ECF15 FATIGUE CRACK PATHS AND RESIDUAL STRESSES IN COLD FORMED RECTANGULAR STRUCTURAL TUBES Sami Heinilä 1 , Timo Björk 1 , Gary Marquis 1 , Mika Bäckström 2 , and Reijo Ilvonen 3 1 Lappeenranta University of Technology, P.O. Box 20, FIN-53851, Lappeenranta, Finland 2 VTT Industrial Systems, 2 P.O. Box 1700, FIN-02044 VTT, Finland 3 Rautaruukki Metform, Harvialantie 420, FIN-13300 Hämeenlinna, Finland Author for correspondence: sami.heinila@lut.fi Abstract Global bending loads in rectangular hollow section (RHS) structural beams create transverse bending stresses along the inside corners of the tubes. In some design applications the fatigue strength of the inside corner becomes critical. Fabricators often desire smaller corner radii both for ascetic reasons and to reduce the machining preparation required for some welded joint configurations. However, smaller corner radii can cause higher residual stresses in the corners and increased fatigue susceptibility due to the larger stress concentration factor. High degrees of cold forming also increase the risk of microcracking. To better understand this behaviour and to help develop guidelines both for RHS manufacturers and end-users, both numerical modelling and fatigue experiments have been performed. Experiments included both full-scale RHS beam testing and testing of RHS sections. The residual stress pattern produced by cold forming has been modelled using elastic-plastic finite element analysis. Predicted residual stresses are consistent with measured values. Fatigue experiments and fracture mechanics analyses have shown that both the fatigue crack growth path and the fatigue life are greatly influenced by the through thickness residual stresses. Introduction A high-strength cold-formed rectangular hollow section structural beam (CFRHS) subjected to cyclic bending loading was found to fail due to fatigue. Applied load and restraint in this case can be modelled as shown in Fig. 1. Initially small discrete flaws along the inner surface of the tube corners grew both in the longitudinal and transverse directions forming one long fatigue crack. Longitudinally, the crack was near the centre support where bending moment was greatest. The crack initiated on the inner surface in the centre of the corner as seen in Fig. 2. As the crack grew in the transverse direction, it gradually turned towards the neutral plane. Eventually the crack path returned to the original direction towards the outer surface forming an S-shaped crack path as seen in the figure. The crack was visible only after it had grown through the wall of the tube. The CFRHS beam material in this case was a fine-grained high- strength structural steel S 650 MC with nominal yield strength of 650 N/mm 2 . Laboratory experiments Small scale specimens Bäckström et al. [1] have reported fatigue results for constant amplitude tensile loading fatigue of mild steel CFRHS beam sections. The reported yield stress for the material was 355 N/mm 2 . All specimens were cut from tubes manufactured according to appropriate EN