Propagation of small fatigue cracks from notched specimens in S355NL steel A. El Malki Alaoui *1 , D. Thevenet **1 et A. Zeghloul ***2 1 MSN. ENSIETA, 2 rue Francois Verny 29806 Brest Cedex 09 France 2 LPMM. Ile du saulcy 57045 Metz Cedex 1 France 1 Introduction Industrial experiments showed, when parts are submitted to cyclic loading, fatigue life in service is not necessarily deter- mined by large cracks period. Indeed, a major part of this fatigue life is constituted by small cracks propagation. This study aims to characterise the short fatigue crack (SC) behaviour and evaluate differences with long fatigue crack (LC) in naval steel (S355NL). 2 Experimental study and results Two kinds of samples were employed. Conventional specimens (SEN ) were used to determine long crack behaviour and then, block notched specimens were dedicated to the short crack behaviour. Conventional sample containing a short slot (4mm in length) was performed by electro-discharged machining (EDM ) using a 290μm diameter wire. Block specimen with a large circular notch located at the center (20mm in radius) was machined without pre-cracking so small fatigue cracks initiated naturally on surface of block notched specimens. Tests were conducted on a naval steel (S355NL) at 35Hz at several stress ratios ranging from 0.1 to 0.5 and at room temperature. Tests were performed on a hydraulic machine with capacity of 100kN. In order to detect and follow fatigue cracks in this material, the plastic replica method and a far-field optical microscope were used. These methods allowed to measure cracks from about 40μm (surface length). We also determined the reference behaviour corresponding to the long crack propagation curve (da/dN - ΔK eff curve)determined by complaisance measurement. 3 Results and discussion Crack propagation rate (da/dN) versus stress intensity factor range ( ΔK or ΔK th,ef f ) was examined. For a given ΔK, short cracks propagate faster than long cracks. Moreover, small cracks propagate for ΔK values lower than the long crack threshold (ΔK th ). Short cracks are often slowed down when they are about 70μm, if they continue, they begin with an increasing rate, and this process can be repeated fig.1 a. Points relating to the short cracks are dispersed in a band, particularly for low ΔK values, which corresponds to initiation and short cracks propagation stage. These dispersions and delays in short cracks propagation are due to interactions between crack and other microstructure elements, three-dimensional variation of crack profile, crack deflection produced by propagation direction change and finally with the closure effect [1] [2]. In order to observe crack profile geometry, we carried out crack shape analysis after fatigue tests with a scanning electron microscope (SEM ) fig.1 b. The purpose is to determine a ratio (a/c) value ; this ratio allows to calculate the stress intensity factor range ΔK and compare to Newman and Radju expression [3]. 4 Numerical part Notch induces a stress concentration factor so the stress gradient was evaluated in the specimen thickness by Finite Element Method (FEM) using Abaqus Standard. By reason of symmetry, we meshed a quarter of the specimen for surface cracks and half of the specimen for the corner cracks fig.2a. For several crack lengths the convergence of J - integral calculation according to the number of contours was checked. For that we carried out a calculation on ten contours, so that each contour presents a layer of element around the crack. The obtained values are constant from the third contour. A calculation of J - integral was performed to determine K values for several crack configurations (corner and surface), shapes (circular and elliptical) and sizes. This study was also extended to long cracks in SEN specimens, and calculations were made in elastic * Corresponding author : e-mail : malkiaab@ensieta.fr, Phone : +33 298 348 807, Fax : +33 298 348 750 ** Corresponding author : e-mail : thevenda@ensieta.fr, Phone : +33 298 348 807, Fax : +33 298 348 750 *** Corresponding author : e-mail : zeghloul@lpmm.univ-metz.fr, Phone : +33 387 315 359, Fax : +33 387 315 366 PAMM · Proc. Appl. Math. Mech. 4, 278–279 (2004) / DOI 10.1002/pamm.200410120 © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim