2
stresses increased with increasing strain rate. More extensive mechanical characterization of stainless steels is still needed to
broaden the application space, particularly in the intermediate- and high-strain-rate tension regime. The primary barrier to
precise characterization in those regimes has been experimental technique development.
In this study, we used a 304L-VAR stainless steel to investigate the upper limit of a fast MTS high-rate servo-hydraulic
machine and the lower limit of the Drop-Hopkinson bar to close the gap at intermediate strain rates. With on conventional
Instron machine and a split Hopkinson tension bar, the 304L- VAR stainless steel was characterized in tension at every order
of magnitude of strain rate from 0.0001 to ~3500 s
−1
. The strain rate effect on the tensile stress-strain response of the 304LVAR stainless steel was determined.
1.2 Material and Specimens
The material investigated in this study was 304L-VAR stainless steel. Two designs were used for the tensile specimens for
quasi-static and dynamic tests, respectively, as shown in Fig. 1.1. The quasi-static tensile specimens (Fig. 1.1a) followed an
ASTM standard E8/E8M-09 with a diameter of 3.18 mm and a gage length of 15.88 mm, making an aspect (length to diameter) ratio of ~5 [4]. Relatively short tensile specimens are needed to achieve stress equilibrium in dynamic tensile tests.
Therefore, the dynamic tensile specimens were designed with the same diameter but a shorter gage length of 6.35 mm, resulting in an aspect ratio of 2. Both quasi-static and dynamic tensile specimens had the same ½ in.-20 threads at both ends and
transitional portion from the gage section to the threaded ends. The only difference between the two specimen geometries
was the gage length. Preliminary test results showed that the specimen size effect was negligible prior to necking. The nominal stress-strain response for the necked specimens with different gage lengths deviated due to highly localized deformation
in the necking region.
1.3 Mechanical Tests at Low, Intermediate, and High Strain Rates
In this study, low-strain-rate tensile tests were conducted with an Instron material test frame under displacement control.
Long tensile specimens shown in Fig. 1.1a were used for all low-strain-rate tests up to 0.1 s
−1
. The force was measured with
a load cell on the top of the test frame to calculate the specimen stress. A laser extensometer was used to measure the deformation of the specimen over the gage section. The engineering stress-strain curve was therefore obtained.
Two experimental apparatus—a fast MTS and a Drop- Hopkinson bar [5]—were employed to cover the intermediate strain
rates. Since specimen size effect is negligible, particularly prior to necking, long specimens (Fig. 1.1a) were used for fast
Fig. 1.1 Tensile specimen design (unit in mm). (a) Long specimen and (b) short specimen
B. Song et al.
stresses increased with increasing strain rate. More extensive mechanical characterization of stainless steels is still needed to
broaden the application space, particularly in the intermediate- and high-strain-rate tension regime. The primary barrier to
precise characterization in those regimes has been experimental technique development.
In this study, we used a 304L-VAR stainless steel to investigate the upper limit of a fast MTS high-rate servo-hydraulic
machine and the lower limit of the Drop-Hopkinson bar to close the gap at intermediate strain rates. With on conventional
Instron machine and a split Hopkinson tension bar, the 304L- VAR stainless steel was characterized in tension at every order
of magnitude of strain rate from 0.0001 to ~3500 s
−1
. The strain rate effect on the tensile stress-strain response of the 304LVAR stainless steel was determined.
1.2 Material and Specimens
The material investigated in this study was 304L-VAR stainless steel. Two designs were used for the tensile specimens for
quasi-static and dynamic tests, respectively, as shown in Fig. 1.1. The quasi-static tensile specimens (Fig. 1.1a) followed an
ASTM standard E8/E8M-09 with a diameter of 3.18 mm and a gage length of 15.88 mm, making an aspect (length to diameter) ratio of ~5 [4]. Relatively short tensile specimens are needed to achieve stress equilibrium in dynamic tensile tests.
Therefore, the dynamic tensile specimens were designed with the same diameter but a shorter gage length of 6.35 mm, resulting in an aspect ratio of 2. Both quasi-static and dynamic tensile specimens had the same ½ in.-20 threads at both ends and
transitional portion from the gage section to the threaded ends. The only difference between the two specimen geometries
was the gage length. Preliminary test results showed that the specimen size effect was negligible prior to necking. The nominal stress-strain response for the necked specimens with different gage lengths deviated due to highly localized deformation
in the necking region.
1.3 Mechanical Tests at Low, Intermediate, and High Strain Rates
In this study, low-strain-rate tensile tests were conducted with an Instron material test frame under displacement control.
Long tensile specimens shown in Fig. 1.1a were used for all low-strain-rate tests up to 0.1 s
−1
. The force was measured with
a load cell on the top of the test frame to calculate the specimen stress. A laser extensometer was used to measure the deformation of the specimen over the gage section. The engineering stress-strain curve was therefore obtained.
Two experimental apparatus—a fast MTS and a Drop- Hopkinson bar [5]—were employed to cover the intermediate strain
rates. Since specimen size effect is negligible, particularly prior to necking, long specimens (Fig. 1.1a) were used for fast
Fig. 1.1 Tensile specimen design (unit in mm). (a) Long specimen and (b) short specimen
B. Song et al.