Seeing the soles of your favorite running shoes wear down after a few months of use can be frustrating. What if there was a way to make them last longer while keeping their cushioning and comfort intact? A recent study takes a deeper look at the tiny structures of the foam used in these shoes to understand exactly how and why they wear down. Using cutting-edge imaging techniques, researchers have uncovered fascinating details about the microscopic changes that lead to the degradation of this material, offering hope for more durable athletic footwear in the future.
Researchers at the 3SR Laboratory (of the Grenoble Alpes University, National Center for Scientific Research and Grenoble Polytechnic Institute), led by Dr. Laurent Orgéas, together with his colleagues Dr. Clara Aimar, Prof. Sabine Rolland du Roscoat and Dr. Lucie Bailly together with Dr. Dimitri Ferré Sentis of Decathlon SEhave investigated the fatigue mechanisms of closed-cell elastomeric foams used in running shoes. Their findings, published in the journal Polymer Testing, reveal important information about how these materials degrade over time and under stress, providing valuable insights for the design of more durable athletic footwear.
The study focused on ethylene-vinyl acetate (EVA) foams, a common material in running shoe midsoles due to its excellent energy-absorbing properties. Despite their widespread use, the degradation process of EVA foams under repeated stress was poorly understood, particularly the link between mechanical fatigue and changes at the cellular level.
To address this issue, the team employed continuous and interrupted cyclic compression testing on EVA foam samples. They used advanced X-ray microtomography to capture detailed 3D images of the foam’s cellular structure before, during, and after fatigue testing. This technique allowed the researchers to observe how the foam’s microstructure evolved under stress, providing a clearer picture of the mechanisms driving fatigue.
The researchers meticulously prepared the foam samples for testing. They started with EVA foam slabs, which they then cut into smaller cylindrical samples. These samples were then subjected to repeated compression cycles to simulate the stresses that running shoe midsoles experience during use. By using continuous and interrupted compression testing, the researchers were able to compare how the foam behaved under different conditions and how it regained its shape after periods of rest.
One of the key findings of the research was the identification of two main fatigue-induced defects: plastic bending and the formation of tears or holes in the cell walls. These defects were found to contribute significantly to the mechanical fatigue of the foam, leading to a partial recovery of the material properties when the cycle was stopped. “Cycle interruption allows the flattening of the cell along the compression axis to be observed with plastic bending and an increase in cell wall tears/holes,” explained Dr. Orgéas.
The research showed that the mechanical properties of the EVA foams degraded in a predictable manner during continuous cycling. A progressive softening of the foam occurred, with significant changes occurring primarily during the first 5000 cycles. This initial rapid degradation was followed by a slower, more consistent decline. The researchers observed that these changes were closely related to the observed microstructural defects, which became more pronounced as the cycles increased.
Furthermore, the study highlights the importance of rest periods in fatigue testing of EVA foams. Samples subjected to interrupted cycles showed a partial recovery of their mechanical properties after each rest period. This recovery was attributed to the viscoelastic nature of the foam and the pressure of the gas trapped within the cells. “These findings suggest that the foam’s ability to partially recover between stress cycles is crucial for its long-term performance,” says Dr. Orgéas.
In terms of practical applications, this research provides valuable insights for the design of more durable running shoes. Understanding the microstructural changes that occur in EVA foams under stress can help manufacturers develop more fatigue-resistant materials. This could lead to athletic footwear that maintains its cushioning and energy-absorbing properties for longer, improving runners’ performance and comfort.
The use of X-ray microtomography was instrumental in this research. This non-destructive imaging technique allowed the scientists to create detailed three-dimensional models of the foam’s internal structure. By comparing images taken before and after fatigue testing, the researchers were able to see how the foam’s internal architecture changed over time. They observed how the foam’s cells, which are initially round and evenly distributed, deform and become irregular with repeated compression. “Three-dimensional imaging gave us a unique insight into the foam’s structural changes at a microscopic level,” noted Dr. Orgéas.
The study also used digital volume correlation, a method that compares images from different stages of the testing process to quantify the 3D deformation field occurring within the foam. This approach allowed the researchers to link these deformation measurements to the degree of cell wall bending and the development of tears or holes with high accuracy. By combining these advanced imaging techniques, the team was able to correlate the mechanical performance of the foam with specific structural changes, offering a comprehensive understanding of the fatigue process.
In conclusion, the research by Dr. Orgéas and his colleagues represents a significant advance in our understanding of fatigue mechanisms in closed-cell elastomeric foams. By linking mechanical fatigue to specific microstructural changes, this research offers a path to the development of more durable and resilient materials for a variety of applications, particularly in sports and athletics.
Journal reference
Aimar, C., Orgéas, L., Rolland du Roscoat, S., Bailly, L., and Ferré Sentis, D. (2023). “Fatigue mechanisms of a closed-cell elastomeric foam: a mechanical and microstructural study by ex situ X-ray microtomography”. Polymer Testing, 128, 108194.
Article name: https://doi.org/10.1016/j.polymertesting.2023.108194
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