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Guidelines for the evaluation of the plane stress fracture toughness of advanced high strength steel sheets in the frame of fracture mechanics
Fracture toughness, from a fracture mechanics point of view, is the property that controls the crack initiation and propagation resistance of a material. It is important to differentiate this definition from the conventional use of the term ‘toughness’, referring to the area under the stress-strain curve of a uniaxial tensile test or the product of the ultimate tensile strength by the total elongation (UTS TE), which is not suitable to describe the material resistance in the presence of pre-existing cracks or defects.
Fracture toughness of ductile engineering materials can be measured in the frame of Elastic-Plastic Fracture Mechanics (EPFM) following the J-integral (giving the value of JC), the J-R curve or the CTOD procedures standardized in ASTM E1820 and ISO 12135. However, these standard methods are intended to characterize the plane strain fracture toughness of metallic materials and, therefore, the defined thickness requirements are not satisfied by thin sheets, such as the sheets used in the automotive industry (1-3 mm). Alternative standards were developed later for the evaluation of the resistance to stable crack extension of thin-gauge materials, the ASTM E2472 and the ISO 22889. These standards propose the use of alternative parameters for fracture resistance characterization, the Crack Tip Opening Angle (CTOA) and the crack opening displacement δ5. These standard methods are experimentally complex, they require expert technical skills for specimen preparation and the crack advance must be monitored during the whole test, which represents one of the main challenges in fracture mechanics testing procedures. Additionally, they usually involve complex data processing and, such as in the case of the CTOA fracture criterion, the use of finite element method analysis, which makes the application of the methods expensive and time-consuming. Thus, alternative simpler and faster experimental approaches, such as the Essential Work of fracture (EWF) methodology or the Kahntype tear tests , have been developed in order to satisfy the growing need of knowing the fracture properties of thin metallic sheets.