Detecting micro-structure and flaws in composites using eddy-current instrumentation

Abstract

Eddy-current inspection of advanced composite materials appears to overcome some of the difficulties typical of ultrasonic inspection. For example, carbon-carbon material can be inspected using eddy-currents without making the sample wet or using any acoustic coupling substances. Eddy-current inspection is very good for detecting fibers and fiber content because the fibers usually conduct electricity very well. Eddy-curre~t measurements can therefore detect broken fibers in many cases where ultrasonic inspection fails to find the damage (broken fibersare not always good at scattering the ultrasonicsl. One of the common arguments against using eddy-currents to inspect composite materials is that they are unable to detect delaminations. This paper presents experimental evidence that suggests that delaminations can be detected with eddy-current inspection. Sabbagh [4] offer theoretical reasoning for the delamination behavior. This paper also presents some experimental data that indicates fiber structure, impact damage, and drilled holes in advanced composite material, including carbon-carbon and graphite-epoxy. Composite materials are of interest to a number of industries for their strength and weight properties. The performance of these materials is obtained by combining strong fibers and a matrix "glue." The resulting material is difficult to inspect because its structure and electrical conductivity are nonhomogeneous. Our experiments apply eddy-current inspection to graphite-epoxy and carbon-carbon composites. We describe our procedure and experimental results for gathering eddy-current measurements that indicate internal features of advanced composite materials. Our measurements of the EMF were made using inductive sensors, excited by various current sources, near samples of material. The EMF measurements, made in the range of 100kHz to SOMHz, indicate features such as fiber tows, impact damage flaws, drilled holes, and weave structure of the material. Laboratory data and model calculations are presented. Computer-controlled electronic instrumentation that uses phase-sensitive techniques measures the amplitude and phase of the induced sensor signal.

Quantitative Electromagnetic Modeling and NDE of Carbon-Carbon Composites

“…multi-layer composite of 36 alterZero-to-peak magnitude is approxrepresent equi-potential contours. Similar characteristics have been observed in the laboratory, some of which have been previously reported [2). Laboratory measurements that demonstrate, at least qualitatively, some of the features of this model calculation are presented in Figure 7.…”

“…The multi-layer model predicted a subtle effect resulting from a simulated delamination. Laboratory data agreed with this prediction in that some simulated delaminations caused a detectable change in transmitted signal [2].…”

“…A delamination was simulated by introducing a new layer of ''air,'' modeled as having very small conductivities in X Y and Z. The simulated delamination was also measured in the laboratory; results have been previously presented [2]. The laboratory ''delaminations'' were thin sheets of Teflon between layers of a multi-layer graphite-epoxy.…”