A thermoelastic technique (SPATE—stress pattern analysis by thermal emission) was used to determine the qualitative stress distributions and detect imbedded delaminations in thin composite beam specimens during vibrational excitation. The capability of the SPATE system to record qualitative stress distribution patterns in a vibrating beam was verified by scanning the beam at its resonance frequencies where the recorded stress patterns matched their corresponding mode shapes. It was found from the SPATE images that self-resonance of the imbedded delaminations occurred when the vibrating frequency of the beam specimen reached a certain value. Results from a finite element model exhibited good agreement with the experimental findings.