## 3 Results and Discussion
### Stress Concentration Around the Impact Indent

Table 3: Residual tensile strength reduction ratio for the impacted UD and TRI GFRP laminates. From the full-field stress distribution obtained from DIC measurement, as shown in Figures 9 and 10, stress distribution ahead of the indentation can be quantitatively compared considering impact conditions, such as impact energy and impactor head shape. In Figure 11, the normalized longitudinal stresses, which are the ratios of the longitudinal stresses to the nominal stress applied to the laminate specimen, ahead of the indent were presented. For the impacted TRI GFRP laminate, as can be found in Figure 11a, the maximum ratios ahead of the pyramidal indent were considerably higher than those ahead of the hemispherical indent, and the ratios were dependent on the impact energy levels. Here, those at the pyramidal indent boundary were 3.3, 2.6 and 1.6 at the impact energy levels of 8 J, 6 J, and 4 J, respectively, while the ratio at the hemispherical indent boundary was in the range of 1.3 to 1.1 Thus, it is found that the longitudinal stress concentration became higher as the impact energy level increased, and the pyramidal impactor had a greater influence on the stress concentration than the hemispherical impactor. In contrast, for the UD GFRP laminate, the maximum ratio at the impact energy of 8 J approached to 1.2, irrespective of the indent shapes, and the ratios for both indent shapes were less than 1.1 at the impact energy level of 6 and 4 J. Therefore, it can be said that, for the impacted UD GFRP laminate, the stress concentration at the impact energy level greater than 6 J was slightly high, but the concentration at all energy levels was relatively insignificant. Dependency of the failure tensile strength of the impacted laminate on the indent shape and impact energy level is shown in Figure 8. For the TRI GFRP laminate impacted with the pyramidal impactor, the failure tensile strength was considerably reduced with increasing impact energy level, which corresponded considerably well to a tendency for the stress concentration around the indent to be higher with increasing impact energy. Moreover, difference in failure tensile strength reductions between the hemispherical and pyramidal indents also corresponded quite well to relative difference in stress concentrations between these two indents. Here, the strengths for the pyramidal and hemispherical indents were reduced by 13-18% and 3-13%, respectively, as can be found from Figure 8. Therefore, it is found that the reduction in the residual strength of the TRI GFRP laminate with increase of impact energy level could be associated with the high stress concentration depending on the impact energy. In comparison, for the UD GFRP laminate, both hemispherical and pyramidal indentations caused an insignificant stress concentration around the indent boundary, less than 1.1, as presented in Figure 11b, and also did not lead to a significant failure strength reduction, which was less than 4%, as can be  
Figure 9: Typical directional stress, longitudinal (\(\sigma_{\text{yy}}\)), transverse (\(\sigma_{\text{xx}}\)) and shear (\(\sigma_{\text{xy}}\)) stresses, distribution in the vicinity of the indents for TRI GFRP laminates (at tensile stress of 600 MPa). found in Figure 8. Therefore, it is found that the impact indents in the UD GFRP laminate generated insignificant stress concentration around the indent, irrespective of indent shape, and also did not lead to significant strength reduction associated with the stress concentration. This may be caused by a relatively low damage generation and progress initiated from the indent in UD laminate, which can be found in Figure 6.  
Figure 11: _Cont._  
Figure 10: Typical directional stress, longitudinal (\(\sigma_{\text{yy}}\)), transverse (\(\sigma_{\text{xx}}\)) and shear (\(\sigma_{\text{xy}}\)) stresses, distribution in the vicinity of the indents for UD GFRP laminates (at tensile stress of 700 MPa).