My first job out of graduate school and post-doctorate work was with the footwear company, NIKE. I was fortunate to hire into the health of the most advanced footwear technology department in the world and I could not have had a more exciting job description. I began by learning how to design NIKE AIR cushioning bladders. After becoming familiar with the design and development process I was able to make my first contribution, which was to pioneer the use of Finite Element Analysis to predict the inflated geometries of NIKE AIR bladders.
The engineers in the NIKE AIR Technology Lab were very proficient at design and had established an efficient process for prototyping new designs-at least for components based on flat film and lay-flat tubing. However, blow molded AIR bladders which were becoming quite popular required building mold tooling prior to prototyping. A design tool that allowed the refinement of a design prior to cutting tools was needed. These molds were expensive and involved several weeks of lead time. The biggest benefit of an FEA method, however, was the ability to predict the geometry of the bladders so that midsole tooling could be made for production. The current practice was to inflate ten AIR bladders, set them aside, allow them to equilibrate for 6-8 weeks and then measure them with calipers. Several mechanisms affected their change in shape but viscoelastic creep was primarily responsible for the time dependent shape change.
There had been several previous attempts to predict the inflated geometries of NIKE AIR bladders prior to my working at the company. It was generally believed that FEA would not be able to predict inflated geometries with sufficient accuracy. Furthermore, the viscoelastic behavior of the urethane elastomers was considered too complicated to characterize. Having been an ardent experimentalist challenging FEA codes for advanced elastic-plastic fracture and rock mechanics problems, I thought for sure that we would be able to utilize FEA for NIKE AIR bladders.
Force deflection data was available for the NIKE AIR bladder material but before I embarked on fitting that data to a hyperelastic model, I was curious to know what strain levels were associated with an inflated bladder. One of the first things I did was to scrounge up some polarizing filters and rig up a crude polariscope to estimate the strain in the film of an inflated bag. My estimate surprised me—less than 1% strain. I verified my measurement using a grid method and sure enough, the strains were really that low. (I also explored using mercury strain gages for directly measuring strains with some success.)
Armed with this simplification, some basic creep data for urethane, and the help of some software vendors, I was able to demonstrate excellent predictive ability of FEA in determining inflated AIR bladder shape and growth over time. The application of FEA helped reduce product development lead times and significantly decreased the cost and time to develop complex blow molded designs. This last figure illustrates how FEA could help identify “hot spots” that would not inflate uniformly. I have since left NIKE and the AIR Technology Lab but not before seeing a whole department grow up to apply FEA to cushioning technology and many other aspects of footwear and athletic equipment.
