TRANSIENT FATIGUE CRACK GROWTH BEHAVIOUR FOLLOWING SINGLE OVERLOADS IN AN AlMgSi ALLOY L. P. Borrego1, J. M. Ferreira2 and J. M. Costa2 1Department of Mechanical Engineering, ISEC/IPC, 3030 Coimbra, Portugal e-mail: luis.borrego@mail.dem.uc.pt 2Department of Mechanical Engineering/FCT, University of Coimbra, 3030 Coimbra, Portugal ABSTRACT Fatigue crack propagation tests with single tensile peak overloads have been performed in 6082-T6 aluminium alloy at various ΔK baseline levels from 4 to 10 MPa m1/2. The tests were carried out at constant ΔK and constant stress ratio R conditions, using MT specimens in a servohydraulic machine at a frequency of 20 Hz. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. The observed transient post overload behaviour is discussed in terms of the overload ratio and ΔK baseline levels. The crack closure parameter U was obtained and compared with the crack growth transients. Plasticity induced closure seems to be the principal mechanism in determining the transient crack growth behaviour following overloads on AlMgSi1 alloy. INTRODUCTION For many fatigue-critical structural parts, fatigue crack propagation under service conditions generally involves random or variable amplitude, rather than constant amplitude loading. Under such loading conditions, the load interaction effects cause a transient behaviour that involves accelerations and/or retardations in fatigue crack growth rate. The load history is therefore a major factor in determining fatigue life. An accurate prediction of fatigue life requires an adequate evaluation of these effects. The 6000 series are very frequently used in this type of structural applications, mainly due to the fact of allying a relatively high strength, good corrosion resistance and high toughness to a good formability and weldability. However, fatigue crack growth under variable load amplitudes in this series of aluminium alloys is not so well understood as in the 7000 and 2000 series. So, in this work was done an analysis of the transient behaviour observed on the crack growth rate due to tensile peak overloads in 6082-T6 aluminium alloy (AlMgSi1). The effects of crack growth retardation following single or multiple peak tensile overloads have been reported in many investigations simply because this type of loading can lead to significant load interaction effects [1-11]. Several mechanisms have been proposed to explain crack growth retardation, which includes models based on residual stress [12], crack closure [13], crack tip blunting [14], strain hardening [15], crack branching [16] and reversed yielding [17]. The precise micromechanisms responsible for these phenomena are not fully understood. In spite of some controversy, the effect of residual plastic deformation, which leads to compressive stresses before the crack-tip and raises the crack opening load on subsequent crack growth
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