Experimental procedure

In this study, the high strain rate test was performed using steel SHPB apparatus for dry specimens and aluminum SHPB apparatus for wet specimens. The intermediate and low strain rate tests were carried out by using MTS 810 system with displacement control.

3.2.1 Dry Specimen Testing

The both dry neat nylon 6 and dry nylon 6/clay nanocomposites were immediately performed under low, intermediate, and high strain rate tests while they left the vacuum oven. The stress and strain relations of dry nylon 6 and dry nylon 6/clay nanocomposites

100Psi was used to push the steel striker bar, and the compression wave was generated in the steel incident bar with 3mm thickness copper pulse shaper attached on the impact surface. The compression wave signals were obtain by a pair of diametrically opposite gages mounted on the middle of the incident bar and the transmission bar. The amplification factors of incident bar channel and transmission bar channel were both set at 1500. The excitation voltages of the Wheatstone bridge circuits were set at 5V.

However, the amplification factor of specimen gage signal was set at 25 under both dry nylon 6 and dry nylon 6/clay specimen test, and the excitation voltages was set at 3V. The sampling rate of oscilloscope was set at 10MHz to record the voltage signals from Wheatstone bridge circuits. The original test data of these materials were recorded by the same way of aluminum specimen test using a digital oscilloscope as shown in Fig. 3.5(a) and (b). Following the same procedure, the incident wave, reflected wave, transmitted wave and specimen gage signals were shifted to same origin of time which the incident pulse reached the incident bar/specimen interface as shown in Fig. 3.6(a) and (b). The histories of P1 and P2 during the SHPB tests were shown in Fig. 3.7(a) and (b). The equilibrium of P1 and P2 indicated that the nylon 6 and nylon 6/clay nanocomposites specimens were homogeneous deformation. In addition, these results also revealed that P1

exhibit greater oscillation than P2. Therefore, P2 was used to extract the stress in the present high strain rate tests.

The specimen strain corresponding to the Hopkinson formula was obtained from the displacements u1(t)on the incident bar/specimen interface and u2(t)on the specimen/the transmission bar interface. However, the strain signal also recorded the histories of specimen deformation by the strain gages mounted on the specimen. Fig. 3.8(a) and (b) show the comparison of the strain histories for dry nylon 6 and dry nylon 6/clay nanocomposites which were obtained by using the Hopkinson bar formula and the strain

the Hopkinson bar theory deviates from that directly measured on the specimen. So the gage result was chosen to construct the more accurate stress and strain relations. The stress-strain curves of dry nylon 6 and dry nylon 6/clay nanocomposites were extracted as shown in Fig. 3.9(a) and (b). In this study, the strain rate about 800/s was measured directly from the specimen.

The mechanical behaviors of dry nylon 6 and dry nylon 6/clay nanocomposites under intermediated and low strain rates were performed using MTS 810 system with displacement control at a stroke rate of 1mm/sec and 0.001mm/s, respectively. A self-adjusting device as shown in Fig. 2.7 was used to eliminate potential bending moments and ensure the specimen to be in full contact with the loading surfaces. During these tests, the stress was obtained from the load cell and the corresponding strain was measured from strain gages mounted on the specimens. The stress and strain histories for each test were recorded using LabVIEW, and the sampling rate of low and intermediate test were set at 2Hz and 200Hz, respectively. The strain could be obtained either from the strain gage directly mounted on the specimen (true strain) or the MTS stroke displacements divided by specimen original length (nominal strain). Fig. 3.10(a) and (b) shows the nominal strain curve and the true strain curve for dry nylon 6 and dry nylon6/clay specimen, respectively, tested at the nominal strain rate of 0.1/s. It is evident that the true strain is quite different from the nominal strain and thus the true strain rate is also different from the nominal strain rate. This discrepancy could be ascribed to the application of the self-adjust fixture in the compression test. Therefore, in this study, the true strain curves were employed for the generation of the stress and strain curves and for the evaluation of strain rate as well. For the experiment conducted at nominal strain rate of 0.0001/s, the measured average true strain rate was 8×10^-5/s. Furthermore, the average true strain rate was 8×10^-2/s corresponding to the experiment at nominal strain rate of 0.1/s.

3.2.2 Wet Specimen Testing

Because the mechanical impedances of wet nylon 6 and wet nylon6/clay nanocomposites are low, the aluminum SHPB apparatus which made by aluminum alloy (6061-T6) was employed in the test to enhance the intensity of the strain signals of the bars.

Moreover, pulse shaper technique was utilized to facilitate the homogeneous deformation of the specimens. It results that the reliable stress and strain curves in small strain ranges can be obtained. In this study, the 5 mm thickness of nylon 6 platelet was selected as a pulse shaper for aluminum SHPB tests. The gas pressure of 30Psi was used to initiate the aluminum striker bar, and the same strain gages of steel SHPB apparatus were also mounted on the middle of the aluminum incident bar and transmission bar. The amplification factors of incident bar channel and transmission bar channel were both set at 1000, and the excitation voltage of the Wheatstone bridge circuits were set at 5V. The amplification factor of specimen gage signal was set at 100 and 50 under wet nylon 6 and wet nylon 6/clay specimen test, respectively. The excitation voltages were set at 3V.

The sampling rate of oscilloscope was also set at 10MHz. Therefore, the stress-strain curves of wet nylon 6 and wet nylon 6/clay nanocomposites were obtained from tests via aluminum SHPB apparatus.

In order to verify the accuracy of the aluminum SHPB apparatus, the dry nylon 6 and nylon 6/clay nanocomposites were examined using this setup. Fig. 3.11(a) and (b) shows the comparisons of stress and strain curves from the steel SHPB and aluminum SHPB apparatus. It was quite obvious that the testing results from aluminum SHPB apparatus were the same as those from steel SHPB apparatus. Then it indicated that the reliable test results could be obtained by using aluminum SHPB apparatus. Therefore, the aluminum SHPB apparatus could be used to investigate the dynamic mechanical properties of wet nylon 6 specimens. These associated results were presented in Appendix A.

by the same procedure of dry specimens. All wet nylon 6 tests were carried out by aluminum SHPB and MTS machine while the moisture absorption of specimens was almost saturate.

3.3 Results and discussion