Impact testing

Three different impact cases are studied in order to examine the effects of FSI on composite structures under dynamic loading. These cases are shown in Fig. 3. First, the dry impact is conducted as the baseline. For the dry impact test, the composite plates are impacted without having any contact with the water. This is shown in Fig. 3(a). Subsequent wet impact tests are undertaken for the same composite plates. In order to avoid any moisture effect on the composite materials, the wet impact testing is performed as soon as the composite plates are submerged into the anechoic water tank. Furthermore, once the wet impact testing is completed, dry impact of the plate is conducted just following the wet impact. The responses of the dry impact tests before and after the wet impact testing are compared. Their results are consistent. By doing so, it can be verified that the composite plates did not absorb any moisture to affect their material properties.

Two different wet impact conditions are considered. The first case has an air-containing rigid box attached to the bottom of the composite plates. The box is completely sealed so that no water permeates into the box when the composite plate with the attached box is submerged into the water of the anechoic water tank. Then, impact loading is applied to the composite plate submerged in water. This is called the air-backed wet impact and is shown in Fig. 3(b). The air-backed composite plate is only in contact with water at the top side where the impact occurs. The other wet impact case is very similar to the previous one except that the air-box is no longer sealed so that water fills the box when the plate and the box are put into the water tank. This is sketched in Fig. 3(c) and called the water-backed wet impact. The water-backed plate is exposed to water on both sides in this case. The same impact conditions, i. e. the same drop weight and height, are applied to the three impact loading cases. The wet impact responses are compared to the dry impact data in order to evaluate the FSI effects.

(a) (b) (c)

Fig. 3. Three different impact conditions with composite plates held in place: (a) dry impact, (b) air-backed wet impact, (c) water-backed wet impact