Fracture toughness to assess the effect of trimming on the fatigue behaviour of high-strength steels for chassis parts

High-strength steels are widely used in vehicle body-in-white, offering a good balance between crashworthiness and lightweight design. The increased requirements of heavier electric vehicles, in terms of fatigue resistance and crashworthiness, highlight that chassis parts have remarkable lightweighting potential. However, applying these grades in chassis parts is not straightforward, as the forming processes, like trimming, may introduce surface defects that compromise the fatigue resistance of the component. This work presents a material selection strategy for the applicability of high-strength steels in chassis parts of electric vehicles. The proposed approach allows the evaluation of the key parameters of the chassis parts in a simple way. The crash performance is evaluated through fracture toughness using the essential work of fracture (EWF) methodology. The method is applied to thin high-strength steel sheets employing double-edge notched tensile specimens (DENT). On the other hand, fatigue performance is investigated in terms of fatigue resistance for both notched and unnotched specimens. The results for different complex-phase and dual-phase steels show a good agreement between the EWF and the fatigue notch factor. The method could help apply high-strength steel to chassis parts, as designers will have a tool to focus the expensive fatigue tests on the best material candidates.Postprint (published version

Fatigue resistance evaluation of high Mn-TWIP steel through damage mechanics: A new method based on stiffness evolution

The work presented here deals with the implementation of a new methodology that allows fast and reliable determination of the fatigue strength. It is based on monitoring the specimen stiffness changes at different stress levels, as an indicator of the evolution of fatigue damage. This new rapid fatigue test uses techniques available in many laboratories, as the DIC (Digital Image Correlation) technique and common extensometers. Moreover, the obtained data are easier to handle than infrared cameras or acoustic emission systems data, and the experimental procedure to determine the fatigue limit is more evident than in the self-heating method. Experiments have been conducted in TWIP (Twinning Induced Plasticity) steel, a material used for lightweighting the structural parts of vehicles. With their excellent energy absorption capacity, TWIP steels can satisfy the part requirements in terms of crash performance, while their high tensile strength can deal with the cyclic loads acting on chassis parts. Therefore, many efforts focus on improving the fatigue strength of TWIP steels through pre-straining and/or surface treatments. However, finding the best way to improve the fatigue resistance requires time and resources that often hinder the development of the material. For this reason, a TWIP steel has been selected to check the new rapid fatigue test. The prediction made using the proposed approach is validated by comparison with conventional staircase results and fatigue crack growth standardised tests. The good agreement allows proposing the new method as a fast and efficient way to determine the fatigue resistance in metals.Peer ReviewedPostprint (published version

Understanding the fatigue notch sensitivity of high-strength steels through fracture toughness

This study presents an innovative approach for selecting high-strength materials for fatigue dimensioning parts, considering both fracture toughness and fatigue performance. Warm and hot forming processes enable the construction of high-strength parts above 1000 MPa with complex geometries, making them suitable for lightweight chassis in automotive and freight applications. This research reveals that high-strength steels can experience up to a 40% reduction in fatigue performance due to manufacturing defects introduced during punching and trimming. Fracture toughness has been proposed as a good indicator of notch sensitivity, with a strong correlation of 0.83 between fracture toughness and fatigue notch sensitivity. Therefore, by combining fracture toughness measurements and fatigue resistance obtained through the rapid fatigue test, it becomes possible to quickly identify the most fatigue-resistant materials to deal with defects. Among the nine materials analysed, warm-formed steels show promising characteristics for lightweight chassis construction, with high fatigue resistance and fracture toughness exceeding the proposed fracture threshold of 250 kJ/m2.Peer ReviewedPostprint (published version

Simulation of metal punching and trimming using minimal experimental characterization

This paper presents a validated finite element modeling approach for simulating shear cutting, needing a minimal amount of experimental characterization. Only one uniaxial tensile test and one force–displacement relationship from a punching experiment are needed for calibration, with maintained prediction accuracy compared to more experimentally demanding approaches. A key ingredient is the observation that the Lode angle parameter is close to zero in the fracture region, postulating that the fracture strain only depends on stress triaxiality, with one free calibration parameter. The true stress–strain behavior is provided from inverse modeling of the tensile test, whereas the fracture model is calibrated using the punching test. The model is verified for different materials by comparing force–displacement curves for punching experiments not used in the calibration. The prediction error for the intrusion is below 4%. A validation is made for two setups. The local residual stresses are measured using Focused Ion-Beam Digital Image Correlation (FIB-DIC). The simulated values are within the experimental bounds. Cut edge morphology and plastic strains obtained by nano-indentation mappings are compared to simulation results, showing a decent agreement. For trimming, the cut edge morphology prediction performance decreases at 17% cutting clearance while it is maintained over the whole range for punching. The predicted hardness values have a mean absolute percentage error below 7.5%. Finally, the effect of element size and remeshing is discussed and quantified. The minimal experimental characterization and simulation effort needed, enables an efficient optimization of the cutting process in the industry.Peer ReviewedPostprint (published version

Numerical simulation of a rapid fatigue test of high Mn-TWIP steel via a high cycle fatigue constitutive law

The generation of reliable data in the high cycle fatigue domain is crucial to support further metallurgic developments of fatigue optimized steel grades. Commonly employed for this aim, traditional standardized characterization methods are expensive and time-consuming. Thus, to circumvent these limitations, different accelerated fatigue testing methodologies have been proposed. In this work, the rapid fatigue test based on stiffness evolution is numerically reproduced using the finite element method for a specific grade of twinning-induced plasticity steel. A high cycle fatigue constitutive law grounded on the continuum damage mechanics framework is employed for this purpose. To adequately capture the material non-linear behavior observed in the experiments, a novel hardening–softening stress–strain curve for damage is proposed. The entire load history in the fatigue domain is modeled. A cycle-jump algorithm is used to improve the computational efficiency of the simulations. It is shown that a reduction of about 55% in the analysis elapsed time is reached by using this algorithm, while the result accuracy is maintained. Finally, the good agreement between numerical and experimental results, revealed by a maximum relative error smaller than 6.0%, evidences the potential of the present constitutive formulation to model the behavior of metals in the high cycle fatigue domain.This work has been done within the framework of the Fatigue4Light (H2020-LC-GV-06-2020) project: “Fatigue modeling and fast testing methodologies to optimize part design and to boost lightweight materials deployment in chassis parts”. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 101006844. The work has been also supported by the Spanish Government program, Spain FPU17/04196. The authors gratefully acknowledge all the received support. Finally, they acknowledge the support received by the Severo Ochoa Centre of Excellence (2019-2023, Spain under the grant CEX2018-000797-S funded by MCIN/AEI/10.13039/501100011033, Spain .Peer ReviewedPostprint (published version

A damage-based fatigue life prediction method for metallic alloys and composites

Tesi en modalitat de compendi de publicacions(English) Fatigue failures in materials have been studied for centuries, with early pioneers like August Wöhler noting that repeated loading, even below the static strength of the material, could lead to structural deterioration. Despite decades of research by notable authors, fatigue remains a complex and challenging issue, accounting for most service failures in metallic and composite structures. Therefore, designing structures to withstand cyclic loads without compromising integrity is crucial. But this process requires conducting numerous fatigue tests to define the appropriate design stress levels for each material and condition. However, determining the fatigue behaviour of metallic alloys and composites through standardised testing methods is often costly and time-consuming. While various techniques have been proposed to expedite testing and enhance the optimisation of materials and components for fatigue resistance, they have not gained wide industry adoption due to limitations in equipment or complex data treatment. Thus, there is an industry need for a testing method that rapidly determines material fatigue resistance, especially in the automotive sector where new designs and developments require results in a short time. To address this challenge, the development of a new testing method for characterising the fatigue resistance of metallic alloys and composites has become essential, as current solutions like ultra-high frequency testing machines or the rapid testing methods using temperature variations are not universal solutions for all these materials. In this thesis, a novel fatigue testing method, named the stiffness method, is introduced to rapidly assess the fatigue resistance of both metallic and composite materials with minimal specimens and in a short timeframe. This approach involves monitoring fatigue damage using different variables, such as inelastic strain in metallic alloys and compliance in composites. These measurements overcome the limitations of other methods by using common extensometers like digital image correlation techniques and contact extensometers. The results obtained through the stiffness method are not only convincing but also more accessible for interpretation and discussion compared to other monitoring techniques, such as temperature dissipation. The effectiveness of this approach has been validated across nineteen metallic materials, including titanium and aluminium alloys, carbon steels, stainless steels, and one carbon-fibre composite. The estimated fatigue limit and high cycle fatigue curve (S-N curve) obtained through the stiffness method align excellently with values derived from standardised tests. This underscores the method as a powerful and efficient tool for swiftly assessing the fatigue behaviour of both metallic alloys and composite materials.Furthermore, this research investigates the fatigue reduction observed in high-strength steels when surface defects are introduced during manufacturing processes such as shearing. This reduction in fatigue resistance is explained by the fatigue notch sensitivity of the material. The results establish a robust correlation between fracture toughness, assessed within the framework of fracture mechanics, and fatigue notch sensitivity in high-strength steels. As a result, fracture toughness coupled with the stiffness method can be a valuable toolkit for selecting materials with superior fatigue resistance. In summary, this work presents an innovative and efficient approach to evaluate the fatigue behaviour of metallic alloys and composite materials, offering significant advantages in terms of time and resource savings. Additionally, it introduces fracture toughness as a valuable indicator for material selection in high-strength steel applications, ultimately contributing to improved fatigue performance.(Català) Les fallades per fatiga, sobretot en materials metàl·lics, han estat estudiades durant segles, amb pioners com August Wöhler que va observar que l'aplicació d'una càrrega repetida molt per sota de la resistència estàtica del material, podia portar a la deterioració i fractura del material. Tot i això, dècades després, la fatiga continua essent un problema complex, responsable de la majoria de les fallades en servei en materials metàl·liques i compostos. De manera que és vital dissenyar estructures que resisteixin càrregues cícliques sense comprometre la seva integritat estructural. Aquest procés però, requereix realitzar nombrosos assaigs de fatiga per definir els nivells de tensió segurs per a cada material i condició. No obstant, determinar el comportament a la fatiga mitjançant els mètodes d'assaig estandarditzats sovint és costós i requereix molt temps. Tot i que històricament s’han proposat diverses tècniques per accelerar els assaigs i optimitzar els materials i components a fatiga, no s'han aplicat generalitzadament a la indústria degut a limitacions en l'equipament, sovint molt car, i a un tractament de dades complex. Així doncs, hi ha la necessitat de desenvolupar un mètode d'assaig que permeti determinar ràpidament la resistència a la fatiga dels materials, especialment en el sector de l'automoció, on els nous dissenys i desenvolupaments requereixen resultats en poc temps. En aquesta tesi, s'introdueix un nou mètode d’assaig de fatiga, anomenat el mètode de la rigidesa, per fer front a aquest repte i avaluar ràpidament la resistència a la fatiga tant de materials metàl·lics com compostos amb un nombre mínim de mostres i en un curt període de temps. Aquest enfocament implica la monitorització del dany per fatiga mitjançant variables diferents, com la deformació inelàstica en aliatges metàl·lics i la compliança en compostos. Aquesta estratègia permet superar les limitacions d'altres mètodes utilitzant extensòmetres comuns a tots els laboratoris de fatiga com per exemple la tècnica de correlació digital d'imatges o extensòmetres de contacte. Els resultats obtinguts mitjançant el mètode de la rigidesa no només són convincents sinó que també són més fàcils d'interpretar i discutir en comparació amb altres tècniques de monitorització com la dissipació de temperatura. Aquest mètode s'ha validat en 19 materials metàl·lics diferents, incloent aliatges de titani i alumini, acers al carboni, acers inoxidables i un compost de fibra de carboni. El límit de fatiga estimat i la corba de fatiga a alt nœmero de cicles (corba S-N) obtinguts mitjançant el mètode de la rigidesa coincideixen amb els valors obtinguts de les proves estandarditzades. Això reforça la validesa del mètode com una eina potent i eficient per avaluar ràpidament el comportament a la fatiga tant d’aliatges metàl·lics com de materials compostos. Per altra banda, s'ha investigat la reducció de la resistència a fatiga observada en acers d'alta resistència quan hi ha defectes superficials dels processos de fabricació de peces metàl·liques com el tall per cisalla. Aquesta reducció de la resistència a la fatiga es pot explicar per la sensibilitat a l'entalla a fatiga del material. Els resultats mostren una correlació robusta entre la tenacitat a la fractura, avaluada en el marc de la mecànica de la fractura, i la sensibilitat a l'entalla a fatiga en acers d'alta resistència. Per tant, la tenacitat a la fractura, juntament amb el mètode de la rigidesa, pot ser una eina molt valuosa per seleccionar materials amb una resistència superior a la fatiga. En resum, aquesta tesi presenta un enfocament innovador i eficaç per avaluar el comportament a la fatiga d'aliatges metàl·lics i materials compostos, oferint avantatges significatius en termes d'estalvi de temps i recursos. A més a més, introdueix la tenacitat a la fractura com a un indicador valuós per a la selecció d'acers d'alta resistència, contribuint en última instància a una millora del rendiment a la fatiga.DOCTORAT EN CIÈNCIA I ENGINYERIA DELS MATERIALS (Pla 2012

Fatigue resistance of press hardened 22MnB5 steels

Press hardening of Boron steels, mainly the 22MnB5 grade, has been implemented during more than a decade in several structural components of the automotive sector, mostly in the body-in-white (BiW). The process combines sheet forming and heat treatment in one single die quenching step, to produce components with complex geometry and strength up to 1600 MPa. In press hardened steels, fatigue behaviour is very sensitive to surface defects or irregularities, either intrinsic or introduced during forming operations such as trimming. This work addresses the understanding and prediction of fatigue resistance of press hardened steels from a Linear Elastic Fracture Mechanics (LEFM) approach. The size of fatigue originating defects has been evaluated and used to estimate the fatigue limit for different surface conditions (coated and uncoated), different coatings (Al-Si and Zn) and different edge conditions (polished and mechanically trimmed)

Fatigue resistance of press hardened 22MnB5 steels

Press hardening of Boron steels, mainly the 22MnB5 grade, has been implemented during more than a decade in several structural components of the automotive sector, mostly in the body-in-white (BiW). The process combines sheet forming and heat treatment in one single die quenching step, to produce components with complex geometry and strength up to 1600 MPa. In press hardened steels, fatigue behaviour is very sensitive to surface defects or irregularities, either intrinsic or introduced during forming operations such as trimming. This work addresses the understanding and prediction of fatigue resistance of press hardened steels from a Linear Elastic Fracture Mechanics (LEFM) approach. The size of fatigue originating defects has been evaluated and used to estimate the fatigue limit for different surface conditions (coated and uncoated), different coatings (Al-Si and Zn) and different edge conditions (polished and mechanically trimmed)