Researchers in the Grainger College of Engineering at the University of Illinois Urbana-Champaign have demonstrated that ‘fatigue’ resistance can be greatly enhanced by controlling how metal plasticity, or irreversible deformation, localises at small scales. It represents a new design strategy for engineering metallic alloys that are resistant to fatigue by leveraging unique deformation processes at the atomic scale.
Metal alloys crack and fail through a mechanism called fatigue when repeatedly loaded and strained. While it’s well known how to design alloys to withstand static loads and pressures, it’s very difficult to design resistance to fatigue because it’s difficult to predict how the underlying cause manifests at the atomic scale.
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‘Transportation, space and energy all create environments where there is risk for fatigue, presenting a challenge to both safety and sustainability,’ said materials science and engineering professor Jean-Charles Stinville. ‘Structural applications that involve high temperatures or radiation need materials resistant to fatigue, and our work shows how to design metal alloys that achieve this.’
Fatigue is governed by how a material accommodates plastic deformation – the irreversible rearrangement of its internal structure under repeated loading. As a material is cyclically loaded and unloaded, localised plastic deformation accumulates, eventually leading to crack initiation. Paradoxically, materials engineered to withstand very high static loads often suffer from reduced fatigue resistance because their microstructure promotes strong localisation of plastic deformation, accelerating damage accumulation.
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‘In alloys, plastic deformation tends to localise into discrete regions, which ultimately become preferential sites for fatigue crack initiation,’ Stinville explained. ‘Because this localisation emerges from complex microstructural and deformation processes interactions, it is difficult to predict where and how it will occur, making it challenging to account for during the engineering design stage.’
Stinville and his collaborators examined whether fatigue resistance can be drastically improved by designing alloys in which plastic deformation is engineered to remain small and uniformly distributed rather than intense and highly localised.
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‘It makes sense intuitively that spreading out the plastic deformation homogeneously reduces the impact of localised deformation, but experimentally demonstrating it was another matter,’ Stinville said. ‘It required new technology capable of scanning large regions at very high resolution combined with theoretical support from density functional theory and ab-initio molecular dynamics simulations.’
The researchers used high-throughput automated high-resolution digital image correlation, a technique developed in Stinville’s laboratory, to map plastic deformation with unprecedented spatial resolution across large material regions. Unlike conventional methods, which must trade field of view for resolution, this approach captures fine-scale deformation over wide areas. These measurements revealed a delocalised mode of plastic deformation involving deformation processes called ‘dynamic plastic delocalisation’. Mechanical testing showed it to be directly associated with greatly enhanced fatigue resistance.
To make sense of the observed structural features, Stinville’s group collaborated with mechanical science and engineering researchers within the group of mechanical science and engineering professor Huseyin Sehitoglu, an expert in the theory and modelling of metal deformation. Computational modelling clarified the roles of chemistry and ordering on the observed delocalised plasticity in the tested materials.
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Now that it has been confirmed that metal chemistry and structure can be used to generate homogeneous plasticity during deformation and therefore greatly improved fatigue resistance, the next step is exploring the potential of this result in material design strategies.
‘Now that the fundamental mechanism has been identified, we can design new alloys chemistry that activates it to produce fatigue resistant alloys,’ Stinville said.
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The research has been published in Nature Communications.