Due to the prevalence of fiber-reinforced composites in industrial applications, the need for damage-detection and characterization has increased. Ultrasonic nondestructive testing (NDT) is a powerful and non-invasive method of detecting flaws and defects in composite materials. Air-coupled and immersion UT techniques are used to quantify the effect of matrix microcracking and delamination within the composite laminate. Detection of matrix microcracks in the laminate is of primary concern since microcracking is one of the initial damage modes resulting in local stiffness reduction. The techniques considered include pulse-echo in immersion, angle-beam through-transmission Lamb wave time-of-flight shift and velocity measurements, and angle-beam backscatter measurements from single microcracks within the composite laminate. The time-of-flight measurements are shown to be a reasonable method for quantifying stiffness reduction in the laminate, however special attention must be provided to the Lamb modes considered and their sensitivity to the independent stiffness components. The angle-beam backscatter technique may be used to detect individual microcracks but poses a limitation on the frequency and depth of inspection. Ultimately, the quantification of microcracking will provide an early descriptor of failure in the laminate.
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