Water Intrusion In Thin-Skinned Composite Honeycomb Sandwich Structures

Thin-skinned composite honeycomb sandwich structures are highly susceptible to water intrusion when used in aircraft applications. To investigate the effects of impact damage and cyclic loads on the susceptibility to water intrusion, thin-skinned composite honeycomb sandwich structures from the rotor blade trailing edge of the McDonnell Douglas Apache and Boeing Chinook helicopters were tested. The Apache and Chinook rotor blade skins were made of glass/epoxy prepreg with a (90/0/90) layup and a (+45/-45) layup, respectively. An apparatus was developed to quantify the air or water flow rate through the skins. Minimum impact and fatigue conditions were determined which would create microcracks large enough to permit water flow through the skins. Damage that was sufficient to permit air and water to pass through was not visible under a 40X optical microscope in many skins. The flow rate was found to be a function of moisture content, damage, applied strain, and pressure difference. For the Chinook rotor blade skins, the peak strains necessary to grow microcracks within one million cyclic loads were well above the strain levels recorded during flight tests.