MODELING OF FRAGMENT LOADS AND EFFECTS ON REINFORCED CONCRETE SLABS

This paper describes a technique used to characterize the loading and damage generated by the primary fragments from cased munitions on reinforced concrete slabs or walls. It has been observed that the damage generated by the fragments, while usually of secondary importance to the airblast, can in some circumstances dominate the response. Test observations indicate that fragments from cased conventional bombs, not only directly load a wall element through momentum transfer, but also create a substantial amount of damage on the wall’s front face. This damage may include slicing of the reinforcement and complete removal of concrete from the front face to depths beyond the front face reinforcement. Single fragment tests have also shown that the momentum transferred is substantially greater than the fragment momentum prior to impact, due to the significant amount of concrete debris ejected from the impact crater. Equivalent forcing functions, in terms of pressure versus time, are derived to approximate both the momentum transfer and the damage generated in the concrete. The calculations used to demonstrate these techniques were performed with the three-dimensional Lagrangian finite element code DYNA3D and a specially modified concrete model which includes the effects of fracture, fracture energy based strain softening, and strain rate. FRAGMENT IMPACT TESTS Figure 1 depicts the type and extent of damage that can occur to a reinforced concrete slab from the fragments generated by a cased bomb. Measurements of the crater depths are given in Figure 2. In tests to measure the effect of single fragments on reinforced concrete slabs, it was observed that the fragment would create a crater in the slab with a significant amount of concrete being ejected from the impact crater; much of the ejecta traveled in a direction normal to the slab (i.e., in the direction opposite that of the fragment). As a consequence, the amount of impulse transmitted to the slab was greater than the initial fragment momentum (i.e., fragment plus ejecta momentum). Transmitted impulses of nearly twice the initial fragment momentum have been observed