No abstract

Abstract. This paper presents a study on the influence of the density gradient profile on the mechanical response of graded polymeric hollow sphere agglomerates under impact loading.Quasi-static, standard split Hopkinson pressure bar (SHPB) tests as well as higher speed direct impact Hopkinson bar tests and Taylor tests are performed on such hollow sphere agglomerates with various density gradient profiles. It is found that the density gradient profile has a rather limited effect on the energy absorption capacity from those tests. It is because the testing velocity performed (<50 m/s) is rather small with respect to its average sound wave speed (around 500 m/s) and the equilibrium stress state can be reached rather quickly. The high impact tests allow to generate a non-equilibrium state condition and the influence of density profiles is clearly observed. Besides, in order to extend this study to the situation beyond our testing limitations, a numerical model is built on the basis of the experimental behaviour data. It confirms the important influence of the density gradient profile under a non-equilibrium stress state situation. This study shows that placing the hardest layer as the first impacted layer and the weakest layer as the last layer has some benefits in terms of maximum energy absorption with a minimum force level transmitted to the protected structures.

In this paper, sandwich plates made from 0.8mm 2024 T3 aluminium alloy skin sheets and graded polymeric hollow sphere cores (having various density gradients) are studied. The experiments at 45m/s were performed with an inversed perforation setup using SHPB system. Quasi-static tests using the same clamping system allow for the rate effect investigation. Numerical simulations are performed in order to get the indispensable local information (which is not experimentally available) to better understand the perforation process. With these experimental and numerical tools, it is found that the gradient profiles change the perforation process under studied impact loading, whereas they have no influence under quasi-static loading. Under impact loading, a competition of two deforming scenarios at the early stage governed the whole process afterwards. One is the global crush of the first layer in contact with the incident skin and the other is the piercing of the incident skin sheet. When the first layer is rather strong, the incident skin sheet breaks and the perforator makes a hole in the core afterwards. When the first layer is rather weak, the skin sheet folds into the core and develops a much more energy consuming process. The best gradient profile in terms of the energy absorption capacity as well as the non-sheet breaking criterion should be a lower first layer and a progressively enhanced core.