Investigation of multiaxial fatigue in the context of turboengine disc applications

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Abstract

Multiaxial fatigue is investigated on two alloys commonly used for compressor and turbine discs. With the objective to assess current design methodologies for discs, the present study involves both experimental and theoretical aspects. Several experimental facilities are used in order to cover a large range of multiaxial stress states, namely tension–compression torsion, tension–internal pressure and biaxial loads applied on a specially designed cruciform specimen. Some results with crack observations are discussed here within a thermo-elastic approach. Further, multiaxial fatigue criteria based on results of cylic viscoplastic calculations are analysed. Different multiaxial fatigue criteria, including those based on octahedral shear stress amplitude combined with hydrostatic pressure, and more recent models based on the critical plane approach are evaluated. The analyses reveal that none of them is able to deliver a correct life time estimation of all the considered uniaxial and multiaxial conditions. A new combined criterion is then proposed that offers a possible compromise and seems to significantly improve the assessments.

Introduction

A critical concern related to fatigue of materials and structures is multiaxial fatigue. Many engineering components that undergo fatigue loading experience multiaxial stress states. Rotating parts in turboengines, like turbine or compressor discs, are typical examples. They involve the interaction of low cycle fatigue (ground-to-ground cycles) and high cycle fatigue (vibratory) in regions with high biaxialities and high mean-stresses. Advances in material testing equipments during last 30 years have enabled to develop multiaxial testing facilities at room and high temperatures. These devices allow us to study the behaviour of materials and structural components by applying loads representative of the service life. The cruciform specimen and the thin-walled tubular specimen are usually used in multiaxial testing facilities. The tubular specimen is a versatile geometry. It can be tested either under axial–torsional loadings, which are the most commonly used multiaxial conditions, or under axial–internal/external pressure loadings. Some reviews of fatigue life investigations using axial–torsional or axial–internal pressure devices can be found in the literature [1], [2], [3], [4]. Testing facilities allowing the combination of axial load, differential pressure and torsion have been also reported in the literature [5], [6], [7], [8]. For the cruciform specimens, the tension–compression loads applied to its each pair of arms can produce a large range of biaxiality values in the central region. However, the stress distribution is usually evaluated using finite element analyses, and depends on several parameters (specimen design, configuration of the loading frame, specimen attachment and actuation method) leading to various testing facilities. A review of the cruciform specimen testing facilities is proposed by Makinde et al. [9], Boehler et al. [10], Hannon and Tiernan [11], and Abu-Fahra et al. [12].

Regarding the theoretical aspects of the multiaxial fatigue, many multiaxial criteria can be found in the literature nowadays. In spite of the number of proposed criteria, no universally accepted approach yet exists. Several good reviews of multiaxial fatigue criteria are available in the literature [13], [14], [15], [16], [17], [18]. These approaches can be divided into three main categories. One popular approach has extended the static yield criteria to fatigue by combining octahedral shear stress amplitude with hydrostatic pressure. By similarity, several strain based criteria have been proposed. Another approach uses energy as a correlating parameter for the multiaxial data. Energy criteria have been proposed and tested by several researchers (namely Garud [19], Ellyin [20], [21] and Radakrishan [22] and more recently Atzori et al. [23]) but present some difficulties for quasi elastic cycles. A third method for multiaxial fatigue life evaluation is to use the critical plane approach [24], [25], [26], [27], [28]. In all cases, as is underlined in the present paper, the largest difficulty with any of such fatigue criteria is to take simultaneously into account both the mean-stress effects (in uniaxial conditions) and the multiaxiality effects.

This paper presents the results of a large study on multiaxial fatigue in the context of compressor or turbine disc applications, involving both experimental and theoretical aspects. The objective of the experimental investigation is to study, under a large range of biaxial stress states, two typical turboengine disc materials: a titanium alloy (TA6V) and a nickel-based alloy (INCO718DA). In addition to standard LCF fatigue test under strain control with various values of strain ratio and HCF tests under stress control, the experimental program includes a significant number of complex multiaxial tests by using alternatively: axial–torsional tests, axial –internal pressure tests and biaxial fatigue tests on cruciform specimens.

Several existing multiaxial fatigue criteria (namely Crossland [29], Sines [30], Smith Watson Topper [24], Brown–Miller [25], Fatemi–Socie [26] and Gonçalvès et al. [31]) are then employed to determine their suitability at correlating the multiaxial fatigue data generated in the experimental program. Regarding the observed limitations, a new criterion is then proposed to estimate shear and equibiaxial fatigue conditions together with mean-stress effects.

Section snippets

Experimental devices to study multiaxiality effects

An extensive test program has been conducted on the facilities of multiaxial fatigue from ONERA and CEAT with two kinds of geometries: tubular specimens and cross-like specimens. Table 1 gathers the different testing facilities of both partners and their mechanical and thermal possibilities. Fig. 1 shows in broad outline the stress domain explored by the experimental devices. We can observe the good complementarity of tests that allow us to investigate a large domain, almost complete except

Cyclic inelastic analysis

The first step of the lifetime analysis is to find the stabilised operating conditions of the structure. The experimental program presented here includes three types of multiaxial tests: tension/torsion in-phase and out-of-phase conditions, tension/internal pressure in-phase conditions, biaxial fatigue loadings. In order to obtain the local stress–strain fields, all the multiaxial conditions have been calculated by various methods, strength of materials rules, (thermo-)elastic finite element

Multiaxial fatigue models

The objective of the present work is to identify reliable multiaxial fatigue models in the context of turbine disc applications. The existing models evaluated here can be classified in two categories: equivalent effective stress models and critical plane models.

Applications and discussion

The presented multiaxial models are examined to evaluate their ability to correlate both uniaxial data and multiaxial data. All the models have been identified through the same uniaxial data base, namely strain and stress fatigue tests at several loading ratio.

Conclusion

Some multiaxial fatigue test facilities have been presented here, involving the characterisation and the identification of multiaxiality effects for fatigue crack initiation in the prospect of turbine disc applications. Associated experimental procedures have been also described, taking into account thermal effects. All these multiaxial test conditions have been evaluated by strength of materials rules and thermo-elastic finite element analysis. Obviously the full cyclic inelastic finite

Acknowledgements

Elisabeth Ostoja Kuczynski, Francois Vogel (from Turbomeca) and Hacène Cherouali (from Snecma) are gratefully acknowledged for their constant collaboration on this research program. The financial support of DGAC (Délégation Générale à l’Aviation Civile) is also gratefully acknowledged.

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