Engineers Figured How Fatigue Cracks Grow and What Stops Their Propagation
The Cornell Fracture group has published the results of an ongoing study on the formation and propagation of fatigue cracks on metals and claims to have identified the mechanism that causes the cracks to grow.
Their paper, titled “Atomic Mechanism of Near Threshold Fatigue Crack Growth in Vacuum,” explains that a molecular dislocation happens near the crack tip and returns to a slightly different location. This cycle repeats numerous times until a fatigue crack forms on the material.
An interesting new finding is that all cracks stop growing after about 20 load cycles following their formation. After testing further in simulations involving structural alloy in a vacuum, the team found that adding new sources of defects like localized strains or nearby cracks won’t change this peculiar mechanical behavior. Instead, the key lay in interacting with the crack tip, forcing atomic bond breakages that allowed the damage to grow.
In summary, cracks can only propagate if the tip continues to be the central point of molecular dislocations. Because these defects are practically random and manifest in different locations, fatigue cracks stop growing on their own after a while. This also means that if dislocations are actively steered away from the tip, engineers can prevent crack propagation and extend the service life of damaged components.
Traditionally, makers of alloys and structural materials in general focus on strength and ductility, while fatigue takes the back seat and is only estimated based on bibliographic hypothesis. Any simulation-based studies available to engineers thus far stopped at ten loading cycles, whereas the Cornell Fracture Group is now conducting simulations that go as far as 180 cycles, more than enough to draw safe conclusions.
Metal makers could use the experimental data to solidify their understanding of the relationship between fatigue crack growth tendency and fundamental material properties (stacking fault energies and elastic moduli) and design fatigue-resistant alloys or provide a better prognosis for their performance on that front.