The combined effect of blast and impact loading against protective structures is still not fully understood. In this paper, the motivation is to investigate the combined effect of pressure and fragment loading on steel plates. To simplify the approach, it is assumed that the fragments strike and perforate the flexible target before the pressure load arrives. These perforations are idealized as pre-formed holes of generalized shapes in thin, ductile Docol 600DL steel plates, which are then subjected to controlled pressure pulses in the laboratory. The tests were carried out in a Pulse Pressure Loading Rig, and two different pressure levels were applied for each plate design. After the tests, the deflections caused by the pressure pulses were measured. Even though the plates were much deformed in some of the tests, no sign of failure was observed. Based on a number of material tests, the parameters in the modified Johnson-Cook constitutive relation were identified. Several sets of non-linear finite element simulations of the pressure loading process were then conducted. The agreement between standard Lagrangian simulations and the experimental results was in general good, but the simulations failed to give an accurate description of the deflection around the pre-formed holes. Fully coupled FSI simulations of the experimental tests were then carried out and these simulations demonstrate significant spatial variation of the pressure load. The FSI simulations also showed that this spatial variation of the pressure load influence the deflection profile, especially in the vicinity of the holes.

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Previous investigations of the penetration and perforation of high-strength steel plates struck by hardened steel projectiles have shown that under certain test conditions the projectile may fracture or even fragment upon impact. Simulations without an accurate failure description for the projectile material will then predict perforation of the target instead of fragmentation of the projectile, and thus underestimate the ballistic limit velocity of the target plate. This paper presents an experimental investigation of the various deformation and fracture modes that may occur in steel projectiles during impact. This is studied by conducting Taylor bar impact tests using 20 mm diameter, 80 mm long, tool steel projectiles with three different hardness values (HRC 19, 40 and 52). A gas gun was used to fire the projectiles into a rigid wall at impact velocities ranging from 100 -350 m/s, and the deformation and fracture processes were captured by a high-speed video camera. From the tests, several different deformation and fracture modes were registered for each hardness value. To investigate the influence of material on the deformation and fracture modes, several series of tensile tests on smooth axisymmetric specimens were carried out to characterise the mechanical properties of the three materials. To gain a deeper understanding of the various processes causing fracture and fragmentation during impact, a metallurgical investigation was conducted.The fracture surfaces of the failed projectiles of different hardness were investigated, and the microstructure was studied for each hardness value.