In the present investigation, a two dimensional finite element model with absorbing boundary condition (B. C. ) has been developed to investigate the ultrasonic wave propagation in orthotropic media. The model can simulate experimental pulse echo or through transmission technique to obtain A-scan data of a domain with or without flaw. The flaws can be cracks, or any inclusion of different material.

In order to obtain simulated A-scan data, free of the effect of spurious reflections from the boundary, the usual practice is to select a large domain. But this procedure may result in huge memory requirement for computation. As an alternative, in this work, absorbing boundary conditions have been implemented on the desired boundary by the use of damping. A consistent damping matrix for the element is defined in terms of the shape functions of the element as

.....(1)

where [C] [N ] being the damping and shape function matrices respectively and

.....(2)

In the present work, the finite element analysis code solves the discretized domain for the displacement field at each time step due to prescribed excitation. An unconditionally stable and second order accurate algorithm [2], popularly known as Theta algorithm, has been implemented for time integration.

The results can either be viewed as variation of displacement of any specified node as a function of time (A-scan representation) or as the displacement over the entire domain at any specified time, termed as snapshot.

The simulation has been validated from time-of-flight considerations and other artifacts of wave propagation such as reflection, scattering and mode conversion in presence of flaws. Simulations have also been performed for orthotropic composite domain and feature evaluation for detection of defects have been performed. After doing FFT on the simulated A-scan data, frequency domain feature analysis is performed. sensitivity of harmonics in presence of different types of flaws are evaluated and the results are in well agreement with experimental observations [3]. The study thus, provides a guideline regarding study of certain harmonics sensitive to certain type of flaw. REFERENCES

1. Y. K. Chow, "Accuracy of Consistent and Lumped Viscous Dampers in Wave Propagation Problems", international Journal for Numerical Methods in Engineering, Vol. 21, pp. 723-732, 1985
2. C. Hoff and P. J. Pahl, "Development of an Implicit Method with Numerical Dissipation from a Generalized Single-Step Algorithm for Structural dynamics". Computer Methods in Applied Mechanics and Engineering, Vol. 67, pp. 367-385, 1988
3. D. Datta, C. V. Venkatesh and N. N. Kishore, "Application of Multi Dimensional Cluster Analysis in Identification of Defects in Fibre Composites", Nondestructive testing and Evaluation, Vol. 12, pp. 197-210, 1995