Compression strength measurement

[eng345]

Hello all,

I have to measure the compression strength of a porous ceramic body (d=L=10 mm) using advanced force gauge, mecmesin and don't have any experience with it. The simple procedure, which I followed, was to measure the force at which the sample started to break, divided by the cross section area of the sample. Can anybody explain me if this is the right method to measure the compression strength (Do I have to use the applied force at which first crack appears or I have to take the maximum applied force/continue to apply the force as long as the applied force can increase)? What is the scope of plotting stress-strain diagram for compression strength measurement?

Thanks in advanced

 

[Lok]

The whole diagram is important. As it tells a lot about the behavior of the tested material.

The first part of the diagram should be a relatively straight line which represents the elastic deformation, then a small to nonexistent (for ceramics) curve to the right for the plastic deformation part and at first crack the force should drop suddenly enough to signal a clear failure (and although you could continue it is not relevant).

Compressive strength is measured in MPa or N/mm² both being expressions of pressure, so the force divided by surface area is fine.
The usually significant compressive strength is just at the elastic/plastic boundary, in the case of ceramics plastic deformation signals the start of micro cracking and all the catastrophic things associated with it.

 

[eng345]

Hello Lok,

thanks for your reply. Basically I am chemist and want to make a comparison of crushing strength of catalyst supports. I red about elastic-plastic deformation in my courses, but I am looking if there is a possibility to make a comparison without have stress-strain diagram (for drawing stress-strain diagram, I have to search software/program). I just took a defination of Crushing strength from "Crush-BK",

"The applied force increases until the solid breaks and collapses into small pieces and eventually powder. The corresponding value of the collapsing force is defined as crush strength."

Do you think that without stress-strain diagram, measuring crushing strength is fine.

thanks

 

[Lok]

Basically I am chemist and want to make a comparison of crushing strength of catalyst supports. I red about elastic-plastic deformation in my courses, but I am looking if there is a possibility to make a comparison without have stress-strain diagram (for drawing stress-strain diagram, I have to search software/program).

You could try a staple sample like NaCl or something whose crush values you can obtain.
Also, how do you obtain your readings, one value, multiple/distance, gauge, electronically?

"The applied force increases until the solid breaks and collapses into small pieces and eventually powder. The corresponding value of the collapsing force is defined as crush strength."

In engineering the yield strength is sought after (that is usually the elastic/plastic boundary) as it gives consistent results and is the point at which structural failure takes place. Crushing something to a fine powder seems to me to necessitate more force (at least energy) at some point in the crushing.
Question: Does your Force value differ from breaking into pieces to crushing into a fine powder? (the first should be greater and more consistent as the second one does not seem consistent at all)

Do you think that without stress-strain diagram, measuring crushing strength is fine.

Being ceramics, their respective plastic deformation part is quite small as they usually break right after elastic deformation. So the value obtained sounds fine. What I am not sure is how much do you travel after first break as this could mean that you can obtain a bigger surface area crushed powder that will need more force to crush further. The Anvils should not travel more that 0.5mm IMHO for a consistent result.

Have fun!

 

[eng345]

The instrument consists of a two compression plates, one is fixed and used as a sample holder, the other plate is manually rorated with the help of screw rod connected to it. The force reading can be read on a electronic display, which display the maximum force applied and hold it until experiment.
thanks

 

[Lok]

The instrument consists of a two compression plates, one is fixed and used as a sample holder, the other plate is manually rorated with the help of screw rod connected to it. The force reading can be read on a electronic display, which display the maximum force applied and hold it until experiment.
thanks

Seems perfectly fine and easy to use. I would say that the values obtained are right. Remember not to go to far with the crushing as barreling could occur at long anvil displacements (the compression plates are sometimes called anvils). This can of course happen only if the anvil tip is larger than 10x10mm.
A small description of the barreling phenomenon is here: http://en.wikipedia.org/wiki/Compression_strength#Deviation_of_engineering_stress_from_true_stress

 

[eng345]

Thanks for your reply
have a nice day