## 3 Results and Discussion
### Stress Concentration Around the Impact Indent  
As can be found from Figure 8, the residual strengths of the impacted GFRP laminates were dependent on the impact conditions such as impact energy and impactor head shape. The impact conditions created various impact damage forms and indent geometries, as shown in Figure 4, which may influence on the failure strength with a highly localized non-uniform stress distribution around the indent. Therefore, information on the stress distribution around the impact intent may be considerably significant to understand a failure response of the impacted laminate. As a result, in this study, local stress distribution around the impact indent on the impacted laminate surface was determined under in-plane tensile loading using DIC technique. [PERSON] et al. (2010) also had measured longitudinal surface strain map on the impacted carbon fiber laminates with DIC technique and locally concentrated strain around the hemispherical indent was measured. Figures 9 and 10 represent typical stress distribution around the impact indent for TRI and UD GFRP laminates, respectively. Here, the stress distribution was determined from the DIC measurement using linear elastic orthotropic relationship. In Figure 9, the directional stress distributions around the pyramidal and hemispherical indent for the TRI GFRP laminate are presented. Here, the full-field stress distributions were obtained at the in-plane tensile stress of 600 MPa which was equivalent to 0.82 \(\sigma_{\text{TS}}\) (tensile strength of the TRI GFRP laminate). As shown in Figure 9, the longitudinal (\(\sigma_{\text{yy}}\)), transverse (\(\sigma_{\text{xx}}\)) and shear (\(\sigma_{\text{xy}}\)) stresses around the pyramidal indent were concentrated in the vicinity of the indent and the concentrated stress became higher with increasing impact energy. The concentration zone envelope of the longitudinal stress, which was developed with an angle of \(\pm 45^{\circ}\), was expanded larger with increasing impact energy. Transverse and shear stresses were concentrated in the vicinity of the indent, and the concentration zones of the respective stresses also were expanded with increasing impact energy. In comparison, for the hemispherical indentation, all directional stresses were not remarkably concentrated, as can be found in Figure 9. Longitudinal, transverse and shear stresses were not relatively highly concentrated in the vicinity of the indent. Figure 10 represents the directional stress distributions around the pyramidal and hemispherical indent for the UD GFRP laminate, which were obtained at the tensile stress of 700 MPa (equivalent to 0.78 \(\sigma_{\text{TS}}\), where \(\sigma_{\text{TS}}\) represents tensile strength of the UD laminate). As shown in Figure 10, the longitudinal (\(\sigma_{\text{yy}}\)) and transverse(\(\sigma_{\text{xx}}\)) stresses around the pyramidal indentation were not relatively remarkably concentrated at the indent edge, unlike those for the TRI laminate, but shear (\(\sigma_{\text{xy}}\)) stress concentration zone was developed, similar to the TRI laminate. Similar to the pyramidal indentation, all directional stress, including the longitudinal, transverse and shear stresses, around the hemispherical indentation were not concentrated around the indent edge. However, lots of line-style concentration zones, normal to the tensile loading direction, of the longitudinal stress, \(\sigma_{\text{yy}}\), for both indents were discontinuously and locally scattered over the testing section, including the indent. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Laminate Type**} & \multicolumn{2}{c}{**TRI**} & \multicolumn{2}{c}{**UD**} \\ \hline \hline
**Impact Energy ()** & **Head Shape** & **Hemispherical** & **Pyramidal** & **Hemispherical** & **Pyramidal** \\ \hline
4 & 0.97 & 0.87 & 0.97 & 1.00 \\
6 & 0.92 & 0.85 & 0.97 & 0.98 \\
8 & 0.87 & 0.82 & 0.96 & 0.86 \\ \hline \hline \end{tabular}
\end{table}