PSI - Issue 42

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

Md Niamul Islam et al. / Procedia Structural Integrity 42 (2022) 785–792

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Fig. 6. Stress-strain curves for nylon CCF (solid) and nylon SCF (dashed; Islam et al., 2021) under tension (black) and compression (red).

Brittle failure was observed in the tensile specimens by inspecting their fracture surface (Fig. 5a) and stress-strain performance in Fig. 6 (black solid line). The tensile modulus of the composite was 13.5 GPa and its tensile strength was 145 MPa, significantly higher than that of the nylon SCF (Islam et al., 2021) – 1.97 GPa and 53.8 MPa, respectively (Fig. 6, black dashed line). This was due to the high strength and stiffness of CCF reinforcement in the loading direction of the composite, which also reduced the elongation at break for the composite, causing its failure at 1.5% compared to 12% of nylon SCF. Other studies on AM CCF nylon-matrix composite showed similar elongation at break (1-2%); however, the tensile strength (900 – 1400 MPa) and stiffness (60 – 100 GPa) were significantly higher thanks to the high volume fraction of fibres (25 – 50%) in the composite structure (Blok et al., 2018; Peng et al., 2019). On the other hand, plastic deformation was observed in the compression test for the nylon CCF samples (Fig. 5b), showing a ductile failure response (Fig. 6, red solid line). The nylon CCF compressed samples flattened in the printing direction, with a buckled side surface (Fig. 5b) indicating a structural response rather than a material one. The compressive stress-strain performance of both composites revealed that nylon CCF had a higher compressive modulus (2.60 GPa) before yield compared to nylon SCF (0.97 GPa) due to the high compressive stiffness of continuous carbon fibre in the loading direction. On the contrary, the compressive strength of nylon CCF was smaller (80.6 MPa) than that of nylon SCF (229.8 MPa), as the long carbon fibres fractured by buckling with increasing strain, reducing the load-bearing capacity of the structure. 3.2. Ballistic impact results The front and back of the nylon CCF plates after the ballistic test are shown in Fig. 7 with damaged areas marked in red. Complete perforation was achieved for the solid plate, with a circular hole at the front of the plate indicating a brittle failure mode, and with delamination failure at the back of the plate, an observation similar to the case of the solid nylon SCF plate (Islam et al., 2021). The damaged areas were measured and summarised in Table 4 for both nylon SCF and nylon CCF plates, with more localised damage observed for the nylon CCF composite plate both at the front and the back of the impact zone. The damaged area of nylon CCF plates at the front was 36% smaller due to the higher stiffness of the structure and the delaminated zone was 54% smaller at the back, as a result of stronger adhesion between the extruded continuous nylon and carbon fibre filaments. Overall, the AM CF-reinforced composite performed better than the AM SF-reinforced composite under dynamic impact loads.