TY - BOOK

T1 - Lifetime Assessment of hot-forged Ti-6Al-4V Aircraft Components based on microstructure and Numerical Analysis

AU - Tan, Wen

N1 - no embargo

PY - 2012

Y1 - 2012

N2 - The present work deals with the fatigue behaviour of Ti-6Al-4V alloy which has undergone different conditions forging and heat treatment. This leads to major variations in the microstructure of the alloy, the influence of which has also been characterised with respect to the fatigue behaviour of the alloy. In addition to static and dynamic testing of specimen, several tests were also carried out on component scale. The results of specimen tests allow characterization of the material behaviour, wherein, the relevant influencing factors such as microstructure, forging condition, effective strain, mean stress and strain, notch effect, conditions of load, low cycle fatigue, and fracture mechanics have been investigated in detail. To check the transferability of results obtained from specimen tests on to the components, a component-like part named “W-Link” was developed which represent geometric boundary conditions predominant in real components. The tests with “W-Link” facilitated, on the one hand, characterization of fatigue behaviour of the component, and on the other hand, the validation of different lifetime assessment methods. Finally, the developed lifetime assessment methods have been applied to the real component – a Lower-Link-Fitting - for the final validation. The fracture surfaces of the specimens, as well as the components were analyzed thoroughly with the help of light microscopy as well as scanning electron microscopy. A large number of micro-sections from defined planes of the components as well as the specimens were studied in detail for developing a microstructure-based analysis. A self compiled MATLAB-Code facilitated the evaluation of the microstructure parameters automatically from the micrographs. Through the extensive analysis, a stark correlation was found between certain microstructural parameters obtained from the MATLAB evaluation and the fatigue behaviour. These parameters include, the primary Alpha content and the “colony size” for the bimodal microstructure and likewise, the width of the Alpha phase on the former beta grain boundaries and the width of the lamellae packet, for the lamellar microstructure. The lifetime estimation based on the results obtained from the specimen tests still has certain undesired discrepancies, and hence, for statistical reasons, an extensive amount of fatigue tests as well as numerical simulations were carried out in this work. The finite element simulation with program package-ABAQUS, used in this work, is based on the cyclic material data obtained from the specimen tests, taking into account the relevant influencing factors in regard to the assembly property state in the simulation (realistic boundary conditions, preload of screws, contact conditions). Based on the material models so derived from the FEM simulations of the specimen, simulations for both components were realised. In the final phase of the lifetime assessments, the effect of material characteristics, component geometry, and the appropriate load-time function were considered in order to estimate the lifetime of the component in terms of different damage models. Thereby, the stress and strain based models derived from the specimen tests and metallographic investigations were developed. These models were validated for both investigated components. Additionally, the finite element fatigue processing program FEMFAT was also applied to assess the local lifetime of the component. By continuously comparing the results from simulations and the laboratory tests, the boundary conditions considered in the simulation were fine tuned to obtain a more precise lifetime assessment model for the components. The investigations contribute to the transferability of fatigue test data from hot-forged specimens to complex components made of alpha-beta titanium alloy Ti-6Al-4V.

AB - The present work deals with the fatigue behaviour of Ti-6Al-4V alloy which has undergone different conditions forging and heat treatment. This leads to major variations in the microstructure of the alloy, the influence of which has also been characterised with respect to the fatigue behaviour of the alloy. In addition to static and dynamic testing of specimen, several tests were also carried out on component scale. The results of specimen tests allow characterization of the material behaviour, wherein, the relevant influencing factors such as microstructure, forging condition, effective strain, mean stress and strain, notch effect, conditions of load, low cycle fatigue, and fracture mechanics have been investigated in detail. To check the transferability of results obtained from specimen tests on to the components, a component-like part named “W-Link” was developed which represent geometric boundary conditions predominant in real components. The tests with “W-Link” facilitated, on the one hand, characterization of fatigue behaviour of the component, and on the other hand, the validation of different lifetime assessment methods. Finally, the developed lifetime assessment methods have been applied to the real component – a Lower-Link-Fitting - for the final validation. The fracture surfaces of the specimens, as well as the components were analyzed thoroughly with the help of light microscopy as well as scanning electron microscopy. A large number of micro-sections from defined planes of the components as well as the specimens were studied in detail for developing a microstructure-based analysis. A self compiled MATLAB-Code facilitated the evaluation of the microstructure parameters automatically from the micrographs. Through the extensive analysis, a stark correlation was found between certain microstructural parameters obtained from the MATLAB evaluation and the fatigue behaviour. These parameters include, the primary Alpha content and the “colony size” for the bimodal microstructure and likewise, the width of the Alpha phase on the former beta grain boundaries and the width of the lamellae packet, for the lamellar microstructure. The lifetime estimation based on the results obtained from the specimen tests still has certain undesired discrepancies, and hence, for statistical reasons, an extensive amount of fatigue tests as well as numerical simulations were carried out in this work. The finite element simulation with program package-ABAQUS, used in this work, is based on the cyclic material data obtained from the specimen tests, taking into account the relevant influencing factors in regard to the assembly property state in the simulation (realistic boundary conditions, preload of screws, contact conditions). Based on the material models so derived from the FEM simulations of the specimen, simulations for both components were realised. In the final phase of the lifetime assessments, the effect of material characteristics, component geometry, and the appropriate load-time function were considered in order to estimate the lifetime of the component in terms of different damage models. Thereby, the stress and strain based models derived from the specimen tests and metallographic investigations were developed. These models were validated for both investigated components. Additionally, the finite element fatigue processing program FEMFAT was also applied to assess the local lifetime of the component. By continuously comparing the results from simulations and the laboratory tests, the boundary conditions considered in the simulation were fine tuned to obtain a more precise lifetime assessment model for the components. The investigations contribute to the transferability of fatigue test data from hot-forged specimens to complex components made of alpha-beta titanium alloy Ti-6Al-4V.

KW - Betriebfestigkeit

KW - Ti-6Al-4V

KW - Flugzeugbau

KW - geschmiedete Bauteile

KW - Ti-6Al-4V

KW - Fatigue

KW - hot-forged

KW - Microstructure

M3 - Doctoral Thesis

ER -