Due to the interest in effective light steel constructions, high-strength steels have gained importance. Different thermal cutting processes are frequently used in the metal processing industry. Besides the weld seams, free cutting edges gain technical and economic relevance as locations for potential fatigue cracks. In this investigation, fatigue tests were carried out on 8-mm-thick samples of S355M and S690Q steels at a stress ratio of R = 0.1. The cutting methods used were oxygen, plasma, laser, and waterjet cutting. Quality improvement methods, like shot peening, grinding, and cutting speed reduction, were applied for some series. The surface roughness was measured to classify the specimens into quality groups according to ISO 9013. The cut edge condition was also characterized by hardness and residual stress measurements. The investigation shows that all tested series exceed the FAT100 class and can be classified in FAT125. Specimens ranged in quality group 2 of ISO 9013 according to the roughness achieve FAT140 regardless of cutting technology or material. According to the ISO 9013, most of the specimens are classified in the quality group 2 and group 3. Fatigue strength results are significantly different in one quality group. No prediction can be made. ISO 9013 has a weak connection to fatigue strength. Quality improvement methods have a significant influence on the fatigue strength and can increase it. Due to reduced cutting speeds, the roughness decreases also. It results in an increase of the fatigue strength in all tested series in this study. In order to make a prediction of the fatigue performance, the standard has to be specified and the cutting process as well as the steel strength should be considered.
The potential of HFMI treatment to increase fatigue life under service loading remains in debate. However, some recent studies show that even under variable amplitude loading (VAL), fatigue strength is increased compared to untreated welds. Discussions generally focus on the stability of initial compressive residual stresses, which may be reduced during VAL due to high peak stresses. In this context, the potential of HFMI treatment is often only attributed to residual stress stability. This study presents further results on the effect of VAL with a P(1/3) and a linear load spectrum on the residual stress stability of HFMI-treated transverse stiffeners (TS) made of mild steel (S355) and high-strength steel (S700M). The impact of random, High-Low and Low-High loading sequences on the fatigue strength as well as on the residual stress behaviour of HFMI-treated joints will be discussed.
Material fatigue is one of the elementary causes of damage in steel construction besides corrosion and abrasion. Design recommendations require that weld seams are placed in less stressed areas due to the crack-sensitive nature of the welded areas. As a result, unwelded areas of the components such as free cut plate edges gain technical and economic relevance as locations for potential fatigue cracks. In the metal processing industry, different thermal cutting processes are frequently used. During the process, unwanted boundary conditions can lead to undesired cuts in the component geometry during the cutting process. These process dysfunctions lead to incorrect components and to rejects. This article presents results of fatigue test data of oxy-fuel thermal cut edges of defect-free and faulty repair-welded samples to investigate the influence of competing notches on the cut edge. Specimens are made from construction steels S355N and S690Q of a 20-mm-thick plate. The presented data shows that the fatigue strength of the damaged cut edges can be recovered by the repair procedure and does not show any reduction of the fatigue strength due to the determined pores or other metallurgical notches of the repaired section.
Kurzfassung Bei der Herstellung von Stahltragwerken nach DIN EN 1090 sowie bei der allgemeinen Materialprüfung ist die Messung der Härte Bestandteil der normgerechten Bauteilprüfung. Dabei wird zumeist die konventionelle stationäre Vickershärteprüfung gefordert. Beispielsweise werden bei der Erzeugung von thermischen Schnittkanten obere Grenzwerte für die Härte vorgeschrieben, die es folglich zu überprüfen gilt. Die Ermittlung der tatsächlich vorliegenden Härte an derartigen technischen Oberflächen stellt sich in der Praxis jedoch als schwierig dar, da die hohen Härten verfahrensbedingt nur in dünnen Schichten vorliegen und die zugänglichen Oberflächen der Schnittkanten Schneidriefen aufweisen. Die umständliche Verfahrensprüfung und die normgerechte Härtemessung an metallografischen Schliffen sind selbstverständlich möglich, aufgrund des hohen experimentellen Aufwands jedoch nicht immer praxistauglich. Im Zusammenhang mit der rechtlichen Produkthaftung, die für jedes gefertigte Bauteil gewährt werden muss, besteht unmittelbarer Bedarf an einem Verfahren zur verlässlichen Ermittlung der Härte, auch an nicht normgerecht vorbereiteten Oberflächen.
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