Glass properties This is a question about the properties and strength of glass. I know that glass is an amorphous solid but I am not sure if that is relevant to my question. My question is that if there's no physical cracks or blemishes, is it possible for glass to be damaged? For example, metal can be damaged by a sharp hit or force to it, as it is evident in a dent. For glass however, if a sharp force hits it and there is no evident crack or scratch, is the glass still in the same condition as it was before it got hit. For instance if I had something in contact with the glass that repeatedly struck it, would it over time weaken (ignore scratching)? (I know that with metal, each successive blow will slightly deform it)
 A: What you're describing is the difference between brittle and ductile behaviour. Most materials show both properties under the appropriate conditions. For example glass becomes ductile as the temperature rises towards the glass transition, while metals become ductile at low temperatures.
A brittle material may be superficially unaffected by a blow, but it is likely that there will be some effect at the atomic scales. If you study the surface with an electron microscope you'll probably find the blow has caused small defects on the surface, and these could nucleate a crack under stress. Alternatively the blow could have caused very small cracks, that again could lead to failure under stress. To what extent this happens depends on the force of the blow.
In metals repeated small deformations can lead to the well known phenomenon of metal fatigue. Analogous processes do happen with brittle materials, though I have to confess this is outside my area of expertise. A quick Google for something like "fatigue in brittle materials" will find lots of related articles like this one.
