PSI - Issue 50

Alexander Eremin et al. / Procedia Structural Integrity 50 (2023) 65–72 Alexander Eremin / Structural Integrity Procedia 00 (2019) 000 – 000

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Aramid fibers are sufficiently inelastic. It has been demonstrated in tensile tests of composite, so a sort of nonlinearity is observed in the graphs. Inelasticity is more apparent for quasi-isotropic composite comparing to orthotr opic. The source of nonlinear deformation behavior is not only due to individual materials’ properties, but also in the layup where the fabric oriented at an angle of 45° to the loading direction undergoes a combined mode of deformation – tension and rotation of the fabric. The curves for CFRP are mainly linear, so the effect of +/-45 layers is minor compared to inherent fiber properties.

Table 2. Mechanical properties of aramid and carbon FRPs with orthotropic and quasi-isotropic layups

Ultimate tensile stress, MPa

Young modulus, GPa

Failure strain, %

Composite type and layup

Poisson’s ratio

Orthotropic layup [0 F ] 7S of aramid fabric Quasi-isotropic layup [45 F /0 F ] 3S of aramid fabric Orthotropic layup [0/90] 5S of carbon fibers Quasi-isotropic layup [45/0/-45/90] 2S of carbon fibers

1.62±0. 08 1.74 ± 0.02 1.47 ± 0.08 1.50 ± 0.05

43.84 ±0.36 29.25 ±0.36 68.21 ± 6.86 46 .45± 1.68

620±12 381 ± 10 1098 ± 81 673 ± 29

0.049±0.0029 0.037 ± 0.018 0.061 ± 0.017 0.252 ± 0.036

Mechanical properties of aramid and carbon based composites are obviously different. The ultimate tensile strength for quasi-isotropic layup compared to orthotropic one was reduced by 49% in both cases, however UTS for CFRP is almost twice higher. The stra ins at failure are not so much different as it was observed for ±45 layups, because the multilayered structure with different orientations suppresses reorientation of fibers and polymer matrix due to compliance. The initial format of the reinforcement also plays a minor role in this case, whenever it is fibers or woven fabric. But it matters for measuring of Poisson’s ratio. Interlocked fiber bundles in aramid fibers do not allow deformation of fibers aligned in transverse direction, so Poisson’s ratio is v ery low – 0.037 for QI AFRC in contrast to 0.252 for very stiff QI CFRP. OT AFRP QI AFRP ε=0.5 %

ε=1.2 %

ε yy

Before failure

OT CFRP

QI CFRP

ε=0.5 %

ε=1.2 %

Before failure

Fig. 5 . Fields of longitudinal (ε yy ) strains for aramid fabric and carbon fiber reinforced polymer composites with orthotropic (OT) and quasi isotropic (QI) layups.

Evolution of strain fields (Fig. 5) is different depending on the material and slightly on the layup of the composite. Orthotropic and quasi-isotropic AFRPs demonstrate early and highly pronounced localization of deformation. The appearance of the AFRP fields is spotted and the spots emerge earlier on the surface of OT AFRP than QI AFRP. The location of spots is the same during loading, so it could not be a peak-locking effect, where spots “move”. The reason for such strain fields lies in the more pliable fibers of aramid, which are easily deformed to the axis of the applied load. At the same time, they experience in-plane shear, rotation and contraction of the