2. PROTECTION AGAINST DYNAMIC DAMAGE A.F. Belikova, S.N. Buravova, Yu.A. Gordopolov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia, e-mail: svburavova@yandex.ru Dynamic loading differs from a quasi-static one by the fact that a characteristic size of loading (product of sound velocity and duration of load) exceeds the sample size. As is known, penetration of an indenter into a target is accompanied by formation of the fan of tensile stains and radial cracks around surface defects that develop simultaneously in the course of cyclic loading. Cracking at dynamic loading has a spallation origin and arise due to interference/focusing of rarefaction waves Spall cracks arise at the meeting of rarefaction waves and are oriented normally to the surface. Another important feature is that, in conditions of dynamic loading, surface defects do not act as seeds for crack birth.

3. Impact of a flyer is accompanied by formation of shock waves, and simultaneously with them arise waves of unloading which source are lateral sides of a shock body. Focusing of side rarefaction waves leads to longitudinal zones of high tensile strains and finally channel (needlelike) cracks. The phenomenon of a channel crack formation accompanies pulse laser loading, detonation spraying. Note that channel cracking was also observed on the surface of gas turbines and marine propellers as a result of cavitation erosion. Rain, dust, and drop erosions also get started from formation of longitudinal cracks.

4. Metallographic section of a steel target after the impact of a bar shaped ledge plate. V~1000 m/sec. Modeling experiments. The material inside of a crack is subject to destruction. Here bright (non-etched) areas testify to change of structure of the steel materials Channel cracks

5. Detonation spraying. V~400 – 700 m/sec Inclusions of sprayed material in the crater bottom show beginning of a channel crack.

6. Lower-speed loading. Laser irradiation impact V~60 -550 m/sec The similar damage morphology of a target at lower velocity and high- velocity loading as well proves a shock physics analysis approach to a problem erosion at slow loading

7. System of longitudinal (cylindrical) cracks in two-dimensional spallation modeling. V~1000 m/sec Sketch of experiment: (1)- explosive charge; (2)-driver (flyer) plate; (3)- sample with channel hole; (4)- ring The channel crack at repeated impact is a source of unloading, the interference of internal and lateral waves of unloading leads to formation of circular cracks.

8. The spall mechanism of the material damage under the contact domain determines the fracture sequence: the channel crack (the first pulse), the cylindrical crack (the second compression- pulse flux), the set of coaxial cracks (the third flux). In the case of a sufficiently high impact velocity, the third pulsed flux producing a new generation of coaxial cylindrical cracks. The fracture sequence 1 2 3 4

9. The above data clearly show that the deepest channel crack is formed at the initial stage of dynamic damage. Therefore, the formation of cylindrical spall cracks can be decelerated or suppressed if we prevent the formation of initial channel crack. To date, non-one-dimensional (focused) spall damage has been studied inadequately, in contrast to the one-dimensional one, arising at an interference of face rarefaction waves when influence of lateral waves is excluded. Our spall model suggests the following principle for protection against dynamic damage. A protective surface layer of material must give rise to the diffraction of shock waves, which would decrease the intensity of rarefaction waves and hamper crack formation. Such protection can be formed by two or three layers of metals with strongly different acoustic properties received by explosive welding. A wavy structure of the weld can be expected to act as a diffraction grating for incident shock waves.