PSI - Issue 57

ScienceDirect Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2022) 000 – 000 Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2022) 000 – 000 Available online at www.sciencedirect.com Procedia Structural Integrity 57 (2024) 386–394

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2452-3216 © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the scientific committee of the Fatigue Design 2023 organizers 10.1016/j.prostr.2024.03.041 2452-3216 © 2023 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the scientific committee of the Fatigue Design 2023 organizers 2452-3216 © 2023 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the scientific committee of the Fatigue Design 2023 organizers © 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0 ) Peer-review under responsibility of the scientific committee of the Fatigue Design 2023 organizers Abstract The decreased fatigue capacity of welded components with increasing plate thickness, known as the size effect, is a well-known phenomenon in most fatigue design standards. The size effect links directly to the probabilistic nature of fatigue failure, where the extent of stress concentrations and stress gradients through the thickness governs the fatigue capacity of the joint. Fatigue design standards and recommendations address the thickness effect by modifying the S-N curve for joints thicker than the reference thickness value; however, the increase in fatigue strength due to the thinness effect is commonly not considered. The probabilistic nature of fatigue failure for welded joints is investigated in the current study using the weakest link modelling approach based on the stress distribution in the welded joint. The modelling approach is evaluated with the effective notch stress method and evaluated against fatigue test data for non-load-carrying welded joints found in the literature. The differences and similarities between the probabilistic and effective notch stress methods give a good insight into how the fatigue strength of welded joints is correlated to the size effect. 1. Introduction Lightweight structures are of great potential for various industries as the thin components have key roles in increasing fuel efficiency, improving performance, and reaching a higher payload capacity, to mention a few benefits. The structure's weight can be reduced significantly by using high-strength steels, thinner-wall components, and a high-stress design and thus reducing environmentalimpact and carbon footprint etc. Although an increase in materialstrength does not generally improve the fatigue performance of as-welded joints, thinning of the structural member may improve. The dimension of welded joints significantly affects the fatigue behaviour of components; the size of the main load-carrying plate, transverse attachment and weld size are the main contributing factors in this matter. This is known as the thickness or size effect. From a statistical point of view, large material volumes may contain more defect zones. Whilst fatigue is the weakest link process, the failure starts from the part of the component which is most prone to failure, weld imperfection and stress concentration regions, for instance, and thereby increasing the number of weak regions could lead to increasing the probability of fatigue failure. In the case of machined components, the defects can be identified as a reason for either surface defects or volume defects, and for welded structures, the defects can be related to the weld length(Deinböck et al., 2020; Pedersen, 2019). Size effect may also be the result of the manufacturing process, namely the technological size effect. This is a result of the fact that the production process may change the mechanical properties of the component due to, e.g., residual stress and angular-axial Fatigue Design 2023 (FatDes 2023) Modelling of Size Effect in Fatigue Strength for Welded Joints using Effective Notch Stress and Probabilistic Methods Mehdi Ghanadi 1 *, Gustav Hultgren 1 , Mattias Clarin 2 , Zuheir Barsoum 1 1 KTH Royal Institute of Technology, Department of Engineering Mechanics Teknikringen 8, SE-100 44 Stockholm, Sweden 2 SSAB Special Steels AB SE-781 84 Borlänge, Sweden Abstract The decreased fatigue capacity of welded components with increasing plate thickness, known as the size effect, is a well-known phenomenon in most fatigue design standards. The size effect links directly to the probabilistic nature of fatigue failure, where the extent of stress concentrations and stress gradients through the thickness governs the fatigue capacity of the joint. Fatigue design standards and recommendations address the thickness effect by modifying the S-N curve for joints thicker than the reference thickness value; however, the increase in fatigue strength due to the thinness effect is commonly not considered. The probabilistic nature of fatigue failure for welded joints is investigated in the current study using the weakest link modelling approach based on the stress distribution in the welded joint. The modelling approach is evaluated with the effective notch stress method and evaluated against fatigue test data for non-load-carrying welded joints found in the literature. The differences and similarities between the probabilistic and effective notch stress methods give a good insight into how the fatigue strength of welded joints is correlated to the size effect. Keywords: Size effect, Probabilistic model, Welded Joints, Fatigue 1. Introduction Lightweight structures are of great potential for various industries as the thin components have key roles in increasing fuel efficiency, improving performance, and reaching a higher payload capacity, to mention a few benefits. The structure's weight can be reduced significantly by using high-strength steels, thinner-wall components, and a high-stress design and thus reducing environmentalimpact and carbon footprint etc. Although an increase in materialstrength does not generally improve the fatigue performance of as-welded joints, thinning of the structural member may improve. The dimension of welded joints significantly affects the fatigue behaviour of components; the size of the main load-carrying plate, transverse attachment and weld size are the main contributing factors in this matter. This is known as the thickness or size effect. From a statistical point of view, large material volumes may contain more defect zones. Whilst fatigue is the weakest link process, the failure starts from the part of the component which is most prone to failure, weld imperfection and stress concentration regions, for instance, and thereby increasing the number of weak regions could lead to increasing the probability of fatigue failure. In the case of machined components, the defects can be identified as a reason for either surface defects or volume defects, and for welded structures, the defects can be related to the weld length(Deinböck et al., 2020; Pedersen, 2019). Size effect may also be the result of the manufacturing process, namely the technological size effect. This is a result of the fact that the production process may change the mechanical properties of the component due to, e.g., residual stress and angular-axial Fatigue Design 2023 (FatDes 2023) Modelling of Size Effect in Fatigue Strength for Welded Joints using Effective Notch Stress and Probabilistic Methods Mehdi Ghanadi 1 *, Gustav Hultgren 1 , Mattias Clarin 2 , Zuheir Barsoum 1 1 KTH Royal Institute of Technology, Department of Engineering Mechanics Teknikringen 8, SE-100 44 Stockholm, Sweden 2 SSAB Special Steels AB SE-781 84 Borlänge, Sweden Keywords: Size effect, Probabilistic model, Welded Joints, Fatigue * Corresponding author. Tel.: +46-70-166 74 28 E-mail address: ghanadi@kth.se * Corresponding author. Tel.: +46-70-166 74 28 E-mail address: ghanadi@kth.se