טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentZiso Ilan
SubjectDynamic Deformation and Mode II Fracture of Maraging 250
steel
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Daniel Rittel


Abstract

Understanding high velocity structural impact requires knowledge of how the materials of these structures behave under dynamical loads. For dynamic loading conditions (as opposed to static loads), thermo-mechanical coupling affects the deformation and the fracture behavior of materials. In general, the mechanical properties of most materials depend on the strain rate, and must therefore be characterized over a broad range of strain rates.

Cylindrical and SCS (Shear Compression Specimen) specimens were used in order to characterize the mechanical properties of Maraging 250 (H900) steel.

The experimental results using the SCS specimen show that the yield stress increases with the strain rate. At a strain rate of 10+4[s-1], the stress yield (flow) increases by about 50% with respect to the quasi-static value. A detailed characterization of the dynamic mechanical properties of Maraging 250 is presented in this work.

Dynamic fracture properties of this material are characterized for shear loading condition (mode II). The value of KIId is determined by a hybrid experimental- numerical stress analysis. The numerical analysis was carried out using the commercial FEM (finite element model) code Ansys LS Dyna, version 8.0. The experiment involved one-point impact of an unsupported "edge crack" specimen, causing shear dynamical loading (Mode II) of the stationary crack tip. The numerical model and calculation of the Dynamic Stress Intensity Factors (DSIF) are based upon the assumption of Linear Elastic Fracture Mechanics (LEFM) and of a small non-linear region near the tip of the crack (Small Scale Yielding - SSY).

The value of the fracture toughness KIId in dynamic shear loading was calculated from the numerical results at specific time (tc) by matching the displacement differential of the crack edges (Crack Opening Displacement - COD) to the analytical asymptotic solution.

Scanning electron fractographic analysis (SEM) of the failed specimens confirm shear failure characteristics. A metallurgical section of an "edge crack" specimen after Mode II dynamical loading also shows evidence of adiabatic shear banding (ASB), as the main failure mechanism. The band thickness range ranges between 5-10μm and its length is between 1-11mm, according to the loading conditions. The failure mode transition is evidenced by the change in crack path according to impact velocity.

This research indicates that the dynamic mode II fracture toughness, KIId, in dynamic shear loading lies within the 55<KIIc<82[MPa(m)0.5] range. This range is obtained for striker impact velocity in the following range: 35<Vstr.c<45[ms-1], respectively.