Combining shear and tensile stress on a glass cylinder
Combining shear and tensile stress on a glass cylinder
(OP)
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.





RE: Combining shear and tensile stress on a glass cylinder
"On the human scale, the laws of Newtonian Physics are non-negotiable"
RE: Combining shear and tensile stress on a glass cylinder
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.
RE: Combining shear and tensile stress on a glass cylinder
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.
RE: Combining shear and tensile stress on a glass cylinder
Glass is a fickle mistress. She needs the soft touch of a Spanish lover not the hard hand of a Russian sailor.
RE: Combining shear and tensile stress on a glass cylinder
"On the human scale, the laws of Newtonian Physics are non-negotiable"