The high replacement costs of gas turbine blades and vanes have created a fast-growing, highly-specialized segment of the repair industry. Moreover, the gas turbine blades failure is a crucial issue, not regarding blades themselves, but for the collateral e ects that this kind of events can generate on the whole machine. In fact, when the blade failure occurs, the debris are carried on by the ue gases stream thus damaging the others blades, both rotor and stator, and others parts and devices along their path. When such a catastrophic sequence of events happens, the machine damage, together with the energy production lost, can easily reach millions of dollars. In order to avoid all this, a series of inspection campaigns is set up with the target of detecting cracks and assessing whether the components are able to continue their service or it is necessary to replace them. When a crack is identi ed its length and growth rate are monitored during the following maintenance control. When said crack, reach a critical length, the modern repairing techniques prescribe to remove the material surrounding it and to replace it by means of some Additive Manufacturing (AM) technique, in order to enlarge the lifetime of said turbine blade. In this case study the base material, which blades are made of, is Ren e 80, a nickel-based superalloy characterised by a coarse grain structure, while the reported material, which is added by means of a welding process, is Inconel 625, another nickel-based superalloy with very good weldability properties. In this thesis work, rst, the Low Cycle Fatigue (LCF) tests criticalities and results on specimens with a repaired blade like geometry are presented. Moreover, the local strain maps obtained by means of Digital Image Correlation (DIC) technique and numerical simulations are compared. Then a new formulation for the J-integral is presented, in particular new values for the h parameter, which is a function of the power-hardening exponent n0, are proposed. In conclusion, a comparison between the experimental and numerical results on the fatigue life predictions and on the estimations of the Equivalent Initial Flaw Size (EIFS) have been performed.
Gli elevati costi di sostituzione delle palette rotoriche e statoriche delle turbine a gas, hanno creato un segmento dell'industria dedicato alle riparazioni altamente specializzato e in rapida crescita. Inoltre, la rottura delle palette di una turbina a gas e una questione cruciale, non solo per quanto riguarda la paletta stessa, ma per gli e etti collaterali che questa tipologia di eventi pu o generare su tutta la macchina. Infatti, quando si veri ca la rottura della paletta, i detriti vengono portati avanti dal usso di gas combusti, danneggiando cos le altre palette, le altre parti e i dispositivi lungo il loro percorso. Quando si veri ca una sequenza di eventi cos catastro ca, il danneggiamento della macchina, unitamente alla perdita di energia prodotta, pu o raggiungere facilmente milioni di dollari. Con l'obiettivo di evitare tutto questo, una serie di campagne di ispezione e messa in atto al ne di individuare le cricche e valutare se i componenti sono in grado di continuare il loro servizio o e necessario sostituirli. Quando viene identi cata una cricca, la sua lunghezza e la sua velocit a di crescita vengono monitorate durante i successivi controlli di manutenzione. Quando questa raggiunge una lunghezza critica, le moderne tecniche di riparazione prescrivono di rimuovere il materiale che la circonda e di sostituirlo per mezzo di qualche tecnica di Additive Manufacturing (AM), per aumentarne la durata. In questo caso, il materiale di base, che costituisce le palette, e il Ren e 80, una superlega a base di nichel caratterizzata da una struttura a grana grossa, mentre il materiale riportato, che viene aggiunto mediante un processo di saldatura, e l'Inconel 625, un'altra superlega a base di nichel con un'ottima saldabilit a.In questa tesi, in primo luogo, vengono presentate le criticit a e i risultati dei test a fatica a basso numero di cicli (LCF), su provini aventi una geometria simile ad una paletta riparata. Inoltre, vengono confrontate le mappe di deformazione locale ottenute mediante la tecnica della Digital Image Correlation (DIC) e le simulazioni numeriche. Successivamente viene presentata una nuova formulazione per il J-integral, in particolare si propongono nuovi valori per il parametro h, che e funzione dell'esponente n0 che descrive la velocit a di incrudimento dei materiali. In conclusione, e stato eseguito un confronto tra i risultati sperimentali e numerici sulla vita a fatica. Inoltre le stime delle dimensioni del difetto iniziale equivalente (Equivalent Initial Flaw Size - EIFS) sono presentate.
J-integral formulation for repaired gas turbine blades under LCF loading condition
MARONGIU, EMANUELE
2016/2017
Abstract
The high replacement costs of gas turbine blades and vanes have created a fast-growing, highly-specialized segment of the repair industry. Moreover, the gas turbine blades failure is a crucial issue, not regarding blades themselves, but for the collateral e ects that this kind of events can generate on the whole machine. In fact, when the blade failure occurs, the debris are carried on by the ue gases stream thus damaging the others blades, both rotor and stator, and others parts and devices along their path. When such a catastrophic sequence of events happens, the machine damage, together with the energy production lost, can easily reach millions of dollars. In order to avoid all this, a series of inspection campaigns is set up with the target of detecting cracks and assessing whether the components are able to continue their service or it is necessary to replace them. When a crack is identi ed its length and growth rate are monitored during the following maintenance control. When said crack, reach a critical length, the modern repairing techniques prescribe to remove the material surrounding it and to replace it by means of some Additive Manufacturing (AM) technique, in order to enlarge the lifetime of said turbine blade. In this case study the base material, which blades are made of, is Ren e 80, a nickel-based superalloy characterised by a coarse grain structure, while the reported material, which is added by means of a welding process, is Inconel 625, another nickel-based superalloy with very good weldability properties. In this thesis work, rst, the Low Cycle Fatigue (LCF) tests criticalities and results on specimens with a repaired blade like geometry are presented. Moreover, the local strain maps obtained by means of Digital Image Correlation (DIC) technique and numerical simulations are compared. Then a new formulation for the J-integral is presented, in particular new values for the h parameter, which is a function of the power-hardening exponent n0, are proposed. In conclusion, a comparison between the experimental and numerical results on the fatigue life predictions and on the estimations of the Equivalent Initial Flaw Size (EIFS) have been performed.| File | Dimensione | Formato | |
|---|---|---|---|
|
2017_07_Marongiu.pdf
non accessibile
Dimensione
31.97 MB
Formato
Adobe PDF
|
31.97 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/135625