High strain rate testing using the SHPB -- Help please

In summary: Maybe there's something you're missing?In summary, the individual project of a part C mechanical engineering student involves high strain-rate testing of a glass filled composite material using a split-Hopkinson pressure bar (SHPB) experiment. The results from the SHPB test rig were compared to external data, but the Young's modulus of the material was found to be significantly different than expected. The student has checked and manipulated the inputs for the program used in the experiment, but the issue remains. The student is seeking advice or help in understanding why this may be happening.
  • #1
Ben_P_1992
7
1
I am a part C mechanical engineering student and require some advice for my individual project, which is based on the high strain-rate testing of a particular glass filled composite material using a split-Hopkinson pressure bar (SHPB) experiment.

I gathered the results on a SHPB test rig and have external data which is being used for comparative analysis to verify my results, however the Young's modulus of the material seems to be way off (It should be about 5-6GPa!) I was hoping somebody had some experience with this experiment or had any ideas as to why this has occurred? Please see the graph below- the gradient of my results should match that of the two curves labelled Finland Test.

https://www.physicsforums.com/attachments/upload_2015-5-2_21-31-21-png.82985/

The data used was gathered from strain gauges mounted on the maraging steel 250 pressure bars. The measured voltage and time was then manipulated with a program (this program has been used in previous experiments and outputted matching results for different materials.) The only inputs required for this program is the elastic wave speed of the pressure bars and their modulus of elasticity as well as the specimen dimensions pre-testing. I am confident that these inputs are correct, and have even played about with the inputs for both (varied the Young's modulus from 160GPa to 220GPa in 10GPa intervals) to see how the Young's modulus is affected, but there is little effect. Please see the graph below:

https://www.physicsforums.com/attachments/upload_2015-5-2_21-37-34-png.82986/

I am very stuck and am not quite sure how to explain why this happening. Any help or advice would be much appreciated.

Thanks,

Ben
 
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  • #2
Hey Ben, sorry I'm afraid I won't be of much help as I haven't made it as far as you, but I just wanted to let you know your pictures are not coming up. Just letting you know so you can fix it so someone who can help you will be able to.
 
  • #3
Thanks for the heads up. The pictures are attached here.https://www.physicsforums.com/attachments/upload_2015-5-2_21-31-21-png.82985/
https://www.physicsforums.com/attachments/upload_2015-5-2_21-37-34-png.82986/
 
Last edited by a moderator:
  • #4
Your images are still broken.
You say that this test setup has been successfully used before, but were those previous materials as soft as yours?
 

1. What is the SHPB testing method and how does it work?

The SHPB (Split Hopkinson Pressure Bar) testing method is a technique used to study the mechanical properties of materials at high strain rates. It works by impacting a specimen with a high velocity striker bar, causing a stress wave to travel through the specimen and be measured by two strain gauges attached to the incident and transmitted bars. This allows for the determination of material properties such as stress, strain, and strain rate.

2. What types of materials can be tested using the SHPB method?

The SHPB method can be used to test a wide range of materials, including metals, ceramics, composites, and polymers. It is particularly useful for studying materials that experience high strain rates, such as those involved in impact or explosion events.

3. How is the data collected and analyzed in SHPB testing?

The data collected in SHPB testing is typically in the form of stress-strain curves, which are generated by the strain gauges attached to the incident and transmitted bars. This data is then analyzed using various mathematical models and techniques to determine material properties such as strain rate sensitivity and dynamic yield strength.

4. What are the advantages of using the SHPB method compared to other testing methods?

The SHPB method offers several advantages over other testing methods, including the ability to test materials at high strain rates (up to 10,000/s), the generation of dynamic stress-strain curves, and the ability to test small specimens. It also allows for the study of materials under realistic conditions, such as impact or explosion events.

5. What are the limitations of SHPB testing?

While the SHPB method is a valuable tool for studying materials at high strain rates, it does have some limitations. These include the complexity of the setup and data analysis, the potential for specimen damage or failure, and the difficulty of testing materials with low strain rate sensitivity. It also requires specialized equipment and expertise, making it less accessible for smaller research groups.

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