Keywords: dynamic, rebound, layers, elastic, soil, damping, sport, Clegg hammer, artificial athlete.

The dynamic impact characteristics of a class of layered structures are computed and a limited laboratory validation is presented. The layers consist of a combination of one or more of the following materials: a concrete base, fine sand layer, and a thin elastic cover. Impact simulates conditions typically arising in construction, as in a sandwiched floor construction or in playing and sports grounds subject to free-falling objects, a player, a ball, etc. Impact tests are carried out with the Clegg hammer [1], thus approximating construction loading conditions. To approximate the other conditions use is made of the Artificial Athlete [2].

The dynamic behaviour of layered structures is complex but for design purposes simplified performance indicators are desirable for describing that behaviour. Preferably a simple, discrete model should be used encompassing some essential measures to describe this behaviour. In this work, the equivalent stiffness and damping of the different layers are therefore represented with an elasto-dynamic spring-dashpot model [3]. The main objective is to show that although the facets of the problem are complex, the simple exact solutions of the type derived by Davies and Karim (1995), provide reasonable possibility to match short-duration peak responses at the point of impact. This will become evident from the test and model results shown despite the variations of the test method and the different test layers.

The results have some novel and important practical implications for layered-structures design under impact. Since the results are of a propriety nature, only limited results are published at this point. Single layer systems consisting of bare concrete, sand or elastic top are calculated first using properties of the test conditions (apparatus, loads and material properties). The peak deceleration and deformation responses are of particular interest and are therefore presented and discussed. These peak values are easily and directly measured by the apparatus. They reflect rebound performance of an impacting object and the structural integrity of the impacted materials. The value of these data for construction, as maintenance measures and for playing quality is evident.

Significance of the various layers to the overall performance indicated by peak deceleration and deformation is also studied. Measured rebound and deformation reflects the equivalent (dynamic) elastic properties representing all of the materials involved as well as apparatus design. More than one layer (2 & 3 layered structures) lead to more complex interactions. A simple mechanical model inevitably ignores some of the features. For example the number and thickness of the various layers are parameters not represented in a discrete model. Despite that complexity and the modelling simplifications one could still derive interesting practical conclusions from matching the various measures provided by multi-layered tests and the spring-damper model.

Experimental results show that sand is of significant influence in composite layered structures, as indicated particularly by the acceleration records of the Clegg hammer tests. The elastic cover layer properties and thickness become more important for controlling the end quality and range of rebound deceleration if sand is not present. This is confirmed by the Clegg hammer peak deceleration measures and by the model predictions for this device. However, the model predictions for deformations measured with the Artificial Athlete do not confirm this finding. Discrepancies indicate either model deficiencies or that this apparatus is not so suitable. Discrepancies in deformation levels could mean that another more complex model should have to be used for predictions involving the Artificial Athlete or that better fit is only likely for elastic material ranges other than those used in the current tests. The Clegg hammer is a simpler device to model since it directly impacts the ground without the cushioning springs used in the Artificial Athlete tester. Matching the measurements of the Clegg hammer and model predictions for the peak deceleration and deformation require good dynamic material and elastic properties. It is also noted that the peak results for rebound velocity are more or less independent of the number and type of layers.

1

B. Clegg, "An impact testing device for in situ base course evaluation", Australian Road Research Bur., Proc. 8, 1-6, 1976.

2

U.E.F.A., "Turf manual: football-related technical requirements", sections 1 & 2, part 2, 1-59, 2003.

3

T.G. Davies, and U.F.A.Karim, "An elastodynamic interpretation of impact test apparatus for soils", Géotechnique 45 (4), 691-700, 1995.

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