A Simple Method to Determine Ductile Fracture Strain in a Tensile Test of Plane Specimen’s

In several practical cases, the ultimate ductile fracture strain determined with tensile test is accepted as a material plasticity measure 1 . In this case, the plasticity has to be defined as an ability of a material to accommodate high permanent strains until fracture appears where this strain reaches certain value called ultimate fracture strain p. The strain value until fracture depends not only on the material type, but also on other several factors, as: strain speed, strain history, material starting structure, temperature, specimen geometry, etc. It is impossible to account for all factors in a single mathematical description, due to a complexity of phenomena and an insufficient state of the art, mainly for phenomena present during a plastic strain. Several experiments 2-5 have demonstrated that the material fracture process strongly depends on the hydrostatic stress. This conclusion has been independently induced based on experiments 6-8 . Recently, several different fracture criteria, including the state of hydrostatic stress, have been developed 8-10 . However, the practical application of above criteria to forecast the fracture during the metal forming process has been feasible thanks the numerical computing methods, which enable to determine the material’s state of stress during the plastic forming process. Currently, the ductile fracture criteria are commonly used when simulating various plastic processing processes 10-13 . However, the practical application of the criteria requires the experimental determination of the ductile fracture strain p value for a given material. This strain is usually determined based on the tensile test, but the determination method indeed is not so obvious, and in several cases even doubtful. In most cases, the tensile test is performed against circular or rectangular cross-section specimens. Considering that the ductile fracture strain p around the fracture zone equals the equivalent strain z in this zone, it can be calculated using the equation: