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Measuring Moisture Levels in Graphite Epoxy Composite Sandwich StructuresGraphite epoxy composite (GEC) materials are used in the construction of rocket fairings, nose cones, interstage adapters, and heat shields due to their high strength and light weight. However, they absorb moisture depending on the environmental conditions they are exposed to prior to launch. Too much moisture absorption can become a problem when temperature and pressure changes experienced during launch cause the water to vaporize. The rapid state change of the water can result in structural failure of the material. In addition, heat and moisture combine to weaken GEC structures. Diffusion models that predict the total accumulated moisture content based on the environmental conditions are one accepted method of determining if the material strength has been reduced to an unacceptable level. However, there currently doesn t exist any field measurement technique to estimate the actual moisture content of a composite structure. A multi-layer diffusion model was constructed with Mathematica to predict moisture absorption and desorption from the GEC sandwich structure. This model is used in conjunction with relative humidity/temperature sensors both on the inside and outside of the material to determine the moisture levels in the structure. Because the core materials have much higher diffusivity than the face sheets, a single relative humidity measurement will accurately reflect the moisture levels in the core. When combined with an external relative humidity measurement, the model can be used to determine the moisture levels in the face sheets. Since diffusion is temperaturedependent, the temperature measurements are used to determine the diffusivity of the face sheets for the model computations.
Document ID
20120000765
Document Type
Other - NASA Tech Brief
Authors
Nurge, Mark
(NASA Kennedy Space Center Cocoa Beach, FL, United States)
Youngquist, Robert
(NASA Kennedy Space Center Cocoa Beach, FL, United States)
Starr, Stanley
(NASA Kennedy Space Center Cocoa Beach, FL, United States)
Date Acquired
August 25, 2013
Publication Date
May 1, 2011
Publication Information
Publication: NASA Tech Brief, May 2011
Subject Category
Man/System Technology And Life Support
Report/Patent Number
KSC-13499
Distribution Limits
Public
Copyright
Public Use Permitted.

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