Mazière, Matthieu (2007) Burst of turboengine disks. PhD thesis Sciences et génie des matériaux, Centre des Matériaux P.M. Fourt, ENSMP p.159.

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Abstract

During design of turboshaft engines, regulation rules impose to manufacturers to prove integrity of rotating parts (disks and compressor impellers) by overspeed experiments : parts should burst under mechanical and thermal loads beyond the rotation speed imposed by the regulation. This requirement guarantees a safety margin of at least 20 % between burst rotation rate and operating conditions.

The regulation evolution will make it possible to use numerical predictions, validated beforehand by experimental testings. Simulations, performed using large deformations elastoplastic finite element calculations, over-estimate at the moment the burst speed of disks designed in Udimet 720, a Nickel based super-alloy.

More reliable predictions of burst speeds required a detailed knowledge of the elastoviscoplastic behavior of the material. The prediction of the burst speed of a rotating disk is obtained by limit analysis. Material parameters which affect the most this limit speed are provided in this work.

For operating conditions the average temperature of disks is close to 500°C. At this temperature, Portevin Le Chatelier (PLC) effect appears during tensile tests on specimens in Udimet 720.

Simulation of this effect requires to use a model taking into account dynamic strain ageing. This model generally implies a localization of strain rate in bands. A localization analysis has been performed in order to use this model for rotating disks.

Two main results are provided in this work about simulation of burst of disks designed in Udimet 720 : (i) at ambient temperature, the burst speed is mostly affected by yield criterion and ultimate stress. (ii) at high temperature (500°C), PLC effect changes the global response of disks without

significantly modifying their burst rotation speed.

This work forms a part of the concerted research project between Turbom´eca, On´era, Snecma and the Centre des Matériaux - Mines Paris - ParisTech entitled ”Durée De Vie” (service life). This project is supported by the DGA and the DPAC.

Table of content

I Introduction 1

I.1 Aims

I.2 Outline

I.3 Notations



II Stability of elastoviscoplastic rotating disks 11

II.1 Introduction

II.2 Stability and uniqueness criteria

II.2.1 Finite strain formulation

II.2.2 Material behavior

II.2.3 Problem formulation

II.2.4 Hill uniqueness and stability conditions

II.2.5 Criteria for rotating disks

II.3 Evaluation of the local critical strain criterion

II.3.1 Simple Tension

II.3.2 Simple Shear

II.3.3 Application

II.4 Simulation of rotating disks

II.4.1 Influence of spin-softening

II.4.2 Influence of yield criterion

II.4.3 Influence of the hardening law

II.4.4 Influence of viscosity

II.5 Conclusion



III Mechanical Behavior of Udimet 720 - 33

III.1 Introduction to Udimet 720

III.1.1 Metallurgy

III.1.2 Processing, heat treatments, and hardening mechanisms

III.2 Mechanical behavior at room temperature

III.2.1 Tensile tests on smooth axisymmetric specimens

III.2.2 Tensile tests on notched axisymmetric specimens

III.2.3 Fractography

III.3 Mechanical behavior at 500°C .

III.3.1 Portevin Le Chatelier effect

III.3.2 Tensile tests on smooth axisymmetric specimens

III.3.3 Tensile tests on notched axisymmetric specimens

III.4 Conclusion



IV Burst prediction of an experimental rotating disk .. 51

IV.1 Introduction

IV.2 Material properties

IV.3 Identification of the Yield parameter n from Notched Tensile test simulation and validation from S-disk residual deformations

IV.4 Numerical modelling of the burst of the B-disk

IV.5 An alternative method to evaluate burst rotation rate of the B-disk



V Identification of material parameters for Udimet 720 at 500°C - 65

V.1 Introduction

V.2 Material model

V.2.1 Constitutive equations

V.2.2 Homogeneous solutions

V.2.3 Material model parameters

V.2.4 Tension of a plate

V.3 Stability analysis

V.3.1 1D linear perturbation

V.3.2 Stability conditions



VI Mesh and time increment sensitivity of localized phenomena for the MacCormick (MC) model 75

VI.1 Introduction

VI.2 Numerical integration of MC constitutive equations

VI.2.1 Runge-Kutta method

VI.2.2 method .

VI.2.3 Control of local time increment and switching method

VI.2.4 Control of global time increment

VI.2.5 Global time increment and method sensitivity

VI.3 Band nomenclature and location indicator

VI.3.1 Band nomenclature

VI.3.2 Numerical detection of bands - The BLI tool

VI.3.3 Evaluation of band width and velocity from the BLI tool

VI.3.4 Application : strain rate sensitivity

VI.4 Mesh sensitivity of localization phenomena

VI.4.1 Qualitative analysis

VI.4.2 Quantitative analysis

VI.5 Conclusion



VII Prediction of critical strain and band orientation. Simulations of axisymmetric specimens 95

VII.1 Introduction

VII.2 Linear perturbation analysis

VII.2.1 Theory

VII.2.2 Prediction of the critical plastic strain

VII.2.3 Estimation of band orientation

VII.3 Simple tension specimens

VII.3.1 Band orientation : symmetry breaking in axisymmetric test samples

VII.3.2 Band type and serration shape

VII.4 Notch tensile specimens

VII.5 Conclusion



VIII Simulation of the Portevin Le Chatelier effect in rotating disks 125

VIII.1 Introduction

VIII.2 Axisymmetric disk simulations

VIII.3 3D disk simulations

VIII.4 Conclusion

Conclusions – Prospects 137

Appendix 143

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