The soles of your favorite running shoes breaking down after just a few months of use can be frustrating. What if there was a way to make them last longer, keeping their cushioning and comfort intact? A recent study dives deep into the tiny structures of the foam used in these shoes to understand exactly how and why they wear out. By using cutting-edge imaging techniques, researchers have uncovered fascinating details about the micro-level changes that lead to the degradation of this material, offering hope for more durable athletic footwear in the future.
Researchers from the 3SR Laboratory (of the Univ. Grenoble Alpes, CNRS and Grenoble INP), led by Dr. Laurent Orgéas, along with colleagues Dr. Clara Aimar, Pr. Sabine Rolland du Roscoat, and Dr. Lucie Bailly together with Dr. Dimitri Ferré Sentis from Decathlon SE, have delved into the fatigue mechanisms of closed-cell elastomeric foams used in running shoes. Their findings, published in the journal Polymer Testing, reveal significant insights into how these materials degrade over time and under stress, providing valuable information for the design of more durable athletic footwear.
The study focused on ethylene-vinyl acetate (EVA) foams, a common material in the midsoles of running shoes due to its excellent energy absorption properties. Despite their widespread use, the degradation process of EVA foams under repeated stress was not well understood, particularly the link between mechanical fatigue and changes at the cellular level.
To address this, the team employed continuous and interrupted cyclic compression tests 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 the fatigue tests. 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 began with slabs of EVA foam, which were then cut into smaller cylindrical samples. These samples were subjected to repeated compression cycles to simulate the stresses experienced by running shoe midsoles during use. By using both continuous and interrupted compression tests, the researchers could compare how the foam behaved under different conditions and how it recovered 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 observed to contribute significantly to the foam’s mechanical fatigue, leading to a partial recovery of the material’s properties when cycling was paused. “Interrupting the cycling allows the observation of the cell flattening along the compression axis with both plastic bending and an increase of tears/holes of cell walls,” explained Dr. Orgéas.
The research showed that the mechanical properties of the EVA foams degraded in a predictable manner during continuous cycling. There was a progressive softening of the foam, with significant changes occurring primarily during the first 5000 cycles. This initial rapid degradation was followed by a slower, steady decline. The researchers noted that these changes were closely linked to the observed microstructural defects, which became more pronounced with increasing cycles.
Furthermore, the study highlighted the importance of rest periods in the fatigue testing of EVA foams. Samples subjected to interrupted cycling showed 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 recover partially between stress cycles is crucial for its long-term performance,” said 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 materials that are more resistant to fatigue. This could lead to athletic footwear that maintains its cushioning and energy absorption properties for longer, improving performance and comfort for runners.
The use of X-ray microtomography was pivotal in this research. This non-destructive imaging technique allowed the scientists to create detailed 3D models of the foam’s internal structure. By comparing images taken before and after the fatigue tests, the researchers could see how the internal architecture of the foam changed over time. They observed how the foam cells, which are initially round and evenly distributed, became deformed and irregular with repeated compression. “The 3D imaging provided us with a unique insight into the foam’s structural changes at a microscopic level,” noted Dr. Orgéas.
The study also utilized digital volume correlation, a method that compares images from different stages of the testing process to quantify the 3D strain field occurring within the foam. This approach enabled the researchers to link these strain measurements with the extent of cell wall bending and the development of tears or holes with high precision. By combining these advanced imaging techniques, the team could 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 step forward in our understanding of the fatigue mechanisms in closed-cell elastomeric foams. By linking mechanical fatigue to specific microstructural changes, this research offers a pathway 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., & Ferré Sentis, D. (2023). “Fatigue mechanisms of a closed cell elastomeric foam: A mechanical and microstructural study using ex situ X-ray microtomography.” Polymer Testing, 128, 108194.
DOI: https://doi.org/10.1016/j.polymertesting.2023.108194
