The experiments on investigation of shock compressibility of heterogeneous anisotropic materials (carbon fiber, fiberglass, textolite and kevlar) when shock wave propagates across and along the fibers were performed using a VISAR laser interferometer, which had a nanosecond time resolution. They are polymeric composite materials consisting of interwoven fibers with a diameter of 5-15 microns and an epoxy matrix as a binder. A distinctive feature of such materials is a strong anisotropy of properties and the sound speed of them along the fibers is several times higher than that one for the transverse direction. The goal of this study is development of targets for experiments at a novel diagnostic system proton microscope (PRIOR) at the TU Darmstadt.
The shock wave profiles were recorded by VISAR laser interferometer. The structure of compression pulse and the shock wave velocity of investigated materials were obtained in each experiment. To study the shock compressibility of materials under high pressure, the explosive propellant charges were used to provide a flat throw of aluminum flyer plates. Their speed varied from 1.13 up to 3.3 km/s. The samples were loaded through aluminum and copper plates.
In anisotropic materials kevlar, textolite and carbon fiber, on the particle velocity profiles the speed jump, followed by oscillations with respect to a certain average value is recorded. These oscillations are due to the heterogeneous structure of the material, which are particularly well visible at low pressure. The velocity profiles for fiberglass at the shock wave propagating across the fibers have two-wave configuration. This difference is due to the fact that in fiberglass, unlike other materials studied, the sound speed across the fibers is higher than the velocity of the shock wave in this direction.
Hugoniot parameters of anisotropic materials were obtained for both orientations of the fibers in the coordinates of the shock wave velocity D – particle velocity u. The particle velocity was calculated from the known speed of the flyer plate and the value of D measured in the experiment. In the investigated pressure range, the experimental data are satisfactorily approximated by linear dependences. Hugoniots of carbon fiber, textolite and Kevlar with transverse direction of the fibers are parallel to each other and differ only in the first coefficient determined by the speed of sound in the material: D=2.53+1.47*u, km/s – for kevlar, D=2.17+1.45*u – for textolite and D=1.70+1.43*u – for carbon fiber. Since the angle of inclination of Hugoniot is determined by the thermal properties of the material, most likely the most significant in this direction are the properties of epoxy resin, which is a common binder in all these materials. The slope of Hugoniot of fiberglass for two directions of the fibers is very different from the slope of that for other materials tested both in the transverse direction and in the longitudinal.
The work is carried out with the financial support of FAIR-Russia Research Center.