In this study, the fatigue behavior of welded steel joints treated with HFMI under constant amplitude loading is investigated. HFMI treatment is a post-welding process aimed at improving the fatigue performance of welded joints. The primary characteristics and the analysis of the factors that most influence a treated weld (such as yield stress σy and stress ratio R) are explored in detail in the following sections. The process involves high-frequency hammering (approximately 90 kHz) directly at the weld toe. The tool used can operate based on different principles, such as electromechanical or pneumatic systems, but the indentor in contact with the material must have a rounded tip with a radius of approximately 2 to 3 mm. When performed correctly, the result is a groove at the weld toe with a radius of approximately 2–3 mm and a depth of 0.2–0.3 mm. This improves the geometry, eliminates defects in the affected area (such as undercuts), and introduces compressive residual stresses. The study was conducted by modeling HFMI-treated welded steel joints with known global and local geometries and applying the Peak Stress Method to geometries with a weld toe radius ρ ̸= 0. The goal of this thesis is to calibrate new fatigue design curves and, if necessary, update existing ones for HFMI-treated joints using the Peak Stress Method, as a function of yield stress range and stress ratio defined in IIW recommendations. Chapter 1 introduces calculation approaches for studying welded joints, distinguishing between global methods, such as the nominal stress method, and local approaches, all based on the theory of linear elastic fracture mechanics, including NSIF, SED, and PSM. Chapter 2 presents some practical examples for 2D and 3D geometries using the local approaches introduced in Chapter 1. This chapter is useful for better understanding the application procedures of these methods and highlighting the differences between them. Chapter 3 provides a concise description of the HFMI treatment, its advantages, and its application limits compared to untreated As-Welded (AW) joints. It offers a detailed analysis of the parameters that influence fatigue strength and discusses the application of the Peak Stress Method (PSM) for this category of joints (with a weld toe radius ρ ̸= 0) for both 2D and 3D geometries. Chapter 4 describes the laboratory experiments conducted to verify whether the secondary stresses induced by misalignments, once the specimen is clamped in the testing machine, vary as a function of the applied force. Chapter 5 contains a collection of all the analyzed papers, providing a description of the joints for each publication, experimental test results, and the outcomes of analyses where the PSM was applied. Chapter 6 compiles all the fatigue design curves calibrated for HFMI-treated welded steel joints up to this point, incorporating new data into previously established curves. Additionally, it applies the stepwise fatigue design approaches recommended by IIW to the PSM. Chapter 7 is the concluding chapter, showcasing the possible agreement between the fatigue design curves calibrated in this and previous thesis projects and those derived using the IIW recommendations applied to the PSM.
Nel presente lavoro, si andrà a studiare il comportamento a fatica di giunti saldati in acciaio trattati con HFMI sollecitati ad ampiezza di carico costante. Il trattamento HFMI è una lavorazione post saldatura, grazie alla quale si va a migliorare il comportamento a fatica del giunto saldato, le caratteristiche principarli e lo studio di quali fattori che influenzano maggiormente una saldatura trattata (tensione di snervamento σy e rapporto di ciclo R), verranno approfonditi in seguito. La lavorazione consiste nel martellamento ad altissima frequenza (mediamente 90 KHz) direttamente al piede cordone di saldatura. Lo strumento utilizzato può avere diversi principi di funzionamento; elettromeccanico o pneumatico per esempio, ma l'indentatore che andrà effettivamente a contatto con il materiale deve presentare una punta raccordata di raggio dell'ordine di 2 o 3 mm. Il risultato quando svolto correttamente è un solco a piede cordone di raggio circa di 2 o 3 mm e profondità mediamente tra i 0.2 e 0.3 mm, comportando a il miglioramento della geometria, l'eliminazione di difetti nell'area interessata (come il sottosquadro) e l'introduzione di tensioni residue di compressione. Lo studio è avventuto andando a modellare i giunti saldati in acciaio trattati HFMI, dei quali si conosceva la geometria globale e locale ed applicandogli il Peak Stress Method, per geometria che presentano un raggio di raccordo ρ ̸= 0. Lo scopo di questa tes,i è quello di calibrare nuove curve di progettazione a fatica ed eveentualmente aggiornare quelle già esistenti, per giunti trattati HFMI per il Peak Stress Method, in funzione di range di tensione di snervamneto e di rapporto di ciclo definite nella raccomandazioni IIW. Il Capitolo 1 ha lo scopo di introdurre degli approcci di calcolo, per lo studio di giunti saldati, si andranno a distinguere in approcci globali, quale il metodo delle tensioni nominali, ed approcci locali tutti basti sulla teoria della meccanica della frattura leneare elastica, NSIF, SED e PSM. Nel Capitolo 2 si raccolgono alcuni esempi applicativi per geometria 2D e 3D, degli approcci locali accennati al capitolo 1. Il capitolo è utile per comprendere al meglio il procedimento di applicazione di questi metodi e mostrare le differenze che possono esserci. Il Capitolo 3 va a descrivere sinteticamnete il trattamento HFMI, i vantaggi ed i limiti applicativi paragonati ad un giunto non trattato As Welded (AW). Vengono descritti con un maggiore dettaglio tutti quei parametri che vanno ad influenzare la resistenza a fatica e l'applicazione del Peak Stress Method (PSM) in uesta categoria di giunto (con raggio di raccordo ρ ̸= 0) sia per geometria 2D che 3D. Nel Capitolo 4 si va a descrivere l'esperienza di laboratorio, svolta al fine di verificare se le tensioni secondarie indotte dai disallineamenti una volta afferrato il provino in macchina vadano a variare in funzione della forza applicata. Il Capitolo 5 contiene la raccolta di tutti gli articoli analizzati, dove per ogni pubblicazione si ha una descrizione del giunto, si riportano i risultati delle prove sperimentali svolte ed i risultati delle analisi dov'è stato applicato il PSM. Il Capitolo 6 va a raccogliere tutte le curve calibrate per giunti saldati in acciaio trattati con HFMI fino a questo momento calibrate, andando ad integrare eventuali nuovi dati analizzati su curve precedentemente tracciate. In più si vanno ad applicare le gradinate per approcci locali raccomandate della IIW al PSM. Il Capitolo 7 è il capitolo conclusivo dove si mostra la possibile concordanza tra le curve di progettazione a fatica calibrate in questo e negli scorsi progetti di tesi e le curve tracciate applicando le raccomandazioni IIW sul PSM.
Stima di vita a fatica di giunti saldati in acciaio soggetti a trattamento HFMI tramite l'applicazione del Peak Stress Method
LONGO, LEONARDO
2023/2024
Abstract
In this study, the fatigue behavior of welded steel joints treated with HFMI under constant amplitude loading is investigated. HFMI treatment is a post-welding process aimed at improving the fatigue performance of welded joints. The primary characteristics and the analysis of the factors that most influence a treated weld (such as yield stress σy and stress ratio R) are explored in detail in the following sections. The process involves high-frequency hammering (approximately 90 kHz) directly at the weld toe. The tool used can operate based on different principles, such as electromechanical or pneumatic systems, but the indentor in contact with the material must have a rounded tip with a radius of approximately 2 to 3 mm. When performed correctly, the result is a groove at the weld toe with a radius of approximately 2–3 mm and a depth of 0.2–0.3 mm. This improves the geometry, eliminates defects in the affected area (such as undercuts), and introduces compressive residual stresses. The study was conducted by modeling HFMI-treated welded steel joints with known global and local geometries and applying the Peak Stress Method to geometries with a weld toe radius ρ ̸= 0. The goal of this thesis is to calibrate new fatigue design curves and, if necessary, update existing ones for HFMI-treated joints using the Peak Stress Method, as a function of yield stress range and stress ratio defined in IIW recommendations. Chapter 1 introduces calculation approaches for studying welded joints, distinguishing between global methods, such as the nominal stress method, and local approaches, all based on the theory of linear elastic fracture mechanics, including NSIF, SED, and PSM. Chapter 2 presents some practical examples for 2D and 3D geometries using the local approaches introduced in Chapter 1. This chapter is useful for better understanding the application procedures of these methods and highlighting the differences between them. Chapter 3 provides a concise description of the HFMI treatment, its advantages, and its application limits compared to untreated As-Welded (AW) joints. It offers a detailed analysis of the parameters that influence fatigue strength and discusses the application of the Peak Stress Method (PSM) for this category of joints (with a weld toe radius ρ ̸= 0) for both 2D and 3D geometries. Chapter 4 describes the laboratory experiments conducted to verify whether the secondary stresses induced by misalignments, once the specimen is clamped in the testing machine, vary as a function of the applied force. Chapter 5 contains a collection of all the analyzed papers, providing a description of the joints for each publication, experimental test results, and the outcomes of analyses where the PSM was applied. Chapter 6 compiles all the fatigue design curves calibrated for HFMI-treated welded steel joints up to this point, incorporating new data into previously established curves. Additionally, it applies the stepwise fatigue design approaches recommended by IIW to the PSM. Chapter 7 is the concluding chapter, showcasing the possible agreement between the fatigue design curves calibrated in this and previous thesis projects and those derived using the IIW recommendations applied to the PSM.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.12608/78639