Combining shear and tensile stress on a glass cylinder

[fdwookie]

I am trying to model the stresses induced by thermal contraction of a conformal coating (epoxies, urathanes) on the glass envelope of a fragile bodied component. The glass is bonded to a metal post and the coating is then applied to the part. The glass is fracturing when the system is cooled. The fractures always start at the glass to metal seal at one end and propagate outwards through the glass envelope into the coating. I modeled this like an ice cream sandwich and have calulated the tensile stress (perpendicular to the longitudinal axis of the part) and the shear stress (length contraction of the epoxy bond at the bond-to-glass interface). To combine the two stresses I was converting the shear to a torque (pivoting around the center of the glass)and then equating Fshear x d1 = Fend x d2 to get a force acting in the radial direction at the end of the glass(where it is fracturing). I am assuming an area equal to the fracture origin when converting the stresses to a force. Once I have the Fend then I can combine the forces. I am posting this to simply get a sanity check. Thanks for any response.

 

[dwallace1971]

If the glass is tempered in any way, then you're really only concerned with max principal stresses at the surface of the part. Model the entire cylinder, apply your thermal loads. Pull the max principal stresses off the surface of your glass, then compare that to your modulus of rupture.

"On the human scale, the laws of Newtonian Physics are non-negotiable"

 

[glass99]

As dwallace said, its all about principal tension stress.

My other comment is: don't use stiff adhesives! Silicone was invented for a reason. Bonding metal to glass is inherently difficult with stiff adhesives because of thermal movements. Dow Corning has a high strength but relatively flexible silicone called TSSA which can work. Shrinkage is a thing to worry about too with stiff/strong adhesives like epoxy. I epoxied a nylatron disc into a glass hole one time, and the glue line thickness was 0.5mm instead of 0.2mm, and the shrinkage caused the glass to break.

 

[fdwookie]

dwallace1971 - no the glass is not tempered and I am coming at this from a failure analysis point of view. We already have the fractured components. I am trying to come up with a model to predict (rule of thumb more like it) if a coating would be safe or not. I already realize from the testing we have done that there is more than just the CTE and modulus at play here. I think though that I can make a simple model based on those two properties. Using them and the geometry of the system I can calculate a tensile and shear stress. The shear stress is the one that varies as the length of the cylinder. The tensile stress is constant along the length of the cylinder. I am looking for a way to combine the stresses at the end of the cylinder where the failures occur. If I have to use Mohr's circle than I will need a little help on how to attack that.

glass99 - you are preaching to the choir. I have spent the better part of the last 10 years trying to get designers and production people to understand that high modulus/high cte material should never be in contact with fragile bodied components without some kind of buffer coating.

 

[glass99]

fdwookie - unless I am missing something about your analysis, glass fails because cracks in the surface open up under tension. Its all about fracture mechanics. Shear has nothing to do with it except insofar as is it creates secondary tension stresses through poisson's ratio or something. Stresses in the body of the material make no difference either. The strength of glass with no surface flaws is gigantic. Note that for AN glass, load duration and humidity play big roles. AN glass is a real bitch to analyze actually. It frequently breaks at a point which did not have the maximum stress.

Glass is a fickle mistress. She needs the soft touch of a Spanish lover not the hard hand of a Russian sailor.

 

[dwallace1971]

Yeah, what he said!

"On the human scale, the laws of Newtonian Physics are non-negotiable"