Finite Element Simulations of Lamb Waves Propagating in Thin Beams Containing Defects
Finlay Casey · 2013
This thesis focuses on modelling Lamb wave propagation and scattering at defects in a thin aluminium beam using the finite element software Autodyn. Previous studies have examined similar problems using different software, yielding promising results. This study will look to assess Autodyn’s capabilies in modelling general Lamb wave propagation as well as scattering behaviour at a defect within a structure.Wave motion was simulated in a 590x25x2mm 6061Al beam model. Simulated phase and group velocities were compared to analytical predictions for a plate of infinite width, as no solution for a beam (with finite width) has been previously developed. Phase velocities determined from the simulations were in good agreement with the analytical solutions, whereas the simulated group velocities slightly overestimated analytical values.Simulations were then carried out for a beam containing a 0.4, 0.6 and 1.2mm through-width, transverse notch. Simulated results were compared to data drawn from a previous study conducted by Alleyne and Cawley [14] based upon a 2D approximation. Reflection and transmission ratios were defined in order to compare the simulated data with that of the previous study. These ratios express the magnitude of the waves transmitted through and reflected from the notch as a fraction of the initial wave pulse. Reflection ratios were shown to increase with notch depth, whereas transmission ratios dropped as the notch size was increased. This result was in line with trends identified in the literature.However, it was noticed that the magnitude of the travelling initial pulse after the notch was greater than it was prior to the defect. This implies that the wave gained energy as it passed through the notch. This phenomenon is not supported by any prior work in this area and is considered to be an anomaly associated with the software itself.From this it was concluded that Autodyn was unable to accurately model wave transmission and reflection in a beam containing a defect. More work is required to further explore Autodyn’s capabilities for modelling ultrasonic waves in structures containing defects.