Failure Strength And Main Factors Analysis Of Carbon Fiber Composites Bolted Joints

Carbon fiber briaded composites has been widely used in aeronautic, astronautic projects due to their excellent mechanical properties, such as high specific strength /rigidity and frictional resistance. With the technique development of the preparation process, the application field of carbon fiber briaded composites has changed from the military domain to civil industry, for instance the automotive industry, construction and sports etc. This is known as the "black gold" composite materials in the future civil fields will have extensive application prospect. Design and analysis of the joint is an important link to connect the whole design of material and structure, is the key of the whole structure design in the application of carbon fiber braided composites. In composite structures, three types of joints are commonly adopted, namely, the mechanically fastened joint, the adhesively bonded joint and the above two kinds of hybrid form. Generally, the bolted joints offer the advantage of allowing disassembly of the inspection structure, repair or replacement of components, and they are not sensitive to surface preparation, service temperature, or humidity. Thus, the bolted joint is still the dominant fastening mechanism used to joint the primary composite structural parts. By comparing with composite laminates, ultra high temperature performance of C/C composite has more advantages in thermal structure, and the 3D six-directional braided composites have more excellent interlaminar properties than laminate. Thus, the motivation of the present research was to study the mechanical properties of bolted joint with C/Cs and 3D six-directional braided composite, expecting that the mechanical properties of thoes structures could fill gaps in the corresponding preperty and more useful reference for engineering application. In view of the above, the research of this paper, which aimed at the C/C composite and 3D six-directional braided composite material as the research object, focused on studying the mechanical properties and failure mechanism of carbon fiber composite bolted joints through experiments and numerical simulation method. The main work includes the following content:Firstly, the tensile fracture of the 3D C/C braided composite structures has been studied through static tensile test. It has been showed that the failure of open structures caused by fiber fracture in tension stress and notch strength was also obtained. An analytic model based on the point stress criterion was established for composite joints for strength prediction. The open hole structures were analyzed using the reliable model. The result showed that the C/Cs is not sensitive to the notch.Then, C/Cs bolted joints were tested in tension load, based on the notch strength and notch sensitivity. Joint strength was tested for single bolted double shear bolted joints, and the effect of lateral restraint on failure load was studied. Failure modes and failure mechanism of joints were obtained. The progressive damage model was established based on Hashin failure criteria and cohesive element. By contrast experiment to determine the reliability of the numerical model, the double lap bolted joints parameters(geometry and torque) were analyzed and some optimal joint parameters were given for engineering application.Single lap single(multi) bolted structure for C/Cs was tested, which have low failure load owing to secondary bending. We analysis the influence of lateral restraint, geometry and joint form on the failure modes and failure loads. Failure load tested with a single row double pitch bolted joint and a double row single pitch bolted joint to provide experimental date for multi-bolted reference design. A numerical model was established, the reliability was analyzed, and the single lap bolted joints parameters(geometry and torque) were discussed. The optimal joint parameters were given for design.Finally, single lap single(multi) bolted for 3D six-direction braided composites was tested. The effect of restraint area and torque on the bolted joints failure load was studied. The failure mechanisms for bolted joint were given. The finite element model based on the characteristic curve and Yamada-sun failure criteria was established. 3D numerical model of the six-directional braided composites joint parameters discussed for single shear bolted joint design.