| 1. |
- Eliasson, Sara, et al.
(författare)
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Numerical fatigue assessment of a cross-ply carbon fiber laminate using a probabilistic framework
- 2024
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Ingår i: Composites Part B. - : Elsevier Ltd. - 1359-8368 .- 1879-1069. ; 281
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Tidskriftsartikel (refereegranskat)abstract
- A probabilistic framework is developed utilizing a two-scale modeling approach to efficiently consider the material variability that is typical for composite materials. The modeling integrates a macro-scale model with effective elastic properties extracted from micro-mechanical simulations. Using a weakest link modeling approach for fatigue assessment the combined effects of defects on fatigue strength in a Carbon Fiber Reinforced Polymer (CFRP) material can be investigated. A full fatigue test program is presented and is used to calibrate the probabilistic fatigue model. By including material variability in the numerical model, the calibrated probabilistic model improves the fatigue life prediction.
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| 2. |
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| 3. |
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| 4. |
- Eliasson, Sara, et al.
(författare)
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Probabilistic fatigue strength assessment of cross-ply laminates : Exploring effects of manufacturing defects through a two-scale modeling approach
- 2024
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Ingår i: Composite structures. - : Elsevier BV. - 0263-8223 .- 1879-1085. ; 330
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Tidskriftsartikel (refereegranskat)abstract
- The study presents a two-scale modeling approach allowing for an efficient fatigue strength evaluation on a macro scale considering a micro-mechanical defect characterization of a Carbon Fiber Reinforced Polymer (CFRP) material. The modeling approach integrates a macro model with the effective elastic properties from micro-mechanical simulations considering voids. This enables the analysis of defects’ influence on material fatigue strength using a probabilistic weakest link approach. A CFRP laminate with a cross-ply layup was investigated. Two simulation case studies demonstrate the effect of void content and size on the characteristic fatigue strength. An experimental investigation was conducted testing the laminates in tension–tension fatigue verifying the predicted numerical behavior. The numerical models identify a difference in the characteristic fatigue strength consistent with the fatigue test results. It is numerically concluded that the investigated CFRP material's fatigue strength is affected by the presence of voids and even with only a slight difference in the global void volume fraction a scatter in fatigue strength is identified.
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| 5. |
- Ghanadi, Mehdi, et al.
(författare)
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Modelling of Size Effect in Fatigue Strength for Welded Joints using Effective Notch Stress and Probabilistic Methods
- 2023
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Ingår i: Fatigue Design 2023, FatDes 2023. - : Elsevier B.V.. ; , s. 386-394
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Konferensbidrag (refereegranskat)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.
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| 6. |
- Hultgren, Gustav, et al.
(författare)
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Digital scanning of welds and influence of sampling resolution on the predicted fatigue performance: modelling, experiment and simulation
- 2021
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Ingår i: Metals. - : MDPI AG. - 2075-4701. ; 11:5
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Tidskriftsartikel (refereegranskat)abstract
- Digital weld quality assurance systems are increasingly used to capture local geometrical variations that can be detrimental for the fatigue strength of welded components. In this study, a method is proposed to determine the required scanning sampling resolution for proper fatigue assessment. Based on FE analysis of laser‐scanned welded joints, fatigue failure probabilities are computed using a Weakest‐link fatigue model with experimentally determined parameters. By down‐sampling of the scanning data in the FE simulations, it is shown that the uncertainty and error in the fatigue failure probability prediction increases with decreased sampling resolution. The required sampling resolution is thereafter determined by setting an allowable error in the predicted failure probability. A sampling resolution of 200 to 250 μm has been shown to be adequate for the fatigue‐loaded welded joints investigated in the current study. The resolution requirements can be directly incorporated in production for continuous quality assurance of welded structures. The proposed probabilistic model used to derive the resolution requirement accurately captures the experimental fatigue strength distribution, with a correlation coefficient of 0.9 between model and experimental failure probabilities. This work therefore brings novelty by deriving sampling resolution requirements based on the influence of stochastic topographical variations on the fatigue strength distribution.
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| 7. |
- Hultgren, Gustav, et al.
(författare)
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Effect of measured local weld geometry and its variability on the local weld stress
- 2023
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Ingår i: Fatigue Design 2023, FatDes 2023. - : Elsevier B.V.. ; , s. 428-436
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Konferensbidrag (refereegranskat)abstract
- The local weld geometry and its variability can significantly affect the fatigue strength of structures, especially for non-load-carrying welds. Standardised definitions, such as sharp transition radii or undercuts, govern stress-raising effects at the weld toe. High-resolution digital tools can nowadays accurately determine these parameters, allowing for studying the impact of geometry variability on fatigue strength. However, real welds rarely exhibit idealised transitions as multiple radii, undercuts, and ripple lines introduce uncertainty in geometry estimations. Numerical simulations of the actual weld geometry, with all its variations, in combination with probabilistic evaluations, have shown great potential for studying the influence of competing notches in the weld. This study compares probabilistic evaluations of 3D scanned welds with analytical relations for stress concentration factors. Results reveal no clear trend between the analytical expressions and the ratio of simulated sectional stress to nominal stress. This highlights the challenge of directly applying existing analytical equations to idealised measurement data from real welds in their as-welded condition for fatigue strength estimations.
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| 8. |
- Hultgren, Gustav
(författare)
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Fatigue and Fracture of High-Strength Steels : Improving Reliability in Strength Assessment
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Structural steel plays a fundamental role in the heavy industry, serving as a key material for numerous load-bearing products and equipment. Its widespread use is attributed to its robustness, resistance to wear, ease of use in construction, and cost-effectiveness. As industries increasingly focus on sustainable development, there is a growing emphasis on efficient material use and the enhancement of component performance. The optimisation of structures, achieved through integrating high-performance materials and appropriate design methodologies, is crucial in advancing product development. Such design strategies should focus on maximising structural capacity while maintaining economic viability. Although the production costs for these optimised structures may be higher, this is often compensated by their reduced operational costs and lower environmental impact. The implementation of high-strength structural steels for lightweight and high-performance structures necessitates a design that can withstand high stress. These materials offer increased static strength and exhibit enhanced fatigue resistance thanks to their advantageous microstructure. However, the full potential of these materials in structural applications is significantly influenced by design decisions and manufacturing techniques. Common production methods, such as welding and cutting, often impede the improvement of fatigue strength in high-performance materials, as numerous standards and guidelines indicate. Therefore, to fully leverage the benefits of high-strength materials, it is crucial to enhance and comprehend the effects of weld quality, cut edge quality, defect tolerance and potential post-weld treatments, ensuring these factors align with the materials' enhanced strength characteristics.The present work investigates aspects that could enhance the reliability of load-bearing structures, thereby facilitating the use of high-stress designs and the integration of high-strength steels. It identifies the quality of welds and cut edges as a key limiting factor. The research thoroughly examines its impact and proposes new recommendations. The defect tolerances are also further studied to understand how defects impact these high-strength materials. The findings offer vital insights for developing improved quality recommendations for welds and cut edges, which are fundamental in effectively utilising high-strength steel.
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| 9. |
- Hultgren, Gustav, et al.
(författare)
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Fatigue assessment in welded joints based on geometrical variations measured by laser scanning
- 2020
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Ingår i: Welding in the World. - : Springer Science and Business Media Deutschland GmbH. - 0043-2288 .- 1878-6669. ; 64:11, s. 1825-1831
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Tidskriftsartikel (refereegranskat)abstract
- In the current study a method to determine the location of fracture initiation for non-load carrying fillet welds based on continuous geometry measurements is proposed. Measurements and weld quality evaluation were carried out on welded specimens using the Winteria® software qWeld. One hundred nineteen specimens were produced, scanned, and fatigue tested until failure. The fracture surfaces have been investigated in order to find the location(s) for most probable point(s) of initiation. These data were then used to fit the proposed model parameters used to predict the point of initiation. Local weld geometry measurements were extracted from the predicted fracture initiation location(s) to analyse the correlation between local weld geometry and fatigue life. It was observed that fatigue life and leg length were positively correlated and that strong correlations exist between the individual geometrical parameters with regard to location of the fatigue crack initiation.
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| 10. |
- Hultgren, Gustav, et al.
(författare)
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Fatigue probability model for AWJ-cut steel including surface roughness and residual stress
- 2021
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Ingår i: Journal of constructional steel research. - : ELSEVIER SCI LTD. - 0143-974X .- 1873-5983. ; 179
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Tidskriftsartikel (refereegranskat)abstract
- An analytical model for the fatigue probability of abrasive waterjet cut high strength steel as a function of surface roughness, surface residual stress, tensile strength and number of cycles to failure is presented. Based on the model, which is valid in the finite and infinite-life high cycle fatigue regime, the influence of the aforementioned parameters on the fatigue strength at different probability levels is studied. For validation, fatigue tests are performed on abrasive waterjet-cut dog-bone specimens manufactured from high-strength steel with a yield strength of 700 MPa. Residual stresses are measured parallel to the loading direction at the inlet, middle and outlet of the cut surface. Surface roughnesses are measured with laser line triangulation as well as a traditional contact stylus method, showing good agreement between both measurement techniques. The proposed probabilistic model shows good agreement with the experimental results with less than 4% error in the predicted mean fatigue limit. Furthermore, the applicability of the presented analytical expression in a probabilistic design framework is demonstrated. An engineering example is introduced demonstrating the implementation of the model in a finite-element simulation, accounting for both multiaxial loading and the statistical size effect. (c) 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
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| 11. |
- Hultgren, Gustav, et al.
(författare)
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Fatigue strength assessment of welded joints incorporating the variability in local weld geometry using a probabilistic framework
- 2023
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Ingår i: International Journal of Fatigue. - : Elsevier BV. - 0142-1123 .- 1879-3452. ; 167
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Tidskriftsartikel (refereegranskat)abstract
- Progress in developing digital quality assurance systems for welded joints has made it possible to accurately measure the local geometry and its variation, making it possible to derive new relations between the geometric variations and fatigue. A probabilistic model for the fatigue strength is here presented based on the actual weld geometry. The novelty lies in that representative stresses can be determined for both the complete weld and sections of the weld. Calibration of the model using 105 fatigue-tested specimens shows a reduced variation in SN-diagrams compared with the nominal stress methods when substantial weld geometry variations are present.
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| 12. |
- Hultgren, Gustav, et al.
(författare)
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Fracture toughness assessment of surface cracks in slender ultra-high-strength steel plates
- 2023
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Ingår i: Engineering Fracture Mechanics. - : Elsevier Ltd. - 0013-7944 .- 1873-7315. ; 289
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Tidskriftsartikel (refereegranskat)abstract
- Safe design against unstable fractures in load-bearing structures is crucial at sub-zero temperatures where the brittle fracture toughness can be unfavourable, especially for high-stress designs incorporating ultra-high-strength steels. The brittle fracture toughness of surface cracks in structural steel with a minimum yield strength of 1300 MPa is, for this reason, tested in the present study at sub-zero temperatures. The realistic flaws are compared with single-edge notched specimens (SEN(B)) from thicker plates with the same chemical composition, using a representative fracture toughness for a three-dimensional crack front according to the Master Curve method. A novel approach determines the latter without considering the local temperature and constraint variation through empirical relations. The experimental result shows a difference in the reference temperature between the two specimen types, which likely is the natural variation of the manufactured materials and/or a loss of constraint due to the difference in the scaled specimen deformation level.
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| 13. |
- Hultgren, Gustav, et al.
(författare)
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Mapping of scatter in fatigue life assessment of welded structures—a round-robin study
- 2021
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Ingår i: Welding in the World. - : Springer Science and Business Media Deutschland GmbH. - 0043-2288 .- 1878-6669. ; 65:9, s. 1841-1855
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Tidskriftsartikel (refereegranskat)abstract
- A round-robin study has been carried out within a national project in Sweden with the addition of an international participant, where several industrial partners and universities are participating. The project aims to identify variation and sources of variation in welding production, map scatter in fatigue life estimation, and define and develop concepts to reduce these, in all steps of product development. The participating organisations were asked to carry out fatigue life assessment of welded box structures, which is a component in load-carrying structures. The estimations of fatigue life have also been compared with fatigue test results. Detailed drawings, loads and material data were also given to the participants. The participants were supposed to use assessment methods based on global and local stresses using the design codes or recommendations they currently use in-house. Differences were identified between both methods and participants using the same codes/recommendations. Applicability and conditions from the cases in the codes were also identified to be differently evaluated between the participants. It could be concluded that for the applied cases the nominal stress method often overestimated the fatigue life and had a high scatter in the estimations by different participants. The effective notch method is conservative in comparison to the life of tested components with little scatter between the results derived by the participants.
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| 14. |
- Rohani Raftar, Hamidreza, et al.
(författare)
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Assessing local stresses in scanned fillet weld geometry using bagged decision trees
- 2024
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Ingår i: Journal of constructional steel research. - : Elsevier BV. - 0143-974X .- 1873-5983. ; 218
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Tidskriftsartikel (refereegranskat)abstract
- This study addresses the limitations of current parametric equations and artificial neural networks (ANNs) in accurately predicting the stress concentration factor (SCF) of fillet welded joints stemming from the simplification of their real weld profiles. To improve the accuracy, this study introduces bagged trees for estimating local stresses. The dataset used as the foundation for training the bagged trees is extracted from the actual weld geometry of T-shaped joints. It is created via a digitalization process involving the extraction of actual geometric parameters from the joints, which are transformed into finite element models (FEMs). These models are then employed to determine the ratio between the simulated sectional stress and the nominal stress (σsec/∆σnom) under an axial loading condition. A comprehensive comparison is carried out among existing parametric equations, ANNs, and the proposed bagged trees. The results emphasize the inadequacy of idealized geometry models in accurately determining local stresses for real weld profiles. In contrast, bagged trees are a promising method for accurately computing sectional weld stresses (σsec) within real weld geometry.
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