This paper studies the reflection and transmissions of Lamb waves incident in plate cracks. The study is performed for one single linear transmission path between one emitter and one receiver. Time signals are acquired for a set of points along the transmission line and, inversely, space signals may be displayed for a set of time instants along the transmission total time, thus obtaining a matrix map of these space-time signals. By a simple representation of this map, it is possible to observe the modes being propagated, because different propagation velocities are separated on the space-time domain. In the presence of defects, it is also possible to observe mode conversion that is dependent on the size geometry and position of that defect. Using a two-dimensional Fourier transform, the information on the space-time domain is converted to the wavenumber-frequency domain, and it is possible to separate the transmitted or reflected waves. It is possible to relate the amplitude of those transmissions and reflections with the size of the cracks. The relation between the methodology of defect size estimation and the establishment of prognostics and health monitoring techniques will be addressed in the paper.