Time Reversal Microwave Methods for Detecting Disbonds in Adhesively Bonded Metal-composite Structures

Abstract

Composites are being increasingly used to replace metals, partially or completely, in aerospace, shipping and automotive industries because of their light weight, corrosion resistance, and mechanical strength. However, the quality of adhesively bonded joints can be severely compromised if there are manufacturing defects such as disbonds or impact damage when in use. These factors can significantly affect the strength of resulting bonded joints and their performance. Hence there is need for detecting these anomalies during manufacturing and maintenance. Time reversal focusing is based on the property of the wave solution to focus back at the source when reversed in time and back propagated. This property has been used in ultrasonic NDT for detecting defects in test objects that act as sources of scattered fields. This paper presents a model based feasibility study that applies time reversal principles to microwave NDE data. Microwave NDE is well suited for dielectric materials, including composites because of the ability of electromagnetic waves to interact with these materials. The scattered field depends on dielectric properties of the medium, hence providing information about the structural integrity of these materials. In this paper microwave NDE is applied to geometry comprising adhesively bonded metal composite structure. Two dimensional Finite Difference Time Domain (FDTD) method is developed for both forward and back propagation of electromagnetic wave field propagation in a test geometry that comprises three layers, namely, composite, epoxy and metal. A Gaussian modulated pulsed source is used to illuminate from the composite side and the reflected field from the metal is measured by a receiver array. The measured microwave fields are time reversed and propagated backwards using the FDTD model. The maximum energy of the back propagated field highlights the defect location. Simulation studies exhibits the feasibility of this approach to detect and characterize disbonds in metal-composite joints.