SAMPE Journal - January/February 2011 - 35

Figure 2. Optical photographs for the fracture surfaces of four HC/SS panels bonded with AF-555M adhesive. The thermal pre-treatment condition of 110°C for 2 hrs was found to yield an adhesive which exhibited a minimum viscosity of approximately 2,200 P at 120°C. The foaming reaction was also significantly suppressed when heated in an oven under vacuum. A four panel test matrix was designed which included two material variables, i.e. as-received vs. pre-treated adhesive and cores with and without perforations. IM7/MTM 45-1 16-ply prepreg, quasi-isotropic face sheets were co-cured with the adhesives in each panel. Results were tabulated in Table 2 and plotted in Figure 1. It is evident from Figure 1 that the pre-treated AF-555M adhesive yielded higher FWT strengths in the HC/SS bonds in both types of Al HC core. These strength values compared favorably to the 7,585 kPa (1,100 psi) reported by the supplier (3M) in similar Al-composite HC/SS bonds2. As-received adhesives with perforated cores yielded a decent value of 8,667 ± 538 kPa (1,257 ± 78 psi). However, a much lower strength value of 6,343 ± 1,110 kPa (920 ± 161 psi) was measured for the non-perforated cores when bonded with the same adhesive. Optical photographs of the fracture surfaces for these four panels are shown in Figure 2. A pair of photographs representing the core and the face sheet sides of the fracture surfaces is included for each panel. Fracture surfaces for the panels bonded with pre-treated AF-555M adhesives exhibited nearzero voids, regardless of the core type used. The adhered fillets on the core side were evenly distributed around the cell walls, and the hexagonal shapes of the fractured adhesive resin adhered to the composite substrate on the face sheet side were clearly defined. The surface fracture was clearly dominated by the cohesive failure mode, resulting in higher FWT strengths. Fracture surfaces for the panels
SAMPE Journal, Volume 47, No. 1, January/February 2011

bonded with as-received AF-555M adhesive contained voids. Voids were particularly evident and abundant in the fractured adhesive resin on the face sheet side. Because of the superior fluidity of as-received AF-555M adhesive, the effect of vacuum was evident in that the composite substrate was visible. For both panels, the hexagonal shape of the fractured adhesive resin on the face sheet was distorted because resin thicknesses were not uniform across the residue. In addition, the adhesive fillets were not evenly adhered to the core walls and the surface fracture was characterized by the adhesive failure mode resulting in lower FWT strength values. Adhesive Bonding Characteristics of XMTA-241/PM15 As-received XMTA-241/PM15 adhesive exhibited a minimum viscosity of 7,100 P at 80°C during a temperature ramp of 1°C (2°F)/min from RT. This viscosity is three orders of magnitude higher than the AF-555M adhesive, and resulted in inferior surface wetting during HC/SS fabrication. This adhesive did not foam under vacuum at elevated temperatures.

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SAMPE Journal - January/February 2011

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