Detection of Vertical Cracks in Carbon Fiber Composites Using an Infrared Line Scanner

Abstract

INTRODUCfIONVertical cracks are among the most problematic types of defects from the thermal nondestructive testing (NDT) point of view. Nowadays, the fast thermal inspection techniques most commonly utilize infrared cameras and pulsed heating provided by flash lamps. The flash lamps illuminate the whole surface of a sample, and the resulting heat flow is mostly perpendicular to the surface. Therefore, the heat signature caused by the vertical cracks may be very small, and the cracks may avoid detection.Techniques like "mirage" [I] and "flying spot" [2] have been utilized for crack detection successfully. Another possible solution might be the use of line heating and an infrared line scanner demonstrated earlier by Hartikainen [3]. This method has been used earlier for detecting delaminations [4] and impact damages [5] in carbon fiber composites. In this paper, the work is continued by applying the method for crack detection in unidirectional carbon fiber composites. The applicability of the method is based on the lateral heat flow generated by the moving line heat source. A vertical crack parallel to the line source will stop or hinder the heat flow, and a detectable heat signature should be formed. However, if the crack is perpendicular to the heating line, it will most probably stay undetected [6]. COMPUTATIONAL MODELThe carbon fiber composite sample used in this study consists of nine unidirectional plies, each of which is 80 11m thick (Fig. 1). The laminate was covered from both sides with wowen 2-D protective skins. The total thickness of the composite plate was approximately 800 11m. In the plate, several 20 mm long and 0.2 mm wide cuts were made to simulate vertical cracks reaching the sample surface. The cuts ranged from one to five plies in depth.

Optimized inspection design for the thermographic characterization of sub-pixel sized through cracks

“…Active themography is particularly well-suited for the detection of cracks penetrating through a thin, plate-like specimen. Preliminary studies demonstrated that, while cracks are among the most problematic types of defect for characterization with thermal NDT, they do block heat flow in the perpendicular direction [9][10][11][12]. This means that the heat from the known thermal input will be obstructed and produce a thermal response significantly different from a similar domain without a crack.…”