Predicting failure of solid mineral materials from the first stage of cyclic stress

Predicting failure of solid mineral materials from the first stage of cyclic stress

Take a wire clip. Now, bend it back and forth in the same place 15, maybe 20 times. The paperclip may have broken before you finished. This is due to the so-called metal fatigue, which occurs when a metal component is subjected to periodic stress until it fails.

While a broken paperclip is a trivial example of metal fatigue, this phenomenon is a huge problem in the wider world. “Most unexpected failures — bridges, aircraft, oil rigs, heart valves — fail with this process,” said Teresa Bullock, a University of California, Santa Barbara professor of materials science, who specializes in the mechanical and environmental performance of materials in harsh environments. Almost any structural metal subjected to periodic stress – deformations, vibrations, temperature extremes, impacts and the like – is weak, with results that can cost hundreds of billions of dollars each year.

To predict and avoid these catastrophic fates, Bullock and fellow researchers at UCLA, the University of Illinois at Urbana-Champaign and the University of Poitiers in France have developed a theory that predicts the limits to which minerals can be subjected to cyclic stress before failure. And they can expect failure from the first cycle. Their research was published in the journal Sciences.

The ability to predict when a metallic component will fail due to cyclic stress has long been a priority when designing an engineering system, whether it is an industrial heart valve or a nuclear power plant. However, according to Bullock, who is also interim dean of the School of Engineering at the University of California, Santa Barbara, the process for making that decision has not changed much in nearly two centuries.

“They take something, they spin it, and they measure the cycles of failure,” she said.

But these empirically driven results often come without the deep quantitative insights that would enable predictions across a wide range of minerals under different conditions. To complicate matters further, failures can often occur after millions or billions of cycles. “If you had to test something for a year or 10 years before it failed, it would be a bit difficult to get enough test results to design against that failure,” Bullock said.

Advanced technologies provide new insights

From the moment a hard metal undergoes its first cycle of stress–usually in tensile first, followed by compression and then back to zero–it is set on a failure path. But often, the damage is not immediately visible to the naked eye. But at the nanometer scale, the damage is there: Atoms in the stressed region of the metal slide against each other, creating patterns of wear called “slip bands.” As the material goes through more cycles, more of these slip bars appear, and eventually a microcrack forms. Additional loading cycles cause the crack to grow until it becomes a macroscopic crack and the metal fails.

“Fatigue strength” is the stress that can be withstand a large number of cycles before it fails, often between a million and a billion. Testing a wide range of engineering metals and alloys using a range of new techniques has enabled researchers to relate measurements from the first cycle of slip localization to the metal’s stress strength in a surprisingly straightforward manner.

“We never expected this correlation to be linear across so many different materials,” she continued. “The set of materials examined are very, very different from each other and they all lie on the same curve.”

At the core of the team’s discovery is the UCSB-developed TriBeam microscope, which allows new, high-resolution approaches to studying sliding bands, along with new ultrasound fatigue testing and multimodal data analysis techniques. “The ability to develop and maintain these advanced tools and combine them with machine learning-assisted analysis within the UCSB infrastructure has been critical,” Bullock said.

According to the study, the location and intensity of the first slip localization events predict when the material will fail and where the crack will begin to form. Key to these expectations is the metal’s “yield strength” – known as the point of no return where the metal is irreversibly deformed during loading.

“The surprising observation is that some of the slip bars that appear during the first half-cycle of stress disappear completely by the end of the cycle,” Bullock explained. “However, a small portion of the bands do not disappear or ‘reverse’ during the first cycle; these were found to be the sites where failure occurred after a billion cycles.”

The high-resolution studies conducted by the researchers also provide insight into the factors that influence the stress strength of a metal, including processing methods and crystal structure – the three-dimensional arrangement of metal atoms. How the atoms slide over each other varies with the way they are stacked. Body-centered cubic arrangements (atoms at each corner and in the center of the cube) experience more scattered slip events, while face-centered cubic (atoms in each corner and on each face of the cube) and closed hexagonal package metals exhibit more localized slips and greater variance in their intensity. . These parameters could explain the differences in the stress lifetime of minerals with different crystal structures, and influence the research team’s theory.

These newly discovered correlations and quantitative insights advance the understanding of metal stress, with the implication being that they can be used to design optimally engineered systems and more accurately predict when and how a metal component will fail.

“If you could predict how a metal would perform from the first cycle, you wouldn’t have to go through all these costly and time-consuming testing methods, and we could make better materials and protect ourselves from disaster,” Bullock said.

Bullock’s research for this study was supported by a US Department of Defense Vannevar Bush College Fellowship (VBFF), which is awarded to approximately 10 faculty members in all fields each year. VBFF supports new ideas out of the box as the researcher’s creativity intersects with the unknown.

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