Study On The Pull-through Characteristics Of Carbon/Epoxy Braided Composite Laminates

Mechanical connection is the main composite structural connection at present.Study on its strength becomes especially important for structural design. Among all failure modes, pull-through failure is caused by the excessive load from bolt along the plate thickness direction. The failure process is complex. Research on the pull-through property is vital for the structural design.In this paper, the characteristics of pull-through damage of braided composite laminates with counterbore holes are investigated from two aspects: its pull-through strength,the influences of hydrothermal conditions, and thickness effect through by tests; The damage process, the influences of the fixture restrain size and the bolt head's diameter through the numerical simulation with ABAQUS finite element software.Experimental studies show that pull-through damage process can be divided into two stages: In the first stage, load and displacement increases linearly until the initial damage occurs, with the corresponding failure load; In the second stage, load and displacement increases nonlinearly, and slowly growth until the laminate fail at last. The pull-through strength is deduced from the failure; Temperature and humidity conditions have significant effects on the strength. The matrix would soften after moisture absorption under the hot and humid conditions(82?,85%),leading to the reduction of bearing properties. Its initial damage occurs earlier and failure load reduces by 38.12%. The matrix becomes brittle under the cool and dry conditions(-55?, dry), and the brittle failure occurs only once. The failure load is the maximum load, and increases by 34.53%. The Failure load increases with the increasing plate thickness, and its maximum failure surface would change from the upper position to the lower position along the laminates' thickness direction.Numerical studies show that the matrix damages earlier than fiber failure. The load drops dramatically due to the fiber failure, indicates that the pull-through strength is controlled by the fiber. Matrix failure mainly occurs in the layer of 90 degrees, 0 degrees and 45 degrees. Fiber failure mainly occurs in the layer of 90 degrees, 0 degrees and a part of-45 degrees. The bearing properties of 45 degrees and-45 degrees layer is higher than 90 degrees and 0 degrees layer. The matrix and fiber tensile failure mainly occur in upper layer, the matrix and fiber compression failure mainly occur in bottom layer. The pull-through fail load decreases with the increasing pore diameter of fixture, when the pore diameter change from 53 mm to63mm. The diameter has less effect on the failure load. The leveling of the bolt countersunk head has great effect on the failure load when bolt head embedded into the laminates' hole, and if the bolt head protrudes from laminates' surface, it has less effect on the failure load.