Impact Damage Assessment in CFRP Composites Using 1-D Electrical Resistance Technique

Abstract

In this work the effectiveness of the one-dimensional (1D) electrical resistance method at sensing low velocity impact damage in AS4/3501-6 carbon fiber polymer matrix laminated composite plate specimens was studied. A fully instrumented experimental setup for characterization of electrical properties has been developed. Two types of laminates were tested: a 16-ply orthotropic [0/45/-45/90]2s laminate (thickness = 2.25 mm) and a 32-ply orthotropic [0/45/-45/90]4s laminate (thickness = 4.5 mm). Single impact and cumulative damage studies were conducted for both the 16- and 32-ply samples. Before and after each impact, the top surface resistance and the oblique resistance (resistance through the thickness of the specimen at an oblique angle) of the impact region were measured and their effectiveness to sense damage was evaluated. I. Introduction Fiber reinforced composite materials are known to be susceptible to low velocity impact damage, which represents a serious design concern for composite structures. Particularly precarious situations arise when no apparent damage at the impacted surface of the composite structure is accompanied by a significant reduction in structure’s strength and stiffness. As current and future aerospace systems rely more and more on fiberreinforced polymer composites, damage detection becomes increasingly important and non-destructive evaluation methods are still highly desirable for such composites. At the same time, adding damage sensing functionality to composites represents an attractive alternative, where a composite material itself may serve as a damage sensor. Carbon fiber polymer matrix composites are electrically conductive. The conductive carbon fibers constitute a conductive network that allows the composite as a whole to be conductive, even though the polymer matrix material is non-conductive. The overall electrical properties of the composites depend not only on the inherent electrical properties of the carbon fibers and their volume fraction in the composite, but also on the connectivity of fibers in composites, sequence of plies, etc. When impact damage occurs, the connectivity of fibers in the individual plies and at the ply interfaces changes, which results in the overall changes in the electrical properties and particularly in the electrical resistance. Therefore, since the conductivity of a CFRP laminate changes with damage, measuring the electrical conductivity of such a composite provides a means of damage sensing. There have been a number of studies on the electrical characterization of carbon fiber polymer matrix composites conducted in the past [1-8] that attempted to monitor changes in the electric resistance and electric field that occur as a result of mechanical damage. Two main techniques have been employed to sense impact