INFLUENCE OF PARENT METAL STRENGTH ON THE FATIGUE STRENGTH OF PARENT MATERIAL WITH MACHINED AND THERMALLY CUT EDGES

摘要

Trends towards more effective and lighter structures have increased the interest in using high strength steels for higher performance and reduced weight. Design measures for avoiding welds in highly stressed regions and the introduction of post-weld improvement of critical welds mean that unwelded sections also have to be assessed in the design of welded structures. In this investigation, hot rolled strip steels in thicknesses of 6-12 mm with minimum yield strengths from 240 to 900 MPa are fatigue tested under constant amplitude pulsating tensile loading. Specimens with as rolled surface and with machined, laser cut, plasma cut and oxygen cut edges are tested. The results are evaluated according to the IIW recommended practice. The surface roughness Rz has been measured for different surface conditions, including ground and as rolled surfaces, and compared to literature data. Thermal cutting methods have been developed appreciably over the years, especially for laser and plasma cutting. This means that the FAT classes evaluated here are higher than those recommended by IIW. The results show that if the quality of cutting can be kept high, the fatigue strength of strip steel, especially strip with laser and plasma cut edges, can be of the same order as that of steel strip with machined edges. Both empirical and physically based models are presented for the influence of surface roughness on the fatigue strength at different steel strength. A tentative recommendation is given for a steel strength enhancement factor for the influence of yield strength on the fatigue resistance at different values of surface roughness.