PSI - Issue 28

M.P. Silva et al. / Procedia Structural Integrity 28 (2020) 2235–2244 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3. Results and discussion Impact tests were carried out considering as variables, not only the material of the sample, but also the impact energy. Figure 3 shows the load–time, energy–time and load-displacement curves for flax laminates under different impact energy. These diagrams represent a typical behaviour occurred for all laminates and agree with the literature (Aslan, Karakuzu, and Okutan 2003; Reis, Ferreira, and Rodrigues 2011). Figure 4 shows the load-time, energy-time and load-displacement curves for all laminates. It is possible to observe in figure 3 and 4, curve oscillations, that according to Abrate (Schoeppner and Abrate 2000), that are related to the sample vibrations that generate an elastic wave, they depends on the stiffness, mass of the specimen and the impactor excitation during the collision moment (Belingardi and Vadori 2002). In detail, in figure 3a and 4a, is possible to observe that the load increases up to a maximum value, Pmax, followed by a drop after the peak load. The maximum load is dependent of the impact energy and represents the peak load value that the composite laminate can tolerate before undergoing major damage. It is also possible to observe in figure 4a that the introduction of 4 and 6 layers of glass fibre (hybridization) also contribute to increase the peak load. In figure 2a it is possible to observe that the maximum load increased with increasing impact energy, this tendency was also verified by other authors (Iqbal et al. 2009). Increase, between 7 and 13 J, were observed around 19.3% for flax laminates. The introduction of glass fibres to the laminate also increases the maximum load, as reported in figure 3a, for an energy of 11 J it is observed an increase of 75.2 % and 178 % form flax laminate to 1x hybrid and 2x hybrid, respectively. Regarding to impact time, same conclusion can be done, impact energy and hybridization prove to influence it, the results shows that there is a large difference between them, the results are summarized in table 2 Figure 2b represents typical energy versus time curves. It is possible to observe that the increase in impact energy promotes a decrease of the elastic recovery and, consequently, the damage increase. Table 2 resumes the elastic recuperation for each laminate. The elastic energy was calculated as the difference between the absorbed energy and the energy at peak load from the graphics presented in Fig. 3b. The beginning of the plateau of the curve coincides with the loss of contact between the impactor and the specimen, so, this energy coincides with that absorbed by the specimen (Gómez-del Rı́o et al. 2005). The elastic energy increases with the introduction of glass fibre layers, (table2) which means that hybrid laminate has more resistance to low velocity impacts than simple flax laminate. Figure 2c and 3c presents the effect of the impact energy and hybridization as well, in the laminate displacement during impact loads. The values show that displacement varies with increasing impact energy, figure 2c reveals that for higher energy values the displacement also increase. In the energy range studied the displacements increase around 47.8 %, between 7 and 13 J, for flax laminates. However, when the glass fibre layers are added (figure 3c) to the composite the displacements decrease, and this is more evident for laminates with 6 layers of glass fibre (2x hybrid). This decrease is around 46 % and 58.4 % respectively which is interesting for applications that need to absorb the impact energy but cannot be allowed to deform so extensively for example, for safety reasons.

Table 2 – Contact time, energy, load and displacement maximum values for all composites at impact energy of 11 J. Composite

Load (kN)

Std. dev.

Contact time (ms)

Std. dev.

Elastic Energy (J)

Std. dev.

Displacement (mm)

Std. dev.

Flax

1.818 0.097 3.172 0.131 5.033 0.378

10.067 5.717 4.267

0.539 0.131 0.062

3.34 4.49 4.71

0.200 0.190 0.070

8.993 4.898 3.753

0.233 0.093 0.104

1x hybrid 2x hybrid

Figure 4 shows the energy profile diagram of the laminates considered in this study where is represented the relationship between impact energy and absorbed energy. It is possible to observe that all data are bellow equal curve which means that absorbed energy are lower than impact energy and the penetration threshold was not reached yet. The data showed in figure 4 can be fitted for all different laminates thru the equation 2. According to Aktas et al. (Aktaş et al. 2009) this equation is more reasonable than the linear relationship between impact energy and the energy absorbed suggested by Shim et al. (Shim, Tan, and Tay 1995)