Nondestructive Inspection of Composite Structures

Project title: Noncontact Composite Damage Inspection and Characterization using Planar Array Capacitive Imaging

Source of support: Office of Naval Research

Carbon fiber-reinforced polymer (CFRP) composites are widely used in Naval aircraft structures. Although they offer significant improvements over conventional metals, they are also susceptible to unique damage modes, such as delamination. Not only is delamination invisible to the naked eye, but they can also be hard to detect and characterize using conventional nondestructive inspection (NDI) technologies such as ultrasonic inspection (UI). The aim of this research is to target the time-consuming elements of current Navy UI methods and to demonstrate improved damage characterization in reduced time, while also lessening the burden on technicians to prepare, inspect, and report component damage. This study will design and validate experimentally a noncontact, 3D, capacitive imaging system for assessing single- and multi-layer delamination in CFRP composite panels. The technique proposed is based on electrical capacitance tomography (ECT), where the electrical permittivity of a sensing region is reconstructed from patterns of noncontact capacitance measurements. Since the permittivity of pristine composites differs drastically than internal damage (e.g., air that fills a delamination), ECT can resolve the spatial location, size, and extent of damage, as well as the surrounding structural features. Conventional ECT uses a circular electrode array and images the cross-section of materials. This project builds on this theoretical framework but proposes planar and curved ECT measurement setups that can be used to scan a structure from one side to characterize subsurface damage and structural features in the vicinity of the electrode array. The imaging system will be used to visualize subsurface damage features and to characterize spatial damage sensing resolution, accuracy, and sensitivity. The result is a rich volumetric dataset that allows direct visualization of subsurface physical properties and damage. The entire project involves close collaboration with NAVAIR In Service Support Center-North Island of the Fleet Readiness Center Southwest in San Diego, CA. By the end of this two-year project, the system will be primed for testing more realistic and complicated damaged parts and can also be used to characterize other defect types. Overall, this proposed project will: (1) reduce inspection time and fleet downtime; (2) eliminate the need for specimen preparation (as is currently needed for ultrasonic NDI); (3) enable direct, 3D, digital permittivity imaging of composite components and subsurface damage features; (4) provide a new capability for single-sided NDI of composite parts with multilayer delamination; and (5) enhance operational readiness. 

Collaborators:

• Naval Air Systems Command, North Island

• Prof. Hyonny Kim, UC San Diego

Peer-Reviewed Publications:

1. G. Fan, S. Gupta, and K. J. Loh, 2018, "Curing and Subsurface Damage Monitoring of Epoxy-based Composites," Structural Health Monitoring (1475-9217), Sage, Online First. DOI: 10.1177/1475921718776612