The load-bearing performance of train bodies under complex service conditions requires accurate evaluation, necessitating substantial analysis of the damage and fracture behavior of thin-walled structures with heterogeneous materials under complex stress states. To address this requirement, this study focuses on MIG-welded thin-walled 6005A-T6 aluminum alloy and proposes a parameter identification method based on the bilevel parallel optimization principle. The welding regions were characterized through metallographic and microhardness tests, and specimens were designed with pre-crack tips located in various welding regions. This enabled the calibration of material parameters from the elastic to the fracture stages for each welding region. By smoothing the material properties at the boundaries of the welding regions based on surface interpolation principles, the complex fracture behaviors, such as slant fractures and V-shaped fractures, were successfully represented. The predicted load–displacement curves closely matched the experimental results, with a relative error in peak force prediction within 8%. The proposed damage characterization method effectively captures material deformation behavior and accurately predicts fracture performance, offering potential refinements to current standards for welding crack propagation tests.

Among several parameters affecting the fatigue life of the welded joints, the size effect and weld quality are the main focus of thecurrent study. Fatigue design technical documents consider the thickness effect by reducing the S-N curve for joints thicker thanthe reference thickness value. Yet, limited attention has been paid to the thinness effect, where the specimens gain life due todecreasing thickness. Understanding the connection between weld profile geometry and fatigue strength is also imperative forassessing welded joints against repeated loads, as geometrical irregularities lead to local stress raising and decrease fatigue life.In this study, several fatigue test series were performed on non-load carrying welded joints with 2- and 16-mm thicknesses underuniaxial constant amplitude loading, resulting in S-N curves to analyse the influence of thickness on fatigue strength. For each testspecimen, the variation of weld shape along the weld seam is measured using a laser scanning tool. Assessment of fatigue testresults and weld profile measured data helps better understand the uncertainty and variation in fatigue life and the relationshipbetween fatigue strength with specimen thickness and the weld quality. The results indicate that there is a beneficial effect fromreducing thickness and increased weld quality.

The fatigue strength of welded joints is influenced by several parameters, among which the size effect, more specifically the thinness effect, and weld quality are investigated in the current study. To this aim, a number of test series were performed on transverse non-load-carrying attachments with 2 mm and 8 mm plate thicknesses under uniaxial tensile constant amplitude loading. Local weld geometrical parameters, along with angular misalignment between plates, were measured with the aid of a laser scanning device. The results were incorporated into the fatigue assessment and weld quality evaluation of welded specimens. The thinness effect is evident in the fatigue strength results, showing an increase of approximately 75% in fatigue strength. This effect is particularly pronounced in thin plates, where weld quality, angular misalignment, have a substantial impact on fatigue strength.

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.

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.

Inom den tunga industrin är strukturellt stål en nödvändig komponent för många lastbärande produkter. Användningen är så utbredd tack vare materialets robusthet, slitstyrka och funktionalitet, dessutom är det kostnadseffektivt. I takt med att industrin alltmer fokuserar på hållbar utveckling, ökar behovet på en effektiv materialanvändning och förbättring av komponenters prestanda. För att främja produktutveckling är det nödvändigt att optimera strukturer, vilket uppnås genom att implementera lämpliga designmetoder med rätt typ av högpresterande material. Sådana designstrategier bör fokusera på att strukturen blir så motståndskraftig som möjligt samtidigt som det förblir ekonomiskt försvarbart. Trots att produktionskostnaderna för dessa optimerade strukturer kan vara högre, kan detta ofta kompenseras av lägre driftskostnader och minskad miljöpåverkan.

Höghållfasta strukturstål erbjuder ökad statisk styrka och uppvisar förbättrad utmattningshållfasthet, tack vare en fördelaktig mikrostruktur. Den fulla potentialen påverkas dock avsevärt i strukturella tillämpningar av design- och tillverkningstekniker. Implementering av dessa typer av material för lättvikts- och högpresterande strukturer kräver därför en design som kan motstå hög spänning. Vanliga produktionsmetoder, så som svetsning och skärning, motverkar ofta förbättringen av utmattningshållfasthet i högpresterande material, vilket även framhävs av dagens standarder och riktlinjer. För att säkerställa att materialens egenskaper kan användas fullt ut, är det avgörande att förbättra och förstå effekterna av svetskvalitet, kantkvalitet vid skärning, defekttolerans och potentiella efterbehandlingar av svetsar.

Det aktuella arbetet undersöker aspekter som kan förbättra tillförlitligheten hos lastbärande strukturer, vilket underlättar användningen av design som tillåter höga spänningar och möjliggör användningen av höghållfasta stål. Kvaliteten hos svetsar och skurna kanter identifieras som en begränsande faktor för användningen av höghållfast stål. Forskningen undersöker dess inverkan och föreslår nya rekommendationer. Defekttoleransen studeras vidare för att förstå hur defekter påverkar dessa höghållfasta material. Resultaten ger viktiga insikter för utvecklingen av förbättrade kvalitetsrekommendationer för svetsar och skurna kanter, vilket sammantaget är grundläggande för att effektivt kunna utnyttja höghållfast stål.

Plate thickness influences the fatigue performance of welded components. In fatigue design standards and recommendations, the thickness effect and fatigue strength reduction have been considered by modifying the S–N curve for plates thicker than a reference thickness. However, increasing fatigue strength due to the thinness effect is often disregarded. The current study focuses on the size effect in fatigue of butt welded and non-load carrying cruciform welded joints under constant amplitude tension load. Literature data is evaluated using the effective notch stress method with a reference radius of 1 mm, which is used for all finite element models to ensure that FAT-value corresponding to a 1 mm notch radius remains constant across all models. A probabilistic assessment of the results using the weakest-link theory is applied to improve the prediction accuracy of thinner members outside the recommended thickness range of the used radius. The method reduces the S–N data scatter in comparison to the variation of test data and shows applicability also for thinner members. A comparison of the size effect for the current method with extrapolated values from standards and recommendations shows a difference in the size effect for thinner members.

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.

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.

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.

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.