Moisture ingression in honeycomb core sandwich panels
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Moisture Ingression in Honeycomb Core Sandwich Panels D. Cise and R.S. Lakes Moisture ingression was studied in several composite sandwich panels, in which hydration was applied over a large surface area at the panel edges. Significant moisture ingression occurred in panels with cores of Korex (based on a substrate of a fiber pulp paper) and IIRP (consisting of a woven-glass-fiber substrate with a polymer coating) of different density. Ingression was more rapid than in panels with hydration applied locally. Ingression followed an exponential pattern in time in most cases, in harmony with diffusion theory.
Keywords
core, honeycomb panels, moisture, sandwich panel, water
1. Introduction Polymer-matrix composite materials are used in many applications such as aircraft and sports equipment, just to mention a few. These materials exhibit high stiffness and strength combined with low density. Epoxy resins used in such materials absorb water from the environment. High temperature can degrade the mechanical properties of the resin, and the degradation is more severe when the resin is also exposed to moisture. Because exposure to fluctuating temperature and moisture is inevitable in aircraft applications, considerable research effort has been devoted to studying the mechanism by which epoxy resin composites absorb and transmit (ingress) moisture and the effect of moisture on their mechanical properties. For example, epoxy-matrix materials and polymer-matrix composites absorb moisture (Ref 1), and the absorption is attributed to the matrix rather than the graphite fibers. Neat resins equilibrate (Ref 2) at higher moisture contents than the graphite-fiber composite. The graphite fibers appear to be nonabsorptive, and the epoxy matrix is the lone contributor to moisture weight gain (Ref 2). Epoxy-matrix material can absorb water up to 5% by weight (Ref 1). Fickian diffusion describes much of the ingression behavior during exposure of graphite-epoxy laminate panels to humidity or immersion in water (Ref 3-5). Water immersion of a graphite-epoxy crossply composite (Ref6) for 20 days at a temperature of 343 K reduced the strength by 13% and the stiffness by 9%. Environments containing humid air can also cause loss of stiffness and strength in graphite-epoxy composite (Ref 7). Hygrothermal histories can, depending on history and polymer material type, either increase or decrease the resistance to matrix cracking (Ref 8). Moisture acts as a plasticizer of the matrix and shifts the glass transition temperature toward lower values (Ref9). The effect is composition dependent (Ref 1) and is of concern because some composites may be used at elevated temperature. Moisture in the polymer matrix causes an increase in mechanical damping (Ref 10). Many studies have been conducted and reviewed (Ref 11) on water transport in D. Cise and R.S. Lakes, Department of Biomedical Engineering, Department of Mechanical Engineering, Center for Laser Science and Engineering, 238 IATL, University of Iowa, Iowa City, IA 52242, USA. (Ad
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