Strain fingerprints will help researchers design better metallic materials

CHAMPAIGN, IL — Engineers can now capture and predict the strength of metallic materials under cyclic loading, or fatigue strength, in hours, not months or years with current methods.

In a new study, researchers at the University of Illinois at Urbana-Champaign report that automated high-resolution electron imaging can capture the nanoscale strain events that lead to metal failure and fracture at the origin of metal failure. The new method helps scientists quickly predict the fatigue strength of any alloy and design new materials for engineered systems subjected to repeated loads for medical, transportation, safety, energy and environmental applications.

The results of the study, conducted by professors of materials science and engineering Jean-Charles Stinville and Marie Charpagne, are published in the journal Science.

Metal and alloy fatigue, such as the repeated bending of a metal paper clip that leads to it breaking, is the root cause of failure in many engineering systems, Stinville said. Defining the relationship between fatigue strength and microstructure is a challenge because metallic materials exhibit complex structures with characteristics ranging from nanometers to centimeters.

“This multi-scale problem is a long-standing problem as we try to observe sparse nanometer-scale events that control macroscopic properties and can only be captured by investigating large areas with fine resolution” , said Charpagne. “The current method of determining the fatigue strength of metals uses traditional mechanical tests which are expensive, time consuming and do not provide a clear picture of the root cause of failure.”

In the current study, the researchers found that the statistical study of nanoscale events that appear on the surface of metal when it is deformed can inform the fatigue resistance of metals. The team is the first to discover this relationship using automated high-resolution digital image correlation collected with a scanning electron microscope, a technique that compiles and compares a series of images recorded during deformation, Stinville said. The researchers demonstrated this relationship on aluminum, cobalt, copper, iron, nickel, steel and heat-resistant alloys used in a wide variety of key engineering applications.

“What is remarkable is that the nanoscale strain events that appear after a single strain cycle correlate with the fatigue strength that tells us how long a metal part will last under a large number of cycles,” Stinville said. “Discovering this correlation is like having access to a unique strain fingerprint that can help us quickly predict the fatigue life of metal parts.”

“Designing metallic materials with higher fatigue resistance means safer, stronger and more durable materials,” Charpagne said. “This work has societal, environmental and economic impacts as it sheds light on micro and nanoscale parameters to accommodate the design of materials with longer lifespans. I believe this work will define a new paradigm in alloy design.

This study was carried out in collaboration with researchers from the University of California, Santa Barbara and the University of Poitiers, France.

The Department of Defense, Office of Naval Research, and Illinois Department of Materials Science and Engineering supported this research.

– This press release was originally published on the University of Illinois at Urbana-Champaign website

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