PSI - Issue 37

M.P. Silva et al. / Procedia Structural Integrity 37 (2022) 841–846 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

843

3

tests were carried out on square section samples with dimensions of 75 x 75 mm. The impact strength was measured using impact energies of 4, 8, 10, 12, 16, 20, 24, and 28 J. This range of energies was selected based in previous studies (P. N. B. Reis et al., 2012) and three specimens were used for each condition, the results presented in terms of average values. After impact tests, the specimens were submitted to fatigue tests to assess the residual fatigue life after exposure to different hostile solutions. The specimens were tested with a span of 60 mm, using an Instron servo-hydraulic testing machine model 1341, and the tests were carried out at room temperature under constant amplitude sinusoidal waveform loading, a stress ratio of R = 0.05, and a frequency of 2 Hz. 3. Results and discussion Low-velocity impact tests were used to assess the impact strength of Kevlar/epoxy laminates at various impact energy. Figure 1 illustrates representative load-time and energy-time curves derived from impact tests conducted at 4 J. These figures indicate the usual profile of all tests, which is quite close to that documented in the literature for comparable laminates (Mortas et al., 2014; P. N. B. Reis et al., 2012). The load-time curves show that the load grows until it reaches a maximum value, P max , after which it drops significantly. These curves contain oscillations that result from the elastic wave and are created by the vibrations of the samples (P. N. B. Reis et al., 2012; Schoeppner & Abrate, 2000). The beginning of the plateau on the energy-time curves corresponds to the loss of contact between the impactor and the specimen, indicating that the impact energy is insufficient to fully enter the specimens. In this case, the impactor collided with the sample and always bounced back. The restored energy is obtained by the difference between the impact energy and the energy given by the plateau (P. N. B. Reis et al., 2013).

a)

b)

Figure 1 - Typical load (a) and energy (b) versus time curves for an impact energy of 4 J.

Finally, the response of laminates to different energy levels is shown in table 1 in terms of maximum impact load, maximum displacement, and restored energy. The average values are indicated and the respective standard deviation for each condition tested as well. In terms of maximum impact load, it is possible to observe that, up to 12 J, this parameter increases with increasing impact energy, but for higher impact energies this increase is more modest. Basically, the increase follows a polynomial law of degree 2, where the maximum load increases around 72 % between the energies of 4 J and 12 J and only around 35 % between 12 J and 28 J. This is in line with the open literature (P. Reis et al., 2013; P. N. B. Reis et al., 2012), in which it is reported that the maximum impact load increases with increasing the impact energy. The maximum impact load should increase almost linearly with increasing impact energy, according to Hosur et al. (Hosur et al., 2007), but because this parameter reflects the maximum value that a composite can tolerate before the most severe damage occurs, this linearity cannot always be observed, as reported, for example, in the study developed by Reis et al (P. Reis et al., 2013). As a result of Gustin et alstudy .'s (Gustin et al., 2005), it is reasonable to conclude that the observed disparities in maximum loads are a result of the varied failure modes included in the laminate.