64 J. Liu et al. 7.4 Results and Discussion The relationship between water absorption ratio and absorption time is shown in Fig. 7.5. The water absorption ratio increased as the time in distilled water increases. Later, the saturated water absorption ratio was reached in about 120 h regardless of the water temperature. It was observed that the average water absorption ratio of the specimen placed in water with higher temperature was significantly higher than that of specimen placed in water with lower temperature. As the water temperature increased 25 ◦C, the increase rate increased by 0.2%, and the saturated water absorption was nearly doubled. Next, the dynamic compressive behavior of CFRP both before and after water absorption was investigated. The striker was launched under the air compressive of 0.1~0.4 MPa. The impact stress-strain curve and strain-strain rate curve were obtained through the one-dimensional stress wave theory. The experimental results both before and after absorption under the air pressure of 0.4 MPa are shown in Fig. 7.6a and b, respectively. The water absorption test specimen absorbed distilled water of 343 K until the water absorption reached 0.4%. The specimen was compressed and the stress increased when an impact load was applied. Afterwards, the stress gradually decreases when the specimen was fractured. The maximum compressive stress was defined as the compressive strength of the CFRP. Similarly, the strain rate increases at the same time as the compression, reaches the maximum value, and gradually decreases. The relationship between the compression strength and strain rate of the material both before and after water absorption is shown in Fig. 7.7. The compressive stresses were proportional to the thickness for the strain rate regardless both before and after water absorption. It is a specific manifestation of the viscoelasticity of the resin matrix of the interlayer of composite fibers under dynamic compression. Moreover, the fracture stress of the specimen that absorbed water was Fig. 7.5 Changes in water absorption ratio against absorption time Fig. 7.6 Dynamic characteristics of specimen before (a) and after (b) water absorption (0.4 MPa)
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