Wetometer for Measurement of Moisture in Composites

A combined experimental and analytic approach to moisture diffusion analysis (MDA) has evolved in proof of principle form as a new instrument termed a moisture profilometer .. The objective in moisture profilometry is the quantitative analysis of the spatial distribution of moisture within composites and definition of localized internal stress fields. An electrolytic cell is utilized to record cumulative moisture evolution and moisture effusion rate. Variation of cell geometry accommodates either laboratory analysis of small composite damage control specimens or field inspection of limited surface area of composite structure in an area scanning mode. An important new aspect of moisture profilometry is calculation of the depth profile of moisture concentration from measurement of surface effusion and rate with time. Statistical estimation theory is applied to this problem and preliminary results indicate that concurrent calculations can generate moisture concentration profiles during the course of moisture effusion measurements. Results of initial instrument design and computational analysis will be demonstrated and discussed

Moisture Diffusion Analysis of Composite Strength Degradation

Moisture diffusion analysis (MDA) has been developed as a new non-destructive evaluation methodology to monitor the strength degradation of graphite-epoxy composite materials. Studies of composite strength degradation due to high moisture identify reversible strength loss due to current moisture content and irreversible strength loss related to prior moisture exposure with microstructure damage. MDA measures current moisture content, directional diffusion coefficients, and moisture concentration profiles. MDA measurement utilizes an electrolytic cell to concurrently record cumulative moisture content and moisture effusion rate. Extension of the diffusion analysis of Shen and Springer isolates directional diffusion coefficients and indicates a high sensitivity of MDA to moisture degradation of the fiber-matrix interface. Extension of the methodology to analyze non-Fickian diffusion shows that MDA can be applied to locate regions of composite strength degradation related to hydrothermal cycling effects. Studies which correlate MDA with ultrasonic response, and interlaminar shear strength of composite laminates subjected to localized moisture damage, show that MDA can be applied to map and locate low strength regions of a composite structure

Kinetics of Moisture Degradation in Graphite-Epoxy Composites

This study forms part of a more general program addressed to developing a better understanding of the mechanisms of interfacial bonding and environmental stability in polymer matrix composites. Several published studies have shown that exposure of graphite fiber reinforced epoxy or polyester composites to water immersion or water vapor at elevated temperature produces decreases in interlaminar shear strength accompanied by rising or falling values of fracture toughness. An approach to the correction of this moisture sensitivity has been suggested by Kaelble based upon a-modified Griffith-Irwin model for interfacial failure and a micro-mechanics model for fracture energy due to fiber debonding

Methods for Detecting Moisture Degradation in Graphite-Epoxy Composites

This presentation reports on the second year of our participation in this NDE program. The purpose of this work is two-fold and this talk divides itself naturally into two segments which relctte to the following objer.tives: To determine the effects of moisture degradation on the mechanical properties of a graphite- epoxy composite under study for use on B-1. To apply promising techniques found in last year\u27s program to detect moisture degradation nondestructively

Detection of Hydrothermal Aging in Composite Materials

In the title of this paper the term hydrothermal , meaning separate or combined conditions of high moisture and temperature, is introduced to describe the complex property degradation processes in two graphite-epoxy composites. It became evident in the course of this study that important hydroelastic stresses degrade composite strength in much the same fashion that thermoelastic stresses do. In fact, the study shows that there are complex internal stress effects within the composite, probably concentrated at the fiber-matrix interface, which depend upon the detailed prior history of moisture-temperature exposure

Polymer Composite Reliability

The structural performance, reliability and durability of polymer composites can now be correlated with three generic classes of internal defects. The first generic class of chemical structure defects (size 10-100Å) that control critical design properties such as glass transition Tg , moisture absorption, and dimensional changes can be controlled by chemical analysis of raw materials prior to manufacture. A second generic class of manufacturing defects (size greater than l0μm) include inclusions, voids and debonds which are related to manufacturing process control and recognized by ultrasonics, optical scanning and other techniques sensitive to interfacial imperfections. The interaction of these two classes of intrinsic defects with environmental and mechanical stresses produces a third class of macroscopic fatigue defects such as interconnected microcracks and macroscopic crack growth which can be detected by visual inspection and ultrasonic emission. The recognition of intrinsic structural defects, and their contributions to polymer composite reliability, represents an important extension in the analytic modeling and reliability predictions for structural polymers, adhesively bonded metals and high strength fiber reinforced composites in which the physical chemistry parameters appear as primary control variables. This discussion introduces and discusses combined deterministic/statistical models for polymer composite reliability. The molecular process which determines the relation between environmental condition and macroscopic structural effect is detailed within such models and provides important criteria for chemical and manufacturing optimization of polymer composite reliability. Experimental data of aging effects on the statistical strength distributions of structure polymers, metal-to-metal joints and reinforced composites are examined and compared with model· predictions