This CWA describes the procedure for the evaluation of the plane stress fracture toughness of thin ductile metallic sheets by means of the EWF methodology. The document provides the guidelines for specimen preparation, testing and data post-processing as well as the limitations of the method.
NOTE 1 The test method proposed in this document is intended to relatively thin metallic sheet materials presenting plane stress conditions, which do not fulfil the thickness requirements described in ISO 12135:2016. It is important noting that toughness values obtained by the present method are thickness-dependent. Therefore, they cannot be considered as an intrinsic material property but a geometry-independent constant for a specific sheet thickness.
NOTE 2 The recommended specimen is the Double Edge Notched Tension (DENT) because of its symmetry and minimal specimen rotation and buckling during the test. The specimens are notched, fatigue pre-cracked and tested up to fracture at a constant displacement rate. Alternatively, a mechanical notching process is described for obtaining sharp-notched DENT specimens. Investigations have shown that EWF results obtained with specimens prepared by means of this mechanical notching process are equivalent to those obtained with fatigue pre-cracked specimens for a range of AHSS. Further analysis is required to confirm the reliability of this procedure for specimen preparation in other materials of lower strength.
NOTE 3 The method requires testing multiple specimens with the same geometry but different crack lengths. From the test, two characteristic parameters are obtained; the specific essential work of fracture, we, and the non-essential plastic work, wp, multiplied by a shape geometry factor β. we is independent of in-plane dimensions and represents the plane stress fracture toughness of thin ductile sheet materials. Since it is obtained from an average of energy values for the complete fracture, it is considered an overall resistance value to stable crack extension, i.e. it contains energetics contributions from crack initiation and propagation resistance. It is also possible determining a single initiation toughness value, wei, which represents the material resistance to crack growth initiation. The parameter βwp depends upon specimen dimensions and, therefore, it is not a material constant.
NOTE 4 Resistance to stable crack extension can be also expressed in terms of a critical crack opening displacement (δc). An empirical relationship between we, δc and flow properties is established.