Comparative Study of Fragment Geometries (Mechancial Project)

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Fragments from explosive device have been and still are a great threat, but has now changed into terrorist attacks involving IED (Improvised Explosive Devise) rather than hostile forces.

This study will show the effects on concrete plates (50 mm depth) impacted by different projectiles, which should replicate fragments hitting buildings. The projectiles used were the 8 mm sphere, the 6 mm sphere, the 8 mm cylinder (RCC) and the FSP (fragment simulating projectile). The thesis was made at request from FOI (Swedish Research Defiance Agency) at Grindsjön and it contains both experiments made there and numerical simulations.

The experiments were conducted at Grindsjön, the projectile were fired in velocities between 800 m/s and 1600 m/s. The 8 mm and RCC obtains higher penetration depth at lower velocity, while the FSP, due to its soft material, will need much higher velocity. Full penetration was obtained at 1510 m/s for the 6 mm sphere, 1310 m/s for the 8 mm sphere and 950 m/s for the RCC.

The simulations were made in LS-DYNA using a meshfree solver (SPH) and the results shows that the RCC creates a bigger initial elastic wave, which will make the concrete block crack more, but it will also make the projectile lose more kinetic energy resulting in lower penetration depth in the concrete. The spherical projectiles have higher penetration depth, but it gives smaller elastic waves resulting in less cracking of the concrete.

The simulations overestimate the penetration depth for the non-flat projectiles while giving good agreements for the flat projectiles, also the damage pattern are consistent with the experiments. An actual fragment from a grenade was obtained and simulated showing that all of the projectile except the RCC shows good agreements and therefore the RCC should not be used in simulating fragments.
Source: KTH
Author: Sedlak, Michal

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