Identifying Defects in Aerospace Composite Sandwich Panels Using High-Definition Distributed Optical Fibre Sensors

Abstract

Automated methods for detecting defects within composite materials are highly desirable in the drive to increase throughput, optimise repair program effectiveness and reduce component replacement. Tap-testing has traditionally been used for detecting defects but does not provide quantitative measurements, requiring secondary techniques such as ultrasound to certify components. This paper reports on an evaluation of the use of a distributed temperature measurement system—high-definition fibre optic sensing (HD-FOS)—to identify and characterise crushed core and disbond defects in carbon fibre reinforced polymer (CFRP)-skin, aluminium-core, sandwich panels. The objective is to identify these defects in a sandwich panel by measuring the heat transfer through the panel thickness. A heater mat is used to rapidly increase the temperature of the panel with the HD-FOS sensor positioned on the top surface, measuring temperature. HD-FOS measurements are made using the Luna optical distributed sensor interrogator (ODISI) 9100 system comprising a sensor fabricated using standard single mode fibre (SMF)-20 of external diameter 250 μm, including the cladding. Results show that areas in which defects are present modulate thermal conductivity, resulting in a lower surface temperature. The resultant data are analysed to identify the length, width and type of defect. The non-invasive technique is amenable to application in challenging operational settings, offering high-resolution visualisation and defect classification.

Accuracy of Distributed Strain Sensing with Single-Mode Fibre in Composite Laminates under Thermal and Vibration Loads

“…Strain measurement is an important method for structural health monitoring (SHM) because the strain distribution usually changes when damage is initiated in a structure [5,6]. An ideal sensor for strain measurement in CFRP is the single-mode fbre (SMF) which is interrogated by an optical frequency domain refectometry (OFDR) system [7]. Tis is because SMFs are light, cheap, easily available, and enable distributed strain sensing with high spatial accuracy.…”

“…Various fbre optical (FO) sensors have been adopted in diferent SHM applications [7][8][9]. For example, buckling detection based on distributed FO sensors [10], a hybrid piezoelectric-fbre optic system based on fbre Bragg gratings (FBG) [11,12], embedded FBG sensors in SHM of complex composite structures [13], and sandwich panel [14].…”

Monitoring of a Highly Flexible Aircraft Model Wing Using Time-Expanded Phase-Sensitive OTDR

“…The interrogation of several FBGs written in a single optical fiber is made possible by time and/or wavelength multiplexing [ 4 , 6 , 7 ]. Despite theoretical studies that affirm the possibility of multiplexing up to one thousand FBGs [ 8 ], in practical terms, the multiplexing of hundreds of FBGs represents a challenge [ 6 , 9 ]. Besides, the FBGs have to be precisely written in the fiber, increasing the cost and complexity of such sensors.…”

Composite Laminate Delamination Detection Using Transient Thermal Conduction Profiles and Machine Learning Based Data Analysis

“…The use of contact sensors and a transmission based heat source has been demonstrated to produce data which can be analysed to yield the transient thermal conduction profile indicative of inter-laminate differences between multi-ply composite laminates [ 1 , 2 , 3 , 4 ]. The approach provides a rapid, low energy (and thus low risk of introducing thermal damage) indication of undocumented modifications to the ply lay-up of a component yielding significant operational benefits within the Maintenance Repair and Overhaul (MRO) segment of the aviation industry.…”