Energy required to shear glass

[keshman87]

Sorry if this is a simple question but it has me stumped.


So I have a window that is fixed into a frame. I need to find out how much energy is required to shear the glass from the frame.

Now I dont really have to worry about the frame, as it can be simplified to just shearing glass basically.

Now I have calculated how much force is required to shear the glass, but im stumped as to work out how much energy is required to break the glass. I've learnt stuff on fracture energy but thats on propagation of cracks and such...

Thanks for any help

 

[IRstuff]

Not sure what you mean by "shear." Glass doesn't "shear."

And why wouldn't it be a function of the adhesive or whatever, that's holding the glass in place?

For that matter, why even go this route? The probability of breaking the glass altogether must be absurdly high.

TTFN

FAQ731-376

 

[keshman87]

yeh I totally undertand where your coming from , but basically im doing a simple simulation on computer and modelling that the glass is fixed solid against the frame, and that it will simply "shear" off the frame and not break

its because it just has to be a simple case so some comparisons can be done

how about if i said it wasnt glass but some other material, is there still a way to calculate the energy

Thanks

 

[CoryPad]

Assuming a flaw-free material, you use strain energy for this. The equation is:

w = 1/2 [·] ([σ]_x[ε]_x + [σ]_y[ε]_y + [σ]_z[ε]_z + [τ]_xy[γ]_xy + [τ]_yz[γ]_yz + [τ]_zx[γ]_zx)

where

w = work (or energy)
[σ] = normal stress
[ε] = normal strain
[τ] = shear stress
[γ] = shear strain

I am not sure of your loading conditions (force introduction, reactions, boundaries), but I believe five of these terms are zero for your case. You need to measure or find a value for the shear strength for your material (or estimate from a tensile strength) and you need the shear modulus (can be calculated using Young's modulus and Poisson's ratio). Then, you can obtain the critical shear strain by equating it to shear modulus divided by shear stress.

However, this will be an overestimation because the glass will have flaws and the fracture energy will be lower than the calculation above.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[IRstuff]

What you're asking for is, I think, impossible in a brittle material.

I recommend that you read up on "shear strength."

TTFN

FAQ731-376

 

[keshman87]

Thanks very much Cory, thats just the kind of equation I was looking for

Yeh I can understand why this would never work in real life with glass, but the boss wants me to find this out just so he can have an idea

 

[vc66]

He wants you to find out about something that couldn't happen in real life? Sounds like my boss...

V