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| 2. |
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| 3. |
- Frómeta, D., et al.
(författare)
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A new cracking resistance index based on fracture mechanics for high strength sheet metal ranking
- 2021
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Konferensbidrag (refereegranskat)abstract
- Driven by current safety and weight reduction policies in the automotive sector, the development of new high strength sheet metal products has experienced unprecedented growth in the last years. With the emergence of these high strength materials, new challenges related to their limited ductility and higher cracking susceptibility have also raised. Accordingly, the development of new fracture criteria accounting for the material's cracking resistance has become unavoidable. In this work, a new cracking resistance index (CRI) based on fracture mechanics is proposed to classify the crack propagation resistance (i.e. the fracture toughness) of high strength metal sheets. The index is based on the fracture energy obtained from tensile tests with sharp-notched specimens. The procedure is very fast and simple, comparable to a conventional tensile test, and it may be used as routine testing for quality control and material selection. The CRI is investigated for several advanced high strength steel (AHSS) sheets of 0.8-1.6 mm thickness with tensile strengths between 800 and 1800 MPa. The results show that the proposed index is suitable to rank high strength steel sheets according to their crack propagation resistance and it can be correlated to the material's crashworthiness and edge cracking resistance.
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| 4. |
- Frómeta, D., et al.
(författare)
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Assessing edge cracking resistance in AHSS automotive parts by the Essential Work of Fracture methodology
- 2017
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Ingår i: Journal of Physics, Conference Series. - : Institute of Physics (IOP). - 1742-6588 .- 1742-6596. ; 896
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Tidskriftsartikel (refereegranskat)abstract
- Lightweight designs and demanding safety requirements in automotive industry areincreasingly promoting the use of Advanced High Strength Steel (AHSS) sheets. Such steelspresent higher strength (above 800 MPa) but lower ductility than conventional steels. Their greatproperties allow the reduction of the thickness of automobile structural components withoutcompromising the safety, but also introduce new challenges to parts manufacturers. Thefabrication of most cold formed components starts from shear cut blanks and, due to the lowerductility of AHSS, edge cracking problems can appear during forming operations, forcing thestop of the production and slowing down the industrial process.Forming Limit Diagrams (FLD) and FEM simulations are very useful tools to predict fractureproblems in zones with high localized strain, but they are not able to predict edge cracking. Ithas been observed that the fracture toughness, measured through the Essential Work of Fracture(EWF) methodology, is a good indicator of the stretch flangeability in AHSS and can help toforesee this type of fractures.In this work, a serial production automotive component has been studied. The componentshowed cracks in some flanged edges when using a dual phase steel. It is shown that theconventional approach to explain formability, based on tensile tests and FLD, fails in theprediction of edge cracking. A new approach, based on fracture mechanics, help to solve theproblem by selecting steel grades with higher fracture toughness, measured by means of EWF.Results confirmed that fracture toughness, in terms of EWF, can be readily used as a materialparameter to rationalize cracking related problems and select AHSS with improved edgecracking resistance.
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| 5. |
- Frómeta, D., et al.
(författare)
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Evaluation of edge formability in high strength sheets through a fracture mechanics approach
- 2019
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Ingår i: Proceedings of the 22nd International ESAFORM Conference on Material Forming. - : American Institute of Physics (AIP).
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Konferensbidrag (refereegranskat)abstract
- Edge fracture prediction in high strength cold formed components still being a challenge for automotive part manufacturers. Even though several experimental methodologies have been proposed in the last years to assess edge formability, the material properties governing edge cracking sensitivity of high strength sheet materials are not clearly defined. This work investigates the correlation between the fracture toughness of various 1000 MPa Dual Phase and Complex Phase steel grades and their edge fracture resistance, evaluated by means of hole expansion tests according to ISO 16630. The good linear correlation observed between these parameters shows that fracture toughness is a reliable indicator of edge cracking resistance in advanced high strength steel sheets. However, it is well known that edge formability does not only depends on the material properties but also on the edge quality. In order to evaluate the effect of the edge condition on edge formability, additional hole tension tests are performed in some of the investigated steel grades with different punch to die clearances. It is shown that steels with greater fracture toughness present higher strain at fracture and lower cutting clearance sensitivity. According to these results, the fracture toughness is proposed as a relevant material property to understand the edge formability of high strength metal sheets.
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| 6. |
- Frómeta, D., et al.
(författare)
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Fracture Resistance of Advanced High-Strength Steel Sheets for Automotive Applications
- 2021
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 52:2, s. 840-856
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Tidskriftsartikel (refereegranskat)abstract
- The fracture resistance of different advanced high-strength steel (AHSS) sheets for automotive applications is investigated through conventional tensile tests, fracture toughness measurements, and hole expansion tests. Different fracture-related parameters, such as the true fracture strain (TFS), the true thickness strain (TTS), the fracture toughness at crack initiation (wie), the specific essential work of fracture (we), and the hole expansion ratio (HER), are assessed. The specific essential work of fracture (we) is shown to be a suitable parameter to evaluate the local formability and fracture resistance of AHSS. The results reveal that fracture toughness cannot be estimated from any of the parameters derived from tensile tests and show the importance of microstructural features on crack propagation resistance. Based on the relation fracture toughness-local formability, a new AHSS classification mapping accounting for global formability and cracking resistance is proposed. Furthermore, a physically motivated fracture criterion for edge-cracking prediction, based on thickness strain measurements in fatigue pre-cracked DENT specimens, is proposed.
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| 7. |
- Frómeta, D., et al.
(författare)
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Fracture toughness measurements to understand local ductility of advanced high strength steels
- 2019
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Ingår i: 38 th International Deep Drawing Research Group Conference. - : Institute of Physics (IOP).
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Konferensbidrag (refereegranskat)abstract
- The determination of the material parameters that best predict the local ductility of high strength sheet materials has become the focus of active research. Even though several correlations have been proposed, they can sometimes be not accurate enough and discussion is still open on this topic. This paper investigates the suitability of different fracture toughness measurements for local ductility prediction in multiple advanced high strength steels (AHSS). Fracture toughness is characterized by means of essential work of fracture and Khan tear tests. The results show that the essential work of fracture, we, correlates well with different local formability (HER, critical bending angle from V-bending tests and local strain at fracture from uniaxial tensile tests) and crash resistance parameters (energy absorbed in axial impact tests). It confirms that fracture toughness, measured in the frame of fracture mechanics, is a relevant material property to rationalize cracking issues associated to the local ductility of AHSS. On the other hand, it is also shown that Khan tear tests, which are conventionally used to evaluate the fracture resistance of thin metal sheets, can overestimate crack propagation resistance and offer a poor prediction ability for local formability and crash performance.
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| 8. |
- Frómeta, D., et al.
(författare)
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Identification of fracture toughness parameters to understand the fracture resistance of advanced high strength sheet steels
- 2020
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 229
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Tidskriftsartikel (refereegranskat)abstract
- The fracture toughness of four advanced high strength steel (AHSS) thin sheets is evaluated through different characterization methodologies, with the aim of identifying the most relevant toughness parameters to describe their fracture resistance. The investigated steels are: a Complex Phase steel, a Dual Phase steel, a Trip-Aided Bainitic Ferritic steel and a Quenching and Partitioning steel. Their crack initiation and propagation resistance is assessed by means of J-integral measurements, essential work of fracture tests and Kahn-type tear tests. The results obtained from the different methodologies are compared and discussed, and the influence of different parameters such as specimen geometry or notch radius is investigated. Crack initiation resistance parameters are shown to be independent of the specimen geometry and the testing method. However, significant differences are found in the crack propagation resistance values. The results show that, when there is a significant energetic contribution from necking during crack propagation, the specific essential work of fracture (we) better describes the overall fracture resistance of thin AHSS sheets than JC. In contrast, energy values obtained from tear tests overestimate the crack propagation resistance and provide a poor estimation of AHSS fracture performance. we is concluded to be the most suitable parameter to describe the global fracture behaviour of AHSS sheets and it is presented as a key property for new material design and optimization.
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| 9. |
- Frómeta, D., et al.
(författare)
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Microstructural effects on fracture toughness of ultra-high strength dual phase sheet steels
- 2021
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Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 802
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Tidskriftsartikel (refereegranskat)abstract
- The influence of microstructure on the fracture toughness of two industrially processed 1000 MPa dual-phase (DP) steel grades is investigated. Crack initiation and propagation resistance are evaluated by means of the essential work of fracture (EWF) methodology and the main damage and fracture mechanisms are investigated. The results are discussed in terms of the proportion and distribution of the different microstructural constituents, which is assessed by scanning electron microscopy (SEM), high-resolution electron backscatter diffraction (HR-EBSD) and nanoindentation hardness measurements. The investigations show that the strain-induced transformation of retained austenite to martensite (TRIP effect), may be detrimental to cracking resistance, even though it increases tensile properties. This phenomenon is attributed to a “brittle†network effect generated by the presence of hard fresh martensite islands in the fracture process zone. The connectivity of the hard secondary phases and the proportion of soft phase (ferrite) also have a major role in fracture toughness. The DP steel with the larger volume fraction of ferrite and homogeneously distributed martensite islands shows significantly higher crack propagation resistance. The contribution of necking to the ductile fracture process is evaluated by means of thickness measurements in fractured DENT specimens and the correlation between the specific essential work of fracture (we) and tensile properties is investigated. It is concluded that the global formability and cracking resistance of high strength DP steels can be balanced through microstructural tailoring. © 2020 The Author(s)
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| 10. |
- Frómeta, D., et al.
(författare)
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New tool to evaluate the fracture resistance of thin high strength metal sheets
- 2020
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Ingår i: International Deep-Drawing Research Group (IDDRG 2020) 26-30 October 2020, Seoul, South Korea. - : Institute of Physics (IOP).
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Konferensbidrag (refereegranskat)abstract
- Fracture toughness has become a key property to predict the fracture performance of high strength metal sheets (edge cracking resistance, crash failure behaviour, local formability, etc.). However, the measurement of the fracture toughness of thin sheets still being challenging, mainly because of complex, expensive and time-consuming specimen preparation. In this work, an innovative tool to readily assess the fracture resistance of thin advanced high strength metal sheets is presented. The device consists of a special cutting tool (punch and die) designed to introduce sharp notches in sheet specimens through a simple shearing process. This new method avoids the need for fatigue pre-cracking procedures and allows measuring the fracture toughness of thin metal sheets with easy and cheap specimen preparation. It has been used in this work to evaluate the crack propagation resistance of four different advanced high strength steel sheets. The obtained toughness values are in good agreement with those measured with fatigue pre-cracked specimens and they show to be suitable to predict edge formability of AHSS sheets.
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| 11. |
- Frómeta, D., et al.
(författare)
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On the correlation between fracture toughness and crash resistance of advanced high strength steels
- 2019
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 205, s. 319-332
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Tidskriftsartikel (refereegranskat)abstract
- Automotive industry players have devoted large efforts to identify the material parameters governing the crash resistance of Advanced High Strength Steels (AHSS). Such knowledge is essential to improve impact performance prediction and optimize new steel development. Nevertheless, there is still an open discussion about which are the most relevant properties on AHSS crashworthiness. In this work, the authors investigate the correlation between the fracture toughness of different AHSS and their crash failure behaviour. Fracture toughness is measured in the frame of fracture mechanics, through the essential work of fracture methodology. Two fracture resistance parameters are characterized: the fracture toughness at cracking initiation, wei, and the essential work of fracture, we. Toughness values are compared with the results of axial impact tests, which are evaluated according to the energy absorbed and the cracking behaviour observed in crash boxes. Results show that fracture toughness permits to describe different crash events in terms of crack initiation and crack propagation and allows ranking AHSS impact resistance; steels with higher we present better crash performance. Therefore, fracture toughness is proposed as a key material property to predict the crash resistance of AHSS and as a relevant design parameter for crash resistant parts.
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| 12. |
- Lara, A., et al.
(författare)
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Sheared edge formability characterization of cold-rolled advanced high strength steels for automotive applications
- 2022
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Ingår i: IOP Conference Series: Materials Science and Engineering. - : Institute of Physics (IOP).
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Konferensbidrag (refereegranskat)abstract
- Edge cracking has become a limiting factor in the use of some advanced high strength steels (AHSS) for high-performance automotive applications. This fact has motivated the development of a multitude of experimental tests for edge formability prediction over the last years. In this sense, the Hole Expansion Test (HET) according to ISO16630 has been established in the automotive industry as a standard procedure for edge cracking sensitivity ranking. However, whereas it may be useful for rapid material screening, the results are often not accurate and reliable enough. Consequently, alternative methods based on Digital Image Correlation (DIC) have been proposed aimed at improving the prediction of edge cracking occurrence during forming and obtaining useful strain data that can be implemented in forming simulations. This paper explores the applicability of different DIC-based methods, such as Half-Specimen Dome Tests, Sheared Edge Tensile Tests, and KWI hole expansion tests with a flat nosed punch, for characterizing the edge formability of three cold-rolled AHSS sheets. The results obtained from the different testing methods are compared and validated with a laboratory-scale demonstrator. Finally, the limitations and advantages of the different methods are discussed.
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| 13. |
- Parareda, S., et al.
(författare)
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Effect of heat treatment conditions on the fatigue resistance of press hardened 22MnB5 steel evaluated through rapid testing technique
- 2021
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Konferensbidrag (refereegranskat)abstract
- Fatigue strength is considered as a crucial parameter for automotive applications subjected to cyclic loads during their long service life, as chassis parts. The high yield stress of press hardened steels poses them as good candidates for lightweight solutions with improved fatigue resistance. However, their high strength leads to an increase in notch sensitivity which can ruin the whole part's integrity. This behaviour was observed in previous works on press hardened steels, where their high fatigue strength was significantly affected by the surface conditions and by heat treatment conditions. Nevertheless, press hardening steels are still good candidates to manufacture complex geometry parts reaching high performance.Aiming at increasing the existing knowledge on the fatigue behaviour of press hardened steels, this paper analyses the fatigue performance of boron steel (22MnB5) under different time austenitizing times. Fatigue resistance is evaluated using a novel rapid fatigue testing technique based on the stiffness evolution. The method permits a fast and reliable determination of the fatigue limit. Based on results obtained with this rapid testing method, the most suitable heat treatment to mitigate fatigue notch sensitivity and then achieving the best fatigue performance for chassis applications is discussed.
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| 15. |
- Sandin, Olle, et al.
(författare)
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Prediction of sheared edge characteristics of advanced high strength steel
- 2022
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Ingår i: IOP Conference Series: Materials Science and Engineering. - : Institute of Physics (IOP).
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Konferensbidrag (refereegranskat)abstract
- In the present work, numerical models are developed for the shearing and cutting process of advanced high strength steel-blanks which can predict the edge morphology in the shear effected zone. A damage model, based on the modified Mohr-Coulomb fracture surface, is calibrated. To increase the predictability of the numerical models, the fracture surface is fine-tuned in areas corresponding to the stress-state of cutting, a methodology called Local calibration of Fracture Surface (LCFS). Four cutting cases with varying clearance are simulated and verified with experimental tests, showing good agreement. It is thus found that the suggested methodology can simulate cutting with adequate accuracy. Furthermore, it is found that solely using plane-stress tensile specimens for calibrating the fracture surface is not enough to obtain numerical models with adequate accuracy.
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| 16. |
- Tarhouni, I., et al.
(författare)
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Assessing the effect of the experimental parameters in the evaluation of the essential work of fracture in high-strength thin sheets
- 2022
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Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 270
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Tidskriftsartikel (refereegranskat)abstract
- The essential work of fracture methodology (EWF) has been successfully adopted to evaluate the fracture toughness of various metals and polymers. However, some aspects of the methodology are still far less understood, such as the influence of the experimental parameters on EWF measurement in thin metal sheets. In the present paper, the ligament range criterion of the EWF approach was revised for several advanced high-strength steels (AHSS). The validity of the upper and lower ligament length limits given by the ESIS protocol is redefined and rationalized according to the necking capability and the plasticity behaviour of the different AHSS grades. The work provides a new criterion to define the minimum ligament length to be tested, based on the minimum distance required by the crack to fully develop the necking capability of the material. The width constraint is too restrictive and has no effect on the deviation from linearity in the upper range. On the other hand, the maximum ligament length is proven to be controlled by the size of the plastic zone as proposed by the ESIS protocol.
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