Quantitative analysis of defects in composite material by means of optical lockin thermography

Abstract

In the aerospace industry, carbon-ber reinforced plastic (CFRP) materials are becoming increasingly popular. Due to mechanical fracture and hence safety related issues, CFRP components must be inspected for defects with non-destructive methods. This thesis focuses on non-destructive testing of CFRP materials with optical lockin thermography. The eld of quantitative analysis of thermographic measurements is enhanced. The data of geometrical parameters e.g. depth, size and shape of defects in structures of globally homogeneous and anisotropic CFRP materials is required for fracture mechanics. To evaluate defects in a quantitative way, image processing algorithms are applied to thermographic phase images in order to get panoramic views of extended aircraft parts and to compare measurements before and after a fatigue load in order to determine potential defect growth. Images of lockin and ultrasound excited thermography are combined with data-fusion techniques to get improved information on defects such as impacts. The image formation process can be modeled through a point-spread function, which depends on the depth of the defect and the modulation frequency. A function is computed by using Green's functions and is adapted to anisotropic materials. The quantities depth, size and shape of a defect are determined through inverse numerical lters. Measurements are compared to numerical simulations and a reconstruction algorithm of planar subsurface defects is validated