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Test method for the determination of a cracking resistance index for advanced high strength steel sheets

The continuous development of new complex multiphase advanced high-strength steel (AHSS) grades for automotive applications has brought the need for alternative formability and fracture performance classification criteria. Owing to their complex microstructures, superior strength and limited ductility compared to conventional mild steels, AHSS are more susceptible to cracking during forming [1-3] or in situations of severe deformation such as in crash scenarios [4-7]. Considering that the main usage of these high-performance sheet materials is for structural and safety-related components, it is obvious that knowing their cracking sensitivity and damage tolerance is essential for their safe implementation in the industry.

Fracture toughness, measured in the frame of fracture mechanics, is the most appropriate property to assess the crack propagation resistance of engineering materials. However, the measurement of the plane stress fracture toughness of metallic sheets is not widely extended in many industrial sectors like the automotive, especially because of the complexity of the Elastic Plastic Fracture Mechanics (EPFM) standardized testing methods. Most of these techniques, such as the J-integral and Crack Tip Opening Displacement (CTOD) procedures described in ASTM E1820 or the determination of a Crack Tip Opening Angle (CTOA) as described in ASTM E2472, are complex, expensive and time-consuming.

In this context, alternative simpler and faster experimental approaches, such as the Essential Work of fracture (EWF) methodology [8] or the Kahn-type tear tests [9], have been developed in order to satisfy the growing need of knowing the fracture properties of thin metallic sheets. For instance, the EWF has been used in several research works to explain the edge fracture sensitivity and crash performance of AHSS and press hardened steels (PHS)[10-13], becoming a relevant property for new high strength sheet materials development and selection.

The present CWA describes a new single-specimen testing method for the determination of a cracking resistance index (CRI) able to classify the crack propagation resistance of high strength metal sheets. The index is derived from the fracture energy obtained from tensile tests with pre-cracked or sharply notched specimens. Based on the good correlation observed between the CRI and the EWF, the CRI is proposed as a useful parameter to estimate the cracking sensitivity of AHSS [14]. The procedure is fast and simple, comparable to a conventional tensile test, and it may be used as an additional routine test for quality control and/or material ranking purposes. The CRI criterion is derived from the EWF methodology with a simplified approach requiring less specimens to be tested and less post-processing work.