Automatic delamination defects detection in radiographic sequences of rocket boosters

Solid rocket motors are routinely examined in real-time X-ray radioscopic mode. In a tangential configuration, the peripheral part of the object is imaged in order to detect the delamination defects between the propellant and the external metal envelope. We have previously developed a real-time imaging system, based on a tomosynthesis algorithm in order to improve the contrast. In this paper, we present a method to perform the automatic detection of delaminations. A single image being not sufficient for robustness, a spatio-temporal aspect is required for the algorithm. In a first step, the proposed method computes the displacement between the current radiograph and a reference one. The images are then registered and compared. On the resulting difference image we apply a smoothed thresholding to obtain an instantaneous confidence image. These images are then cumulated over time to get maps of probability of a defect pixel. Defects are then enhanced thanks to a neighbourhood validation. Thus the method combines spatial and temporal aspects in both detection and characterization. Several experimental tests have been performed, on both true and simulated radiographs of boosters and of other cylindrical objects.