Nondestructive inspection and quantification of carbon fiber laminates for barely visible impact damage and adhesive layer thickness measurements.

Abstract

Carbon fiber laminates are used extensively across numerous industries. Their high strength to weight ratio allows for weight savings over metallic alternatives without compromising structural integrity. The layered structure of carbon fiber laminates constitutes the need for new testing procedures to quantify quality and inspect for defects, for both manufacturing and in-service. This research uses a novel high resolution ultrasonic non-destructive testing (NDT) technique to evaluate, two similar, but unique scientific contributions, barely visible impact damage (BVID) and adhesive layer thickness. An in-house drop weight impact device is used to produce BVID. Inherent to carbon fiber laminates, damage may exist beneath the impact surface that is undetectable by basic visual inspection. Traditional mechanical testing techniques are not appropriate for evaluating damage as there is a desire to keep the composite in-service, to drive down costs. Traditional tests are destructive, rendering the component useless after testing. The first scientific contribution of this dissertation is the ability to form three-dimensional damage profiles in-situ via the introduced NDT technique. Profiles are paired with a finite element analysis of the component, and true damage geometry is shown to cause significantly greater localized normalized maximum von Mises stress than that observed using high resolution surface imaging. The second scientific contribution of this dissertation is characterization of bondline thickness for bonded composites. Adhesive bonds allow a uniform stress distribution between bonded components and eliminate stress concentrations associated with traditional rivets. Uncertainty in quality, specifically bond thickness, prevents extensive use for primary structures. Traditional mechanical testing techniques qualifying bond thickness or uniformity is not appropriate as they destroy the bond making the component not fit for service. The technique presented non-destructively obtains the adhesive layer thickness for uniform and non-uniform bonds. This dissertation culminates by demonstrating the BVID and bondline measurements on both an immersion and a portable ultrasonic scanning system. A portable scanning device is developed for real world applications and allows for future efforts to focus on Federal Aviation Administration certifications to use the developed BVID and bond thickness techniques in industry.