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3. Experiments

3.1 Fabrication

The composite tubes were fabricated by tubular braiding machine (Murata Machinery, Ltd.) using 48 carbon prepreg yarns (Nippon oil corporation, in which fiber is the carbon yarn (TORAYCA T700-6k) and the resin is modified epoxy (Nippon oil corporation; resin impregnated ratio is 35wt%)).

For the fabricating process, prepreg yarns were braided around the mandrel with 20mm diameter. Then PP tape was wrapped around the preforms stacked with a few layer up to the predefined thickness of the braided tube and it was cured in an oven at 80oC for half an hour and at 130oC for 1.5 hours. The photograph and specification of the fabricated tubes are shown in Figure 3 and Table 1, respectively. The number of stacking layers is decreased with increase in the braiding angle to keep the same thickness of composite tube with different braiding angle since the thickness of one layer increases with increase in the braiding angle.

3.2 Bending test

The four-point bending test was performed by using the pulley unit and the metal solid-core bar as shown in Figure 4. The pulley unit and the round bar are capable of decreasing the stress concentration generated at the point of support and loading nose.

The rosette gage was stuck on the surface of the center and bottom part of the specimen to measure the local tensile strain of the tube and to calculate the principal strain. The bending test was performed by using an INSTRON universal testing machine with a span length of 300mm and cross-head speed of 5mm/min.

3.3 Results of bending test

The results of bending test were shown in Table 2. The bending modulus and strength were decreased with increase in the braiding angle.

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