SAMPE Journal - January/February 2011 - 34

bagged according to common practices of vacuum bag curing. The composite prepreg face sheet and the adhesive film were co-cured in an oven according to the cure cycle provided by the supplier for a given prepreg/adhesive combination. Measurement of Flatwise Tensile (FWT) Strength Each co-cured HC/SS panel was cut to yield three 5 cm by 5 cm (2” by 2”) FWT test specimens. Face sheets of each specimen were lightly cleaned and roughened by sand blasting using a 220 grit size Figure 1. Effects of adhesive treatment and core type on the FWT and 275 kPa (40 psi) nozzle pressure (Econoline strength of AF-555M adhesive. Inc, Grand Haven, MI), cleaned by a damp cloth and dried in an oven at 75°C (160°F) for 1 hour. The plate fixture on a rheometer (ARES, TA Instruments, Inc.)11. pre-treated specimens were then bonded between two 5 cm The lower plate was oscillated at a fixed strain of 2% and a (2”) aluminum blocks using Hysol 9395 two-part adhesive. fixed angular frequency of 10 rad/sec while the upper plate The adhesive was allowed to cure at RT overnight, followed was attached to a transducer which recorded the resultant by oven curing at 75°C (170°F) for 1 hr. FWT strength meatorques. The adhesive or matrix resin films were trimmed to surements were conducted on an MTS test frame according disks of 2.54 cm in diameter. For the materials studied, four to ASTM C297 specifications12. to six disks were stacked up for a specimen of 1.2 to 1.5 mm in thickness and then loaded between the parallel plates. Optical Micrographic Examination Fracture surfaces of the failed FWT specimens were exThe oven was heated according to the vendor recommended temperature cycle for a given specimen. Storage (G’) and loss amined with a Canon EOS D60 camera using macro photo (G”) moduli were obtained as a function of time (t). These lens MP-E, 1-5x, and characterized according to ASTM Dmoduli were then converted to the complex viscosity h*(t). 5573 specifications13. Fracture surface morphology was The specimen’s softening point, minimum viscosity, gelation also investigated by scanning electron microscope (Hitachi temperature, and the processing window were identified from S-3700N SEM). the measurements. Results and Discussion Fabrication of Honeycomb Sandwich Structure (HC/SS) Adhesive Bonding Characteristics of As-received vs. Pretreated AF-555M Panel As-received AF-555M adhesive exhibited a minimum visThe quasi-isotropic 16-ply composite face sheet, 20 cm by 20 cm (8” by 8”), [-45/90/45/0]2s, was assembled by stack- cosity of 290 Poise (P) at 120°C during a temperature ramp ing up 16 plies of uni-directional prepreg plies. Adhesive films of 1°C (2°F)/min from RT. Such a low viscosity rendered were also trimmed to the same size. Both adhesive and pre- superior processability for this adhesive. However, when stacked face sheets were individually sealed in a plastic bag heated to elevated temperatures the AF-555M foamed under vacuum. The origin of the foaming reaction was unknown, and stored in a freezer at -20°C (-10°F) before use. A pre-stacked 16-ply prepreg face sheet was laid on a steel but was dependent upon the heating rate and vacuum level. tool plate covered with Kapton® film and release cloth, fol- Thermal pre-treatment will stiffen the material by partially lowed by a pre-trimmed adhesive film of the same size. A 2.54 advancing the degree of cure of the matrix adhesive, and cm (1”) thick H/C core sheet was then laid down on top of thus inhibit/retard the foaming reaction by depressing the the adhesive film, followed by another layer of adhesive film growth rates of air and gaseous volatiles. It is conceivable and 16-ply pre-stacked face sheet. Finally, a layer of release that severe thermal pre-treatment will effectively increase film and a steel caul plate were placed on top to complete viscosity of the adhesive to an extent that becomes unbondthe assembly. The assembly was dammed on four sides and able in the HC/SS fabrication. Table 2. AF-555M bond strengths and fracture failure modes of HC/SS panels.

SAMPE Journal, Volume 47, No. 1, January/February 2011

SAMPE Journal - January/February 2011

Table of Contents for the Digital Edition of SAMPE Journal - January/February 2011