The implication of thin-walled structures has recently gained attention in the shipbuilding of large-size constructions. Although this allows big ships to reduce their weight, several researches have shown that thin-walled structures generally exhibit severe welding-induced imperfections, which considerably affect the state of stress experienced by steel-made, 3 to 4 mm-thick plates. Currently, the IIW recommendations provide a stress magnification factor k_m to estimate the Hot-Spot Structural stress for the fatigue design of welded components. Such factor considers eccentricity, linear lateral deflection and straightening effects under tensile load. However, its validity is confined to plates with thickness of at least 5 mm, since the k_m formula neglects the initial curvature effect, which is actually critical for the stress-state of thin plates. In this regard, the aim of this thesis is to provide an analytical model for the k_m factor that includes the initial curvature effect in evaluating the Hot-Spot structural stress due to welding-induced distortions. For this purpose, the study used a second-order, non-linear Beam Theory. Based on this theory, a model was designed by describing the welding-induced deflection only along the longitudinal direction of a plate. The study resulted in a concise analytical model for the k_m factor. The geometrically non-linear Finite Element Analysis (FEA), provided by the solver ABAQUS, validated the model for 4 mm-thick beam and plate strip elements, each of which assuming 27 configurations with different distortions and tensile loads. Results showed less than 2% and 4% error for beam and plate strip, respectively. As the plate strip represents a plate with infinitely-rigid stiffener effect on lateral edges, a second plate element was modelled to simulate non-rigid lateral edges in order to extend the applicability of the model. A maximum underestimation of the k_m factor of about 8% for the second plate was observed. Both its centerline and peak stress assessment showed mean errors around the 3%. In conclusion, the thesis confirms the dominating role of the welding-induced initial curvature effect in the thin-plate structural assessment and validates a suitable analytical model for the stress magnification factor k_m. Nevertheless, although the analytical model has been validated by FEA, experimental validation is recommended for future works.
Di recente, l'utilizzo di strutture a pareti sottili ha permesso significativi miglioramenti nel design di costruzioni di grande dimensioni come le navi da crociera. Nonostante notevoli benefici in termini di peso e gestione degli spazi interni, le strutture a pareti sottili sono particolarmente sensibili alla formazione di imperfezioni geometriche durante i processi di saldatura. Attualmente, le raccomandazioni fornite dal IIW per l'analisi a fatica di componenti saldati sono valide per pareti con spessore minimo di 5 mm, le quali mostrano deformazioni lineari. In caso di spessori di circa 3-4 mm, oltre i difetti osservabili su pareti spesse, bisogna considerare la curvatura della distorsione. Questa tesi presenta un modello analitico per il fattore di amplificazione del Hot-Spot structural stress nell'area di saldatura, includendo l'effetto dovuto alla non linearità della curvatura, quando il piatto viene sottoposto ad uno stress di tensione assiale. Utilizzando una teoria di secondo ordine delle travi e il principio di sovrapposizione degli effetti, tale modello fornisce una stima dello stato di stress del piatto in direzione longitudinale consistente (ovvero con un errore minore del 5%) con i risultati delle simulazioni numeriche fornite dal solver ABAQUS. L'applicabilità del modello viene estesa attraverso lo studio di strutture con diverse condizioni al contorno, ed un'analisi parametrica dimostra il ruolo predominante della curvatura nell'amplificazione dello stress nell'area di saldatura. Poichè il modello è convalidato sulla base di analisi con elementi finiti (FEA), studi sperimentali sono necessari per confermare la validità del modello.
Stress magnification factor for thin plates with welding-induced distortions
MANCINI, FEDERICA
2018/2019
Abstract
The implication of thin-walled structures has recently gained attention in the shipbuilding of large-size constructions. Although this allows big ships to reduce their weight, several researches have shown that thin-walled structures generally exhibit severe welding-induced imperfections, which considerably affect the state of stress experienced by steel-made, 3 to 4 mm-thick plates. Currently, the IIW recommendations provide a stress magnification factor k_m to estimate the Hot-Spot Structural stress for the fatigue design of welded components. Such factor considers eccentricity, linear lateral deflection and straightening effects under tensile load. However, its validity is confined to plates with thickness of at least 5 mm, since the k_m formula neglects the initial curvature effect, which is actually critical for the stress-state of thin plates. In this regard, the aim of this thesis is to provide an analytical model for the k_m factor that includes the initial curvature effect in evaluating the Hot-Spot structural stress due to welding-induced distortions. For this purpose, the study used a second-order, non-linear Beam Theory. Based on this theory, a model was designed by describing the welding-induced deflection only along the longitudinal direction of a plate. The study resulted in a concise analytical model for the k_m factor. The geometrically non-linear Finite Element Analysis (FEA), provided by the solver ABAQUS, validated the model for 4 mm-thick beam and plate strip elements, each of which assuming 27 configurations with different distortions and tensile loads. Results showed less than 2% and 4% error for beam and plate strip, respectively. As the plate strip represents a plate with infinitely-rigid stiffener effect on lateral edges, a second plate element was modelled to simulate non-rigid lateral edges in order to extend the applicability of the model. A maximum underestimation of the k_m factor of about 8% for the second plate was observed. Both its centerline and peak stress assessment showed mean errors around the 3%. In conclusion, the thesis confirms the dominating role of the welding-induced initial curvature effect in the thin-plate structural assessment and validates a suitable analytical model for the stress magnification factor k_m. Nevertheless, although the analytical model has been validated by FEA, experimental validation is recommended for future works.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/149720