REVIEW ON FINITE ELEMENT APPROACHES IN STRUCTURAL ANALYSIS OF AERO-ENGINE BLADE DISC ATTACHMENTS FOR FATIGUE LIFE ESTIMATION AND FAILURE ANALYSIS

Abstract

Aero-engine rotors are critical components, as their failure during service has a direct consequence on aircraft flight safety. Failure critical regions such as blade-disc attachment, assembly holes, weld areas and hub are located in the compressor and turbine stages of the rotor assembly and it is essential to validate and certify their design life. Of the several failure critical regions, blade-disc attachment is considered as the most critical element in the fatigue analysis of a high performance aero-engine. Conventionally, a dovetail and fir tree attachment is used for securing the blades with the disc (both in the compressor and turbine stage), although integral blades and disk is used in some stages of compressor rotor. During the operation of aero engine, blade disc attachment endures frequent occurrence of high frequency, low amplitude stress due to vibratory stress. In addition to the above, variation of centrifugal stress, bending stress and thermal stress corresponding to various flight phases (such as take-off, cruise and landing) lead to high amplitude, low cyclic fatigue (LCF) loading. Analysis of state of stress in the blade-disc attachment is a challenging task as it involves unknown boundary conditions, varying levels of contact stresses and clearances at the interfaces of the blade disc assembly. Photo elastic method was used earlier for analyzing the stresses at the blade disc assembly. Finite element method is widely used in the recent times for the analysis of stresses and strains at the blade disc attachment, as well as to evaluate the dynamic characteristics of rotors leading to failure life prediction. This paper reviews the different approaches adopted during finite element analysis of the blade-disc attachment, their advantages and limitations and identifies the state of art. Based on the review, the following may be summarized; a)Modern finite element codes have matured to a stage where the interface conditions between blade root and disc with thermal mechanical loading at the blade disc attachment can be effectively handled starting from a simple analysis which assumes infinite coefficient of friction at the interface to complete modeling and assessment of the extent of interface motion. b) A three dimensional approach to model the problem, although two-dimensional representation may still play an important role in the design process by the way of reduced design and development cycle time. c) Sub-modeling (global-local) approaches to predict the high stress gradient near the contact edges of blade disc attachment. d) Integration of finite element analysis with metallurgical examination in failure analysis of aero engine rotors.