130 A.M. Elmahdy et al. 13.2 Experimental Procedures 13.2.1 Specimen Description The composite specimens used are glass fiber reinforced polyesters. Glass fibers are bidirectional weave cloth of volume fiber 53.3 %. The plate used is made up of eight layers of thickness 0.25 mm each. Specimen strips are cut into dimensions of 120 mm by 30 mm with a total thickness of 4 mm. 13.2.2 Impact Device Description The repeated impact testing device utilizes a pneumatic system to apply the impacts, as shown in Fig. 13.1. It consists of an air compressor (1) of power 1.5 hp which has a working pressure range of 1 bar up to 5 bars. The capacity of the attached air tank is 24 liters. The compressed air passes through a hose to an air filter (2) and a solenoid valve (3). The solenoid valve is connected to a double action air cylinder (4), which uses the air pressure from the compressor to actuate and provide the force needed for impact. The air cylinder diameter is 70 mm and the maximum stroke of its piston is 100 mm. The solenoid valve controls the repeated action of the impact. It takes its signal and power from a control unit (5), which is programmed to provide specific fixed timing signals to open and close the outputs of the valve. The force of the air cylinder’s piston is transferred to a system of connecting rods (6), which is attached to the impactor (7). The impactor hemispherical nose is 12 mm in diameter and the distance between the impactor nose and the specimen – referred to as ‘distance X’- ranges from 5 to 60 mm. Different specimen dimensions and materials can be used and fixed in the specimen fixation (8), up to 120 mm in length and 100 mm in width. A photoelectric switch sensor (9) and a linear variable displacement transducer (10) are attached to the device to enable automated measurements of the number of impacts and the deflections of the specimens respectively. Both sensors are controlled via a data acquisition card (11) NI-USB 6212, which is connected to a personal computer (12) to display and record the measurements by a means of an interface designed using LabVIEW program. When the pressurized air flows into the air cylinder, the control unit sends intermittent signals to the solenoid valve which allows the air to flow in each side of the air cylinder, one at a time, causing the repeated impact force on the specimen. The impact begins with a forward stroke, causing a sudden impact force, then the impactor stays in contact with the specimen for a certain contact time while applying force in the same time. Finally, the impact cycle ends with the return stroke, which releases the impactor away from the specimen. Figure 13.2 illustrates the load cycle of the used impact device. The impact load cycle takes approximately 8.5 seconds to be applied, making the device capable of delivering approximately seven impacts per minute. Fig. 13.1 Components of the repeated impact testing device
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