Quantitative Determination of Porosity by Active Thermography

We present a new approach based on Active Thermography by which it is possible to produce images of porosity in Carbon Fibre Reinforced Plastics that correspond striking well with results obtained by ultrasonic testing. We applied the method of Pulsed Thermography by using flashes of light for the generation of heat. The evolution of surface temperatures which depends on the diffusivity of the sample and on the sample's geometry could be well fitted by means of a heat conduction model. Images of porosity are generated by means of an evaluation by which the influence of the light intensity distribution on the data is eliminated. Thus corrections with respect to the lateral distribution of the heat generation are not necessary, nor are emissivity corrections. Corrections due to the influence of geometry, however, had to be taken into account. The quantitative evaluation of the porosity is based on its linear relation to the diffusivity. Images of porosity obtained thermographically are compared with corresponding images as obtained by state of the art ultrasonic testing. We show that the thermographic method exhibits a better sensitivity and resolution of porosity. Measurements have been performed on samples with average porosities between 1 and 5 %. With some modifications the method can be applied for the quantitative characterization of delaminations of multi-layered samples as well.