Mechanical Property And Bonding Methods Of GF/PP Sandwich Structures

Composite sandwich structure has been widely used in aerospace and other fields due to its high specific stiffness strength and multi-functional properties.The interface performance of the face-core bonding has become an important constraint to the thermosetting composite lattice sandwich structure.For low surface energy thermoplastic composites,existing adhesives do not meet the interface performance requirements of the sandwich structure,and it is necessary to develop a more effective face core connection method.Compared to thermoset composites,thermoplastic composites have more core joining options available for their high damage tolerance and repeated heating characteristic.In this paper,the mechanical properties of glass fiber reinforced polypropylene(GF/PP)composites under different joining processes were studied in terms of theoretical design,fabrication process and experimental characterization.The main contributions of this thesis are listed as follows:(Ⅰ)Hot-pressing and hot-melting methods were developed for thermoplastic composite sandwich structures.GF/PP corrugated sandwich panel was designed and fabricated.The flatwise compression properties of GF/PP corrugated sandwich panel with different relative density were experimentally studied and compared.Based on the theory of composite buckling,the prediction expression on the flatwise compressive strength on fabricated structure was derived.The ABAQUS VUMAT user subroutine was developed considering the typical failure mode and stiffness degradation criterion to simulate the mechanical response of the sandwich structure under flatwise compression.(Ⅱ)Mechanical properties and failure mechanism GF/PP laminate with adhesive bonding,mechanical bonding,hybrid bonding and resistance welding was systematically studied and compared.A mechanical bonding method using polypropylene stick as bolt was proposed.The joint interface performance and reinforcement mechanism of the structure were studied by single lap shear experiments.A resistance welding technique with new heating element was developed,and an evaluation frame on resistance welding process evaluation and an optimization method on process parameter were established.(Ⅲ)Polypropylene stick reinforcement technique was brought out to improve the facesheet/core bonding strength of GF/PP curved sandwich beam,which effectively suppresses the debonding between facesheet and core.Considering the buckling,crushing of the facesheet and core layer and debonding,the failure prediction of the curved axial,circular and polypropylene stick reinforced axial corrugated sandwich beams under threepoint bending was theoretically derived and the failure maps were furthermore drawn which well predicted the failure modes of fabricated specimens.Moreover,the influence of the debonding zone on the edgewise compression properties of the curved axial corrugated sandwich panel was numerically studied.(Ⅳ)Based on interlocking technique and hierarchical design,a simple and fast technique has been proposed to fabricate hierarchical thermoplastic composite(GF/PP)honeycomb cylindrical structure(HTCHCS).The axial compression performance and failure mechanism of HTCHCS with different configurations were experimentally.Aiming at both reducing the initial peak load and improving the crushing efficiency of the structure,the arrangement of the axial rib was optimized.The deformation recovery behavior and reloading property of t HTCHCS were also studied.