Bearing Properties Characterization And Analysis Of Bolted Joint Composite Laminate Structure And Research On The Fixture

As the wide application of fiber reinforced resin matrix composites, composite structures have developed from the initial non load-carrying structure into the primary load-carrying structure. Investigations showed that when structure failure and destruction occurred, it happened at the joints at a 60-80 percent of possibility. Thus, the design of the connection between the composite parts, especially the widely used bolt connection design is particularly important. Because the stress state around the bolt hole of the composite laminate structure is very complicated, the failure mode of the composite laminate is complicated and varied.All along, how to reasonably determine the permissible value of bolted joint structure, and improve the reliability and security of bolted joint composite structure have been the research hotspot of structural designers. Thus, a comparative analysis and improved design of the bearing properties testing standard of bolted joint composite laminate structure, the establishment of macro and microscopic combined numerical analysis method, the accurate testing, characterizing and analyzing of the bearing properties of bolted joint composite laminate structure will have important guiding significance to realizing the security and reliability design of bolted joint composite structure.Based on experimental and numerical analysis, firstly, this paper is going to make comparative analysis from several perspectives on the current bearing properties testing standard of bolted joint composite laminate structure, clarify scientific and reasonable testing methods, accurately evaluate the bearing properties of bolted joint composite laminate structure. Secondly, the macro-micro gradual damage numerical analysis method for the bolted joint composite laminate structure is set up combined with the improved Generalized Method of Cells. So, the destruction mechanism of bolted joint composite laminate structure is revealed from the perspective of mesoscopic. And the influence factors on the performance of the bolted joint composite laminate are researched. Finally, the effect of test fixture on the testing results is characterized and is analyzed. And the test fixture is researched and is improved further. The concrete research contents are as followed:1、Experimental research and analysis on the standard testing method for the bearing properties of bolted joint composite laminate structure is done. Firstly, the testing for the bearing response of the bolted joint composite laminate structure is done in accordance with the standard testing method such as the United States standard ASTM D5961, the national standard GB/T 7559 and the aviation standard HB7070. And in this process, the comparison and analysis of the three kinds of testing standard HB7070, GB7559, ASTM D5961-A is done. It gives the reason that the measurement result of the ASTM D5961-A testing standard is high. Meanwhile, it reveals the mechanism that the single shear bearing strength value is lower than that of the double shear bearing strength value.2、The macro-micro numerical method for predicting the bearing strength of the bolted joint composite laminate structure is created. Firstly, the relationship between the macro performance of composite and the microscopic performance of component material is established combined with the improved GMC. The macro-micro gradual damage numerical model for the bolted joint composite laminate structure is created based on the general finite element analysis software and its user defined subroutines. The effects of the mesh density, the failure criterion of constituent materials and the decay factor of materials properties on the numerical results are researched. And in order to verify and modify the numerical model, the numerical results are compared with the testing results. It lays the foundation for the subsequent characterization analysis and improvement of the test fixture.3、Characterize and analyze the bearing properties of the double-shear bolted joint composite laminate structure and research on the test fixture. Aim at the ASTM D 5961-A standard testing method, the macro-micro gradual damage numerical model considering the properties of constituent materials is created for the single bolted joint and double-shear composite laminate structure. Based on the model, the influence of microstructure of composite laminate on the bearing strength of bolted joint composite laminate structure is given for the tensile test of the bolted joint composite laminate structure. It shows that the initial strength of the simulation results and the experimental data are in good agreement.4、Characterize and analyze the bearing properties of the single-shear bolted joint composite laminate structure and research on the test fixture. According to the ASTM D5961-B testing standard, the test for the bearing properties of the single bolted joint single-shear composite laminate structure is done. The results are compared with the properties of the double-shear bolted joint composite laminate structure based on the ASTM D5961-A testing standard in order to analyze the difference of construct properties under the condition of different testing standard. The test fixture of the ASTM D5961-B testing standard is improved on several aspects. The effective stiffness of the fixture is increased, the single shear loading state is ensured and the scatter of the test data is effectively reduced.