PSI - Issue 28

James Allum et al. / Procedia Structural Integrity 28 (2020) 591–601 J.Allum et al. / Structural Integrity Procedia 00 (2019) 000–000

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3.5. Stress-strain behaviour The stress-strain plots of typical specimens of each type are given in Fig. 8. As highlighted previously, all specimens had similar UTS, with a significantly greater strain-at-fracture level of the F NG specimen, compared to both the F G and Z specimens. The stress-strain plot indicates some notable variation in the fracture characteristics of these types. While all demonstrated a peak UTS close to their yield point, the F NG specimens had greater plasticity. Specimens visibly stretched and narrowed under tension. This characteristic (not seen in the F G and Z specimens) was due to the ability of the extruded filaments, aligned in the direction of loading and unhindered by grooves, to deform plastically at increased extension. All Z specimens failed at an interfacial region with sudden and immediate fracture. F G specimens shared this characteristic, all fracturing transversally at a manually grooved region. This similarity indicates that the grooved filament-scale features common for the F G and Z specimens are regions of stress concentration, with these features responsible for sudden fracture at lower strain values.

Fig. 8. Stress-strain curves of one representative specimen for each specimen type.

3.6. Fractography Comparison of fracture surfaces demonstrated their similarity for all specimen types (Fig. 9 (a) to (c)). This fractographic evidence showed that the interfacial bond (Fig. 9 (c)) did not possess any distinguishing material properties different from those of the bulk extruded filament (Fig. 9 (a) and (b)). The increased plasticity of the F NG specimens was evidenced by the presence of shear lips (consistent with observations made in a previous study by the authors [20], which used a pre-cracked fracture-toughness specimen as opposed to tensile-testing specimen) in Fig. 9 (a) (labelled). Shear lips are not evident on the F G specimens (Fig. 9 (b)). The same is true for the Z specimens (Fig. 9 (c)), in which the extruded filament orientation prohibited the formation of shear lips. It is hypothesised that the formation of shear lips in F NG increased the plasticity by enabling extended deformation in the affected regions, prolonging the overall displacement to fracture of the specimens. 3.7. Summary The reduction in mechanical performance of the F G specimens, from that characteristic of the F NG, to that of the Z, by the introduction of manual grooves, demonstrates that the filament-scale geometry was the predominant cause of mechanical anisotropy. Anisotropy was not caused by a weak interfacial bond; in fact, the similarity in strength of F NG , F G and Z indicated that the material in the interface between layers (filaments) in Z specimens was equivalent to the bulk extruded filament. The mechanical characteristics shared by the F G and Z specimens demonstrated that their mechanical performance was reduced as a result of grooves (natural in Z and manually introduced in F G ), which reduced their load-bearing area. The diminished mechanical performance observed was a result of geometrical