THE APPLICATION OF THE TIME-OF-FLIGHT-DIFFRACTION INSPECTION TECHNIQUE ON

GRP STRUCTURES

Johann OosthuizenChris StantonZach McCann

AEA Technology plc, PO Box 4416, Edenvale 1610Tel: (011) 450 2324, Fax: (011) 450 2327, email: aeat@worldonline.co.za

ABSTRACT

Time-of-flight-diffraction (ToFD) is an ultrasonic inspection method originallydeveloped by the United Kingdom Atomic Energy Authority (UKAEA) for the non-destructive evaluation of components in the nuclear industry. The method relies onthe diffraction of ultrasonic energies from 'corners' and 'ends' of internal structures(primarily defects) in a component being tested.

ToFD is now recognised as the most rapid, versatile and reliable method of ultrasonicNDT available to industry today. ToFD was originally developed as a method ofaccurately sizing and monitoring the through-wall extent of welding defects and in-service flaws, primarily in steel components. Maurice Silk, its inventor, released thefirst report for publication in 1974, stating the main principles of the technique. Theseoriginal principles have not needed revision since, which is a clear indication of theintegrity of the technique.

AEA Technology has now adapted the ToFD technique and developed inspectionprocedures to locate, identify and size defects in glass reinforced plastic (GRP)structures. The procedures cover defects such as delamination, flange cracks,fractures, cracks, tears, incorrect lamination, disbonding, lack of adhesive, materialdegradation, porosity & voids, inclusions and foreign bodies, and incorrectdimensions.

Tests have shown that ToFD can accurately identify and size defects in GRPlaminates of practical construction and thickness. The technique also allows for lay-up verification, as it is possible to identify the individual layers in the laminates.

INTRODUCTION

Overview of NDT techniques [1]

In general all of the conceivable NDT techniques available for the inspection fordefects/flaws in Polymer Matrix Composites (PMC’s) have been researched,developed or applied in practice. From the various information searches it wouldappear that there are 3 NDT techniques for composites that are currently mostprevalent in the literature: these techniques are ultrasonics, thermography andshearography. Note that this reflects current R&D effort and a different set of NDTtechniques would be selected on the basis of practical application in the field.

The ultrasonic inspection technique has by far been the subject of the majority ofthese activities and reflects the belief that the technique has the potential to addressmost of the inspection problems presented by PMCs. Ultrasonics is seen as the onlyreliable technique for the inspection of thick composite laminates. Ultrasonics is theprincipal technique used for the inspection of adhesive bonds in PMCs and there aremany examples of use of the ultrasonic technique for the inspection of a variety ofbonded components (e.g. bonded doubler plates on aircraft, automotivecomponents). The ultrasonic NDT technique is regarded as a relatively fastinspection technique, however, large area in-service inspection applications requirecareful consideration. Unlike inspection during manufacture (when insertion orsquirter coupling methods can be readily applied), in-service inspections need toconsider the effective ‘coupling’ of the ultrasound into the component and thescanning of the inspection site. The display of inspection results can be verypowerful and informative, in particular the data presentation of a C-scan inspectionrevealing delaminations in a composite laminate.

Thermography is also very well advanced as an inspection technique for PMCs, inparticular for the inspection of thin laminates. The attraction of the technique lies is inthe speed of inspection, however, the capabilities of the technique to reliably detectthe range of expected defects/flaws in PMCs, and its application to thicker laminatestructures, is not fully understood.

Another rapid inspection technique, shearography, appears to be capable ofinspecting large areas in real time and is being practically used for the inspection ofcomposite structures and pressure vessels. Shearography senses out-of-planesurface displacements in response to an applied load, the surface displacementsbeing indicative of flaws/defects in the material. The most effective means ofapplying load to the structure has been found to be thermal and surface vacuumtechniques and a number of propriety systems now exist for shearographyinspection. However it is questionable whether shearography is yet sufficientlydeveloped to be considered a practical field inspection method.

In addition to these 3 externally applied techniques there has been considerable workundertaken into the research and development of embedded devices (e.g. fibreoptics, embedded piezoelectric transducers etc.) used for the ‘health monitoring’ ofcomposite structures. It is anticipated that this approach to the in-serviceinspection/integrity measurement of composites will continue to be developed inparallel with the improvements in the capabilities and understanding of the popularNDT inspection techniques.

The Time of Flight Diffraction Method

The ToFD technique uses a single probe pair in a transmitter-receiver arrangement.Longitudinal probes are usually applied for transmitting and receiving the ultrasoundthrough the material that is being inspected (Fig. 1).

When the ultrasound is incident at linear discontinuity such as a crack, diffractiontakes place at its extremities in addition to the normal reflected wave. This diffractedenergy is emitted over a wide angular range and is assumed to originate at theextremities of the flaw. This is in marked contrast with conventional ultrasonictechniques, which rely on the amount of energy reflected by discontinuities.

Fig. 1. The ToFD principle

The ToFD method detects a surface (lateral) wave travelling directly between theprobes and also a backwall echo from energies that reach the back of the testpiecewithout interference from defects. The diffracted signals are received via the receiverprobe and are evaluated with the Ultrasonic System to greyscale images (Fig. 2).

Fig. 2. Greyscale image of a ToFD scan

The study of this phenomenon has led to the successful use of the ToFD method onmetallic components for:• Flaw Detection: as signals may be recorded from a range of flaws;• Flaw Sizing: since the spatial (or time) separation of the diffracted waves is

directly related to the height of the flaw.

Application of an adapted ToFD method to GRP Laminates.

AEA Technology (SA) has now adapted the ToFD technique and developedinspection procedures to locate, identify and size defects in GRP structures. Thiswas accomplished through extensive experimentation with probe settings andmanipulation. In order to verify the accuracy of the ToFD technique when inspectingGRP laminates test pieces with representative defects were manufactured.

Test laminate 1 consisted of 20 layers of 450g/m2 CSM with a NCS 901PA-resinsystem. Delaminations and foreign objects were included in the laminate and withthe size and laminar positions recorded. The manufactured defect positions arepresented in Table 1.

Table 1. Laminate Defect Schedule

Layer Manufactured Defects Other Defects2 • Delamination (φ55mm)

• Metal inclusion (φ2 × 23mm)4 None6 None8 Metal inclusion (35o/d × 19i/d)

10 Delamination (105×50mm)12 None14 Delamination (φ95mm)16 None18 None20 None

• 3 mm slot × 4 mm deep• 10 mm slot × 7 mm

deep• 18 mm slot × 7 mm

deep

The ultrasonic inspection trials were performed using standard ToFD equipment usedfor the examination of plates and piping. The ToFD probes were operating in the‘Pitch and Catch” mode with a semi-automated encoding system. The Microplus unitwas set to capture A-scans every 0.5mm over the full length of each scan. Datacapturing was carried out across the defects from both sides of the plate.

The scans produced using the ToFD equipment were able to adequately image theartificially induced delaminations, foreign bodies, machined notches and themechanically induced delamination which had been worked into the test laminate.The orientation of the scan did not affect the results achieved. Typical inspectiondata obtained from the inspection technique is shown in Fig. 3.

Fig. 3. Greyscale image of Defects within a GRP Laminate

During the analysis of the grey-scale image the type of defects can be identified. Thedimensions and depths of the flaws in the test laminates could be determined bymaking use if the Microplus processing facilities. Identification of the defects areshown in Fig. 4 with the defect sizes presented in graphical form in Fig. 5.

Fig. 4. Defect identification

The through-thickness dimensions shown are consistent with the layer number atwhich the defects were placed.

Fig. 5. Dimensional data obtained from the Grey-scale image

Other applications in the GRP industry

• Laminate verification: The inspection technique allows individual layers within theGRP laminate to be identified from the images obtained during the inspection.The inspection technique could therefor be used as a laminate lay-up verificationmethod.

• Detection of debonding of the lining: Since it is possible to detect significant andabrupt shifts in the backwall, the technique can be used to locate the debondingof linings.

• Nozzle-to-shell cracks and delaminations: Specific probe and scanner set-upscan be achieved for complex geometry items by manufacturing job-specific probeholders to enable the equipment to focus within the area of interest.

References

1. “Generic procedure for the Ultrasonic Inspection of FRP Composites”, Draftdocument by AEA Technology, National NDT Centre, UK, August 2000.