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Tech Insight: Numerical Simulation of Guided Wave Inspection

TWI’s experts use computational numerical simulations to model the propagation of ultrasound waves through complex geometries. The ability to accurately predict ultrasound behaviour can be used to improve the design of ultrasound transducers, signal processing and image quality, reducing scattering and allowing for improved imaging across cross-sectional imaging planes in real time.

Finite element analysis (FEA) and numerical modelling are used to simulate the behaviour and propagation of guided waves in structures such as pipes and rails, and the interaction of these waves with defects.

TWI has conducted a range of different projects across a variety of industry sectors, whether as core research for the benefit of our Industrial Members in different industries or as dedicated projects for individual Members or groups of companies.

Core Research Programme (CRP)

The core research programme (CRP) involves the creation of projects to address challenges faced by our Industrial Members, advancing technologies and processes and finding new solutions for industry problems. These projects are jointly funded by our Industrial Members, with the outcomes made available to all of our Members.

- Finite Element Analysis of Guided Waves in Pipe/Rail Flaws

At the time of this 2003 CRP project, the use of guided waves to test pipes non-destructively was becoming an established technique for industry, allowing tens of metres of pipe to be screened from a ring of transducers located at a single position. This project was created to assess whether a finite element technique could also be applied to detecting flaws in rails. This included quantifying the reflection characteristics of guided waves from flaws in pipes as well as validating the wave propagation modelling technique against experimentally measured transmission data in a steel plate. Our team also quantified both the propagation characteristics and reflectivity of guided waves from flaws in rails with a view to applying the technique to rails in the future.

- Improved Guided Wave Technology for Pipe Defect Detection

This 2007 project sought to advance long range ultrasonic testing by transforming guided wave inspection from a predominately screening procedure to one that allowed for exact screening and sizing. This would not only reduce the time and cost of inspection, but it was hoped to also allow for the control of the focused beam to ‘steer’ it around bends in pipes, opening up the application of the inspection technique to previously inaccessible regions of pipework. Modelling the focusing of guided waves was integral to this work and the potential to size corrosion damage. Finding an effective focusing technique for guided waves allowed our teams to determine the applicability of the technique for a range of pipe geometries.

- Sizing Locally Thinned Areas and Guided Wave Pipe Inspection

Continuing our development of guided wave technology, this 2008 CRP project used a combination of theoretical modelling and experiments to quantify the effects of pipe bends on guided waves so that optimised frequency selection or algorithms for a phased array style excitation could be used to overcome the effects of the bend. In addition, the project sought a technique for sizing locally thinned area defects in straight pipes using guided waves to better distinguish between uniform circumferential thinning and a severe patch of localised corrosion at one circumferential position.

- Long-Range Guided Wave Inspection Beyond Pipe Bends

Having successfully validated modelling technologies for studying the effect of pipe bends on guided waves and developed novel flaw sizing techniques, this 2009 CRP project aimed to use the insight modelling provides to quantify and gain an understanding of the behaviour of guided wave propagation in and beyond pipe bends. The project investigated the use of modelling to quantify the effects of a range of pipe bend angles on guided waves and developed signal reconstruction techniques for overcoming the effects of pipe bends on the propagation of guided waves.

- Long-Range Guided Wave Pipe Modelling and Inspection

Continuing to build upon the findings of the previous CRP projects, this 2012 project recognised the potential of guided wave inspection for unpiggable sections of pipeline, such as at cased road crossings where pipe bends distort the received signals. A combination of finite element analysis and experimentation were used to understand the behaviour of guided waves in a relatively tight pipe bend in order to develop a technique for correcting signal distortion caused by propagation around a bend.

- Quantitative Guided Wave Inspection of Pipes

This 2018 CRP work investigated a range of guided wave modes that had, up until that point, not been used for screening areas of pipe for corrosion (Figure 1). The aim was to see if these previously ignored modes, which all interact differently with different sizes and shape of defect, could capture information that can be used to detect smaller defects and provide a more quantitative assessment of them (Figure 2). This work informed future developments for inspection software to create time savings for the application of emerging quantitative guided wave inspection techniques. The effect of a 90° bend on the guided wave signals in a pipe was investigated for several wave modes and the effect of the bend on an axisymmetric signal was also calculated for four different geometries of bend elbow and welds. Finite element analysis was used to test a time reversal technique for producing a desired signal beyond a pipe bend and a new technique for measuring the axial extent of a flaw was developed and successfully experimentally validated.

- Guided Wave Inspection of Plate-like Structures

Moving away from pipe inspection, this 2019 CRP investigated the use of guided wave inspection for medium sized plate-like components that are inaccessible or hidden, ranging from flat plate structures such as bridges and storage tank floors to large diameter cylinders, like storage tanks walls, pressure vessels and wind turbine support structures. With these inspections, the use of the usual array of transducers as deployed for pipeline inspection was not applicable, so our experts developed and evaluated an ‘omnidirectional SH0 transducer’ capable of successfully ultrasonic guided wave testing plate-like components. This development was aided by finite element analysis ahead of prototyping and testing (Figure 3).

- Guided Wave Flaw Sizing for Pipe Inspection in the Field

This 2020 project investigated flaw sizing for otherwise inaccessible areas to provide reliable information about the dimensions of any flaws detected. Being able to quantify the severity of an anomaly would reduce the requirement for costly intervention such as excavation and inspection to determine the flaw size in a buried pipeline. Being able to determine the physical theoretical capability of the flaw sizing technique would provide important guidance for future guided wave tooling design. This project recommended hardware improvements as well as ascertaining the ability to measure flaw sizes at welds or beyond pipe supports. An experiment carried out for a flaw at a weld showed agreement between the finite element models and the experiment (Figures 4-5).

- Guided Wave Focusing for Pipeline Inspection in the Field

This 2020 CRP project continued from the previous one, with the aim of progressing the use of ultrasonic guided waves beyond simple pipe geometries with constant wall thicknesses. TWI’s experts used a combination of finite element analysis and experimentation to identify, develop and validate an improved focusing capability in situations such as pipes with weld cap geometry, pipe supports, pipe branches, or changes in thickness (Figures 6-7). Four state-of the-art guided wave focusing techniques were assessed, with the most successful being synthetic focusing (achieved through post-processing raw data from an unfocused test) and the analytical dispersion-removal (AD-focus) active focusing method (involving tailored inputs to different segments of the guided wave transmitting tool). Finite element modelling showed that the AD-focus method was successful in a range of set-ups, including different tool configurations (individual points, octants, multiple rings, collar gap), pipe sizes, focal angles, focal distances, and a number of flexural wave modes. These findings were validated by lab-based experiments, showing excellent agreement between model and experiment, giving high confidence in the other modelling results, and in the use of modelling for the development and refinement of guided wave technology.

- Signal Processing Techniques for Guided Wave Inspection of Buried Pipelines

Also In 2020, our experts addressed guided wave signal processing techniques for buried pipelines. Many pipelines are buried in the ground for safety or aesthetic reasons and in these cases non-metallic coatings are widely used to protect the pipe from corrosion. External coatings, which are often viscoelastic in nature, reduce the test range due to absorption of the sound energy into the coating, with different coating types and thicknesses having different effects on the inspection range. In addition, imperfections in the bonding conditions between the coating and the pipe and corrosion in the pipe can increase noise levels and degrade the signal-to-noise ratio (SNR). Our team investigated SNR enhancement through improved signal processing methods for coated pipe data to increase the capability of guided wave inspection.

Joint Industry Projects

Our joint industry projects (JIPs) allow interested parties to come together as project sponsors, pooling resources for greater research outcomes, while also being given exclusive access to the results and the opportunity to guide the direction of the projects themselves. These projects typically focus on challenges faced by specific industry sectors, providing industry-relevant information, guidance and solutions.

- Use of Long Range Guided Wave Ultrasonic Testing for Fitness-for-Service Determination of Pipelines

Announced in 2009, this JIP was created to integrate flaw sizing with fitness-for-service procedures and extend flaw sizing capability to a wider range of pipe diameters. The accuracy of these assessments were then established via validation tests to create validated techniques for the examination of difficult to inspect areas. The evidence that was gathered was used to support the use of the technology for presentation to regulatory bodies.

You can find out more about TWI’s work with the numerical simulation of guided wave inspection, including the support we can provide for Industrial Members, here:

https://www.twi-global.com/what-we-do/services-and-support/asset-management/finite-element-analysis/numerical-simulation-of-guided-wave-inspection

Figure 1. Guided wave inspection equipment
Figure 1. Guided wave inspection equipment
Figure 2. Comparison of the measured axial extent of a flaw from experimental data with the actual axial extent of the flaw, for a range of flaw sizes and shapes
Figure 2. Comparison of the measured axial extent of a flaw from experimental data with the actual axial extent of the flaw, for a range of flaw sizes and shapes
Figure 3. Test set-up - the prototype omnidirectional transducer, mounted at the centre of the plate, and the scanning vibrometer system, used to measure the ultrasound propagation
Figure 3. Test set-up - the prototype omnidirectional transducer, mounted at the centre of the plate, and the scanning vibrometer system, used to measure the ultrasound propagation
Figure 4. Finite element model used to simulate external corrosion at a weld in a 6-inch Schedule 40 steel pipe
Figure 4. Finite element model used to simulate external corrosion at a weld in a 6-inch Schedule 40 steel pipe
Figure 5. 8-inch pipe trial with flaws at welds
Figure 5. 8-inch pipe trial with flaws at welds
Figure 6. Experimental set-up for laser vibrometer experiments at TWI on a 6-inch diameter steel pipe
Figure 6. Experimental set-up for laser vibrometer experiments at TWI on a 6-inch diameter steel pipe
Figure 7. Example results from laser vibrometer when the standard active focusing technique was applied to a 6-inch schedule 40 pipe
Figure 7. Example results from laser vibrometer when the standard active focusing technique was applied to a 6-inch schedule 40 pipe
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