Split-Hopkinson Pressure Bar: Dynamic Compression
Split-Hopkinson Pressure Bar: Dynamic Compression
Moradi, Arvin, Chatoyer Haynes
Group 47 12/01/2016
Abstract—Performing a dynamic compression testing on specimens and analyzing the stress-strain response is the objective of this article. Additionally, these results are compared to a quasi-static result provided by the instructor. This experiment is performed by benefiting from a Split-Hopkinson Pressure Bar (SHPB) set up. When a wave travels through the incident and the transmission bar, the speed would not change due to similar material’s property. The change in material because of the placement of a specimen of other material, changes the wave’s behavior that passes through. The yield strength of the brass washer is reported to be 536±12.80 MPa when the ultimate strength for the marble cube is calculated to be 153±3.35 MPa in the SHPB test.
Index Terms—Dynamic compression, Split-Hopkinson Pressure Bar, Stress-strain, Quasi-Static test, Universal Testing Machine.
INTRODUCTION
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HIS article discusses the behavior of a material under compressive load via stress-strain analysis in a Split-Hopkinson Pressure Bar (SHPB) settings. To complete the settings, two strain gages are arranged on the SHPB in a half bridge configuration. These data are then compared to a quasi-static test performed on the same specimens from Instron testing.
Several components are needed to complete this experiment. SHPB which includes a striker bar along with two dimension-like bars (incident and transmission bars), two strain gages, a Tacuna Amplifier, a digital oscilloscope and the two specimens that will be tested in this experiment [1].
Once the setup is ready for testing, the specimens are placed in between the incident and the transmission bars one at a time and the experiment is performed. Using mechanics of materials fundamentals, the behavior of the specimens used is studied using the wave generated in the SHPB.
Split-Hopkinson Pressure Bar
Two dimension-like bars are aligned next to each other in a way that the bars are free to translate in one direction – they can move towards or away from each other. On one side, a striker bar with similar thickness, but shorter in length is placed so that it can also only translate in one direction (similar to the incident and the transmission bars).
The incident and the transmission bars are free on each side and their transitional movement is not restricted with any fixed end. Fig. 1 displays the HSPB utilized in the lab for this experiment.
[pic 1]
Fig. 1. The striker bar along with the incident and transmission bars are limited to translate to or away from each other [1].
To perform a dynamic compression test, a specimen is placed in between the two symmetrical bars, and via the striker bar the compressive load is applied to the specimen and its deformation is then studied. Fig. 2 displays the orientation of each component in detail.
[pic 2]
Fig. 2. The strain gages are placed in the center of each symmetrical bar [1].
Specimens
For this experiment, one brittle and one ductile material are chosen to perform the dynamic testing on them. The brass washer as the ductile material, after going under compressive load, experiences a diameter change and the marble cube breaks as expected for a brittle material. Fig 3. Displays the specimens used in this experiment.
[pic 3] [pic 4]
Fig. 3. A ¼” Brass washer on the right and a ~1 Marble cube on the left. [2][pic 5]
Wave Propagation
As an omnipresent object, waves are all around us in different forms such as light waves, sound waves and so on. In this experiment, the behavior of the specimens that are placed in the HSPB and undergo compressive loads, are studied via the behavioral examination of the waves that passes through these specimens. There are two types of waves: longitudinal and shear waves.
Longitudinal Waves
Known as the primary waves, longitudinal waves travel faster than any other waves and the motion of the wave is parallel to the motion of the particle. This is the type of wave that we examine for this experiment. The velocity associated with these waves are calculated using (1) with regards to the material properties of the object that the waves are going through, where E is the Young’s modulus and ρ is the density of the object. This is performed on both specimens in this experiment.