Modelling of cracking sites/development in axial dovetail joints of aero-engine compressor discs

This paper presents a methodology to predict probable sites of fretting fatigue cracking in axial dovetail joints of an aero-engine compressor. The paper attempts to tackle at once the problems of the numerical analysis of a typical loading, the interface frictional behaviour, the plasticity induced and the possible fretting fatigue crack initiation and their further development. Incremental two-dimensional elastic plastic finite element analysis was the tool to simulate the macro mechanics behaviour of the fretted surfaces between blades and discs during normal engine cycles. A friction coefficient of 0.25 was assumed. One cycle included disc acceleration, full blade loading and unloading and final deceleration. In general, the state of stress and strain was found multi-axial and non-proportional everywhere within the considered model. Fatigue surface cracks were assumed likely to initiate in regions of cyclic plasticity along the plane which experienced the maximum shear stress range and to develop in regions of tension rather than compression. Thus, fatigue crack initiation are probable on the fretted surfaces neighbouring to the joint edges nearest to the dovetail notch base either on the disc side or on the blade side. Critical initiated cracks can be recognised by their sites having sufficient cyclic tensile stress ranges. The probability of developing the initiated cracks is discussed. Multiple cracking sites are probable. Initiated cracks are expected to grow in stage I mode II along local planes of maximum shear stress range which have different orientation depending on the local stress state and its variation with time. Some of those cracks are expected to become dormant before reaching stage II mode I crack due to the existence of local compressive stress field. Stage II cracking is controlled by the maximum tensile stress range. One or two of the remaining stage II cracks become dominant and in the worst case can lead to catastrophic failure. The present work suggests a possible surface cracking mechanism for the formation of relatively large material particles, which other investigators in the literature experimentally found filling the mouth of cracks initiated by fretting fatigue. The basis of that mechanism is the sign of the local maximum shear stress range and the relative position of its plane compared with the plane of its kinked development stage II tensile crack. (c) 2006 Elsevier Ltd. All rights reserved.