Narrow Results

Modeling of serrated chip formation is important to provide guidance for further development of this process and predict the quality of machined parts. The simulation of serrated chip formation is hard to achieve due to the lack of a proper material model which represents real phenomenon within a narrow adiabatic shear band. In the present study, a material constitutive law containing strain hardening and thermal softening was proposed to predict adiabatic shear localization within serrated chips. The outcomes of FEM simulation show the capability to predict serrated chip formation. The effective strain distributions and the forces are computed within this study.

Dynamic mechanical behaviors of Mg-Gd-Y series extruded magnesium alloy and its σ-εcurves are measured by using of Hopkinson pressure bar technique. According to the concept of efficiency of absorption energy, this thesis compares Mg-Gd-Y series extruded alloy with ZK60 extruded magnesium alloy. Being obtained similitude numbers by means of normalizing processing for governing equations of the continuum mechanics and according to the viewpoint of equal density of area, the comparative experiment of anti-projectile capabilities is elaborately designed between Mg-Gd-Y alloy and the 7A52 aluminum alloy. The result has validated that anti-projectile capability of Mg-Gd-Y alloy is better than those of 7A52 alloy under the condition of equal density of area. The relative technique approaches for improving anti-projectile capability of Mg-Gd-Y alloy are put forward making use of similitude numbers. Finally, the basic characters of effect of adiabatic shear are revealed by micro analysis.

This study describes an improved method used to determine the dynamic shear failure of materials under blast loading conditions. The proposed method is a convenient and effective and generates material failure along the controlled fracture trajectories under uniform uniaxial stress conditions at a strain rate of $10^4$s${}^{-1}$ to $10^5$s${}^{-1}$. The method was developed by performing experiments on AISI 1045 steel with heat treatment. Specimens were filled with composition B explosives and detonated, and fragments were recovered. Most of the macroscopic shear cracks originated at the notch tips and connected adjacent internal and external notches. The phenomena of multipath and bifurcate shear fracture trajectories were also observed. Fractography analysis of the fragments provided data on damage and failure mechanism and revealed that dynamic shear fracture was characterized by microvoids and microcracks. The adiabatic shear band of AISI 1045 steel was detected on the fracture surface boundary. This finding indicates that adiabatic shear localization is the underlying failure mechanism of the studied material.

The ultrafine-grained interstitial free (UFG-IF) steel was fabricated using the equal channel angular pressing (ECAP) method. Transmission electronic microscope (TEM) observations showed that the grains were refined to sub-micrometer scale and the average grain size was several hundred nanometers (nm). Uniaxial compression tests were conducted over a wide range of strain rates (up to 6000/s) and temperatures (from −196${}^{\circ }$C to 300${}^{\circ }$C) to understand the dependence of mechanical behaviors on the strain rate and temperature. It is found that the UFG-IF steel showed a weak strain rate sensitivity and displayed a strong temperature sensitivity only within a very narrow temperature range. Microstructural observations on the specimens deformed dynamically at temperatures below −20${}^{\circ }$C via scanning electronic microscopy (SEM) and TEM indicated the occurrence of adiabatic shear localization. In order to explain the reason why adiabatic shear localization formed at the low temperatures and high strain rates, the susceptibility factor to adiabatic shear band (ASB) was estimated. It is proposed that more heat generated during the adiabatic plastic deformation process due to the enhanced flow stress of the UFG-IF steel at lower temperatures may play a dominant role in the formation of ASB. Meanwhile, the experimental results also provide strong evidence that for the UFG-IF steel the dynamic recrystallization which results from the severe plastic deformation and high temperature rise only occurs at the later stage of ASB evolution. With regard to the formation of ASB in the UFG IF steel, the localized plastic flow at a lower stress level indicates that localized deformation happens firstly and the temperature rise in the localized shearing zone leads to the final plastic instability.