A novel approach for thermographic Non-Destructive Evaluation (NDE) of laminated polymer composite structures is presented. The technique is based on Lock-In Thermography (LIT), which traditionally uses an external heat source. Here a new means of internal heating, via a lightweight embedded actuator capable of providing highly repeatable and uniform heating is presented. The equipment cost, complexity and hence size is reduced by removing the need for high power external thermal excitation. Instead, the necessary temperature modulation of the internal actuator is achieved with a compact and low-cost Arduino controlled circuit. The size and cost of the equipment is further reduced by demonstrating that a miniature printed circuit board mounted thermal core type micro-bolometer can be used effectively for LIT. The performance of the thermal core is quantitatively compared with the more expensive and bulky traditional infra-red cameras. It is shown that the thermal core can detect defects with similar overall performance as a cooled photon detector and an uncooled micro-bolometer. The low-cost, compact nature and small mass of the thermal core offers great potential in thermographic inspection opening the possibility of deploying devices permanently on structures in conjunction with the embedded actuators for in-situ monitoring in the service environment.

Lock-in thermography using miniature infra-red cameras and integrated actuators for defect identification in composite materials

Application of TNDE is difficult, if not impossible, in the case of materials having a low emissivity. It can be shown that radiation measurement cannot be successfully performed if material emissivity is smaller than 20% of black-body emissivity (Gaussorgues 1984b, p 421). This is, for instance, the case for materials having highly reflective surfaces, such as bare metal sheets. The bonded aluminium laminate samples studied in Chaps 1 and 4 and which are used in aeronautic and transport industries are good examples of such materials.

Inspection of Materials with Low Emissivity by Thermal Transfer Imaging

Honeycomb Sandwich Composites (HSCs) have been widely used in aerospace and aircraft industries because of their high temperature and corrosion resistance, etc. As defects and damage to HSCs are unavoidable, non-destructive testing (NDT) is essential to ensure the reliability of HSCs. A series of artificial flat-bottomed holes (FBHs) defects of HSCs were prepared for pulse thermography (PT) and LFM-Chirp-Square pulse-compression thermography (PuCT). LFM-Chirp-Square PuCT has the advantage of frequency modulation over PT to detect different depth ranges of FBHs defects. In order to improve the efficiency of defect detection, pulse phase thermography (PPT) and total harmonic distortion (THD) techniques were applied to process the infrared image sequences. The results show that THD processing technique outperforms the PPT processing technique in improving the signal-to-noise ratio (SNR) from feature images. The combination of LFM-Chirp-Square PuCT and THD techniques can be applied to improve the defect detection identification results in HSCs. 

LFM-Chirp-Square pulse-compression thermography for debonding defects detection in honeycomb sandwich composites based on THD-processing technique

The article discusses research in the field of using resistive heating elements cut out from conductive film as thermal stimulation sources for performing fast and effective active infrared inspection of aircraft parts made of composite materials consisting of carbon fibers and a polymer matrix. These heating elements can be added once only for the purpose of inspection, or they can be integrated into the structure of the composite parts. The suitability of their use for thermographic inspection of monolithic laminates as well as sandwich structures was experimentally verified.

Infrared Thermographic Inspection of Aerospace CFRP Laminates and Sandwiches: Resistive Heating Element as a Source of Thermal Stimulation

The honeycomb sandwich structures with carbon composite face sheets combined with aluminium or Nomex® honeycomb core are extensively used in the aircraft airframe structure. They provide some key b...

Tomáš Chlebeček

Papers

Testing of the heating element integrated into the honeycomb sandwich structure for active thermography inspection