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Brittle vs. Elasticity Youngs Modulus

The Engineering Blog


I have a perverse pleasure in watching dash-cam crash videos on YouTube.

It’s not just ‘armchair-malice’; I enjoy the engineering mechanics of the interactions in the same way I enjoy billiard balls bouncing of each other like a newtons cradle in two dimensions.

I especially enjoy watching glass smashing in a collision; often exploding at impact and it got me thinking about why some things are brittle and some are elastic.

1a

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).

2a       3a

In the case of something very elastic, like a rubber band, releasing it will return it to it’s original size.

In this context the engineering term for the force applied is “Stress” and the stretching is referred to as “Strain”
But if we stretch the band too far, it ceases to be elastic and will either stretch and deform or snap. When we pass the point of elasticity, we reach the Yield Point. It’s marked “P” on the diagram above.

Finally, we get to Youngs Modulus. Youngs Modulus is the measure of elasticity. It’s a measure of the steepness of the graph.

5a

The blue line in the diagram above is representative of glass, while the purple line at the bottom might be our rubber band.

Here’s some examples of some common Youngs Modulus figures for common materials in MPa

Steel          206,000
Copper      120,000
Rubber       0.4 to 1.6

An understanding of Elasticity suddenly brings into sharp relief the demise of all those windscreens and why the steel bodies bend.

https://www.youtube.com/watch?v=kSVej8wFH-4
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