where m is the quality of hammer and h is the height of hammer before free-hall. In the experiments, m is 2 kg and h is various from 0.1 to 0.6 m in step of 0.1 m. Thus, the impact energy W can be calculated as 2, 4, 6, 8, 10 and 12 J in sequence. General impact characteristics for CFRP structures are illustrated in Fig. 4. The small energy impact can result in a concave on the surface of the specimen.The big energy impacts can lead to not only a concave, but also the descending area outside of concavity, some protruding structure on the edge of concavity and on the rear side of sample. Fig. 5a and
b shows the front side and rear side of 12 J impacted laminate using microscope. Clearly, some protruding structures are around the concavity on the front side while the protruding structures are in the middle of impact spot on the rear side. As shown in Fig. 5c–e for rear sides of 10 J, 8 J and 6 J impacted laminates, the protruding structures are decreased as the impact energy decrease. There is no protruding structure found on the rear sides of 2 J and 4 J impacted laminates.