| Due to excellent mechanical and acoustical properties,composite sandwich has been widely used in the construction of ships.Embedded fiber Bragg grating(FBG)sensors is found to be one of the most ideal and reliable solutions for structural health monitoring of composite ships,due to the characteristic of laminated manufacturing of composite and the demand for internal damage monitoring of structures.However,the presence of the embedded FBG sensors may cause geometric discontinuity and stress concentration.The damage and failure of sandwich structures with embedding FBG sensors has been studied in this paper,which works as a foundation for structural health monitoring of sandwich structure with embedding FBG sensors.Firstly,the numerical simulation method of resin pocket geometry is analyzed and modified,and the resin pocket length is simulated with the modified numerical simulation method.Then,the finite element model of sandwich panel with embedding FBG sensor is established to study the damage and failure modes of the panel under tensile,compressive and bending loads respectively.The results show that when the tensile and compressive loads are perpendicular to the direction of FBG sensor,the first damage is interface debonding between resin pocket and face.The presence of resin pocket causes the discontinuity of sandwich structures in the direction of loads.The stress concentration factor in the face in the load direction exceeds 3.When the tensile and compressive load direction is parallel to the direction of the FBG sensor,the embedded FBG sensor has almost no effect on the damage and failure of the sandwich panel,and the strength of the sandwich panel depends on the strength of the face.Under bending load,embedding FBG sensor has almost no effect on the damage and failure of the sandwich beam,and the flexural stiffness decreases rapidly because of PVC foam failure,which results in the failure of the sandwich beam.Then,the effects of resin pocket length,the diameter of FBG sensor,and the embedding method on the damage and failure of the sandwich panel are analyzed respectively.The results show that when the load direction is perpendicular to the direction of FBG sensor,the disturbance of the FBG sensor to the thickness of the sandwich panel is an important factor causing the stress concentration in the load direction in the face.Increasing the resin pocket length or reducing the diameter of FBG sensor can reduce the stress concentration factor in the load direction and increase the strength of the sandwich panel.Changing the embedding method of FBG sensor can effectively reduce the impact on the strength of sandwich panel.Half embedded in the face and half in the core has least impact on the sandwich panel.Finally,the FBG sensor is embedded into the marine sandwich L-joint specimen to study the influence of embedded FBG sensors on the structural damage and failure.The results show that with embedding the FBG sensor,the interface debonding between the resin pocket and the face occurs at a relatively small load,resulting in a "step" or "disappearence" phenomenon of the FBG sensor data,but it has little effect on the damage and failure of the sandwich structure because the debonding does not extend in a wide range under static load. |
