Tell-tale signs of fatigue appear early in metals

Fatigue in metals
A typical microstructure (grain structure at the micron scale) of metallic material. (Courtesy: J Stinville)

The tiniest defects in a metal can cause it to fail, but predicting when this so-called metal fatigue will occur is difficult. Now, however, researchers in the US and France have found that the fatigue strength of a metallic material can be calculated after just a single loading cycle thanks to a new nanometre-resolution digital image correlation technique. The technique enabled the team to observe incipient weak points known as slip localizations on the surface of a wide range of alloys and could guide the design of fatigue-resistance alloys in the future.

“We found that after the first fatigue deformation cycle, the amplitude of the plastic localization events that developed determines the fatigue strength of metallic materials,” explains Jean-Charles Stinville, a materials scientist and engineer at the University of Illinois at Urbana-Champaign who led the research. “This observation identifies the origin of fatigue failure and paves the way to rapidly predicting fatigue strength, which measures how many times a certain amount of stress can be applied to a material before it fails.”

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When a metal is under load, linear defects known as dislocations move through its crystal lattice, causing the atoms to slip over each other and the metal to deform. These surface slip localizations are where stress concentrates and they act as nucleation sites for cracks that grow progressively with further loading cycles, eventually causing failure.

Fatigue strength and yield strength

Researchers currently measure the strength of a metal or alloy by testing samples under different cyclic loading conditions to calculate the highest stress the material can withstand for a given number of cycles – a complex and time-consuming method. In the new work, Stinville and colleagues found that the (irreversible) slip localization after the first loading cycle was linearly correlated to the fatigue strength of different metals. This correlation provides a way to rapidly predict the fatigue strength of metals, they say.

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In their work, they subjected a variety of face-centred cubic (FCC), hexagonal close-packed (HCP) and body-centred cubic (BCC) metallic materials to cyclic deformation and observed their behaviour on the nanometre scale at the earliest stages of cycling. They observed that the characteristics of slip events varied as a function of the crystal structure and microstructure. For instance, slip localization is usually less intense in BCC metallic materials, explaining their better fatigue strength in comparison to FCC and HCP materials.

“Our experimental tool allows for statistical quantification of nanometre-scale deformation events that are involved during the deformation of metals,” Stinville tells Physics World. “Their characteristics inform the structure’s and microstructure’s effects on the mechanical properties, and in particular fatigue strength.”

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Such analysis is helpful for identifying alloys that show exceptional or unusual behaviour and provides a different approach for identifying fatigue-resistant alloys quickly and simply, he adds.

The researchers say they now plan to extend their technique to loading conditions under extreme environments, such as high and cryogenic temperatures. “Such measurements will be useful for identifying metals for transport applications and those that require extreme temperatures,” Stinville says.

They report their work in Science.

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