| 1. |
- Casellas, Daniel, et al.
(författare)
-
A fracture mechanics approach to develop high crash resistant microstructures by press hardening
- 2017
-
Ingår i: 6th International Conference Hot Sheet Metal Forming of High-Performance Steel CHS2. - Warrendale, PA : Association for Iron & Steel Technology, AIST. - 9781935117667 ; , s. 101-107
-
Konferensbidrag (refereegranskat)abstract
- Crashworthiness is a relevant engineering property for car parts. However it is not easy to measure at laboratory scale and complex impact tests have to be carried out to determine it. Crash resistance for high strength steel is commonly evaluated in terms of cracking pattern and energy absorption in crashed specimens. Accordingly, the material resistance to crack propagation, i.e. the fracture toughness, could be used to rank crashworthiness. It has been proved in a previous work by the authors, so the measure of fracture toughness, in the frame of fracture mechanics in small laboratory specimens, would allow determining the best microstructure for crash resistance parts. Press hardening offers the possibility to obtain a wide range of microstructural configurations, with different mechanical properties. So the aim of this work is to evaluate the fracture toughness following the essential work of fracture methodology for ferrite-pearlite, bainite, ferrite-bainite, martensite and martensite-bainite microstructures. Results showed that bainitic microstructures have high fracture toughness, similar to TWIP and CP steels, which allows pointing them as potential candidates for obtaining high crash resistance in parts manufactured by press hardening.
|
|
| 2. |
- Casellas, Daniel, et al.
(författare)
-
Fracture resistance of tailor tempered microstructures obtained by different press hardening conditions
- 2015
-
Ingår i: Hot Sheet Metal Forming of High-Performance Steel 5th International Conference. - Auerbach : Verlag Wissenschaftliche Scripten. - 9783957350237 ; , s. 221-229
-
Konferensbidrag (refereegranskat)abstract
- Tailored tempering can produce different amounts of martensite, upper and lower bainite and ferrite depending on the press hardening conditions are usually obtained. The tensile properties of such microstructures are quite well known, but the intrinsic fracture properties, as fracture toughness, have not yet been characterized. This is mainly related the experimental difficulty to measure fracture toughness in thin sheets. Recently, the authors have proposed to measure it through the application of the Essential Work of Fracture methodology. The knowledge of facture toughness would give valuable information to increase their applicability in automotive components and would also allow a further understanding of fracture and crack propagation mechanisms in B steel hardened at different cooling conditions. Thus the aim of this paper is to determine the fracture toughness in microstructures of a 22MnB5 steels obtained by tailored tempering. Results indicate that the EWF methodology can be applied to measure fracture toughness and the energy for crack initiation in ferrite-pearlite and ferrite-bainite microstructures. Fracture initiation energy is the same in ferrite-pearlite and ferrite-bainite microstructures, which will indicate that ferrite has a clear effect on fracture initiation. Moreover, fractography results reveal the effect of inclusion content on fracture energies
|
|
| 3. |
- Casellas, Daniel, et al.
(författare)
-
Fracture Toughness to Understand Stretch-Flangeability and Edge Cracking Resistance in AHSS
- 2017
-
Ingår i: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 48A:1, s. 86-94
-
Tidskriftsartikel (refereegranskat)abstract
- The edge fracture is considered as a high risk for automotive parts, especially for parts made of advanced high strength steels (AHSS). The limited ductility of AHSS makes them more sensitive to the edge damage. The traditional approaches, such as those based on ductility measurements or forming limit diagrams, are unable to predict this type of fractures. Thus, stretch-flangeability has become an important formability parameter in addition to tensile and formability properties. The damage induced in sheared edges in AHSS parts affects stretch-flangeability, because the generated microcracks propagate from the edge. Accordingly, a fracture mechanics approach may be followed to characterize the crack propagation resistance. With this aim, this work addresses the applicability of fracture toughness as a tool to understand crack-related problems, as stretch-flangeability and edge cracking, in different AHSS grades. Fracture toughness was determined by following the essential work of fracture methodology and stretch-flangeability was characterized by means of hole expansions tests. Results show a good correlation between stretch-flangeability and fracture toughness. It allows postulating fracture toughness, measured by the essential work of fracture methodology, as a key material property to rationalize crack propagation phenomena in AHSS.
|
|
| 4. |
- Frómeta, D., et al.
(författare)
-
Assessing edge cracking resistance in AHSS automotive parts by the Essential Work of Fracture methodology
- 2017
-
Ingår i: Journal of Physics, Conference Series. - : Institute of Physics (IOP). - 1742-6588 .- 1742-6596. ; 896
-
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.
|
|
| 5. |
- Frómeta, D., et al.
(författare)
-
Evaluation of edge formability in high strength sheets through a fracture mechanics approach
- 2019
-
Ingår i: Proceedings of the 22nd International ESAFORM Conference on Material Forming. - : American Institute of Physics (AIP).
-
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.
|
|
| 6. |
- Frómeta, David, et al.
(författare)
-
Fracture Toughness Evaluation of Thick Press Hardened 22MnB5 Sheets for High Crash Performance Applications in Trucks
- 2019
-
Ingår i: Hot sheet metal forming of high-performance steel. - : Wissenschaftliche Scripten. ; , s. 113-121
-
Konferensbidrag (refereegranskat)abstract
- The present work investigates the fracture toughness of a thick press hardened 22MnB5 steel sheet, in order to assess its applicability for truck safety parts. The fracture toughness is evaluated in the frame of the elastic plastic fracture mechanics (EPFM), in terms of essential work if fracture (EWF) and J-integral. Digital Image Correlation (DIC) measuring techniques are used to measure crack tip opening displacement and crack extension. It is shown that EWF presents some limitations for thick high strength metal sheet applications, making necessary the use of conventional EPFM procedures. Digital image analysis has shown to be very useful in fracture toughness characterization of thick press hardened steel sheets. The differences observed between different methodologies are discussed and results are compared to toughness values of different AHSS grades.
|
|
| 7. |
- Frómeta, D., et al.
(författare)
-
Fracture toughness measurements to understand local ductility of advanced high strength steels
- 2019
-
Ingår i: 38 th International Deep Drawing Research Group Conference. - : Institute of Physics (IOP).
-
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.
|
|
| 8. |
- Frómeta, D., et al.
(författare)
-
On the correlation between fracture toughness and crash resistance of advanced high strength steels
- 2019
-
Ingår i: Engineering Fracture Mechanics. - : Elsevier. - 0013-7944 .- 1873-7315. ; 205, s. 319-332
-
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.
|
|
| 9. |
- Golling, Stefan, et al.
(författare)
-
Determination of the essential work of fracture at high strain rates
- 2017
-
Ingår i: 6th International Conference Hot Sheet Metal Forming of High-Performance Steel CHS2. - Warrendale, PA : Association for Iron & Steel Technology, AIST. - 9781935117667 ; , s. 261-269
-
Konferensbidrag (refereegranskat)abstract
- During the last decades, the use of ultra-high strength steel (UHSS) has increased as its favorable ratio between strength and mass allows the design of lighter body-in-white while maintaining passenger safety. Modeling impact loads of components made of UHS steel requires reliable descriptions of the material deformation and fracture behavior.Traditional stress or strain based fracture criteria are used in finite element modeling. A different approach in modeling fracture in components uses the fracture energy as a model parameter.Fracture toughness is difficult to measure in thin sheets; a method termed Essential Work of Fracture (EWF) provides the possibility to determine the fracture toughness in sheet metal. With knowledge of the fracture toughness the understanding of fracture behavior and crack propagation in ultra-high strength steel can be increased. The obtained EWF is related to the fracture energy and can be used in numerical models as a material parameter.In the present work results from preliminary testing are shown and a discussion on cross-head speed and strain rate in the critical specimen cross section is given. The use of digital image correlation provides information about the displacement field in the vicinity of the notch and hence about the strain- and strain rate distribution. Furthermore, the difficulties in reliable measurement of force and elongation in high speed tensile testing machines are elucidated. Issues encountered during the development of the high-speed DENT specimen are not limited to the specific geometry presented in this paper.The present work aims at the development of a test specimen to obtain the Essential Work of Fracture (EWF) at high test speed. This work contributes to the overall goal to model fracture behavior and crack propagation, dependent on the strain rate. For the investigation, a high-speed tensile testing machine equipped with an in-house developed load cell and an optical elongation measurement system was used with a high-speed camera to obtain data for digital image correlation.
|
|
| 10. |
- Golling, Stefan, et al.
(författare)
-
Influence of microstructure on the fracture toughness of hot stamped boron steel
- 2019
-
Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 743, s. 529-539
-
Tidskriftsartikel (refereegranskat)abstract
- The automotive industry's desire for weight reduction while maintaining crashworthiness demands development of materials and material properties within the economic framework of consumers. The industrial process of hot stamping provides a technique to utilize steel in an efficient way. In hot stamping, microstructural characteristics of a steel blank are influenced by controlling the cooling rate. Hot stamping has become a prevalent method for lightweight solutions in car bodies without sacrificing passenger safety. The process of hot stamping applies sequential forming and quenching in a single production step. During the cooling of the blank, various microstructures can be formed depending on the cooling rate or holding temperature. Special tooling allows the application of different cooling rates within the same blank. Thus, the microstructure and mechanical properties can be influenced in designated areas of a blank.Fracture toughness properties of sheet metal are necessary to better understand fracture initiation and crack propagation during crash loading as well as improve crashworthiness predictions. This paper focus on fracture toughness of low-alloyed boron steel sheet common in the automotive industry. A heat treatment process is used to form different microstructures, predominately consisting of one single phase or mixed microstructures with two distinct phases. The fracture toughness of the present microstructures is evaluated using the Essential Work of Fracture methodology. Results are discussed in terms of the different microstructures obtained and the consequent part performance.Results show a strong connection between microstructure and fracture toughness. The bainitic grade shows favorable fracture toughness while a mixed microstructure of bainite and martensite shows a very brittle fracture behavior. A post heat treatment in the form of paint bake curing shows a negligible effect on fracture toughness of martensite.
|
|
