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How Metal Responds to a Strike: Surprising Results on Heat Generation and Energy Storage

“Strike while the iron is hot” is a well-known adage meant to inspire immediate action. Researcher Juan Carlos Nieto-Fuentes is spreading a slightly different message: Strike and the iron is hot.

Nieto-Fuentes is a researcher at the University Carlos III of Madrid in Spain studying what happens to metal objects during high-speed collisions. The more we know about how metal responds to such impacts, the better equipped we are to design vehicles that can protect passengers in a crash, armor that can shield soldiers from enemy strikes, and metal structures that can withstand shocks. One of those responses is generating heat.

From top to bottom, this series of thermal images reveal the temperature increase of a metal cylinder (tantalum) during high-speed deformation. The colors go from cold (black) to hot (white). Credit: Daniel Rittel.

In research recently published in the American Physical Society’s journal Physical Review Letters, conducted while Nieto-Fuentes was working at the Israel Institute of Technology (Technion),
Nieto-Fuentes and a team that includes colleagues from Technion and the Nuclear Research Center Negev (NRCN) in Israel made a surprising discovery that could help us better understand–and maybe someday prevent–metal failures due to such impacts.

Physicists like to consider impact situations in terms of energy. When you strike a metal object, you transfer the energy of that impact into the material. The energy has to go somewhere, and the usual explanation is that some of it goes into heat generation and the rest is stored in the material.

If you were to zoom way in on a metal, you’d see that the material consists of a nice, orderly arrangement of atoms. Well, mostly. All metals contain defects, like tiny voids no larger than the width of a human hair. Although they are tiny–on the scale of the space between atoms–defects can cause big problems. In general, the more defects there are in a material, the more likely the material is to fail. Cracks, dents, and other indications of material failure result from high concentrations of defects.

Left: Metal failure in a kitchen knife after one drop too many. Credit: Photo by Arne Kuilman.
Right: A farrier works with a horseshoe after heating it in a forge to soften the metal. Credit: Photo by Jonathan Bean on Unsplash.

Remember how impact energy doesn’t go entirely into generating heat? The remaining energy is stored in the metal as defects.

In light of this, generating heat might sound like a safer response to high-speed impacts, but heat causes problems too. Most metals get weaker with heat, that’s why materials like silver and iron are first heated and then shaped and cooled. To put it simply, high impact collisions are a problem for metal because heat and defects can accelerate their failure.

In this new research, the Technion team performed a series of experiments on small metal cylinders. In each iteration, a cylinder was placed end-to-end between two longer steel bars. A third steel bar then rammed into the end of the first bar, generating a shock wave that traveled through it and hit the cylinder. In response, the cylinder did two things—generate heat and deform.

Sketch of the experimental design. Credit: J. C. Nieto-Fuentes, S. Osovski, D. Rittel, to be published in MethodsX.

The researchers captured the heat generated by the cylinder with an infrared detector and its deformation over time with the help of the second steel bar. The technology doesn’t exist to watch defects form and move in real-time during such an experiment but measuring the deformation of the material over time, something called the strain rate should be a good substitute.

According to well-accepted theoretical models, the fraction of impact energy transformed into heat depends on the strain and strain rate experienced by the material. That held true for these experiments; the heat produced by an impact varied predictably with strain rate.

Taking things one step further, the researchers collaborated with Arie Venkert at NRCN to analyze cross-sections of the impacted cylinders with high-resolution microscopes. Based on the defect concentrations and patterns, also known as the microstructure of a sample, they calculated the amount of impact energy stored in each metal cylinder.

Since microstructure governs deformation, the team expected microstructure and energy storage to correspond directly to the strain rate. Furthermore, as the well-accepted model predicts, they expected microstructure to vary predictably with both heat generation and energy storage.

But the data didn’t support this. Cylinders with the same observed microstructure generated different amounts of heat while retaining similar mechanical characteristics (stress-strain curves), regardless of strain rate.

Cylinder specimens next to an Israeli shekel for scale. The cylinders are 6 mm in length and diameter. Credit: J. C. Nieto-Fuentes, S. Osovski, D. Rittel.

At the time, Nieto-Fuentes was a graduate student working under professors Daniel Rittel and Shmuel Osovski at Technion. They spent hours trying to understand these results, often over lunches of hummus and Iberian ham–a culinary reflection of their respective cultures, Rittel and Osovski from Israel and Nieto-Fuentes from Spain. On weekends they exchanged frantic emails and phone calls, all ending in the same way, “What’s going on here? Everything seems to be similar and yet, when you strike the sample harder, the heat generation is completely different.”

About six months later they converged on an answer. The problem wasn’t with the experiment or the analysis. To their surprise, it was rooted in the basic assumption of a direct relationship between impact

energy, stored energy, microstructure, and heat generation. Models based on this assumption match experimental results in many situations, but not when the object deforms quickly, as in this case.

“The direct relationship between heat generation and microstructure under impact is not so straightforward,” Nieto-Fuentes explains. “The resulting microstructure doesn’t set the heat, rather [it is set by] the continuous evolution toward a final state.” In other words, something happens while the microstructure changes that isn’t captured in the energy-based model of the situation.

Since we don’t have the technology to monitor how microstructures evolve under these conditions yet, the team is trying to uncover what that “something” is in other ways, primarily through modeling techniques. Hopefully, their work will pique the interest of other scientists as well, and the research community will strike at this question while the iron is hot.


--Kendra Redmond

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