New insights on metal fatigue process

Scientists from Johns Hopkins University have found a new method to detect cracks in metals before they propagate and lead to failure.

Metallic materials tend to experience a certain degree of deterioration when they are subjected to repeated loading. This process is known as fatigue and causes structural damage associated with the initiation and propagation of microscopic cracks.

Metal fatigue is classified into 3 stages:

1. Crack Initiation: Microcracks open at the tips of microstructural flaws in the metal’s structure.
2. Crack Propagation: Repeated loading can cause the growth or the coalescence of microcracks which propagate and become a threat to the structural integrity of the metal.
3. Failure: Crack accumulation leads to the material's breakdown which is typically not accompanied by plastic deformation.

A great proportion of metal structures and facilities is subjected to repeated or cyclic loading. From bridges that bear the load of vehicles to airplanes that are impacted by air pressure differentials. Material fatigue is a common failure pattern that is hazardous when not detected. 

In the study, recently published in Science, researchers focused on the origins of the fatigue process in nickel. The team studied the microscopic cracks that evolved into shear bands leading to the material's failure by utilizing high-resolution techniques. “Fatigue failure plagues all metals and mitigating it is of great importance,” Jaafar A. El-Awady, co-author of the study and a mechanical engineering professor at the Johns Hopkins Whiting School of Engineering, stated.

According to their observations, there is a connection between the initial microcrack development to the final location of the fracture that leads to failure. This is a very important finding since the prediction of where large cracks will evolve is enabled. Therefore, a failure can be prevented based on the detection of the microcracks.

“We’re able now to have a more fundamental understanding about what leads up to cracks. The practical implication is that it will allow us to understand and predict when or how the material is going to fail," Prof. El-Awady, said.

Scientists also focus on another aspect to emphasize the importance of thoroughly understanding microscopic crack initiation and propagation mechanism. In industry, companies tend to replace many metal parts in fear of fatigue without being certain about the condition of the material. Common tests do not focus on the initial stages of crack development and are conducted in larger samples than those actually used.

"The component could actually be fine and never fail but they throw it away anyway solely on the bases of statistical arguments. That’s a huge waste of money,” Prof. El-Awady, mentioned. Therefore, it is of great importance to assess the evolution of fatigue to completely understand when a component is prone to failure and is no more functional. 

Sources: Johns Hopkins University, Sciencedirect