Lightweight and with great resistance to corrosion, aluminum offers very desirable characteristics when it comes to vehicle construction. Where it can run into trouble, however, is the development of weak points via repeated, alternating stress (imagine bending a paper clip back and forth over and over until it breaks). Scientists in Australia have come up with a solution to this so-called “failure by fatigue,” modifying the microstructure of aluminum alloys so that they can heal these weak points themselves.
"Eighty per cent of all engineering alloy failures are due to fatigue,” says Monash University’s Professor Christopher Hutchinson, who led the research.”Fatigue is failure due to an alternating stress and is a big deal in the manufacturing and engineering industry.”
The research carried out by Hutchinson and his team is described as the first of its kind, and focuses on the underlying reason for this fatigue called precipitate free zones (PFZs). These are weak links that form in aluminum alloys through alternating stress, which start out as tiny spots of plasticity and go on to form cracks before eventually fracturing the material.
Hutchinson and his team of engineers sought to intervene in the early stages of this process, by harnessing the mechanical energy that is generated during alternating stress. More specifically, the team came up with a way of capturing new particles that form as stress is applied to the material, and use those to reinforce the weak points and significantly delay the emergence of cracks and fractures.
This is achieved through a “training” process that mimics the strains placed on the material, although at higher stress than usual, which is repeated through several hundred cycles. This leads to a higher concentration of fine particles in the weak zones that enhances the yield and tensile strength of the material, which can then heal itself during operation.
"In this respect, the structure is trained and the training schedule is used to heal the PFZs that would otherwise represent the weak points,” says Hutchinson. “The approach is general and could be applied to other precipitate hardened alloys containing PFZs for which fatigue performance is an important consideration."
The researchers say that modifying the starting microstructure in this way could significantly improve the fatigue life of aluminum alloys. They also note that high-strength aluminum alloys, which have notoriously poor fatigue strength, stand to gain the most and could have their fatigue life extended by as much as 25 times.
"Our research has demonstrated a conceptual change in the microstructural design of aluminum alloys for dynamic loading applications," Hutchinson says. "Instead of designing a strong microstructure and hoping it remains stable for as long as possible during fatigue loading, we recognized that the microstructure will be changed by the dynamic loading and, hence, designed a starting microstructure (that may have lower static strength) that will change in such a way that its fatigue performance is significantly improved.
The research was published in the journal Nature Communications.
Source: Monash University