Measurement of Composite Fiber Volume Fraction Using Thermal and Ultrasonic Inspection Techniques

Abstract

Many methods exist for the experimental determination of fiber volume fraction (FVF) in graphite epoxy composites. The most commonly used method of determining composite FVF involves the removal of the matrix by burn off or acid digestion. In addition to being destructive, this technique is time consuming and requires the disposal of toxic waste. Also this technique can be operator dependent (Cilley et al., 1974). Recent work (Zalameda et al. 1992) has shown that the FVF can be determined by measuring thermal diffusivity assuming negligible porosity levels. In addition, work has been done in ultrasonically determining porosity in composites by using a frequency dependent relative attenuation measurement (Hughes et al., 1987). The objective of this work is to develop a nondestructive technique to determine FVF using a dual inspection methodology. The relationship between thermal diffusivity and fiber, matrix and void volume fractions is described in a one dimensional heat flow model where the void volume fraction is determined ultrasonically. The use of a phase lag technique is implemented to make quantitative measurements of thermal diffusivity. These measurements were on composite plates with varying FVF. Diffusivity measurements indicated a nonlinear relation between FVF and measured diffusivity with values ranging from .003 to .007 cm^2/ sec. In addition to the thermal measurements, frequency dependent relative attenuation ultrasonic measurements were made within the area to be destructively tested. Results have shown an approximate linear correlation between porosity and attenuation. Results will be presented on 16 and 32 ply composite plates with lay ups of [0/90] 4s and [0/90] 8s and with FVF values ranging from 50 to 75 percent. A comparison of the measurement results to destructive testing is shown and the implementation of the thermal and ultrasonic measurement techniques is described.