Using the Laser Hammer

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Peening materials has historically been accomplished by impacts from hard materials. Now it can be done with lasers. (Jean-Christophe, Michel, Delagnes/)

In general, cracks form and propagate in a material only when that material is under tension. This makes sense because the opposite force, that of compression, tends to push cracks closed. It also makes experiential sense because when we bend an object, it nearly always fails on the outside (the tension side) of the bend.

Practical people soon discovered that “peening” a highly stressed part in the area where cracking typically occurred could postpone or even prevent such cracking. Tapping or striking a metal surface with the spherical face of a familiar ball-peen hammer can produce part-spherical dents, placing the material in compression. It is this residual compression that can extend a part’s fatigue life, for when the part is stressed by applied load, no tension can occur in the peened area until the residual compression is relieved.

Shot Peening and Fillet Rolling

The more familiar forms of this “metal improvement” are shot peening and rolling. In the first, an air-driven blizzard of hard steel shot is directed at the area to be treated; The second (rolling) is typically applied to a crankshaft, to the radius or fillet where a journal joins a crank web. Here, a fairly thin disc of hard steel with a smoothly radiused edge is forcibly pressed against the fillet as the part is rotated around the journal’s axis.

I have personal experience with the benefits of shot peening. When we encountered cracked crankshaft discs in the final three seasons of racing Yamaha’s TZ750 two-stroke, I would have the disc crack-checked by Magnaflux; if the part passed, I would then have the area which typically cracked shot peened by a specialist who knew how much peening intensity was enough. We never had another disc failure. As for fillet rolling, I have recently described how BSA was able to stop premature failures of its newly designed parallel-twin crankshafts by using the technique.

Now there is a dramatic new method of peening whose effect can penetrate usefully deeper into metals than the two I’ve mentioned: peening by laser shock. It had its beginnings in the 1960s, and it matured as a useful industrial process in the 1990s. GE now uses it for turbine blade surfaces.

Laser Shock Peening

Laser shock peening works by directing a brief but intense pulse of laser radiation at a surface to be treated. The pulse’s intensity converts material at the surface into a high-pressure ball of plasma. (Plasma is what results when intense applied energy strips the electrons off the atoms in a material, creating a very hot high-pressure gas of energetic bare atomic nuclei.) In the instant during which that ball of extreme pressure expands outward, it also exerts that extreme pressure against the surface, driving a powerful wave of compression into it.

The current industrial systems use a more complex and effective method: A layer of dark material is applied to the surface to absorb the laser radiation, and a transparent layer of mass, which can be plain old water, is applied over that.

Why the need for the transparent layer? Allow this illustration: An enthusiastic young prospector wants to split a promising boulder to see if there’s anything valuable inside. He lays a quarter stick of dynamite on the boulder and sets it off. Nothing. A more experienced person, alarmed by the bang, arrives with advice: “Fetch me a couple handfuls of mud from that stream and I’ll show you something interesting.”

The youth complies. The old-timer sets another quarter stick on the boulder, then covers it with the mud. Upon ignition, click, the boulder cleaves in half. The mud, for a brief instant confining the explosion, reflects enough energy into the rock to split it.

Spalling

Something similar but much less pleasant can happen when a certain type of explosive shell strikes the hull of an armored vehicle. The bursting shell sends a compression wave of such intensity through the armor that when it reflects from the armor’s inner surface as a wave of tension, a piece of that armor—a “spall”—is detached. As it carries a great deal of energy, the spall does terrible things to the tank’s interior…and its occupants.

For a picture of how this happens, visit your CEO’s corner office. On his desk you may find a common executive toy consisting of several steel balls, all suspended in a frame at the same height by pairs of tiny wires, each ball in contact with its neighbor(s). Pull back a ball at the end of this group and let it swing to strike its neighbor. The energy of that modest impact travels through the set of balls, sending the ball at the far end swinging while the others remain motionless.

In the most effective implementation of laser-shock peening, two layers of material are applied to the surface to be peened: 1) a dark material to absorb the energy of the laser pulse and be transformed into high-pressure plasma, and 2) a transparent layer immediately above it through which the pulse can pass, which acts in the same fashion as the mud in our earlier example.

By these means and with a laser pulse of sufficient briefness and intensity, we can achieve surface pressures in the range of 150,000 to 1,500,000 psi. They are highly effective in generating residual compressive stress in the material being treated, extending deeper into it than can the mechanical methods previously used. Naturally, everything has to be right to achieve this desirable effect—that was achieved by engineering.

Peening to Create Energy

Since many of us are coming to the conclusion that just covering more and more farmland with wind and solar may not satisfy all our energy needs, there is more interest in such things as hot-fusion energy. One of the schemes for achieving this is basically to use laser shock to peen a tiny pellet containing hydrogen isotopes from all directions. A spherical array of pulsed lasers is aimed at its central point, a pellet is dropped in, and as it passes through the center of this device, the lasers fire. The entire outer surface of the pellet is converted into plasma at tremendous pressure. The plasma expands in all directions, including inward. That inward pressure compresses the material in the pellet to such an extent that the hydrogen isotopes reach the temperature and pressure at which nuclear fusion begins.

The notional future version of such a machine would cycle steadily, fusing pellet after pellet, releasing tremendous energy. The task that remains is to collect that energy, convert it into electricity, and put it on your monthly power bill. Charge the battery on your neighbor-friendly silent electric motocrosser or trials bike, and use the farmland to grow food.

Peening and the Hydrogen Bomb

This process of using radiation to compress matter (radiation in this case contributing the “R” in “laser”) suggests some earlier history, and indeed the concept is central to the hydrogen bomb. Around 1950 US physicists were trying to find a way to compress a “fusion package,” which is really just a much larger “pellet” from the above example, in order to initiate within it a fusion reaction many times more powerful than that of an atomic (fission) bomb.

They tried to achieve this compression within a spherical compression geometry like that of the Trinity test, which surrounded a fissile plutonium core with a spherical system of chemical explosive lenses. Their purpose was to generate a highly uniform inward wave of compression moving at roughly five miles per second. It worked to initiate fission, but the fusion researchers were unable to achieve fusion with a spherical compression system.

A fresh understanding came when they realized that compression could be created by radiation. When an atomic bomb begins to fission, much of its prompt energy release is a massive flux of soft X-rays. What if, they reasoned, we could collect that radiation and by some means direct it uniformly onto the entire surface of the fusion package? A way was found, and a fusion bomb was created.

Conceptually, the fusion bomb is closely analogous to engineering a way to apply laser shock peening over the entire surface of an object, in this case the fusion package, simultaneously.

How did we get here from TZ750 flywheel cracking? The journey is possible because everything is connected by physics.

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