1. UK Research Centre in NDE Guided Wave Tomography for Accurate Corrosion Mapping in Inaccessible Areas Francesco Simonetti, Peter Cawley, Mike Lowe Imperial College

2. UK Research Centre in NDE Motivation Objective: Evaluate the maximum depth of corrosion patches at pipe supports (~1m), especially long supports on eg slug catcher lines. Use low frequency guided waves (cannot use ray tomography) 2 rings of transducers on either side of the support Large diameter pipe  simulation and experiments on plates

3. UK Research Centre in NDE Circular array of sensors Source Propagating incident field Defect Propagation of the incident field towards the defect Total field = incident + scattered fields Scattered field Diffraction Tomography (1)

4. UK Research Centre in NDE Diffraction Tomography (2) V0V0 V1V1 V0V0 Total field = incident field + scattered field Input to the diffraction tomography algorithm

5. UK Research Centre in NDE Diffraction Tomography (3) Dispersion curves in a plate (large diameter pipe assumption) 0123 1000 2250 3500 4750 6000 Frequency.thickness (MHz.mm) Group velocity (m/s) S 0 around 1.75 MHz.mm A 0 around 0.50 MHz.mm Does not consider: Mode conversion Reduced stiffness Reduced mass

6. UK Research Centre in NDE Finite Element Reconstruction Polychromatic reconstruction between 45 to 55 kHz : Simulated incident and scattered fields Multiple scattering Grating lobes

7. UK Research Centre in NDE Finite Element Reconstruction (2) Polychromatic reconstruction between 45 to 55 kHz : Simulated incident and scattered fields

8. UK Research Centre in NDE Experimental Reconstruction Experimental Setup 64 transducer array 50% deep tapered defect 10 mm aluminium plate

9. UK Research Centre in NDE Experimental Reconstruction Experimental polychromatic reconstruction between 45 to 55 kHz : No incident field subtraction – Only transmission data of total field Artefacts due to the scattering from the array of transducers

10. UK Research Centre in NDE Experimental Reconstruction Experimental polychromatic reconstruction between 45 to 55 kHz : No incident field subtraction – Only transmission data of total field – Laser Measurement

11. UK Research Centre in NDE Practical Issues: Contact Condition Guided wave tomography uses phase and amplitude of transmitted signals. Contact conditions can alter amplitude if: The pipe is coated with attenuative materials e.g. bitumen Corrosion scale forms a layer that effectively welds the pipe to the support The support is acoustically coupled to the pipe for the selected guided wave and frequency

12. UK Research Centre in NDE Practical Issues: Scattering Model Diffraction Tomography is based on the Born approximation that is valid if the defect size is comparable to and the thickness loss is small Travel Time Tomography uses the ray theory that is suitable if the defect is >>  and the distance between sensors L is not too large TTT image DT imageTrue Model

13. UK Research Centre in NDE Practical Issues: Scattering Model Travel Time Tomography does not work with small defects or defects that are less than  ( L) apart TTT image DT imageTrue Model

14. UK Research Centre in NDE Main Tasks Conduct field tests to identify the guided modes and testing frequencies that are less sensitive to contact conditions Develop a hybrid approach that combines diffraction and travel-time tomography to cover the realistic spectrum of corrosion defect extent and depths Convert from circular array to ‘cross-hole’ – two rings of transducers either side of support Develop reproducible, easily detachable transduction that does not produce scattering artefacts and build system Test in lab and field