Kwanghyun Ahn

Strain hardening model of pure titanium considering effects of deformation twinning

This paper is concerned with the effect of deformation twinning on the strain hardening behavior of commercially pure titanium during the compressive loading. In accordance with many studies on titanium, the strain hardening behavior of titanium during compression has different characteristics from those of general metallic materials. It has been reported that the strain hardening rate of titanium during compression can be divided into three stages. In the first stage, the strain hardening rate decreases as the strain increases due to dynamic recovery. Following the first stage, however, a sudden increase in the strain hardening rate is observed in the second stage. It is well known that the occurrence of the second stage is due to the generation of deformation twinning. After the second stage, the strain hardening rate decreases again as the strain increases in the third stage. In this paper, a strain hardening model that can represent the three stages of strain hardening is proposed based on the investigated effect of deformation twinning on the strain hardening behavior of titanium. The electron backscatter diffraction (EBSD) analyses are conducted to quantify the twin volume fraction with increase of compressive plastic strain, which provide fundamental frame of the hardening model for titanium.

Evaluation of Crash Energy Absorption Capacity of a Tearing Tube

This paper deals with the numerical prediction of energy absorption capacity of a tearing tube, which can absorb the crash energy through expanding and axial splitting processes. It is important to consider the dynamic material behavior and fracture characteristics of tube material in order to simulate the deformation behavior of a tearing tube accurately. Uniaxial tensile tests are performed in order to obtain the flow stress curve and the fracture strain of a tube material according to the strain rate. Quasi-static tensile tests were carried out in the range of strain rates from 0.001/sec to 0.01/sec using a static tensile testing machine. Dynamic tensile tests were conducted at strain rates ranged from 0.1/sec to 300/sec using a high speed material testing machine. These dynamic material properties are utilized to finite element analysis through a user material subroutine of ABAQUS/Explicit. Especially, ductile fracture criteria are adopted in order to describe the fracture characteristics of a tube material during axial splitting process. Dynamic tearing cases are simulated and the reliability of numerical results is verified by comparing them with the experimental results in dynamic tearing tests. The simulated results accurately predict the onset of fracture in axial splitting process and the energy absorption capacity has a good agreement with experimental results.

Fracture modelling of DP780 sheets using a hybrid experimental-numerical method and two-dimensional digital image correlation

This paper is concerned with the construction of fracture envelopes of DP780 sheets using two methods: a hybrid experimental-numerical method; two-dimensional digital image correlation (2D-DIC). For the hybrid method, four types of ductile fracture tests were carried out covering a wide range of stress states on specimens: with a central hole; two symmetric circular notches; flat grooved; and diagonally double-notched. Based on the fracture strain and loading paths identified with finite element simulation, a fracture envelope was obtained by employing the three-parameter modified Mohr-Coulomb fracture model. In addition, the fracture surface strain was directly measured using 2D-DIC. Loading histories of each test were extracted from a surface element of a three dimensional finite element model. The comparison of fracture envelopes constructed by the two methods reveals that there is little difference. Thus, it can be concluded that 2D-DIC is applicable to fracture modelling of DP780 sheets despite the assumption of the plane stress condition even after necking.

Dynamic Hardening Equation of Nickel-Based Superalloy Inconel 718

The dynamic response of the turbine blade materials is indispensable for analysis of erosions of turbine blades as a result of impulsive loading associated with gas flow. This paper is concerned with the dynamic hardening equation of the Nickel-based superalloy Inconel 718 which is widely used in the high speed turbine blade. Reported representative dynamic hardening equations have been constructed and evaluated using the dynamic hardening characteristics of the Inconel 718. Dynamic hardening characteristics of the Inconel 718 have been obtained by uniaxial tensile tests and SHPB tests. Uniaxial tensile tests have been performed with the variation of the strain rate from 0.001/sec to 100/sec and SHPB tests have been conducted at the strain rate ranging up to 4000/sec. Several existing models have been constructed and evaluated for Johnson-Cook model, Zerilli-Armstrong model, Preston-Tonks-Wallace model, modified Johnson-Cook model, and modified Khan-Huang model using test results at various strain rate conditions. The most applicable equation for the Inconel 718 has been suggested by comparison of constructed results.

Development of Buffer Stopper Using Progressive Compression Process for Crash Energy Absorption of a Railway Vehicle

A railway buffer stopper is a crash energy absorbing device which is installed at the end of rail lines in order to prevent derailment. To stop a train which cannot otherwise stop at the rail end due to some kind of trouble, this device should be installed in a railway station or depot where the rail line finishes. Friction type buffer stoppers have been installed at railway stations in most European countries. These devices can absorb the crash energy of the train by using the friction between a friction element and the rail when the train crashes into the buffer stopper. This paper proposes a new concept for the buffer stopper which can replace the friction type buffer stopper. More economical and effective buffer stopper can be suggested by using a progressive compression process. When the train crashes into the new type buffer stopper, a metal strip installed at the rail is progressively compressed and absorbs the kinetic energy of the train through its own plastic deformation. The feasibility of the buffer stopper using the progressive compression process is first investigated using numerical analysis and then the design and concept for the new system are suggested by a parametric study. The design of the progressive compression type has been verified by a series of experiments.