Lock-in Thermography for Process Integrated Non-Destructive Evaluation of Carbon Fibre Reinforced Aircraft Stuctures

Kurzfassung

Reliable process integrated quality assurance for the manufacturing of large-scale CFRP aircraft structures, such as fuselage shells, cargo doors or components of a central wing box, belongs to one of the most challenging objectives for nondestructive testing. Beside the well known key criteria like non-contact measurement and imaging options as well as the maximum failure detection resolution and defect selectivity even more aspects have to be considered: the penetration depth into the component’s material, the scanning and measurement speed, the coverage area as well as the accessibility of the structure (inspection condition with single side or two side accessibility). The probably most significant requirements for process integrated non-destructive inspection are the measurement time and data evaluation. Therefore the coverage area and the measurement speed are quite important. For example common ultrasound inspection screens the component’s surface line by line until the whole area is scanned. This requires quite a lot of time and as soon as the component’s shape includes curved or ribbed areas, it becomes difficult to receive proper signals of the material’s interior structure. In contrast, the main benefit of optical inspection methods is the coverage of relatively large areas within one exposure time but on the other hand only features which are located near the component’s surface can be detected. However, lock-in thermography seems to be a good compromise of all these boundary conditions as it is monitoring the time dependent change of thermal waves that are emitted by the structure component. Depending on the excitation frequency of the induced thermal waves, it is possible to collect information of the material’s microstructural features from different depths and to cover relatively large areas within one measurement cycle. A single side accessibility of the structure component is absolutely sufficient and measurement time as well as data acquisition and interpretation can be performed within the order of minutes. Detecting and localizing material defects or significant microstructural changes is relatively easy since changes in the material’s thermal diffusivity are clearly distinguishable in the visual data mapping of the component’s surface. As previous work has shown, even a proceeding resin front of an infiltration process can be detected and visually displayed. Focusing the benefits and potentials it is shown that lock-in thermography is a valuable non-destructive inspection tool that can be used as part of a process integrated quality assurance system for CFRP structure component manufacturing.