WEB Detection of defects in fiber composite materials with the active lock-in thermographyWednesday (29.04.2020) 15:20 - 15:40 Room 1
Detection of defects in fiber composite materials
with the active lock-in thermography
It is important to be able to exclude damage to the material to guarantee component functions. Therefore, they must be detected. For fiber composites, this can be achieved non-destructively with active lock-in thermography. The method is based on the response of the component to light excitation. The optical waves are translated into thermal waves in the component. Local differences in the sample interfere with the conductivity of the waves and are reflected on the sample surface by temperature differences. These differences are measured contactless with an infrared camera.
The aim of this work is the automated image analysis of the active lock-in thermography of glass fiber reinforced thermoplastics. For this purpose, schematics of the defect images are required which suppose a clear and meaningful image interpretation. By knowing the influence of the process parameters on material damage, the proportion of rejects can be reduced by readjusting the process.
Up to now, 100 tensile specimens measuring 205x25x1.5 mm made of glass-fibre reinforced thermoplastics have been examined by means of active thermography. The specimen material was prepared with defined defect patterns such as fiber breakage, fiber displacement, delamination and insufficient consolidation in the uppermost and middle fabric layers. The experimental arrangement of the two halogen lamps, the infrared camera and the samples, the excitation frequency, the number of measurements and the measurement arrangements of reflection and transmission were varied and tested.
Subsequently, all samples were systematically excited by halogen spots in three measuring periods with a frequency of 0.05 Hz each. The measuring time was approx. 37 seconds. The camera stood half a meter directly in front of the sample. To the left and to the right of the camera, the halogen spotlights were positioned slightly higher at the same distance from the sample. A large number of material damage was visible on the phase images. The optical quality of the camera, the characteristics and local position of the defects, the excitation frequency and the number of measurements are of decisive importance for the detection of the defects.