Involved people: Amina Alaoui (CERMMO, Ecole Nationale des Ponts et Chaussées), Hicham Alaoui (DEA student of the UMLV), Michel Couprie
At the time of a road accident, most of the energy of the shock can be absorbed by deformable materials placed in the coachworks or the helmets. These materials are in general polymer foams. Laboratory CERMMO of the ENPC makes a study of the deformations of these materials under the effect of shocks, with an aim of selecting the compositions and the dimensions best adapted to the various conditions of use.
The experiments carried out initially consist in dopping a weight on a sample of material, of roughly cubic form. While varying the drop height, one can obtain different speeds at the time of the shock. The material is filmed during the shock by means of a fast camera (up to 10000 images/second). To facilitate the processing by image analysis, a grid is painted on the sample (fig. 15(a)). The sequence of images obtained during an experiment must be treated in order to obtain the trajectories, in two dimensions, of points located on the marked face of the sample.
The first stage of the processing consists of an extraction of the clear areas of the marking. In the second stage, the points (centers of the extracted areas) obtained from the nth image must be put in correspondence with the points resulting from image n+1, in order to be able to reconstitute the trajectories (fig. 15(b)). One of the major difficulties is that, dimensions of the sample decreasing with crushing, certain clear areas can disappear during the sequence. The following stages are devoted to the calculation, the analysis and the display of the results: individual trajectories, field of the displacement vectors, isovalue curves of the deformations in x, y and module.
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