Tomographic reconstruction of elastic constants in composite materials using numerical and experimental laser ultrasonic data

Nondestructive characterization of composite materials has been attempted using the concept of ultrasonic tomography. A new method, Composite-ART (C-ART) is proposed for measuring the 2-d distribution of elastic constants. This leads to a nonlinear relation between the time-of-flight and the field variables. The solution approach employs a nonlinear least-square minimization with appropriate constraints to attain a reliable solution. The method turns out be a multiple parameter reconstruction as the elastic constants characterize a pixel. Extensive studies using simulated data are carried out to establish the robustness of the method. These studies provide the guidelines with regards to choice of the relaxation parameter and an initial guess as the controlling parameters for the reconstruction of a cross-section. The sensitivity of the controlling parameters towards the solution behavior is also discussed in detail. The experimental studies were done using laser-based ultrasound as the source radiation allowing one to exploit the point source and point detector approximation. Unidirectional composites with defects were scanned and good quality reconstructions are obtained. It is observed that a low relaxation parameter helps in presence of noise due to measurement error.