Real-Time Debonding Monitoring of Composite Repaired Materials via Electrical, Acoustic, and Thermographic Methods

Abstract

Electrical properties of composite materials have been thoroughly investigated recently for the detection and monitoring of damage in Carbon Fibre Reinforced Polymers (CFRPs) under mechanical loading. Carbon Nanotubes (CNTs) are incorporated in the polymer matrix of CFRPs for the enhancement of their electrical properties. The electrical properties have shown to be sensitive to the damage state of the material and hence their monitoring provides the profile of their structural deterioration. The aim of the paper is the cross validation and benchmarking of an Electrical Potential Change Monitoring (EPCM) technique against Acoustic Emission (AE) and Lock-in Thermography (LT). All techniques successfully identified damage and its propagation. Thermography was more efficient in quantifying damage and describe dynamically the debond topology, as it provided full 2D imaging of the debond in real time. EPCM was both successful in providing quantitative information on debond propagation and its directionality. AE provided consistent information on damage propagation. All techniques identified three stages in the fatigue life of the interrogated coupons. The representation of the fatigue behavior as a function of life fraction, the correlation of AE data with EPCM and LT data and most importantly the consistent behavior of all tested coupons allowed for both the direct and indirect cross correlation of all employed methodologies, which consistently identified all aforementioned fatigue life stages.

Nondestructive Testing of Polymers and Polymer–Matrix Composites

“…A combination of techniques can be used either as a validation method for one or the other, as a benchmark, or the increased evidence of indications can be explored for a more detailed description of damage, for example by data fusion (327,328). Combinations reported include, among others, DSPI, X-ray CT and TSA (317); ultrasonic C-scan and shearography (318); ultrasound, thermography, vibration testing and embedded sensors (319); IRT and AE (329); IRT and DSPI (330); AE, ESPI and Superconducting Quantum Interference Device (SQUID) current mapping (331), and Electrical Potential Change method, AE and Lock-in Thermography applied to debonding monitoring of composite repairs (332). Combinations of NDT or NDE methods and their application raise two important issues.…”

On the fatigue response of a bonded repaired aerospace composite using thermography

“…1) has been widely employed in the past in various modes to characterize and analyse the response of materials during cyclic mechanical loading. Thermomechanical coupling by LT or the so called Thermoelastic Stress Analysis (TSA) [3,8,15,16,[22][23][24][25], has been used to construct stress maps for both composite and metal materials under cyclic mechanical loading. Stress maps allow for the determination of the location of damage initiation and its subsequent evolution.…”

“…In this work, thermomechanical coupling was compared with simultaneously recorded acoustic emission signals. In a later study of the team, Grammatikos et al [15], the thermomechanical coupling observed in bonded repaired composites was studied in comparison with both acoustic emission signals and electrical measurements. More recently, the same principle was used in combination with Lamb waves technique, to study the bonded repair integrity of a patch repaired real helicopter wing by AgustaWestland (SW-4) [25].…”

“…body (paint in this work ε = 0.97-1), ensures for optimum recording data and hence facilitates thermographic analysis [14,15,25,51].…”

“…In previous works of the authors, LT was employed to assess the patch repair efficiency on aluminium substrates [14], combined with other structural health monitoring techniques [15] as well as to rapidly estimate fatigue limit in ceramic matrix composites [16]. In this work, LT was employed to assess the bonded repair integrity on artificially damaged composite substrates when subjected to cyclic mechanical loading.…”