Nondestructive Evaluation (NDE) Techniques Assessment for Graphite/Epoxy (Gr/Ep) Composite Overwrapped Pressure Vessels

NDE methods evaluated included eddy current measurements, ultrasonics, radiography, acoustic emission monitoring, acoustic-ultrasonics, and interferometric (e.g., shearography) methods. The major interest was in the detection of damage to COPVs caused by impacts at energy levels that would leave no visible scar on the COPV surface. Each of the above-mentioned NDE methods was assessed with regards to its utility for the detection of impact damage to COPVs and its applicability in the field. Three COPVs were impacted at various energies and then inspected using each of the NDE methods. The results for each method are presented, followed by a summary comparing the advantages and disadvantages. Several NDE techniques were shown to be effective for detecting impact damage sites on graphite/epoxy overwrapped pressure vessels even if the impact energy was below the threshold for creating visible surface damage. These techniques (ultrasound, radiography, thermography, shearography, and eddy current testing) are sensitive because a small dent is left in the liner following the impact. The impact damage detection thresholds for the various NDE techniques employed depend on the size and shape of the vessel. The tup characteristics and the internal pressure of the vessel during impact also affect the degree of damage and, hence, the detectability of the impact site. The size of the indication associated with a flaw is also dependent on the technique used. Visual inspection can lead to an underestimate or lack of detection of the damage. For both thermography and shearography, consistent sizing could be accomplished through the use of calibration standards and a consistent test set-up. The ultrasonic and eddy current results are most likely accurate measures of how much of the liner was debonded.