DYNAMIC STUDY OF FOOTWEAR MATERIALS SIMULATING REAL LOADS

Abstract

The study of footwear materials has been traditionally based on the determination of the stiffness and the use of the shore A level or in the shock absorbing capacity. In most of the papers ody one of those parameters is determined in spite of the relation between them is not clear; a quite soft material doesn't have necessary to be shock absorbing and a shock absorbing material can be quite rigid. Besides the study of footwear materials has to face two main problem. The first one is that most of the materials used in footwear under the loads occurring in the sport movements, specially in running or jumping, are no more linear because of the high level of the forces developed in those movements. This means that the rigidity of the material depends on the load and increases with load. The second problem is owed to the viscoelastic behaviour of most of those materials. The rigidity of a viscoelastic material increases as the frequency increases. For these reasons the study of footwear materials must be done by simulating the forces occurring in the movements developed in the sport the footwear is conceived for. This paper presents a new methodology of study of footwear materials bawd on the determination of the loads applied and its simulation by means of a dynamic testing machine. Both the rigidity and the shock absorbing capacity of the materials are investigated a a function of frequency. This method permits lo know not only the behaviour of the material in real situations but also which frequencies are the ones preferably absorbed. This is specially interesting because of the general estimation that high frequencies are related with injuries located in the articular cartilage. Some materials used in running shoes are studied and both the influence of thickness and composition aw investigated. The results presented are compared with the previously published obtained when the loads applied are perfect sinus waves. The results show that while the rigidity always increases with frequency with a rate depending on the material, the shock absorbing capacity variation with frequency depends on the material. The shock absorbing capacity of the material a a function of thickness results show that as the thickness increases the shock absorbing capacity increases too. For the rigidity the influence of the thickness in the bottoming out of the material is clearly showed. The thickness analysis also shorn that it is possible to obtain a optimum thickness determined when a thicker material doesn't means a significant increase in shock absorbing capacity and a lowering of rigidity.