Glasses crack effects

All else being equal, a toughening effect occurs if the crack tilts or twists away from a planar geometry because this reduces the net crack driving force. In homogeneous materials such as glass, cracks tend to propagate in... 

The glass transition effect arises from a consideration of the speed of crack growth, which in turn depends on the velocity of sound in the material (3). In a plastic matrix having Young s modulus equal to 3 GPa, the velocity of a crack under impact conditions is about 620 m/s. If the crack radius is about 1000 A, an equivalent cyclic deformation frequency of about lO Hz can be calculated. 

Anstice P. D. and Beaumont P.W.R. (1981). Hygrothermal aging effects on the micromechanisms of crack extension in glass fiber and carbon fiber composites. In Proc. ICF 5 (Francois D. et al.. eds.). Pergamon Press, Oxford, Vol. I, pp. 473-483. 

Apparently two opposing effects play a part On the one hand the glass fibres take over a part of the stress, thus partially releasing the stress on the polymer. On the other hand, fibres cause, in particular at their ends, stress concentrations, which can initiate crack formation. 

Explode a small charge in contact with a block of glass in the image path. The internal multiple cracking caused by the charge makes the glass effectively opaque... 

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