10/5/2026
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The experiment was designed to study friction. It turned out to be a sensitive gauge of how a contact wears while the interface is sliding.
Friction at the joints of engineering structures controls how they damp vibration, how they fatigue, and how reliable they are, yet the contact surfaces are hard to observe while they operate. Researchers have now shown that a single strong pulse, sent down a row of touching steel spheres, can report on a contact while it is still sliding.
The work was led by Alfredo Fantetti of Imperial College London, with University of Illinois mechanical engineering PhD student Jean Myung Jung, professor Kathryn Matlack, and professor Alexander Vakakis.
Their technique relies on a “nonlinear solitary wave” — a single, compact pulse of force that travels through a chain of spheres without spreading out, much like the click that passes through a Newton’s cradle. How fast the wave-pulse travels changes based on how stiff the contacts along its path are, so its speed carries information about the surfaces it touches.
In the experiment, the chain of spheres interacts with an oscillating block, creating friction between the chain and the boundary. Laser sensors measure the time it takes for the pulse to travel to the block and bounce back.
As the block kept sliding, the wave-pulse began returning faster. After about 30 minutes of continuous sliding, the pulse was traveling roughly 5% faster than at the start, a sign that the contact had become noticeably stiffer. Using simulations and microscopy of the surfaces, the team showed this change in the wave-pulse speed was directly related to wear between the surfaces.
“We originally designed the experiment to study friction, and how the pulse responds when the contact is sticking versus sliding,” Jung said. “What it turned out to be good at is tracking wear. Seeing the contact change while it was still sliding was the exciting part.”
The pulse was largely insensitive to whether the surfaces were sticking or sliding when it arrived. It presses on the contact far harder than the load holding the surfaces together, so it briefly locks them in place. That makes it a cleaner measure of wear than conventional ultrasound, whose signals can change for several reasons at once.
Because the pulse concentrates its force on a near-point contact, it can reach small contacts that are difficult to isolate with ultrasound. Solitary-wave methods have already been used to assess adhesive joints, cement curing, and implant stability. This study extends them to contacts that are dynamically sliding.
The work has important applications in in situ monitoring of frictional interfaces. For example, a compact probe that reads a contact’s stiffness as it wears could be used for real-time monitoring of joints in engineering structures.
This is the first laboratory demonstration. Absolute readings still depend on chain calibration, alignment, and contact history, and the researchers’ next step is to probe contacts located beyond the end of the chain.
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The material is based on work supported by the Office of Naval Research (ONR) under Grant No. N00014-24-1-2632. Fantetti acknowledges funding from the Imperial College Research Fellowship scheme.
The paper, “High-rate interaction between strongly nonlinear solitary waves and an oscillating frictional boundary of a granular crystal,” published in Extreme Mechanics Letters, is available online.