ABSTRACT REFERENCE NUMBER: ICF100277PR EFFECT OF NOTCH SHARPNESS AND COLD EXPANSION ON THE FATIGUE CRACK ARREST A.AMROUCHE*, G.MESMACQUE*, R.GHFIRI*, A.TALHA*, A.IMAD** * Laboratoire de Mécanique de Lille URA CNRS 1441, IUT A GMP Le recueil. Rue de la recherche BP 179, 59653 Villeneuve d’Ascq France ** Laboratoire d’Etudes des Structures, Ecole des Hautes Etudes Industrielles, 13 rue de Toul, 59046 Lille Cedex, France ABSTRACT A fatigue crack can be stopped by a decrease of the notch sharpness and by residual compressive stresses at the crack tip [1]. The decrease of the notch sharpness is generated by a drilling at the crack tip and the residual compressive stresses by a cold expansion. This work is about two aluminium alloys which are used in land transport components. Three drilling radius and five expansion rate were performed at the crack tip. The required number of cycles to obtain a new crack initiation, the crack propagation rate and the crack tip opening displacement are studied and analysed from the different cases. The necessary number of cycles to obtain a new crack initiation is called Time Life. The curves time life versus the applied stress are similar to the Wöhler curve and the same use can be done. The residual compressive stresses and the plastic zone size are determined from a numerical calculation code . Without any cold expansion the fatigue life is controlled by the local stress concentration factor. In this case the fatigue life can be explained from a power law of the hole radius[2]. The crack propagation rate is minimum at the crack initiation and is depending on the hole radius too. After the new crack initiation we find again the crack propagation law of the initial material. By practising a cold expansion, the time life is strongly improved and the minimum crack propagation rate is lower [3,4,5,6]. We can observe a limit in the time life improving exceptif the crack expansion degree become too high the time life strongly decreases. The cold expansion brings residual stresses, plastic deformations and the local microstructure is modified. The fatigue life depends on the cold expansion degree but it is difficult to identify the separated effect of residual stresses, of the plastic deformation and of the microstructure. KEYWORDS Notch sharpness, cold expansion, time life, residual stresses, crack propagation INTRODUCTION The propagation of fatigue crack often leads to fracture of components. Therefore it is possible to extend the time life by decreasing the notch sharpness and introducing residual compressive stresses [6]. An usual technique to decrease the notch sharpness is the drilling at the crack tip and to induce residual stresses is the cold expansion. The expansion degree is defined by the relation DCE % = 100*(3- p)/ where p is the drilling radius. By this way we can stop the crack propagation for a more or less time which is called “Time life”. After a new crack initiation the crack propagation rate is lower than before drilling and that contribute to the security working.
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