## 3 Results and Discussion
### Reduction of Residual Static Strength  
As shown in Figure 4, Figure 5, Figure 6, the low-velocity impact created various forms of internal damage patterns, including matrix crack, fiber breakage, delamination, and debonding, and indents on the impacted surface of the GFRP laminates. These indents and damage can cause significant strength reduction, which also can lead to unexpected failure of the laminates [13].  
Figure 6: Typical impact damage patterns created by a hemispherical impactor for UD laminate (at impact energy level of 8 J).  
Figure 7: Schematic representation of the dominant impact damage patterns created in TRI and UD GFRP laminates.  
In this study, the dependency of the residual strength of the impacted GFRP laminates on the laminate structure, impact energy, and indent shape was examined, which is represented in Figure 8. Here, the residual in-plane tensile strength of the impacted TRI and UD laminates was determined as a failure strength obtained from in-plane tensile test of the impacted laminates, and obtained from duplicate tests. In Figure 8, the strengths were normalized with the tensile strength of the respective laminates, which was displayed as the residual strength ratio, \(\sigma_{\text{f}}\)/\(\sigma_{\text{TS}}\). Table 3 lists the numeric values of the ratio. As can be found from Figure 8, the residual strength for the GFRP laminates was reduced with increasing impact energy. For the TRI GFRP laminates impacted with hemispherical and pyramidal impactors, the ratios were linearly decreased with increasing impact energy, irrespective of impactor head shape, where the decreasing rates were 0.0236/J and 0.0126/J, respectively. The reduction of the ratios, ranging from 0.87 to 0.82, for the pyramidal indentation was considerably lower than that, ranging from 0.97 to 0.82, for the hemispherical indent. Therefore, it can be said that the residual strength for the TRI GFRP laminate may be considerably more sensitive to the pyramidal impact indent, compared to the hemispherical impact indent. This strength reduction with increasing impact energy is likely to be closely related to the indentation dimension increment with increasing impact energy, which is shown in Figure 4. The indentation depth and width on the impacted surface were created larger with increasing impact energy, as shown in Figure 4. It had been reported that the impact residual strength decreased as the impact energy and impact indent dimension for CFRP laminates increased [13].  
For the UD GFRP laminate impacted with the hemispherical impactor, the residual strength was reduced by 3-4%, and was not considerably changed with increasing impact energy. Contrary to the hemispherical indent, the residual strength of the laminate impacted with pyramidal impactor was relatively greatly reduced. Especially, the strength reduction ratio for the UD laminate impacted at less than 6 J was nearly similar to that for the hemispherical indent, but, at impact energy of 8 J, the ratio dropped down to 86 %. This change of the strength ratio may also be closely related to the dramatic change in pyramidal indentation dimension at greater than impact energy of 6 J. As can be found in Figure 4, the pyramidal indentation dimension was dramatically increased over the impact energy range of more than 6 J.  
Figure 8: Dependency of residual strength of the impacted GFRP laminates on the impact energy and impact indent shape.  
Therefore, it is found that the residual strength of the TRI GFRP laminate is much more sensitive to the impact damage, compared to the UD laminate, and the pyramidal impact indent has greater influence on the strength than the hemispherical impact indent.