טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentChen Sagi
SubjectUnraveling Damage Processes at the Microstructural Level
DepartmentDepartment of Mechanical Engineering
Supervisor Dr. Shmuel Osovski
Full Thesis textFull thesis text - English Version


Abstract

The need for strong and yet lightweight materials will always be part of our everyday life. With that, one essential requirement is that of strength and high damage tolerance. The advanced equipment that is accessible in research labs, such as sophisticated microscopes and high-performance computing, allows us to study the origin and the evolution of damage in metals. Specifically, ductile fracture is a process involving microvoids nucleation and growth during which a local material section gradually loses its strength, ending abruptly by void coalescence. This process is studied in this research using numerical techniques to model the damage mechanisms at the microstructural level of a deformed metal. The widely used GTN model for ductile fracture describes the damage growth in metals subjected to hydrostatic pressure, whereas the response to shear deformation is still under investigation. In this study, the main and elementary feature representative of ductile damage (a spherical empty void), is studied over the macro and the micro scales, focusing on the case of shear deformations.

In this research, numerical observations of the damage evolution around voids that are subjected to shear have been obtained and correlated with features of the fracture surface. For that purpose, two approaches are utilized: full structural modeling, and micro modeling of the fracture surface.

         The full structural modeling is done following a cylindrical specimen with a 45-degree groove to enforce high shear deformations while applying compressive load to the specimen (an opposite response is achieved with applying tension). Embedding a bulk spherical pore inside the gauge can bring new evidence regarding the damage evolution under shear/compression and shear/tension deformations around such heterogeneities that may naturally exist in the material.

         From a microstructural point of view, a voided material subjected to shear is studied by employing cell calculations, in which the void is placed into an infinite periodic matrix and the far-field external loading applied to the cell is given through a prescribed set of boundary conditions. It is shown that when considering damage evolution in the matrix material, two failure scenarios can be achieved: (1) void collapse, and (2) fracture initiation. In (1), the void gets flattened into a micro-crack and the fracture forms through shear localizations that are developed between those micro-cracks. In (2), fracture originates at the void’s interface and follows a shear localization pattern between adjacent voids.

Finally, it was found that shearing a void will eventually promote damage through hydrostatically tensioned regions that develop around it. The shape evolution of the void can be divided into two consecutive stages. At first, the symmetry of the spherical void breaks, and the void becomes elongated in the direction of maximum shear. Next, with further shearing, the void continues to elongate while decreasing its (empty) volume. Two scenarios for the failure are observed and discussed: (1) fracture starts at the apex of the void and (2) the void flattened to a micro-crack.