Quasi-isentropic compression using functionally graded materials in gas gun and explosive driven systems

Abstract

Results of hydrodynamic simulations of dynamic compression experiments performed by impact loading of materials are reported. The simulations pertain to a new approach for generating quasi-isentropic compression using functionally graded materials (FGM). First of all, we focus on quasi-isentropic compression waves generated by a constant velocity impactor (similar to that from a gas gun). Quasi-isentropic compression is characterized from the temporal profiles of pressure at target surface and fluid velocity at target-window interface generated from different functional forms of density variation along the FGM flyer. It is shown that quadratic FGM is the best option for increasing rise time of pressure pulse. Secondly, FGM induced quasi-isentropic compressions are studied by accelerating the impactor with high explosive (HE) driven shocks in both the cases when impactor is in contact with the target (contact geometry) and separated from target by air gap (flyer geometry). The study reveals that nearly isentropic pressure profiles can be realized with quadratic FGM impactor in flyer geometry. Contact geometry produces a distinct initial pressure jump which is undesirable for isentropic compression. Finally, quasi-isentropic compressions are observed for spherically symmetric FGM flyers driven by HE. Here also it is seen that quadratic FGM impactor serves better in improving the peak impact velocity at shell target. Thus, based on the study, we conclude that quadratic FGM is the right choice for generating quasi-isentropic compression. The parameters of the FGM impactors, optimized using a genetic algorithm code, are also presented for both planar and spherical systems.