This research will discuss the behavior and damage response, naming initiation and propagation, of a 3D printed sandwich composite through the use of acoustic emission technique under indentation test. Re-entrant honeycomb (also known as auxetic honeycomb) is used as the core material. The structures are manufactured using polylactic acid reinforced with flax fibers. This type of material could be found throughout a variety of domains of applications such as automobile, aeronautic and aerospace. Thus, it is of high interest to study the mechanical response of this structure under indentation test to determine its response under such solicitation. To follow the invisible damage initiation and damage propagation in the sandwich composite, acoustic emission (AE) technique is used. The detected AE data signals are then post-processed in agreement with an unsupervised pattern algorithm. The classification of the AE signals is made through the use of the K-mean algorithm. The amplitude of the AE data points varies between 40 and 80 dB indicating invisible damage initiation and propagation. The analysis shows that there are three different classes of events: the core cracking, the matrix cracking and the fiber/matrix debonding. The effect of each damage mechanism on the total failure of the sandwich composite is determined. Moreover, microscopic observations on the failed surfaces are performed using the scanning electron microscope (SEM). Thus, AE classes are related to each damage mechanism.