Heat brings new insights

Using thermography to reliably detect defects

The latest generation of passenger aircraft is constructed largely from modern, high-performance composite materials. Carbon fiber composites (CFC) in particular are much more robust and rigid than aluminum and a lot lighter, holding out the promise of significant reductions in fuel consumption and CO2 emissions. To live up to the high safety standards that apply to aviation, accurate methods of checking the condition of materials in ongoing flight operations are required. Here non-destructive testing (NDT) plays a critical role, as it allows the quality of a fabricated part or aircraft structure to be extensively tested without inflicting any damage on the material itself in the process. One of the most important surface-oriented NDT technologies is infrared thermography (IRT), a (heat-) imaging method under which high-resolution infrared cameras render concealed defects visible, enabling them to be detected.

Non-destructive testing methods have long been used to examine metal aircraft structural components. But in the case of structural components made from composite materials, use of this technology remains highly challenging due to the complexity of both the material and the potential failures. As a result, until recently there was no efficient way of safely, completely and reliably examining a structure built from composite materials for damage and faults that was commensurate with the scale and variety of ways that they are used in modern commercial aircraft.

Lufthansa Technik has teamed up with AT – Automation Technology and the Institute of Plastics and Composite Materials, the Institute of Telecommunication Engineering and the Institute of Modeling and Computation at the Hamburg University of Technology (TUHH) to research the "Optimization of NDT methods for composite materials". The aim of the project is to understand the performance boundaries of modern non-destructive testing methods based on active thermography as used by aircraft manufacturers and maintenance organizations. It should then be possible to significantly improve the reliability of defect detection and the efficiency of test procedures in operational use.

Under active (heat flux) thermography, the surface of the objects to be measured is heated by a brief thermal pulse from a heat source (e.g. a halogen lamp). The heat then moves from the surface to the interior of the material. If any faults or defect areas with lower thermal conductivity are lurking below the surface, the heat is prevented from spreading. As a result, the surface above these defective areas remains hot for longer, enabling infrared cameras to bring out these temperature differences in a visible manner. In the case of composite materials, for example, concrete indications of unspectacular damage or of defects such as delaminations can be detected in this way in ribs and stringers. Active thermography offers many advantages. Thus, with this technology it is possible to examine relatively large areas (0.25 to 0.5 square meters) in a single pass, it is scalable to any size and can be applied either in the appropriate workshop or, using mobile units, directly on the aircraft fuselage.

The "Optimization of NDT methods for composite materials" research project, which is supported by the Federal Ministry of Education and Research and, has been running from November 2009 to October 2013, has already produced its first important results. Thus it has been possible to gain extensive insights and experience with the new methodology and to acquire a better understanding of the material properties. In parallel to this, software for the automated detection of defects has been developed. Although the method is currently still limited to materials not exceeding four millimeters in thickness, it is already possible to systematically examine some 80 percent of the fuselage of a Boeing 787 with it. If operation of the latest aircraft types predominantly built from CFCs is to comply with aviation law requirements, a reliable technology such as this and a defect detection method that is both proven in practice and officially certified are essential.