Joosef Leppänen

Fragment impacts into concrete

Massive concrete withstand blast wave and fragment impacts effectively, and consequently, reinforced concrete is commonly used for protective structures. An important issue is how the fragment impacts influences the concrete. It is well known that the behaviour of concrete exposed to fragment impacts leads to damage in the form of severe cracking as well as spalling. When fragment penetrates concrete deeply, scabbing may occur at the reverse side of the wall, or even perforation. However, the knowledge of how the fragment impacts influences the material properties of concrete are not well known. To study this, experiments have been carried out, in order to examine to what extent the concrete, at various distances, is affected by the fragments. The fragments, which were spherical and 4 mm in radius, were shot against concrete blocks with dimensions of 750 x 750 x 500 mm3 at a speed of approximately 1 650 m/s. The depth of penetration and crater size was measured. Thereafter, the concrete blocks were cut in two halves and the global macro cracking observed. Uniaxial compressive and splitting tests were made from cylinders, which were drilled out from various positions of the block. Furthermore, specimen from the blocks were thin grinded and micro cracking was analyzed with microscope. Experiments presented here shows that the damage from fragment impacts is localized in the very front of the impact zone. The concrete strength below this zone, though, was hardly affected at all. This indicates that is possible to distinguish between the global load effects and the local damage effects, which is caused by the fragment impacts. Consequently, it might be possible to separate the loads from blast wave and fragment impacts, in design; and hence, the structure could be analyzed as a pre-damaged structure with decreased effective depth or width.

Finite element analyses of concrete penetration with a steel projectile. Comparison of Lagrangian and Eulerian techniques

When a projectile impacts a concrete target, it penetrates the concrete, severe crushing occurs and a crater forms. When concrete is subjected to impact loading, the strength will increase due to the strain rate and confinement effects. This paper presents finite element analyses of concrete perforation with a steel projectile. To predict the perforation depth of the projectile and the crater size of the concrete, material models are required where the strain rate effect, large deformations and triaxial stress states are taken into account. By using the Lagrangian technique, large deformations will lead to excessive grid distortions and tangling; therefore an erosion algorithm is needed. By using the Eulerian technique, the problems of grid tangling are overcome. The analyses are performed with the nonlinear finite element program AUTODYN and the results are compared to experiments concerning the depth of penetration and the crater diameter.