Shear Strengths of Glass Ceramics 1

[Otibix]

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

 

[3DDave]

How well does it perform with a scratch vs unscratched? Ordinary glass loses 99+% of it's strength due to small surface imperfections, but some sintered ceramics probably don't change quite as much.

Ordinary freshly drawn optical fiber has a tensile yield strength of nearly 1 million psi; touch it with a finger and the damage from that slight contact drops it to around 5000 psi.

The shear strength of metals will be better identified because while individual defects in the material might allow initial dislocations, the overall failure is from the statistical failure of billions (trillions?) of atoms along the slip plane. In ceramics it might be that one tiny area fails and creates a stress riser that leads to catastrophic failure and you are not seeing shear strength as much as the exaggeration of stress concentration in a brittle material.

For example, the fracture toughness of mild steel is nearly 100X that of the ceramic which suggests 100X the sensitivity to scratches and other defects.

 

[EdStainless]

I used to work with engineering ceramics (a bunch of grades) and all that we could define was modulus, density, and a rough compressive strength.
Sorry but I think that you stand no chance of this.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Otibix]

Thanks for you input both.

We know that the fuses will work and have plenty of margin against the limiting load. Based on our testing +2sd on normal distribution we still have a fos of 2. Problem is is we where to get that one idealic fuse without defect, if it was very much higher we wouldn't stand a chance...

Are you aware of any empirical rules which can be applied to justify a ball park mechanical strength. We know the upper bounds flexural strength and compressive strengths of the material?

 

[SWComposites]

The data sheet you linked has strength values.
Post a sketch of the fuse configuration.
Fuses generally are defined to have a very narrow and defined strength range. A material with a lot of scatter does not seem appropriate.

 

[Otibix]

I should correct and say there's a lot of scatter but all of it is far below our limits. The fuses are 3/4mm pins without any grooves or reduced sections. Used in a typical knuckle joint setup with 5mm tick eye