High-strength low-alloy steels (HSLA) are gaining popularity in structural applications in which weight reduction is of interest, such as heavy duty machinery, bridges, and offshore structures. Since the fatigue behavior of welds appears to be almost independent of the base material and displays little improvement when more resistant steel grades are employed, the use of bolted joints is an alternative joining technique which can lead to an increased fatigue performance of HSLA connections. Manufacturing a hole to allocate the fastener elements is an unavoidable step in bolted elements and it might induce defects and tensile residual stresses that could affect its fatigue behavior. This paper studies and compares several mechanical (punching, drilling, and waterjet-cut) and thermal (plasma and laser-cut) hole-making procedures in HSLA structural plates. A series of 63 uniaxial fatigue tests was completed, covering three HSLA grades produced by thermomechanically controlled process (TMCP) with yield strength ranging from 500 to 960 MPa. Samples were tested at single load level, which was considered representative in HSLA typical applications, according to the input received from end users. The manufactured holes were examined by means of optical and electron microscopy, 3D point measurement, micro hardness tests, X-ray diffraction, and electron backscatter diffraction (EBSD). The results give insight on cutting processes in HSLA and indicate how the fatigue failure is dominated by macro defects rather than by the steel grade. It was shown that the higher yield strength of the HSLA grades did not lead to a higher fatigue life. Best fatigue results were achieved with laser-cut specimens while punched samples withstood the lowest amount of cycles. @en

High strength steels (HSS) offer a unique opportunity to reduce the weight of civil and off-shore structures and heavy-duty machinery. Such equipment is subjected to continuous cyclic loading and therefore fatigue failure often happens where concentrations of stresses are present, the latter most of the time being caused by connection details. Previous studies have demonstrated that an increase in yield strength does not lead to a proportional increase in fatigue resistance, particularly in welded connections. In that regard, the utilisation of bolted joints is often proposed as an alternative to welded joints. The hole-making procedure is an essential factor in the assessment of the fatigue resistance of bolted joints, since different hole-making techniques yield different surface qualities and residual stresses, which consequently impact the final fatigue limit. This work addresses the effect of the main mechanical (punching, drilling and waterjet) and thermal (plasma and laser) cutting techniques on the fatigue performance of HSS plates containing holes. A series of fatigue tests with moderately thick plates made of S500MC were carried out using different hole-making techniques. The surface, geometry and residual damage of every hole-making procedure were studied by means of optical evaluation, hardness tests and roughness evaluation, and large differences were observed: drilling was found to produce the most geometrically accurate hole and the smoothest surface finish; while laser and waterjet cutting displayed the best fatigue performance. The results of this research can provide an estimation of the applicability of each hole-making process to this particular HSS grade.

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