Stress waves due to detonation of a cylindrical charge at a free surface

Abstract

Using the dynamic photoelasticity method, we have studied stress-wave fields due to explosion of a single charge at the surface of a plate. The known theoretical solutions for the plane cases of action of a concentrated normal impulse and a tangential force [1, 2] enable us to construct the stress-wave pattern in a semiinfinite elastic medium for the cotTesponding type of action.Comparison of the experimental data from explosion of a eylin&ieal surface charge, in plates of optically sensitive material based on g40-M epoxy resin, with the theory for the impulse from a concentrated normal force applied to the boundary of an elastic half-plane [1] reveal good qualitative agreement between the experimental and theoretical patterns [3]. These results indicate that the dynamic photoelasticity method can be used to study stress waves due to an explosion in an elastic medium.However, if the depth of immersion of the charge in the material is small, the calculations become more complicated, and this makes it difficult to get numerical results. Our present aim was to make an experimental study of the stress and displacement fields, in particular, the maximum tangential stresses, and also the damping of the amplitudes of the main types of stress waves due to explosion of a point charge at the surface of a plate.This problem was solved experimentally by the optical-polarization method, with high-speed kinephotography to record the stress wave. We used flat models, made of ~40-M, 300 • 250 x 3 mm in size. The load was imposed by detonating a cylindrical charge of lead azide, 15 mg in weight; the ratio of the wavelength X to the plate thickness d was X/d ~ 10. The charge was placed at a distance from the free surface equal to its own diameter.Since the depth of the charge in the material has a marked influence on the stress distribution near the free surface [3], the results of estimates of the wave field are valid only for the type of charge configuration under investigation.Charge K wave~ I "v. 5~ ~rnax~'~ OC ~0 ~ Fig. 1. Scheme of recording stress and displacement fringe patterns, in photoregistration mode, along the rays.The scheme for recording the wave patterns of interference fringes and displacements in the plate is shown in Fig. 1.The stress waves were recorded by superhigh-speed photography with the camera equipment at a distance F= r/Xeomp = 1-8.5 from the charge, at 10" intervals, in time-lapse conditions simultaneously over the whole field of the part of the model under study. The radial displacements U r of the plate were recorded in tl~ee directions, a= 0 ~ a=45 ~ and a= 90", and the vertical displacements of the surface were measured at distancesT= 1-7 from the charge center.It is known that in general, when an impulsive load is applied to the free surface of a plate, the longitudinal wave P is propagated with velocity Cs, the transverse wave S and conical wave K with MISI, Moscow.