A Child's Guide To Laser Peening

Gather 'round children, it's time you learned about laser peening.

Yes - laser peening. That's P-E-E-N-I-N-G. Not that other word you're thinking of!

Now this might sound like some boring subject for adults only, but laser peening is actually lots of fun. It involves robots and airplanes and really big lasers. You interested yet? Let's discover more...

Add Some Mettle To Your Metal

To understand laser peening you first need to understand metal. What is metal? Metal is that hard, solid material we use to make hard, solid things. Your bike is made of metal. Your parent's car, too. Knives and forks, nickels and dimes, bridges and trains - you'll find metal in everyday objects large and small.

We make things from metal because it's durable and strong. Metal objects can lift really heavy loads. They hold their shape well and last for many years.

But they don't last forever.

Even the strongest metal objects eventually wear down and break. Those years of heavy loads lead to something called "metal fatigue".

What is metal fatigue?

We've already talked about metal, so let's explore the word fatigue. Fatigue is another word for being tired. It's what you feel after running around outside all day. You're tired and sweaty and sore. Your body is fatigued from all that hard work!

Well, metals get tired too. If a metal object works hard for a long time, eventually it starts to wear down. The metal develops little cracks, and those cracks grow until they look something like this:

That's metal fatigue. Have you ever played with a metal paperclip? You can bend it back and forth a few times, but if you do it too much the metal will break.

Don't worry. It's just a paperclip! Metal fatigue in paperclips is not a big deal. But what about metal fatigue in skyscrapers or airplanes? That is a big deal. Airplanes have lots of metal parts that work really hard. If something breaks during a flight, it's dangerous for the people on board.

That's why scientists and engineers study metal fatigue, and they work to find ways to prevent it.

These guys are on the case.

So how do we prevent metal fatigue? To answer that question, we need to talk about stress.

Let's Talk About Stress

We've all experienced stress before: That feeling of pressure or strain. You might feel stress before a big test in school, or visiting the doctor to get a shot.

Metals feel stress too, but in different ways. Stress in metals is a physical force - like a pulling or a squeezing as the metal does its job. Let me explain.

Take a look at this construction crane:

It's big and tall and made of metal. The metal is strong, but the structure is heavy, and the metal holding it together is under constant stress.

The stress we're worried about - the type that causes metal fatigue - is called tensile stress. Tensile stress is a force that tries to pull the metal apart, like an intense game of tug-o-war.

Lucky for us and our crane, the structure is built to withstand lots of tensile stress. But over a long enough time, all that pulling and tension slowly wears the metal down. Fatigue sets in and cracks begin to form, and before you know it that crane ends up looking like this:

Yikes! Hope everybody's okay!

To avoid these metal fatigue failures, engineers try to limit the tensile stress on critical metal parts. Fortunately, tensile stress isn't the only kind present in metals, and we can use a different type of stress to make the metal stronger.

Behold The Power Of Compression!

If tensile stress is the culprit behind metal fatigue, then compressive stress is the solution. Rather than pulling objects apart, compressive stress squeezes them together like a big, cozy hug.

Ahhhh, compressive stress... :)

Metals are very resistant to being squeezed. Go ahead, pull out a coin and try squeezing it with your hand. Not easy, is it? Over time, a compressed metal object can flatten or warp, but it usually won't develop the same fatigue cracks you get from pulling and tension.

Because of this, engineers love compressive stress. It protects their metal parts against cracking. They love it so much they add compressive stress to all their important parts. How do they do that? With a process called peening.

Oh yeah - peening!

Peening is a technique for compressing metal parts to make them stronger. It's been around a long time. Have you ever seen this type of hammer?

It's called a "ball peen hammer", and that round knob is used to bash metal surfaces to add compressive stress. Peening with a hammer is hard work, so people developed another technique called shot peening.

Shot peening replaces the hammer with thousands of little balls. The balls are fired at the metal surface, and each one acts as a tiny peening hammer that adds a bit of compressive stress to the part. Shot peening looks something like this:

Shot peening is great, and it's been used for decades to enhance metal. But shot peening has limits. It squeezes the metal surface a tiny bit, but that compression doesn't go very deep.

This becomes a problem for really hard-working parts like those we find in airplane engines.

Have you ever flown in an airplane? It's very exciting when those powerful engines lift you off the ground. Airplane engines generate incredible force. The world's most powerful engine can produce over 100,000 horsepower. That's a lot of horses!

Don't look at us...

But it's not really horses doing the work. Each engine is actually made up of hundreds of metal blades like this:

These blades are called airfoils, and they spin very quickly to move air and produce thrust. Airfoils spin so fast they experience a lot of tensile stress. Over time, they can crack and break, which is BAD. A broken airfoil damages the engine and puts the plane in danger.

But fear not! Engineers apply peening to these parts to reduce the tensile stress. Sometimes shot peening does the job, but sometimes it doesn't go deep enough.

The best protection against cracking is compressive stress deep within the metal. This is particularly important for engine airfoils and other high-stress parts.

So how do we generate deeper compressive stress...?

Come on, you know this...

With laser peening!

Yes - laser peening. The subject of this lesson. Now that we understand metals, metal fatigue, and stress, we can understand how laser peening works and why it's so important. This is where it gets fun, kids! (If you weren't having fun already.)

So What Is Laser Peening?

Laser peening is a method for adding compressive stress to metals to prevent fatigue. Much like shot peening or a ball peen hammer, laser peening involves striking the metal surface to compress the material. However, instead of using hammers or little metal balls, laser peening compresses the metal with focused beams of light.

I know, it sounds crazy.

You might have seen a laser in action before. Maybe you saw them in a light show or a science fiction movie. When most people think of lasers, they imagine something like this:

Pretty cool! But the lasers we're talking about look more like this:

A laser is just a concentrated beam of light. Kind of like a very focused light bulb. Some lasers are extremely powerful, like a thousand light bulbs turned on at once.

"But wait a minute..." You say. "How do you compress metal with a beam of light?"

I'm glad you asked! But the answer gets a little complicated.

Laser peening delivers a high-energy light beam to the metal surface. The laser is very powerful, and it hits the metal surface with a lot of energy. So much energy, in fact, that it creates a small explosion.

Not quite but we're on the right track. Laser peening actually produces an explosion of plasma. So, what the heck is plasma? Maybe you recognize this:

Or this:

Plasma is an extremely hot gas that contains so much energy the atoms separate into individual particles. It takes lots of energy and the right conditions to produce plasma. Lightning bolts are made of plasma. So are stars. Our sun is just a giant ball of roiling plasma.

Beautiful, isn't it?

Here on earth, we can use lasers to create plasma explosions for metal improvement. As you can imagine, it's a lot of energy in a small space. The plasma has so much energy it just wants to burst out in all directions, kind of like how you feel on the last day of school.

By expanding so quickly, the plasma exerts a strong force on the metal surface. During laser peening, we make that force even stronger by flowing water over the area. The water acts like a cover that traps the expanding plasma and holds it against the metal. Think of it as a heavy blanket keeping the plasma from exploding everywhere.

Is this really necessary?

Sorry buddy, but it is necessary. That blanket of water raises the plasma pressure on the surface of the metal. In fact, the pressure generated during laser peening can reach one million pounds per square inch! That's like a hundred elephants standing on a matchbook.

It's a lot of pressure.

What's more, it happens very quickly. Each laser pulse and plasma burst takes place in just a few nanoseconds. A nanosecond is one billionth of a second. So, if you take the time to say, "One Mississippi", and divide that into a billion parts, you'll have one nanosecond. It's crazy small.

Meanwhile, the energy and pressure are crazy big. So, we have this high-energy, high-pressure plasma exploding on the metal surface for just a few short nanoseconds. The result is a powerful shockwave in the metal. So, what the heck is a shockwave?

Well, you've probably seen water waves before. Have you ever been to the beach?

These waves are created by energy moving through the water. They might be caused by wind or undersea currents, but they are basically just bands of energy that make the water rise and fall.

Since water is a fluid, it sloshes and moves around quite easily. You can even create little waves yourself just by tossing a stone.

While it's easy to make waves in water, it's a lot more difficult in solid metal. You need to hit the metal with a lot of energy to produce a significant shockwave. You need something powerful and focused, something that generates a lot of pressure in a short amount of time. Something like...

A plasma explosion!

Now it all makes sense. Laser energy generates a plasma burst, which sends a shockwave into the metal. Problem solved!

But wait - what does this have to do with metal fatigue? Are these shockwaves actually good for the metal?

You're asking some tough questions, kid. The truth is that the shockwave alters the metal - in a good way - through a process called plastic deformation.

Plastic defor-what?

Yeah, plastic deformation. It's about as complicated as it sounds but I'll attempt a simple explanation. Put on your science hat! (Although you probably should have been wearing it this whole time.)

A Brief History Of Plastic Deformation

"Deformation" is really just a big word that means something changes shape. Deformation is usually the result of some pressure or force, and it comes in two types: elastic and plastic.

With elastic deformation, an object changes shape under pressure but returns to normal when that pressure is removed. Rubber bands are elastic. So are springs. A balloon can be elastic as long as you don't pop it.

Plastic deformation is more permanent. This means an object changes shape under pressure and does not change back. This applies to many objects. If you bend a paperclip it just stays that way. It doesn't snap back on its own like a rubber band.

So how do we use plastic deformation to prevent metal fatigue? Once again, it comes back to compressive stress.

I knew it!

Of course you did. You're a smart kid. You'll be laser peening on your own before long.

As the shockwave moves through the material, it causes plastic deformation. This means the metal permanently changes shape due to the energy of the passing wave. It doesn't happen to the whole object, just the small area we hit with our laser.

This small region is deformed by the shockwave, causing it to stretch and push outward. But, the surrounding material is not affected by the shockwave, so it resists and pushes back. So, one area is pushing out, while the rest is pushing back...

What do we end up with when things push against each other?

Compressive.

Residual.

Stress.

Can you see it now?

Metals crack due to tensile stress.

Engineers fight this with compressive stress.

Laser peening adds compressive stress to metal parts.

Laser peening uses light energy to create a plasma explosion.

The plasma generates high pressure and a shockwave.

The shockwave stretches the metal, pushing it outward.

The surrounding metal pushes back.

The result is permanent compressive stress in the part, which prevents cracking and fatigue.

Whew.

We did it! We learned laser peening!

Congratulations everyone. We should celebrate.

Seriously, this is difficult stuff. There are lots of people who don't understand metal fatigue, peening, or residual stress. Go and ask your mom how plastic deformation works. Seriously, try it...

Ummmm...

Like I said, it's complicated. But thanks to smart engineers and powerful lasers, the important metal parts we depend on are stronger than ever.

One Last Thing Before We Go...

To wrap this all up, it's important to understand why laser peening is so effective at stopping metal fatigue. After all, if engineers can do the same thing with hammers or shot peening, why do we need a laser in the first place?

Good question.

As tensile stress pulls metal objects apart, compressive stress in the material holds them together. Shot peening and hammer peening add a thin layer of compression to the surface, but if you go down just a little bit into the metal that compressive stress disappears.

The real power of laser peening comes from how deep it puts compression into the metal. Deeper compressive stress makes it harder for cracks to form, and the more compressive stress in a part the greater tensile stress it can withstand.

Laser peening regularly generates compressive stress TEN TIMES deeper than shot peening. You might even say laser peening is ten times better than shot peening. (You said it, not me.)

Simply put, laser peening generates a more powerful shockwave that goes deeper into the metal. This creates deeper compressive stresses which provide greater protection against cracks.

And isn't that what we all want at the end of the day?

See, even mom's catching on.

I hope you've enjoyed this guide to laser peening. If you'd like to learn more, you should talk to the experts at LSP Technologies. They've been laser peening for over twenty years. They practically invented it!

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