PSI - Issue 28

6

T. Stergiou et al. / Structural Integrity Procedia 00 (2019) 000–000

Theodosios Stergiou et al. / Procedia Structural Integrity 28 (2020) 1258–1266

1263

Fig. 2. Model dimensions (in mm) used in the finite-element analysis.

3.1 Contact definition Under impact, the aluminium substrate experiences an impulse loading, fully characterised through a force-time relationship. This section illustrates the force-time response for coating thicknesses of 1.2 mm and 7 mm, compared to the monolithic-target case. The related data were acquired at a frequency of 44 MHz, to ensure sufficient contact force resolution during the impact event. The force measurements were considered up to the time required for the projectile to cover a distance equal to its diameter, for each coating case. For the purpose of this analysis, the force in the direction normal to the impacting plate was considered ( z -direction). The introduction of polyurea front coating produced alterations to the force-time profile, evident in Fig. 3. In contrast to the monolithic aluminium plate impact, with the initial peak followed by peaks of gradually decreasing magnitude, the introduction of polyurea resulted in a dominant peak, followed by a second peak of considerable magnitude. The time interval between the peaks for the monolithic and laminate cases was also different and sensitive to the polymer layer’s thickness. For the case of the monolithic target, the proximity of the peaks demonstrates that the projectile and the target were in contact multiple times after initial impact, and the degree of target deflection was characterised by the time interval between those peaks. On the other hand, a recoil after impact of the laminate target was more profound, growing with increasing coating thickness, presented by an increase in time interval between the first and second peaks for the 1.2 mm and 7 mm coating thickness cases. Such features are due to the increase in mass on the impact zone, resulting in higher force magnitudes than that observed for the monolithic target. The two discrete peaks in the laminate cases indicate two dominant contact instances. This was confirmed by the analyses, where the second peak corresponded to the instance where the projectile reaches the plate, which experienced an acceleration as a result to the initial impact. In contrast to the initial impact, where a normal contact occurred, the second projectile-plate interaction took place at an angle, as a result to local plastic deformation in the plate and the formation of petals prior to the second interaction. Such a behaviour is different to the impact of the monolithic metallic plate, with multiple interactions between the projectile and target. The target’s resistance to the projectile’s motion was initially with a decaying magnitude, as a result of the global bending response and deformation of the target. The global bending response was followed by a spring-back of the plate, responsible for the increase in the interfacial normal force that is evident after 30 μs. The magnitude of the normal force after spring-back was comparable to that at initial impact.