Effects of laser cutting on the cut-edge properties of structural steel S355N subjected to high-cycle fatigue

Laser cutting is a relatively new technology in the steel structures field if compared with other process like welding. The complexity of physical mechanisms dominant the laser cutting process plays a fundamental role on the alteration of material properties of the heat affected zone and consequently on the structural response of elements subjected to high-cycle fatigue applications. Moreover, in the last years an important evolution was observed in steel structures execution through the standard EN 1090 that introduced both the concept of execution class and minimum requirements for structural elements. The limitations of the local hardness HV10 on the cutting surface, imposed by the standard EN 1090 for high-cycle fatigue applications, certainly is an important parameter but it is not enough to explain the behaviour of structural elements. The proposed study is focused on the investigation of correlations between relevant material properties altered by laser cutting process and the response of both material and structural joints subjected to high-cycle fatigue loadings. In order to analyse the performances of the laser cutting we compared several properties of the cutting surface machined by mechanical and laser cutting. In detail, microstructures and material properties in terms of microhardness, hardness and residual stresses were investigated for steel elements of S355N. In addition, to correlate material properties and structural response, high-cycle fatigue tests were carried out both on material test coupons and on structural details by elemental bolted connections. The main results observed through the tests confirm that specimens made with mechanical cutting exhibit a better behaviour than specimens treated with laser cutting, though all specimens satisfied EN1090-2 requirements. The microstructures and material properties of laser cutting resulted to be favourable. Code requirements were also met for high-cycle fatigue applications. More precisely, the fatigue category proposed by Eurocode 3 for drilled holes, i.e. FAT90, resulted to be adequate for realistic elemental bolted connections made with laser cut.