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Shear Strength of Glass Ceramics

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Otibix

Mechanical
Mar 8, 2024
14
Background:Long story short, we have a design that utilizes a glass ceramic (Macor(R)) type mechanical fuse that will shear prior to a defined load. This was essentially designed through trial and error and now we are in a position where we need to substantiate the design. We undertook testing of the ceramic fuses and there was (predictably) a lot of scatter - Also.

Problem Statement:Does anyone have any experience determining the shear strength of glass ceramics. Notably, this is the way they are usually used, therefore they tend not to have defined properties. The ceramic we are using can be found at Link.

Currently the only shear strength value we can find is in a research report which isn't clear about how this property has been identified, and provides a result almost 5 standard deviations away from our test data suggests...

We are currently trying to arrange further testing but we would like (if we can find an appropriate method) to back this up with a calculated/estimated shear strength figure. Is there any empirical or failure theories you would recommend to go alongside the test data?

Thanks for your help,

Otibix
 
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It's a probability function, and not a pretty one. NASA did a lot of research on characterizing ceramic failures (think Space Shuttle tiles) and modelling them stochastically to predict failures. One of my professors at U of Washington, Dr. Bollard, was part of the effort some 30+ years ago. Brilliant guy, may he RIP, but that stuff was so opaque to my brain that I decided to work on something simpler and got my Master's being an RA for a project that built a working ram accelerator instead. Should tell you what you are up against.

Every mechanical "fuse" I've ever worked with has used metallic materials, from shear bolts to burst disks, and even then, exhaustive testing of the material lots would be required to verify the mechanical properties of interest so that the end item met its specifications.
 
I agree, We may have been led down a poorly laid path, but this remains the hand we are delt. If you can point us in the direction of any of the materials you mentioned, or have any thoughts how we can estimate an idealised figure, that would be helpful.

I had hoped to use mohrs failure criterion to predict an upper bounds, but with no idea on the internal angle of friction, or the cohesion of glass ceramics (couldn't find any literature on this) I'm a bit stumped... We are happy with our test results and we have 2x margin against our + 2sd figure... I also heard rumours about empirically applying tresca and Von miese to the compressive and flexural strength, but couldn't find any solid reading on the subject ( and get 2 vastly different results)
 
Why ceramic ? high temperature ? corrosion resistance ?
 
Correct, also the fuses needed to be small, finding a material which was brittle enough with a high enough yield and low enough uts for the scenario was proving difficult to the point that the only materials we could find to implement where too strong to machine to the small diameters required or not available within the project timeline...

Essentially, we started with the thought that ceramics don't effectively yield, so we are designing to one criteria rather than minimum yield and maximum uts. As mentioned, the design works in practice but we just need to firm the paper aide
 
Sorry, I can't figure out how to delete?
 
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