Fracture Mechanics

Rastanking

Hi everyone,

I have a problem that I hope someone can help me with, the question is.

A mass of 500kg induces a tensile stress in a glass sheet restrained at the top. Given that the fracture strength of sound glass is 170MPa what is the maximum length of slit that can be tolerated without fracture.

The mass is 200mm wide, 2mm deep and has a crack of 2a in the centre

so using the formula = δ√∏a = √EGc

I get this δ = (500 x 9.81) / 200 x 2

So first question is because the mass is suspended at the top I'm assuming that the 9.81 is gravity and if so the second question appears, from the formula how do I know which figures to use?

To me the formula states ∏xa, I'm assuming that's area and E = Youngs Modulus of Elasticity and Gc is Critical energy release rate.

Any help would be appreciated.