The Measurement of Porosity in Composite Materials Using Infrared Thermography

Abstract

The presence of porosity in polymeric matrix composites degrades the en gineering performance, and for many critical applications it is necessary to ensure that the porosity of the as-manufactured composites is minimized. Nondestructive evaluation (NDE) techniques, such as ultrasonics or X-ray, are often used to inspect composites for porosity and non-conforming parts are rejected. An infrared thermographic technique is described here, which can be used to determine the degree of porosity in polymeric matrix composite materials. The technique consists of heating one face of the part with a high power laser and observing the thermal response on the other face as the heat diffuses through the part. Internal defects such as porosity modify the apparent diffusivity and reduce the rate and quantity of heat transferred to the opposite face. Experiments were conducted on carbon-epoxy samples with a range of known porosities and varying thick nesses and the thermal response was measured. The results show that for the high porosity samples smaller temperature rises, with longer times to reach the maximum were observed on the opposite face. An analysis of the heat transfer through the samples is presented and this analysis shows how the thermal diffusivity influences the temperature rise observed on the opposite face. The results from this analysis were used to interpret the experimental thermal response in terms of an apparent diffusivity and a good correlation was obtained between the diffusivity and the porosities for the samples tested.