Influence of mechanical anisotropy on micro-voids and ductile fracture onset and evolution in high-strength low alloyed steels

In this paper results of a wide and innovative mechanical assessment, that was performed on large diameter spiral line pipes for gas transportation, are reported. The anisotropic material hardening has been characterized by tensile (smooth and notched specimens), torsion, and compression tests. Tests were performed in the pipe of the pipe with specimens machined along several orientations, taking into account the pipe through thickness direction. The influence of different triaxiality stress states on anisotropic behavior of the material have also been analyzed by means of tensile tests on notched specimens. After the experiments, the material was assessed by measuring the void distribution on the material as is, and on many deformed and fractured specimens, including tensile tests at different triaxiality, and torsion tests. The results showed that in such a class of materials, the experimental void fraction is fully negligible and not related to the applied plastic strain, even at the fracture proximity. As a consequence it can be concluded that, the plastic softening hypothesis may be dropped and damage due to void evolution hypothesis is not adequate.