Ageing infrastructure is one of the imperative issues worldwide for sustainable development. The need to quickly find creative solutions to address the significant economic implications and the potential risks to public safety is well understood. Hence, the development of robust, cost-efficient and reliable technologies to provide in-situ safety diagnosis of structures is of upmost importance. A local damage at a critical section of a structure can lead to catastrophic failure, which has been demonstrated in a number of structural failures in the past. Apart from the essential requirement that inspection system must be reliable to incipient damages, two highly desirable features are capacity for graphical representation and the ability to evaluate damage quantitatively. Our research group is working on the problem of using Lamb waves to achieve quantitative imaging of the local damage in plate-like structures.
Lamb waves (named after Horace Lamb who first described them in 1917) are also known as guided plate waves. They are a kind of ultrasonic wave that propagates along thin-walled structures with free boundaries, such as plate or shell. They are either symmetric or anti-symmetric in their form. The average displacement of symmetric Lamb waves over the thickness of the structure is in the longitudinal direction, whereas the average displacement of anti-symmetric Lamb waves is in the transverse direction. We are developing a Lamb wave tomographic imaging approach for in-situ safety inspection of structures. Lamb wave tomographic imaging is similar to X-ray computed tomography (CT) but instead of using ionising radiation it employs Lamb waves that can be transmitted across the inspection area from a remote and accessible transducer location. While the interaction of photons with matter can be described by simple ray models in X-ray CT, scattering diffraction and refraction phenomena characterise the encoding of the mechanical property information of Lamb wave signals. Diffraction and refraction add considerable complexity to the problem of retrieving Lamb wave tomographic imaging and managing these phenomena present the main challenge in developing this approach. Therefore at a fundamental level, we are currently investigating Lamb wave propagation and scattering characteristics at different types of damages. We have gained a fundamental insight into this physical phenomenon and are currently developing a general approach of Lamb wave diffraction tomography, which can provide a quantitative imaging of the damage.
The research team has already carried out an extensive investigation. Finite element analysis and laser Doppler vibrometer were used to numerically and experimentally investigate the physical phenomenon of the Lamb wave propagation and scattering phenomena at damages in metallic and carbon fibre reinforced composite laminates. The findings improved the fundamental physical insights of Lamb waves in different types of damage, e.g. corrosion in metallic plate and delamination in composite laminate, which provided a strong theoretical basis in developing a quantitative and cost effective damage imaging technique. A diffraction tomography has been developed and we are currently evaluating its performance using finite element simulation data and experimental verification will be carried out as well.
This research could result in an imaging technique for identifying incipient damage in structures. The technique not only quantitatively identifies the location, size and shape of the damage, but also provides a graphical presentation of the damage detection results to assist engineers who are in making judgments about the remedial work.
1. Ng, CT. (2015) "A Two-stage Approach for Quantitative Imaging of Laminar Damage in Plates Using Lamb Waves".
Earthquakes and Structures An International Journal, 8(4): 821-841.
2. Ng, CT. (2015) "On the Accuracy of Analytical Modelling of Lamb Wave Scattering at Delaminations in Multilayered
Isotropic Plates". International Journal of Structural Stability and Dynamics, 15(7): 1-12.
3. Ng, CT, Veidt, M. Rose, LRF, Wang, CH. (2012) "Analytical and Finite Element Prediction of Lamb Wave Scattering at
Delaminations in Quasi-isotropic Composite Laminates". Journal of Sound and Vibration, 33(22): 4870-4883.
4. Ng, CT. (2012) "Scattering of the Fundamental Anti-symmetric Lamb Wave at Delaminations in Composite
Laminates". Journal of the Acoustical Society of America, 132(1): 115-123.