טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentRotbaum Yonathan
SubjectDynamic Tensile Necking: Influence of Geometrical
Imperfections
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Daniel Rittel
Full Thesis textFull thesis text - English Version


Abstract

This thesis deals with the influence of geometrical imperfections in dynamic tensile specimens. The subject is addressed using experimental and numerical means throughout the main steps of the work.

The study starts with an investigation of feasibility of long tensile specimen for dynamic experiments, emphasizing the issues of specimen's equilibrium and uniformity of the stress and the strain fields in the gauge. The investigation will show the superiority of long tensile specimen on the short, common specimen, in terms of uniformity of the strain, which in turn affects the accuracy of experimental stress-strain curves.

We report experimental evidence which shows how the locus of the neck inception changes as a function of the boundary conditions. This observation was modeled using finite elements simulations. In addition it will be shown that at sufficiently high impact velocities, the flow stress in the necking area becomes significantly higher than in the rest of the specimen, so that while the former resists deformation (strain hardening), it transfers the load to the latter. The result will be the formation of a local neck and failure away from the first neck.

The same effect of neck retardation will be investigated again for 10% notched tensile specimen using quasi-static and dynamic loading. Counterintuitive results will be reported, namely that dynamic failure will occur away from the notch, despite its being a strong stress concentrator. The contribution of the material response will be presented, emphasizing constitutive parameters such as strain rate hardening. The same effect will be shown to be active when the material properties are perturbed only at the local level, as in the case of machining of the notch, which in itself may again be sufficient to stabilize the structure vs. local failure until a neck forms elsewhere.

While many of the physical observations are quite counterintuitive with respect to the engineering views of stress concentration's effect, the present work rationalizes those observations and also provides guidelines for the designers of dynamically tensioned structures that may contain notches or similar flaws.