Formulae and units used to describe the electrical properties of insulators

When the stress at the tip of the crack is equal to the cohesive stress calculated in Section S4.2.1, the bonds will break, so increasing the length of the crack and causing fracture to occur. Equating these values, we obtain 

the material will fracture when the stress is given by  

A growing crack does not have a smooth elhptical cross-section. However, a value of the radius can be estimated by considering that the forces between the atoms is very small at separations of about 4ro, giving p a value of about 2ro- Using this value, we obtain 

Griffith originally made an estimate of this type (but in a slightly different way) and deduced 

3 Formulae and units used to describe the electrical properties of insulators 

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FORMULAE AND UNITS USED TO DESCRIBE THE ELECTRICAL PROPERTIES OF INSULATORS 549... 

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