What is Fracture Toughness
Fracture toughness is defined as the energy spent in the creation of two surfaces at the crack tip that give rise to crack propagation. The ASTM E1820 describes the standard procedure to evaluate the fracture toughness of ductile engineering materials, as AHSS, by means of the traditional J-integral and Crack Tip Opening Displacement (CTOD) measurements.
Nonetheless, it is intended to plane strain fracture toughness characterisation and the defined specimen thickness requirements are not satisfied for thin sheets as AHSS thin sheets (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.
However, those standards methodologies are complex and involve exhaustive specimen preparation, rigorous data treatment and the measurement of the crack advance during the tests, which is one of the main difficulties in fracture toughness measurement.