The use of S₀ mode Lamb waves generated at low frequency range for measuring the distribution of micro-cracks occurred in plate-like structures

In 2009, a railway manufacturer signed a $217 million contract with the Land Transport Authority of Singapore to provide new trains for that nation’s Mass Rapid Transit (MRT) system. After the new trains were delivered, the MRT found surface cracks in some of the 35 subway trains’ bodies. Further analysis found that these cracks occurred due to impurities introduced during manufacture of the aluminum alloy plates. The 35 trains were rejected and sent back to the manufacturer for replacement. During the manufacture of composite materials, impurities can be introduced. Ignoring such impurities may result in the formation of numerous micro-cracks due to fatigue, and these cracks can ultimately make the materials break into pieces. Recently guided wave has become a promising method to inspect the integrity of metal plates. When applying guided wave to inspect composite materials in plate-like structures, multiple wave modes will be generated. The occurrence of multi-wave modes is more frequent at high frequency ranges, which host high order antisymmetric (A₁, A₂, A₃ … A_n) wave modes as well as symmetric modes (S₁, S₂, S₃ … S_n). Different type of wave mode will propagate within the plate at different phase velocity, making the identification of defect location occurred in the plate very difficult. A number of methods have been proposed to separate various wave modes so that a particular wave mode with a steady velocity can be used to analyze the reflected wave signal for locating each defect position. Alternatively, a simple method can be employed is the use of the two fundamental modes, A₀ and S₀ at low frequency range. At this low frequency range, basically there are only two wave modes with distinguishable velocities. S₀ is traveling nearly double the velocity as compared to that of A₀. Hence, it is much easier to determine the location of defect by examining the temporal waveforms generated by the reflections of these two modes. In this paper, the S₀ mode Lamb waves is used to detect the existence of defects. The results show that S₀ is promising in detecting micro damages by revealing the growth of the second harmonic of S₀. The amplitudes of second harmonic reflection and transmission generated from the primary S₀ mode Lamb wave interacting with a localized microstructural damage were measured. The length and width of a microscale damage are found to be related to the growth of the second harmonic components. An index was also designed to provide a quantitatively evaluation on the nonlinearity of the inspected material.