Modeling transient radiation of ultrasonic transducers in anisotropic materials including wave attenuation

The use of anisotropic materials such as fiber-reinforced composites and columnar-grained stainless steels as primary structural components has resulted in a critical need for reliable and effective NDE techniques. In interpreting the wave amplitudes recorded in ultrasonic NDE experiments, the complicated nature of wave propagation in these media as well as wave attenuation have to be considered. Modeling will reveal the quantitative features of these phenomena and thus help in the optimization of conventional and in the development of new NDE techniques. In this contribution, a point source superposition technique is applied to model transducer-radiated transient wavefields assuming anisotropic material and attenuation symmetry. The presented formulation involves characteristic quantities obtained from plane wave theory and the respective point source directivities. For composite materials, the viscoelasticity is taken into consideration thru a complex, frequency-dependent stiffness tensor. For obtaining the pulsed solutions, the harmonic radiation is calculated at many frequencies and then this data is numerically Fourier transformed into the time domain. Two problems are addressed in detail: (i) the generation and propagation of quasi-shear vertical waves using commercial angle beam probes is considered for transversely isotropic weld metal specimens. The calculated wavefronts are employed to determine the transducer directivity patterns as a function of the grain alignment. (ii) For a unidirectional composite exhibiting orthotropic material symmetry, the effect of viscoelasticity on the propagating quasi-longitudinal waves is examined for various insonification directions.