Imke Weich

Fatigue Behaviour of Welded High-Strength Steels after High Frequency Mechanical Post-Weld Treatments

Investigations have been carried out regarding the fatigue strength of welded details improved by high frequency treatment methods. These methods increase the fatigue strength by cold forming of the surface, so that the weld toe is smoothened, the surface hardness is increased and compressive residual stresses are present up to a depth of 1 to 1.5 mm. In this paper, the surface residual stresses at the weld toe are investigated before and after different loading conditions and for different steel grades. It is shown that only high tensile fatigue loading can lead to a slight residual stress relaxation for low-strength steels. The fatigue crack behaviour is analysed in more detail. The crack propagation rates with and without surface treatment are investigated, using defined lines of rest. The study shows that crack propagation in the edge layers is reduced. Several cracks may start in the UIT-treated zone but will not propagate further, until one final crack, often close to the edge of the specimens, will lead to failure. The results of fatigue tests for butt welds and longitudinal stiffeners improved by high frequency hammer peening are presented. The fatigue strength is seen to be doubled. For high-strength steels, the improvement at different load levels is identical, but for lower-strength steels, high stress ranges lead to reduced improvement. This fact results in flatter SN-curves and can be explained by the lower maximum of residual stresses and residual stress relaxation.

Henry Granjon Prize Competition 2009 Winner Category C: “Design and Structural Integrity” EDGE Layer Condition and Fatigue Strength of welds improved by mechanical post-weld treatment

In this paper, investigations about the beneficial effect of high frequency peening methods on the fatigue strength of welds are reported. Recent investigations prove that these methods are effective means of improving the fatigue strength of welded details. In the reported studies, the mode of functioning of these methods, as well as their beneficial effects on fatigue strength, have been experimentally and numerically analyzed and quantified. The results of the experimental studies show that the benefit of high frequency peening methods relies on compressive residual stresses and surface hardening, caused by plastic deformation of the edge layers at the weld toes. The experimentally-determined fatigue strength of new welds and that of already fatigue-pre-loaded welds before the treatment is improved by 80 to 100 percent, as compared with the as-welded condition. Based on the findings, a design model for post-weld treated welds is developed, which allows for the consideration of the beneficial effect of the post-weld treatment for the design of steel structures. The increased fatigue strength is considered in a local design concept, by introducing improvement factors for the local fatigue strength. The local compressive stresses are considered, as well as the local stress ratio.

Effects of High-Frequency Peening Methods on the Surface Layers and the Fatigue Strength of Welded Details

Investigations have been carried out regarding the influence of high-frequency peening methods on the fatigue strength. These methods cause plastifications at the weld toe, which change the local weld toe geometry and produce compressive residual stresses at the surface as well as surface hardening. Two different techniques, High-Frequency Impact Treatment (HiFIT) and Ultrasonic Impact Treatment (UIT), are compared. Laser measurements of the weld seam prove that both methods change the overall weld toe radii to a more uniform shape. Residual stress measurements verify the introduction of compressive residual stresses at least up to a depth of 1–1.5 mm which can reach values up to the yield strength. Further analyses show that the local effects increase the fatigue strength. Crack detection methods prove that, due to the material mechanical effects, the crack initiation and surface layer propagation phases are extended. These effects lead to a significant increase of the fatigue strength.

Extension of Life Time of Welded Fatigue Loaded Structures

Research has been initiated on the application of weld improvement methods to increase the fatigue life of offshore wind energy plants and other fatigue loaded structures. The effect of the application of Ultrasonic Peening (UP) and Ultrasonic Impact Treatment (UIT) has been compared to the effect of shot peening. Further investigations focussed on the improvement effect of UP on preloaded details whereas preloading has been defined as the application of the full design fatigue loading. The fatigue tests prove that the three analysed improvement methods lift the fatigue strength of virgin and also of preloaded welded notch details strongly.

Characteristics of High Frequency Peening Methods and their Effects on the Fatigue Strength of Welded Details

Research has been initiated on the effects of high frequency peening methods on the fatigue strength. These methods combine an improvement of weld toe profile with an initiation of compressive residual stresses and surface hardening. The effects of two techniques, High Frequency Impact Treatment (HiFIT) and Ultrasonic Impact Treatemnt (UIT) are compared. Laser measurements of the weld seam prove that both methods increase the overall weld toe radii. Further, residual stress measurements verify the introduction of compressive residual stresses at least up to a depth of 1 mm. The values meet the yield strength combined with an increase of the surface hardness. These material mechanical effects cause an increased crack resistance. Crack detection methods prove that the material mechanical effects yield to a retarded crack initiation. Experimental results show that these effects lead to a significant increase of the fatigue strength and reduced slopes of the SN-curves.