## 1 Introduction  
It has been revealed that composite structures are considerably sensitive to damage caused by accidental impact events, especially those of a low velocity, during fabrication and in service [1], and the damage may not be easy to inspect visually or with Non-Destructive Evaluation (NDE) use. Therefore, in order to apply the composite laminates in the structures reliably and cost-effectively, it may be important to understand the impact response of the laminates. The complex nature and anisotropic properties of the polymeric composites can lead to the formation of various types of damage, which may be associated with failure in complex patterns. Under the low-velocity impact loading, damage patterns, including delamination, matrix cracking, debonding, and fiber breakage, which are similar to common damage modes in composite laminates, occur, and the damage is significantly dependent on the impact energy levels and impactor shapes [2; 3; 4; 5; 6]. The consequences of the damage may greatly reduce the stiffness and residual strength of the laminates. Mechanical performance of the impacted composite laminates could be characterized through inspecting the damage by nondestructive techniques such as ultrasonic C-scan, X-radiography and infrared thermography. The threshold of barely visible impact damage in the laminates was investigated with the ND techniques [7], and [PERSON] et al. [8; 9] assessed the low-velocity impact damage induced at four different impact energy levels with ultrasonic C-scan and three-dimensional Digital Image Correlation (3D DIC) system, and analyzed the damage evolution and failure mechanism with experimental and numerical approaches. Impact damage strength degradation and progress of the damage were also investigated. [PERSON] et al. [10] evaluated the residual pseudo-ductile behavior in tension for carbon fiber laminates, and reported that the damage area increased progressively as the impact and absorbed energy increased, and the full-field longitudinal strain distribution was measured by the DIC (Digital Image Correlation) system. [PERSON] et al. [11] analyzed the local impact resistance of the composite laminate with fracture mechanics model as a laminate containing a hole of the same diameter as the impacting diameter. It was also reported that the residual strength could be modelled as a function of indentation depth [12] and was dependent on impact energy for the carbon fiber composites [12; 13]. [PERSON] et al. [5] investigated the effect of impactor shape on the resulting damage and residual strength for thin woven carbon/epoxy laminates, and it was found that the blunter impactor caused the large damage area, and the residual tensile strength for the impacted specimens was significantly reduced. The dependency of residual strength on the impact energy was investigated by modelling the strength as a function of impact energy [12], and it was revealed that the strength of Carbon Fiber-Reinforced Polymer (CFRP) laminate decreased as the impact energy increased [13]. Even for the plain weave Glass Fiber-Reinforced Polymer (GFRP) composite laminates, [PERSON] et al. [14] showed that the residual tensile strength decreased with the increase in the impact energy due to the increase of the damage region, and tensile properties like strain at failure and elastic modulus also had the same dependency. Compression after impact (CAI) strength for the composite laminates also was showed to have the same dependency on the impact energy [15]. In this study, the effect of impact conditions, including impact energy and indent shape, and layer structure on the impact response for the GFRP laminates after impact was investigated with measurement of stress concentration around the indents. In order to examine the dependency of the residual strength for the impacted GFRP laminates on the impact conditions, an impact indent was introduced on the two different layer-structured GFRP laminates at different energy levels with pyramidal and hemispherical impactors.