טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentDolinski Michael
SubjectModeling Adiabatic Shear Failure from Energy Principles
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Daniel Rittel
Dr. Avraham Dorogoy
Full Thesis textFull thesis text - English Version


Abstract

When a material is dynamically loaded, bands of intense inhomogeneous deformation may sometimes appear. These bands are called adiabatic shear bands (ASB) and they appear in metals, polymers and granular materials alike. According to the conventional wisdom, the phenomenon occurs due to thermo-mechanical coupling effect, namely when the softening rate is higher than the hardening rate. This instability point is characterized by a critical strain () which is often considered as the failure criterion. Recent evidence contradicts the usage of a critical failure strain, showing that the temperature has a negligible effect on the inception of the phenomenon. Therefore, a new failure criterion is presented in this work (inspired by the work of Rittel et al. (2006)). This criterion is based on the plastic strain energy density.

This research focuses on:

(1) Formulating a criterion for initiation and propagation of adiabatic shear bands, based on plastic strain energy density. The criterion should be easy to implement numerically, with a minimal number of adjustable parameters.

(2) Assessing analytically the failure criterion, and providing a procedure of measurement and calibration of its parameters from experimental data.

(3) Verification and validation of failure criterion by comparing numerical simulations to laboratory experimental results.

(4) Performing full scale penetration experiments at different impact velocities for additional validation and verification of the proposed criterion.

The formulated failure criterion has three parameters, two of them can be measured from global experimental stress-strain curves, and the third one is measurable from the local (inside the band) stress-strain curve. Alternatively, the parameters can be obtained from adjustments based on numerical simulations. This criterion determines both the initiation and propagation of the shear band. In other words, when a critical strain energy density is met, the material starts to loose its load bearing capability with ongoing local damage, until negligible residual strength is achieved. It is shown that the damage evolution has an exponentially decaying behavior in terms of strain.

Numerical simulations of four selected laboratory experiments show an excellent agreement with the experimental results, both qualitatively and quantitatively. Moreover, numerical simulations of ballistic penetration experiments show that the proposed failure criterion reproduces the whole range of impact velocities with a single set of parameters. By contrast, the same simulations were performed with the conventional critical strain criterion. Here, the results were much less satisfactory and required a constant adaptation of the criterion parameters to roughly reproduce the attained results. These last results emphasize again the advantages of an energy based criterion.