ELI5: Modulus of Elasticity - incredibly high values for wood?

Submitted by u193 t3_11dqiai in explainlikeimfive

I looked up the modulus of elasticity for Southern Pine and it's 1,800,000 PSI. Looking up the definition of what the MOE is at first makes sense I thought; however, when putting it into perspective, this equates to 450 cars (if weighing 4,000lbs each) stacked on a 1-inch by 1-inch area which seems incredible. I am assuming that I must just be not understanding something about what this is. Can someone please ELI5.

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Red_AtNight t1_jaa7pho wrote

Modulus of elasticity is the relationship between applied stress, and strain.

If you try to bend wood by applying force to it, you are applying a stress, and the amount that it bends is called the strain. 1.8 million PSI is not actually that high of a modulus of elasticity, considering steel's is more like 30 million PSI.

Basically it tells you that wood is resistant to being deformed - but steel is significantly more resistant to being deformed.

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breckenridgeback t1_jaaayq0 wrote

OP might be confusing a high elastic modulus with a high [yield](https://en.wikipedia.org/wiki/Yield_(engineering)) point. In this case, since wood is a relatively stiff material, it will yield long before it deforms by very much (here's a table with yield strengths for wood boards).

To rephrase for OP: the elastic modulus tells you how much wood deforms, but it doesn't tell you anything about when it will bend out of shape permanently or break. A wood board can't support 450 cars, but not because it gets squashed, because the load exceeds the yield strength of wood.

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baggier t1_jaac8mv wrote

This. The modulus of elasticity is the force need to stretch something (hypothetically) 100% . Most things cant stretch 100% so you stretch it by say 1% and multiply the result by 100

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u193 OP t1_jaafj6r wrote

So does that mean that it would take 1.8 million PSI to stretch the wood to double its length; however, it would yield long before that?

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ZackyZack t1_jaajcma wrote

Pretty much, yes

Off the top of my head, I dunno how much sooner, but the "double its length" is spot on

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WakkaBomb t1_jaar2zh wrote

Which is also why we have 350,000 ton steel sky scrapers instead of 1 million ton stacks of wood

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Turnip45 t1_jaagc5n wrote

Modulus of elasticity is a measure of how much something stretches when a force is applied provided you don’t stress it beyond its elastic capacity. It often correlates with strength, but not always.

To calculate how much something will stretch under a force:

Extension = Force x Length / (E x Area)

Wood has a decent modules of elasticity for its weight, but it’s E is lot less than most building materials. Something like 2-8% that of steel depending of species.

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