Thermal Ndt & E of Composite Aircraft Repairs

The objective of this study was to investigate different defects and material systems (notch or delamination under carbon and boron composite patching bonded with FM73 adhesive film to the surface of Al 2024-T3) using several thermal non-destructive testing and evaluation techniques. Active thermographic approaches, using, firstly, a simple heat excitation source with an infrared camera, secondly, an integrated pulsed thermographic system, thirdly, pulsed phase thermography and lastly, thermal modelling, were used in the inspection of the composite aircraft repaired panels. In all situations the subsurface defects, were positioned intentionally. After the detection of the defects, representative images obtained from the thermographic investigation underwent quantitative analysis with the intention of obtaining information about the defects in space and in time. Introduction: The purpose of a repair on a damaged area is to transfer through the patch the applied load(s) from one side to the other, by-passing the defected area. Such defects in the past were repaired by the use of metal patches, joined mechanically by the use of pins or screws. Since these repairs implied the formation of stress and strain concentrated areas around the fasteners, leading to additional structural problems, the alternative technique of using composite patches by employing adhesive films in order to attach the repair to the metallic structure is an important maintenance approach. In this technology, the patch is commonly formed from carbon or boron composite, applied onto a wider area of the defect on the metallic structure, with the direction of the fibres in parallel to the direction of the load(s) [1]. Composites offer a large number of advantages over the conventional aircraft material of aluminium; higher specific strength and stiffness, superior corrosion resistance, improved fatigue performance, etc. A considerable amount of work has been conducted using various non-destructive testing techniques in the detection and identification of defects in aircraft structures [2-4]. Thermography is one of the latest non-contact and nondestructive techniques that can be used effectively for the assessment of aircraft materials. Active thermographic approaches, using, firstly, a simple heat excitation source with an infrared camera [5], secondly, pulsed thermography (PT), thirdly, pulsed phase thermography (PPT) and lastly, thermal modelling, were used in the inspection of the composite aircraft repaired panels. PT is a popular thermal stimulation technique where the surface under investigation is pulse heated (time period of heating varying from a few milliseconds for high conductive materials such as metals to a few seconds for low conductive materials such as composites) using one or more pulse heating sources and the resulting thermal transient at the surface is monitored using a thermal camera [6]. PPT combines the pulsed acquisition procedure of PT with the phase/frequency concepts of lock-in thermography for which specimens are submitted to a periodical excitation [7]. Finally, as far as the mathematical thermal modeling is concerned, a specific software (ThermoCalc 3D) was used [8], in order to obtain information about specific defects (notches and delamination) in space and in time. Experimental: Three different experimental set-ups were used for the inspection of multi-ply carbon or boron composite patches. Firstly, a simple heat excitation source (hot air gun) with an infrared camera (Avio TVS 2300 Mk II ST, 3-5.4 μm) was employed. Secondly, PT, using an integrated pulsed thermographic system (Thermoscope) employing a medium wave (3–5 μm) infrared camera (Merlin by Indigo) was used. The system has an attached integrated flash heating system (power output from lamp is 2KJ in 2 to 5 ms). A sampling frequency of 7.5 Hz was used on a 320x256 pixel array. Thirdly, PPT using a focal plane array infrared camera (Santa Barbara Focal Plane SBF125, 3 to 5 μm) working at a sampling frequency of 22.55 Hz on a 320x256 pixel array was utilised. Two high power flashes (Balcar FX 60), giving 6.4 KJ for 15 ms each, were used as heating sources. The thickness of each ply of the composite patches was 125 μm. Description of the investigated samples are shown in table 1, whilst the actual dimensions of the samples and their defects are presented in table 2. Sample Description of Sample A Simulation of delamination (Teflon 25 mm x 25 mm between 3rd & 4th ply) on boron composite patch on Al 2024-T3 panel P1 Carbon composite patch on notch on Al 2024-T3 panel Table 1: Description of investigated panels Sample X-axis (mm) Y-axis (mm) No. of Plies Defect Dimensions (mm) Total Area (mm2) Defect Area (mm) Defect (%) A 70 150 6 25 x 25 10500 625 5.95 P1 65 160 6 10 x 1 10400 10 0.09 Table 2: Dimensions of samples and their defects Furthermore, mathematical thermal modelling, using specific software, was also attempted in order to obtain information about the defects (delamination and notch) in space and in time. The ThermoCalc-3D software, which has been developed for simulating thermal non-destructive testing problems where transient temperature signals over subsurface features are of a primary interest, was used. The thermo-physical properties [9] and heating time parameters shown in tables 3 and 4 respectively were used for the modelled composite panels. Thermal Conductivity (WmK) Material X axis Y & Z axes Heat Capacity (JKgK) Density (Kgm) Carbon Composite 7 0.8 120