Als Kriterium der Unterscheidung zwischen der Phase des Anrisses und des stabilen Rissfortschritts wurde der Spannungsabstandsansatz (engl. critical distance method) verwendet. Die Untersuchungen zeigten, dass die Schwingfestigkeitssteigerung bei den gekerbten Proben aus S355J2+N in erster Linie auf die Randschichtverfestigung zurückzuführen sind, während bei den gekerbten Proben aus S960QL fast aus-schließlich die induzierte Druckeigenspannung für den Schwingfestigkeitsgewinn verantwortlich ist

Determination of Loading and Residual Stresses on Offshore Jacket Structures by X-ray Diffraction

As basements of offshore wind turbines (OWTs) in deep water (>50 m), jacket structures are an economic alternative to monopiles. For this reason, the structural durability of jackets has become more important. In such structures, welded tubular joints are weak points for fatigue design. The harmful effect of tensile residual stresses in welding joints is well known. For these reasons, the residual stresses and the loading stresses of offshore jacket structures were determined by X-ray diffraction (XRD) using a mobile diffractometer. This allows us to directly determine the load stress at the fatigue-critical locations, namely at the weld toe at the testing rig. High tensile residual stresses up to 250 MPa were determined in a welded (and unloaded) condition. At a loaded structure (10,000 load cycles), a lower residual stress level was determined. During loading, a local increase in the stress at the welded joint that is between 1.4 and 4 times higher than the applied nominal stress was determined. Furthermore, it is shown that additional treatment (grinding and clean blasting) influences the local stress state significantly

Identification of material properties for finite element simulation of the deep rolling process applied to welded joints

During butt welding of structural steels, an inhomogeneous material state across the weld occurs that has a detrimental influence on fatigue strength. In the presented study, the identification of Lemaitre Chaboche elastoviscoplastic model parameters is shown. The identification was conducted for base material, heat affected zone and filler material of submerged arc butt welded 1.8813 (S355MLO) structural steel. The derived constitutive model was integrated into a finite element simulation of the deep rolling process, which has not been investigated before for the post treatment of welded joints. Mechanical process loads were derived, giving first explanations of the deformation behavior

Fatigue Performance of High- and Low-Strength Repaired Welded Steel Joints

Large portions of infrastructure buildings, for example highway- and railway bridges, are steel constructions and reach the end of their service life, as a reason of an increase of traffic volume. As lifetime extension of a commonly used weld detail (transverse stiffener) of these structures, a validated approach for the weld repair was proposed in this study. For this, welded joints made of S355J2+N and S960QL steels were subjected to cyclic loading until a pre-determined crack depth was reached. The cracks were detected by non-destructive testing methods and repaired by removal of the material around the crack and re-welding with the gas metal arc welding (GMAW). Then, the specimens were subjected to cyclic loading again. The hardness, the weld geometry, and the residual stress state was investigated for both the original- and the repaired conditions. It was determined that nearly all repaired specimens reached at least the fatigue life of the original specimen

Fatigue Performance of High- and Low-Strength Repaired Welded Steel Joints

Large portions of infrastructure buildings, for example highway- and railway bridges, are steel constructions and reach the end of their service life, as a reason of an increase of traffic volume. As lifetime extension of a commonly used weld detail (transverse stiffener) of these structures, a validated approach for the weld repair was proposed in this study. For this, welded joints made of S355J2+N and S960QL steels were subjected to cyclic loading until a pre-determined crack depth was reached. The cracks were detected by non-destructive testing methods and repaired by removal of the material around the crack and re-welding with the gas metal arc welding (GMAW). Then, the specimens were subjected to cyclic loading again. The hardness, the weld geometry, and the residual stress state was investigated for both the original- and the repaired conditions. It was determined that nearly all repaired specimens reached at least the fatigue life of the original specimen

Residual stress analysis of butt welds made of additive an tradionally manufactured 316L stainless steel plates

Due to the limited building volume of powder bed additive manufactured (AM) methods, combining several AM parts for a larger part through a welding joint may be required. Through AM processes, Laser powder bed fusion (LPBF) has a great potential because it enables the production of nearly fulldensity components. But the residual stresses are still an issue in the wide application of this method. In this study residual stress analysis of butt joints made of 316L AM steel plates compared to conventional rolled steel plates were performed by X-ray diffraction with two different analysis methods: The commonly used sin² psi-method and the comparably new cos alpha-method. Complex residual stress states were determined at the welds made of AM steel plates compared to the welds made of conventional steel plates. High tensile residual stresses were determined in the AM plates depending on the layer orientation, but high compressive residual stresses were measured in the rolled steel plates. However, the residual stress level in the heat affected zone (HAZ) of the weld was comparably low in the AM steel plates and similar to the welds made of rolled steel plates. The residual stress analysis with the cos alpha-method showed the advantage of the comparable short measurement time based on small radiation time compared to the conventional sin² psi-method. A high influence of the layer orientation and manufacturing process was determined on the residual stress state at the base material. Close to the weld, relatively small differences in residual stress state between the investigated conditions were measured by both methods

The influence of coverage for high frequency mechanical impact treatment of different steel grades

info:eu-repo/semantics/publishe

Application of different simulation approaches to numerically optimize high-frequency mechanical impact (HFMI) post-treatment process

info:eu-repo/semantics/publishe