Damage Detection in Polymer Matrix Composites by Analysis of Polymer-Water Interactions Using Near-Infrared Spectroscopy

Abstract

Detection of early stage damage in polymer matrix composites (PMCs) is a challenge for current non-destructive examination (NDE) methods. New techniques based on sub-micron scale effects may enable identification and characterization of early, non-visible damage. Moisture, ubiquitous in most service environments and readily absorbed by PMCs, may present an opportunity for identifying and characterizing such damage. Water molecules absorbed naturally within PMCs exist either as ‘bound water’ (interacting with the polymer matrix via secondary bonding mechanisms), or as free water (having negligible external bonding interactions). Early-stage physical damage within a PMC takes the form of nano to micron scale free volume within the polymer matrix. This new free volume represents an opportunity for water to occupy and form clusters within the host polymer without directly interacting with the polymer matrix chemically or physically. Therefore, relative to a pristine region, a damaged region would contain a much higher concentration of free water. This altered spatial distribution of free water creates an opportunity for damage identification and characterization. This study explores this possibility by investigating the moisture state in undamaged and 3-joule impactdamaged 16-ply epoxy/glass fiber composite laminates. The specimens are exposed to moisture via immersion in deionized water to simulate long-term moisture exposure in a humid environment. The state of absorbed water is characterized quantitatively via near-infrared spectroscopy. Results show a significantly higher ratio of free-to-bound water in the 3-Joule damaged specimen when compared to the undamaged specimen at the same moisture content. This indicates the existence of free water within voids associated with matrix micro-cracks and delaminations created due to impact. Results from this study suggests that analysis of the nature of polymer-water interaction could be leveraged in developing a technique for damage detection even at very low damage levels in PMCs.