Results and discussions

The stress and strain curves of dry neat nylon 6 and dry nylon 6/clay nanocomposites at strain rate ranges from 8×10^-5/s to 800/s were shown in Fig. 3.12(a) and (b). The constitutive relations exhibit an apparently linear elastic range followed by a nearly perfect plastic behavior. It was shown that, for dry neat nylon 6 and dry nylon 6/clay nanocomposites, the linear elastic ranges increased when the strain rate increases.

However, the slopes of the linear portions were almost the same within the tested strain rate range, which indicated that the Young’s moduli were not sensitive to strain rate.

Figs. 3.13-3.16 show that the comparison of stress and strain relations of dry neat nylon 6 and dry nylon 6/clay nanocomposites at different strain rate tests, and these results indicated that the Young’s modulus and yielding stress of nylon 6/clay were higher than those of neat nylon 6. The Young’s modulus for low and intermediate strain tests were determined based on the experimental data with strain range up to 0.5% using a linear function. However, the stress-strain curves of steel SHPB tests under small strain range were more fluctuant than those of aluminum SHPB tests. The Young’s modulus of dry nylon 6 and dry nylon 6/clay under high strain rate tests were evaluated from the stress-strain curves of aluminum SHPB tests in this study. The all values of the Young’s modulus under different strain rates were summarized in Table 3.1. It reveals that the supplement of 5 wt% organoclay in the dry nylon 6 nanocomposites can improve the stiffness up to 32% with the tested strain ranges.

3.3.2 Wet Specimen Results

Fig. 3.17(a) and (b) shows the stress and strain curves of wet neat nylon 6 and wet nylon 6/clay nanocomposites at strain rate ranges from 8×10^-5/s to 500/s. It was shown that these curves are almost nonlinear except that measured at strain rate of 500/s.

Theoretically, the Young’s modulus should be determined from the slope of the stress and strain curves at the initial portion. However, due to the nonlinearity, it becomes a challenging task to decide the suitable initial strain range for the evaluation of the Young’s modulus. In this study, the experimental data with the strain range of 0.05%, 0.1%, 0.2%, and 0.3%, respectively were selected and the corresponding Young’s modulus were evaluated by linear curve-fitting as shown, respectively, in Figs 3.18-3.21. It was indicated that for the strain range within 0.05%, the experimental data is lacking and scattering, which prevents the correct interpretation of the Young’s modulus. The similar result was observed in the case within strain range of 0.1%. By comparing the results with strain range of 0.2% and 0.3%, it was observed that the Young’s modulus is decreasing with the increase of the strain range implying that, in the 0.3% strain level, the nonlinearity is somehow present. In view of the forgoing, the experimental data with strain range of 0.2% was adopted for the determination of the Young’s modulus of nanocomposites and nylon 6 specimens with true strain rate up to 0.08/s. More results regarding to the determination of Young’s modulus of nylon6 and nylon6/clay nanocomposites in terms of different strain ranges at true strain rate of 8×10^-5/s were presented in Appendix B and summarized in Table 3.2. For true strain rate of 0.08/s, based on the experimental data with 0.2% strain level, the Young’s modulus of wet nylon6 and wet nylon 6/clay nanocomposites were calculated and illustrated, respectively in Fig. 3.22(a) and (b).

However, for high strain rate, the initial portion (strain less than 0.5%) of the stress and strain curves is quite oscillating and unsuitable for the determination of the Young’s

stress-strain curves demonstrate apparently larger linear range than those obtained in the low strain rate. Therefore, we resort the stress and strain curves with strain level up to 0.5% for evaluating the Young’s modulus. The associated results for nylon6 and nylon6/clay nanocomposites were shown, respectively in Fig. 3.23(a) and (b). By following the same procedure, the Young’s modulus of wet nylon 6 and nylon 6/clay nanocomposites at different strain rates were calculated and the average results were presented in Table 3.3. It was depicted that for the wet nylon6 and wet nylon 6/clay nanocomposites, the Young’s modulus increases with the increment of the strain rate. In addition, for each strain rate, the nylon6/clay nanocomposites exhibit higher stiffness than the nylon 6. The enhancement can be achieved up to 43% at the strain rate of 8×10^-5/s.

Based on out current results, it is interesting to mention that the enhancement seems not to be affected significantly by the strain rate. The complete stress and strain relations of wet nylon 6 and wet nylon 6/clay nanocomposites at three different strain rates were illustrated in Figs. 3.24-3.26. Thus, the inclusion of the organoclay can effectively improve the stiffness of the wet nylon6 in both linear and nonlinear ranges.

3.3.3 Moisture effects

In order to investigate the moisture effect on nylon6/clay nanocomposites, the stress and strain relations shown in Figs. 3.12(b) and 3.17(b) were re-plotted in Figs. 3.27-3.29 in terms of dry and wet samples. It was revealed that for each strain rate, the dry sample always demonstrate superior mechanical response such as stiffness and yielding stress than the wet one. In addition, based on the results summarized in Table 3.1 and Table 3.3, it indicated that, at low strain rate, the Young’s modulus of wet nylon6/clay nanocomposites is only 1/4 of that in dry case. Thus, it should be of concern that with the presence of moisture, the mechanical properties of nylon6/clay nanocomposites would be distorted