The structural health monitoring of structures during active use (in service) has long been of interest to the NDE community. One technique uses passive ultrasound or Acoustic Emission (AE). However, the interpretation of the AE signals is difficult especially when the operator tries to distinguish between the growth of harmless micro-cracks and the development of harmful delaminations. This paper focuses on two types of structures, i.e., aluminum plates such as used in wing structures in aircraft and graphite plates such as encountered in aircraft disc brakes where carbon-carbon composite is used . The objective in this work is to distinguish the acoustic emissions (AE) caused by delaminations from those associated with microcracking. The technical approach is to use finite element methods (FEM) to simulate AE from sources represented by piezoelectric wafers embedded in the composites. In flat panels of graphite and aluminum-alloy AE waveforms were modeled from transverse cracks and longitudinal delaminations. The results show distinct differences in the amplitudes, durations and frequency content creating a potential avenue for distinguishing between these two flaw types.