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Babcock has achieved a significant step forward with a new technique allowing non-destructive examination (NDE) of composite repairs and the underlying substrate – something they describe as a valuable advance allowing repairs to be monitored for assurance of long term performance.
The company, combining its composite technology and non-destruction examination expertise, explain that they have developed an advanced ultrasonic method for the examination of fibre reinforced composites, which allows defects to be identified in sufficient detail to enable verification for quality assurance purposes and long term asset integrity management.
According to Babcock, the quality of the consolidated laminate stack making up the composite repair, in addition to the bond line performance (where the laminate bonds with the substrate) , are integral design constraints in offering a composite repair or strengthening solution. Any defects, and in particular voids, in the laminate will compromise the structural integrity of the designed matrix.
With the new method, Babcock say they can now identify discontinuities, flaws, air pockets, creases or foreign bodies in laminate or the area between the substrate and laminate that could affect the reliable performance or lifetime of the composite repair. The Babcock examination method involves an innovative application of existing equipment modified in-house, underpinned with extensive testing by highly experienced engineers.
Babcock explains that trials of the new method have been undertaken on multiple laminates constructed by Babcock’s composites technology, manufactured from various different resins reinforced with glass and carbon fibres. They say the most striking capability revealed by these trials when compared to existing composite NDE techniques is the clarity of the images produced, showing the individual plies of the laminate. Whereas previous composite NDE methods have had the capability to identify some defects under a narrow band of conditions, Babcock claims their method is potentially capable of identifying the actual type of defect. The trials also proved that examinations can be carried out through substrates of mild steel up to 10 mm thick to examine the laminate and bond line.
Additionally, results from the trials showed that the metallic substrate below the repair can also be clearly seen, along with a good indication of its thickness. In designing a composite repair the substrate is not always assumed to be redundant, so the ability to monitor any further substrate degradation can be a useful means of risk assessment, where structural integrity is important.
Babcock says the data produced by the inspection technique allows the production of images showing cross sections through the structure of the composite in three planes; the surface and two sides. The scan data can be used to provide verification for quality assurance purposes at any time following curing or hardening of the composite repair.
Taking this new development forward, Babcock Composites Technology General Manager David Coumbe points out that further work is now needed to quantify the results and prove the accuracy of the substrate thickness measurement. “Currently, in order to produce an accurate thickness measurement of both the composite and the substrate, two scans are required taking into account the different speeds of sound in the two materials,” he explains.
“Having said that, with the equipment setup for the examination of the composite we can clearly view the substrate, and identify any variations in thickness. Using dedicated analysis software we can effectively ‘zoom in’ through the material viewing each ply within the laminate and view areas of particular interest.”
He continues, “With the new technique we can provide customers with the confidence that structural rehabilitation using composites can be assured as a permanent solution (with the often considerable time and cost savings that brings), rather than a temporary fix to be replaced at a later date with conventional material components.
“With the ability to monitor, and to react to emerging defects should they arise, we are now able to offer long term asset integrity management of our solutions.”
Composite repairs using Fibre Reinforced Plastics (FRP), a matrix of fibre and resins in a layering process, have been increasingly recognised in a range of industries, including the defence, marine, oil and gas, and rail sectors, for their high value in restoring full structural strength and pressure integrity to corroded structure, plant or other components without requiring costly shutdown or replacement. These repairs are designed with a quantified, demonstrable design life according to the application needs (which could be anything from a temporary repair of a few months, to 20 years or more).
However, Babcock explains that monitoring the condition of the substrate, and the quality of the repair itself (which is a highly skilled, specialist procedure), has traditionally been difficult as most existing NDT methods were developed for the examination of homogeneous metallic materials. Additionally the nature and configuration of the repairs, compounded by the inability to see through the laminate, has made the employment of conventional non-destructive tests challenging.
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