The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Composite materials, in particular panels used in aeronautical applications, proximate to the engines, are intended for operation at temperatures below a maximum threshold. These panels, whether forming one single layer or used in a sandwich structure comprising a central core such as honeycomb covered with two external skins or face sheets, include unique mechanical characteristics and a reduced mass, and may take various shapes.
However, subsequent to thermal events comprising an increase of temperature above a maximum threshold, and depending on the exposure times, some damage to the materials of the panels may occur, which may reduce the structural strength characteristics. Thus, it is desirable to be able to characterize the level of the material damage during maintenance operations in order to increase safety.
In order to carry out inspection on these composite materials, it is known to apply a primary paint over the surface, in particular a polyurethane paint, which will undergo color transformations during thermal events.
A first known method for damage assessment consists in visually studying the discoloration of the primary paint in order to deduce therefrom some level of evolution of the material. This method gives a qualitative indication, but does not allow for an accurate quantitative measurement of the damage.
Another known non-destructive method, in particular presented by the documents EP-A1-2343531 and US-A1-20110001047, performs an analysis of the discoloration of a primary paint by a technique based on infrared spectrometry.
In particular, the second mentioned document presents the following method: after having carried out a calibration model, infrared radiation is emitted on a point of the surface of the material to be analyzed; then, the diffused reflection spectrum of this surface is measured; and by considering on this spectrum a criterion which is compared to the calibration model, a history of the thermal or chemical aspects of the coating is assessed, in order to deduce therefrom a condition of the composite material supporting it.
Nonetheless, this method is complex to carry out, and the obtained results highly depend on the implementation of the primary paint. In particular, thickness variations of the coating layer, of its polymerization cycle, of the number of deposited layers or of the surface condition, will disturb the infrared reflection. The results as well as the conclusions related to the damage to the material are not accurate.
Other known non-destructive methods for assessing the damage to the materials use in particular ultrasonic or thermographic conventional techniques which allow in particular detecting detachments between the layers of the materials. Thus, it is possible, for example, to control the detachment between the skin and the core of a panel comprising a sandwich structure. Nonetheless, this method does not allow assessing the condition of the material by itself.