The soles of your favorite running shoes can be frustrating after just a few months of use. What if there is a way to make them last longer and keep them cushioned and full comfort? A recent study delved into the tiny structure of the foam used in these shoes to understand exactly how they wear out. By using cutting-edge imaging techniques, researchers have discovered fascinating details about the microscopic changes that lead to the degradation of this material, offering hope for a more durable sports footwear in the future.
3SR Laboratory led by Dr. Laurent Orgéas (researchers in the 3SR Laboratory of Grenoble Alpes, CNRS and Grenoble INP) and colleague Dr. Clara Aimar, PR. Sabine Rolland du Roscoat and Dr. Lucie Bailly and Dr. Dimitri Ferré Sentis of Decathlon SE both delve into the fatigue mechanism of closed-cell elastic foam used in running shoes. Their findings, published in the journal Polymer Testing, reveal how these materials are downgraded over time and stress, providing valuable information for designing more durable sports footwear.
The study focuses on ethylene ethylene vinyl ester (EVA) foam, a common material in the middle of running shoes due to its excellent energy absorption properties. Despite their widespread use, the degradation process of EVA foam under repeated stress is not clear, especially the link between mechanical fatigue and changes in cellular levels.
To address this problem, the team used continuous and interrupted annular compression tests on EVA foam samples. They used advanced X-ray microtransmission to capture detailed 3D images of foam cell structure before, during and after fatigue tests. The technology allows researchers to observe how the microstructure of the foam evolves under pressure, thus giving a clearer understanding of the mechanisms driving fatigue.
The researchers carefully prepared foam samples for testing. They start with a flat plate of EVA foam and are then cut into smaller cylindrical samples. These samples were subjected to repeated compression cycles to simulate the stresses experienced in the mid-bottom of the running shoe during use. By using continuous and interrupted compression tests, researchers can compare how the foam performs under different conditions and how it recovers its shape after rest.
One of the main findings of the study was the identification of two major fatigue-induced defects: plastic bending and tearing or formation of holes in the cell wall. These defects were observed to contribute significantly to the mechanical fatigue of the foam, resulting in partial recovery of the material properties during suspension of circulation. “The interruption of the cycle can increase flattening of cells flattened along the compression axis by plastic bending and tearing/holes of the cell walls,” explains Dr. Orgéas.
Studies have shown that the mechanical properties of EVA foam degrade in a predictable manner during continuous cycles. The foam has gradually softened, with major changes mainly taking place. After the initial rapid degradation, the steady decline rate is slower. The researchers noted that these changes were closely related to observed microstructure defects, which became more obvious as the cycles increased.
Furthermore, the study highlights the importance of rest time in EVA foam fatigue testing. After each break, samples subjected to interrupted cycles showed partial recovery of their mechanical properties. This recovery is attributed to the viscoelasticity of the foam and the pressure of the gas trapped in the cells. “These findings suggest that the ability of foam to partially recover between stress cycles is critical to its long-term performance,” Dr. Orgéas said.
In practical applications, this study provides valuable insights into designing more durable running shoes. Understanding the microstructure changes occurring in EVA foams under pressure can help manufacturers develop more fatigue-resistant materials. This may result in sports footwear, thereby maintaining its cushioning and energy absorption properties to improve runner performance and comfort.
In this study, the use of X-ray microtransmission volume is critical. This lossless imaging technology allows scientists to create detailed 3D models of the internal structure of the foam. By comparing images taken before and after the fatigue test, the researchers can see how the internal architecture of the foam changes over time. They observed how foam cells, which were originally round and evenly distributed, deformed and irregular with repeated compression. “3D imaging provides us with unique insights into the structural changes of foams at the microscopic level,” Dr. Orgéas noted.
The study also utilizes digital volume correlation, which compares images from different stages of the test process to quantify the 3D strain field that occurs in the foam. This approach allows researchers to link these strain measurements to the degree of cell wall bend and the development of high-precision tears or pores. By combining these advanced imaging techniques, the team can correlate the mechanical properties of the foam with specific structural changes, thus giving a comprehensive understanding of the fatigue process.
In summary, the research of Dr. Orgéas and colleagues represents an important step in our understanding of the fatigue mechanisms in closed-cell elastic foams. By linking mechanical fatigue to specific microstructure changes, this study has developed a variety of applications for the development of more durable and elastic materials, especially in sports and sports.
Journal reference
Aimar, C. , Orgéas, L. , Rolland du Roscoat, S., Bailly, L. , & Ferré Sentis, D. (2023). “Fatiency Mechanisms of Closed Cell Elastic Foams: Mechanical and Microstructure Study Using EX X-ray Microscopic Imaging.” Polymer Testing, 128, 108194.
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