Fatigue behavior of the foamLoad and displacement are monitored throughout the test at a rate of 150Hz. The data collected are then analyzed using a custom MatLab code that calculates several key parameters. The load-displacement curve is fit using a 6th-order Fourier function (best fitting function) and integrated to calculate the energy input to the sample. Initial tests on the first two samples identified issues with the experimental procedure: it was difficult to duplicate the initial loading conditions for each test, making it difficult to clearly differentiate changes from one test to another. In order to improve the reproducibility of the tests, a pre-load of 20N was applied on the second pair of test samples. Experiments revealed that first 100 cycles of each test are responsible for over half of the observed decrease in the calculated variables (energy, stiffness, …)! The hysteresis energy ratio is between 0.22 and 0.25 for all four samples. The mean stiffness of the EVA foam is between 60N/mm and 97N/mm and is increasing from one test to another. The comparison of the two types of recovery showed only a small difference in performance. While the mean maximum energy of the sample with regular recovery was increasing an average of +1.4% between each test, it was decreasing by -1% for the vacuum-recovered sample!

The main problem encountered during the execution of this project was an load cell too coarse to allow load-control testing of the samples. In order to improve the quality of the measurements and to obtain better results it is advised to build a custom testing machine specifically designed for foam testing. To better simulate running, a machine with a mass dropped from a selectable height at frequencies that mimic running, as mentioned in the ASTM standards (ASTM F1614-99), would be preferred. This would always input the same energy to the sample. Including a non-contact displacement sensor to allow measurement of foam recovery over time would provide precious additional information. As shown by the SEM image, a few cracks and holes have been detected in the structure of the foam after fatigue testing. It would be interesting to analyze the location of those elements in the sample to see if they are close to the side in contact with the tup or if the damage in the sample is regular. Even though we measured a noticeable difference of behavior between the two types of recovery, in the shoe the midsole is constrained between several other layers of materials which may restraint the effect of a vacuum.

CONCLUSION

Master’s Thesis of Science HES-SO in Engineering

Fatigue Performance of Ethylene Vinyl Acetate Foam Midsole Materials

Samuel De CastroProfessors in charge : Prof. Dr. Didier Maillefert and Prof. Dr. Thomas Mackin

HEIG-VD // Cal Poly

In collaboration with Nike®

DESCRIPTION

Crack in the wall of a cell (Lufo48_10x15k_V)

RESULTS

EVA foam sample in the Instron 1331 machine

In the specific field of running shoes, the main goals of the footwear are: (1) to reduce the shock transmitted from the ground to the legs, and (2) to provide motion control. Research has shown that, over the course of a single run, the shock absorption properties change dramatically. The role of footwear science consists of finding the best materials to achieve the goals stated above throughout a product’s design lifespan. The most commonly used materials for athletic shoe midsoles are foamed Polyurethane (PU) or Ethylene Vinyl Acetate (EVA). Analyzing the fatigue behavior of running shoes in situ is complicated by the biomechanics of running, wherein runners exhibit a broad range of styles that require accounting for a large number of parameters. One solution is to conduct experiment in a lab on foam samples using testing methods that simulate, more broadly, the stress and strain applied to the foam in a running shoe.

OBJECTIVESThe broad aim of this study is to improve our understanding of the fatigue mechanics of EVA foams. Therefore the study focused on the fatigue behavior of EVA foam samples provided by Nike®.

Repeated load-unload cycles (15’000 cycles per day) were conducted on EVA foam samples cut into squares measuring 80x80x20mm over a span of 10 (2 samples) and 14 days (2 samples). The samples were loaded at a frequency of1.7Hz, corresponding to the highest mean stride frequency found in the literature. As the recovery of the foam between each run is an important parameter, one sample was recovered in air under standard atmospheric conditions while another was placed in a vacuum chamber at a gagel pressure of approximately -65kPa.

Wrinkled cells and holes in an EVA foam sample after 750km run [1]

[1] R. Verdejo and N. J. Mills, « Heel–shoe Interactions and the Durability of EVA Foam Running-Shoe Midsoles » , September 2004, DOI : 10.1016/j.jbiomech.2003.12.022.

• Analyze the EVA foam mechanical behavior to determine if there is a significant evolution from one test to another

• Compare the results of the different types of recovery to understand the role of vacuum recovery

• Use Scanning Electron Microscopy (SEM) to analyze the foam microstructure for evidence of deterioration [1]

Scanning Electron Microscopy (SEM)As shown in the figures below, there is distinct evidence of fatigue deterioration in the EVA foam microstructure. Holes and cracks appear in the cell walls of fatigued samples. We can assume that these changes in the material structure were created by the repeated cycles and not the freeze fracturing process because these same features are not seen in the pre-fatigued samples.

Hole in the wall of a cell (Lufo48_10x15k_V2)

Crack in the wall of a cell (Lufo48_10x15k_R2)

Hysteresis Energy Ratio(Lufo48_10x15k_V2)

Strength and Energy(Lufo48_10x15k_V2)