Composite materials have become increasingly vital in engineering structures, including spacecraft, airplanes, automobiles, boats, bridges, and buildings, due to their remarkable properties such as high strength, stiffness, fatigue resistance, light weight, and low thermal expansion. Additionally, structural health monitoring is crucial and indispensable for enhancing the safety and consistency of structures by deploying reliable sensors. Traditional sensors, mainly relying on an external power supply/battery, have many limitations regarding flexibility, shape adaptability, and sensitivity. Notably, flexoelectricity is an emergent electro-mechanical phenomenon found in all dielectrics due to inhomogeneous deformation, i.e., strain gradients. Furthermore, additive manufacturing/3D printing technology is becoming increasingly popular for generating complex 3D geometry using computers. In this context, we present the design and fabrication of a flexible, and shape-adaptable 3D printed flexoelectric sensor based on soft elastomeric polymer which exhibits excellent mechano-electrical sensitivity as well as the capability of accurately detecting the damages of composite structures from its early stage to progression and finally catastrophic failure, without the need for batteries or external power supplies. Thus, this new class of strain sensor can provide notable and fascinating information on in-situ defect monitoring in self-powered mode, stepping toward the advanced structural health monitoring application.