Interacciones entre especies

Close cell foams, such as Ethylene vinyl acetate (EVA) foam are widely used in engineering, sport and biomedical fields (Mills et al, 2003; Moreu and Mills, 2004). EVA foam is made up of tiny gas bubbles, which give the material a unique mechanical behaviour, which is difficult to characterise. A typical application for EVA foam is in footwear, where a range of EVA materials of different hardness are used (Verdejo and Mills, 2004; Gu et al 2011). As illustrated in Figure 2.23, sports shoes can be broken down into 4 main areas; upper shoe, inserts, mid-soles and the outsole. While the upper shoe functions are mainly to hold the sports shoe together, it also helps to stabilize the foot during running (Gu et al 2011). The midsole of a sports shoe, which commonly made by EVA foam, aims to help spread the impact forces so the peak ground reaction force is not placed directly to the foot, leg and knee, which is how most injuries have occurred (Gu, 2010). The indentation resistance of the material directly reflect the deformation of the material under load and the comfort of the shoe, etc. EVA foam could provide the mid-sole cushioning properties. Detailed properties of the foam is very important for both material development and the design. In a product development process, the engineer has to select material form a range of product, in some case combination of material has to be used. Figure 2.24 illustrate an example in developing midsole using different EVA foams (Gu, 2011). In this case, EVA foams of different hardness is used in different areas of the midsole. The effect of these different designs has to be simulated with finite element modelling as shown in Figure 2.25, which requires detailed material parameters.

Given the wide application of EVA and the use of indentation testing, it is important to explore the use of ANN in indentation of EVA materials. Two main areas need to be studied for both industrial application and academic research. One area is direct estimation of the indentation curves for material of known properties, this will help the engineer/researcher to estimate the potential performance when comparing different

EVA foam is governed by two parameters (equation 2.4) (as compared to metallic materials, the P-h curve of which can be represented by only one coefficient (equation 2.1)), so it very difficult to tell how the material is going to perform under indentation as the two parameter influence the P-h curves in a different way. FE modelling has to be performed which is not very convenient in particular when the software or modelling expertise are not available. In addition, an ANN program validated with standard indentation tests may be able to be transferred into more complex loading to study the effect of material properties such as foot-shoe interaction. Another area to be investigated is to inverse estimate the material parameters from indentation tests as it is much more easier than standard approach (i.e. combination of shear and compression tests), it will be a great advantage if the material parameter can be predicted inversely form indentation tests as a quick way of predicting the properties with full confidence. As detailed in section 2.2 and illustrated in Figure 2.7, use of standard tests are complicated and time consuming. In addition, it will be difficult to conduct materials testing at different temperatures with the setup of standard tests.

There are some challenges for both research directions (i.e. direct P-h curve prediction and inverse material property estimation). The material represents a much more challenging research topic than metal materials. The indentation curve of metals can be represented curvature (as in Eq. 2.1), but for EVA foams, a more complex way has to be used to represent the data. Polynomial fitting could be way to describe the curves, but there are issue of nonuniqueness in using polynomial fitting. More challengingly, the choice of mathematical representation of the curve has to be properly selected to be able to aid the direct or inverse engineering. In addition, the accuracy/robustness in direct (predict indentation curves from known properties) and inverse (predict properties from indentation curves) has to be established. The practice in some of the published work in materials oriented projects has been focus on limited number of testing cases, which could not satisfy the need of materials research and development or to be used by material researchers without direct experience in ANN. There may be non-uniqueness issues, where more than one set material parameters fit the target of P-h curves; this and the uncertainty

associated with this has been a major barrier in preventing wider application of inverse program. One direction of this work is to explore situation where testing data of different situation (e.g. indenter size. thickness) can be used to improve the robustness. Another aim on program development is to establish methodology and computational tool that is capable of mapping out all possible solution in a direct/inverse program, which will ultimate give the user the confidence in the inverse results and select the optimum/best available solution based on pre-knowledge or experience in case that no unique properties can be defined, this will be suitable for the nature of materials testing. The material behaviour of EVA material is representative of a range of materials including biomedical materials, so the program developed and evaluated with EVA as a model material will be transferable to other material systems in the future based on the research framework (including the use of different ANN tools (nftool, nntol and code based), programming language and curve fitting/searching approach to be established in this project). Some of these will be highlighted in the discussion and future work section.

Figure 2.23 Schematics to show the structure of the sport shoe design and the application

of EVA materials (Nigg et al., 2006).

Upper

Insole

Midsole (EVA)

(a) Images showing the midsole and different materials.

Figure 2.24 Manufacture of the midsole with different materials to illustrate the use of

Figure 2.25 FE model for simulating the effect of EVA foams on shoe heel interaction (Gu et al 2010) to highlight the importance of EVA testing and properties.