Study of inner surface behavior and structural integrity of composite liquid hydrogen (LH2) outer tank using a robotic crawler

Abstract

Inspection of composite liquid hydrogen tanks (LH2) for aviation is highly challenging using current non-destructive testing techniques. Also, manufacturing of these composite LH2 tanks is challenging due to thermal stresses, making post-manufacturing and in-service inspection essential for safe operations. The external insulating layer and restricted access to the tank’s interior are the reasons for this. To inspect the inner curved surfaces of a carbon fiber-reinforced polymer (CFRP) composite tank, a compact, high-payload-ratio ultrasonic probe-based inspection using an autonomous robotic crawler was developed. The developed device offers a workable solution for challenging LH2 tank inspection situations, as it is designed to function through tank apertures as small as 250–300 mm in diameter. For accurate localization on the uneven inner surfaces of the composite LH2 tank, the robot utilizes wheel encoders and features wheeled locomotion. A phase array-based wheel probe for ultrasonic examination, operating at 5 MHz, is used in conjunction with a spring load to ensure proper coupling with the surface. To perform straight-line motion on a CFRP composite surface under dynamic conditions at various speeds, this work examines proportional-integral-derivative tuning for a crawler, combined with an ultrasonic phased array probe, and determines an optimized speed for composite LH2 inspection. The device demonstrates the detection of possible ply drop-off and surface irregularity damage. The findings are important, as an investigation has been conducted to assess the quality of ultrasonic scans on the curved surface with varying thicknesses of the composite tank.