Today, application of polymeric composites and sandwich panels has increased in the industry due to their lower weight to volume ratio and also better mechanical properties in comparison with metals used in automotive and marine industries in diverse structures. Detection of failure initiation and examination of failure mechanism in composites, especially for sandwich, panels are state of art. In this research, the Acoustic Emission (AE), as a non-destructive testing method, was applied to estimate the residual strength of the polyester/glass fiber sandwich pannel with polyurethane foam with 3 different lay-up techniques. Sandwich panels were placed in 3 different energy levels under a low velocity impact and, then, with a three-point bending test, their bending strength was evaluated using the acoustic Emission. By simultaneously analyzing the acoustic data and examining the force-displacement diagrams obtained from the bending test and their correlation, the remained strength of the sandwich panels, priorly damaged by impacts of different energy levels, is estimated. For this purpose, the accumulated acoustic energy during bending and strain energy from the force-displacement diagrams have been used to calculate the recently presented Sentry function of pre-damaged samples to compare with a virgin case without previous defect. The results show that there is a direct relationship between Sentry function data as a new indicator of residual strength and accumulated energy of acoustic data that contains the effects of various failure mechanisms. In the largest destroyed sample with fiber layout of 90 and 45 degrees with respect to bending direction containing a maximum pre-impact of 60 Jules, the highest strength drop was up to 27% compared to the virgin sample.