Analysis of stress effect on Lamb wave propagation in isotropic plates

This paper presents an analysis of the stress effect on Lamb wave propagation in isotropic plates based on the theory of nonlinear elasticity. In this study the plates are assumed to be initially isotropic hyperelastic and subjected to homogeneous stress. The theory of small deformations superimposed on large deformations is used to derive the acoustoelastic dispersion equations for both symmetric and anti-symmetric modes of Lamb waves. Different magnitudes of the inhomogeneous stress are considered in this study. The results of the theoretical predications show that the acoustoelastic effect of isotropy plates subjected to a realistic level of applied stresses is quite significant, especially for higher order Lamb wave modes near the cut-off frequencies.C. T. Ng, M. Mohabuth & A. Kotouso

Experimental study on local plastic collapse in a plate weakened by two collinear cracks

2015 the 4th International Conference on Advances in Mechanics Engineering (ICAME 2015)Fracture and fatigue assessment of structures weakened by multiple site damage (MSD) currently represents a challenging problem. Here, we communicate the outcomes of an experimental study conducted on a plate weakened by two closely-spaced collinear cracks, which represents the simplest form of MSD. The experimental results are used to verify the predictions of a recently developed theoretical approach.Munawwar Mohabuth, Donghoon Chang and Aditya Khann

Large acoustoelastic effect for Lamb waves propagating in an incompressible elastic plate

Abstract not availableMunawwar Mohabuth, Andrei Kotousov, Ching-Tai N

Effect of uniaxial stress on the propagation of higher-order Lamb wave modes

Abstract not availableMunawwar Mohabuth, Andrei Kotousov, Ching-Tai N

Experimental observation of cumulative second harmonic generation of Lamb wave modes

Non-linear Lamb waves have been employed in recent years to monitor microstructural changes in thin-walled metallic plate and shell structures. The extreme sensitivity of this technique is advantageous for detection, but also leads to high variability in measurements taken. The measurement of the material nonlinearity is complicated by several factors, such as the nonlinearity present in measurement equipment, transducer position and angle, as well as the coupling conditions at the transducer-specimen interface. The present article reports experimental results obtained for the inherent material nonlinearity in a large undamaged and unloaded Aluminium plate. The need for improved robustness of the experimental procedure is identified.M. Mohabuth, A. Kotousov, A. Khanna and C.-T. N

Implication of changing loading conditions on structural health monitoring utilising guided waves

Structural health monitoring systems based on guided waves typically utilise a network of embedded or permanently attached sensors, allowing for the continuous detection of damage remote from a sensor location. The presence of damage is often diagnosed by analysing the residual signals from the structure after subtracting damage-free reference data. However, variations in environmental and operational conditions such as temperature, humidity, applied or thermally-induced stresses affect the measured residuals. A previously developed acoustoelastic formulation is here extended and employed as the basis for a simplified analytical model to estimate the effect of applied or thermally-induced stresses on the propagation characteristics of the fundamental Lamb wave modes. It is noted that there are special combinations of frequency, biaxial stress ratio and direction of wave propagation for which there is no change in the phase velocity of the fundamental anti-symmetric mode. The implication of these results in devising effective strategies to mitigate the effect of stress induced variations in guided-wave damage diagnostics is briefly discussed.Munawwar Mohabuth, Andrei Kotousov, Ching-Tai Ng and LR Francis Ros

Measurement of residual stresses in rails using Rayleigh waves

This paper presents a new nondestructive approach for evaluating the residual longitudinal stresses in rails. The developed approach utilises the acoustoelastic effect to infer the longitudinal stress from the measured speed of Rayleigh waves propagating along the longitudinal direction. The measured Rayleigh wave speed along the longitudinal direction is shown to vary significantly across the height of the rail section, which can be directly correlated to the residual stress profile in the rail section. Unlike existing residual stress measurement techniques, such as hole-drilling or sectioning, the developed approach can be potentially applied for the in-situ residual stress measurement, without taking the rail out of service.James Martin Hughes, James Vidler, Aditya Khanna, Munawwar Mohabuth, Andrei Kotousov and Ching-Tai N

Mechanics and evaluation of early damage

Proceedings of the ICMFM XIXThis chapter describes the microstructural mechanisms leading to damage and the formation of fatigue cracks as well as the methods available to monitor these processes. The evaluation of early damage is especially important for structures with long service life spans, where the crack nucleation stage can dominate the total fatigue life.Andrei Kotousov, James Vidler, James Hughes, Aditya Khanna, Ching-Tai Ng and Munawwar Mohabut

Comparative evaluation of in situ stress monitoring with Rayleigh waves

The in situ monitoring of stresses provides a crucial input for residual life prognosis and is an integral part of structural health monitoring systems. Stress monitoring is generally achieved by utilising the acoustoelastic effect, which relates the speed of elastic waves in a solid, typically longitudinal and shear waves, to the stress state. A major shortcoming of methods based on the acoustoelastic effect is their poor sensitivity. Another shortcoming of acoustoelastic methods is associated with the rapid attenuation of bulk waves in the propagation medium, requiring the use of dense sensor networks. The purpose of this article is twofold: to demonstrate the application of Rayleigh (guided) waves rather than bulk waves towards stress monitoring based on acoustoelasticity, and to propose a new method for stress monitoring based on the rate of accumulation of the second harmonic of large-amplitude Rayleigh waves. An experimental study is conducted using the cross-correlation signal processing technique to increase the accuracy of determining Rayleigh wave speeds when compared with traditional methods. This demonstrates the feasibility of Rayleigh wave–based acoustoelastic structural health monitoring systems, which could easily be integrated with existing sensor networks. Second harmonic generation is then investigated to demonstrate the sensitivity of higher order harmonics to stress-induced nonlinearities. The outcomes of this study demonstrate that the sensitivity of the new second harmonic generation method is several orders of magnitude greater than the acoustoelastic method, making the proposed method more suitable for development for online stress monitoring of in-service structures.James Martin Hughes, James Vidler, Ching-Tai Ng, Aditya Khanna, Munawwar Mohabuth, LR Francis Rose, Andrei Kotouso

Effect of central and non-central frequency components on the quality of damage imaging

Accurate image reconstruction of damage through Lamb wave diffraction tomography (LWDT) requires substantial information of scatter field. This can be achieved using transducer network to collect the scatter field data. However, this requires a large number of transducers that creates logistical constraints for the practical applications of the technique. Various methods have been developed to improve the practicability of LWDT. One of the main approaches is to employ data at multiple frequencies within the bandwidth of the excitation signal. The objective of this study is to investigate the performance of using the data at non-central frequencies to reconstruct the damage image using LWDT. This provides an understanding on the influence of data at each individual frequency in the damage image reconstruction.In this paper, a series of numerical case studies with consideration of different damage sizes and shapes are carried out. Different non-central frequencies data is used to reconstruct the damage image. The results show that using the data at different non-central frequencies leads to different qualities of the reconstructed damage images. The quality of these reconstructed damage images are then compared to investigate the information contained of the data at each individual frequency. The study shows that the non-central frequencies data can provide additional information in the damage image reconstruction. Overall, the results of this study provide insights into the influences of the data at different frequencies, which is essential to advance the developments of the LWDT.Gnana Teja Pudipeddi, Ching-Tai Ng, Andrei Kotouso