PSI - Issue 37

Md Niamul Islam et al. / Procedia Structural Integrity 37 (2022) 217–224 Md Niamul Islam et al. / Structural Integrity Procedia 00 (2019) 000 – 000

223

7

revealed fracture lines parallel and perpendicular to the direction of the extruded fibres, with a significantly wider delamination zone (Fig. 8b, red circle). Therefore, brittle fracture and delamination were identified as the main failure modes of this polymer composite under ballistic impact. Previous impact test studies on AM nylon and reinforced nylon were performed in Caminero et al. (2018), but mostly in the form of Izod or Charpy impact test. So, as a comparison of ballistic impact on AM plates, Desu et al. (2020) implemented a drop-weight impact test for unreinforced solid unidirectional PLA plates of 2.5 mm thickness. A 16 mm hemispherical tip was used as an impactor of 4.4 kg, and a similar plate-fixture setup was used. For high energy impact of 5 J, a complete perforation and localised damage of the plate was observed similar to those in the nylon SCF plates. However, no delamination was witnessed as PLA exhibited a more brittle behaviour than nylon SCF. 3.2. Numerical modelling Numerical results of the impact of the laminated target with a spherical projectile at a velocity of 100 m s -1 are presented in Fig. 9. Similar to the experiment, a circular perforation at the front of the plate (Fig. 9a) was achieved, indicating that Hashin damage model predicted adequately the damage initiation. However, the side view showed a ductile damage evolution (Fig. 9b); therefore, brittle damage evolution needs to be implemented in the model. Also, no delamination or fracture line was observed at the back of the plate, since delamination was not accounted for in FE simulations as a failure mode. In the future work, cohesive surface elements will be used for this purpose.

Fig. 9. Numerical modelling results for ballistic impact using FEA at 100 m s -1 : (a) front side; (b) back side; (c) side view.

4. Conclusion AM-produced plates made of nylon reinforced with short carbon fibre were investigated under ballistic impact in this study. The material was fabricated into a quasi-isotropic structure with fused deposition modelling. Quasi-static tensile and compression tests were carried out to characterise the material. The tensile modulus (1.97 GPa) was almost two times higher than the compressive modulus (0.97 GPa) but the tensile strength (53.8 MPa) was significantly lower than the compressive one (229.8 MPa). The deformation and fracture of the 3D printed material revealed an elastic-brittle behaviour. Dynamic mechanical analysis in 3-point bending was also carried with a frequency sweep. The low tan delta value obtained (0.08) further confirmed that the polymer composite was highly elastic. In parallel, ballistic impact tests were carried out on solid 2.5 mm thick AM plates using a gas gun with spherical steel projectiles. The impact velocity ranged between 95 m s -1 to 120 m s -1 , with the respective impact energy varying between 14 J and 20 J. Complete perforation of the plate was achieved at these velocities. The front of the plate experienced penetration, resulting in an almost circular hole, while the back showed significant delamination along with fracture. The ballistic experimental conditions were modelled with finite elements using the results from quasi-static experiments and employing Hashin damage model via a user subroutine. Simulations at an impact velocity of 100 m s -1 revealed a similar perforation at the front of the plate while brittle damage evolution and delamination at the back of the plate were not reproduced in the current version of the model.