Detection study of delamination defects in composite cylinders based on CT scanning and Digital Shear Speckle Interferometry

Carbon fiber reinforced composite hydrogen storage cylinders are the essential components for hydrogen fuel cell vehicles., offering advantages such as low weight, high hydrogen storage density, and excellent fatigue resistance. However, due to the complex manufacturing processes, defects such as voids, delaminations, and wrinkles are often unavoidable, leading to complex local mechanical responses during service. These defects compromise the overall structural integrity of the cylinders and reduce their service life. Currently, nondestructive testing (NDT) techniques used for detecting internal defects in carbon fiber reinforced composite hydrogen storage cylinders include industrial CT, ultrasonic testing, and acoustic emission techniques. However, due to the multilayer structure of composite cylinders, acoustic signals may attenuate, making specific defect analysis challenging and increasing the likelihood of missed defects. Although industrial CT can accurately measure and locate defects, it is costly, inefficient, and involves certain radiation exposure, making it impractical for real-time inspection in production or operational environments. Consequently, there is a lack of appropriate evaluation methods during the production and service stages of these cylinders, leading to conservative designs and high costs. To address it, this study explores efficient and reliable defect detection and evaluation methods for carbon fiber reinforced composite hydrogen storage cylinders through experimental investigations using industrial CT and digital shear speckle interferometry.

First, industrial CT was used to perform tomographic scans on the carbon fiber-reinforced composite hydrogen storage cylinders. The brightness in the CT images is proportional to the material density, allowing for easier identification of delamination defects within the fiber-wound layers. Additionally, digital shear speckle interferometry based on the Michelson interferometer principle was employed to measure the surface deformation of the cylinders during hydraulic loading. As the internal pressure of the cylinder increases, the regions of surface deformation induced by internal defects gradually become apparent, manifesting as multiple pairs of alternating light and dark "butterfly" shear fringe patterns. Finally, the results obtained from the digital shear speckle interferometry were quantitatively analyzed to determine the out-of-plane displacement gradients and the distribution of out-of-plane displacements on the cylinders surface.

The industrial CT scans of the hydrogen storage cylinders revealed a significant presence of delamination defects within the cylinders, characterized by a wide range of sizes and random locations, with a large distribution area that has a non-negligible impact on the overall performance of the cylinders. The "butterfly" shear fringe patterns induced by defects, as measured by digital shear speckle interferometry, exhibited a high degree of overlap and consistency in terms of location and size with the delamination defects identified by the CT scans. Quantitative analysis of the digital shear speckle interferometry results showed that variations in the density of shear fringes under the same pressure differential reflect the differing effects of internal non-uniform delamination defects on the out-of-plane displacement of the cylinder surface. Under a pressure differential of approximately 0.626% of the working pressure, shear fringe distortion signals induced by delamination defects were observable, revealing micrometer-scale changes in out-of-plane displacement characteristics.Conclusions: Industrial CT was conducted on the hydrogen storage cylinders to analyze the distribution of sizes and locations of delamination defects, revealing that delamination is the predominant type of manufacturing defect within the fiber-wound layers of the cylinders. Integrating digital shear speckle interferometry measurements during the hydraulic testing of the cylinders, it was observed that the defect-induced "butterfly" shear fringe patterns closely matched the delamination defects identified by CT scans in both location and size. This consistency validates the applicability of digital shear speckle interferometry for non-destructive testing of the cylinders. The digital shear speckle interferometry allows for non-contact monitoring of surface deformations, providing real-time data support. When combined with hydraulic testing, it aids in evaluating the service performance and safety of the hydrogen storage cylinders.