Hello! In my research, I am interested in waves but in shocks as well. Practical situations may involve ships bumping against bridge piles or icebergs, or car accidents and even rockets crash or blast. There is thus a need for fast dynamic experiments allowing the mechanical characterization of materials. In this research topic, I will show you how it is possible to determine the dynamic properties of various materials under controllable fast loadings! In the case of car accidents, the shock leads to fast dynamic loads and very large strain rates. Such large strains cannot be reached on traditional loading devices! We thus need specific fast loading devices as the Hopkinson device. I am going to explain the basic principles of this experiment. The Hopkinson device is composed of very long bars in which waves propagate. A pressurized system launches an impactor on the so-called incident bar. The generated wave propagates into the incident bar and reaches a specimen to be studied. Part of the waves is transmitted to a so-called transmitted bar and part is reflected into the incident bar. The analysis of the signals recorded on the bars allows the comparison of the incident, reflected and transmitted waves! How do these waves interact at the bar/specimen interfaces? The bar properties are denoted with index 1 and the specimen properties with index 2. Considering the first interface, the continuity of displacement reads u_1 = u_2 leading to incident wave + reflected wave equals transmitted wave. Similarly, for the continuity of normal forces, T_1=T_2 leading to T_I+T_R=T_T. What is the influence of these various waves? We consider two falling bars - the bottom bar is stiff and the upper bar is softer. We may assess the contrast effect at the interface. When falling on the ground, the shock wave propagates into the bottom bar and is reflected or transmitted at the interface. The wave transmitted to the upper bar governs the rebound conditions with respect to the mechanical contrast. Be careful! For large contrasts, we notice that the rebound may be very very strong! What is the link with the property contrast? Here is the specimen in the Hopkinson device: the waves are now reflected at both interfaces! The sum of the incident and reflected waves is equal to the transmitted wave: u_i+u_R=u_T for displacements and T_i+T_R=T_T for normal forces. Measuring these waves on the bars, we may determine the specimen properties! The measured displacement ratio u_T/u_i is equal to 4 chi / (1+chi)^2, where chi is the impedance ratio. Since chi = rho_1 C_1/rho_2 C_2, it allows the calculation of the wave velocity in the specimen C_2. So, what is new? Hopkinson bars allow this classical identifications but at very large strain rates!! Here is the global view of the device: the impactor is launched on the incident bar. The incident and reflected waves are measured by strain gauges on the incident bar. The transmitted wave is measured in the transmitted bar. We can now determine the ratio between the transmitted and incident waves. How do the various waves propagate within the specimen? At the first interface, the incident wave is reflected and transmitted in the specimen. At the second interface, the stress wave is partly reflected in the specimen and partly transmitted to the second bar. As shown by the blue triangles, the axial stress progressively becomes uniform in the specimen. We may thus study the behavior of the specimen but at very high strain rates! What are then the forces at the bar specimen interfaces? The left hand force T_in is plotted in black and the right hand force T_out in red. At first, T_in increases with a zero value for T_out. At 50 micro-seconds, both forces are balanced and we may study the specimen behavior during the loading phase. Around 200 micro-seconds, the unloading phase begins and T_in decreases. After a while, T_out decreases as well. We thus have two transient phases (for loading and unloading) and two equilibrium phases in which the fast dynamic behavior of the specimen may be directly studied! We may then use the expressions of the axial stress and strain in the bars. The axial stress equals rho_1 C_1 (u_1)dot. The axial strain reads: (u_1)dot / c_1. Furthermore, at equilibrium, T_out and T_in are equal. It means that E_1 S_1 epsilon_T = E_1 S_1 (epsilon_i+epsilon_R). In the specimen, the axial strain may then be determined by integrating the strain difference from one interface to the other, namely eps_out-eps_in, w.r.t time. At equilibrium (see above), the axial strain may then be determined from the integrated reflected wave only. The strain rate in the specimen is obtained by derivation of the previous expression. Finally, the axial stress depends on the ratio of the cross-section areas S_2 by S_1 and the sum of the strains epsilon_i+epsilon_R+epsilon_T. At equilibrium, the axial stress in the specimen may then be determined from the transmitted wave only. The fast dynamic behavior of the specimen is now determined! We may apply this experimental method to glass breakage, to foam dynamics, oh no, not that type of foam!! And various cases involving fast planet dynamics! Let us now summarize this video! I have shown experimental researches on fast dynamics using a special device called Hopkinson bars. It involves various waves propagating in very long bars and a very short specimen! Many important applications are allowed. Crash-test is one of them. In this case, it is obvious that fast dynamics and high strain rates are at work!