A technique for polymer composite damage detection by analysis of polymer-water interactions and damage-dependent hysteresis

Polymer composite materials are increasingly being adopted across civil infrastructure, oil & gas, marine, automotive, and aerospace industries. Achieving ubiquitous adoption would require new advances in development of non-destructive examination techniques which are cost effective, simple to utilize, and sensitive enough to ascertain the structural integrity of composite parts, especially in the absence of visible damage. A common characteristic of these materials is their tendency to absorb measurable moisture in nearly all operating environments. This absorbed moisture either becomes bound to the polymer network via secondary bonding interactions or exists as free water with negligible interactions. Damage creates fractures and new internal free volume where water molecules can exist in the latter state. Thus, a novel basis for nondestructive examination in polymer composites which leverages this locally-higher concentration of free water in damaged areas is proposed. Effects of humidity or precipitation-driven fluctuations in moisture content on water state distributions is also investigated. Polymer-water interactions—determining the free or bound state of water—are characterized by near-infrared spectroscopy and microwave-range dielectric properties. Results show a direct correlation between the extent of local damage and higher relative levels of free water at damage sites. They also indicate moisture variation has a significant effect on the water state distributions in damaged polymer composites. Hence, presenting an opportunity for reliable detection of localized damage by analysis of polymer-water interactions and damage-dependent hysteresis.