Brittle vs. Elasticity – Youngs Modulus

Embrittlement is a big deal. Embrittlement of steel meant the Titanic cracked rather than bent. Embrittlement of rubber O-rings destroyed the Challenger shuttle.

So why does glass break and steel bend? 

Most engineering materials in the world are crystalline. That is, their molecules are arranged in regular rows and columns. When impacted by force, rows of molecules can slide over one another and hence bend.
(Side bar. Alloyed metals can be stronger than pure, because different sized molecules in the crystal, can stop these molecules from sliding)

Glass is not crystalline; it’s an amorphous blob (sadly somewhat like my body). It was a molten liquid that was ‘frozen’ before it had a chance to form a crystal pattern. So, with no rows and columns, when it’s impacted the molecules can’t slide over each other, and the weakly-bonded blob, disintegrates.

I thought of it again when I bit into my Gluten Free Shrewsberry biscuit and it crumbled everywhere (shout out to the TGF bakery in Sydenham).

We know that there’s a measure for hardness (Shore and Rockwell etc) but how do we measure brittleness?
Most objects are somewhat elastic for the reasons discussed above. If we apply a stretching force of a certain amount, the object will stretch by a fixed amount in a linear fashion. This is Hookes Law (no reference to Peter Pan).