On the measurement of fracture toughness to understand the cracking resistance of advanced high strength steel sheets
- Autores: David Frómeta Gutiérrez
- Directores de la Tesis: Daniel Casellas Padró (dir. tes.), Jessica Calvo Muñoz (codir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2021
- Idioma: español
- Materias:
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- Resumen
Automotive designers are constantly facing new challenges to meet the more and more stringent safety and CO2 emission legislations. Concerning the latter, vehicle lightweighting has become one of the main goals of the automotive industry, not only to reduce fuel consumption in fuel-powered cars but also to enhance the battery range in electric vehicles. At the same time, weight reduction cannot be attained at the expense of passenger’s safety in case of a crash. Hence, it is important to select the best-suited strategies to find the optimum balance between weight reduction and crashworthiness. In this sense, Advanced High Strength Steels (AHSS) have been positioned as one of the most effective solutions to this demand. AHSS present very high strength and high crash performance, which allows reducing vehicle mass while maintaining the safety of the occupants. These outstanding mechanical properties have promoted their widespread implementation for structural and crash-relevant automobile components. However, the application of AHSS have introduced new challenges related to their limited ductility and cracking resistance. Premature cracking during edge forming operations (edge cracking) or the occurrence and propagation of cracks under impact loading are some of the common cracking related issues in processing and implementation of AHSS.
To face these problems, the development of new approaches to properly characterize the cracking resistance of AHSS has become unavoidable since conventional failure criteria based on uniaxial tensile properties and forming limit curves fail to describe cracking related phenomena. In this thesis, a fracture mechanics-based approach is proposed to rationalize and understand the crack initiation and propagation resistance of AHSS. Results have been correlated with edge cracking resistance and crash behaviour of a broad range of advanced high strength sheet steels.
Fracture toughness is evaluated in the frame of fracture mechanics through different testing methods, such as the essential work of fracture, the J-integral and the Kahn-type tear tests. The relationship between the obtained fracture toughness parameters as well as the limitations of the different methods have been discussed. High-resolution video extensometry and Digital Image Correlation (DIC) techniques were used to investigate the fracture behaviour of the different steels. Edge cracking resistance is characterized by standard hole expansion tests and DIC-assisted hole tension tests. Crashworthiness is assessed through laboratory impact resistance tests. The influence of microstructural constituents on the crack propagation resistance of AHSS is also assessed.
The results show that fracture toughness, in particular the specific essential work of fracture (we), is a suitable material property to understand the cracking behaviour of AHSS and rank the material’s resistance to different crack-related failures, such as edge fracture or crack propagation during a crash event. These conclusions are based on the good correlation established between we and the results from edge cracking and impact resistance tests. On the other hand, the experimental observations show that we can be used to discern the role of microstructural constituents on the fracture behaviour of AHSS.
It is pointed out that proper microstructural design cannot be only focused on tensile properties since they do not inform about cracking resistance.
According to all the experimental findings, the fracture toughness is considered as a relevant material property for AHSS design and performance classification. In line with this, a new classification system, considering global ductility and fracture toughness, is proposed for a more comprehensive description of the overall formability and fracture behaviour of AHSS.
