The dynamic deformation behaviors of structural metallic materials by external load have been mainly considered in a view point of ensuring their reliability and safety in accordance with the high speed of mechanical systems. In order to understand the deformation and fracture behaviors of machine/structure under impact loading the deformation behaviors of structural materials under high strain rates should be investigated, which should be used for the numerical analysis of the behaviors. In this study, the SHPB test and the Taylor bar impact test corresponding to the strain rate range of 103 ~ 104 s-1 were carried out to investigate the dynamic deformation behavior of metallic material, respectively. As specimens, 5 kinds of materials with three different crystal structures; AISI4340 steel, HSA800 steel, Hi-Mn steel, OFHC copper and Ti-6Al-4V alloy were supplied, respectively.
Firstly, on the SHPB test apparatus, it could be successfully configured a LabVIEW program for data acquisition and processing during SHPB impact test. The developed scheme provided a flexible user interface which gives an easier handling during SHPB test. From SHPB impact tests, the parameters for dynamic hardening model of plastic flow behavior at high strain rate range could be obtained. The curve fitting using both standard Johnson-Cook model and Modified Johnson-Cook one which are representative dynamic hardening models was also conducted. The effects of strain rate on the material responses have been investigated, particularly on the flow stress and strain rate relation. Generally, the flow stress versus logarithm strain rate relationship of the three high strength steel was non-linear, this is the reason why the standard Johnson-Cook model did not provide a good fit in simulating these relation. Therefore, the non-linear relation of strain rate forms in the modified Johnson-Cook model could fit the relationship well for all materials investigated.
On the other hand, at the higher strain rates than the SHPB tests, Taylor bar impact tests method equipped with 8 Ch high-speed photography system were used. The contact duration time of projectile was obtained by plotting the instantaneous length of projectile with elapsed time using a series of photos taken during Taylor bar impact. Then the determination of dynamic yield strength and the normalized strain rate was possible. The dynamic deformation behaviors of various material projectiles were investigated. According to the materials adopted which showed different strain hardening or softening behavior, they represented different bulging at impact surface and in deformed shapes.
Numerical analysis using a commercial AUTODYN S/W for the deformation behavior during Taylor bar impact test was performed with parameter values obtained from the SHPB impact test. The results were compared with the experimental results obtained by Taylor impact test. It was tried to estimate the dynamic deformation and to investigate the applicability of dynamic hardening model adopted on metallic materials at higher strain rate range than 104 s-1.
As a whole, it was possible to understand the effects of strain rate hardening on the dynamic plastic flow behaviors it metallic materials adopted which have different crystal structures. Finally, in could be found that by combining the Taylor impact test results with numerical analysis, it was possible to predict the dynamic flow behavior in metallic materials at high strain rate region exceeding 104 s-1.