## 3 Results and Discussion
### Observation of Low-Velocity Impact Damage Pattern  
As shown in Figures 3 and 4, the impact indent geometry was dependent on the impactor head shape, laminate direction and impact energy level. The impact damage mode generated below the impact indent under the impact may be another significant consideration for the GFRP composite design. In general, major impact damage modes have been reported to include matrix cracking, delamination and fiber breakage [8; 9].  
In this study, the damage pattern for the TRI and UD GFRP laminates impacted with different head-shaped impactors at different energy levels was examined. Figures 5 and 6 represent typical impact damage patterns generated beneath the indents in TRI and UD laminates. The patterns were observed on the sectional planes along the longitudinal and transvers direction of the laminates with FE SEM (Field Emission Scanning Electronic Microscope, JEOL). Figure 5 represents damage patterns created from the pyramidal impactor for the TRI GFRP laminate, where those were incurred at the impact energy level of 8 J. Here, a 3D sectional view, assembling the damage aspects on the front, longitudinal and transverse plane, and three magnified views A, B and C, representing the damage patterns produced at the local areas A, B and C on these three directional planes, respectively, are displayed. On the impacted front plane, as can be seen in the magnified view A, fiber breakage and matrix splitting were found. These modes may be induced by stress concentration at the contact of the impactor edges. On the longitudinal and transverse planes, various damage modes, including matrix cracking, fiber breakage, debonding and delamination, were generated, as shown in the magnified views B and C in Figure 5. In this study, the damage patterns from the pyramidal impactor in UD GFRP laminate also were observed, and it was found that the dominant patterns were considerably identical to the damage patterns observed in TRI laminate. Typical damage patterns from the hemispherical impactor are presented in Figure 6. In Figure 6, the damage patterns created in the UD GFRP laminate at impact energy of 8 J are displayed. Here, like Figure 5, a 3D sectional view assembling the damage aspects along the three directions and three magnified views A, B and C on these three directional planes, respectively, are represented. Contrary to the damage from the pyramidal impactor, the damage from the hemispherical impactor was not created just beneath the impact indent, as can be found in Figure 6. Matrix cracking and debonding between matrix and fiber were found below the indent, as shown in the magnified views B and C of Figure 6.  
Figure 5: Typical impact damage patterns created by a pyramidal impactor for TRI laminate (at impact energy level of 8 J).  
Based on the SEM observation presented in Figures 5 and 6, dominant impact damage patterns, which can be created in the GFRP laminates, can be schematically represented as shown in Figure 7. The dominant impact damage modes are dependent on the impactor head shape and laminate structure, but it can be said that the damage patterns, such as matrix cracking, delamination, debonding and fiber breakage, predominantly occurred in GFRP laminates.