Poisson’s Ratio vs Young’s Modulus: What They Mean and Why They Matter

At Istar Machining, we work with many materials. We need to know how they act when we pull or push them. Two big ideas help us understand this: Poisson’s Ratio and Young’s Modulus. Let’s learn about them in a way that is easy to get.

What Are These Big Words?

When we pull on a thing:

• Young’s Modulus tells us how hard it is to stretch
• Poisson’s Ratio tells us how much it gets skinny when we stretch it

Think of a rubber band. It is easy to stretch (low Young’s Modulus). When you pull it, it gets skinnier in the middle (high Poisson’s Ratio).

Now think of a steel bar like we use in our CNC machining service. It is hard to stretch (high Young’s Modulus). When you pull it, it barely gets skinnier (medium Poisson’s Ratio).

Young’s Modulus: The Stretch Number

Young’s Modulus is a big number that tells us how stiff a thing is. We write it as E.

• What it measures: How hard it is to make something longer
• Big E = Hard to stretch (like steel or diamond)
• Small E = Easy to stretch (like rubber)
• Units: We measure it in GPa (that’s a lot of pressure!)

At Istar Machining, we need to know this when we make parts from metal or other stuff.

Poisson’s Ratio: The Skinny Number

Poisson’s Ratio tells us how much skinnier something gets when we pull it. We write it as ν (the Greek letter nu).

• What it measures: How much thinner something gets when stretched
• ν = 0.5 means it keeps the same size overall (like rubber)
• ν = 0 means it doesn’t get skinny at all (like cork)
• ν < 0 means it gets FATTER when pulled! (weird materials called auxetics)
• No units: It’s just a number, not measured in inches or anything

When we do precision CNC milling, we need to think about this number too.

Big Table of Materials

Here is a table that shows both numbers for stuff we work with at Istar Machining:

Why Do These Numbers Matter?

When we make parts at Istar Machining, we need to know how they will act:

1. If we make a spring: We need something with low E (easy to stretch)
2. If we make a tool: We need something with high E (hard to stretch)
3. If we make a seal: We need something with high ν (keeps its size)
4. If we make a shock part: We might want low or even negative ν

Real-Life Examples

Steel vs. Rubber

• Steel (E = 200 GPa, ν = 0.3)
  • Very hard to stretch
  • Gets a little skinny when pulled
  • Good for strong parts
• Rubber (E = 0.01 GPa, ν = 0.5)
  • Very easy to stretch
  • Gets very skinny when pulled
  • Good for parts that need to bend

Cool Example: Cork

Cork has a Poisson’s Ratio near zero. That means when you push on a cork, it doesn’t bulge out the sides! That’s why it’s good for wine bottles – it can squish down without pushing too hard on the glass.

In our CNC prototype machining work, we sometimes make parts that need to act like cork.

When Both Numbers Work Together

Some math shows how these numbers work together:

• Shear Modulus (G) = E / [2(1+ν)]
• Bulk Modulus (K) = E / [3(1-2ν)]

These tell us how materials act when we twist or squeeze them all over.

What Happens in Real Jobs?

Airplane Wings

When making airplane parts with aluminum CNC machining, we need to know:

• Young’s Modulus (E) tells us how much the wing will bend in the wind
• Poisson’s Ratio (ν) tells us how the shape will change

Medical Parts

For medical CNC machining, we often make parts that go in the body:

• Bone has E about 20 GPa and ν about 0.3
• If we make a fake bone part, it needs to have the same numbers or it won’t work right

Fun Facts About These Numbers

• Most materials have ν between 0 and 0.5
• Rubber has ν almost exactly 0.5, which means its size stays the same overall
• Some foams have negative ν, which means they get fatter when pulled!
• Diamond has the highest E of any natural material

Wrong Ideas People Have

Wrong Idea #1: “Poisson’s Ratio is always positive.”

Truth: Some special materials have negative Poisson’s Ratio. They get fatter when you pull them!

Wrong Idea #2: “Young’s Modulus is all that matters for strength.”

Truth: A material can have high E but still break easy. Other numbers matter too!

Questions People Ask

How We Use This at Istar Machining

When we do custom CNC machining, we think about these numbers all the time:

1. Picking the right material: Different jobs need different E and ν
2. Making parts that fit together: We need to know how much parts will change shape
3. Testing parts: We can check if our parts have the right E and ν

Real Case Study: Car Parts

A car maker asked us to make parts for a new car. We used our automotive CNC machining skills. The parts had to:

• Be strong (high E)
• Not get too skinny when pulled (medium ν)
• Not break when hit (good toughness)

We picked a special steel with E = 205 GPa and ν = 0.29, and the parts worked great!

Why You Need Both Numbers

You might wonder: “Why do we need two numbers? Isn’t one enough?”

The answer is no! These two numbers tell us different things:

• E tells us about stretching
• ν tells us about getting skinny

Both matter for making good parts.

Materials with Weird Numbers

Some materials have very special numbers:

• Cork: ν ≈ 0 (doesn’t get skinnier when pressed)
• Auxetic foams: ν < 0 (get fatter when pulled)
• Diamond: E = 1220 GPa (extremely hard to stretch)
• Jelly: E very low (super easy to squish)

At Istar Machining, we sometimes work with these weird materials for special jobs.

What to Remember

The big ideas to take home are:

1. Young’s Modulus (E) tells how hard it is to stretch something
2. Poisson’s Ratio (ν) tells how much skinnier it gets when stretched
3. Both numbers help us pick the right material for a job
4. Materials act different based on these numbers

Next time you see a rubber band or a steel beam, think about these numbers!

How to Learn More

If you want to know more about how materials work and how we use them in machining, you can:

1. Talk to our team at Istar Machining
2. Ask us about our CNC parts machining services
3. Learn about our work with different materials

We hope this helps you understand these important ideas. Remember, knowing how materials work helps us make better things!

[^1]: Data for material properties comes from engineering handbooks and material science references. [^2]: Young’s Modulus is measured in GPa (gigapascals), which is a measure of pressure or stress. [^3]: Poisson’s Ratio is a dimensionless number, meaning it has no units. [^4]: Some specialized materials like auxetic foams have negative Poisson’s Ratio values, which is contrary to what we observe in most common materials. [^5]: At Istar Machining, we consider these material properties when selecting optimal materials for specific applications in our CNC manufacturing processes.