2.2. Sample preparation
The double hat shaped tubes were fabricated using the bladder
molding process, which basically consisted of preparing carbon
and resin preform, adding adhesive at overlap areas, placing into
a metal mold, exerting pressure from the inside with an inflatable
bladder and curing within a hot press. The Toray plain weave carbon
fiber T300/epoxy prepreg was chosen for the fabrication. The
fiber volume fraction is around 56%. The average tensile strength
and modulus, determined from the tests on the 6-ply laminates
according to ASTM D3039-76, is 590 MPa and 56 GPa, respectively.
As shown in Fig. 2a, the prepreg was laid smoothly by hand into
each half of mold, which allowed higher pressure, better surface
finish and better control of tube dimensions. The thin thermoplastic
film bladder was placed in the middle of the tube, and the
Loctite Hysol 9432 adhesive was deposited on the overlap areas
before halves of molds closed. The closed molds were heated up
to around 150 C inside a hot press, and a pressure of 1.25 MPa
was applied in the bladder to load the preforms into molds (see
Fig. 2b). Then the fabricated tube was cured about 45 min to
provide optimum strength and hardness, allowing the overlap
edges of the tube to be bonded together firmly. Finally, the desired
length of specimen was sectioned from the cured tube. In this
study, three sets of molds with different overlap edge thickness
were used to fabricate the 3-ply, 6-ply and 9-ply specimens,
respectively.