## 3 Results and Discussion  
### Impact Indent Geometry  
In this study, two different indent shapes were introduced in the respective TRI and UD GFRP laminates using pyramidal and hemispherical head-shaped impactors at three different impact energy levels of 4, 6 and 8 J. Figure 3a,b show typical indent patterns from the pyramidal and hemispherical head impactors, respectively, on the impacted surface for both laminates. Cross-section height profiles of the respective indentations were measured using a surface roughness tester (Surftest, Mitutoyo), as shown in Figure 3c,d. Here, the profile was measured repeatedly three times. Profiles of the respective indentations had the same appearances as the corresponding impactor head shapes, irrespective of the laminate structures, and the typical pyramidal and hemispherical indent profiles are presented in Figure 3c,d. The dimensions of the indents were varied with the impact energy levels. However, for the pyramidal indent, the profile near the indent tip was partly flat, as shown in Figure 3c, which may result from the presence of fiber breaks which would be caused by a locally dynamic impact process. Fiber fracture damage in the composite laminate under impact load was revealed to be mainly affected by high local stress concentration near the impact point [18; 19]. Impact load in the composite laminate can produce high local stress concentration near the impact point and indentation, and the energy can be dissipated through a damage process consisting of fiber fracture beneath the impact point and delamination in the locally deformed area. It was reported that the fiber breaks can increase with increasing velocity, depending on the severity of local deformation and fragmentation [18; 19]. From these profile measurements, dependency of the impact indent geometry on the impactor shape, impact energy level and laminate direction could be investigated. In this study, the maximum depth, \(\rm z_{max}\), and width, \(\rm w_{max}\), of the respective indents were determined, which are plotted in Figure 4 and listed in Table 2. Here, the maximum depth and width are the maximum depth of the indent from the impacted surface and a square side length or diameter of the indent base at the impacted surface of the specimen, respectively. As found in Figure 4, the maximum depth, \(\rm z_{max}\), of the pyramidal indent for the TRI laminate was not dependent on the impact energy, while for the UD laminate, it was not varied at the relatively low impact energy level, but increased slightly with increasing impact energy at the relative high impact energy, 8 J. The depth, \(\rm z_{max}\), for the TRI and UD GFRP laminates, was within a range of between 111 to 117 \(\upmu\)m, and between 164 to 207 \(\upmu\)m, respectively. For the hemispherical indent, the depth, \(\rm z_{max}\), was nearly linearly dependent on the impact energy with a rate of 8 \(\upmu\)m/J within a range of 46-77 \(\upmu\)m and 10 \(\upmu\)m/J within a range of 63-102 \(\upmu\)m for the TRI and UD laminates, respectively. Comparing the depth for both indents, \(\rm z_{max}\) of the pyramidal indent was relatively larger than that of the hemispherical indent for both laminates and all impact energy levels. \(\rm z_{max}\) of the hemispherical indent ranged from 46 to 102 \(\upmu\)m for both laminates, while \(\rm z_{max}\) for the pyramidal indent from 111 to 207 \(\upmu\)m. Therefore, it can be found that the depth of the pyramidal indent is more than twice as deep as the hemispherical indent depth. The width, \(\rm w_{max}\), of both indent shapes for the TRI laminate was within a range of between 4.39 and 5.94 mm and nearly linearly dependent on the impact energy level with a rate of 0.36 mm/J. For the UD laminate, \(\rm w_{max}\) of both indent shapes also increased with increasing impact energy level, but \(\rm w_{max}\) of the pyramidal indent became larger than that of the hemispherical indent with increasing impact energy.